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1701404 (1 of 16) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Low Work Function Surface Modifiers for Solution-Processed Electronics: A Review

Xiang Peng, Lin Hu, Fei Qin, Yinhua Zhou,* and Paul K. Chu*

DOI: 10.1002/admi.201701404

electron injection or extraction, whereas a high work function metal such as Au is employed for hole injection or extraction. Since the low work function metallic elec-trodes are normally chemically reactive, they are readily oxidized in air (oxygen) and water when serving as cathodes. Air-stable electrodes with a low work function are more challenging compared to ones with a high work function.

Figure 1 shows the physics of band bending when a metal electrode is in contact with an n-type semiconductor to illustrate the importance of a low work function for the electrode. When the work function (WF = Evac − EF) of the metal electrode is larger than that of the n-type semiconductor, electron flows from the semiconductor to the metal electrode. Unfavorable band bending for electron

collection is formed at the interface in this case. By contrast, if the work function of the metal electrode is lower than that of the n-type semiconductor (Figure 1b), the opposite band bending is formed to favor electron collection from the semiconductor to the electrode. Therefore, an electrode with a sufficiently low work function is crucial to efficient electron injection or collec-tion. Surface modification has been shown to be effective in reducing the work function of the electrodes to facilitate elec-tron injection and collection,[7] and enhanced electron collec-tion or injection improves the performance of organic electron devices. The proper surface modification strategy can produce air-stable electron collection or injection electrodes combined with high work function electrode/surface modifiers. This approach also enables diverse design of novel-device architec-tures for flexible electronics and printed electronics.

Several types of low work function surface modifiers have been developed for the cathode interface. Inorganic metal oxides and alkali metal salts are effective low work function modifiers. N-type metal oxides such as titanium oxide (TiOx)[8] and zinc oxide (ZnO)[9] are often used as cathode interlayers due to the excellent charge mobility and electron selectivity. The metal oxide film with a typical thickness of more than 10 nm possesses a low work function when coating on a high work function electrode and high-temperature annealing is normally required. Alkali metal salts like LiF,[10] Cs2CO3,[11] and Li2CO3

[12] can also be incorporated into the cathode interlayer to reduce the electrode work function. Considering the intrinsic insu-lating properties, the thickness must be carefully controlled to ensure efficient electron extraction and injection. Another example is organic molecules grafted with ionic or polar

Interfaces with a low work function are crucial to electron injection and col-lection in semiconducting devices such as diodes and transistors, and many types of surface modifiers have been developed to tailor the work functions of electronic devices. In this review, the basic and working mechanisms of low work function surface modifiers covering the surface dipole, low intrinsic bulk work function, doping, and self-doping are described. Efficient solution-processed surface modifiers developed lately to modify target surfaces are described and recent development of solution-processed semiconducting devices including organic (quantum dots and perovskite) solar cells, light-emitting diodes, field-effect transistors, and organic photodetectors is summarized. Finally, the prospects and strategies to produce efficient and reliable low work function modifiers for solution-processed semiconducting devices compatible with low-cost fabrication techniques such as printing are presented.

Dr. X. Peng, Dr. Y. H. Zhou, Prof. P. K. ChuDepartment of Physics and Department of Materials Science and EngineeringCity University of Hong KongTat Chee Avenue, Kowloon, Hong Kong, ChinaE-mail: [email protected]; [email protected]. Hu, F. Qin, Dr. Y. H. ZhouWuhan National Laboratory for OptoelectronicsSchool of Optical and Electronic InformationHuazhong University of Science and TechnologyWuhan 430074, China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201701404.

Hall of Fame Article

1. Introduction

Solution-processed and printable electronics have been attracting enormous attention due to low-cost, light-weight, flexible, and wearable applications,[1] and there has been much technological advance in pertinent devices including organic [quantum dots (QDs) and perovskite] solar cells (OSCs),[1,2] light-emitting diodes (LEDs),[3] field-effect transistors (FETs),[4] and organic photodetectors (OPDs).[5] In these semiconductor-based optoelectronic devices, electrons and holes are extracted from or injected into the organic semiconductors. The energy levels between the organic semiconductor and corresponding electrode must be properly matched in order to yield effi-cient charge injection or extraction.[6] Generally, a low work function (WF) metal, such as Ca, Ba, Mg, etc., is used for

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pendant groups such as fullerene-end-capped poly(ethylene glycol) (PEG-C60),[13] poly[(9,9-bis(3′-(N,Ndimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene) (PFN),[14] polyethylenimine (PEI), and ethoxylated polyethyleneimine (PEIE).[7a] These organic surface modifiers are directly coated on the conductive electrode forming an ultrathin layer (<10 nm) that chemically or physically adsorbs onto the conductor sur-face. The pendant ionic or polar groups not only render the molecules with orthogonal solubility to the organic semicon-ductor, but also create strong interfacial dipoles on the electrode surface to decrease the work function. In addition, organic sur-face modifiers are solution-processed at room temperature and the process is compatible with high-throughput manufacturing techniques such as printing.

In this review, we first discuss the basic and working mecha-nisms of low work function organic surface modifiers covering topics such as the surface dipole, doping, and self-doping. Solution-processed surface modifiers including conjugated and nonconjugated polymers as well as small molecules will be summarized. Preparation and application of solution-processed low work function surfaces to semiconducting devices such as organic (quantum dots and perovskite) solar cells, light-emitting diodes, field-effect transistors, and organic photodetectors are discussed. Finally, the future and outlook of low work function modifiers are discussed from the perspective of performance, processing, and reliability of solution-processed semicon-ducting devices and compatibility with large-scale fabrication.

2. Mechanisms

2.1. Interface Dipoles and Molecular Dipoles

Organic low work function surface modifiers tend to contain ionic or polar pendant groups such as ethylene glycol, amino, quaternary ammonium salt, and phosphonate.[15] These polar functional groups chemically or physically adsorb onto the elec-trode surface to create strong interface and/or molecular dipoles/electrical field to lower the work function of the electrode. Figure 1c shows the diagram of the surface dipole that produces an electric field and reduces the work function of the electrode. The vacuum energy level shifts downward. The dipoles can be interface dipoles (μID) formed at the interface between the modifier molecular and electrode surface and/or (ii) molecular dipoles (μMD) caused by the dipole in the polar modifier.

