design of diketopyrrolopyrrole (dpp)‐based small...

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Design of Diketopyrrolopyrrole (DPP)-Based Small Molecules for Organic-Solar-Cell Applications

Ailing Tang, Chuanlang Zhan,* Jiannian Yao, and Erjun Zhou*

DOI: 10.1002/adma.201600013

1. Introduction

Organic semiconductors including small molecules and poly-mers have gained a lot of attention over the last few decades due to their low-cost, light-weight, easy modification and flex-ible manufacturing.[1] As a result, polymer–fullerene-based bulk-heterojunction (BHJ)-type organic solar cells (OSCs) have made remarkable improvement, especially in the last decade, and the power conversion efficiencies (PCEs) of exceeding 10% have been realized.[2] Meanwhile, small-molecule–fullerene-based OSCs have also exhibited PCEs as high as ≈10%.[3] On the other hand, the inherent shortcomings of fullerene deriva-tives, such as the need for high purity, weak absorption in the long-wavelength region, restricted electronic tuning and meta-stable morphology, force the development of novel alternative

After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modifica-tion, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modi-fications, such as the variations of donor–acceptor building blocks, alkyl sub-stitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic small molecules.

Dr. A. Tang, Prof. E. ZhouCAS Key Laboratory of Nanosystem and Hierarchical FabricationCAS Center for Excellence in NanoscienceNational Center for Nanoscience and TechnologyBeijing 100190, P. R. ChinaE-mail: [email protected]. J. Yao, Prof. C. ZhanBeijing National Laboratory of Molecular ScienceInstitute of ChemistryChinese Academy of SciencesBeijing 100190, P. R. ChinaE-mail: [email protected]. E. ZhouYangtze River Delta Academy of Nanotechnology and Industry Development ResearchJiaxing, Zhejiang Province 314000, P. R. China

electron acceptors. By judicious materials design and device optimization, the PCEs for the single-layer, fullerene-free OSCs have been increased up to 8%[4] (polymer as electron acceptors) and 11% (small mol-ecule as electron acceptors),[5] respectively. Recently, 1,4-diketopyrrolo[3,4-c] pyrrole (DPP) emerged as a very promising elec-tron-deficient motif to construct above-mentioned four kinds of photovoltaic materials.

Diketopyrrolopyrrole (or pyrrolo[3,4-c]-pyrrole-1,4-dione, as shown in Scheme 1) as a new pigment class was first obtained in 1974.[6] DPP-containing polymers have been subsequently reported since from 1993.[7] However, it was not until

2008 that DPP-based small molecules and polymers were applied in solution-processed organic photovoltaics (OPV) for the first time by Nguyen and Janssen, respectively.[8] At the same time, the first DPP-based polymer semiconductor for organic thin-film transistors (OTFTs) was reported by Win-newisser et al.[9] Since then, DPP as a promising building block, has been widely applied for design of the high perfor-mance organic semiconductors. Through the energy level engi-neering, the affording DPP derivatives can be used as p-type, n-type, or ambipolar organic semiconductors. In the field of OSCs, DPP-based polymer electron donors have achieved remarkable PCEs as high as 9.4%.[10] DPP-based conjugated polymers often exhibit an broad optical absorption, even up to 1000 nm,[11] and good film-forming characteristics as well as film morphology, resulting in high short-circuit current (Jsc) and good fill factor (FF) in solar cells. On the other hand, DPP-containing small-molecule materials could offer advan-tages over DPP-based polymers in terms of ease of synthesis and purification, uniform and defined molecular structure, less batch-to-batch variation, and higher open-circuit voltage (Voc). In addition, DPP-based small molecules possess a greater tendency to self-assemble into ordered domains, which leads to high charge-carrier mobilities. At present, the PCEs of the DPP-containing small-molecule donors have reached up to 8%.[12] Moreover, DPP derivatives also have served as an electron acceptor instead of the conventional fullerene deriva-tives in BHJ OSCs, and recently, DPP-based small-molecule acceptor and polymer acceptors showed gratifying progress with high PCEs of 5.1% and 2.9%, respectively.[13]

In OTFTs, they also exhibited promising charge trans-port properties. The record high hole mobility values of 17.8 cm2 V−1 s−1 and the electron mobility values of 4.34 cm2 V−1 s−1 were attained for p-type[14] and n-type[15]

Adv. Mater. 2016, DOI: 10.1002/adma.201600013

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some DPP-based polymer showed excellent ambipolar charge transport properties with high hole and electron mobilities (hole/electron mobilities: 8.84/4.34 cm2 V−1 s−1).[15] And certain DPP-containing small-molecule semiconductors also revealed very promising performance in field-effect transistors with a high hole mobility of 0.7 cm2 V−1 s−1[16] and electron mobility of 0.96 cm2 V−1 s−1.[17]

This impressive growth of the DPP-based organic semicon-ductors in the past decades has attracted a lot of attention and hence triggered the emergence of a huge literatures in this area. Several recent review articles focused on the application of the DPP-based organic semiconductor, especially polymers, for fluo-rescent probes, OTFTs and OSCs.[11a,18] However, very limited review articles summarized the research progresses of the DPP-based photovoltaic small molecules. For example, in 2012, Tian et al. reported the development of organic materials containing DPP unit in the field of BHJ OSCs and dye-sensitized solar cells.[19] Zhan et al. listed the part of DPP-based small molecules in his review article entitled “Small-Molecule Semiconductors for High-Efficiency Organic Photovoltaics”.[20] Thus, a compre-hensive review focused on expounding the relationship between the structural design, optoelectronic properties and photovoltaic performance for the DPP-based small molecules is needed.

In this progress report, we try to exhibit the structure–prop-erty relationships by analyzing the reported DPP conaining small molecules and predict the challenges and prospects of the development of this kind of materials. The molecular engineering, including design of conjugated backbone, choice of alkyl side chains, and modification of end-capped groups have been applied for improving the optoelectrical properties of DPP-based small molecules, as shown in Figure 1. These structural modifications concentrated on optimizing the light-harvesting ability, molecular energy levels, solution-processi-bility as well as the film-forming characteristic. For example, modulating the conjugated building blocks principally could control not only the light-harvesting ability and molecular energy levels by intramolecular charge tranfer, but also the film-forming ability by adjusting the intermolecular packing. Besides the solubilization effect of the alkyl side chains, their length, branching position and symmetry could also act on the molecular packing and crystallinity, which in turn affect the film morphology. Furthermore, it have been proved that the end groups in DPP-based small molecules play a vital role on the optoelectronic properties, carries genera-tion, transportation and collection. Combining the molecular engineering with the device optimization, the PCEs of DPP-based small molecules have been improved from the original 2.3%[8a] to 8%[12] (as electron donors), and the original 1% to 5.1%[13] (as electron acceptors) respectively.

2. The Synthesis and Properties of DPP Building Block

2.1. Synthesis of DPP Monomer

In 1974, DPP, as an unexpected by-product, was first reported by Farnum et al. from the classical reformatsky reaction with

benzonitrile, ethylbromoacetate and zinc as reactants. In this work, they isolated this highly insoluble, thermodynamically stable, brilliant red and crystalline by-products in 5–20% yield.[6] In order to improve product yield, several optimized synthesis

Ailing Tang, received her Ph.D. degree in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2014 under the supervision of Prof. Jiannian Yao and Prof. Chuanlang Zhan. She is cur-rently an assistant professor at the CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center

for Nanoscience and Technology (NCNST), China. Her research interests include the design and synthesis of organic small molecules for organic solar cells and the device fabrication.

Chuanlang Zhan, received his Ph.D. degree from the Institute of Photographic Chemistry, Chinese Academy of Sciences in 2000 under the supervision of Prof. Duoyuan Wang, and joined CAS Key Laboratory of organic Solids, Institute of Chemistry, Chinese Academy of Sciences (ICCAS) as a postdoctoral fellow. He was promoted as

an associate professor in 2002 and a professor in 2012 at ICCAS. His research activities focus on synthesis and self-assembly of functional molecular materials, solar energy conversion, and solar cells.

Erjun Zhou, received his Ph.D. degree in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2007 under the supervision of Prof. Yongfang Li. From 2007 to 2014, he worked in Japan with Prof. Kazuhito Hashimoto and Prof. Keisuke Tajima as a postdoctoral fellow/research scientist at

the JST, the University of Tokyo and RIKEN. In 2014, he joined National Center for Nanoscience and Technology (NCNST), China as a professor. His research interests are the design, synthesis, and characterization of organic and polymeric functional materials for optoelectronic and photovoltaic applications.



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routes have been reported. For example, in 1988, Iqbal et al. utilized benzonitrile to react with succinate esters in the pre-sent of the base, as shown in Scheme 1. This synthetic route increased the reaction yield up to 60–70%, which promoted the commercial production and application of DPP pigments.[21] Numerous DPP derivatives (as shown in Scheme 1) have since been synthesized by changing the benzonitrile with other aro-matic nitriles, such as furan,[22] thiophene,[23] selenophene,[24] thiazole,[25] thieno[3,2-b]thiophene,[12,26] pyridine,[27] etc.

2.2. Molecular Structure and Properties of DPP Monomer

The infrared spectra (IR) showed that the structure of DPP core contained free functional groups CO, NH, C=C, forming bicyclic lactam rings.[6] The X-ray structure analyses obtained by Mizuguchi and co-workers (Figure 2) indicate that the DPP core is virtually planar and present strong intermolecular interaction

in the solid state including hydrogen bonding (between N–H and O) and π–π interactions (between the adjacent bicyclic lactam rings).[28] The highly ordered molecular stacking in the solid state benefited the intermolecular charge hopping, which endow DPP dyes with high hole and electron mobili-ties. Accordingly, many DPP-containing polymers showed high hole mobilities values above 10 cm2 V−1 s−1 and high electron mobilities up to 4 cm2 V−1 s−1. DPP-based small-molecule semi-conductors also showed high hole/electron mobilities around 1 cm2 V−1 s−1.

Most of the DPP dyes show strong absorption with high εmax (molar extinction coefficients) values as well as high quantum yield of fluorescence. The solid state absorption maxima of DPP can be strongly induced by the conversion of the substi-tutes, which has allowed the development of DPP dyes covered a large color area. For example, as shown in Figure 3, Patil et al. reported three DPP-based derivatives with varying the electron-donating ability of the donor groups, such as benzene (PDPP-Hex), thiophene (TDPP-Hex) and selenophene (SeDPP-Hex).[29] It is obvious that all of them showed high εmax around 1–3 × 104 m−1 cm−1 and a gradual bathochromic shift of both π–π* and the low energy band on going from PDPP-Hex to TDPP-Hex to SeDPP-Hex. Fluorescence quantum yield meas-urements reveal that PDPP-Hex exhibited a high photolumi-nescence quantum yield (PLQY) of 85%, while TDPPHex and SeDPP-Hex showed PLQY of 79% and 66%, respectively.

Benefiting from the bicyclic lactam rings, DPP dyes show strong electron-withdrawing ability, and can be widely used as an electron-deficient building block in donor–acceptor (D–A) synthetic strategy. For example, as shown in Figure 4,



Scheme 1. Chemical structures of DPP chromophores and the classic synthesis route.

Figure 1. The explanatory drawing of the structure–property–performance relationships in the DPP-based organic solar cells.


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Li and co-workers calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of three DPP-based molecules (a DPP core being flanked with thiophene (DBT), benzene (DBP), and pyridine (DBPy)).[27a] The simulation results showed the deep HOMO around −5.1 to −5.4 eV and LUMO energy levels around −2.5 to −2.8 eV, respectively, indicating that DPP is a stronger electron-withdrawing building block.[27a] Additionally, DPP dyes possess exceptional photochemical, mechanical and thermal stability. These outstanding semiconducting properties have primarily promoted DPP derivatives to be applied in fluo-rescent probes,[18e] solid-state dye lasers,[30] organic field effect transistor (OFET),[31] and OSCs,[19,32] etc.

2.3. The Synthetic Procedure for DPP-Containing Small-Molecule Semiconductors

DPP has been regarded as an interesting block in D–A syn-thetic strategy due to its possibility of making various synthetic

modifications. On the other hand, its struc-ture allows to attach soluble alkyl chains (either linear or branched) on the 3,4-position (i.e., the lactam N-atoms) for getting solution-processed materials. More importantly, there is also enormous potential to attach various blocks to the 2,5-position of the DPP moiety for affording promising organic semiconduc-tors. Taking dithienyl-DPP as an example, Scheme 2 displays the possible synthetic mod-ifications of DPP monomer. Firstly, the alkyl chains were attached to the amide position by N-alkylation to yield the soluble dialkyl-DPP monomer (2). Subsequently, the bromina-

tion of aryl units was a key step to ensuring the occurrence of the subsequent palladium-catalyzed coupling reaction. Direct bromination with a molar equivalent or two molar equivalent N-bromosuccinimide (NBS) is possible to yield monobromi-nated 3 or dibrominated 4. Then, Pd-catalyzed Stille (or Suzuki) coupling reaction between brominated dialkyl-DPP (3 or 4) and a molar equivalent aryl-tin compounds (or aryl-boron rea-gents) furnished molecule 5 or 6 in a good yield, respectively. Compound 5 can be converted to the desired intermediate 6 by further bromination reaction. Finally, a variety of novel π-extended conjugated backbone could be achieved by typical Stille (or Suzuki) reaction, which is performed with all types of brominated dialkyl-DPP (3, 4 or 6) and aryl-tin compounds (or aryl-boron reagents). As shown in Scheme 2, DPP-based small molecules could be classified as follows: a central DPP chromo-phore flanked with two π-conjugated units as arms, forming the single-DPP-type conjugated backbones; two or multi DPP chromophores flanked on a central π-conjugated unit to get double-DPP or multi-DPP-type conjugated backbones.



Figure 2. a) The chemical structure of the DPP-Ph and the intermolecular interactions between DPP-Ph in the crystal lattice: b) hydrogen bonds; c) π–π stacking. b,c) Reproduced with permission[18i] Copyright 1992, Wiley-VCH.

Figure 3. a) Chemical structures of three DPP molecules; b) Absorption spectra in toluene; c) Photograph taken to show the variation in the emitting behavior of the three different DPP derivatives under UV-light illumination (≈365 nm); d) the emission spectra of the three DPP molecules in toluene. Reproduced with permission.[29] Copyright 2014, The Royal Society of Chemistry.


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3. DPP-Based Photovoltaic Small Molecules as Electron Donors

As mentioned above, according to the number of the DPP units, in principle, DPP-based small molecules can be divided into three types: single-DPP, double-DPP and multi-DPP respectively. We will introduce reported DPP-based photovoltaic small mol-ecules by means of this classification.

3.1. Single-DPP-Type Photovoltaic Small Molecules

According to the different arm building blocks, we further classify single DPP small molecules to three types. Scheme 3, 4, and 5 show the chemical structures of the small molecules with DPP chromophores as core. Table 1–3 summarize the maximum absorp-tion wavelength, frontier orbital energy levels, hole mobilities and the photovoltaic performances of these molecules.

3.1.1. DPP Chromophore as Core with Non-Fused Building Units as Arms

In 2008, DPP-based small-molecule donor (a1, as shown in Scheme 3) was applied in solution-processed BHJ OSCs for the first time by Nguyen et al.[8a] This molecule consisted a diketopyrrolopyrrole core with t-butoxy carbonyl (t-Boc) groups on the N,N-positions as alkyl chains and two dithiophene units arms, which showed a broad absorp-tion band with a λmax at 724 nm in solid film. A PCE of 2.33% was recorded when blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Soon after, they used the 2-ethyl-hexyl groups instead of the t-Boc groups as the alkyl chains on the lactam nitrogen atoms.[33] The resulting compound a2 exhib-ited deeper HOMO levels, and more ordered molecular orientation, higher hole mobilities. Blended with[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), a2 showed improved device performance with a PCE of 3%. In 2011, Reynolds et al. designed an amphi-philic DPP-based π-conjugated oligomer (a3) along with terminal lipophilic dodecyl chains and lateral hydrophilic triglyme chains and explore how the self-assembly behavior of the alkyl chains acted on device perfor-mance. The experiment results of particular p-channel OFET indicate that the introduc-tion of flexible triglyme chains did not depre-ciate the device performance, but rather



Figure 4. The optimized HOMO/LUMO electron distribution diagrams and geometry of DBT-Me, DBP-Me, and DBPy-Me obtained by computer simulation with density functional theory (DFT) calculations. Reproduced with permission.[27a] Copyright 2014, Wiley-VCH.

Scheme 2. Synthetic procedure for different kinds of DPP-containing small-molecules semiconductors.


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impart desirable solubility for purification and processing. On the contrary, the PCE of a3-based BHJ OSCs was only 0.68%, lower than that of a2-based solar cells, likely due to undesirable

phase separation.[34] The large phase separation come from the strong tendency to self-assemble into oversize fibril width through synergistic solvophobic effects and π–π interactions.



Scheme 3. Chemical structures of small-molecule donors with DPP as core and non-fused π-conjugated building units as arms.


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Scheme 4. Chemical structure of small-molecule donors with DPP as core with fused building units as arms.


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In 2014, Janssen et al. synthesized three isomeric π-conjugated molecules (a4a–c) based on diketopyrrolopyrrole and thiophene substituted with hexyl side chains in different position. They found that the position of the side chains could affect film mor-phology, and the kinetics of crystallization and crystallite orien-tation. Learning from the thin film’s absorption spectra, they observed that a4a aligned in a J-type configuration with hexyl at the 3-position, the formation of a H-type configuration for the 4-position in a4b, whereas an intermediate aggregation is found for a4c with hexyl at the 5-position. Furthermore, a4a exhibited a high degree of ordering and a4b showed a certain freedom of orientation with a tight π–π stacking distance, whereas a4c gave a long π–π stacking separation. As a result, a4b gave the highest PCE of 3.3% with the highest photocurrent and fill factor (FF).[35]

Zhan and Yao et al. compared the difference of the end-capped octylthiophene and the octylselenophene groups and

they found that with respect to the octylthiophene unit (a4d) attached onto the DPP core, the introduction of a polarized octylselenophene (a5a) can enhance the self-assembly prop-erties of the small molecules and improve the photovoltaic performances.[36] Luscombe et al. investigated the difference of the device performance by replacing hexylbithiophene end groups with tetradecylselenophene (a5b) and tetradecylbise-lenophene (a6). Because of strong internal electron transfer between the selenophenes and the DPP core, they exhibited broad optical absorption than that of the hexylbithiophene end groups. Though the addition of selenophene rings in a6 resulted in an increase in planarity and a red-shift of the absorp-tion spectra with a lower bandgap than that of a5b, a6 showed a poor PCE than that of a5b. The reason was that the active layers for a6 had too large domains and high roughness values to limit the hole mobility, in turn, the overall device performance.[37] Ko



Scheme 5. Chemical structure of small-molecule donors with DPP as core with ethenyl or ethynyl-linked building units as arms.


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Table 1. Optical and electronic properties, hole mobilities, and OPV performance of DPP-based small-molecule donors with DPP as core and non-fused π-conjugated building units as arms.

