Materials Design Considerations for Charge Generation in OrganicSolar CellsStoichko D. Dimitrov and James R. Durrant*

Centre for Plastic Electronics and Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom

ABSTRACT: This article reviews some of our recent progresson materials design guidelines for photoinduced chargegeneration in bulk-heterojunction organic solar cells. Overthe last 7 years, our group has employed transient absorptionmeasurement to determine the relative quantum yields of long-lived polaron pairs for over 300 different organic Donor/Acceptor blend films. We have shown that this optical assay ofcharge separation can be a strong indicator of photocurrentgeneration efficiency in complete devices. In this review, weconsider the lessons that can be drawn from these studiesconcerning the parameters that determine efficiency of this photoinduced charge separation in such solar cells. We consistentlyfind, from studies of several materials series, that the energy offset driving charge separation is a key determinant of the efficiencyof this charge generation, and thereby photocurrent generation. Moreover, we find that the magnitude of the energy offsetrequired to drive charge separation, and the strength of this energetic dependence, varies substantially between materials classes.In particular, copolymers such as diketopyrrolopyrrole- and thiazolothiazole-based polymers are found to be capable of drivingcharge separation in blends with PCBM at much lower energy offsets than polythiophenes, such as P3HT, while replacement ofPCBM with more crystalline perylene diimide acceptors is also observed to reduce the energy offset requirement for chargeseparation. We go on to discuss the role of film microstructure in also determining the efficiency of charge separation, includingthe role of mixed and pure domains, PCBM exciton diffusion limitations and the role of material crystallinity in modulatingmaterial energetics, thereby providing additional energy offsets that can stabilize the spatial separation of charges. Other factorsconsidered include the role of Coulombically bound polaron pair or charge transfer states, device electric fields, charge carriermobilities, triplet excitons, and photon energy. We discuss briefly a model for charge separation consistent with these and otherobservations. We conclude by summarizing the materials design guidelines for efficient charge photogeneration that can be drawnfrom these studies.

KEYWORDS: polymer, fullerene, transient absorption spectroscopy, charge transfer states, photocurrent, charge separation

■ INTRODUCTION

Organic solar cells (OSC) are showing rapid advances inefficiency and potential for technological application. The mostefficient single-junction polymer/fullerene OSC are nowapproaching 10% power conversion efficiencies under labo-ratory conditions,1−8 with increasing confidence that efficien-cies of 15% or higher will be achievable at least with multi-junction devices. These advances in efficiency have largely beenachieved by the synthesis and testing of new materials anddevice architectures, including, in particular, new photoactivelayer materials whose energy levels have been modulated toachieve higher output voltages and/or improved photocurrentgeneration due to lower optical bandgaps.1,2,4,7,9 However, atpresent, our ability to predict device function from materialsstructure remains relatively limited. As a consequence, the vastmajority of new materials tested have not performed betterthan the ongoing state of the art, typically achieving sig-nificantly lower device performance than that predicted bysimple, energy level based, device models.10,11 A key challengefor this field is therefore the development of improved materialsdesign guidelines to increase the effectiveness of materials

synthesis and screening programmes. In this review, we addressone aspect of this materials design challenge, namely, thematerials design requirements for efficient photocurrent gen-eration and, in particular, the requirements for a high quantumyield photoinduced charge separation.In practice, for most new photoactive layer materials, the

quantum efficiency of photocurrent generation in optimizedsolar cells is found to be significantly below unity. Identifyingthe cause(s) of this underperformance in photocurrent genera-tion is therefore a key consideration for materials design. Theefficiency of photocurrent generation in OSC can be brokendown into four factors, the efficiencies of photon absorption(ηabs), exciton diffusion to the donor/acceptor (D/A) interface(ηdiff), charge separation at this interface (ηsep), and chargecollection by the external circuit (ηcoll) (see Figure 1).

12−14 The

Special Issue: Celebrating Twenty-Five Years of Chemistry ofMaterials

Received: July 17, 2013Revised: September 9, 2013

Review

pubs.acs.org/cm

© XXXX American Chemical Society A dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXX

development of new, lower bandgap polymers has resulted insignificant, and easily measured, improvements in ηabs, althoughat the expense of lower energies of the resultant photoexcitedstates (excitons). Similarly, the bulk-heterojunction (BHJ)strategy, based on an interpenetrating blend of donor andacceptor domains, has proved, for many photoactive layers,to be remarkably effective at enabling efficient exciton diffusionto the D/A interface. In addition, for many devices and, inparticular, for blends of donor polymers with reasonable holemobilities blended with the soluble fullerene acceptor PC60BM([6,6]-phenyl C61-butyric acid methyl ester), the chargecollection efficiency ηcoll is found to approach unity at shortcircuit (it should be noted that the efficiency of charge col-lection is, however, a key consideration for overall device effi-ciency, with the competing process of nongeminate recombi-nation limiting the voltage output, and often the fill factor,of most devices).11,15−20 In some cases, a tension between ηcolland ηabs is observed as a function of photoactive thickness, withefficient charge collection only being obtained for filmthicknesses too thin for efficient light absorption.21−23 How-ever, in general, it appears that, for many new photoactivelayers and, in particular, blends of new donor materials withPCBM, the observation of suboptimal photocurrent densitiesoften derives neither from inefficiencies in ηabs, ηdiff, nor ηcoll.Rather, as we discuss in this review, such suboptimal pho-tocurrent densities often result from limitations in chargeseparation efficiency, ηsep. As such, this review focuses uponevidence that the efficiency of charge separation indeed limitsphotocurrent generation in many OSC and, more importantly,upon the material parameters that determine this efficiency.Understanding of the device physics of OSC has improved