Surface modifiers based on polymers containing simple ali-phatic amine groups (PEI and PEIE) to reduce the work func-tion of conductors including metals, metal oxides, conducting polymers, and graphene have been prepared.[7a] The polymer surface modifiers are processed in air from a diluted solution in environmentally friendly solvents such as water or alcohol. The large concentration of amine groups (primary, secondary, and tertiary) in the polymer structure yields a high pH in aqueous and methoxyethanol solutions. After deposition of the ultrathin layer PEIE on the conductors, the work functions are reduced significantly from 4.95 to 3.32 eV for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), 4.40 to 3.30 eV for indium tin oxide (ITO), and 4.70 to 3.40 eV for Au.[7a] The neutral amine groups primarily physisorb onto the

electrode surface and reduction of the work function is a syn-ergistic effect stemming from the ethylamine molecular dipole (μMD) along the direction perpendicular to the surface and the dipole (μID) formed at the interface between the modifier mole-cules and electrode surface.[7a] The contribution of μID is attrib-uted to partial electron transfer from the amine-containing molecules to the electrode surface. It has been demonstrated that the amine and hydroxyl groups can spontaneously adsorb

Xiang Peng received his PhD in materials science from City University of Hong Kong and is a postdoc fellow in Prof. Paul Chu’s research group in City University of Hong Kong. His research interests focus on synthesis of functional nanomaterials and fabrica-tion of electrochemical energy related devices for superca-pacitors, Li-ion batteries, and electrocatalysis applications.

Yinhua Zhou received his PhD (2008) in chemistry from Jilin University with Prof. Wenjing Tian. He spent one year in Prof. Olle Inganäs group in Linköping University as a visiting PhD student (2007–2008). Afterward, he worked as a postdoctoral fellow at Georgia Institute of Technology (2009–2013) with Prof. Bernard Kippelen. In

October of 2013, he joined Huazhong University of Science and Technology as a professor. In 2017, he worked as a research fellow in Prof. Paul K. Chu’s group. His research interest includes conducting polymers, organic photovol-taics, and printed electronics.

Paul K. Chu received his PhD in chemistry from Cornell University. He is Chair Professor of Materials Engineering in the Department of Physics and Department of Materials Science and Engineering at City University of Hong Kong. He is also Fellow of the Hong Kong Academy of Engineering Sciences (HKAES) and serves

on the membership committee. His research interests are quite diverse encompassing plasma surface engineering, materials science and engineering, and functional materials.





onto the ZnO surface resulting in dipolar polarization.[12] PEI and polyallylamine with protonated amines can also reduce the work function of ITO and enhance the electrical contact in the inverted OSCs device as reported by Kang et al.[16] The dipoles were believed to stem from the positively charged amines (pro-tonated amines) of the cationic nonconjugated polyelectrolytes interacting with the negatively charged terminal oxygen ions of the ITO surface. Although it is still debatable whether efficient dipoles are produced by pronated or nonprotonated nitrogen, the strategies of intermolecule and intramolecule dipoles are effective in reducing the surface work function of electrodes.

For the PEI (or PEIE) modified conducting polymer elec-trode PEDOT:PSS, the work function reduction by PEI (about 1.4–1.6 eV) is larger than that on the ITO or metal (about 1.1–1.3 eV).[7a] Li et al.[17] have suggested that this is because of chemical reduction of the surface layer of PEDOT:PSS by PEI. The PEDOT component in the water-dispersed PEDOT:PSS is p-type and synthesized by oxidative polymerization. Amines (PEI, methyamine, etc.) have strong electron-donating proper-ties and are able to reduce the PEDOT:PSS.[18] Chemical reduc-tion by amines shifts up the Fermi level of the surface of the PEDOT:PSS. Therefore, a larger work function is observed from the PEDOT:PSS electrode compared to metals and oxides when modified by PEIE or PEI.

2.2. Doping and Self-Doping

Doping and self-doping, which are efficient strategies to increase the electrical conductivity and charge carrier density,

shift the Fermi level, fill the traps in organic semiconduc-tors, and are employed to fabricate efficient optoelectronic devices.[19] Figure 1d shows the diagram of the doping shifting up the Fermi level of the n-type semiconductor after electron transfer from the n-dopants to the semiconductor. When the doped layer is inserted between the electrode and active semi-conducting layer, the contact between the doped layer and elec-trode is smooth, since both have large carrier concentrations. In addition, the doped layer elevates the Femi level of the semicon-ductor leading to band bending (Figure 1b) that favors electron collection. The thickness of the doped layer can have a broad range (up to 100 nm) if it is sufficiently conductive for efficient devices. This is an important advantage for printed electronics.

Li et al. have investigated doping of fullerene via anion-induced electron transfer and the implication to FET devices.[20] The phenyl-c61-butyric acid methyl ester (PCBM) and common tetrabutylammonium salts (TBAX, X = F, OH, AcO, Br, I) are processed into a blend solution in common organic solvents. Electron transfer between the anions of TBAXs and n-type sem-iconductor induces effective doping without the need for harsh activation conditions. Moreover, the doping concentration can be modulated by adjusting the solvent polarity and state of blending in the solution or solid. These observations provide evidence of the mechanism of possible interfacial doping of PCBM through the aforementioned low work function surface modifiers in the devices. Chen et al. have developed a series of stable and highly conductive self-n-doped fullerene derivatives linked with a different number of ammonium groups and var-ious anions (I− and Br−).[21] The electronic and spatial structures of fullerene ammonium iodide are systematically studied to


Figure 1. Schematic energy-level diagram of a metal/semiconductor (n-type) before and after contact. After contact, band bending occurs at the interface. The bending direction depends on the Fermi level difference of the metal electrode and the semicnoductor. a) When the work function, WF = Evac − EF, of the metal electrode (EF,m) is larger than that of the semiconductor (EF,s), bending is not favorable for electron collection. b) When the work function of the metal electrode is smaller than that of the semiconductor, bending is favorable for electron collection. c) Diagram of surface dipole reducing the WF (shifting the Evac downward). d) Diagram of doping the interfacial layer shifting the Fermi level up and reducing the work function.




elucidate the cross self-n-doping. Electron transfer from iodide to the core fullerene is observed to produce n-doping and the strong anion–π interactions between iodide and fullerene cores stabilize the n-doped fullerene. As referred to as the PEI modifier, it has been reported that PEI impurities can induce n-doping reactions in organic semiconductors.[18a] The vapor produced from the PEI sample containing N-based impu-rity molecules (ethyleneimine dimer or trimer) with a strong reducing characteristic accounts for the primary mechanism responsible for efficient electron injection. The volatile molec-ular reducing agent from the PEI sample migrates throughout the bulk of the organic materials, and hence, both the ener-getics at the interface and electrical conductivity of the organic semiconductor bulk are changed. Owing to the large density of electron-donating amine groups, PEI has also been successfully used to n-dope carbon nanotubes and graphene.[22]

3. Surface Modifiers

A low work function surface is a critical part of an optoelec-tronic device. The low work function can be achieved with active metals such as Ba, Ca, etc., and n-type metal oxides such as ZnO, TiOx, and so on. In addition, based on the dipole and doping (including self-doping) mechanisms, many organic small mole-cules and polymers have been developed to produce low work function interfaces. Here, we summarize recent advances per-taining to the design of solution-processed low work function surface modifiers for electronic devices. The chemical structure of some surface modifiers is listed in Figure 2.