λmaxa)

[nm]HOMO/LUMOb)

[eV]μh

c) [cm2 V−1 s−1]

Active Layer Jsc [mA cm−2]

Voc [V]

FF PCE [%]

Ref.

a1 742 −5.03/−3.00 5 × 10−7 (S) a1:PC61BM = 7:3 8.42 0.67 0.45 2.33 [8a]

a2 720 −5.20/−3.70 1.0 × 10−4 (S) a2:PC71BM = 1:1 9.2 0.75 0.44 3.0 [33]

a3 750 −5.50/−3.90 3.4 × 10−3 (F) a3:PC61BM = 1:1 2.41 0.55 0.55 0.68 [34]

a4a ≈680 – – a4a:PC71BM = 2:1 5.89 0.85 0.50 2.48 [35]

a4b ≈555 – – a4b:PC71BM = 2:1 7.47 0.84 0.53 3.30 [35]

a4c ≈600 – – a4c:PC71BM = 2:1 5.30 0.79 0.45 1.90 [35]

a4d 659 −5.06/−3.38 3.2 × 10−4 (S) a4d:PC61BM = 3:1 5.59 0.78 0.44 1.90 [36]

a5a 669 −5.01/−3.38 3.0 × 10−4 (S) a5a:PC61BM = 3:1 5.81 0.86 0.46 2.33 [36]

a5b 695 −5.46/−3.46 4 × 10−5 (F) a5b:PC61BM = 1:1 4.90 0.77 0.41 1.53 [37]

a6 722 −5.39/−3.65 2 × 10−5 (F) a6:PC61BM = 1:1 3.40 0.70 0.36 0.84 [37]

a7 648 −4.88/−3.40 4.99 × 10−5 (S) a7:PC71BM = 1:0.9 5.73 0.59 0.45 1.50 [38]

a8a 625 −5.17/−3.51 2.01 × 10−6 (S) a8a:PC71BM = 3:2 11.17 0.69 0.56 4.32 [39]

a8b 620 −5.16/−3.49 0.96 × 10−6 (S) a8b:PC71BM = 3:2 5.29 0.67 0.28 1.00 [39]

a9 610 −5.33/−3.60 9.64 × 10−6 (S) a9:PC71BM = 3:2 8.55 0.94 0.50 4.02 [40]

a10 690 −5.33/−3.58 4.65 × 10−5 (S) a10:PC71BM = 1.5:1 11.4 0.89 0.53 5.37 [41]

a11 635d) −5.17/−3.50 – a11:PC71BM = 1:1 13.6 0.72 0.48 4.73 [42]

a12 675d) −4.82/−3.25 – a12:PC71BM = 1:1.5 12.24 0.69 0.51 4.30 [43]

a13a 699 −5.12/−3.33 – a13a:PC71BM = 1:1 3.12 0.76 0.46 1.09 [44]

a13b 705 −5.05/−3.27 – a13b:PC71BM = 1:1 5.09 0.76 0.53 2.05 [44]

a14a 639 −5.22/−3.44 4.9 × 10−5 (S) a14a:PC61BM = 1:1 1.94 0.74 0.32 0.46 [45]

a14b 652 −5.25/−3.49 6.94 × 10−5 (S) a14b:PC61BM = 1:2 2.19 0.89 0.28 0.55 [45]

a14c 655 −5.18/−3.44 7.94 × 10−5 (S) a14c:PC61BM = 1:1 4.05 0.88 0.45 1.59 [45]

a14d 637 −5.50/−3.69 4.2 × 10−3 (S) a14d:PC71BM = 1:1 8.27 0.93 0.54 4.2 [46]

a14e 651 −5.48/−3.68 – a14e:PC71BM = 1:1 3.73 0.93 0.35 1.2 [46]

a15 ≈700 −5.25/−3.58 – P3HT:PC61BM:a15 = 1:0.8:5% 9.84 0.60 0.58 3.37 [47]

a16a 550d) −5.22/−3.18 4.56 × 10−4 (F) a16a:PC71BM = 1:1 8.06 0.78 0.52 3.23 [48]

a16b 625d) −5.08/−3.14 6.34 × 10−4 (F) a1 6b:PC71BM = 1:1 10.63 0.72 0.60 4.65 [48]

a17a 563 −5.28/−3.24 5.8 × 10−4 (S) a17a:PC71BM = 1:1 7.34 0.90 0.46 3.04 [49]

a17b 588 −5.62/−3.456 3.58 × 10−5 (S) a17b:PC71BM = 1:1 9.98 0.86 0.54 4.63 [50]

a17c 614 −5.51/−3.464 1.03 × 10−5 (S) a17c:PC71BM = 1:1 8.88 0.90 0.52 4.15 [50]

a17d 632 −5.40/−3.416 8.89 × 10−5 (S) a17d:PC71BM = 1:1 10.86 0.84 0.60 5.47 [50]

a17e 624 −5.54/−3.524 6.46 × 10−5 (S) a17e:PC71BM = 1:1 9.92 0.88 0.56 4.88 [50]

a18 610 −5.20/−3.20 – a18:Vinazene(1) = 1:1 2.30 1.23 0.28 0.80 [51]

a18 610 –5.20/−3.20 – a18:Vinazene(2) = 1:1 2.76 1.08 0.37 1.10 [51]

a19a 619 −5.28/−3.38 7.49 × 10−5 (S) a19a:PC61BM = 1:2 7.07 0.97 0.30 2.06 [52]

a19b 671 −5.25/−3.48 1.51 × 10−4 (S) a19b:PC61BM = 1:2 14.86 0.93 0.43 5.94 [52]

a20a 664 −5.44/−3.40 – a20a:PC71BM = 1:1 1.1 0.86 0.40 0.37 [53]

a20b 656 −5.39/−3.71 – a20b:PC71BM = 1:1 0.7 0.63 0.32 0.15 [53]

a20c 633 −5.33/−3.47 3.2 × 10−5 (S) a20c:PC71BM = 1:1 12.60 0.83 0.44 4.57 [53]

a20d 666 −5.28/−3.57 1.1 × 10−5 (S) a20d:PC71BM = 1:1 7.0 0.81 0.50 2.83 [53]

a21 724 −4.61/−3.08 – a21:PC71BM = 1:1 4.2 0.47 0.40 0.79 [54]

a22 615 −5.11/−3.29 – a22:PC71BM = 1:1 4.5 0.87 0.37 1.45 [54]

a23 654 −5.30/−3.59 – a23:PC71BM = 1:1 2.7 0.50 0.41 0.55 [54]

a)In film; b)Estimated from the cyclic voltammetry data; c)F and S: measured by field effect transistor (FET) or space-charge-limited current (SCLC) method; d)In solution.


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λmaxa)

[nm]HOMO/LUMOb)

[eV]μh

c) [cm2 V−1 s−1]

Active layer Jsc [mA cm−2]

Voc [V]

FF PCE [%]

Ref.

b1a 658 −5.20/−3.40 3 × 10−5 (S) b1a:PC71BM = 3:2 10 0.92 0.48 4.40 [55,56]

b1a 620d) −5.65/−3.88 – b1a:C60 3.22 0.66 0.32 0.75 [57]

b1b 676 −5.19/−3.47 4.3 × 10−5 (F) b1b:PC71BM = 1:1 5.70 0.76 0.33 1.43 [58]

b1b 616d) −5.69/−3.86 – b1b:C60 0.13 0.39 0.19 0.01 [57]

b1c 648 −5.07/−3.33 1.5 × 10−5 (F) b1c:PC71BM = 3:7 4.31 0.81 0.30 1.03 [58]

b1d 628d) −5.71/−3.85 – b1d:C60 7.90 0.72 0.39 2.50 [57]

b1e 624d) −5.75/−3.86 – b1e:C60 3.30 0.60 0.39 0.80 [57]

b2 680 −4.98/−3.38 9.0 × 10−5 (S) b2:PC61BM = 3:1 4.51 0.79 0.36 1.30 [36]

b3 600 −5.20/−3.40 – b3:PC71BM = 1:4 4.3 0.73 0.31 1.3 [59]

b4 610 −5.20/−3.50 – b4:PC71BM = 1:4 6.2 0.81 0.30 1.7 [59]

b5 598 −5.30/−3.20 – b5:PC71BM = 1:4 3.2 0.73 0.29 0.7 [59]

b6a 600 −5.20/−3.20 – b6a:PC71BM = 2:1 5.7 0.77 0.55 2.7 [59]

b6b – −5.20/−3.50 – b6b:PC71BM = 2:1 8.3 0.76 0.58 3.7 [59]

b6c – −5.20/−3.20 – b6c:PC71BM = 2:1 6.6 0.78 0.48 2.4 [59]

b6d – −5.20/−3.20 – – – – – – [59]

b7a 606 −5.17/−3.36 1.09 × 10−5 (S) b7a:PC71BM = 1:2 1.86 0.69 0.39 0.39 [60]

b7b 682 −5.04/−3.36 1.36 × 10−5 (S) b7b:PC71BM = 1:4 3.69 0.64 0.40 0.95 [60]

b8 722 −5.25/−3.50 0.06 (F) b8:PC71BM = 1:1 2.40 0.78 0.48 0.89 [61]

b9 651 −5.35/−3.61 – b9:PC71BM = 1:1 6.83 0.88 0.43 2.59 [62]

b9 647 −5.18/−3.47 1.24 × 10−4 (S) b9:PC61BM = 3:2 4.06 0.84 0.51 2.61 [63]

b10 681 −5.29/−3.60 – b10:PC71BM = 1:1 6.57 0.81 0.45 2.37 [62]

b11 663 −5.20/−3.56 2.7 × 10−4 (S) b11:PC61BM = 3:2 4.18 0.92 0.54 3.15 [63]

b12 625 −5.14/−3.20 8.9 × 10−4 (S) a12:PC71BM = 1:1 10.82 0.82 0.56 4.96 [49]

b13 ≈590 −5.36/−3.56 – b13:PC71BM = 1:1 1.15 0.78 0.24 0.23 [64]

b14 ≈675 −5.35/−3.54 – b124:PC71BM = 1:1 2.25 0.81 0.27 0.52 [64]

b15a 698 −5.16/−3.60 4.14 × 10−4 (S) b15a:PC71BM = 1:1 4.22 0.78 0.27 0.91 [65]

b15b 743 −5.17/−3.67 7.75 × 10−3 (S) b15b:PC71BM = 1:1 3.44 0.78 0.57 1.52 [65]

b15c 728 −5.13/−3.58 1.19 × 10−3 (S) b15c:PC71BM = 1:1 9.66 0.64 0.46 2.85 [65]

b16 645 −5.31/−3.60 1 × 10−4 (F) b16:PC71BM = 1:0.75 14.60 0.63 0.58 5.30 [66]

b17 646 −5.30/−3.62 1 × 10−4 (F) b17:PC71BM = 1:0.75 13.10 0.65 0.52 4.40 [66]

b18a 643 −5.31/−3.57 – b18a:PC71BM = 1:3 3.39 0.71 0.27 0.65 [67]

b18b 668 −5.25/−3.44 – b18b:PC71BM = 1:3 4.55 0.53 0.26 0.64 [68]

b18c 792 −5.01/−3.73 2.0 × 10−3 (S) b18c:PC71BM = 1:2.2 13.39 0.73 0.37 3.62 [68]

b19 694 −4.90/−3.33 – b19:PC71BM = 1.5:1 5.00 0.55 0.38 1.04 [69]

b20 571 −5.33/−3.56 3.90 × 10−6 (S) b20:PC61BM = 1:1 3.65 0.71 0.40 1.05 [70]

b21 592 −5.32/−3.61 1.29 × 10−4 (S) b21:PC61BM = 1:1 7.46 0.86 0.37 2.36 [70]

b22 722 −4.87/−3.47 8.29 × 10−5 (S) b22:PC61BM = 1:1 2.18 0.64 0.52 0.74 [70]

b23a 729 −5.20/−3.26 1.0 × 10−5 (S) b23a: PC71BM = 1:3 8.4 0.80 0.40 2.7 [71]

b23b 726 −5.23/−3.27 9.7 × 10−5 (S) b23b: PC71BM = 1:1 10.5 0.74 0.69 5.4 [71]

b23c 721 −5.19/−3.25 1.37 × 10−4 (S) b23c: PC71BM = 1:2 9.88 0.76 0.69 5.21 [25b]

b23d 728 −5.16/−3.27 1.42 × 10−4 (S) b23d: PC71BM = 1:2 10.51 0.76 0.61 4.83 [25b]

b23e 725 −5.25/−3.37 2.40 × 10−4 (S) b18e: PC71BM = 1:2 10.75 0.89 0.73 7.00 [25b]

a)In film; b)Estimated from the cyclic voltammetry data; c)F and S: measured by FET or SCLC method; d)In soluion.




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et al. designed molecule a7 as a novel derivative of diketopyrro-lopyrrole–thiophene with triphenylamine(TPA) end group and they investigated the effect of TPA moiety. Compared to the mol-ecule that did not have TPA end groups, a7 showed red-shifted absorption and significantly higher molar absorption coefficient, as well as high-lying HOMO levels. Moreover, TPA contributed to a better hole mobility and efficiencies, showing a PCE of 1.5% with a Jsc = 5.73 mA cm−2, a FF = 0.45, and a Voc = 0.59 V.[38]

Recently, Zhang and Gong et al. carried out a study to com-pare the effects of halogenation on the photovoltaic properties by the design of DPP-based molecules containing the identical conjugated framework and different terminal fluorine (a8a) and chlorine (a8b) atoms. With the electronegative fluorine atom, the a8a displayed a higher thermal stability and stronger inter-molecular interactions as well as a lower LUMO energy level than that of a8b. Furthermore, a8a-based blended film showed a higher charge-carrier mobility and a stronger tendency to crystallize, which were expected to lead to a higher PCE of 4.32%.[39] The low solubility of a8b resulted in the formation of the inferior film quality and in turn the low PCE.

In addition, the device performance of DPP-based materials was also investigated by introducing some electron-deficient substitutes. In 2013, Shi and Chen et al. designed an ester functionalized DPP derivative (a9) by utilizing two thiophene-2-carboxylate segments as end-capped units. Because the elec-tronegative of the ester substituent, a9 showed the deep HOMO energy level and narrow optical bandgap and gave a very high open-circuit voltage (Voc) of 0.94 V and a promising PCE of 4.02%.[40] As we know, the oxygen atom in the furan ring is

smaller than the sulfur atom in the thiophene ring and could provide a more planar molecular structure, thus, they further chose furan-2-carboxylate instead of thiophene-2-carboxylate as terminals, affording molecule a10. As expected, a10 gave an intensive π–π stacking and improved film morphology. The hole mobility of a10 was one order of magnitude higher than that of a9 and the PCE of 10 was increased up to 5.37%.[41] Wang et al. applied a DPP unit flanked by two 3,3′′-dioctyl-terthiophene units as core and two electron-deficient octyl cyanoacetate units as terminals to synthesize molecule a11. The end-capped octyl ayanoacetate in a11 was used to enhance intramolecular charge transfer, narrow the optical bandgap as well as improve solu-bility. With 3% 1,8-diiodooctane (DIO) incorporated into the a11:PC71BM-based solar cells, short-circuit current (Jsc) of the resulting blend film was significantly increased to 13.6 mA cm−2 and the PCE was increased up to 4.73%.[42] Dithiafulvalene (DTF) as a donor unit has been used for construction of the light-absorbing dye applied in dye-sensitized solar cell due to its unique optoelectronic properties and effective charge separa-tion. In 2015, Singh et al. reported a novel small molecule, a12, with D–π–A–π–D molecular structure composed of DTF donor, DPP acceptor and oligothiophene-based π-linker. Absorption spectrum of blend film showed strong absorption band in vis-ible and NIR region with tailing up to 1100 nm. Device com-posed with blend a12:PC71BM in a weight ratio of 1:1.5 gave a high PCE of 4.30% with Jsc of 12.24 mA cm−2, Voc of 0.69 V and FF of 0.51.[43] In 2016, Chen et al. introduced two electron-withdrawing groups (octyl cyanoacetate and 3-octyl rhodanine) at the flank of the central DPP core and designed two small



Table 3. Optical and electronic properties, mobilities, and OPV performance of single-DPP-type donors.

λmaxa)

[nm]HOMO/LUMOb)

[eV]μh

c) [cm2 V−1 s−1]


Voc [V]

FF PCE [%]

Ref.

c1 638 −5.24/−3.50 – c1:PCBM = 1:1d) 2.19 0.46 0.34 0.30 [72]

c2 670 −5.13/−3.46 – c2:PCBM = 1:1d) 7.65 0.86 0.51 3.40 [30]

c3 658 −5.25/−3.40 8.5 × 10−5 (S) c3: A = 1:1 8.0 0.90 0.54 3.90 [73]

c4 590 −5.51/−3.73 – c4:PC71BM = 1:4 2.38 0.79 0.27 0.51 [74]

c5a 682 −5.72/−4.03 – c5a:PC71BM = 7:3 8.89 0.85 0.42 3.15 [74]

c5b 682 −5.72/−4.03 – c5b:PC71BM = 7:3 6.20 0.87 0.42 2.25 [74]

c6 603 −4.98/−3.68 6.80 × 10−6 (S) c6:PC71BM = 1:3 2.79 0.92 0.27 0.69 [75]

c7 642 −5.17/−3.51 2.56 × 10−4 (S) c7:PC71BM = 1:3 3.92 0.77 0.57 1.71 [75]

c8 643 −5.10/−3.35 6.17 × 10−5 (S) c8:PC61BM = 1:2 8.62 0.88 0.36 2.74 [76]

c9 653 −5.14/−3.40 1.20 × 10−4 (S) c9:PC61BM = 1:2 10.30 0.93 0.32 3.10 [76]

c10 661 −5.07/−3.37 2.68 × 10−4 (S) c10:PC61BM = 1:2 11.90 0.84 0.38 3.76 [76]

c11 666 −5.12/−3.41 4.46 × 10−5 (S) c11:PC61BM = 1:2 9.73 0.90 0.33 2.92 [76]

c12 678 −5.00/−3.31 5.12 × 10−6 (S) c12:PC61BM = 1:2 6.60 0.49 0.46 1.48 [77]

c13 676 −5.13/−3.48 6.90 × 10−6 (S) c13:PC61BM = 1:2 7.67 0.84 0.31 1.99 [77]

c14 640 −5.17/−3.39 2.61 × 10−5 (S) c14:PC61BM = 1:2 7.17 0.89 0.35 2.33 [77]

c15 638 −5.26/−3.51 3.05 × 10−5 (S) c15:PC61BM = 1:2 9.04 0.98 0.35 3.10 [77]

c16 652 −5.26/−3.77 2.90 × 10−4 (S) c15:PC61BM = 2:3 9.28 0.77 0.61 4.39 [78]

c17 653 −5.27/−3.74 5.80 × 10−5 (S) c17:PC61BM = 2:3 8.18 0.73 0.6459 3.86 [79]

c18 691 −5.36/−3.69 7.71 × 10−5 (S) c18:PC61BM = 1:1 9.24 0.87 0.63 5.07 [79]

a)In film; b)Estimated from the cyclic voltammetry data; c)S: measured by SCLC method; d)Bilayer solar cells.


