polymer solar cells: recent approaches and achievements

Polymer Solar Cells: Recent Approaches and Achievements

Riccardo Po,† Michele Maggini,‡ and Nadia Camaioni§,*Centro ricerche per le energie non conVenzionali, Istituto ENI Donegani, ENI S.p.A., Via G. Fauser 4,28100 NoVara, Italy, Dipartimento di Scienze Chimiche, UniVersita di PadoVa, Via F. Marzolo 1,35131 PadoVa, Italy, and Istituto per la Sintesi Organica e la FotoreattiVita (CNR-ISOF), Consiglio Nazionaledelle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy

ReceiVed: June 30, 2009; ReVised Manuscript ReceiVed: NoVember 6, 2009

Polymersolar cells have gained wide interest in the past few years for their potential in the field of large-areaand low-cost photovoltaic devices. Thanks to rather simple treatments developed in the new millennium, themorphology of polymer solar cells has been optimized at the nanoscale level, leading to high efficient charge-carrier photogeneration and collection. Power conversion efficiency up to 6% and 6.5% have been reportedin the literature for solution-processed polymer solar cells in single-junction and tandem configuration,respectively, and a record efficiency of 6.77% has been recently announced. After an introduction into theoperational principles and device structure of polymer solar cells, this paper provides an overview of thelast-years research activity. In particular, the different treatments successfully performed on polymer activelayers, and their beneficial effects on the overall device efficiency are discussed. Subsequently, some significantexamples of photoactive materials will be examined, outlining the foremost structure-properties relationships.Some directions for further enhancement of the performance of polymer solar cells will be also introduced,mainly through the fine-tuning of the electronic properties of the active materials.

1. Introduction

Currently, silicon solar cells cover more than 85% of themarket of photovoltaics.1 However, silicon cells technology isstill not cheap enough to allow a wide diffusion of photovoltaicenergy conversion in the absence of government incentives. Forthis reason huge efforts of research and development have beenspent in the last years to find alternative and improved solutionsin this field. Polymer solar cells (PSC) represent a newtechnology that in the midlong term could lead to affordableenergy. In addition, PSC are lightweight and can be madeflexible, opening the possibility for a range of new applications;large-area, pliable devices can be fabricated easily and inex-pensively, by employing cost-effective techniques like, forinstance, inkjet or screen printing, and slot-die, gravure, or spraycoating. On the other hand, issues like limited efficiency andstability have to be solved before going through the stages ofindustrial production and commercialization.

The mechanism underneath the operation of a polymer (or anorganic) solar cell exhibits, of course, many similarities with thatof inorganic cells, but also some distinctions, arising from a fewimportant different characteristics of the materials involved:

1. While inorganic semiconductors exhibit a band structure,organic semiconductors possess discrete energy levels (molec-ular orbitals). Nevertheless, the term “bandgap” is oftenimproperly used for organic semiconductors.

2. When a bound hole-electron pair (exciton) is generatedin an inorganic semiconductor, its immediate dissociation is

observed. Excitons in organic semiconductors are tightly bound(binding energy of around 0.3-0.5 eV) and their dissociationmust be promoted in some way avoiding radiative recombination.

3. Compared to inorganics, charge carrier mobilities in organicsemiconductors are very low.

4. Light absorption coefficients of organic materials are muchhigher than those of inorganics.

The architecture of a typical single-junction PSC is sketchedin Figure 1a, together with a simplified current-voltage curveunder illumination (Figure 1b) and the scheme of the energylevels of the different components (Figure 1c). The core of thecell is the photoactive layer, which is generally composed by ap-type electron-donor compound (D) and an n-type electron-acceptor compound (A). Both A and D are organic π-conjugatedmaterials, and either one or the other (or both) is a polymer.The photoactive layer, typically around 100-200 nm in thick-ness, is interposed between the electrodes; additional layers ofelectron or hole transporting materials can be present.

The photoinduced charge transfer is the photophysical processresponsible for the cell working. Ideally, the overall process ofelectricity generation consists of five steps:

1. Photoexcitation of the absorber material(s) causes thepromotion of electrons from the ground state, approximated2

by the highest occupied molecular orbital (HOMO), to theexcited state, approximated by the lowest unoccupied molecularorbital (LUMO). Excitons are generated.

2. Excitons produced within a diffusion length3 from the D/Ainterface will have the chance to reach it before decaying,radiatively or not.

3. If the offsets of the energy levels of the D and the Amaterials are higher than the exciton binding energy, excitonsdissociate at the D/A interface. Excitons photogenerated in thedonor side will dissociate by transferring the electron to theLUMO level of the acceptor and retaining the positive charge,while those created in the other side will transfer the hole to

* To whom correspondence should be addressed. E-mail:[email protected]. Tel.: +39 051 6399779. Fax: +39 051 6399844.

† Istituto ENI Donegani, ENI S.p.A.‡ Dipartimento di Scienze Chimiche, Universita di Padova.§ Istituto per la Sintesi Organica e la Fotoreattivita, Consiglio Nazionale

delle Ricerche.

J. Phys. Chem. C 2010, 114, 695–706 695

10.1021/jp9061362 2010 American Chemical SocietyPublished on Web 12/08/2009

the HOMO of the donor while retaining the negative charge.This step leads to the formation of free charge carriers.

4. The charge carriers diffuse to the electrodes through therespective materials (electrons in the acceptor and holes in thedonor).