3.1. Nonconjugated Polymer Surface Modifiers

Polyethylene oxide (PEO) with the structure of ITO/PEO/active layer/PEDOT has been prepared in vacuum-free inverted semi-transparent OSCs.[23] Ultraviolet photoemission spectroscopy confirms that the PEO converts the ITO to the electron collec-tion electrode by reducing the work function of the ITO by up to 0.5 eV.[23] This is the first demonstration of inverted OSCs with ITO modified by a polymer as the cathode, although the efficiency of the OSCs is only about 0.7% due to inefficient active materials, low conductivity of the PEDOT:PSS electrode, and semi-trans-parency with limited absorption. Inspired by this piece of work, other polymers have been demonstrated as modifiers to reduce the work function of ITO or metal electrodes for efficient elec-tron collection. For example, aliphatic amine-containing polymer modifiers, PEI and PEIE, have been shown to universally reduce the work function of most conductors including metals, metal oxides, conducting polymers, graphene, and ITO by about 1 eV.[7a] In-depth studies indicate that the polymer modifiers physically adsorb onto the conductors due to partial charge transfer and intramolecule dipole independent of the surface chemistry. Low work function modification can be performed by spin coating, printing, or immersing the conductors in the solution. The low work function interfaces exhibit good air stability. PEI or PEIE is commonly used in organic solar cells, organic LEDs, organic field-effect transistors, organic photodetectors, perovskite solar cells (PerSCs), perovskite LEDs, and inorganic quantum dot LEDs.[16,24]

Park et al.[25] have incorporated halogen ions into the PEIE that yields the nonconjugated polyelectrolyte of PEIEH+X (X = Cl−, Br−, and I−) and PEIEH+X can dope PCBM films. With Au as the electrode, electron injection is improved after PEIEH+X is inserted between the PCBM and Au elec-trode. The electron mobility and Ion/Ioff of the PCBM-based transistors increase to greater than 10−2 cm2 V−1 s−1 and 105, respectively. Poly(ionic liquid) (block polymer: polystyrene and 1-(4-vinylbenzyl)-3-methyl-imidazolium chloride polymerized by reversible addition–fragmentation chain transfer) has also been applied to reduce the work function for electron collec-tion in organic solar cell devices.[26]

3.2. Conjugated Polymer Surface Modifiers

Water and alcohol soluble conjugated polyelectrolytes com-posed of the π-conjugated backbone and surfactant-like side groups such as amine, ammonium, phosphate, sulfonic, and zwitterionic groups constitute another class of surface modifiers. These materials facilitate charge extraction and injection in polymer light-emitting diodes. Alcohol-soluble conjugated polyelectrolyte PFN has been used to modify the ITO cathode in an inverted OSCs[14] and a certified efficiency of 9.2% has been attained from the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-eth-ylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):PC71BM active layer in an inverted device. The ultrathin PFN (10 nm) on the ITO surface reduces the work function of ITO from 4.7 to 4.1 eV and improves the ohmic contact with the active layer due to the interface dipole at the ITO surface. Transport and collec-tion of photogenerated charge carriers are effectively improved with the dipoles. Though the backbone of the PFN is a conju-gated structure, the thickness of the PFN layers is restricted to less than 10 nm. To reduce the sensitivity of the modifica-tion layers, several aminofunctionalized conjugated polymers have been synthesized and adopted as the cathode interlayer in inverted OSCs.[27] For example,[27a] a Hg-containing deriva-tive of amino-functionalized conjugated polymers (PFEN-Hg) exhibits orthogonal solvent processability and good film for-mation. It can also reduce the work function of the ITO because the aminofunctionalized side chains facilitate for-mation of the desired dipole with the ITO substrate and Hg enhances the noncovalent Hg–Hg interactions and increases the packing of the PFEN-Hg thin film to produce good elec-tron transport and collection. Good device performance has been observed from the PFEN-Hg with a wide range of film thicknesses. Other novel conductive interface materials that can be easily processed into a variety of thicknesses without significantly affecting the device performance have been devel-oped, for example, n-type conjugated polyelectrolyte poly-2,5-bis(2-octyldodecyl)-3,6-bis(thiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-2,5-bis[6-(N,N,N-trimethylammonium)hexyl]-3,6-bis(thiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione (PDPPNBr) as cathode modifier in OSCs.[28] Because of the electron-deficient nature of the diketopyrrolopyrrole (DPP) backbone and planar structure, PDPPNBr possesses high con-ductivity and electron mobility. PDPPNBr with a thicknesss up to 30 nm still delivers high-performance in solar cells. Two





naphthalene diimide-based water/alcohol-soluble conjugated polymers, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-5,5′-bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N′-di(2-ethylhexyl)imide] (PNDIT-F3N) and poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-5,5′-bis(2,2′-thiophene)-2,6-naphthalene-1 , 4 , 5 , 8 - t e t r a c a b o x y l i c - N , N ′ - d i ( 2 - e t h y l h e x y l ) i m i d e ]dibromide (PNDIT-F3N-Br), have also been developed.[29] The π-delocalized planar naphthalene diimide backbone ensures good electron mobility and the amino or ammonium

functionalized side chains endow them with unique interface modification capability for electron collection. Electron spin resonance reveals the doping effects on PC71BM and self-doping effects on PNDIT-F3N and PNDIT-F3N-Br. The syner-getic effects improve the device performance significantly to 9.7% and 10.11% for the PTB7-Th/PC71BM and PffBT4T-2OD/PC71BM systems, respectively.[29] With PffBT4T-2OD/PC71BM as the active layer, a prominent power conversion efficiency of over 8% has been achieved when the PNDIT-F3N reaches 100 nm.


Figure 2. Chemical structures of some low-work function surface modifiers.