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iew molecules, a13a and a13b. As expected, the two molecules with

in films exhibited broad and red shift solar absorption ranging from 300 to 900 nm with optical bandgaps of around 1.40 eV. After carefully device optimization, the BHJ solar-cell devices based on the a13a and a13b as donors and PC71BM as acceptors gave a PCE of 2.05% and 1.09%, respectively. The low PCEs were attributed to the large domain size and phase separa-tion. Hence, it is necessary to investigated the intrinsic packing behaviors of the molecules, which play a great role on the mor-phology forming.[44]

Besides the aforementioned five-membered aromatic ter-minals, a various functionalized six-membered aromatic rings also have been chosen as the terminals, such as benzene (a14a), 4-fluorobenzene (a14b), n-butylbenzene (a14c) etc. By compar-ison of a14a-, a14b-, and a14c-based solar cells, Shi et al. observed that terminal n-butyl in a14c can promote molecular crystal-lization and lead to the formation of finer film morphology, affording the best photovoltaic performance (PCE = 1.59%).[45] In 2013, Adachi et al. investigated the effect of the length of terminal alkyl chains (hexyl vs dodecyl) on photo voltaic per-formance based on compounds a14d and a14e.[46] Because the short alkyl chain can introduce the liquid-crystalline char-acteristics to direct molecular self-assembly. It was found that the solar cells based on a14d with hexyl showed significantly increased Jsc and FF caused by fine film morphology. Yang et al. synthesized a DPP-cored small molecule comprising electron-rich dimethylphenlyamine moieties, namely a15, to be incorpo-rated into a poly(3-hexylthiophene) (P3HT): PC61BM-based BHJ solar cell. The three-dimensional bulky side chains in the main backbone contributed to form amorphous structure and sup-press aggregation at P3HT:PC61BM interfaces, which promoted the photocurrent generation through the effective dissociation of the generated excitons in the P3HT phase at the absorp-tion region.[47] In 2014, Chandrasekharam and Sharma et al. explored the role of the fluoro- (a16a) and alkoxy- (a16b) substi-tution on the device performance. Because of the stronger elec-tron-donating ability of the dibutyloxy phenyl unit than that of flurophenyl unit, the HOMO level of a16a was deeper than that of a16b. Compared to a16b-based blend film, a16a-based blend films produced continuous interpenetrating networks, which facilitated efficient exciton dissociation and charge transport, leading to the increase of the hole mobility and PCE (4.65% vs 3.33%).[48] At the same year, they further chose three bulky 2,4,6-triisopropylphenyl as terminal units to achieve molecule a17a and a PCE of the BHJ solar cells using a17a:PC71BM was 3.04%.[49] In 2015, they synthesized four mole cules (a17b–e), all of which contained thiophene ended DPP as central core and different donors i.e., mesitylene (a17b), 1-isopropoxy-2-meth-ylbenzene (a17c), 1,2,3-trimethoxy-5-methylbenzene (a17d) and 1,3-di-tertbutylbenzene (a17e) as arms shown in Scheme 3. The end-capped group exerted influence on the intrinsic electronic properties of chromophore or their solution phase conforma-tions. Hence, all four molecules showed different absorption peaks and energy levels. The absorption ranges of a17d and a17e were broader than that of a17b and a17c, which indicated that the size of end groups had a significant influence on the molecular ordering and π–π stacking in thin film. The trend in the variation of HOMO energy levels was consistent with the trend in the electron-donating ability of the donor units. OSCs

based on these small molecules achieved moderate PCEs in the range of 2.86–3.55%. After the optimization with solvent and thermal annealing, the BHJ devices, based on a17d:PC71BM and a17e:PC71BM showed improved PCEs of 5.47% and 4.88%, respectively, which came from the improved charge-carrier mobility, crystallinity, and light-harvesting ability of the active layer.[50]

Nguyen et al. chose the phenyl attached DPP (3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) instead of the thiophene attached DPP (3,6-di(thiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4(2H,5H)-dione) as the core with hexylbithiophene as arms, affording molecule a18.[51] The investigation of the a18: Vinazene heterojunctions indicated that the high Voc of 1.08 V was benefited from the appropriate LUMO level while the low Jsc was attributed to the poor film morphology.

In 2014, Yin et al. designed two novel asymmetrical push–pull small molecules (a19a and a19b), using ethynylbenzene as the π-bridge to connect TPA and DPP as a fundamental dipolar D–π–A structure. As a control, a19b was capped with cyanophenyl at the end of DPP unit. Compared to a19a, a19b showed extended spectral regions with effectively reduced bandgap and the reasonably deepened HOMO level, resulting in a higher Jsc of 14.86 mA cm−2 with a higher PCE of 5.94%.[52] In 2015, Stupp et al. reported the effect of hydrogen bonding on active-layer morphology and solar-cell efficiency by introducing the cyanovinyl amide (a20a and a20c) or ester groups (a20b and a20d) at the ends of the molecules. Four molecules were designed to be symmetric (a20a and a20b) or asymmetric (a20c and a20d). The amide groups can provide hydrogen-bonding ability and offer a second driving force for self-assembly beyond π–π stacking. By comparing two asymmetric derivatives, they found that a20d revealed greater crystallinity and π–π stacking while a20c formed short fiber-like supramolecular aggregates with much smaller domain sizes and less order. Interestingly, devices fabricated with the amide–fullerene combination have a greater Jsc leading to a 50% increase in PCE. The same conclu-sion can be draw in the symmetric system (a20a and a20b). In great contrast, the PCE values for the symmetric donors were lower than 1%. The strong phase separation between the donor molecules and PC71BM resulted in large aggregates formation (≈500 nm) in the blended films, which explained the low Jsc observed in devices.[53]

Seo et al. reported the electronic properties of five DPP deriv-atives as a function of alkyl chains and the conjugated units attached to the DPP core. The materials are divided into two groups: a1, a2, a21 and a21, a22, a23, where the substituting alkyl chains and the conjugated units attached to the DPP core are varied, respectively. For the a21, the first absorption peak occured at 568 nm, 87 nm blue-shifted compared to those of the a1 and a2. The side chains of a1 and a2 are solvated more favorably by the solvent molecules than that of a21, which con-tributed to more expanded structures in the films, resulted in a red-shift in the absorption spectrum. As expected, the side alkyl chains have little influence on the bandgap (Eg) values since they have the same conjugated backbone. However, for a22 and a21, modification of conjugated aryl groups next to the DPP core resulted in blue-shifted absorption and a slight increase in bandgap. Based on the UPS results, substituting the thiophene units with phenyl or pyridine units induced a clear increase




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of IP, due to relatively lower electron-donating ability of both phenyl and pyridyl ring than a thienyl ring. Accordingly, the highest Voc (0.87 eV) is obtained from a22:PC71BM device, while the lowest Voc (0.47 eV) is obtained from the a21:PC71BM cell.[54]

3.1.2. DPP as Core with Fused Building Units as Arms

As mentioned in Section 3.1.1, a PCE of 3% for DPP-thiophene-based solar cells has been achieved by using two terthiophenes to functionalize 2-ethylhexyl-substituted DPP core in Nguyen’s group. Thereafter, they utilized a fused benzofuran system (b1a, as shown in Scheme 4) to replace the terminal hexylbithio-phene units as arms. The introduction of the highly conjugated structure and the electronegative oxygen atom both stabilized the HOMO level, affording an increased Voc of 0.9 V with a PCE of 4.4% for b1a:PC71BM devices.[55] In 2011, by studying the charge recombination processes, they further found the b1a:PC61BM devices yielded the PCE of 5.2% when annealed at 110 °C under an illumination intensity of 11 mW cm−2.[56] To further evaluate the effect of the heteroatoms in the aro-matic end-groups on the physical properties and device perfor-mance, three kind of aromatic end-groups were implemented including benzofuran, benzothiophene and 1-methyl-indole (b1a–c). They found that the heteroatom substitutions gave rise to the changes of intra- and intermolecular interactions, which further resulted in different solubility, crystallinity, film morphology, charge-carrier mobility as well as photovoltaic per-formance. Within this series, b1a with a moderate solubility provided the best film morphology to yield the highest PCE among the three DPP derivatives. b1b with low solubility had high tendency to crystallize, forming large phase separation and thus, a low PCE. While b1c with high solubility showed low homogeneity of the blended film, resulting in low mobility and device performance.[58] In 2012, Leriche and Roncali et al. have analysed the effects of composition of the side-chain on the elec-tronic properties of the DPP system based on four compounds (b1a,b and b1d,e) containing various combinations of benzo-furan, benzothiophene, thiophene and furan units. They found that the composition of the side-chain had little effect on the energy levels, but strongly affects the sensitivity of the materials toward thermal treatment and thus indirectly the performances of the resulting solar cells. As a result, b1a leads to the highest PCE of 2.50% with a Jsc close to 8.0 mA cm−2. Because of the low intensity of the incident photon to electron conversion effi-ciencie (IPCE) response, b1b, b1d, and b1e led to low Jsc and PCEs.[57] The octylthieno[3,2-b]thiophene (b2) was also selected as the terminals to attach onto the DPP core by Zhan et al. It is found that the FF of the b2:PC61BM-based solar cells was commonly inferior (<40%) for each donor/acceptor ratio, which can be ascribed to the low packing order and macroscale phase separation with domain sizes of more than 100 nm.[36] Fréchet and co-workers reported a series of DPP-cored small molecules with different electron-rich end-groups (b3–b6) to illustrate the effect of the planarity of the end groups on the device per-formance. It is found that the highly planar C2-pyrene as the small-molecule end-group can induce to form tight, aligned crystal packing and fine morphology by π–π interactions. So the resulting b6b-based devices gave the highest PCE of 4.1%

with a FF of 0.58.[59] Zhu and co-workers designed two low-bandgap small molecules with D–A and D–A–D frameworks, namely b7a, and b7b, in which TPA and DPP were used as donor (D) and acceptor (A) segments, respectively. b7b showed significantly red-shifted absorption profile than b7a, which was attributed to the stronger donor–acceptor effect of the D–A–D framework than the D–A framework. Moreover, a symmetrical TPA terminal in D–A–D-type b7b resulted in an increasing HOMO energy level. As excepted, symmetrical D–A–D frame-work made its corresponding molecule exhibit the increased PCE (0.95% vs 0.39%) and Jsc (3.69 vs 1.86 mA cm−2) values in the BHJ–OSCs.[60] Sonar et al. reported a benzothiadiazole end-capped small molecule, b8, using thiophene flanked DPP as a central conjugated fused aromatic building block. Incorpo-ration of electron-withdrawing benzothiadiazole groups at the end of DPP core endowed b8 strong donor–acceptor interac-tions and solid state ordering. Thus a significant red shift about 120 nm was observed when comparing solution to solid state absorbance. However, a low PCE of 0.9% was observed[61] for b8: PC71BM system.

The DPP-cored materials with naphthalene (b9), 5-(2-naphtyl)-thienyl (b10), 6-fluoro-2-naphthyl (b11) as the end groups were presented by Ding’s research group[62] and Zhu’ group,[63] respectively. b9 showed strong aggregation absorp-tion at 625–725 nm probably due to the remarkable aggregate formation induced by the stronger intermolecular interactions between naphthalene end groups. In comparison, b10 showed relatively weak intermolecular interactions due to reduced coplanarity caused by additional hexylthiophene units.[62] While more efficient intermolecular stacking in solid state and lower HOMO and LUMO levels was observed in b11, benefited from the monofluorination of the naphthyl group. Consequently, b11 afforded an improved PCE of 3.0% with a higher Voc of 0.92 V.[63] Two indole units was also flanked to the center DPP core as terminal units by Chandrasekharam et al., affording b12.[49] The strong electron-donating property of indole facili-tated the intramolecular charge transfer between the indole and DPP core, which made b12 exhibit broad band absorption, low optical bandgap and high hole mobility. As a result, a high PCE of 4.96% has been achieved.[49] Kozma and co-workers intro-duced electron-rich dibenzofuran (b13) and acenaphtene (b14) appending to the DPP core, respectively. Because of their crystal-lization features, both of them showed large crystalline domains in the blended films which hindered the charge generation and transportation, giving a poor Jsc and FF as well as a low PCE below 1%.[64] In 2014, Zhan and Yao et al. selected DPP as core and two 4,8-di-2-(2-ethylhexyl)-thienyl-benzo[1,2-b:4,5-b′]-dithiophene (thienyl-BDT) as arms, forming a model backbone (b15a). Then they chose octyl-2-cyano-3-(thiophen-2-yl)acrylate (b15b), and 2-hexylbithiophene (b15c) as end groups to attach on the flank of the model backbone, respectively. It is observed that the end-capped aromatic groups produced different com-patibility between donors and PC71BM, in turn the film mor-phology. As a result, the electron-deficient and planar units of octyl-2-cyano-3-(thiophen-2-yl)acrylate could increase hole mobility and enhance FF while the poorer planarity of 2-hexylb-ithiophene could increase Jsc with fine film morphology.[65]

Some DPP-cored dumbbell-shaped electron donor mate-rials have also been reported. In 2013, Leclerc and Ziessel




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iew et al. designed two dumbbell-shaped electron donor materials

(b16 and b17) with DPP as core and triazatruxene as arms. The end-capped triazatruxene could promote end-to-end π–π interactions in adjacent molecules, affording fine active layer morphology and a high Jsc of 14.6 mA cm−2. Both para- and meta- isomers have comparable optical properties and energy level as well as charge transport. They inferred the slightly lower efficiency obtained for b17 resulted from the higher series resistance, and consequently lower FF.[66] Three novel triads (b18a–c) based on a DPP central core and two electron-withdrawing boron-dipyrromethene (BODIPY) terminals were synthesized by Skabara’s[67] and Zhan’s[68] groups, respectively. 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene was chosen as arms (b18a and b18b) by Skabara et al. and these OPVs demonstrated modest PCEs of 0.65%.[67] Zhan et al. used hexylbithiophene-substituted BODIPY as arms (b18c) to enhance absorption and improve the crystallinity, achieving a significant increase in the PCE from 0.65% to 3.62%.[68] In order to enhance the absorption of the DPP-cored electron donor materials, Sastre-Santos et al. combined two units of phthalocyanines covalently attached to the opposite positions of a DPP unit, yielding com-pound b19. As a result, b19 provided highly light absorbing in a broad visible/near-infrared spectral region. Furthermore, the strong donor nature of the phthalocyanine contributed to enhance the push–pull nature of the molecule, forming enhanced intermolecular interaction. However, due to the low homogeneity of the blended film, a low PCE of 1.04% with a very poor Jsc and FF were obtained.[69] Recently, a series of small molecules (b20-b22) contained a diketopyrrolopyrrole core as the central acceptor unit, two oligomeric benzene or thiophene as the donor, and two isoindigo or benzothienoiso-indigo as the weaker acceptor, were synthesized by Yang et al. to investigate the influence of the number of the thiophene rings on the optical, electrochemical and photovoltaic properties. The absorption showed a general trend of red-shift with increased number of thiophene while the HOMO levels increased as a result of substitution benzene with thiophene as donor. In com-parison, the PCE was increased from 1.07% for b20 to 2.36% for b21 as the number of thiophene increased from zero to two. However, the PCE decreased significantly when the number of thiophene increased to four in b22. Hence, b21 showed the highest PCE of 2.36% with the highest Voc and Jsc, resulting from the increased crystallinity and charge-carrier mobility.[70]

To combine the most desirable properties from both small molecules and polymers, Wang and co-workers designed a series of extended π-conjugated backbone structures com-posed of multiple electron-donating (D) moieties and elec-tron-accepting moieties (A). Based on this, they synthesized five extended π-conjugated oligomers (b23a-e) using DPP derivatives as the central stronger acceptor unit, two indaceno-dithiophene as electron-donating π-conjugated bridges, two benzothiadiazole derivatives as the second and weaker acceptor, and two n-hexyl-substituted bithiophenes as the terminal groups.[25b,71] They systematically studied on how the specific chemical structures in π-extended molecule skeleton affected the performance of the materials in two aspects: i) variation of electron affinities of the acceptor units through replacing the two thiophenes by two thiazoles on the DPP core or replacing the benzothiadiazole by difluorobenzothiadiazole to control

the molecular energy levels, optical properties, and Voc; ii) vari-ation of the length of the π-bridge unit between two acceptor chromophores by an extra thiophene π-bridge to change the light-harvesting ability, and Jsc. Through molecular modifica-tion, oligomer b23e gave the best PCE of 7.00% with very high FF of 0.73 and high Voc of 0.89 V, which marked the highest PCE value reported for the OSCs based on a single-DPP unit.[25b]

3.1.3. DPP as Core with Ethynyl or Ethenyl-Linked Building Units as Arms

The incorporation of ethynylene or ethenylene π-spacers offered a viable approach to induce planarity, enhance π-conjugation along the polymer chain and thus charge-carrier mobility.[80] Therefore, as typical π-linkage units, they have also been widely applied in DPP-based small-molecule semiconductors. Scheme 5 and Table 3 summarize the application of ethynyl- and ethenyl-π bridge and elaborate the effects of ethynyl and ethenyl-linkage on the device performance of the DPP-based OSCs.

As shown in Scheme 5, Hany and co-workers investigated the influence of a vinylene group on the film formation based on molecules c1 and c2. They demonstrated the vinylene group could increase the co-planarization of c2 and enhance molec-ular aggregation and crystallization, affording an extended domains in the blended film formation. This film morphology was in favor for the efficient charge generation and extrac-tion, leading to the improvement of the PCE for c2 from 0.3% to 3.4%.[72] Sharma and Thomas et al. designed a new DPP-based small-molecule donor (c3) containing cyanovinylene as π-bridge and electron-withdrawing 4-nitrophenyl (CN) as termi-nals. Its device performance was investigated by using modi-fied PCBM (F and A) as electron acceptor. The devices based on c3:A blends gave the highest PCE of 3.9%, when casted from the tetrahydrofuran (THF)/DIO cosolvent.[73]

To explore the effect of acetylene-bridge, Park et al. designed molecule c4 with DPP as core and pyrene as terminals and molecule c5 containing acetylene as π-bridge between DPP and pyrene. Based on the hybridization theory, the acetylene-incorporation assisted to reduce the bandgap and increase the ionization potential. Accordingly, c5 showed smaller bandgap, deeper HOMO level, and stronger light absorption. As a result, the solution-processed OSCs based on c5a showed an increased PCE of 3.15% with enhanced Jsc and Voc.[74] Wu and Ouyang et al. utilized two DPP-based organic molecules functional-ized with phenanthrene (c6) and ethynylphenanthrene (c7) to explain the role of C–C triple bond. They found that triple bond can increase the rigidity by avoiding the steric hindrance between DPP and phenanthrene, which conduced to increase the inter-molecular interaction and improve the intermolec-ular charge transport.[75] Li et al. also systemically studied the π-linkage effects on the OPV performance by means of careful molecular design of c8–c15. In one of their works, four linkage units, single bond, vinylene, acetylene and acrylonitrile, were introduced to form a D–π–A–π–D-type structure consisting of DPP-based electron-withdrawing group and triphenylamine-based electron-donating unit. Within this series, c9 with vinylene as the π-linkage exhibited better molecular coplanarity and stronger intermolecular interactions, reaching a relatively




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higher PCE of 3.76%, while c10 and c11 with acetylene and acrylonitrile as π-linkages exhibited relatively deeper HOMO levels caused by their electron-withdrawing effect, giving a higher Voc of 0.93 V and 0.90 V, respectively.[76] In the other work, they chose the alkylated carbazole and fluorene as ter-minal electron-donating groups and ethynyl as π-linkage to syn-thesize c13 and c15. In order to explain this question, c12 and c14 without ethynyl as π-linkage were used as comparison task, respectively. The similar conclusion was observed in this work. Owing to the enhanced molecular coplanarity and decreased HOMO levels and bandgap caused by the ethynyl-linkage, compounds c13 and c15 exhibited increased Voc of 0.84 V and 0.98 V and improved PCE of 1.99% and 3.10%, respectively.[77]

In 2015, Zhu et al. synthesized an linear molecule c16 in which the DPP core and the 9-(1-(3-dodecylthienyl)ethynyl)-anthracen-10-yl end groups are bridged through an acetylenic bond. The incorporation of the electron-withdrawing acetylenic bond can help decrease the HOMO level and the conjugated cylinder-like π-electron density is more adaptable to confor-mational and steric constraints, retaining the planarity of the affording conjugated backbone. The c16-based BHJ solar cells exhibited a modest PCE of 4.39%.[78] Later, with respect to molecule c17 which is an analogue of c16 with a longer alkyl chain on the DPP core, they further introduced 1,4-phenylene moiety between the DPP core and the 9-(1-(3-dodecylthienyl)-ethynyl)anthracen-10-yl end groups through an acetylenic bond, affording c18, to improve the photovoltaic performance. The high aromatic resonance stabilization energy of 1,4-phe-nylene unit facilitates π-electrons to localize within the phe-nylene ring rather than delocalized over the whole molecule, so that the intramolecular charge transfer is partly confined with the strong electron-donating thiophene moieties away from the electron-accepting DPP core. Thus, molecule c18 exhibited a deeper HOMO level, a strong and broad absorption as well as the enhanced hole mobility, leading to an improved PCE of 5.07% with a increased Voc and Jsc.[79]

In view of the above analysis, we can conclude that compared to direct C–C bonding as bridge, the ethenyl- and ethynyl-con-taining bridges could restrict the intramolecular rotations to increase the rigidity of molecules and improve the molecular planarity. Differently, relative to the double bond as the linkage, the absorption at higher energy region can be blue-shifted and LUMO levels can be lowered by the presence of more electron-egative character of triple bond spacers. In short, the incorpora-tion of ethynylene or ethenylene-spacers leads to enhance the intermolecular interaction and increase carriers mobilities as well as photovoltaic performance.