significantly in recent years, with semiempirical device modelsnow able to reconstruct the open circuit voltage, and in somecases the fill factor, of a device relatively well.18,20,24−29 How-ever, models to calculate the photocurrent density of devicesfrom materials parameters are relatively limited. Most deviceefficiency predictions assume unity efficiency of excitondiffusion ηdiff to the D/A interface (i.e., that the domain sizesare much less than the exciton diffusion length) and thatunity efficiency of charge separation ηsep is achieved providedthat the energy offset between the polymer and fullerenelowest unoccupied molecular orbitals (LUMO) is at least

0.3 eV.11,30,31 Such a simplistic view of photocurrent gen-eration, however, is clearly unable to explain the large variationsin photocurrent generation efficiency observed betweendifferent photoactive layer materials. One additional factorthat has been identified as potentially limiting photocurrentgeneration is the formation of Coulombically bound charges atthe D/A interface.12,14,32−38 The low dielectric constant oforganic semiconductors (εr = 3−4) suggests that such boundstates could have coulomb binding energies of several hundredmeV (≫ kBT). There have been extensive studies of suchbound states, which we will refer to herein as bound polaronpair states (BPP). We note that such states have also beenreferred to as charge transfer (CT) states (due to the obser-vation of sub-bandgap optical absorption or emission) orexciplex states (due to the observation of exciplex-likeemission). Several studies have provided evidence that thesestates can undergo relatively rapid charge recombination (ontime scales from picoseconds to ∼100 ns),35,36,39−42 with thisrecombination pathway potentially being a key limitation onthe efficiency of charge separation, and thereby photocurrentgeneration.12,37,38,41−59 The extent to which this chargerecombination channel at the D/A interface indeed competeswith the charge separation process in OSC, and the conse-quences of this for materials design, is a subject of extensivediscussion in the current OSC literature.35,37,53,54,60−63

In this review, we discuss recent advances in our under-standing of charge photogeneration in polymer/fullerene OSC,with the aim of developing empirical materials design rules forefficient photocurrent generation. We start by introducing theprimary experimental approach we have employed, transientabsorption spectroscopy (TAS), and considering the evidencethat this approach is indeed an effective assay of charge sep-aration efficiency in OSC. We then focus on a key con-sideration for the thermodynamic efficiency of such solar cells,namely the correlation between energetic offset driving chargeseparation and the quantum efficiency of this charge separationprocess ηsep. This is followed by a discussion of the role of filmmicrostructure and material crystallinity, again focusing uponηsep. We then discuss other factors we observe to influence ηsep.These considerations are then summarized and discussed interms of empirical materials design guidelines for efficientcharge photogeneration for OSC.

Background to Photoinduced Charge Generation inOSC. Photocurrent generation in OSC is fundamentally dif-ferent from that of the standard p−n junction solar cells. Theorigin of this difference lies in the low permittivity of organicmaterials and their inherent energetic and structural disorder.Unlike most inorganic semiconductors, light absorption bypolymer and fullerene films generates localized excited statesthat are tightly bound electron and hole pairs. The bindingenergies of these so-called excitons can be more than an orderof magnitude higher than the thermal energy, therefore makingtheir separation a key issue for efficient photocurrent gen-eration. This issue was successfully addressed by Tang, who in1986 first used a heterojunction of two organic materials withdifferent electron donor and acceptor properties to provide anenergetic driving force for the dissociation of the photo-generated excitons.64 This breakthrough device was based on abilayer heterojunction of copper phthalocyanine and a perylenetetracarboxylic derivative sandwiched between transparent ITOand Ag electrodes, and achieved ∼1% power conversionefficiency (PCE).