3.3. Small Molecule Surface Modifiers

Small molecule surface modifiers are attractive because of advan-tages such as easy purification, monodispersity, well defined structures, and better batch-to-batch reproducibility. Self-assembly small-molecule monolayers are widely used to tune the work func-tion based on the chemical or physical interactions between the small molecules and target surface.[30] The work function increase or decrease depends on the direction of the dipole moment. The monolayer of aminopropyltrimethoxysilane (APTES) is able to reduce the work function of ZnO by about 1 eV.[30d] APTES chemisorbs onto the ZnO surface due to the SiO chemical bonding. Besides the self-assembled molecular dipole, doped or self-doped small molecules are used to construct low work function interfaces. Doping increases the charge carrier concen-tration and electrical conductivity and shifts up the Fermi level. For example, when 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a dopant is mixed with PCBM, it can increase the conductivity of PCBM by over three orders of magnitude and shift the work function from 4.9 to 4.4 eV.[19d] Electron-donating groups (such as amine, acridine) are attached to the electron-deficient molecules (such as fullerene, perylene diimide) to form self-doping with a large electron carrier concentration. For example, fullerene-end-capped poly(ethylene glycol) (PEG-C60) has been synthesized and added to the poly(3-hexylthiophene) P3HT/PC61BM blend in OSCs.[13] The self-segregated PEG-C60 molecules migrate to the surface of the P3HT/PC61BM active layer to form a nanoscale layer between the active layer and metal cathode. The increased open-circuit voltage (VOC) and short-circuit current density (JSC) after addition of PEG-C60 are attributed to the interfacial dipole moment. Moreover, PEG-C60 enables the active layer to form an ideal vertical morphology and the self-assembled PEG-C60 buffer layer enhances the device performance and air stability. Fullerene derivative modifiers have been developed by attaching polar groups such as phosphoric esters, amines, cationic ammonium, etc., to fullerene. Two solution-processed fulleropyrrolidines with amine (C60-N) and zwitterionic (C60-SB) substituents which reduce the work functions of Au, Cu, and Ag have been pre-pared[31] and high-power conversion efficiency exceeding 8.5% is achieved from organic photovoltaics independent of the cathode selection (Al, Ag, Cu, or Au). The devices prepared with the C60-N and C60-SB layers with thicknesses between 5 and 55 nm show high efficiency. PDIN and PDINO are two other efficient organic small molecule cathode interlayers based on perylene diimides (PDI) as the core and amino (PDIN) or amino N-oxide (PDINO) as the terminal substituent.[32] Benefitting from the extended planar structure of the PDI units, the two interlayers have high conductivity of ≈10−5 S cm–1 rendering them efficient for a wide thickness range between 6 and 25 nm. The work function tuning effect of the two PDI-based interlayers allows high work function metals (Au or Ag) for the cathode. The conventional device based on PTB7:PC71BM as the active layer and PDINO/Al as the top cathode exhibits an efficiency of 8.24%, which is higher than that of the corresponding Ca/Al-based device (6.98%).

A nonconjugated small molecule electrolyte 4,4′-(((methyl(4-sulphonatobutyl)ammonio)bis(propane-3,1-diyl))bis(dimethyl-ammoniumdiyl))bis-(butane-1-sulphonate) (MSAPBS) has also been designed and synthesized via a quantitative quaterniza-tion reaction.[33] The MSAPBS offers good ohmic contact for

photogenerated charge carrier collection and the active layer has optimal horizontal phase separation and vertical phase gradation. Single-junction devices comprising PTB7:PC71BM as the active layer boast an efficiency of over 10%. Ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) is also used to effectively modify ITO or ZnO to reduce the work function for devices.[34] Organic solar cells with the [BMIM]BF4 modified ITO deliver high performance and fill factors and the performance can be further enhanced when ITO/ZnO/[BMIM]BF4 is used for electron collection.

4. Applications in Optoelectronic Devices

4.1. Organic Solar Cells and Perovskite Solar Cells

Both OSCs and PerSCs are diodes. High and low work func-tions are both needed to create the potential and determine the device polarity. Surface modifiers as interfacial layers are typically introduced between the electrodes and active layer to produce sufficiently high and low work functions for effi-cient charge collection. The interfacial layer plays several roles: (1) tuning the work function of the electrode to improve the charge collection, (2) improving the selectivity toward holes or electrons while blocking the other and minimizing charge recombination at the interface, (3) enhancing light harvesting by introducing optical spacers, and (4) improving the device sta-bility. Here, we summarize some applications of low work func-tion interfaces in OSCs and PerSCs. Efforts have been made to improve the device performance by introducing surface modi-fiers to both single-junction and tandem solar cells.

4.1.1. Single-Junction Cell

Single organic and perovskite solar cells have a similar sand-wiched configuration consisting of the bottom transparent conducting oxide electrode, light absorption layer, charge trans-porting layers (if needed), and top metal electrode. The com-munity defines the device as the “conventional structure” and “inverted structure” based on the electrical polarity. In the organic solar cells community, the device is called “inverted structure” when the bottom electrode on the substrates is used for electron collection (top electrode is for hole collection) and “conventional structure” when the bottom electrode on the substrates is used for hole collection (bottom electrode is for electron collection). For perovskite solar cells, the definition of “inverted structure” and “conventional structure” is opposite to that of organic solar cells. That is, in the “inverted structure,” the bottom electrode on the substrate is for hole collection and the top electrode is for electron collection. In the “convention structure,” the bottom electrode is for electron collection and the top electrode is for hole collection.

Several materials have been adopted to produce low-work function electrode by forming a dipole layer at the interface. Among the various interfacial materials, PEIE and PEI are the most widely used to reduce the work function of ITO and PEDOT:PSS electrodes due to their effectiveness and com-mercial availability at a low cost.[35] The work function of ITO





decreases to 3.6 eV from 4.6 eV after deposition of a thin layer PEIE (about 10 nm) for efficient electron collection in inverted organic solar cells. PEIE is cost effective and can be easily pro-cessed under ambient conditions from solutions in environ-mentally friendly solvents such as water or alcohol. By spin coating PEIE onto the ITO as the cathode and MoO3 as the hole extraction buffer layer underneath the Ag top electrode, the solar cells with P3HT and indene-C60 bis-adduct (ICBA) as the active layer yield a power conversion efficiency (PCE) of 5.9%.[7a] In a follow-up study, PEIE is introduced on ZnO to enhance the effi-ciency of the cell by lowering the work function of ZnO and the inverted structure of small molecules OSCs with PCE of 7.88% is produced using 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5] thiadiazole) [p-DTS(FBTTh2)2] and PC71BM as the donor and the acceptor, respectively.[36] Moreover, both the surface roughness and conductive band energy of ZnO can be tuned by depositing a thin electron-rich PEI layer on the top and high PCE of 8.9% is obtained for the PTB7:PC71BM devices.[37]

Besides PEI and PEIE, a conjugated polymer, PFN, has been proven as efficient polymer interfacial material.[38] The OSCs with the PFN interlayer show significant and simultaneous enhancement leading to a PCE of 8.37% for the PTB7:PC71BM devices.[39] Solar cells with an inverted structure based on PTB7:PC71BM have an efficiency of 9.2%. A thin layer of PFN (10 nm) on the ITO surface reduces the work function of ITO from 4.7 to 4.1 eV and improves electron collection by enhancing the ohmic contact with the active layer.[14] Based on PFN, a series of polymers have been designed as surface modifiers.[40] An aminofunctionalized conjugated metallopolymer PFEN-Hg has been prepared as the cathode interlayer in inverted OSCs. The PBT7:PC71BM solar cells with a wide range of film thicknesses deliver improved performance of 9.11%. PFEN-Hg reduces the work function of ITO and the Hg–Hg interactions promote strong stacking of the polymer.[27a]

In addition to the interface materials discussed above, fullerene-based surface modifiers provide another choice because they match the lowest unoccupied molecular orbital (LUMO) energy level of common acceptor materials such as PCBM and ICBA. A series of fullerene-based self-assembled monolayers (C60-SAM) containing different anchoring groups such as catechol, carboxylic acid, and phosphonic acid have been fabricated. When they are used to modify ZnO, the weaker acids like carboxylic acid and catechol-based C60-SAMs can be assembled with the ZnO. The device efficiency is improved considerably because the interface dipole formed from the different anchoring groups enhances photoin-duced charge transfer and reduces loss from recombination.[41] A bis-adduct fullerene mate-rial (C60-bis) that functions as efficient elec-tron selective materials has been reported.[42] Fulleropyrrolidinium iodide is mixed with a small amount of PEIE (0.8% w/w ratio) to produce high conductivity, orthogonal solvent processability, and good tunability for the elec-trode work function. PCE of around 9.6% is

achieved from inverted devices with PBDTT-TT:PC71BM layers[43] and a summary of the device characteristics of typical OSCs with different low work function surface modifiers is shown in Table 1.