5. The charges reach the electrodes and are collected.Other more elaborate mechanisms (charge-separate state model,4

ground state charge-transfer complex model5) are mentioned forreference but will not be discussed here. Furthermore, it must bestressed that the simplified HOMO-LUMO scheme of Figure 1ccould not account for the real complexity of the situation and thatthe energy levels of pure compounds may change at the interfacewhen the donor and the acceptor are put into contact.6

Going back to the simple mechanism, in each of the abovesteps several phenomena can take place that decrease theefficiency of the global process, so that only a limited portionof the photons reaching the cell are able to generate “useful”charge carriers. Thus, the optimization of each step is funda-mental to extract as much energy as possible from the device.This optimization encompasses the development of improvedapproaches in the design of materials and in the devicestructures, as will be shown hereinafter.

The energy levels of the donor and the acceptor can be tunedthrough the design of their molecular structure (bandgap engineer-ing).7 The matching of these energy levels (and of the Fermi levelsof the electrodes), according to the scheme in Figure 1c, is thekey to make the photoinduced charge-transfer process smooth asmuch as possible. To allow exciton dissociation, a driving forceexceeding the Coulombic attraction of the hole-electron pair isrequired;8 to this purpose, energy offsets ∆ELUMO and ∆EHOMO of0.3-0.5 eV are necessary to overcome the exciton binding energy9

and to avoid the undesired process of back charge transfer.Furthermore, to increase the fraction of absorbed light, and thusthe number of photons contributing to the short-circuit current (Isc)of polymer solar cells, the absorbing components should have alow energy gap (bandgap), taking into account that the maximumof the solar spectrum is placed at around 650 nm (1.9 eV). On theother hand, since it has been demonstrated10,11 that the thermody-namic limit of the open-circuit voltage (Voc) is proportional to thedifference LUMOA - HOMOD, a high energy gap of the donorwould favor this condition. Therefore, the best compromise betweenhigh Isc and high Voc must be found.

The other factor heavily affecting the performance of PSC isthe nanomorphology of the photoactive layer. Since the lifetimeof the exciton is short, its diffusion length in organic materialsis only about 10-20 nm.3 This means that the exciton mustreach the D/A interface to give the charge transfer withoutundergoing to a radiative or nonradiative decay. Thus, the donorand acceptor phases should self-organize to form nanodomainswith dimensions comparable to the exciton diffusion length. Toincrease the possibility for the exciton to reach the interface,the donor-acceptor contact area must be as large as possible.Furthermore, once the exciton is formed and dissociated, thehole and the electron must drift to the electrodes within theirlifetimes. Again, the phase morphology is critical to formpercolated pathways to the contacts. These considerationssuggest that the photoactive layer should be thin, but forabsorbing most of the solar light thick layers would be preferred.

During the years, the structure of the active layer evolved tomeet at best the above-mentioned requirements and find the bestcompromise. From the simple, original bilayer arrangement in∼100 nm donor-acceptor films (Figure 2a), through a diffusedbilayer heterojunction (Figure 2b), the disordered bulk hetero-junction (BHJ)12,13 (Figure 2c) is currently the state-of-the-artin PSC. In a BHJ, donor and acceptor are intimately mixed toform an interpenetrating phase network at the nanoscale level.Methods to control the degree of phase segregation have beendeveloped and will be described hereinafter. Furthermore, theperformance of a PSC is affected not only by the electronicproperties of the active materials and by the nanomorphologyof the active layer but also by the electrical contacts thatcontribute substantially to solar cell efficiency. Indeed, theworkfunctions of the electrodes determine whether the contactwith the organic material is ohmic or blocking, while theirdifference provides the charge carriers with the built-in potential,necessary to reach the electrodes. The cathode is made with alow workfunction metal (usually aluminum), whereas a highworkfunction material is used for the anode. One of theelectrodes must be transparent; in principle, semitransparencycan be attained with very thin metallic films, but the preferredsolution is by far the use of a transparent conductive oxide,indium-tin oxide (ITO, acting as anode) in most cases.

The simplified scheme in Figure 1a does not includeadditional functional layers, also called buffer layers, whose roleis to favor charge collection at the electrodes.14 Although theexact mechanism of action of those buffer layers is still debated,the final result is an increase of the cell efficiency. An ultrathinlayer of LiF (less than 1 nm) below the aluminum cathodeimproves the contact and lowers the Al workfunction.15 On theother hand, a layer of conductive poly(3,4-ethylenedioxy)thio-phene/poly(styrene sulfonate) (PEDOT-PSS) above the ITOsubstrate improves the workfunction of the anode (usually, ITO/PEDOT-PSS system is regarded as the “anode”), providing abetter matching with the HOMOD energy level. It also acts asan electron-blocking layer and smooths the roughness of ITOsurface, thus minimizing the occurrence of short-circuits.

There are several ways to fabricate PSC with architecturessimilar to that depicted in Figure 1a, which make use ofdeposition techniques depending on the materials used. Thetransparent conductive oxide is deposited through sputtering;PEDOT-PSS buffer layer and the photoactive layer are depositedby wet techniques, e.g., spin-coating for small area devices orprinting techniques for larger areas in continuous processes. Themetal cathode is usually deposited through thermal evaporation,although printing techniques can be applied in the case ofconductive metal pastes. The devices ought to be encapsulated

Riccardo Po studied Industrial Chemistry at the University of Pisa, wherehe received the MSc in 1988. Subsequently he moved to the EnichemResearch Center in Novara, where he joined the Materials Department,working on polymer synthesis and modification. In 1999 he became Managerof the Polymer Chemistry and Physics Department. In 2007 the ResearchCenter was incorporated by ENI SpA and he was appointed Manager ofthe Solar Energy Department and put in charge of the research activitieson polymer solar cells.