3.2. Double-DPP Ttype Photovoltaic Small Molecules

3.2.1. The Linear Small-Molecule Donors with DPP as End Groups

In 2011, Marks and co-workers firstly adopted a new strategy which utilized a weak donor as core and DPP-based electron-withdrawing units as arms to afford an acceptor–donor–acceptor (A–D–A) motif. All kinds of linear small-molecule donors with DPP as end-capped arms emerged one after another since then, as shown in Scheme 6. Table 4 lists the optoelectronic

properties, mobilities, and OPV performance of the linear small-molecule donors with DPP as end groups.

Marks and co-workers introduced a linear 3,8-bis(2-ethyl-hexyloxy)naphtho[1,2-b:5,6-b′]dithiophene (NDT) as core and flanked the NDT core with DPP units, yielding compound d1. This pull–push architecture afforded d1 a broad, high oscil-lator strength visible absorption, ordered molecular packing, and exceptional hole mobility. When combined with the electron acceptor PC61BM, a PCE of 4.06% was achieved.[81] Thereafter, they utilized a “zig-zag” 4,9-bis(2-ethylhexyloxy)-naphtho[1,2-b:5,6-b′]dithiophene (zNDT) instead of linear NDT as electron-donating core and achieved molecule d2. The nonlinear shape of zNDT provided d2 a compact π-system and diminished conjugation, which endowed d2 upshifted frontier molecular orbital energies, increased hole mobility, and enhanced PCE of 4.7%.[82] Jo et al. synthesized a series of simple structured A–D–A-type small molecules (d3–d10) by introducing various electron donor units with different electron-donating power, such as thiophene and phenylene, thienothiophene, naphthalene etc.[83] An interesting aspect was revealed that the weak donating unit (phenylene or naphtha-lene) gave rise to deeper HOMO energy levels than that strong donating unit (thiophene or thienothiophene) and the fused aromatic ring (thienothiophene or naphthalene) with extended planar structure exhibited higher hole mobility than that non-fused aromatic ring (e.g., thiophene or phenylene). Accordingly, the introduction of naphthalene with weak donating power and planar structure could afford d9 a promising PCE of 4.4%.[83b] In 2015, they chose the electron-donating dithienopyran as core and DPP as arms, producing compound d10. Molecule d10 showed strong light absorption with a low bandgap and high crystallinity, exhibiting high Jsc and hole mobility over 0.1 cm2 V−1 s−1. When blended with PC71BM, the device showed a high PCE of 6.88% with a high Jsc of 14.1 mA cm−2. Further-more, the d10 also exhibited a promising PCE of 4.8% when a polymeric electron acceptor, P(NDI2OD-T2), was chosen.[85]

Zhu and his co-workers further investigated the influence of the planar aryl hydrocarbon core and its linkage position on photo voltaic properties by using a different substituted (9,10- and 2,6-substitued) anthracene (An) unit as the hydrocarbon core and two DPP units as arms, namely, d11 and d12. It was observed that d12 with 2,6-substituted anthracene exhibited the signifi-cantly red-shifted absorption profile and high HOMO level rela-tive to that of d11 with 9,10-substituted anthracene. As a result, d12-based solution-processing OSCs displayed a remarkably high PCE of 5.44% with 1% DIO additive.[86] Li et al. also examined the effect of novel aromatic donor units on the properties of DPP-based molecules by introducing alkylated carbazole (d13), diphenylamine (d14), fluorene (d15) and phenothiazine (d16) as center unit, respectively. Enhanced intramolecular charge transfer transition band and increased λmax as well as extended absorption spectral range of compounds from d13 to d16 were observed. In fact, d13 revealed a relatively higher PCE of 1.50%, while the PCEs of the others were below 1%. This is because d13 exhibited more efficient photo induced charge separation than that of the other three materials.[87] In 2012, Marks and co-workers reported another three acetylenic materials with the rigid and planar electron-rich 2,2′-ethyne-1,2-diylbis[3-(alk-1-yn-1-yl)thio-phene] (EBT) as core and DPP as arms by changing the length of




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Scheme 6. Chemical structures of linear small-molecule donors with DPP as end groups.


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Table 4. Optical and electronic properties, mobilities, and OPV performance of double-DPP-based donors.

λmaxa)

[nm]HOMO/LUMOb)

[eV]μh

c) [cm2 V−1 s−1]


Voc [V]

FF PCE [%]

Ref.

d1 676 −5.40/−3.68 3.28 × 10−3 (F) d1:PC61BM = 1.5:1 11.27 0.84 0.42 4.06 [81]

d2 610 −5.11/−3.39 0.057 (F) d2:PC61BM = 1.5:1 11.7 0.75 0.50 4.4 [82]

d3 628 −5.19/−3.66 6.0 × 10−5 (S) d3:PC71BM = 1:1 7.40 0.84 0.37 2.31 [83a]

d4 638 −5.17/−3.68 2.5 × 10−5 (S) d4:PC71BM = 1:1 4.30 0.80 0.43 1.49 [83a]

d4 633 −5.29/−3.43 2.4 × 10−5 (S) d4:PC71BM = 1:1 7.48 0.82 0.54 3.33 [84]

d5 612 −5.31/−3.65 8.8 × 10−5 (S) d5:PC71BM = 1.25:1 9.09 0.93 0.47 4.01 [84]

d6 643 −5.14/−3.55 5.1 × 10−4 (S) d6:PC71BM = 1:1 6.8 0.78 0.57 3.0 [83b]

d6 623 −5.20/−3.30 7.4 × 10−5 (S) d6:PC71BM = 1:1 6.57 0.79 0.50 2.58 [84]

d7 702 −5.11/−3.56 7.7 × 10−4 (S) d7:PC71BM = 1:1 9.3 0.81 0.53 4.0 [84]

d8 620 −5.21/−3.57 6.1 × 10−4 (S) d8:PC71BM = 1.5:1 8.3 0.86 0.53 3.8 [84]

d9 650 −5.18/−3.58 1.1 × 10−3 (S) d9:PC71BM = 1:1 9.5 0.87 0.53 4.4 [84]

d10 658 −5.23/−3.73 0.18 (F,N) d10:PC71BM = 1:3 14.12 0.80 0.61 6.88 [85]

d10 658 −5.23/−3.73 0.18 (F,N) d10:P(NDI2OD-T2) = 1:2 10.42 0.82 0.58 4.82 [85]

d11 575 −5.38/−3.35 2.19 × 10−6 (S) d11:PC71BM = 1:5 4.09 0.97 0.32 1.45 [86]

d12 662 −5.19/−3.25 4.02 × 10−4 (S) d12:PC71BM = 1:1 11.90 0.82 0.55 5.44 [86]

d13 644 −5.03/−3.32 – d13:PC61BM = 1:2 4.12 0.66 0.44 1.50 [87]

d14 618 −5.01/−3.33 – d14:PC61BM = 1:2 1.95 0.64 0.34 0.53 [87]

d15 618 −5.16/−3.36 – d15:PC61BM = 1:2 3.17 0.66 0.30 0.78 [87]

d16 626 −5.04/−3.34 – d16:PC61BM = 1:2 2.63 0.65 0.35 0.75 [87]

d17a 626 −5.46/−3.64 0.10 (F) d17a:PC61BM = 1:1 6.67 0.72 0.36 1.70 [88]

d17b 626 −5.52/−3.78 0.08 (F) d17b:PC61BM = 1:1 5.64 0.75 0.40 1.68 [88]

d17c 623 −5.44/−3.66 0.04 (F) d17c:PC61BM = 1:1 2.37 0.63 0.29 0.43 [88]

d18a 610 −5.30/−3.44 5.9 × 10−6 (S) d18a:PC71BM = 1:1 5.22 0.76 0.55 2.19 [84]

d18b 670 −5.29/−3.27 – d18b:PC71BM = 1:1 8.72 0.78 0.44 2.85 [89]

d18b 667 −5.13/−3.65 1.1 × 10−3 (S) d18b:bis-PDI-T-EG = 1:1 3.46 0.95 0.41 1.34 [90]

d19a 679 −5.15/−3.44 4.67 × 10−1 (S) d19a:PC71BM = 1:1 11.86 0.72 0.62 5.29 [91]

d19a 679 −5.15/−3.44 2.2 × 10−2 (S) d19a:bis-PDI-T-EG = 1.2:1 4.66 0.92 0.47 2.01 [90]

d19a 680 −5.23/−3.46 1.6 × 10−3 (F) d19a:PC61BM = 1:1 11.97 0.84 0.58 5.79 [92]

d19a 626 −5.23/−3.46 3.0 × 10−3 (S) d19a:IEIC = 1:1 8.24 0.90 0.54 4.00 [93]

d19b 624 −5.28/−3.49 1.1 × 10−2 (S) d19b:IEIC = 1:1 10.87 0.94 0.59 6.03 [93]

d20 620 – 1.8 × 10−4 (S) d20:N2200 = 1:1 7.60 0.82 0.60 3.74 [94]

d21a 620 −5.80/−3.83 2.7 × 10−4 (S) d21a:PC61BM = 1:1 5.50 0.88 0.64 3.10 [95]

d21b 615 −5.79/−3.77 3.7 × 10−4 (S) d21b:PC61BM = 1:1 11.4 0.80 0.60 5.50 [95]

d22 671 −5.20/−3.64 1.7 × 10−5 (S) d22:PC71BM = 1:1 9.85 0.71 0.57 3.90 [96]

d22 671 −5.20/−3.64 1.6 × 10−3 (S) d22:bis-PDI-T-EG = 1:1 4.60 0.83 0.43 1.62 [90]

d23 600 −5.16/−3.44 1.6 × 10−2 (S) d23:bis-PDI-T-EG = 1:1 3.79 0.83 0.51 1.62 [97]

d24 620 −5.11/−3.32 1.27 × 10−3 (S) d24:PC71BM = 1:1.5 8.53 0.88 0.38 2.82 [98]

d25 615 −5.12/−3.17 1.1 × 10−2 (S) d25:PC71BM = 1:1 10.4 0.72 0.54 4.02 [99]

d26 629 −5.10/−3.58 2.0 × 10−5 (S) d26:PC71BM = 1:1 6.82 0.77 0.46 2.39 [100]

d27 648 −4.96/−3.67 1.33 × 10−3 (S) d27:PC71BM = 1.5:1 10.69 0.71 0.48 3.69 [100]

d28 627 −5.24/−3.61 9.0 × 10−2 (F) d28:PC61BM = 1:1 6.98 0.72 0.44 2.21 [101]

d29 598 −5.36/−3.61 1.0 × 10−2 (F) d29:PC61BM = 1:1 9.59 0.82 0.38 3.03 [101]

d30 695 −5.62/−3.74 3.8 × 10−5 (S) d30:PC61BM = 1:1 9.70 0.89 0.56 4.80 [102]

d30 695 −5.62/−3.74 9.2 × 10−2 (S) d30:SdiPBI = 2:1 6.10 0.92 0.42 2.40 [102]

d30 695 −5.62/−3.74 3.6 × 10−2 (S) d30:SdiPBI-S = 1:1 6.70 0.96 0.53 3.50 [102]


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the alkyl chains on EBT (d17a-c). The solar cells using d17a with shorter n-hexyl chains as electron donor and PC61BM as the elec-tron acceptor exhibited PCEs of 1.70% after thermal annealing at 100 °C. Devices fabricated with compound d17b bearing longer side chains exhibited optimal PCE of 1.68% without annealing. Nevertheless, compared to d17a and d17b, molecule d17c has the lowest PCE of 0.43%. They found compounds d17a and d17b with linear side chains showed closer π–π stacking, which assisted in maintaining high levels of film organization, promoting slightly better device performance. While the bulkier branched alkyl substituents in d17c disrupted crystalline and produced greater disorder and less dense π–π stacking, which in turn hindered forming fine film morphology for efficient photogenerated charge-carrier collection and transport, consistent with the low PCE.[88]

In 2013, benzo[1,2-b:4,5-b′]dithiophene (BDT), a promising electron-donating unit to construct photovoltaic polymers, was also applied in DPP-based small molecules. Tu et al. reported a series of A–D–A-based small molecules consisting of DPP as electron-withdrawing units and thiophene (d4), bithio-phene (d6) and dodecyloxy-substituted BDT (d18a) as elec-tron-donating core, respectively.[84] The differential scanning calorimetry (DSC) experiments revealed that d6 possessed the best crystallization ability due to the varied molecular confor-mations. d18a showed the optimal coplanarity and tended to form strong π–π stacking. As a result, both d6:PC71BM and d18a:PC71BM blend films with DIO processing showed crystal nanofibrils with respective widths of 10–15 nm and 15–20 nm and lengths of 30–100 nm and 70–180 nm. In contrast, d4:PC71BM blend film formed smaller nanofibrils, contributed to efficient charge separation and transporta-tion, therefore leading to higher Jsc and PCE.[84] Nguyen et al. selected 2-ethylhexyloxy-substituted BDT as the core instead of dodecyl oxy in d18a, affording molecule d18b and achieved a modest PCE of 2.85% for the d18b:PC71BM-based device.[89] In 2014, Zhan et al. also synthesized the same compound d18b

and used a PDI dimer of bis-PDI-T-EG as electron acceptor to fabricate solar cells, yielding a PCE of 1.34%. They thought the low PCE may be attributed to its poor film morphology caused by the intermolecular strong O-O interaction in the processing solvent.[90] Then, they chose the 5-alkylthiophene-2-yl-substituted BDT (d19a) as core to replace the alkyloxy-substituted BDT in DPP-based A–D–A-type small molecule. With judicious device optimization, when using PC71BM as the electron acceptor and o-DCB as the parent solvent with 0.7% DIO as the additive, they obtained a Jsc of 11.86 mA cm−2, a Voc of 0.72 V, a FF of 62%, and a high PCE of 5.29%.[91] Further, they investigated the device performance with the d19a/bis-PDI-T-EG as active layer, the best device exhibited a PCE of 2.01%.[90] In addition, Zhan et al. found BHJ devices based on d19a/PC61BM showed an improved PCEs up to 5.79% after thermal annealing at 110 °C for 10 min.[92] In 2015, they also used d19a as electron donor and IEIC as the electron acceptor to fabricate the fullerene-free organic solar cells and attained a moderate PCE of 4%. Then they chose linear alkylthio substituents to replace the 2-ethyl-hexyl at the center BDT unit to improve the device perfor-mance. The linear alkylthio substitutions in d19b improved the crystallinity and unexpectedly reserved small phase separation domains in the blend, affording an improved PCE of 6.03% relative to the d19a:IEIC-based devices.[93] In 2015, Liu and Inganäs et al. designed a thieno[2,3-f ]benzofuran (TBF) moiety as core and DPP as flanked arm and yielded d20. Replacing the sulfur atom with an oxygen atom in TBF endowed d20 a better co-planarity and π–π stacking and a deeper HOMO level than that of 5-alkylthiophene-2-yl-substituted BDT. The opti-mized device based on the d20:N2200 gave a PCE of 3.74%.[94] Zhang and Liu et al. studied the effect of heteroatom substi-tution on molecular aggregation and the resulting photo voltaic device performance through molecular design. First, they used a 2-ethylhexyloxy-benzene-substituted benzo[1,2-b:4,5-b′]di-thio-phene. (BDTP) moiety as the molecular core to synthesize d21a.

Table 4. Continued.



λmaxa)

[nm]HOMO/LUMOb)

[eV]μh

c) [cm2 V−1 s−1]


Voc [V]

FF PCE [%]

Ref.

d31 663 −5.44/−3.69 1.05 × 10−4 (S) d31:PC71BM = 1.5:1 9.00 0.84 0.52 3.97 [103]

d32a 635 −5.15/−3.45 8.2 × 10−4 (S) d32a:PC71BM = 3:4 9.32 0.74 0.55 3.76 [104]

d32b 635 −5.15/−3.45 4.07 × 10−4 (S) d32b:PC71BM = 1:1 4.62 0.73 0.55 1.83 [104]

d32c 635 −5.15/−3.45 2.33 × 10−4 (S) d32c:PC71BM = 3:4 0.45 0.64 0.42 0.12 [104]

d33a 620 −5.07/−3.21 2.7 × 10−5 (S) d33a:PCBM = 1:1 5.10 0.82 0.38 1.60 [105]

d33b 650 −5.07/−3.21 2.8 × 10−5 (S) d33b:PCBM = 1:1 3.70 0.70 0.47 1.20 [105]

d33c 618 −5.07/−3.21 8.7 × 10−4 (S) d33c:PCBM = 1:1 7.50 0.77 0.60 3.50 [105]

d33d 613 −5.07/−3.21 4.3 × 10−4 (F) – – – – – [105]

d34a 612 −5.31/−3.45 3.4 × 10−5 d34a:PC61BM = 6:4 9.90 0.85 0.513 4.35 [106]

d34b 607 −5.33/−3.47 1.7 × 10−5 d34b:PC61BM = 6:4 7.30 0.82 0.452 2.69 [106]

d35 790 −5.18/−3.39 4.6 × 10−5 (S) d35:PC61BM = 1:1 11.88 0.80 0.50 4.78 [107a]

d36a 813 −5.07/−3.60 4.68 × 10−4 (S) d36a:PC61BM = 1:1.2 16.00 0.71 0.64 7.23 [107b]

d36b 803 −5.09/−3.55 2.11 × 10−4 (S) d36b:PC61BM = 1:1.2 10.52 0.81 0.500 4.26 [108]

d36c 842 −5.07/−3.61 3.51 × 10−4 (S) d36c:PC61BM = 1:1.2 14.93 0.71 0.547 5.81 [108]

d37 802 −5.14/−3.76 4.85 × 10−4 (S) d37:PC61BM = 1:1.2 16.76 0.78 0.62 8.08 [107c]

a)In film. b)Estimated from the cyclic voltammetry data. c)F and S: measured by FET or SCLC method; N: the neat film.