A typical PerSC possesses a sandwiched configuration sim-ilar to that of OSCs and so the knowledge gained from inter-facial engineering of OSCs can be extended to PerSCs. By employing PEIE to reduce the work function of ITO and doping titanium dioxide (TiO2) with yttrium (Y-TiO2) to enhance the carrier concentration, carrier injection into the carrier transport layers is facilitated and good carrier extraction is maintained at the electrodes yielding cell efficiency up to 19.3%.[24a] PCBM is a representative material for the electron transport layer in PerSCs because it has excellent electron accepting properties. However, the mismatch between LUMO of PCBM and work function of the metal often causes difficulty of charge extraction leading to the poor device performance. To match the energy level, PEIE, PFN-based materials, and fullerene-based interface modifiers are applied to the PCBM/metal electrode interface. Efficient inverted PerSCs have been fabricated by inserting an ultrathin layer of PEIE or poly[3-(6-trimethylammoniumhexyl)thiophene] between the PCBM and Ag electrode. The PCE with PEIE modifiers reaches 12% compared to 8.5% of the devices without the interfacial layer.[44] The C60-bis surfactant has been used to modify the energy level at the PCBM/electrode inter-face in PerSCs[45] and a layer of PFN has been prepared to form the ohmic contact to assist electron extraction from PCBM to Al giving rise to an enhanced fill factor (FF). The optimal device shows PCE of 17.1% and FF of 80%.[46] Because of the mismatch between the PCBM and electrode, an alternative electron transport layer (PFN-2TNDI) is introduced to replace the PCBM. The backbone of the conjugated polymer is com-posed of amine modified fluorene and conjugated naphtha-lene diimide. The polymer can passivate the surface traps of perovskite to improve electron extraction as well as reduce the work function of the metal electrode. As a result of the dual function, the PCE of devices based on PFN-2TNDI increases by about 30% compared to devices based on PCBM.[47]


Table 1. Summary of device characteristics of some OSCs with low-work function surface modifiers reported in the literature.

Device structure VOC [V] JSC [mA cm−2] FF [%] PCE [%] Ref.

ITO/PEIE/P3HT:ICBA/MoO3/Ag 0.81 11.0 66 5.9 [7a]

PES/PH1000/PEI/P3HT:ICBA/PH100 0.80 6.5 54 2.8 [35]

ITO/ZnO/PEIE/p-DTS(FBTTh2)2):PC71BM/

MoO3/Ag

0.77 15.2 67 7.9 [36]

ITO/PEDOT:PSS/PTB7:PC71BM/PFN/Al 0.76 15.7 70 8.4 [39]

ITO/PFN/PTB7:PC71BM/MoO3/Al 0.75 17.4 70 9.2 [14]

ITO/ZnO/PFN-Br/PBDT-DTNT:PC71BM/

MoO3/Ag

0.75 17.4 61 8.4 [40a]

ITO/PFN-V/PTB7-th:PCBM/MoO3/Al 0.80 17.6 65 9.2 [40b]

ITO/PFEN-Hg/PTB7:PC71BM/MoO3/Al 0.74 17.4 71 9.4 [27a]

ITO/PEDOT:PSS/PffBT4T-OD:PC71BM/

PNDIT-F3N-Br/Ag

0.77 17.9 73 10.4 [29]

ITO/PEDOT:PSS/PIDT-PhanQ:PC71BM/

C60-bis/Ag

0.88 11.2 60 5.8 [42]

ITO/PEDOT:PSS/PCE-10:PC71BM/C60-N/Ag 0.78 16.8 71 9.3 [31]




4.1.2. Tandem Solar Cells

Photons with lower energy than the energy bandgap of the active materials are not absorbed and do not contribute to energy con-version thus resulting in transmission loss. On the other hand, photons with energies larger than the energy bandgap generate hot charge carriers giving rise to thermal loss.[48] To overcome the limitation of single-junction devices, tandem devices can simultaneously achieve large absorption and reduce thermal losses by stacking active layers with different bandgaps.

In a tandem device, the charge recombination layer (CRL) plays an essential role. The CRL needs to possess a large work function contrast between the top and bottom surfaces for electron and hole collection and therefore, low work function surface modifiers are necessary for CRL to collect electrons effectively. By modifying PEDOT:PSS with PEIE, a recombi-nation layer with low optical absorption, high electrical con-ductivity, and large work function contrast of 1.3 eV between the top and bottom interfaces has been prepared and the recombination layer is robust enough to withstand the solu-tion-based processing conditions of the top sub-cell. The rep-resentative cell shows FF of 72% and PCE of 8.2% with the CRL based on the active layers of [1,2-b:4,5-b0]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C):PCBM/P3HT:ICBA (Figure 3a,b).[49] The same CRL (PEDOT:PSS/PEIE) is adopted by organic tandem solar cells to demonstrate the feasibility of all-solution-pro-cessed and all-plastic seven-junction solar cells (Figure 3c,d).[50] Recently, a new CRL (ZnO/PCP-Na) has been developed for high-performance solar cells, where PCP-Na is poly[2,6-(4,4-bis-sodiumpropylsulfonate)-4H-cyclopenta-[2,1-b;3,4-b′]-dithiophene-alt-biphenylborate].[51] Self-doping enables the effectiveness of the CRL in tandem cells. Two sub-cells with complementary photoresponse spectra have been designed and optimized and the solution-processed and fullerene-free tandem OSCs show an average PCE greater than 13% (Figure 3e).[52]

Perovskite-containing tandem solar cells are attractive because of the high efficiency and easy processing and the most common tandem devices are monolithic perovskite/crystalline silicon tandem solar cells. An organic PEIE/PCBM/indium zinc oxide bilayer has been adopted as CRL to avoid annealing and with spiro-OMeTAD as the hole transport layer where spiro- OMeTAD is 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine) 9,9′-spirobifluorene, MoOx as the buffer layer, and sputtered indium oxide/indium tin oxide (IO:H/ITO) stack as the trans-parent electrode in the PerSC top-cell. The monolithically integrated PerSC/Si tandem solar cell shows PCE as high as 21.2%.[53] Besides Si solar cells, organic solar cells have been investigated as the bottom cells in the monolithically inte-grated tandem solar cells. An infrared (IR)-sensitive block pol-ymer, PBSeDTEG8, with excellent thermal tolerance is intro-duced as the bottom sub-cell. The all-solution processed TiO2/PFN/PEDOT:PSS is the CRL which functions as electrical con-nection and facilitates efficient charge recombination. This yields a PerSC/polymer monolithically integrated tandem solar cell with 10.23% PCE.[51a] Another interesting tandem configu-ration is the PerSC-1/PerSC-2 all-perovskite monolithically integrated tandem solar cell.[54] PEI doped PCBM is used in

the CRL to increase the efficiency of electron collection.[54a] With optimization of the perovskite layers and the CRL, the monolithic perovskite–perovskite tandem solar cell shows a high VOC of 1.98 V and stabilized PCE of 18.5% with improved sub-cell characteristics and efficient CRL of C60/bis-C60/ITO.[54c]