Figure 1. Simplified illustration of the typical architecture of a BHJpolymer/fullerene organic solar cell. The diagram depicts the fourprocesses leading to photocurrent generation in the photoactive layer:light absorption (by the polymer in this case), exciton diffusion to D/Ainterface, charge separation at this interface, and charge collection atthe electrodes. The diagram illustrates both finely intermixed andrelatively pure polymer and fullerene domains, as well as variations inlocal material crystallinity; the relative proportions of these structuralfeatures are strongly dependent upon the choice of photoactive layermaterials and film processing conditions.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXB

The photocurrent of most organic bilayer heterojunctiondevices, such as that reported by Tang et al.,64 is, however,limited by the short exciton diffusion length of organicmaterials. Polymers and fullerenes, for example, are typicallyreported to have exciton diffusion lengths shorter than 10 nm,thus substantially limiting the thickness of the active layer andits light harvesting properties.65−69 One approach to addressingthis limitation is the use of BHJ films; this approach has provedparticularly effective for polymer/fullerene-based devices.70

These are typically fabricated as random bicontinuous inter-penetrating networks of a p-type polymer and n-type fullerenein which the D/A heterojunction area is very high; as a result,the likelihood of excitons reaching the D/A interface is sub-stantially increased. Figure 1 shows a standard device archi-tecture excluding hole and electron blocking layer, which arenot discussed in this review. The active layer of the device ispresented as a polymer/fullerene BHJ with pure and intermixeddomains and variations in polymer crystallinity; the relativeproportions of pure and intermixed domains, and materialcrystallinity, are thought to vary substantially between differentdonor and acceptor materials and film processing conditions.OSC based on such BHJ architectures are now achieving closeto 10% PCE.1−8

Transient Absorption Assay of Charge Photogenera-tion Efficiency. The primary experimental technique we haveemployed to assay the efficiency of charge generation in D/Ablend films and solar cells is nanosecond to millisecond TAS.This is a relatively straightforward pump−probe technique thatmonitors the transient absorption of photogenerated chargecarriers, with the primary requirement being a high detectionsensitivity, enabling signal magnitudes to be measured reliablywith low light excitation densities, thereby avoiding nonlinearand saturation effects in the active layer.56,71−73 Nanosecondlaser pulses are used as an excitation source, while the output ofa tungsten lamp probes the changes in the optical density of thefilm or device induced by the laser excitation. A Si or InGaAsphotodiode, connected to an amplifier and oscilloscope is usedfor detecting and recording the transient absorption signals onnanosecond to millisecond time scales. On this time scale,dissociated polarons can be identified by their characteristicpower law decay dynamics and nonlinear behavior at highexcitation densities, indicative of nongeminate recombina-tion.72,74 In contrast, geminate recombination, for an exampleof Coulombically bound polaron pair states, exhibits distinctexponential (or stretched exponential) dynamics, linearbehavior as a function of excitation density, and relativelyfast decay times (picoseconds to ∼ a hundred nano-seconds).35−37,39,47,75,76 Triplet states, which can also contrib-ute to the transient absorption signals on these time scales, canbe often distinguished from these polaron states by their moreexponential decay dynamics and quenching in the presence ofmolecular oxygen.77 As such, the magnitude of the transientpolaron absorption (ΔOD = change in the optical density) canbe employed as a direct, at least semiquantitative, assay of thequantum efficiency of generation of dissociated charges. Thereare, of course, limitations to this approach, including for ex-ample differences in polaron absorption coefficient (whichmeans that comparison of polaron yields can only be donequantitatively when comparing blends of similar chemicalcomposition) and the potential for some nongeminate recom-bination to occur on time scales faster than the instrumentresponse (avoided or minimized by the use of sufficientlylow excitation densities such that the temporal onset of

nongeminate recombination occurs within the resolved timerange; see, for example, ref 40). The simplicity of this approachhas allowed us to undertake comparative studies of the effi-ciency of charge generation for a large number of photoactivelayer materials, blend compositions, and processing conditions,now comprising over 300 such distinct film types (including 70different donors and 20 different acceptors, as well as a range ofblend compositions and film processing conditions). Theresults of many of these studies have been publishedpreviously.7,37,40,56,78−88 This large body of data has allowedus to identify statistically significant empirical materials cor-relations that can be difficult to observe from smaller data sets.In this review, we aim to summarize some of the key findings ofthese studies.We have reported strong evidence that our transient

absorption assay of charge generation in blend films is indeeda good indicator of the efficiency of photocurrent generation indevices. In particular, we have observed a clear correlationbetween the amplitude of our transient absorption assay ofpolaron yield, ΔOD, and photocurrent density JSC/maximalIQE for a broad range of polymer/PCBM blend films over 3orders of magnitude in signal size (Figure 2).89 This correlation

is clear evidence not only of the effectiveness of our empiricalapproach as an indicator of photocurrent generation but alsothat, for many such blend films, ηcoll is not a key factordetermining JSC. It should be noted that this correlationbetween ΔOD and JSC is not observed for all photoactivelayers; for example, it breaks down for donor/acceptor blendsemploying perylene diimide acceptors and for donor poly-mers with low hole mobilities (specifically FET mobilities of<10−4 cm2 V−1 s−1), in both cases assigned to nongeminatelosses during collection limiting photocurrent generation inthese blend films.86,89 In addition, for some, notably amorphousblend films, the device photocurrent is observed to be sig-nificantly larger than that predicted from our film ΔOD assay;we have shown that this behavior is associated with a strongelectric field dependence of ηsep, as we discuss further later.90