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Subsequently, they designed a 2-ethylhexyloxy-benzene-sub-stituted benzo[1,2-b:4,5-b′]difuran (BDFP) moiety via oxygen atom substitution on the BDTP moiety and thus synthesized d21b. Compared with d21a, the tendency of d21b to aggregate was well reduced in organic solvents, which resulted in the formation of more appropriately sized domains in blend films of d21b:PC61BM, facilitating charge generation. Devices based on d21b showed much higher photocurrents than d21a and attained a high PCE of 5.5%.[95] d22 was synthesized by intro-ducing electron-donating alkylthienyl flanked on a central BDT chromophore. Compared with d19a, d22 showed a twisted con-jugated backbone and decreased π–π-stacking strength, aggre-gation ability, and crystallite size. As a result, d22 displayed a modest PCE of 3.90% with PC71BM as electron acceptor[96] and a PCE of 1.62% was attained for the device of d22:bis-PDI-T-EG.[90] In order to further extend the π-conjugated backbone, Zhan et al. incorporated two thiophene-separated BDT units as core and synthesized d23. Still, with bis-PDI-T-EG as the acceptor, the constructed devices showed a best efficiency of 1.6% by using 2% DIO as the additive.[97]

In 2014, Zhan et al. used a rigid and coplanar indaceno[2,1-b:6,5-b′]dithiophene (IDT) as core to give a novel bipolar A–D–A-type compound d24. d24 exhibited strong absorption in solution with high maximum molar extinction coefficient. The estimated HOMO and LUMO energy levels were −5.11 and −3.32 eV, which was appropriate to match the energy levels of PC71BM acceptor and P3HT donor, respec-tively. Hence, it can be used as a donor with PC71BM acceptor or acceptor material with P3HT donor, respectively. The OSCs based on the d24:PC71BM blend exhibited a PCE of 2.82%.[98] Marks et al. reported a DPP functionalized tetrathienothio-phene (TTA) small molecule, d25. The large fused TTA core provided strong intermolecular S–S interactions and excellent molecular planarity, which favored a formation of close-packed thin-film morphologies for efficient charge transport, yielding sizable hole mobility of 0.01 cm2 V−1 s−1 and a high PCE of 4.0% in inverted OSC device.[99] Pentathiophene (PTA) and pyrro-lemodified pentathiophene (NPTA) were chosen as the central donor unit in DPP–donor–DPP-type compounds, respectively, namely d26 and d27, by Shi et al. Compared to d26 without alkyl chain substitution on the central sulfur atom of the PTA unit, d27 with a dodecyl chain on the central nitrogen atom of the NPTA unit exhibited improved crystallinity and better mis-cibility with PC71BM, which endowed d27 higher hole mobility and better film morphology, giving rise to a higher PCE of 3.83%.[100] Choi et al. synthesized new DPP-based small mol-ecules (d28 and d29), containing (E)-1,2-di(thiophen-2-yl)ethene and naphtho[1,2-b:5,6-b′]dithiophene (NDT), respectively, as electron-donating moieties. The NDT unit has a weaker elec-tron-donating ability than the TVT unit, which resulted the d29 showed the blue-shifted absorption and decreased HOMO level. In addition, d28 exhibited higher carrier mobility than d29, benefited from its better molecular planarity and smaller π–π stacking distance. The BHJ OSC devices based on d28 and d29 gave maximum PCEs of 2.21% and 3.03%, respectively. They thought that the PC61BM aggregates penetrated into the conjugated molecular aggregates of d29 because of the slightly twisted structure of the molecule, which induced the higher Jsc value and PCE.[101] Li et al. designed a small-molecular donor,

d30, incorporating binary electron-deficient units, DPP-based arms and pentacyclic aromatic bislactam-based core. They chose a fullerene derivative (PC61BM) and two non-fullerene perylene-bisimide-based acceptors as electron acceptors to construct organic solar cells, respectively. The results showed that fullerene-based solar cells provided a PCE of up to 4.8%, while the non-fullerene solar cells also exhibited promising PCEs of 2.4% and 3.5%, respectively. The slightly lower PCE of the non-fullerene solar cells was mainly due to the low elec-tron mobilities and the large phase separation.[102] Zhu et al. synthesized a novel A1–A–A1-type small molecule (d31), in which benzodi(pyridothiophene) (BDPT) was used as a novel weak central acceptor (A) unit and DPP was used as terminal acceptor (A1) units. The pentacyclic BDPT aromatic unit can form big conjugated and planar small molecule with the DPP unit, giving rise to the broad absorption spectrum, low HOMO energy level and enhanced π–π stacking and crystallinity. Hence, a PCE of 3.97% with a Voc of 0.84 V, a Jsc of 9.0 mA cm−2 and a FF of 52.37% was obtained.[103]

From what we described above, it would be reasonable to believe that the conjugated backbone design play an impor-tant role in determining the device performance. Modulating the nature of donor–acceptor building blocks could principally control the backbone co-planarity, bandgap, energy levels, and the molecular packing. For example, according to the molecular orbital theory, the stronger donating unit gave rise to higher HOMO energy levels and more red-shifted absorption spec-trum than that strong donating unit. The fused aromatic ring with extended planar structure provided excellent molecular planarity, strong intermolecular interactions, which contrib-uted to form close-packed thin-film morphologies for efficient charge transport, exhibiting higher hole mobility than that non-fused aromatic ring.

In 2014, Zhan et al. studied the effect of anchoring termi-nals on the film morphology and device performances by choosing DPP-BDT-DPP as a model backbone and varying the anchoring groups [C5H11 (d32a), COOCH3 (d32b), and SiCH3(OSiCH3)2 (d32c)] terminated on the N-substituted alkyl-chain spacer of the DPP units. They observed for the first time that manipulating the dipolar anchoring groups termi-nated on the alkyl-chain spacer could improve photocurrent generation. The polar COOCH3 anchoring terminal leaded to an enhancement in Jsc (4.62 vs 9.32 mA cm−2), whereas the value of Jsc sharply decreased to 0.45 mA cm−2 if adopting a SiCH3(OSiCH3)2 as another polar anchoring group.[104] Through contrastive analysis, the changes in Jsc were associ-ated with changes in the π–π stacking distance and the phase size as well as molecular orientations. Changing the terminal anchoring group from SiCH3(OSiCH3)2 to C5H11 and then to COOCH3 led to significant decreases in the π–π stacking distance and the phase size. And different molec-ular orientation was observed: tilted orientation for COOCH3, misaligned packing for C5H11, and edge-on orientation for SiCH3(OSiCH3)2. Compared to the edge-on orientation for SiCH3(OSiCH3)2, the titled orientation for COOCH3 con-tributed to the out-of-plane hole transport in solar cells, leading to the increase of the Jsc. Kim et al. synthesized a series of ADA-type small molecules (d33a–d) by controlling the type of the alkyl chains on dithienosilole (DTS) electron-donating




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Scheme 7. Chemical structures of linear small-molecule donors with DPP as bridged arms.


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moiety and two DPP electron-withdrawing moieties. The exper-iment data indicated the shorter alkyl groups on the DTS core facilitated well-matched molecular packing, whereas the slightly longer alkyl chains on the DTS core prevented close molecular packing, yielding molecular disorder in the resulting films.

Accordingly, the d33a and d33b-based films were highly crystal-line with a primary edge-on orientation, whereas the d33c and d33d were inclined to adopt radial distribution of molecular ori-entations.[105] As a result, d33a and d33b-based OFET devices showed higher hole mobilities caused by the highly crystalline



Table 5. Optical and electronic properties, mobilities, and OPV performance of double-DPP-based donors.

λmaxa)

[nm]HOMO/LUMO b)

[eV]μh

c) [cm2 V−1 s−1]


Voc [V]

FF PCE [%]

Ref.

e1 670 −5.00/−3.60 – e1:PC71BM = 1:1 7.67 0.57 0.50 2.19 [109]

e2 678 −5.23/−3.61 3.6 × 10−2 (F) e2:PC61BM = 1:0.8 14.21 0.66 0.40 3.76 [110]

e3 656 −5.16/−3.65 1.1 × 10−2 (F) e3:PC61BM = 1:1 11.49 0.60 0.46 3.19 [110]

e4a 750 −5.16/−3.64 1.01 × 10−5 (S) e4a:PC71BM = 1:1 6.08 0.65 0.60 2.36 [111]

e4b 750 −5.15/−3.61 1.28 × 10−5 (S) e4b:PC71BM = 1:1 13.39 0.69 0.56 5.12 [111]

e5 738 −5.11/−3.59 – e5:PC71BM = 1:1 4.12 0.67 0.58 1.62 [89]

e6 749 −5.07/−3.77 – e6:PC71BM = 1:1 2.2 0.51 0.66 0.74 [89]

e7 705 −4.98/−3.36 – e7:PC71BM = 3:7 6.2 0.78 0.52 2.51 [89]

e8 717 −5.20/−3.60 – e8:PC71BM = 7:3 3.49 0.80 0.53 1.46 [89]

e9 683 −5.20/−3.49 – e9:PC71BM = 1:1 2.07 0.73 0.51 0.77 [89]

e10 697 −4.97/−3.49 e10:PC71BM = 1:1 2.88 0.67 0.38 0.74 [89]

e11 684 −5.19/−3.64 3.7 × 10−6 (S) e11:PC71BM = 1:1 3.00 0.84 0.51 1.28 [89]

e12 697 −4.97/−3.29 – e12:PC71BM = 1:1 10.60 0.73 0.45 3.50 [112]

e13 698 −4.96/−3.27 – e13:PC71BM = 1:1 10.30 0.71 0.52 3.80 [112]

e14 691 −4.98/−3.31 – e14:PC71BM = 1:1 11.90 0.69 0.57 4.60 [112]

e15 634 −4.95/−3.32 – e15:PC71BM = 1:1 10.40 0.67 0.58 4.00 [112]

e16 650 −5.21/−3.39 4.3 × 10−5 (S) e16:PC71BM = 1:3 8.08 0.89 0.45 3.26 [113]

e17 712 −5.36/−3.77 1.8 × 10−3 (S) e17:PC71BM = 1:1 12.20 0.76 0.62 5.90 [114]

e18 675 −5.46/−3.77 2.4 × 10−7 (S) e18:PC71BM = 1:1 4.10 0.86 0.54 1.90 [114]

e19 675 −5.32/−3.80 3.7 × 10−7 (S) e19:PC71BM = 1:1 8.16 0.66 0.43 2.40 [114]

e20 665 −5.24/−3.70 3.7 × 10−7 (S) e20:PC71BM = 1:1 8.17 0.66 0.40 2.30 [114]

e21 720 −5.13/−3.63 7.76 × 10−4 (S) e21:PC71BM = 1:1.5 8.35 0.67 0.57 3.20 [115]

e22 720 −5.08/−3.60 9.77 × 10−4 (S) e22:PC71BM = 1:1.5 15.64 0.62 0.59 5.77 [115]

e23a 701 −5.15/−3.58 2.23 × 10−6 (S) e23a:PC71BM = 1:1 10.21 0.70 0.51 3.30 [116]

e23b 710 −5.15/−3.59 7.76 × 10−6 (S) e23b:PC71BM = 1:1 12.18 0.69 0.58 4.64 [116]

e23c 715 −5.18/−3.63 2.37 × 10−4 (S) e23c:PC71BM = 1:1 13.60 0.69 0.64 5.87 [116]

e24 716 −5.16/−3.60 – e24:PC61BM = 1:1 11.40 0.67 0.53 4.04 [117]

e25 663 −5.12/−3.37 – e25:PC71BM = 1:1 8.58 0.68 0.48 2.80 [117]

e26 640.1 −5.18/−3.55 9.0 × 10−5 (F) e26:PC71BM = 1:3 9.00 0.86 0.39 3.00 [118]

e27 671.1 −5.19/−3.59 4.0 × 10−4 (F) e27:PC71BM = 1.5:1 8.37 0.91 0.49 3.71 [118]

e28 730 −5.27/−3.68 1.2 × 10−2 (F) a28:PC71BM = 1:1 0.25 0.48 0.34 0.04 [119]

e29 633 −5.21/−3.43 2.55 × 10−4 (S) a29:PC71BM = 1:5 6.15 0.78 0.40 1.94 [60]

e30 647 −5.25/−3.49 8.47 × 10−4 (S) a30:PC71BM = 1:1 11.13 0.85 0.60 5.67 [120]

e31 670 −5.17/−3.47 2.88 × 10−4 (S) a31:PC71BM = 1:1 15.35 0.66 0.5789 5.88 [120]

e32 697 −5.03/−3.37 2.33 × 10−4 (S) a32:PC71BM = 1:1 10.77 0.62 0.6394 4.26 [120]

e33 698 −5.27/−3.34 8.42 × 10−5 (S) a33:PC61BM = 3:2 5.63 0.65 0.66 2.41 [121]

e34 708 −5.30/−3.16 3.27 × 10−5 (S) a34:PC71BM = 1:3 10.55 0.85 0.39 3.22 [121]

e35 747 −5.42/−3.45 5.75 × 10−5 (S) a35:PC71BM = 1:1 2.21 0.88 0.49 0.95 [121]

e36 771 −5.35/−3.44 3.53 × 10−5 (S) a36:PC71BM = 1:2 2.11 0.81 0.39 0.67 [121]

a)In film. b)Estimated from the cyclic voltammetry data. c)F and S: measured by FET or SCLC method.


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the highest PCE, attributed to significant face-on molecular packing. The solubility of d33d was too poor to fabricate the solar cells. Kim et al. coupled the fused dithieno[2,3-d:2′,3′-d′]-benzo[1,2-b:4,5-b′]dithiophene (DTBDT) structure with DPP moieties to generate highly planar d34a and d34b. Differently, 2-ethylhexyl groups were chosen as the alkyl side chains on DTBDT in d34a while 2-butyloctyl groups were selected as side chains on DTBDT for d34b. Both molecules displayed the similar optical properties and HOMO levels. The d34a:PC61BM blend films showed nanoscopic (≈10 nm) bicontinuous inter-penetrating domains with a high crystallinity, giving a higher PCE of 4.35%. In contrast, the d34b molecule with longer linear alkyl chains tended to form more aggregates and larger domains, which reduced the device PCE (<3%).[106]

Porphyrin analogues have extensive π-conjugated surfaces and show high efficiency in photosynthesis. Peng and co-workers designed a series of porphyrin-based small molecules (d35–d37).[107] They conjugated a porphyrin core with two DPP arms by ethynylene linkages to increase the planarity of the backbone, which facilitated the intramolecular charge trans-port. By changing the 3,5-di(dodecyloxy)-phenyl groups on por-phyrin zinc in d35 with 4-octyloxy-phenyl groups in d36a, the PCE was increased from 4.78% to 7.23%.[107a,b] This is because the long chains of the dodecyloxy groups at the 3,5-positions of the phenyl ring in d35 protruded out of the porphyrin plane and suppressed the intermolecular π–π stacking, leading to a relatively poor photovoltaic performance. Less bulky 4-octyloxy-phenyl groups at the porphyrin periphery of d36a effectively promoted its intermolecular interactions in the solid state, facilitating intramolecular charge transport and thus affording an increased PCE. In 2016, they further synthesized two por-phyrin small molecules substituted with either furan- or sele-nophene-linked diketopyrrolopyrrole units, named as d36b and d36c. Combined with the previously described thiophene analogue (d36a), they discussed the impacts of the O and Se chalcogen atoms in the linking 5-membered ring in these donor materials on the performance of organic solar cells. On the one hand, the heavy atoms broadened the absorption and narrowed the bandgap by tuning the energy levels regularly. On the other hand, the selenophene containing analogue showed better miscibility and smaller phase separation with PC61BM than the furan analogue. As a result, the optimized organic solar cells based on the selenophene containing species showed a higher PCE of 5.8% than that of furan-based devices with a PCE of 4.3%.[108] Further 5-(2-ethylhexyl)thienyl was attached to the porphyrin and the affording molecule d37 showed high permittivity and good morphology, contributed to a generation of a good EQE and low energy loss. Finally, a higher PCE of 8% was achieved for d37 after adding 1% pyridine additive and thermally annealing, which is the highest PCE among the DPP-based small-molecule solar cells.[107c]

In a word, careful manipulations of the alkyl-chains by altering the shape, length, substitution position, etc., can greatly improve the solubility and intra- and intermolecular forces, which then act on the improvement of nanostructures. It has been demonstrated that linear side chains have a stronger pro-pensity for aggregation in film while unnecessarily bulky or excessive side chains could increase rotational angle of the

backbone through steric hindrance effect, which in turn lead to loose molecular π-packing. The substitution position of the alkyl chains can exert an influence on the optical and electronic prop-erties as well as molecular packing in thin films through tuning the steric hindrance effect or rotational angle. What’s more, it was found that functionalized side chains with some dipolar units could bring about drastic changes in intermolecular inter-actions and in turn affect the orientation of π-stacking with respect to the substrate, leading to different carrier mobilities.

3.2.2. The Linear Small-Molecule Donors with DPP as Bridged Units

As mentioned, although a various length π-bridge conjugated group between DPPs have been investigated, effective con-jugation could not always be improved accordingly. Recently, the introduction of end-capped units at the flank of the DPP–Donor–DPP-type backbone was regarded as a promising and common approach for extending the π-conjugated backbone. The resulting backbone can be named as Ar–DPP–Donor–DPP–Ar, as shown in Scheme 7. Table 5 lists the optical and electronic properties, mobilities, and OPV performance of Ar–DPP–Donor–DPP–Ar-based donors. In this Section, we emphasize on clarifying either the effect of the center donor units on the device performance or the effects of the end groups on the device performance.