4.2. Organic, Perovskite, and Quantum-Dots Light Emitting Diodes

LEDs consist of a sandwiched structure in which the light-emis-sion layer is deposited between the cathode and anode. Elec-trons and holes can be injected into the emissive layer from the corresponding electrodes through the interfaces. The charge injection at the interfaces directly affects the efficiency of the LEDs and interface modulation is also critical to the device per-formance. Low-work function modification benefits electron injection in the LEDs. Some examples of adopting low work function surface modifiers to enhance the electron-efficiency and device performance are summarized below. Low-work function modifiers which have been demonstrated in different types of LEDs include organic light emitting diodes (OLEDs), polymer light emitting diodes (PLED), perovskite light emitting diodes (PerLEDs), and QDs light emitting diodes (QLEDs).

In 2004, Cao and co-workers reported the application of a thin layer of polymer PFN and its derivatives (PFN-Br) inserted between the electrode and the emissive polymer layer. The LED device delivered performance as well as the device with Ba/Al the electron-injection electrode.[55] This has initiated the application of interfacial polymers to create dipoles and improve the device performance. Afterward, similar dipole poly-mers have been developed to improve electron injection. Jen and co-workers[56] have reported that another polymer PFN-OH can form dipoles between the emissive layer and Al cathode by solution processing to improve the device performance. Furthermore, it is found that processing solvents for PFN-OH are crucial to the device performance because the processing solvents influence the aggregation and morphology of the PFN-OH on the emissive layer. The film cast from ethanol is quite smooth while the film cast from the water/ethanol mix-ture shows rough and aggregate features. The aggregate of PFN-OH delivers higher performance than that with smooth and uniform PFN-OH. The better performance with the PFN-OH aggregates is attributed to the better hole-blocking ability.[56] The white OLEDs show a maximum quantum effi-ciency of 10.7% and maximum power efficiency of 13.2 lm W–1 that is even higher than the device with the Ba/Al electrode. PEI and PEIE which have been widely used in organic solar cells have also been proven to be very effective in enhancing the performance of LEDs. For example, insertion of the a PEIE layer between the electrode and the electron transport mate-rials can enhance the efficiency of the phosphorescent LEDs.[7a] PEI is deposited on ITO or ZnO to fabricate inverted OLEDs where the bottom electrode is for electron injection and the top electrode is for hole injection.[57] The polymer modifier not only improves electron injection from the ZnO to the emissive layer by decreasing the work function, but also blocks quenching of excitons and increases the luminous efficiency. The LEDs with





PEI display improved current efficiency (CE) of 13.5 cd A−1 com-pared to OLEDs with Cs2CO3 interfacial layer (Figure 4c,d).[57a] The PEIE modifier has also been used to fabricate efficient

stretchable LEDs.[58] PEI is deposited on the GO-AgNW/PUA composite to produce the electron-injection electrode, in which GO-AgNW is graphene oxide-silver nanowire composite and


Figure 3. a,b) Tandem solar cells with PEDOT:PSS/PEIE as the ICL and the J–V characteristics. (a,b) Reproduced with permission.[49] Copyright 2012, The Royal Society of Chemistry. c) Conductivity tuning of the PEDOT:PSS/PEI ICL. d) All-solution-processed all-plastic multijunction (7-junction, 22 layers) cells displaying VOC of 5.37 V. (c,d) Reproduced with permission.[50] Copyright 2016, The Royal Society of Chemistry. e) Solution-processed fullerene-free tandem OSCs with optimized sub-cells and CRL delivering a record PCE of 13.8%. Reproduced with permission.[52] Copyright 2017, American Chemical Society.





Figure 4. a) Schematic drawing and b) optical photographs of a fully stretchable PLED using the GO-AgNW/PUA composite electrode as both the anode and cathode. Reproduced with permission.[58] Copyright 2014, American Chemical Society. c,d) IPLEDs structure using PEI and PEIE on the ZnO layer and cur-rent efficiency versus voltage characteristics of the inverted PLEDs with various interlayers. (a–d) Reproduced with permission.[57a] e) Schematic illustration and cross-sectional TEM image of the multilayered PerLED, and f) flat-band energy level diagram (MAPbI3−xClx:CH3NH3PbI3−xClx, MAPbBr3:CH3NH3PbBr3). (e,f) Reproduced with permission.[24d] g) Cross-sectional STEM image showing the NFPI7 perovskite MQW LED device architecture. h) Photographs of green and red MQW LEDs with the logo of the IAM, and the corresponding CIE coordinates. (g,h) Reproduced with permission.[61] Copyright 2016, Nature.




PUA is polyurethane acrylate. The device can survive after 100 stretching cycles between 0 and 40% strain and can be stretched up to 130% linear strain at room temperature (Figure 4a,b). The low work function modifier PEI has also been applied to tandem PLEDs to improve electron injection.[59]

Perovskites (MAPbX3), a new class of promising materials for high efficiency solar cells, are potential light-emitting devices due to the high photoluminescence quantum efficiency, good color purity, and high charge mobility.[60] A PerLED structure incorporating a multifunctional PEI interlayer between the oxide electron transport layer and perovskite emissive layer has been fabricated.[24d,61] The PEI layer facilitates the formation of high-quality perovskite and reduces the work function of the cathode for electron injection into the hybrid perovskites. These two advan-tages improve the device performance and the CH3NH3PbI3-xClx-based near-infrared (NIR) PerLEDs show an internal quantum efficiency of 15.7% at a high radiance of 28 W sr−1 m−2 and low voltage of 2.2 V (Figure 4e,f).[24d] A solution-processed PerLED based on self-organized multiple quantum wells (MQWs) has excellent film morphology. The MQW-based LED with the ZnO/PEI electron transport layer exhibits a high external quantum effi-ciency of up to 11.7%, good stability, and exceptional high power performance with an energy conversion efficiency of 5.5% at a current density of 100 mA cm−2 (Figure 4g,h).[61]

QLEDs, another kind of light emitting devices, have emerged as alternatives to OLEDs for next-generation displays due to unique properties such as the size-dependent energy bandgap, narrow spectral emission bandwidths, and compatibility with solution processes. PEIE modified ZnO nanoparticles have been suggested as the electron transport layer in the inverted structure of red CdSe–ZnS based QLEDs. The work function of ZnO decreases from 3.58 to 2.87 eV, thus facilitating electron injection into the CdSe–ZnS QD emissive layer and charge bal-ance of the QD emitter. As a result, the device exhibits a low turn-on voltage of 2.5 V and CE of 1.53 cd A−1.[62]