Notwithstanding these exceptions, in general, we observe a

Figure 2. Correlation between a ΔOD assay of the yield of dissociatedpolarons measured by transient absorption spectroscopy of thin blendfilms and JSC (×) and IQErel (squares) measured in complete devicesfor a series of polymer/PCBM blends as a function of polymer, blendcomposition, and thermal annealing. Filled circles correspond toblends with two low hole mobility polymers, where the low holemobility results in low photocurrent due to collection losses, the opencircle corresponds to the indenofluorene polymer IF8TBTT,89 and, forthis polymer, the deviation from the correlation line is assigned to fielddependent charge generation. Reprinted with permission from ref 89.Copyright 2010, WILEY-VCH Verlag GmbH & Co. KGaA,WeinheimQ.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXC

clear, at least semiquantitative, correlation between ΔOD andJSC for many photoactive layers. As such, for the rest of thisreview, we focus primarily upon the parameters that we observeempirically to determine the magnitude of our ΔOD assay ofpolaron generation.It should be noted that our transient absorption assay of

polaron yield does not distinguish between trapped and freepolarons. There is extensive evidence that most organic pho-toactive layers exhibit an exponential tail of states extendingenergetically into the bandgap of the layer, below the mobilityedge of at least one material (most notably the donor polymer).Following charge separation, polarons will relax down thisdensity of states to the film quasi-Fermi levels. Both non-geminate recombination and transport of photogeneratedcharge carriers have been suggested to occur primarily throughthermally driven detrapping of these energetically trappedcarriers.22,91,92

■ ENERGETICS OF CHARGE GENERATIONDefinitions and Measurements of Energy Offset. From

an efficiency viewpoint, the energy loss associated with pho-toinduced charge separation is a critical consideration. Theenergy difference between the optical bandgap of the deviceand the maximal free energy of the photogenerated chargecarriers, as measured by its open circuit voltage, is a keymeasure of thermodynamic efficiency. In another viewpoint,charge separation is necessary to give the photogeneratedspecies sufficient lifetime to be collected by the external circuitas an external photocurrent; however, driving and stabilizingthis charge separation requires energy losses. This balancebetween kinetics and energetics is a key cause of why increasesin device photocurrent output are often achieved at the expenseof cell voltage, or vice versa.There is currently considerable variation in the literature over

how the energy difference driving charge separation in OSCshould be defined and how it can be measured.93 We willconsider first different definitions of this energy offset and thenhow these can be measured. For simplicity, we will only con-sider charge separation from polymer excitons. Chargeseparation from fullerene excitons is discussed briefly and fol-lows similar energetic arguments.Figure 3 illustrates two different descriptions of the ener-

getics of charge separation, one based on the electronic orbitalenergies and the other on state energies. In the electronicorbital picture, the energy offset driving charge separation isoften described as the LUMO−LUMO level offset ΔELUMO. Ithas been widely reported that ΔELUMO > 0.3 eV is required todrive charge separation, although we note that there is verylittle experimental evidence for this specific energy offsetrequirement. This 0.3 eV energy offset requirement has beendescribed as being necessary to overcome the binding energyof the excitons generated in organic semiconductor films,typically estimated at 0.3 eV.11,13 This exciton binding energyis also considered the reason for the optical bandgap (Eg) ofmost organic semiconductors being less than their elec-tronic bandgap. It should be noted that, in this viewpoint, aΔELUMO > 0.3 eV requirement corresponds to an approximatelyzero energy difference between the exciton energy (as ap-proximated by Eg) and the blend electronic bandgap (IP−EA,where IP is the electron donor ionization potential and EA isthe electron acceptor electron affinity). As such, this require-ment ΔELUMO > 0.3 eV is equivalent to requiring that theenergy offset driving charge separation, after accounting for

the exciton binding energy, to be greater than zero. Analternative viewpoint is to consider state energies, as illustratedin Figure 3b. As we discuss, we use the energy difference ΔECSbetween the polymer exciton, ES1, and the polaron energies IPand EA ΔECS = (IP−EA) − ES1, as a measure of the energyoffset driving charge separation. ΔECS is related to the ΔELUMOby ΔECS = ΔELUMO − Eexc

b. Eexcb represents the exciton binding

energy. Also illustrated on both diagrams is the energy of BPPstates EBPP, with their coulomb binding energy relative toIP−EA being given by EBPP

b.The above discussion employs IP and EA as measures of the

polaron energies. For semiconductors, these correspond to theconduction and valence band edge enthalpies. It is important todistinguish these energies from the free energies of electronsand holes in the blend, as defined by the electron and holequasi-Fermi levels. The splitting of the quasi-Fermi levels bylight irradiation determines the voltage output of the device(often referred to, at open circuit, as eVOC, in the absence ofother voltage losses) and corresponds to the energy stored bythe photogenerated electrons and holes following thermal-isation with these Fermi levels (CSTR in Figure 3b). In general,the quasi-Fermi levels lie within the electronic bandgap of thefilm, such that the free energy of photogenerated polaron pairsis less than the electronic bandgap of the film. This difference inenergy between IP−EA and eVOC derives most simply from theincrease in entropy of the electrons and holes as they separatefrom the interface (readily calculated from simple degeneracyarguments to be several hundred meV),13 as well as thermalrelaxation/trapping processes as the polarons relax down theintraband density of states discussed above. The free energy lost