In 2012, Nguyen and co-workers reported a π-extended small molecule consisting of a dithienothiophene (DTT) and two hexylthiophene end-capped DPP chromophores (e1). They dem-onstrated that using 1-chloronaphthalene (CN) as processing addi-tive gave rise to an increase in the PCE from 0.5% to 2.19%. The addition of 1% CN appeared to suppress aggregation and formed smooth surface and desired phase separation, leading to dramati-cally increased Jsc.[109] With thiophene as end groups, Choi et al. investigated the influence of the conjugation length of the center electron-donating units on the device performance by introducing different numbers of thiophene bridged units (terthiophene, e2; quarterthiophene, e3). The longer thiophene bridge between the DPP dimer in e3 showed an increase of the rotational freedom and disorder of the system, resulted in weak intermolecular inter-actions and a blue-shifted absorption. In contrast, e2 displayed strong intermolecular interactions, giving rise to uniform order of crystalline domains in the thin film. Thus, compared with e3, e2 possessed an optimized conjugation length and provided a higher carrier mobility of 3.6 × 10−2 cm2 V−1 s−1 and higher PCE of 3.76%.[110] Shin et al. designed two new low-bandgap small molecules, e4a and e4b, with different terminal side chain. e4a was flanked by hexyl at head and tail of backbone while e4b was free attachment. Though both of them exhibited similar optical, electrochemical properties and charge-carrier mobilities, different device performances were attained for each other. e4b-based blend film performed optimal film morphology leading to higher Jsc, thus a higher PCE of 5.12% for e4b was achieved than that of e4a (PCE = 2.36%).[111]

Nguyen et al. synthesized a series of materials based on an architecture consisting of two DPP cores with different aro-matic π-bridges between the DPP units and varied end-capping groups (e5–e11).[89] The center aromatic π-bridges contained alkoxy-BDT, benzo[c][1,2,5]thiadiazole (BT), fluorene (FL), DTS,




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and hydrolyzed dithieno(3,2-b;2′,3′-d)silole (SOHDT) cores etc. And various terminal groups include 2-benzofuran and hexylbithiophene, hexylthiophene etc. Comparing the mate-rials e5, e6 and e7 with the same hexylbithiophene end-capping groups revealed how core groups affected device properties. Although the BT core possessed a broader absorption spectrum and greater potential for photocurrent, e6 showed the lowest Jsc. The electron deficient nature of BT endowed e6 a too deep LUMO level, closed to that of the PC71BM, which reduced the ability to generate photocurrent from excitons generated in the donor phase. The comparison of the compounds e8–e11 displayed the effect of the central core-group on the device properties when 2-benzofuran was employed as end-capping group. Within this class, the most promising material consisted of a hydrolyzed DTS core which exhibited desirable proper-ties including an absorption onset of 800 nm and high Voc of over 0.85 V. The fluorene-cored material produced the largest Voc while the photocurrent onset was the shortest out of the series. A combination of studies based on e5 and e9, they found 2-benzofuran could lead to deeper energy levels than hexylbith-iophene, giving rise to a larger Voc, while hexylbithiophene con-tributed to yield lower series resistance, larger Jsc.[89] Janssen et al. synthesized four structurally related molecules (e12–e15) in which the two electron deficient DPP moieties were sepa-rated by an aromatic conjugated terthiophene and end-capped with 2-(thiophen-2-yl)benzofuran or 2-(thiophen-2-yl)-benzo[b]-thiophene. They found that tailoring of the molecular structure mainly affected the solubility and, in turn the processing condi-tions. Under carefully adjusted processing conditions, e14 with the lowest solubility in 1,1,2,2-tetrachloroethane (TCE) solution gave the highest PCE of 4.6%, while the other bis-DPP mole-cules gave lower PCEs of 3.6–4.0%.[112]

Shi and Chen et al. selected two kinds of aromatic six-mem-bered rings (fluorene as core and benzene as terminals) to con-nect two DPP moieties which could provide moderate dihedral angles between DPP and aromatic six-membered rings. As a result, the twisted molecular conformation for e16 contributed to the formation of suitable energy levels and fine phase separa-tion domains.[113] The impacts of these different central cores on the optoelectronic properties and photovoltaic performance were also investigated by Yasuda and Adachi et al. e17–e20 were designed by linking two DPP units with alkylthienyl-substituted BDT, indacenodithiophene (IDT), thiophene, or isoindigo cores and introduced hexyl-substituted phenylthiophene end-capping groups at both extremities to expand the π-conjugated structure and to facilitate film-forming ability. Among the four oligomers, e17 with a alkylthienyl-substituted BDT core possessed broad absorption and excellent nanostructured film-forming ability, exhibiting the highest hole mobility and an outstanding PCE of up to 5.9%. In contrast, the inverted solar cells based on e18, e19, and e20 delivered inferior PCEs of 1.8%, 2.3%, and 2.1%, respectively. The main reason was that e18 and e20 showed much lower hole mobilities on the order of 10−7 cm2 V−1 s−1, which are nearly four orders of magnitude lower than that of e17, caused by the poorer packing order of the e18- and e20-based thin films.[114]

Recently, Zhan and Yao et al. studied the effects of two kinds of terminals on the solar-cell performance. The aliphatic n-butyl (n-Bu) unit in e21 and non-alkyl, branched aromatic and

electron-donating diphenylamine (DPA) in e22 were selected as end-capping groups on a diketopyrrolopyrrole-based linear backbone. They found that the DPA end-functions contributed to enhance the light-harvesting capacity, improve the charge dissociation, and reduce the recombination loss, all of which leaded to more carriers being collected by the electrode. Accord-ingly, the photocurrent of e22 significantly increased from 8.35 to 15.64 mA cm−2 and the PCE from 3.2 to 5.8%.[115] Based on a series of narrow-bandgap, π-conjugated small molecules containing DPP electron acceptor units coupled with BDT electron donors, Hwang et al. controlled the nanoscale mor-phologies of the photoactive layers of the photovoltaic cells by end-group functionalization of the small molecules with fluo-rine derivatives (e23a with hydrogen atom as a reference, e23b with fluorine atom as end groups and e23c with trifluoromethyl as end groups). The e23b and e23c films showed red-shifted UV absorption peaks with wide shoulder region (at 700–800 nm), compared to those of the e23a film, implying stronger inter-molecular interactions in the e23b and e23c chains than those in the e23a chains. The calculated theoretical electron affini-ties indicated that the fluoromethyl substituents contributed to stabilize both the HOMO and LUMO levels, in contrast, fluoride substitutions had only marginal impact on the frontier orbitals. The PCEs dramatically increased from 3.65% for the e23a:PC71BM device, to 4.96% for the e23b:PC71BM device, and to 6.00% for the e23c:PC71BM device. Among the device para-meters, the Voc values remained constant while the Jsc and FF increased by their turns. This is because the fluorinated mole-cules packed better than the nonfluorinated molecules, forming an improved film structure during the fast solvent evaporation that occurs in spin-casting and the face-on conformation in the blend film increased by their turns, which were responsible for the increased Jsc values and hole mobilities. In addition, the ratio between the hole and electron mobilities in the active layer were decreased from e23a to e23c, leading to improved FFs of OPVs. In particular, strong dipole–dipole interactions induced by the electronegative fluorine atom at the end of the backbone exerted a significant influence on the molecular structure, packing, and morphology of the thin films.[116]

Wan et al. synthesized two DPP-based small molecules, e24 and e25, which contained dialkoxy-substituted BDT or dioctyl-tertthiophene (3T) as the central donor units and TPA as the terminals. The BDT units endowed the e24 largely red-shifted absorption onset and relatively deep HOMO energy level than that of e25. As a result, the solar cells based on e24/PC61BM displayed a high PCE of 4.04% after annealed at 130 °C for 10 min.[117] Chen and co-workers compared the effect of the benzene core in e26 and biphenyl-DPP core in e27 on device performance. Compared to e26 with benzene core, e27 with π-extended conjugated biphenyl-DPP core showed red-shifted absorption and lower bandgaps as well as a longer-range-ordered crystalline structure with a sharper diffraction peak in the small angle region, which was ascribed to that the pres-ence of a DPP unit at the center of e27 brought out stronger intermolecular interactions than e26. It was found that the hole mobilities and PCE for e27-based device was higher than that of e26, accompanied with higher FF and Voc.[118] Segura et al. syn-thesized a dicyanovinylene-terminated bidiketopyrrolopyrrole semiconductor (e28). The combination of an extended




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electron-donating oligothiophene with the strong electron-with-drawing DPP-dicyanovinylene groups endowed e28 ambipolar TFT response with balanced electron and hole mobilities of

0.02 cm2 V−1 s−1. But it displayed very poor photovoltaic perfor-mance caused by the intrusion of triplets in the carrier forma-tion process.[119]



Scheme 8. Chemical structures of star-shaped small-molecule donors with DPP as arms.


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In 2015, Zhu and Yang et al. designed a narrow-bandgap small molecule, e29, with D–A–B–A–D frameworks, in which in which TPA, 3,6-dithienyldiketopyrrolopyrrole and 9,10-anthra-cene (AN) were used as donor (D), acceptor (A) and bridge (B) segments, respectively. The moderate PCE of 1.94% and Jsc of 6.15 mA cm−2 were observed in the e29-based solar cell.[60] In order to systematically investigate the effect of the central aryl (Ar) fused ring and the terminal units on the photovoltaic perfor-mance of small molecules, they used 2,7-pyrene (Py) as a central Ar core, DPP as the armed A unit and 2-octylthiophene (Th) or triphenyl-amine (TPA) as the terminal D group to construct the A–Ar–A-type moleucle of e30 and the Ar(A–D)2-type molecules of e31 and e32. Attaching the electron-rich 2-octylthiophene and triphenylamine units can not only extend π-conjugated system and increase electron-donating ability as well as extend absorption bands, but also cause their molecules to be oxi-dized easier and result in an increasing HOMO energy level. e31 based on the 2-octylthiophene terminal exhibited better photovoltaic performance than e30 while e32 containing the triphenylamine terminal displayed decreasing photovoltaic per-formance. They found the e31 blend film displayed the highest photoresponse efficiency, accompanied with the uniform morphology and appropriate domain sizes, giving the highest PCE of 5.88%.[120] For better understanding the influence of different conjugated molecular terminals on the properties of DPP–Phenyl–DPP-based small molecules, they synthesized four novel small molecules, e33–e36 with DPP-phenyl-DPP as the core, donor units of 2-octylthiophene (Th), pyrene (Py), acceptor units octyl 2-cyano-3-(thiophen-2-yl)-acrylate (ThCN)

and 3-octyl-5-(thiophen-2-ylmethylene)rodanine (ThR) as the terminals, respectively. The materials with acceptor terminal units (ThCN or ThR) had much deeper HOMO energy levels than those with donor terminal units (Py or Th), which helped to increase the Voc. Among them, compound e36 had the min-imum optical bandgap of 1.48 eV, attributed to the strongest electron-withdrawing property of the ThR. A maximum PCE of 3.22%, a Jsc of 10.55 mA cm−2, a Voc of 0.85 V, and FF of 39% were obtained in the e34/PC71BM based solar cells, benefited from its finest phase separation and highest EQE over 40% in a broad range from 374 to 664 nm. Compound e33 showed the highest μh of 8.42 × 10−5 cm2 V−1 s−1, which can be attributed to the good solubility and the plane of thiophene terminals. The poor solubility of terminal acceptors of ThCN and ThR resulted e35 and e36-based devices lower PCEs.[121]

Through the above analysis of the data, it is found that end-capped groups attached to the flank of the above-mentioned backbones benefited to further extend the conjugated back-bone and modify the HOMO/LUMO energy levels as well as absorption properties. The attached end-capped groups could implement the accessional inductive and resonance effects by adding electron-donating or electron-withdrawing units. In general, electron-donating groups benefit to increase the HOMO energy level, whereas electron-withdrawing groups contribute to reduce the LUMO energy level. Moreover, a few work demonstrated that the attached end-capped groups could regulate the intermolecular interaction between the donor and acceptor, which acts on the carries generation, transportation and collection.



Table 6. Optical and electronic properties, mobilities, and OPV performance of multi-DPP-based donors.

λmaxa)

[nm]HOMO/LUMO b)

[eV]μh

c) [cm2 V−1 s−1]

Active layer Jsc [mA cm−2]

Voc [V]

FF PCE [%]

Ref.

f1 620 −5.20/−3.35 – f1:PC61BM = 1:2 4.88 0.58 0.34 0.97 [122]

f2 622 −5.15/−3.60 5.21 × 10−7 (S) f2:PC71BM = 1:3 7.92 0.66 0.35 1.81 [122]

f2 618 −5.18/−3.25 3.58 × 10−3 (S) f2:PC71BM = 1:2 5.08 0.77 0.35 1.38 [123]

f3a 607 −5.13/−3.32 1.16 × 10−2 (S) f3a:PC71BM = 1:4 10.06 0.81 0.36 2.95 [123]

f3a 596 −5.65/−3.86 5.2 × 10−5 (S) f3a:PC71BM = 1:3 8.95 0.89 0.36 2.91 [124]

f3b 592 −5.60/−3.84 4.0 × 10−5 (S) f3b:PC71BM = 1:2 10.17 0.75 0.58 4.39 [124]

f4a 645 −5.11/−3.39 8.63 × 10−5 (S) f4a:PC61BM = 1:1 7.94 0.72 0.52 2.98 [125]

f4b 647 −5.13/−3.43 2.42 × 10−5 (S) f4b:PC61BM = 1:1 5.14 0.76 0.42 1.63 [125]

f4c 653 −5.09/−3.41 4.43 × 10−5 (S) f4c:PC61BM = 1:2 5.83 0.80 0.42 1.98 [125]

f5 580 −4.98/−3.48 1.09 × 10−5 (S) f5:PC71BM = 1:2 1.86 0.69 0.39 0.39 [126]

f6 578 −4.98/−3.46 1.52 × 10−4 (S) f6:PC71BM = 1:2 5.59 0.73 0.37 1.50 [126]

f7 601 −5.03/−3.45 1.52 × 10−4 (S) f7:PC71BM = 1:2 9.27 0.78 0.48 3.42 [126]

f8 673 −4.97/−3.46 1.67 × 10−4 (S) f8:PC71BM = 1:2.5 8.95 0.80 0.51 3.67 [127]

f9 628 −4.89/−3.44 1.99 × 10−4 (F) f9:PC71BM = 1:1 9.48 0.64 0.63 3.81 [128]

f10 640 −5.36/−3.35 3.58 × 10−4 (S) f10:N2200 = 1:1 8.59 0.87 0.62 4.64 [129]

f10 640 −5.36/−3.35 2.20 × 10−4 (S) f10:PC71BM = 1:1 7.69 0.89 0.50 3.56 [129]

f11 640 −5.45/−3.41 2.21 × 10−4 (S) f11:N2200 = 1:1 8.14 0.86 0.54 4.02 [129]

f11 640 −5.45/−3.41 1.65 × 10−4 (S) f11:PC71BM = 1:1 7.12 0.89 0.49 3.22 [129]



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iew 3.3. Multi-DPP-Type Photovoltaic Small Molecules

3.3.1. The Star-Shaped and V-Shaped Small-Molecule Donors with DPP as Arms

Triphenylamine (TPA) possesses a special propeller starburst-like molecular structure, which endows it good electron-donating and high charge-transporting properties. Some researchers covalently linked TPA and DPP as backbone, including star-shaped molecules with TPA as core and push–pull molecules with TPA as the terminal group, to study their photovoltaic performance, as shown in Scheme 8. Table 6 pro-vides a summary of optoelectronic properties, hole mobilities as well as OPV data.

In 2012, Chu et al. synthesized two new starburst D–A mol-ecules (f1 and f2) featuring TPA units as the core and DPP units as the acceptor moieties in the arms, in 1:2 and 1:3 ratios, respectively. This backbone showed broad absorption features with the absorption bands extended up to 793 nm. Further-more the number of accepting arms played a key role in deter-mining their optical and photovoltaic properties. f2 with three DPP arms exhibited broader and stronger absorption, resulted in a higher PCE of 1.81%.[122] Zhan and co-workers intro-duced a thiophene ring as the conjugated π-bridge between the TPA core and DPP arms, namely f3a, for comparative studies with f2. Studies revealed that embedding the thiophene ring enlarged the conjugation, thus enhanced the light-harvesting ability and hole mobility, leading to a dramatically enhanced PCE with respect to that of f2 (2.95% vs 1.38%).[123] In 2016, Stupp et al. synthesized two tripodal “star-shaped” compounds f3a and f3b based on diketopyrrolopyrrole (DPP) side chains for solution-processed BHJ OSCs. they found that f3a with branched alkyl chain (2-ethylhexyl) substituents on DPP units could not aggregate in solution or form crystalline domains in thin films, whereas linear (docedecyl) alkyl chains in f3b could promote the formation of one-dimensional (1D) nanowires and more crystalline domains in the solid state. As a result, 50% PCE improvement was observed for compound f3b with the linear alkyl chains, which was attributed to the significant increase in the FF of devices resulting from a reduction of trap states.[124]

Shi et al. also used TPA as the core, DPP as the arm and attached unsubstituted or substituted benzene rings (phenyl, f4a; 4-fluoro-phenyl, f4b; 4-n-butyl-phenyl, f4c) at the end of DPP unit to fine-tune the energy levels and aggregation state of the starburst molecules. Among three small molecules, end-capping groups (phenyl) without any substituent afforded strong intermolecular orbital overlap and gave a higher mobility, while the introduction of electron-withdrawing fluorine (F) to the 4-position of phenyl group contributed to decrease the HOMO energy, whereas 4-butylphene with the largest end-group’s size generated the poorest molecular packing. These results made f4a-based OSCs exhibit the highest PCE of 2.98%.[125] Similarly, Zhu et al. attached phenanthrene (PN) as the planar arene terminal to the TPA-DPP-based backbone to provide a favorable end-to-end π–π interaction and manipulated the number of accepting arms step by step to control the molec-ular energy levels, attained f6–f8. In order to illustrate the role of the terminal phenanthrene, f5 with TPA as donor core and

DPP as acceptor arms was also synthesized. Accompanied with the increase of the number of accepting arms, the strength of the absorption obviously enhanced, favoring the photocurrent generation.[126] As a result, the PCE of the solar cells increased gradually. The higher PCE of 3.42% were obtained in the solu-tion-processed f7/PC71BM-based solar cells while f8 exhibited the highest PCE of 3.67% with an increased FF of 51%.[127] A new V-shaped small molecule, f9, containing TPA core, two DPP arms and two end-capped hexylbithiophene units, was designed by Tao et al. With a broad absorption, a low optical bandgap of 1.61 eV, an optimized PCE of 3.81% with high FF of 63% was achieved via adding 0.1% DIO as solvent additive to form well-defined phase-separated film morphology.[128]

Recently, Chen and Jen et al. utilized an atom efficient direct arylation reaction to connect four DPP electron-accepting units onto either a tetraphenyl-methane or a tetraphenyl-silane core and achieved two novel 3D-shaped donor molecules (named as f10 and f11). Benefited from the complementary light absorp-tion of f10 and f11 donor with N2200 polymer acceptor and balanced charge mobilities, both f10 and f11 can yield efficient photocurrent generation and improved FF when blending with linear N2200 polymer acceptor than blending with PC71BM acceptor. Therefore, the PCEs of 4.64% and 4.02% with N2200 blends surpassed those obtained from PC71BM (3.56% and 3.22%, respectively).[129]

3.3.2. C3- or C4-Symmetric, Y-Shaped, and X-Shaped Small-Molecule Donors with DPP as Arms

Increasing the coplanarity of molecular backbone generally enhances the intramolecular charge transfer and produces strong π–π stacking, contributed to improve the charge-carrier transport ability and increase light-harvesting ability. As shown in Scheme 9, in this Section, we focus on discussing the influ-ence of the center planar building blocks on photovoltaic per-formance in DPP-based small-molecule solar cells, such as 1,3,5-trithienyl phenyl, triazine as well as pyrene etc. Table 7 summarizes the corresponding optoelectronic properties, hole mobilities and OPV data.