Michele Maggini earned a Laurea in Chemistry from the University ofPadova in 1984. From 1986 through 1988 he was Research Associate atThe University of Chicago. After one year in the industry and eight yearsas a scientist of the Italian Council of Research (CNR), in 2000 Magginibecame Full Professor of Organic Chemistry at the University of Padova.

Nadia Camaioni received her MSc (Laurea) in Physics in 1985 at theUniversity of Bologna. She was appointed research scientist at CNR-ISOFInstitute in 1994 and senior researcher in 2002. She is the author of morethan 90 scientific publications in the field of physical chemistry and materialsscience. Her scientific interests are concerned with the optoelectronicproperties of organic semiconductors and her research activity is mainlyfocused on the transport properties of functional organic materials and theirinvestigation in solution-processed solar cells.

696 J. Phys. Chem. C, Vol. 114, No. 2, 2010 Po et al.

to achieve long durability. On the lab scale, glass slides andbicomponent glues are commonly used; more complex tech-niques must be applied in continuous processes, where flexiblesubstrates are exploited. To ensure an effective isolation fromair and moisture, high-barrier packaging films must be used.

The current state-of-the-art most efficient PSC are based onsoluble derivatives of fullerene as electron-acceptors, in par-ticular [6,6]-phenyl-C61-butyric acid methyl ester16 (PCBM,Figure 3) and [6,6]-phenyl-C71-butyric acid methyl ester(PC70BM, Figure 3). Fullerenes have a set of truly uniquecharacteristics such as high electron affinity17 as well as highelectron mobility,18 that make them very good acceptor com-ponents in BHJ solar cells. Importantly, the photoinducedelectron transfer from excited donors is orders of magnitudefaster than back-transfer or exciton decay.19 As for electrondonors, a wide range of different conjugated polymers andoligomers have been considered.20 Poly(3-alkylthiophenes) andtheir variations are very popular donor materials,21 able to afforddevices with almost 5.5% power conversion efficiency (PCE);however, the certified highest efficiency reported in the literatureis 6%,22 for single-junction cells based on PCDTBT23 as donorand PC70BM as acceptor (Figure 3). Even higher PCE values(up to 6.5%)24 have been demonstrated for all-solution processedtandem solar cells,25 that is, for more complex device structuresin which two individual subcells are stacked and, usually, series-connected. Currently, the certified record efficiency for polymersolar cells is 6.77%, recently announced26 and there is still plenty

of room for efficiency improvement. Indeed, it has been shownthat, in principle, up to 10% and 15% efficiencies can beachieved, for single-junction and tandem PSC respectively,through the molecular design of materials with optimizedelectronic properties.27

2. Rapid Progress over the Last Years

The last 7-8 years have seen a rapid progression of theperformance of polymer solar cells, mainly due to the achieve-ment of a fine control over the nanomorphology of the deviceactive layer. Most of the research activity has been focused on

Figure 1. (a) Typical architecture of a single-junction polymer solar cell; (b) current-voltage curve under illumination; (c) energy levels diagram.The photovoltaic parameters extracted from the current-voltage curve: short-circuit current (Isc); open-circuit voltage (Voc); maximum output power(Pmax). The fill factor (FF) represents the ratio (Pmax)/(Voc × Isc); the power conversion efficiency (PCE or η) is defined by the ratio Pmax/Pin ) (Voc

× Isc × FF)/Pin, where Pin is the incident irradiation power.

Figure 2. Structures of the photoactive layer (dark gray, donor; lightgray, acceptor; white, anode; black, cathode): (a) bilayer heterojunction;(b) diffused bilayer heterojunction; (c) bulk heterojunction.

Figure 3. Molecular structures of [6,6]-phenyl-C61-butyric acid methylester (PCBM), [6,6]-phenyl C71-butyric acid methyl ester (PC70BM),poly(3-hexylthiophene) (P3HT), and poly[N-9′′-hepta decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole] (PCDT-BT).

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the investigation of active layers made of regioregular poly(3-hexylthiophene) (P3HT, Figure 3) and PCBM. Previously, until2002, poly(phenylenevinylene) (PPV) derivatives were mainlyused as absorbers and electron donor components.28 Despite thepromising charge transport properties29-31 and better solar lightharvesting of P3HT, PPV derivatives were preferred becauseof their superior film-forming properties with respect to poly(3-alkylthiophenes).32

Controlling the Nanoscale Morphology of BHJ Cells. Thewide interest in P3HT was triggered by the first results showingthe dramatic effect of thermal annealing processes on P3HT:fullerene active layers.33,34 A 3-fold increase of both short-circuitcurrent and power conversion efficiency (from 0.86 to 2.52 mAcm-2 and from 0.82 to 2.50%, respectively, under white lightirradiation of ∼20 mW cm-2, Figure 4) was reported for BHJsolar cells made of P3HT and a fulleropyrrolidine (N-methyl-2-[(3′,4′-dibenzyloxy)phenyl]fulleropyrrolidine, FPY) upon amild thermal treatment at 55 °C.33

Thermal annealing of PSC based on P3HT:PCBM activelayers have been reported for annealing temperatures rangingbetween 75 and 230 °C,35-42 and power conversion efficienciesup to 5% have been demonstrated through the optimization ofannealing temperature and time, achieved by annealing at 150°C for 30 min.36 Parallel to the relevant enhancement of cellperformance, a remarkable effect of the annealing processes ontheopticalpropertiesoftheactivelayerisalwaysobserved.33,34,39,40,42