Figure 3. Energy level diagrams of a polymer/fullerene D/A interfaceduring photocurrent generation shown as (a) an electronic orbitalenergy diagram illustrating material conduction and valence bandenergies and (b) a state energy diagram illustrating the relativeenergies of singlet exciton (S1) and charge separated (CS) statesrelative to ground state (S0). Both diagrams also include illustration ofthe energetics of bound polaron pair states. Eexc

b is the binding energyof the singlet exciton; EBPP

b is the equivalent binding energy of theBPP states. ΔELUMO is the donor/acceptor LUMO−LUMO offset. EF

e

and EFh are the electron and hole quasi-Fermi levels of the blend under

device operation; the splitting of these levels corresponds to the freeenergy of photogenerated charge carriers after thermalisation to theseFermi levels (this thermally relaxed charge separated state is illustratedas CSTR in part b). At open circuit, the energy of CSTR corresponds toeVOC (in the absence of other voltage losses). ΔECS is the enthalpydifference driving charge separation defined as the difference inenthalpy between the singlet exciton energy (ES1) and the enthalpy ofthe charge separated polarons at their respective material band edges,as given by IP−EA. ΔGCS corresponds to the overall free energydifference associated with charge photogeneration, including theenthalpy difference ΔECS and the entropy gain/energetic lossesresulting from thermalization with the film quasi-Fermi levels.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXD

during this overall charge separation process to polaronsthermalized with the film quasi-Fermi levels is illustrated inFigure 3b as ΔGCS.

94 We note that in our initial discussion ofthe energetics of charge separation,13,56 we did not make thisdistinction between ΔECS and ΔGCS.In terms of experimental measurements of the various energy

offsets illustrated in Figure 3 as driving charge separation, here-in, we employ ΔECS = (IP−EA) − ES1 where ES1 is measuredfrom optical spectroscopy data and IP and EA fromphotoelectron spectroscopy or cyclic voltammetry data. Wenote that these measurement techniques have the potential forsignificant systematic errors (for example due to the presence ofan exponential density of tail states complicating thedetermination of the band ‘edge’); as such, we consider relativemagnitudes of ΔECS rather than its absolute value. An alter-native energy offset measurement used in the literature is basedon electroluminescence of emissive interfacial charge transferstates, which determines the difference in energy between theemissive exciton and CT states.95 A third measure employs thedifference between the optical bandgap and open circuit voltageof the device (Eg − eVOC), this assay corresponds (at opencircuit) to ΔGCS, as defined above, and differs from ΔECS bythe energy loss associated with entropy gain and trapping.96 Wenote that this latter, free energy assay of energetic offsetdepends upon the magnitude of the quasi-Fermi level splittinggenerated under solar irradiation and will therefore dependupon several additional factors, including the irradiation in-tensity and the nongeminate recombination rate constant.Exciton Quenching versus Polaron Generation: The

Impact of Materials Energetics. Charge separation indonor/acceptor blend systems can be most simply describedas electron transfer from donor excitons to generate positiveand negative polarons (for acceptor excitons, this correspondsto a hole transfer process). In this simple picture, the efficiencyof exciton quenching at the D/A interface should correlatedirectly with the yield of photogenerated charges. There is nowextensive evidence that this correlation is not observed. In theextreme case of donor polymers with very low optical ban-dgaps, blending with PCBM does not result in strong polymerphotoluminescence quenching, indicative of unfavorableenergetics for exciton quenching at the D/A interface. How-ever, for almost all the donor polymers we have studied, wehave observed remarkably efficient polymer photoluminescencequenching (PLQ) following blending with PCBM, typically inexcess of 90%, and often >98% (in this regard, P3HT is ratheranomalous, exhibiting modest PLQ (70−90%) attributed toits more complete phase segregation and the resultant excitondecay losses during diffusion to the P3HT/PCBM inter-face).46,56,79,97 Despite this remarkably ubiquitous highefficiency of polymer PLQ, indicative that polymer excitonsare indeed being quenched at polymer/fullerene interfaces, theyields of dissociated photogenerated charges, as assayed byour ΔOD measurement, are observed to vary very significantlybetween blend films. This distinction between consistentlyefficient PLQ and strongly varying yields of charge generationwas first reported by Ohkita et al. in a study of a series of poly-thiophene based polymers blended with PCBM, as illustrated inFigure 4,56 and has since been observed for several otherpolymer/fullerene, polymer/perylene, and polymer/quantumdot systems.40,46,80,86,98 This observation is important for anumber of reasons. First, it indicates that photoluminescencequenching is not a reliable assay of charge generation in organicblend films. Second, as the yield of dissociated charges does not

correlate with the efficiency of exciton quenching, it suggeststhe presence of a competing pathway following excitonquenching at the interface that competes with the generationof long-lived dissociated charges. As we discuss later on, there isnow extensive evidence that this competing pathway involvesthe formation of interfacial BPP states.13,35,36,41,42,47