In 2013, Zhan and Yao et al. designed a novel planar back-bone in which 1,3,5-trithienyl phenyl as electron donor core, DPP as the acceptor arms, and thiophene or thienyl-BDT as alternative bridged units to adjust the molecular configuration and conjugation, thereby obtaining a series of C2- or C3-sym-metric molecules, g1–g5. They found that the C3-symmetric molecules exhibited more favorable optoelectronic properties than those of C2-symmetric molecules and the thienyl-BDT-bridged molecules exhibited better optoelectronic properties than their counterparts with the thiophene bridge. Because the introduction of strong electron-donating and 2D-extended thienyl-BDT bridge enhanced the intramolecular charge transfer and intermolecular interactions, leading to particu-larly improved light-harvesting ability and hole mobility. As a result, g5 showed the highest efficiency of 3.60 % with 3 % DIO as the solvent additive and thermal annealing at 100 °C for 10 min.[130] In 2014, Chen and co-workers used 1,3,5-phenyl as the electron donor core and DPP as electron acceptor arms and attached planar pyrene as end-capped units to the end of DPP,




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achieving molecule g6. Compared with the molecule g2, g6 with pyrene as terminals gave a higher PCE (2.47%), ascribed to its extended conjugated length and decreased bandgaps as well as a favorable end-to-end π–π interaction.[118]

In 2015, Lee et al. reported three novel small molecules (g7, g8, and g9) containing a planarized triazine as the central core, 2,5-thienyl DPP or 1,4-phenylene DPP as the π-conjugated bridge, and t-butyl-substituted triphenylamine or t-butyl-sub-stituted carbazole as the end groups, respectively. Benefited to the excellent optoelectronic properties, the lower energy bandgap and higher charge transfer capacity of 2,5-thienyl DPP unit, the solar cells based on g7 with 2,5-thienyl DPP branches were much better than those of the solar cells based on the

one with 1,4-phenylene DPP branches. The stronger electron-donating character and more efficient π–π* charge transfer of tert-butyl-substituted triphenylamine endowed g8 a red-shifted absorption band in favor for the photocurrent generation. The material g8 showed higher PCE and Jsc than that of g9 with car-bazole as end groups.[131]

In 2014, Chen and co-workers designed molecule g10 con-sisting of a center planar pyrene and DPP-based π-conjugated bridge as well as phenyl terminals. Because of the steric-hindrance DPP-based arms, g10 showed twisted structures and weak π–π stacking. As a result, g10/PC71BM film had an under-developed phase separation and unclear boundaries, leading to a lower PCE (1.86%) with the lower Jsc and FF.[118]

Scheme 9. Chemical structures of X- and Y-shaped small-molecule donors with DPP as arms.


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3.3.3. Linear Small-Molecule Donors with DPP as Both Core and Arms

Besides the above-mentioned multi-DPP, there should be a kind of special linear multi-DPP-type small molecules, where DPP segments were used as both core and arms. In 2013, Nguyen et al. designed this novel molecular architecture with three DPP chromophores (tri-DPP) in the conjugated backbone, employing a DPP-phenyl group as the central moiety instead of the common electron-donating conjugated moieties (Scheme 10).[132] The incorporation of an extra electron-deficient DPP unit as core could benefit to decrease the HOMO energy levels. Moreover, the resultant phenyl-thiophene linkages could cause a slight twist to the conjugate backbone’s conformation, which further deepen the HOMO energy levels. By changing the alkyl sub-stituents attached on the central phenyl-DPP with either 2-ethyl-hexyl (h1a) or n-hexyl (h1b) alkyl substituents, they synthesized two tri-DPP molecules. These different alkyl substitutions on

the central DPP moiety imparted the h1a and h1b. As shown in Table 8, the determined HOMO levels were −5.4 and −5.2 eV for h1a and h1b, respectively. The higher HOMO value of h1b was probably attributed to its less steric n-hexyl substitutions on the central DPP unit, which contributed to form a more planar conformation in the solid state, as well as much higher tendency of crystallization in h1b films. As a result, the as-cast film of h1b showed distinctive rod-like crystallites while that of h1a exhib-ited a featureless morphology, indicating much greater degree of crystallinity for h1b-based blended film. Accordingly, h1b pro-vided the optimized PCEs of 5.5%, while h1a gave a lower PCE of 4.8%. These results suggested that this strategy of tri-DPP is a promising approach to synthesize high-performance donor molecules. It is known that bisphenyl DPP unit is noncoplanar with a large dihedral angle of ≈20–40° due to the steric repul-sion between the phenyl ring and the bisthiophene DPP core. In order to achieve a coplanar structure, in 2016, Wantz and Li et al. reported a new DPP oligomer (h2) consisting of three bisthiophene DPP units. Even though the coplanar molecular geometry of h2 contributed to achieve greater crystallinity and improve charge transport, the OPV devices using the donor/acceptor blends of h2 and PC71BM exhibited low PCE of 0.72%. They demonstrated that h2 and PC71BM were unable to intermix effectively, resulting in oversized crystalline phases which are detrimental for exciton diffusion.[133] To further improve the solar-cell performance of tri-DPP, it is needed to either modify its chemical structure or explore other processing conditions.

3.4. Photovoltaic Small Molecules Containing DPP Analogues

To improve the properties of DPP by structure modification, some DPP analogues were designed and synthesized. In 2015, thiophene-fused diketopyrrolopyrrole unit (7H-pyrrolo[3,4-a]-thieno[3,2-g]indolizine-7,10(9H)-dione) (as shown in Scheme 11) has been designed as an electron-withdrawing unit by Chen and Zhan et al.[134] Compared with thiophene-substituted DPP, the fusion of a thiophene ring via a carbon–carbon double bond into a DPP unit can not only extend the conjugation backbone but also strengthen the molecular co-planarity, which is in favor for

Table 7. Optical and electronic properties, mobilities, and OPV performance of multi-DPP-based donors.

λmaxa)

[nm]HOMO/LUMO b)

[eV]μh

c) [cm2 V−1 s−1]


Voc [V]

FF PCE [%]

Ref.

g1 627 −5.15/−3.60 4.4 × 10−4 (S) g1:PC71BM = 1:2 3.87 0.70 0.30 0.81 [130]

g2 627 −5.18/−3.53 1.05 × 10−3 (S) g2:PC71BM = 1:4 4.80 0.68 0.33 1.16 [130]

g3 610 −5.13/−3.60 1.75 × 10−3 (S) g3:PC71BM = 1:5 4.45 0.84 0.33 1.24 [130]

g4 650 −5.14/−3.60 1.79 × 10−2 (S) g4:PC71BM = 2:1 5.57 0.74 0.45 1.86 [130]

g5 643 −5.13/−3.57 4.24 × 10−2 (S) g5:PC71BM = 1:1 7.69 0.77 0.59 3.60 [130]

g6 655.8 −5.22/−3.49 3.1 × 10−4 (F) g6:PC71BM = 1:3 7.23 0.92 0.37 2.47 [118]

g7 640 −5.26/−3.68 – g7:PC71BM = 1:1.5 6.34 0.73 0.34 1.57 [131]

g8 523 −5.47/−3.41 – g8:PC71BM = 1:1.5 5.85 0.61 0.32 1.14 [131]

g9 514 −5.51/−3.42 – g9:PC71BM = 1:1.5 2.75 0.50 0.28 0.39 [131]

g10 620.5 −5.26/−3.61 1.6 × 10−3 (F) g10:PC71BM = 1:3 5.83 0.94 0.34 1.86 [118]


Scheme 10. Chemical structures of small-molecule donors with DPP as both core and arms.


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high efficient charge transportation and broad light absorption. Choosing thiophene-fused diketopyrrolopyrrole unit as the elec-tron acceptor arms and DTS unit as a donor core constructed a new kind of A–D–A-type molecule (i1) for solution-processed solar cells. As shown in Table 8, the optimized photovoltaic device with PC71BM as electron acceptor exhibited a relatively high PCE of 4.28% but a low Voc of 0.63 V. The relatively high HOMO level was against the improvement of Voc. The structural modification needs to be further optimized to enhance Voc.[134]

Marks and co-workers produced pyrrolo[3,4-c]pyrrol-1(2H)-one by chemical transformation of the lactam group in DPP to the lactim derivatives, namely, O-alkylated DPPs. Then they attached O-alkylated DPPs onto the flank of the electron-rich fused tetrathienoacene core, affording compound i2.[99] Because O-alkylated DPPs are known to have smaller energy gaps and lower-lying HOMO energies than the N-alkylated ones, the resulting compound i2 attained red-shifted absorp-tion and compressed optical bandgap. In fact, the i2:PC71BM films exhibited less electron density contrast nor less ordering, implying the absence of discrete small molecule and PC71BM domains, but rather intimate mixtures with PC71BM. In terms of OSC performance, this microstructure was likely to increase both geminate and nongeminate recombination losses, thus leading to low Jsc and PCE. As shown in Table 8, the i2 isomer, despite good OTFT mobility, was found to give rise to poor PCE below 1%.[99]

Roncali et al. reported two DPP analogues by replacing the two keto groups of DPP by one and two thioketo groups. They used these DPP analogues as core and two fused benzo-furan as arms to attain two new molecules, i3 and i4. Surprisingly, i3 was obtained as oil and showed a limited stability. Hence, i3 could not be evaluated in OSC devices. They demonstrated that the replacement of the keto groups of DPP by thioketo ones resulted in the dramatically decrease of the photovoltaic activity with a PCE below 0.001%.[57] This phenomenon could be related to the very low LUMO level of −4.14 eV, which decreased the driving force of photoinduced electron transfer to PC61BM. On the other hand, the

quenching of luminescence suggested a considerable reduc-tion of the lifetime of the singlet excited state which can also contribute to reduce the probability of photoinduced electron transfer. From these reported small molecules based on DPP analogues, we think it might be not a good strategy to modify of diketo group in DPP.

4. DPP-Based Small Molecules as Electron Acceptors

DPP molecules with strong electron-withdrawing ability, high electron mobility and low-lying LUMO energy levels should be also used, in principle, as electron acceptors in BHJ OSCs. Scheme 12 summarizes the structures of the DPP-based electron acceptors and Table 9 exhibits the relative optical and electronic properties, mobilities, and OPV performance.

In 2010, Chen et al. designed four DPP-based elec-tron acceptors by introducing some electron-withdrawing end-capping groups on the flank of DPP core, such as

Scheme 11. Chemical structures of small molecules containing DPP analogues.

Table 8. Optical and electronic properties, mobilities, and OPV performance of DPP-based donors.

λmaxa)

[nm]HOMO/LUMO b)

[eV]μh

c) [cm2 V−1 s−1]


Voc [V]

FF PCE [%]

Ref.

h1a 629 −5.40/−3.75 4.0 × 10−5 (S) h1a:PC71BM = 1:1 8.98 0.89 0.606 4.8 [132]

h1b 633 −5.20/−3.56 6.6 × 10−4 (S) h1b:PC71BM = 2:3 10.40 0.86 0.617 5.5 [132]

h2 725 −5.34/−3.99 1.22 × 10−3 (F) h2:PC71BM = 1:1 2.15 0.67 0.50 0.72 [133]

i1 750 −4.99/−3.55 2.33 × 10−5 (S) i1:PC71BM = 1:1.5 11.06 0.63 0.589 4.13 [134]

i2 643 −5.06/−3.28 1.8 × 10−2 (F) i2:PC71BM = 1:1 1.67 0.60 0.482 0.48 [99]

i3 688d) −5.66/−4.04 – – – – – – [57]

i4 718d) −5.68/−4.14 – i4:PC61BM = 3:2 0.007 0.40 0.22 <0.001 [57]

a)In film. b)Estimated from the cyclic voltammetry data. c)F and S: measured by FET or SCLC method. d)In CH2Cl2 solution.


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trifluoromethylphenyl (j1a), trans-2-[4-(trifluoromethyl)phenyl]vinyl (j1b), triflu orophenyl (j1c) as well as ditrifluorometh-ylphenyl (j1d). Among all the solar cells, the best PCE of 1%

was attained for j1a:P3HT-based solar cells, attributed to the highest Voc of 0.81 V. j1b showed a broad and red-shifted absorption band as well as narrow bandgap, attributed to

Scheme 12. Chemical structures of DPP-based electron acceptors.


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Table 9. Optical and electronic properties, mobilities, and OPV performance of DPP-based small-molecule acceptors.

λmaxa)

[nm]HOMO/LUMO b)

[eV]μe

c) [cm2 V−1 s−1]


Voc [V]

FF PCE [%]

Ref.

j1a 599 −5.26/−3.52 – P3HT:j1a = 1:2 2.36 0.81 0.52 1.00 [135]

j1b 592 −5.18/−3.55 – P3HT:j1b = 1:2 1.70 0.64 0.53 0.58 [135]

j1c 595 −5.31/−3.68 – P3HT:j1c = 1:2 2.70 0.65 0.32 0.56 [135]

j1d 569 −5.28/−3.57 – P3HT:j1d = 1:2 0.85 0.71 0.23 0.14 [135]

j1e 670 −5.38/−3.63 – P3HT:j1e = 1:1 1.64 0.56 0.50 0.47 [136]

j1f 577 −5.52/−3.71 – P3HT:j1f = 1:1 0.80 0.73 0.39 0.23 [136]

j1g 577 5.37/−3.57 – P3HT:j1g = 1:1 1.18 0.94 0.48 0.54 [136]

j2a 510 −5.55/−3.59 – P3HT:j2a = 1:1.5 0.79 0.85 0.25 0.17 [137]

j2b 580 −5.90/−4.09 3 × 10−3 (F) P3HT:j2b = 1:1 1.93 0.52 0.31 0.31 [137]

j2c 615 −5.39/−3.68 – P3HT:j2c = 1:2 0.87 0.85 0.32 0.24 [137]

j2d 636 −5.54/−3.94 2 × 10−3 (F) P3HT:j2d = 1:2 0.86 0.60 0.29 0.15 [137]

j2e 677 −5.88/−4.13 2.3 × 10−5 (S) P3HT:j2e = 1:2 5.91 0.89 0.50 3.28 [138]

j2f 592 −5.23/−3.44 – DTS(FBTTh2)2:j2f = 1:1 0.64 0.92 0.27 0.46 [139]

j2f 592 −5.23/−3.44 – P3HT:j2f = 1:1 1.40 0.81 0.37 0.33 [139]

j3a 683d) −5.54/−4.22 4.53 × 10−6 (S) P:j3a = 1:2 8.23 0.92 0.52 3.90 [140]

j3b 738d) −5.64/−4.36 7.84 × 10−6 (S) P:j3a = 1:2 10.21 0.86 0.56 4.95 [140]

j4a 618 −5.26/−3.26 6.8 × 10−6 (S) P3HT:j4a = 1:1 2.68 1.18 0.38 1.20 [141]

j4b 607 −5.12/−3.32 1 × 10−4 (S) P3HT:j4b = 1:1 1.33 1.14 0.41 0.62 [123]

j5 622 −5.30/−3.28 2.8 × 10−5 (S) P3HT:j5 = 1.2:1 4.91 0.97 0.43 2.05 [142]

j6 620 −5.11/−3.32 8.16 × 10−5 (S) P3HT:j6 = 1.2:1 1.43 1.17 0.50 0.83 [98]

j7 599 −5.30/−3.50 8e-6(TOF) P3HT:j7 = 1:1 2.42 1.10 0.45 1.2 [143]

j8 615 −5.56/−3.74 – P3HT:j8 = 1:1.2 3.16 1.17 0.62 2.30 [144]

j9 549 4.92/−3.46 – P3HT:j9 = 1:1 2.15 1.05 0.45 1.02 [145]

j10 645 −5.06/−3.71 1 × 10−6 (O) P3HT:j10 = 1:1 2.06 1.08 0.52 1.16 [146]

j11a 645 −5.70/−4.18 2.4 × 10−4 (S) PTB7:j11a = 1:2 7.77 0.83 0.47 3.03 [13]

j11b 645 −5.85/−4.33 8.2 × 10−4 (S) PTB7:j11b = 1:2 12.10 0.81 0.51 5.00 [13]

j12 602 −5.16/−3.52 1.4 × 10−5 (S) P3HT:j12 = 1:1.5 1.45 1.10 0.30 0.48 [147]

j13 573 −5.18/−3.65 3.6 × 10−5 (S) P3HT:j13 = 1:1.5 1.54 1.13 0.37 0.65 [147]

j14a 650 −5.21/−3.39 2.8 × 10−4 (S) P3HT:j14a = 1:1 5.35 1.18 0.50 3.17 [113]

j14b 643d) −5.31/−3.65 2.68 × 10−3 (S) P3HT:j14b = 1:3 6.25 0.97 0.39 2.37 [148]

j15 612 −5.22/−3.52 – P3HT:j15 = 1:1 1.91 0.97 0.48 0.90 [149]

j16a 631 −5.24/−3.47 4.63 × 10−6 (S) P3HT:j16a = 2:1 4.18 1.06 0.46 2.05 [149]

j16b 669 −5.29/−3.53 2.05 × 10−5 (S) P3HT:j16b = 2:1 5.88 0.90 0.51 2.69 [149]

j17a 611 −5.27/−3.41 5.64 × 10−7 (S) P3HT:j17a = 2:1 2.93 1.06 0.42 1.33 [150]

j17b 651 −5.30/−3.43 1.30 × 10−6 (S) P3HT:j17b = 2:1 3.73 0.87 0.56 1.84 [150]

j18a 622 −5.26/−3.60 – P3HT:j18a = 1:1 6.96 1.10 0.48 3.63 [151]

j18b 629 −5.24/−3.55 – P3HT:j18b = 1:1 5.21 0.88 0.41 1.87 [151]

j18c 631 −5.23/−3.57 – P3HT:j18c = 1:1 3.88 0.91 0.40 1.42 [151]

j18d 650 −5.26/−3.51 1.29 × 10−4 (S) P3HT:j18d = 2:1 8.29 1.14 0.55 5.16 [152]

j19a 616 −5.30/−3.58 – P3HT:j19a = 1:1 2.37 0.75 0.43 0.76 [152]

j19b 620 −5.29/−3.56 – P3HT:j19b = 1:1 1.21 0.79 0.29 0.28 [152]

j19c 618 −5.29/−3.57 – P3HT:j19c = 1:1 1.96 0.70 0.33 0.45 [152]

a)In film. b)Estimated from the cyclic voltammetry data. c)F and S: measured by FET or SCLC method; TOF: measured by using an oscilloscope by varying the bias voltage. d)In solution.