The effect on the absorption spectra is 2-fold, as shown inFigure 5 for P3HT:FPY: (i) an overall enhancement of theabsorption intensity and (ii) a red-shift of the optical absorptionwith a better resolved vibronic structure of the polymer phase.This spectral evolution is the result of an increased crystallizationof P3HT,43 induced by the enhanced molecular mobility duringthe annealing process, that provides an improved overlap of theabsorption spectrum with the solar emission. This is alsoreflected by the external quantum efficiency (EQE, defined asthe ratio of the number of collected carriers to the number ofincoming photons) of the annealed devices.34,38,42 In Figure 5the comparison of the EQE of P3HT:FPY solar cells beforeand after a thermal treatment at 55 °C for 30 min is also shown.Though the EQE is enhanced for all wavelengths as a result ofthe thermal treatment (Figure 5, top panel), its most dramaticchange has been observed in the spectral region related to thevibronic structure of P3HT, arising as a consequence of theheating process (Figure 5, bottom panel).

The enhanced crystallinity of P3HT:PCBM active layers uponthermal annealing, as demonstrated by X-ray diffraction,36,40

improves the transport properties of the active layer, leading toa reduced series resistance39 and an increased fill factor of theannealed devices. A further evidence of the improved crystal-linity of the donor phase in annealed solar cells is given by theevolution of the cyclic voltammogram of the active layer.44,45

The onset of P3HT oxidation decreases upon annealing (asshown in Figure 6 for a P3HT:FPY film), indicating a lowerionization potential (IP) for the polymer. The decrease of IP isconsistent with the enhanced P3HT crystallinity upon annealing,

Figure 4. Current-voltage characteristics of a cell made of P3HT:FPY (3:2 w/w) before (full line) and after (dotted line) a thermaltreatment of the device at 55 °C for 30 min. White-light irradiationpower: 20 mW cm2. Modified from ref 33 by permission of The RoyalSociety of Chemistry.

Figure 5. Top: absorption spectra of a spin-coated P3HT:FPY (3:2w/w) film from a chloroform solution before (solid line) and after(dotted line) a thermal treatment at 55 °C for 30 min; external quantumefficiency of a PHT:FPY based-cell before (open circles) and after (solidcircles) the thermal treatment. Bottom: ratio of the external quantumefficiency after the treatment to that before the treatment (solid circles);the absorption spectrum of the active layer after the thermal treatment(dotted line) is also shown for a better comparison. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced from ref 34 withpermission.

Figure 6. Cyclic voltammograms of a 100 nm thick film of P3HT:FPY (3:2 w/w) in 0.1 M TBAP in acetonitrile before (solid line) andafter (dotted line) a thermal treatment at 55 °C for 30 min. Sweep rate:50 mV/s.


as an improved crystallinity is expected to increase the planarityof the polymer backbone which, in turn, enhances the spatialdelocalization of the polymer polaron, thus lowering the polymerIP.45 The reduction of the donor IP could account for thereduction of the open-circuit voltage often observed in annealedcells.33-35,37,39

Thermal treatments have profound effects on the blendmorphology and a lot of work has been carried out tocharacterize the evolution of active layer morphology uponheating. Usually, as-cast P3HT:PCBM films appear featurelessand homogeneous, without any obvious phase segregation,whereas a coarser film texture is observed upon annealing.39

Transmission electron microscopy investigations have shownthe formation of long crystalline P3HT fibrils upon heating,leading to a nanoscale lateral phase separation and to anorganization of P3HT and PCBM with a large interfacial areafor efficient charge generation and crystalline order for improvedtransport properties.43,46 It is noteworthy that the beneficialeffects of thermal annealing processes on the performance ofBHJ solar cells have been also demonstrated for active layersmade of donors and acceptors different from P3HT47,48 andfullerenes,49 respectively.

Microwave irradiation50,51 has been recently proposed as analternative heating method to enhance the performance of PSC.X-ray diffraction analysis indicated that microwave annealinginduces the crystallization of the polymer phase in P3HT:PCBMactive layers50 with spectral changes that are similar to thoseobserved upon thermal annealing.51 Improved photovoltaicperformance have been demonstrated for P3HT:PCBM devicesthrough microwave treatments, with power conversion efficiencyup to 3%.51

The optimum morphology for the active layer of PSC canbe also successfully promoted at room temperature, through thecontrol of the drying time of the active layer, that is by “solventannealing”41,52-54 or by “solvent vapour annealing”.55 Duringthe spin-coating deposition, the rapid quenching of the solventdisrupts the planar conformation and ordering of the P3HTchains; however, the intrinsic driving force for self-organizationof P3HT can lead to the recovery of the ordered structure, ifthe growth rate of the active layer from solution to solid stateis controlled. Solvent annealing of P3HT:PCBM active layersresults inimprovedopticalpropertiesandenhancedcrystallinity.52,55

For the related devices this translates in a reduced seriesresistance, improved and balanced transport properties,52,53,55 andenhanced PCE. However, the best performance of solvent-annealed devices has been achieved upon an additional thermalannealing process,52 performed on completed devices, i.e., afterthe deposition of the top electrode (the so-called “post-production” thermal annealing). Indeed, it has been observedor demonstrated by different groups34,36,39 that postproductionthermal annealing processes are more effective in enhancingBHJ solar cells performance, indicating that their effects arenot limited to the active layer properties but also extended tothe interface with the top cathode.36

An “annealing-free” approach to achieve an optimizednanoscale morphology is through the construction of orderedprecursors in the solution used for film deposition. It has beenshown that the use of appropriate additives into the host solventrepresents a powerful and simple method for controlling themorphology of PSC,56-60 even when annealing treatments arenot effective in enhancing the device performance.56 Indeed,mixtures of solvents, in which the donor and the acceptor havedifferent solubilities, induce the formation of ordered aggregatesdirectly in solution, leading to an optimum segregated D/A

morphology during the film deposition. The efficiency ofnonannealed PSC made of a low bandgap polymer (PCPDTBT,vide infra) and PC70BM increased from 2.8% to 5.5% by simplyincorporating a few volume percent of 1,8-octanedithiol intothe host solvent (chlorobenzene) without any further annealing.56

The impressive variation of the polymer phase morphology ina PCPDTBT: PC70BM film deposited with the additive is shownin Figure 7.