We have conducted several studies to investigate how theyield of dissociated charges varies with material ener-getics.40,79,80,86,88 We have not observed strong correlationswith either the absolute exciton or polaron pair energies, norwith the magnitudes of these energies relative to estimates ofmaterial triplet state energies (further discussion of this point isfound later on). However, we have observed a strong cor-relation between our polaron quantum yield assay, ΔOD, andthe difference in energy between the exciton and polaron pairs,as defined as ΔECS = (IP−EA) − ES1. This is illustrated inFigure 4 for the data reported by Ohkita et al.,56 which showsthat, for this series of donor polymers, the charge photo-generation yields increases with 2 orders of magnitude for a300 meV increase in ΔECS. This observation strongly indicatesthat the energy offset ΔECS is a key determinant of chargeseparation efficiency ηsep and thereby photocurrent generationin OSC.We have since carried out studies with several series of

polymer/acceptor films to investigate the generality of the

Figure 4. Spectroscopic analyses of exciton quenching and chargegeneration yields as a function of the energy offset driving chargeseparation ΔECS for a series of seven polythiophene/PCBM (19:1)blend films. (a) Polymer photoluminescence quenching data relativeto neat polymer films, showing consistently high PLQ efficiencyplotted as a function of ΔECS. (b) Transient absorption assay of theyield of dissociated charges determined from the magnitude of theΔOD signals at 1 μs delay time. Molecular structures of four of thepolymers are included in part b. All data have been corrected forvariations in the absorption at the excitation wavelength. Adapted withpermission from ref 13. Copyright 2010, American Chemical Society.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXE

correlation between ΔECS and ηsep.37,40,86,89 While clear cor-

relations were not observed in all cases, indicative of otherfactors influencing ηsep within the particular series studied inaddition to ΔECS (such as differences in film microstructure, asdiscussed later), we have successfully reproduced the clearcorrelation between ΔECS and our ΔOD assay of ηsep for sixdistinct material series.40,56,78−80,86,88 Typical data from threesuch series are shown in Figure 5, a fourth, employing PDI

acceptors is discussed later (Figure 8). Based on these results,we conclude that the energy offset ΔECS is indeed a key factordetermining the charge separation yields in organic donor/acceptor blends. Haque et al. have also recently reported a sim-ilar correlation for a series of polymer/quantum dot blends.98

In addition, Janssen et al. have reported a similar energetic cor-relation for a series of diketopyrrolopyrrole (DPP) polymers,employing an EQE assay for charge generation and Eg − eVOCas an assay of the energy offset driving charge separation.96

These observations provide clear evidence that ΔECS is, ingeneral, a key determinant of charge separation efficiency inOSC.In addition to demonstrating the reproducibility of the

correlation between ΔECS and ΔOD within a materials series, itis also apparent from Figure 5 that the magnitudes of ΔECSneeded to achieve efficient charge photogeneration vary sub-stantially between materials series. For example, it is apparentthat the DPP-based polymer/PC70BM blend films (black-filledtriangles in Figure 5) show efficient charge separation at energyoffsets almost 1 eV lower than those required to achievecomparable yields of charge separation for the polythiophene/PCBM blend films. We have reported a similar reduction in theenergy offset required to drive charge separation for blend filmsemploying the donor polymer PCPDTBT relative topolythiophene polymers.78,86,88 At present, it is unclear whether

this striking effect derives from a reduction in coulomb bindingenergy of the BPP, an increase in local dielectric constant,a difference in polymer internal dipole,99 greater mixing ofcharge transfer and exciton states, or another parameter.50

Our observation that the energy offset required to driveefficient charge separation may be modulated by choice of classof donor polymers is of key significance for optimization of thesolar cell device efficiency. It clearly indicates that the widelyused assumption that ΔELUMO > 0.3 eV for efficient chargegeneration is material dependent. For the polythiophene-basedpolymers blended with PCBM, the energy offset ΔECS requiredto drive charge separation is ∼0.9 eV, almost half the photon en-ergy (and corresponding to ΔELUMO = 1.2 eV, using ΔELUMO =ΔECS + Eexc

b and assuming Eexcb = 0.3 eV). This energy loss is

therefore the dominant energetic loss limiting device efficiencyin such devices. In contrast, for several of the more recentlydeveloped low bandgap copolymers such as DPP-TT-T, theenergy offset ΔECS required to drive charge separation ap-proaches ∼0 eV (corresponding indeed to ΔELUMO = 0.3 eV,assuming Eexc

b = 0.3 eV).1,37,88 This reduction in the energyoffset with new donor polymers is likely to be one of the keyfactors behind the impressive advances in device efficiency inrecent years.