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higher conjugation length caused by vinyl linkage. Moreover, the extended conjugated block brought out the reduction of oxi-dation onset, affording j1b the highest HOMO value (−5.18 eV) with the lowest LUMO value (−3.55 eV) compared to other derivatives. Accordingly, j1b gave the lowest Voc of 0.64 V and the PCE was around 0.58%. j1d displayed the more stabilized LUMO energy level, induced by the two strong electron-with-drawing trifluoromethyl groups.[135] In 2015, phenyl rings sub-stituted with electron-withdrawing cyanide groups in different positions were chosen as end groups to attach onto the flank of the center DPP core by Lim et al., affording three compounds j1e with p-benzonitrile as terminals, j1f with m-benzonitrile as terminals and j1g with o-benzonitrile as terminals. The physical properties and OPV characteristics varied with the substitution pattern (p, m, and o-position) of the cyanide group. The j1e-based film exhibited more red-shifted absorption, probably due to the high degree of molecular aggregation. The maximum absorption coefficient of j1e was almost two times higher than that of j1f and j1g. The improved absorption may facili-tate photocurrent generation in BHJ cells. In addition, j1g had the highest LUMO energy level than that of the others, which resulted in the highest Voc. As a result, P3HT: j1e-based device showed a better Jsc of 1.64 mA cm−2 than that of P3HT:j1g-based device (1.19 mA cm−2), while the device for P3HT:j1g had a higher Voc of 1.09 V than that of the device for P3HT:j1e device (0.56 V).[136] In addition, Janssen and co-workers reported four DPP-based small-molecule acceptors (j2a–d) that were designed by combining the electron-withdrawing aldehyde groups at the either end of DPP core. The aldehydes-substituted j2b and j2d had a much lower reduction potential and higher oxidation potential than their parent compounds (j2a and j2c). When applied in BHJ solar cells with P3HT as the donor material, device efficiencies were in the range of 0.15–0.31%, limited by low FF and Jsc.[137] Poor morphology with relatively coarse domains with sizes of several hundred nanometers may explain the strongly field-dependent photogeneration efficiencies and the low FFs. They inferred that further optimization of DPP-type acceptor materials should primarily focus on improving the film morphology when blended with P3HT. In 2015, one acetylene-bridged molecule j2e, built by grafting phthalim-ides on DPP blocks, have been synthesized by Blanchard and Cabanetos et al. Such acetylenic linkages indeed permitted the extension of the effective conjugation while lowering the energy level of the frontier orbitals and reducing the steric interactions between thiophene units and phenyl rings. Once blended with P3HT, the PCE of ca. 3.3% were achieved, which is the highest PCE for the single-DPP-type acceptor.[138] At the same year, Welch et al. designed a phthalimide-based organic π-conjugated small-molecule acceptor (j2f) with DPP as a central building bock and phthalimide as arms. When blended with electron donor DTS(FBTTh2)2, a PCE of 0.46% could be attained with a high Voc of 0.92 V while using P3HT as electron donor, a PCE of 0.33% with a low Voc of 0.81 V was achieved.[139] In 2016, two small molecules composed of 1,1,4,4-tetracyanobuta-1,3-diene-substituted DPPs denoted as unsymmetrical j3a and sym-metrical j3b were synthesized by Sharma et al. Both of them exhibited low bandgap with excellent absorption ability from 500–800 and 500–900 nm for j3a and j3b, respectively, which were complementary to conjugated polymer P. In addition,

they also showed appropriate energy levels with polymer P. The OSCs based on P:j3a and P:j3b showed PCEs of 3.90% and 4.95%, respectively. The values of IPCE of the j3b-based devices was larger and broader than that of j3a, which resulted in the higher Jsc and PCE for j3b-based devices.[140]

In 2012, Zhan et al. developed a novel 3D star-shaped acceptor comprising TPA core and DPP arms (j4a). j4a exhib-ited excellent thermal stability, strong absorption, and matched energy levels with P3HT. Solution-processed BHJ OSCs based on P3HT:j4a showed the PCE as high as 1.20% after annealing at 150 °C for 10 min, with very high Voc up to 1.18 V.[141] Zhan and Yao et al. embedded a thiophene ring between the TPA core and DPP arms to yield j4b, and the best P3HT:j4b-based device exhibited a PCE of 0.63% with a high Voc of 1.14 V.[123]

Given that silole moiety exhibits low-lying LUMO and good electron-transporting properties, in 2013, Zhan and Li et al. utilized the dibenzosilole (DBS) to design a novel linear non-fullerene acceptor in which DBS as core and DPP as arms (j5). Contributed to its broad and strong absorption, appropriate energy levels and moderate electron mobility, the P3HT:j5-based solar cells exhibited the PCE as high as 2.05%.[142] Later, they chose IDT instead of DBS as core and achieved a new electron acceptor, j6. j6 showed complementary absorption from 500 to 700 nm with a high molar extinction coefficient and matched energy levels with respect to that of P3HT. Solar cells based on P3HT/j6 yielded a PCE of 0.83% with a high Voc of 1.17 V after thermal annealing.[98] Gupta and co-workers designed a novel nonfullerene electron acceptor with fluorene (FL) as a central core and DPP capped at both the ends (j7). The affording mol-ecule j7 provided highly conjugated donor–acceptor chromo-phores with good planarity and high thermal stability. The BHJ devices based on a P3HT:j7 blend (1:1) after annealing yielded a notable PCE of 1.20% with a high Voc of 1.10 V.[143] Later, they used carbazole as the central core instead of fluorene, achieving compound j8. j8 afforded a 2.30% PCE with a high Voc (1.17 V) when tested with the conventional donor polymer P3HT in solu-tion-processable bulk-heterojunction devices.[144] In 2014, they chose naphthalenediimide (NDI) as the central core with DPP terminal units (j9).[145] The NDI unit possesses a planar conju-gated structure and strong electron-withdrawing property and has been regarded as an active component for electron-acceptor semiconductors. Hence, j9 exerted strong and broad absorption tailing into the near infra-red region, appropriate energy levels matched with P3HT. P3HT:j9 (1:1)-based solar cells showed PCEs of 1.02% with a high Voc of 1.05 V.[145] At the same year, they designed a DPP-based electron acceptor (j10) where bulky peripheral DPP substituents were connected through a small sized benzo[c][1,2,5]thiadiazole (BT) core. The connection of DPP and BT functionalities can provide highly conjugated backbone with good planarity and appropriate energy levels matching with P3HT. P3HT:j10-based solar cells gave a PCE of 1.16%.[146] In 2015, Jo et al. synthesized a new electron acceptor, j11a, comprising benzo[c][1,2,5]thiadiazole core (BT), DPP arms and thiophene bridges. In order to investigate the effect of fluorine substitution on the photovoltaic perfor-mance, they replaced the central BT unit with fluoro-substituted benzo[c] [1,2,5]thiadiazole (j11b). Because j11a and j11b exhibited appropriate energy levels matching with PTB7, they used PTB7 as an electron donor polymer to investigate the according device


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performance. Accordingly, a PCE of 3.03% was observed for the PTB7/j11a-based devices. The PTB7/j11b-based device exhib-ited a higher PCE of 5.00% with a higher Jsc of 12 mA cm−2, than that of the PTB7/j11a-based device. The improved performance for PTB7/j11b-based device was attributed to the red-shifted absorption, increased electron mobilities, suppressed bimo-lecular recombination, fibril-like nanostructure. It’s worth men-tioning that j11b is highly promising as an electron acceptor in non-fullerene OSCs.[13] Shi and Yu et al. investigated the rela-tionship between geometrical structures and photovoltaic prop-erties by designing molecules j12 and j13. j12 was composed of the central phenyl moiety flanked with two DPP units while j13 used a dimethylphenyl moiety as core. Two methyl groups attached to the benzene ring could increase the torsional angles between the central moiety and the DPP units, resulted in more blue-shifted absorption bands and better miscibility with P3HT. As a result, j13-based OSCs showed a better PCE of 0.65% than that from j12-based OSCs (0.48%).[147] In 2015, Shi and Chen et al. selected two kinds of aromatic six-membered rings (fluorene and benzene) to connect two DPP moieties, affording j14a. The moderate dihedral angles between DPP and aromatic six-membered rings afforded twisted molecular conformation, which provided fine phase separation domains in the blend films. Thus, the resulting OSCs with P3HT:j14a exhibited a high PCE of 3.17% with a Jsc of 5.35 mA cm−2, an extremely large Voc of 1.18 V.[113] Given the deep LUMO level of the electron acceptor can help split excitons and generate cur-rent, Chen et al. chose thiophene-2-carbonitrile to replace the benzene in j14a, affording j14b, which contributed to decrease the LUMO level of the electron acceptor materials. Owing to the electronegativity of the thiophene-2-carbonitrile group, j14b had a LUMO of −3.65 eV and a narrow bandgap of 1.66 eV. Organic solar cells (OSCs) with j14b as the acceptor and P3HT as the donor showed a PCE of 2.37% with a Voc of 0.97 V, a Jsc of 6.25 mA cm−2, and a FF of 0.39. [148]

[2,2]paracyclophane framework was introduced to construct nonfullerene electron acceptors by Zhang et al., in which four DPP moieties were linked at different positions (para- or meta- position) of [2,2]paracyclophane (j16[149] and j17[150]). Meanwhile, they changed the end-capped groups on the DPP moiety with trifluorophenyl (i.e., j16a and j16b, j17a and j17b), to finely tune frontier energy levels. j15 was synthesized as a reference, which possessed a close HOMO/LUMO energies with respect to those of j16b. Using P3HT as the donor, j15 led to poor photovoltaic performance with a PCE of 0.90%, whereas a high PCE of 2.69% was achieved for the blending thin film of j16b with P3HT.[149] A PCE of 1.84% was obtained for j17b-based device after thermal annealing.[150] The transan-nular structure through space π–π interaction of [2,2] paracy-clophane endowed j16 and j17 a cylindrical and rigid structure, which was beneficial for their dense packing and thus forma-tion of molecular domains. Recently, Wan et al. reported a series of DPP-based electron acceptors in which four DPP units attached to a spirobifluorene (SF) center (j18).[151] A designed cross-shaped molecular geometry helped in suppressing strong intermolecular aggregation in the P3HT: j18 blend, leading to efficient non-fullerene PSCs. Branched 2-ethylhexyl, linear n-octyl, and n-dodecyl alkyl side chains were chosen as substitu-ents to functionalize the N,N-positions of the DPP moiety to

tune molecular interactions. The choice of different side chains resulted in quite different physical properties and drastically different film morphology. Among them, the branched 2-eth-ylhexyl group provided j18a less crystalline and interacted less with P3HT than that of the linear alkyl chains, resulted in mod-erate crystallization, nano scaled phase separation (20–30 nm). Thus, the P3HT:j18a blends displayed the highest PCE of up to 3.63%, whereas P3HT:j18b and P3HT:j18c blend films led to modest PCEs of 1.87% and 1.42%, respectively.[151] Recently, they further installed j18a by introducing four phenyl rings as the end-capped units, achieving molecule j18d. After proper optimization of the device fabrication, a maximum PCE of 5.16% with an extremely high Voc of 1.14 V was achieved. More importantly, the P3HT: j18d devices exhibited much better thermal stability than the P3HT: PC61BM ones.[152] They also used an acetylene bridge to couple DPP units with SF, yielding j19a–c. The introduction of C–C triple bonds resulted in slightly blue-shifted absorption and increased bandgaps, leading to far inferior device performance in the range of 0.28–0.76% for j19a–c-based solar cells.[152]

DPP units possess excellent optoelectronic characteristics such as strong light absorption, highly conjugated and coplanar structure, and good photochemical stability. The natrue of the strong electron-withdrawing ability, high electron mobility and low-lying LUMO energy levels endowed DPP dyes a chance to be electron acceptors in BHJ OSCs. The convenient synthesis of DPP units endows them with adjustable energy levels and an extended light absorption range. Compared to other non-fullerene electron acceptors, such as PDI-based derivatives, ITIC, ITIC-Th et al., with PCEs > 8%, most DPP-based non-fullerene organic solar cells exhibited unsatisfactory Jsc values of < 10 mA cm−2 and FF values <50%, which were far below that of ITIC-based non-fullerene organic solar cells, probably because of its low charge transporting property and poor blend film mor-phology. Most of the DPP-based electron acceptors with slightly high LUMO levels could only match with the wide-bandgap electron donors, such as P3HT. Hence, in order to improve the Jsc and FF, it is necessary to further decrease the LUMO levels of the DPP-based electron acceptors to form a complemen-tary energy level alignment with the narrow bandgap electron donors. In addition, to regulate the compatibility between DPP-based electron acceptors and the electron donors by molecular design is an effective method to realize fine film morphology.

5. DPP-Modified Fullerene Derivatives

In view of the strong absorption, excellent solubility and good electron transportation properties for DPP moiety, there are several reports in which DPP moieties were connected to fuller-enes to improve the solubility and extend the absorption spectra of fullerene derivatives. This kind of fullerene–DPP dyes could be used as electron acceptor or as single component to fabricate organic solar cells. The related molecular structures are shown in Scheme 13 and the corresponding optoelectronic properties, mobilities, and OPV performance are listed in Table 10.

In 2010, Janssen et al. presented two diketopyrrolopyrrole-fullerene-based small-bandgap oligomers (k1 and k2) with DPP as core and C60 as the end-capped units. They found ultrafast


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energy transfer occurred from DPP to C60 (≈0.5 ps), followed by the charge separation and formation of the triplets once photoexcitation of the oligomer.[153] In 2011, Morin and co-workers found that the introduction of an ethynyl-bridged DPP on the C60 cage (k3) could increase the LUMO energy level, which

was beneficial for BHJ solar cells to improve Voc.[154] Ma and Tamayo et al. reported a series of oligothiophene–DPP–fullerene triads (k4a–c) by increasing the number of the carbon atoms on the alkyl linker between the DPP and PCBM. All of them showed high solubility, strong and broad absorption spectra

Scheme 13. Chemical structures of DPP–fullerene small molecules.

Table 10. Optical and electronic properties, mobilities, and OPV performance of DPP-modified fullerene.

λmaxa)

[nm]HOMO/LUMO b)

[eV]μh

c) [cm2 V−1 s−1]


Voc [V]

FF PCE [%]

Ref.

k1 555 −5.18/−3.09 – – – – – – [153]

k2 617 −5.01/−3.20 – – – – – – [153]

k3 571 -6.59/−4.08 – – – – – – [154]

k4a 630 −5.07/−3.68 4.5 × 10−4 (F) k4a 1.30 0.71 0.25 0.23 [155]

k4a 630 −5.07/−3.68 3.7 × 10−7 (S) P3HT:k4a = 1.5:1 3.38 0.55 0.31 0.58 [155]

k4b 630 −5.05/−3.67 6.1 × 10−4 (F) k4b 1.43 0.73 0.26 0.27 [155]

k4b 630 −5.05/−3.67 1.4 × 10−6 (S) P3HT:k4b = 1.5:1 6.47 0.64 0.44 1.81 [155]

k4c 630 −5.07/−3.68 1.5 × 10−3 (F) k4c 2.67 0.66 0.28 0.49 [155]

k4c 630 −5.07/−3.68 4.6 × 10−6 (S) P3HT:k4c = 1.5:1 7.60 0.65 0.49 2.41 [155]

k5a 650 −5.24/−3.69 – k5a 0.69 0.60 0.25 0.10 [156]

k5b 650 −5.13/−3.67 – k5b 3.17 0.56 0.33 0.56 [156]

k5c 650 −5.05/−3.69 – k5c 4.79 0.51 0.46 1.11 [156]



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coverage (250–800 nm), as well as suitable molecular orbital energy levels. The PCE of single-component OSCs approached 0.5%, while a higher PCE of 2.41% was achieved for OSCs con-taining P3HT as the electron donor and k4c as the electron acceptor.[155] On the other hand, Hashimoto and Tajima et al. also synthesized a series of oligothiophene–diketopyrrolopyr-role–fullerene triads (k5a–c) by changing the number of thio-phene rings in the donor part to tune the structure of the films. As the increase of the number of thiophene units from 4T to 8T, the absorption spectra were broaden and absorption edges red-shifted to the long wavelength. Likewise, the energy levels of the HOMO in the k5a–c became higher-lying as the number of thiophenes increased. After device optimization, the single-component device of k5c showed the highest PCE of 1.11%.[156] They thought the realization of an interpenetrating bicontin-uous nanoscale networks, especially the continuous phase for hole transport, should be the largest challenge for this kinds of single-component solar cells.

6. Conclusions and Outlook

Since the first DPP-based small molecule for solution-pro-cessed OSCs was reported in 2008, the PCEs of OSCs based on DPP-containing small molecule as electron donors or electron acceptor have been improved to 8% and 5%, respectively. In this article, we have discussed nearly 200 small molecules incorpo-rating various kinds of DPP moieties and reviewed all the avail-able information revealing the effect of structure modification on their optoelectronic properties and device performance.

Modulating the nature of donor–acceptor building blocks, for the D-A synthetic strategy, is an effective approach to obtain high-efficiency DPP-based small molecules, which principally could not only control absorption properties, bandgap, energy levels, but also determine the molecular packing and film morphology. The commonly used building blocks included non-fused five-membered aromatic rings, six-membered aro-matic rings, and fused heterocyclic rings etc. Three kinds of fundamental backbones were achieved: single-DPP-type with two π-conjugated units flanked on a central DPP chromophore; double-DPP- or multi-DPP-type with two or multi DPP chromo-phores flanked on a central π-conjugated units. After intensively device optimization, the PCE for single-DPP-type backbones has been improved up to 7%, and the highest PCE of double-DPP-type backbones has been reached up to 8%, as well as a PCE of 4.6% was achieved for multi-DPP-type backbone.

End-capped groups have been attached to the flank of the above-mentioned backbones to further extend the conjugated backbone and modify the HOMO/LUMO energy levels as well as absorption properties. Moreover, design of end-capped groups plays a role in adjusting the intermolecular interactions between the donor and acceptor, which may induce the charge separation at the D/A interface, in turn act on the photocurrent generation. The strategy of utilizing functional end-capped groups has not yet been extensively studied and further designing the end-capped groups might promote the increase of PCE for DPP-based small-molecule OSCs.

Side-chain engineering by altering the shape, length, sub-stitution position, and so on, can afford the DPP-based small

molecules with good solubility for purification and device fab-rication. In fact, the side chains also play an important role in controlling the molecular coplanarity, crystalline organization, and thin-film morphology. The existing DPP-based small mole-cules contains symmetrical soluble alkyl chains (either linear or branched) on the 3,4-position (i.e., the lactam N-atoms) for get-ting solution-processed materials. Attaching some functional-ized aryl side chains on the 3,4-position may help achieve extra intermolecular interactions.

Though the highest PCE of the DPP-based small mole-cules have reached up to 8%, the types of the high-efficiency DPP-containing materials still are limited. Through the anal-ysis of the device parameters for all the DPP-based organic solar cells, it can be found that most of the present examples showed low Jsc around 9-11 mA cm−2 and FF ≤60%, leading to modest PCEs of 6%. So, to achieve the high PCE above 9%, the current task is to increase the Jsc and FF while maintaining a large Voc in DPP-based solar cells. From the discussion in this review, a synergy of the molecular engineering strate-gies toward the π-conjugated backbone, end-capped groups and side chains as well as modified DPP derivatives can constitute an effective strategy for acquiring excellent device performance of DPP-based small molecules. The affording DPP backbones with multi extended π-conjugated functional units could combine the most desirable properties from both small molecules and polymers, achieving high Jsc, FF as well as PCE. In addition, the device fabrication technology, such as ternary materials system, tandem device structure, inter-facial engineering and morphology optimization etc. could help to improve the photovoltaic performance further for the practical application. There is reason to believe that DPP as an excellent pigment molecule has a bright future in the field of organic solar cells by integrated molecular, interficial and device engineering.

AcknowledgementsThis work was supported by the National Natural Science Foundation of China (Nos. 51473040, 51673048, 21504019, 21602040), the National Natural Science Foundation of Beijing (No. 2162045), and the Chinese Academy of Sciences (QYZDB-SSW-SLH033).

Received: January 3, 2016Revised: May 11, 2016

Published online:

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