Also “solid” additives could be effective in improving themorphology of BHJ solar cells, as demonstrated for a copolymerbased on thienothiophene and pentathiophene units, used ascrystallization nucleating agent in P3HT:PCBM active layers.61

The addition of a few percent by weight of the polymer additiveto the P3HT:PCBM solution increased the degree of crystallinityof P3HT and enhanced the performance of the related solar cells.

The control of the nanoscale morphology of the active layeris crucial to ensure optimum charge-carrier photogeneration andextraction. During the last years it has been widely demonstratedthat the strong enhancement of power conversion efficiencyachieved trough different approaches (Figure 8) (thermal an-nealing, solvent annealing, additives) originates from thedevelopment of a morphology with an optimum phase segrega-tion with crystalline domains of different composition. Thatoptimum morphology likely consists of a combination of verticaland lateral phase segregation, as suggested by different groups,42,62

with a donor-rich concentration toward the device anode andacceptor-rich toward the cathode. That gradient concentrationis highly beneficial for solar cells performance, because of theimprovement of the electrode selectivity leading to higher valuesof fill factor.

“Ordered” Bulk-Heterojunction Cells. The active layermorphology is a key factor for the efficiency of BHJ solar cells,because the inherently disordered structure of the blends canlead to both the reduction of carrier mobilities and therecombination between charges of opposite polarity. The idealmorphology to balance exciton dissociation and charge transportrequirements is a columnar segregated structure, perpendicularto the device electrodes, in which the size of each donor oracceptor section is within an exciton diffusion length.3 Thatvertically aligned D/A structure realizes a fully ordered com-bination of vertical and lateral phase segregation, with inde-pendent and straight pathways to the appropriate electrodes for

Figure 7. TEM images of PCPDTBT:PC70BM films, after the selectiveremoval of PC70BM, deposited (a) without and (b) with 1,8-octanedithioladditive. Modified from ref 57 by permission of The AmericanChemical Society.


the two types of carriers. It provides an effective excitondissociation and, at the same time, greatly reduces charge lossesby recombination.

The ideal vertically aligned D/A structure could be accessible(i) through a full-organic approach or (ii) through a hybridorganic-inorganic approach. Some attempts have been madeto self-organize ordered organic D/A bicontinuous architectures.To this purpose, D/A block-copolymers8,63,64 as well as hydrogen-bonding interactions65 have been proposed. Another innovativeconcept for self-assembling D/A nanostructures suitable forcharge separation and transport has been recently proposed byWicklein and co-workers: an organogel acceptor molecule buildsnanostructures in the presence of a donor polymer.66

A very nice, all-organic, approach for obtaining verticallyaligned active layers has been recently proposed by Wang etal.67 P3HT/PCBM core-shell nanorod structures (Figure 9) werefabricated by using a melt-assisted wetting of porous aluminatemplates. The average diameter and length of the nanorods,with a PCBM-rich core and a P3HT-rich shell, were 65 and110 nm, respectively. Solar cells based on the array of verticallyaligned P3HT/PCBM nanorods showed an overall efficiencyof 2.0%, upon thermal annealing at 120 °C. This approach couldbe promising for achieving high efficiency polymer solar cells,if easy methods for film patterning at the nanoscale level willbe developed.

A few groups have proposed the fabrication of verticallyaligned architectures by combining an inorganic semiconductor(nanorod/nanofibril array or a mesoporous inorganic layer) withan organic semiconductor. The inorganic nanostructure acts asa “rigid” template for the infiltration of the organic semiconduc-tor, thus realizing the desired architecture for the active layerof solar cells. Just a few attempts have been reported on hybridsolar cells, in which a conjugated polymer is infiltrated into arigid nanostructured template made of titania68-70 or ZnO.71,72

These scarce and preliminary studies have shown the feasibilityof the hybrid approach; however, poor conversion efficiencieshave been reported so far.

In conclusion of this section, the steep increase of theperformance of PSC achieved during the last years is mainly

due to the development of a variety of protocols for obtainingan optimum phase-segregated D/A morphology. Currently, mostefficient PSC exhibit an internal quantum efficiency (IQE,defined as the ratio of the number of collected carriers to thenumber of absorbed photons) approaching 100%,22 indicatingthat the best morphology, providing both high efficient chargecarriers generation and collection, is attained in those cases. Anearly unity IQE seems to leave little space for furtheroptimization; however, the protocols until now developed andoptimized for particular D/A pairs, mainly for P3HT:PCBM,could not be effective if only one of the two components ischanged in the blend. However, theoretical investigations haveclearly shown that new materials with improved and optimizedelectronic properties are required to further improve the ef-ficiency of PSC; thus the fine control of the active layermorphology will continue to be one of the main issues in thedevelopment of efficient cells.