Dependence of Charge Photogeneration Yield uponPhoton Energy. The previous section discusses data wherethe energetics of charge separation were modulated by chang-ing the materials in the photoactive layer. We now considerdata employing a complementary approach, modulating thephoton energy used to excite a single material system. In thisapproach, we employed a low-band gap polymer/fullereneblend with a very low ΔECS, such that bandgap excitationresulted in relatively low yields of separated charges.37

The results of transient absorption experiments as a functionof excitation wavelength (shown in Figure 6) revealed that, forthis blend, the charge photogeneration yield increases withphoton excitation energy above the optical bandgap. Com-plementing this observation, pump−push photocurrent spec-troscopy demonstrated that this increase in the yield ofdissociated charges with photon energy correlated with areduction in the yield of interfacial BPP states. Bakulin et al.also compared directly the yield of dissociated charges vs BPPstate formation in three different related polymer/fullereneblend films, thereby comparing the hole (PCBM excitation)and electron (polymer excitation) transfer contributions.36 Astrong correlation between driving energy and bound stategeneration was observed independent of the material excited.Experimental evidence that the photon energy can also mod-

ulate the efficiency of charge separation, alongside the materialsenergetics discussed in the previous section, has also beenreported in two other studies (although in at least one caserather controversially60−62).100,101 In our own studies, we haveonly observed a clear photon energy dependence for two blendsystems with particularly small energy offsets. In general, such aphoton energy dependency requires charge separation toproceed on a time scale faster than exciton thermal relaxationand therefore is unlikely to be observed in all systems, inagreement with our observation that, for most blend systemswe have studied, we have not observed any photon energydependence of charge separation yield. Notwithstanding thiscaveat, these data therefore provide further evidence that theyield of dissociated charges is strongly dependent upon thedifference in energy between the exciton initiating charge sep-aration and the dissociated polaron pairs.

Figure 5. Plots of the yield of dissociated charges, as determined fromtransient absorption signal amplitudes, against −ΔECS for threedifferent series of donor polymers blended with PCBM. A strongcorrelation of the yields and ΔECS is observed for the three differentseries of D/A blend films, presented in the figure using different colorcodes. DPP-based copolymers blended with PC70BM at 1:2 w/w areshown with black triangles. Polythiophene-based polymers, such asP3HT, blended with PCBM at 1:1 w/w are shown with blue opensquares. Thiazolothiazole-based polymers blended (1:1 w/w) withPCBM and ICBA are shown in red. For each series, the largest ΔODsignal is normalized to one to facilitate comparison between series, asdifferences in polaron extinction coefficients between polymer seriesare possible. The electron affinities of the PC70BM and PC60BM wereassumed to be 3.7 eV. Data were taken from refs 40, 86 and 88. Forthe polythiophene series, the ΔOD data have been extrapolated to atime delay of 100 ns to facilitate comparison with the other data series.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXF

Model for Energetic Dependence. We turn now todiscuss briefly a model of charge separation based on ourexperimental observation that the yield of dissociated chargesincreases with ΔECS. A key consideration for this model is theempirical observation that, even for blend films with smallenergy offsets such that the yield of dissociated polarons dropssubstantially, the efficiency of exciton quenching at the interfacecan still be very high (often approaching unity). This observationclearly implies a competing loss pathway, which results in chargecarrier recombination after exciton dissociation and whichappears to be particularly dominant for small energy offsets.For the purpose of this discussion, we will only consider

charge separation across a D/A interface consisting of well-defined polymer and fullerene phases (see the following textfor discussion of more realistic interface structures). The model,as illustrated in Figure 7, is based on that originally proposedin Ohkita et al.,56 but modified to take into account recentultrafast spectroscopy and theoretical studies, including, inparticular, those by the Friend group.33,35,100,101 A key com-ponent of the model is the presence of Coulombically boundpolaron pair states at the D/A interface that function as a

recombination loss pathway. The model assumes that theelectron is transferred adiabatically from the polymer exciton tothe fullerene acceptor without an initial loss of energy(neglecting any exciton diffusion processes). The electron istherefore injected with excess energy, corresponding to ΔECSabove the bulk band edges, or ΔECS + Eg

BPP above the BPPstate energy (we have previously referred to this initial statewith excess energy as a ‘hot state‘). There is then a competitionbetween motion of the electron away from the D/A interface,corresponding to spatial charge separation, and loss of theexcess energy through thermalisation/electronic relaxationprocesses. This ‘hot’ electron motion versus thermalizationpicture is analogous to the Onsager model for autoionization insolution, as we have discussed previously.13,102 As for Onsager,stable charge separation is only achieved if the electron escapesthe coulomb attraction of the positive polaron residing on thepolymer before it loses its excess energy.102