4.3. Field-Effect Transistors

Organic field-effect transistors (OFETs) that use organic semi-conductors as the channel have progressed significantly in recent years. The organic semiconductors can be fabricated by vacuum deposition or by solution processing. OFETs have potential appli-cations in flexible electronic devices including displays, inte-grated circuits, memory devices, and sensors, and are convenient tools to study transport properties of semiconducting materials. The field-effect mobility values of the device with solution-pro-cessed organic channels are larger than those found from amor-phous silicon (a-Si) field-effect transistors.[63] The typical OFET is composed of following basic elements: a thin semiconductor film, a dielectric layer, and three electrodes. The gate electrode is separated from the semiconductor film by the dielectric layer. The other two electrodes are the source and drain electrodes. They contact the semiconductor film at a short channel length, and when a bias voltage is applied to the gate, charges aggregate at the interface between the semiconductor and dielectric layer forming a conducting channel allowing current to flow across the source and drain electrodes. Therefore, the source–drain contacts in FETs affect the device operation and proper tuning of

the work function of the metal electrodes to improve the charge injection to semiconductor layer is of great importance.

To improve electron injection, PEIE modification has been performed on the Au electrode to produce n-channel OFETs as well as metal oxide based transistors.[7a,64] The PEIE modi-fied Au source and drain electrode yield improved injection as well as good air stability. For the n-channel OFETs with the PEIE coated Au source and drain electrodes and an organic semiconductor of poly[N,N′-bis(2-octyldodecyl)-naphthalene-1,4:5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) [P(NDI2OD-T2)] (also called N2200), the threshold voltage (VTH) drops from 4.5 to 0.4 V. The average mobility increases from 0.04 to 0.1 cm2 V−1 s−1 compared to the reference devices without the PEIE modification. Metal oxide based transis-tors which have high mobility and optical transparency in the visible region have attracted large interest. For the n-channel metal oxide InGaZnO (IGZO) TFTs, VTH of the IGZO thin-film transistors (TFTs) drops from 38.7 to 1.5 V and the mobility increases from 0.004 to 1.2 cm2 V−1 s−1 in the devices with the PEIE-modified electrodes (Figure 5) compared to the reference IGZO devices without PEIE modification.[7a]

PEI has been applied as an effective n-type dopant to increase the mobility of n-type metal oxide and polymer in FETs.[54a,65] PEI is introduced to dope metal oxide and indium oxide (In2O3) to enhance the transistor performance especially the mobility. PEI doping of In2O3 films not only impedes crys-tallization and controls the carrier concentration, but also acts as electron dopants and/or scattering centers. The In2O3 doped with PEI on a 300 nm SiO2 gate dielectric substrate yields an electron mobility as high as 9 cm2 V−1 s−1 and excellent on/off ratio of 107. Except for the metal oxide, PEI plays a more apparent role in doping the polymer semiconductor.[65c] Nano-pores enhanced n-doping of polymer poly(diketopyrrolopyrrole-thiophene-thieno[3,2,b]thiophenethiophene) (DPP2T-TT) doped by PEI has been demonstrated. Doping with PEI at a high concentration enhances the n-channel characteristics via passi-vating the p-channel behavior. An order of magnitude enhance-ment in the electron mobility has been obtained compared to that without PEI.[66] Furthermore, PEI can convert p-type poly-mers into unipolar n-type polymers. A small amount of PEI added to several ambipolar and p-type polymer semiconductors convert them into unipolar n-type OFETs devices. The electron-rich amine radical groups in PEI are thought to raise the Fermi level, fill the electron traps, and act as traps for holes to produce unipolar n-channel OFETs with improved electron mobility.[65b]

3D perovskites (e.g., MAPbI3) based FETs are challenging because of ion migration, since the mobility of perovskite FETs is low at room temperature and they are prone to degradation during tests. Gold source–drain contacts have been modified with the polymer surface modification layer PEIE (2 nm) to reduce the work function of gold. The grain size of the perov-skite thin films inside the channel region increases after doping PEIE due to the enhanced amine–amine interaction from the PEIE-treated electrodes. The room temperature mobility of the FET increases from 0.01 cm2 V−1 s−1 for the bare gold contact to 0.5 cm2 V−1 s−1 for the PEIE-treated electrode in addition to the enhanced device stability.[67]

PEI and PEIE are effective in the n-channel organic and metal-oxides field effect transistors via surface modification as





well as effective doping. Doping is also important, since it can tune the charge carrier concentration, electronic conductivity, as well as Fermi energy. Particularly, there are limited effective solution-processed in the literature. PEIE and PEI are soluble in polar solvents such as water or alcohol and it is challenging to uniformly mix and dope organic materials that dissolve in non-polar solvents. Recently, an effective amidine-type n-dopant of DBU has been developed.[19d] The DBU liquid mixes well with the nonpolar solvents and universally dopes electron-acceptors such as fullerenes and other nonfullerene acceptors. OFETs have been fabricated with PC61BM and DBU-doped PC61BM as the channel and Au as the source/drain electrodes. Vth of the OFETs decreases from 33.5 to 8.2 V and the electron mobility increases from 5.35 × 10−3 cm2 V−1 s−1 to 5.94 × 10−2 cm2 V−1 s−1 when the doping ratio is 0.1 wt% compared to the undoped PCBM film.

4.4. Photodetectors (PDs)

PDs are the crucial components in imaging systems, dig-ital photography equipment, environmental surveillance

instruments, communication systems, and biological sensing apparatus. Typically, PDs with two terminals are operated in the photovoltaic or photoconductive mode. Here, we focus on the photovoltaic-mode PDs which are particularly interesting due to the low operating voltage and fast response. Solution-processed bulk organic heterojunctions or recently emerging metal-halide perovskites are discussed. Based on the solution-processed low work function modifiers, device architecture and processing techniques (such as fully printed) can be realized for low-cost and flexible organic and perovskite photodetectors.[68]

Organic semiconductors have many advantages in PDs, since they offer the possibility of large-area coverage and flex-ible devices by means of solution-based and low-temperature processes. Low-work function modifier PEIE directly put on the ITO lowers the work function of high-performance solution-processed organic photodetectors with an inverted structure.[24b] On top of the active layer (PBDTTT-C and PC71BM), PEDOT:PSS/Ag is used as the high-work func-tion electrode. The device shows small dark currents under a reverse bias. The thin layer of PEIE is effective in providing the surface with a low work function and leads to small dark


Figure 5. a) Device structure of N2200 OTFTs. b) Transfer characteristics of N2200 OTFTs with and without PEIE. c) Device structure of IGZO TFTs. d) Transfer characteristics of IGZO TFTs with and without PEIE. Reproduced with permission.[7a] Copyright 2012, Science.