3. Recent Approaches in the Materials Design

During the last years the attention has been mainly focusedon the control of the nanomorphology of the BHJ active layer,in particular of P3HT:PCBM active layer, and power conversionefficiencies up to 6% have been achieved. Despite the effortsdevoted to overcome the efficiency limitations of PSC,22,36,52,73,74

which still hamper their full commercialization, there is plentyof room for efficiency increase through: (i) the identification ofcandidate D/A materials with improved and optimized electronicproperties; (ii) the fine control of the morphology of the relatedD/A active layers. Several approaches toward the optimizationof the morphology have been already discussed. However,morphology is largely determined by the chemical nature ofthe donor and acceptor components, by their solubility andmutual affinity, so also these factors have to be considered inthe molecular design.

The strategies for improving the electronic properties of theactive materials are indicated by the definition of the powerconversion efficiency. Apart the “quality” of the current-voltage

Figure 8. Approaches to control the nanomorphology of polymer solarcells.

Figure 9. (a) Cartoon representation of the well-ordered P3HT/PCBMnanorod structures. (b) SEM image of the P3HT/PCBM nanorodstructures. The length of the rods is 110 nm. Modified from ref 67 bypermission of The Institute Of Physics Publishing Ltd.


characteristic of the solar cell (the diode quality, not discussedhere, but greatly affecting all the photovoltaic parameters), theenhancement of PCE passes through the enhancement of theshort-circuit current and of the open-circuit voltage. Indeed, thesynthesis of new molecules is moving along two main axis: (i)the development of materials with bandgap structure providinga better optical absorption in the red and infrared portions ofthe solar spectrum (P3HT:PCBM active layers can only harvestsphotons with wavelength below 650 nm); (ii) the developmentof D/A pairs for which the difference between the LUMO levelof the acceptor and the HOMO of the donor is maximized(Figure 1c). The first approach, the low bandgap approach, couldlead to enhanced Isc, if the absorption of the D/A active layer isbroad enough to efficiently collect also photons with higherenergy. Through the second approach, D/A pairs for solar cellsexhibiting higher Voc values (Figure 1c) are obtained. The energylevel engineering is currently considered among the mostpromising strategies to improve BHJ solar cells efficiency,27,75

provided that the D/A pairs (i) exhibit a very efficient photo-induced charge transfer, (ii) have an adequate and complemen-tary absorption cross-section covering most of the solar spec-trum, (iii) demix to produce 20-30 nm size domains for creatingand delivering separated charges to the electrodes, and (iv)exhibit high and balanced charge carrier mobilities.

The molecular design of D/A pairs for high efficiency PSChas to meet a lot of optoelectronic requirements, other than anexcellent processability from solution, very high chemical purity,etc. To this end, an extensive characterization of the newlysynthesized materials is required, involving multidisciplinaryexpertise (Figure 10), to assess their potentials as promisingdonors or acceptors for polymer solar cells. Chemists are makinga great effort in the direction of energy level engineering and avariety of fullerene derivatives and p-type conjugated polymers(vide infra) have been proposed as functional materials towardhigh efficiency PSC.

Energy Level Engineering of Fullerenes. To date, theacceptor materials that gave polymer solar cells with PCE inthe 4-6% range are mostly soluble derivatives of [60]fullereneand [70]fullerene, despite the fact that fullerenes do notrepresent, in terms of HOMO/LUMO levels and optical absorp-tion, the ideal choice for the majority of the donor polymers.27

Higher fullerenes have significantly larger absorption crosssections at longer wavelengths than their [60]fullerene coun-terparts, which complement the absorption profile of the donor.However, the use of fullerenes higher in molecular weight than[70]fullerene,76 with current donor materials and processingconditions, did not contribute to major improvements of deviceperformance. On the other hand, trimetallic nitride endohedralfullerenes, such as Ih-Sc3N@C80

77 and Lu3N@C80,78 showed thepotential to replace [60]fullerenes and [70]fullerenes as optimalelectron acceptor in solar cells. The latter gave PCE higher than4% with interesting perspectives toward higher efficiencies.Also, functionalized graphene has been recently employed asacceptor component in polymer solar cells79 with PCE of around1%, although more work is needed to assess its potential infuture-generation organic photovoltaic materials.

Over the past few years, the benchmark acceptor materialfor polymer solar cells has been PCBM, although a wide varietyof other functionalized analogues and heteroanalogues have beenproposed to improve processability or HOMO/LUMO levels10,80-90

(Figure 11).In PCBM, most of the peculiar electronic properties of native

[60]fullerene are preserved or slightly affected, for adequate cellperformance. Furthermore, the structure of the phenyl-butyrateaddend provides enough solubility to process PCBM in a varietyof solvents for a fine-tuning of the level of supersaturation,which is a physical parameter playing a fundamental role toestablish D/A phase separation in polymer/fullerene blends.Kronholm and Hummelen14 published a comprehensive accounton fullerene-based acceptors, in which the impact on solar cells

Figure 10. Nonexhaustive list of investigation techniques required for an extended characterization of active materials for polymer (organic) solarcells.


performance of physical parameters such as solubility, super-saturation thermodynamics, precipitation kinetics and miscibilitywith donor materials, is reviewed. Once the optimal solvent wasfound for a given donor material, the solubility of the fullerenecomponent seemed, at a first glance, a simple and obviousparameter to consider for morphology control toward cellimprovement. However, attempts to increase the acceptorsolubility, by changing, for instance, the methyl ester of PCBM,gave no major PCE improvements.89 In a recent study, Troshinand co-workers80 observed a general dependence of all solarcell parameters on the solubility of the fullerene acceptor for awide variety of methanofullerenes bearing different organicmoieties. The authors concluded that the best material combina-tion for a performing BHJ solar cell is that where donor andacceptor components are of adequate and similar solubility inthe solvent used for the deposition of the active layer. A varietyof functionalized fullerenes have been synthesized in the pastfew years and tested in BHJ solar cells by several groups34,88,91,92

without significant PCE improvements, if compared to PCBM.Quite surprisingly, PCBM shows an adequate solubility forsolution processing; it produces a desired nanomorphology incombination with donor material counterparts, such as P3HT,in chlorinated aromatic solvents, after a relatively mild thermaltreatment. This suggests that changing addend moieties on thefullerene sphere is unlikely to produce a real breakthrough incell performance.14

New avenues to highly efficient BHJ solar cells would bethose producing substantial changes in the properties of thefullerene π-system. To this end, synthetic efforts have beenstrengthened to design novel fullerene acceptors with higher-lying LUMO levels than PCBM, to increase the VOC that, forthe P3HT:PCBM pair, is around 0.6 V.10,22,78,84,85 It has beenreported that for an optimized ∆ELUMO of ∼0.3 eV, a PCEaround 10% can be realistic.27,75 Kooistra and co-workersshowed that the LUMO level can be increased by addingelectron-donating moieties, such as methoxy- or methylthio-,to the PCBM phenyl ring.85 However, this produces only a shiftsmaller than 100 meV. More recently the same group reporteda better solution to LUMO variation by using PCBM bisad-ducts.84 Alternative to diazoalkane cycloaddition, the additionof Grignard/organolithium reagents to [60]fullerene gives accessto 1,2- and 1,4-di(organo)fullerenes in very high yield and

excellent purity.93 Grignard chemistry may give a better chanceto influence [60]fullerene electronic properties if compared toother addition reactions, because it allows us to install functionalgroups directly on the carbon atoms of the spheroid.

Energy Level Engineering of Electron-Donor Materials.Many efforts are devoted toward a rational design of newconjugated polymers with the long-term objective to overcomethe problems suffered by the benchmark P3HT because of itsbandgap close to 2 eV. The need of low bandgap (LBG)polymers, especially in the 1.4-1.9 eV region, for a better matchwith the solar spectrum stimulated the synthesis of a variety ofpromising structures that have been tested in polymer solar cells,mostly coupled with [60]fullerene or [70]fullerene as acceptors.In many cases, the LBG polymers were copolymers made ofalternating electron-rich and electron-poor building blocks.94

Figure 12 shows a nonexhaustive collection of LBG semicon-ducting polymer structures that are considered promising donormaterials for polymer solar cells.

Thiophene, fluorene, carbazole, thiadiazole, and quinoxalineare among the constituent building blocks of LBG copolymersthat have been used as donors. The synthesis, characterization,and impact on practical devices of these new polymers havebeen recently reviewed.14,75 Among them, it is worth mentioningalso the metallopolyyne polymers that are π-conjugated materi-als in which a metal atom is integrated into the polymerbackbone. Wong and co-workers used platinum polyynes, ofgeneral formula [-Pt(PBu3)2sCtCsXsCtC-]n where X )arylene or heteroarylene, in BHJ solar cells.102 Although someaspects of the metalated polyyne performance are still contro-versial,103 these materials are interesting candidates to explorethe role played by triplet excited states and triplet excitons incharge generation and fate.104,105

Despite the wide variety of performing LBG polymers thathave been produced in the past few years, the PCE of prototypedevices is still moderate. Indeed, the highest reported efficiencyfor LBG polymer/fullerene single-junction cells is 5.5%,achieved with the blend PCPDTBT:PC70BM,56 while that forP3HT:PCBM cells is 5.4% (certified by NREL).106 Consideringthat, differently from PCBM, PC70BM slightly contributes inharvesting solar light, the two values seem comparable. Whyare LBG polymer solar cells slow to develop into high-efficiencydevices? As a matter of fact, with chemical synthesis one can

Figure 11. Selected functionalized fullerenes, other than PCBM and PC70BM, that have been used in polymer solar cells.


lower the bandgap of a conjugated polymer by moving eitherthe HOMO level toward the vacuum level or the LUMO levelaway from the vacuum level (Figure 1c). However, the formeraction produces a detrimental reduction of VOC, whereas thelatter could push the ∆ELUMO away from the minimum energyoffset, which is required for electron transfer. This scenario isfurther complicated by the fullerene accepting material whoseLUMO level, which is fairly difficult to raise,84,85 limits that ofthe polymer.

Lowering the bandgap of a conjugated polymer to absorbvisible and near-infrared radiation, while providing efficientcharge generation, transport, and collection in a polymer solarcell is still a big challenge. In this connection, it should be notedthat LBG materials are particularly important to build efficienttandem solar cells,25 in which stacked individual subcells withdifferent bandgaps are required. Indeed, photons not absorbedin the front cell are transmitted to the second one, absorbingthe higher-energy portion of the solar spectrum, thus leadingto higher PCE due to the absorption of a larger portion of thesolar spectrum.

A variety of other approaches toward new and potentiallyuseful π-conjugated materials, recently proposed in the literature,should be mentioned for reference, such as thermocleavableLBG polymers that improve devices stability107 and graded-bandgap polymers for efficient light harvesting.108 Also, func-tional dendritic thiophene oligomers,109 long-wavelength squaraineabsorbers,110 or LBG oligomers111 have shown promises aseffective donor component in BHJ solar cells.