currents by lowering the reverse-bias charge injection from the cathode.[24b] The most promising aspect of organic semiconduc-tors is that they are printable at low temperature compared to their inorganic counterparts. Fully printed flexible OPD arrays have been proposed.[68] The cathode is formed by a blade-coated layer of PEDOT:PSS on a polyethylene naphthalate flexible sub-strate. To reduce the work function of the cathode, the PEIE layer is blade-coated on the PEDOT:PSS and the active layer blade-coated on top of the cathode is a bulk-heterojunction (BHJ) blend of PCDTBT and PC71BM. The device is completed by screen printing an array of anodes of PEDOT:PSS into the desired shape and the fabrication process is an all-solution one employing printing (Figure 6a,b). The fully printed device shows a detectivity on the order of 1013 cm Hz0.5 W−1.[68] The transfer-printing method is also powerful in fabricating high-sensitivity OPDs. A film is deposited on the substrate and trans-ferred onto the target substrate. This method avoids washing or damage issues arising from the use of nonorthogonal solvents, since the film is transferred onto the target surface in dryness. Both the conducting polymer electrode and active layers can be fabricated by this technique. Xiong et al.[69] have recently

reported a transfer-printed conducting polymer that can serve as the top electrode instead of the vacuum-deposited metal electrode and the photo detector responds to incident light on both sides through the bottom or top electrode. PEIE is used to adjust the work function of the ITO bottom electrode. The photo detector shows a responsivity of 0.37 A W−1 at 850 nm and small dark current density of 3.0 nA cm−2 at −0.2 V based on the NIR active layer of PMDPP3T:PC61BM (Figure 6c,d).

A long charge carrier lifetime and diffusion length are observed from lead-halide perovskite films indicating small recombination of charge carriers in the bulk films thus also promi sing for photo detector applications. A hybrid solu-tion-processed perovskite photodetector with the water/alcohol-soluble conjugated polymer, PFN, as the interfacial layer has been produced. The PFN with 2,9-dimethyl-4,7-di-phenyl-1,10-phenanthroline shows a comparative capacity of hole blocking. The photo detector exhibits a large detectivity approaching 1014 Jones, linear dynamic range over 100 deci-bels (dB), and fast photoresponse with a 3 dB bandwidth up to 3 MH at room temperature.[70] PEIE is employed to manipu-late the crystallites with a perovskite size and overall degree of

Figure 6. a) Fabrication process of all-printed OPDs. b) Band diagram of all-printed OPD structure (inverted geometry with PEDOT:PSS top anode) and other partially printed devices. (a,b) Reproduced with permission.[68] c) Device structure of the double-side responsive near-infrared photodetector: glass/ITO/PEIE/PMDPP3T:PC61BM/PEDOT:PSS. d) Energy diagram of each layer used in the photodetector. (c,d) Reproduced with permission.[69] Copyright 2016, Royal Society of Chemistry.





disorder to control the junction electrical properties to realize charge collection narrowing. Based on the mixture of PEIE and organohalide perovskite or lead halide semiconductor, the green and blue photodiodes show full-width at half-maximum of less than 100 nm.[71]

5. Summary and Outlook

Tremendous progress has been made recently in solution-pro-cessed organic electronics such as organic (quantum dots and perovskite) solar cells, light-emitting diodes, field-effect transis-tors, and organic photodetectors. The device performance and stability can be improved by optimizing the interfaces (metal/organic, dielectric/organic, and organic/organic) for good elec-trical contact. In this paper, recent advances pertaining to low work function surface modifiers for solution-processed elec-tronics are reviewed and the mechanisms responsible for the improved device performance are discussed. Modifiers that chemically or physically adsorb onto the target surface due to the grafted ionic or polar groups form intrinsic molecular dipoles as well as interface dipoles between the modifiers and metal or metal oxide electrodes resulting in work func-tion reduction. In addition, doping and self-doping have been implemented to produce efficient interlayers that reduce the sensitivity of the layer thickness enabling printing processing. These surface modifiers reduce the work function of the elec-trodes to improve electron collection and injection. In the meantime, these modifiers also create doping which enhances the charge carrier concentration and fills the trap states at the interface. For example, when PEIE is used as a modifier on top of ITO or ZnO as an electron-collecting electrode in fullerene-containing solar cells, doping of PEIE in the fullerene[54a] pas-sivates the defects at the ITO/active layer or ZnO/active layer to increase the power conversion efficiency.[36] In addition, water or alcohol-derived modifiers yield films with a high surface energy beneficial for deposition of the active layer on top.

Many types of materials including metal oxides, conjugated or nonconjugated polymer electrolytes, self-doped polymer and small molecules, and self-assembled functional molecules have been explored to decrease the electrode work functions. Each type has advantages and disadvantages regarding the electrical, optical, chemical, mechanical properties, and processing. Dif-ferent surface modifiers can be adopted to achieve target prop-erties based on the request of the performance, processing, and reliability of the target devices. To develop low work function surfaces and interfaces, three properties (performance, pro-cessing, and reliability) should be carefully considered.

i. Performance: The work function of the surface produced by the modifiers must be sufficiently low to enable efficient electron injection from the electrode to the semiconductor or extraction from the semiconductor to electrode. For the com-monly seen organic electron-accepting semiconductors such as fullerene, a work function of less than 4 eV is generally sufficient for electron injection.

ii. Processing: Roll-to-roll printing is ideal for solution-pro-cessed electronics, including the low-work function modifi-ers or layers. High conductivity is desired for the layers to

reduce the film thickness sensitivity of device performance with printing processing. The layers should have conductiv-ity of 100–1000 S cm−1 or even higher. In this case, the layer itself can act also as a conducting electrode. It should be a low work function electrode with one-step printing which simplifies device processing. Like PEDOT:PSS as the high work function layer, it has high conductivity up to thousands S cm−1 and is printable and transparent. Therefore, it is the most widely used high work function interface and electrode in optoelectronics. Some materials with a low work function as well as high conductivity up to 1000 S cm−1 are certainly desirable.

iii. Reliability and stability: Oxygen and humidity stability is important for processing or printing in air and also directly influences the reliability of the devices and increases the cost due to stringent encapsulation. Possible reactions between the surface modifiers and active layer may occur. For exam-ple, the PEI or PEIE low work function modifiers may re-act with the donor–acceptor type nonfullerene acceptor thus compromising the performance of the solar cells.

AcknowledgementsThe work was jointly supported by the National Natural Science Foundation of China (Grant Nos. 21474035 and 51403071), and City University of Hong Kong Applied Research Grants (ARG) Nos. 9667122 and 9667144. This article is part of the Advanced Materials Interfaces Hall of Fame article series, which highlights the work of top interface and surface scientists.

Conflict of InterestThe authors declare no conflict of interest.

Keywordslow work function, printed electronics, solution-processed electronics, surface modification

Received: October 28, 2017Revised: February 24, 2018

Published online: April 3, 2018

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