As a concluding remark, it is worth stressing that the D/Apair should be regarded as a single entity; to this purpose, linkedD-A molecular structures have been also proposed during theyears (vide infra). The development of a new donor cannot bedone without taking into account the nature of the acceptorintended to be matched to, and vice versa. As explained at thebeginning of this section, the achievement of high Voc dramati-cally depends on both the pair components, and even the “right”donor could fail in the performance if matched to the “wrong”acceptor (and vice versa). However, an increased open-circuitvoltage of 200-300 mV (Table 1) as well as an enhancedefficiency22 have been already achieved, with respect to thebenchmark P3HT:PCBM, through the energy level engineering.The further optimization will be a continuing process and theefficiency of PSC could approach the predicted 10% throughlittle steps forward.

Linked D-A Molecular Structures. Blending D and Acomponents is, currently, the most effective approach to efficientPSC. Other approaches to generate BHJ solar cells include theso-called double-cable concept,112,113 supramolecular p-nheterojunctions,114,115 and the self-assembly of semiconductordiblock copolymers.116-119 Double cables are ambipolar materi-als in which functionalized fullerenes (n-cable) are covalentlylinked to a conjugated polymer backbone (p-cable). Althoughtheir photovoltaic performance is far from being attractive forthe market, double cables synthesis, characterization and testingin solar cells shed light on fundamental issues, such as chargetransport and phase separation, paving the way toward func-

Figure12. Molecularstructuresofselectedlowbandgapcopolymersthathavebeenusedinpolymersolarcells:(1)poly(cyclopentadithiophene-benzothiadiazole):1a (PCPDTBT),95 1b,96 1c;97 (2) poly(indolocarbazole-oligothienylbenzothiadiazole);98 (3) poly(thienothiophene-benzodithiophene);99 (4)poly(thiophene-thienylenevinylene;52 (5) poly(bithiophene-thiadiazoloquinoxaline);100 (6) APFO-Green 9.101

TABLE 1: Some Examples Showing the Variation of Voc with the Energy Levels of the D and A Components of BHJ SolarCells

donor:acceptor HOMOD (eV) LUMOA (eV) LUMOA - HOMOD (eV) Voc (V) ref

P3HT:PCBM -5.1 -4.3 0.8 0.63 36P3HT:bisPCBM -5.1 -4.2a 0.9 0.73 84P3HT:Lu3N@C80 -5.1 -4.0a 1.1 0.81 78PCDTBT:PC70BM -5.5 -4.3 1.2 0.88 22oligothiophene:PCBM -5.5 -4.3 1.2 0.93 109

a Estimated from the shift of the first reduction potential with respect to that of PCBM.


tionalized diblock copolymers as active layers instead of D/Ablends. In the all-conjugated version,118 block copolymers offerthe unique opportunity to fine-tuning either the HOMO andLUMO energy levels of the conjugated blocks or the morphol-ogy of the active layer.8,63,64

The D-A linked approaches also include the use of a varietyof molecular structures, made of a functionalized fullerenecovalently linked to oligomeric, conjugated architectures. Since1999120 many molecular diads and triads have been studied121-123

to elucidate fundamental issues such as the nature, ratio andorientation of the A and D moieties as well as the nature/lengthof their connecting fragment.124,125 A PCE around 1.3% is thehighest reported value for a dyad-based, single-component solarcell to date.126

4. Summary and Outlook

Three elements will have a crucial impact on the success ofpolymer solar cells as an emerging technology in photovoltaics:(i) efficiency, (ii) durability, and (iii) cost.

The first issue has been extensively discussed thus far. Afterthe pioneer studies dating back to 16 years ago, over the last 8years the efficiency of PSC has undergone an impressiveprogression127 that seems still far from coming to an end. Thespaces for improvement lie mainly in two areas. The first oneconcerns the optimization of the materials used for lightharvesting and carrier generation, transport, and collection, bytuning their optical and electronic properties. The second areapertains to new device architectural designs and to the controlof nanoscale morphology, ensuring the maximum light absorp-tion, the exciton dissociation, the transport of the carriers intwo distinct phases, and their extraction at the contacts. Hugeefforts in research and development, both in industry and inacademia, are devoted to these subjects.

Currently PSC are already capable of reasonably high powerconversion efficiencies; up to 10% for single junction devices27

and up to 15% for tandem cells128 are technically feasible bysimply optimizing the donor materials, as shown by theoreticalconsiderations.

As for durability of polymer solar cells, it has been reportedthat flexible modules based on P3HT:PCBM, encapsulated withcommercial barrier coatings and tested under outdoor (over 1year) or accelerated conditions, did not show significantperformance losses.129

The overall cost of PSC is affected by two factors: costsassociated with materials and costs arising from fabrication. Thecontribution given by materials cost should be reasonably small,in consideration of the very low amounts required to obtain filmsonly a few hundreds nanometers thick. Organic semiconductorscan be very easily solution-processed on large areas by applyingprinting techniques with high throughput (i.e., roll-to-rollprocessing).130 Such techniques are inherently cheap. Further-more, the low cost of PSC balances the lower efficiency anddurability as compared to inorganic solar cells, making thistechnology economically competitive. A cost of electricity of0.1 €/(kW h) in Middle-Europe is expected according to adetailed economic analysis.75 In the past few years a numberof companies interested in the production of polymer photo-voltaics started their activity131,132 and the first commercialdevices are going to be introduced on the market.131

Acknowledgment. N.C. thanks ENI SpA (contract number4700007315). M.M. thanks MUR (FIRB RBNE033KMA),Fondazione CARIPARO (MISCHA project), and University ofPadova (HELIOS project) for financial support.

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