A large energy offset ΔECS results in injection of electronswith a large excess energy, facilitating their escape from thecoulomb attraction of the polymer polaron. This increasedescape probability may derive both from the longer time takento thermalize this large excess energy and from the poten-tially greater wave function delocalization present for stateswell above the band edge, as has been proposed by severalgroups.14,35,39,43,100,103−107 For example, theoretical calculationshave provided evidence that higher energy BPP or CT statescan be more delocalized and hence more prone to disso-ciation.8,100,103 In contrast, for a small energy ΔECS, the injectedelectron does not have sufficient excess energy to escape thecoulomb attraction with the positive polaron, and therefore, itbecomes trapped at the interface, forming a bound interfacialBPP state. This bound BPP subsequently either decays directlyto ground or, for systems with material triplet excitons lyingbelow the BPP state in energy, undergoes intersystem crossing

Figure 6. (top graph) Charge photogeneration quantum yields of aBTT-DPP/PCBM blend film as estimated by TAS, recorded at thepolymer polaron band (1150 nm) at a 0.2 μs time delay (red circles)as a function of excitation wavelength λexc. Data was normalized fordifferences in film absorption at λexc. IQE for photocurrent generationof the corresponding device is presented with a black line; it showssimilar increases in the charge yield with excitation wavelength. Inset:PL quenching of the BTT-DPP/PCBM blend film plotted as afunction of excitation wavelength. (bottom graph) Results of pump−push photocurrent (δJ/J) measurements on BTT-DPP/PCBM devicesat different excitation wavelengths. The decays correspond to relaxedBPP state formation and recombination, thus showing that higheryields of relaxed BPP states are generated by longer wavelengthexcitation. Reprinted with permission from ref 37. Copyright 2012,American Chemical Society.

Figure 7. Energy level diagram depicting a model of chargephotogeneration at organic D/A heterojunctions. The model isillustrated for two different initial exciton energies (SA and SB) relativeto the polaron energies, corresponding for example to singlet excitonsof two donor polymers with different optical bandgaps, or twodifferent excitation wavelengths of the BTT-DPP polymer. Excitationof the higher energy exciton (SA) results in electron transfer (kET) tothe acceptor with sufficient excess energy to avoid formation of aCoulombically bound interfacial polaron pair (BPPR) but rather leadsto formation of charge separated states (CS), which are subsequentlystabilized (kTR) by charge migration away from the D/A interface,resulting in an increase in state degeneracy (entropy) and electronicrelaxation into lower energy polaron states in the film (CSTR). Incontrast, for the lower energy exciton (SB), the injected electron doesnot have sufficient excess energy to escape the coulomb attraction ofthe polymer polaron and therefore results in formation of boundpolaron pair states that subsequently recombine (kR) radiatively ornonradiatively to ground or to lower lying polymer/acceptor tripletstates.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXG

to the triplet BPP state, followed by charge recombination totriplet excitons.The model illustrated in Figure 7 suggests an inverse cor-

relation between the yields of separated charges and inter-facially bound states, as evidenced by the data in Figure 6.Analogous inverse correlations have also been observed bet-ween the intensity of CT state emission and EQE or polaron forma-tion, again consistent with a competition between generation ofbound interfacial states and free charges.33,35,37,41,57,76,108,109

We note, however, that there remains significant controversy inthe literature over the nature and role of these BPP or CTstates in organic solar cells, and significantly more experimentaland theoretical work is necessary to establish fully the detailedmechanism of charge separation in these devices. For example,an optimum driving energy for charge separation, consistentwith Marcus nonadiabatic electron transfer theory, has beenobserved for charge separation from fullerene excitons in diluteblends with donor polymers.111 In addition, there is increasingevidence that sub-bandgap excitation of low optical densitycharge transfer (CT) transitions can result in efficient photo-current generation, suggesting that the states populated in suchexperiments do not correspond to the interfacially bound statesresponsible for the energetic dependencies of charge separationdiscussed above, as we discuss further towards the end of thisarticle.53,112−115

We note that following the initial charge separation, theenergy of the electron and hole is further reduced as the chargecarriers thermalize to the film quasi-Fermi levels, as alsoillustrated in Figure 7. For efficient solar cell materials thatachieve charge separation with ΔECS approaching 0 meV, thisthermalization/trapping process is actually the dominantenergy loss process associated with the overall charge pho-togeneration process13,32 and is a key factor in stabilizing thecharge separation in these devices. The magnitude of this over-all free energy loss, ΔGCS, is dependent upon device operatingcondition (being largest at short circuit conditions) and alsodepends (at open circuit conditions) upon the rate constant fornongeminate recombination. However, we do not discuss theseenergetic losses further in this review, as our focus is on thequantum efficiency of photocurrent generation, which isdetermined by efficiency of the initial charge separation, ratherthan by these energetic loss processes.In terms of materials design rules, the model illustrated in

Figure 7 has several important implications. In addition to po-tentially explaining the origin of the energy offset dependencewe observe empirically, it suggests several other parametersare likely to influence the efficiency of charge separation. Theseinclude the BPP state binding energy Eg

BPP, the extent ofdelocalization of the electron and hole wave functions after/during charge transfer at the interface, the electron mobility, thethermalization time of the excess energy, the physical structureof the interface, including any local variations in energetics orcomposition, the domain structure and material composition inthe blend film, and the presence of interface or moleculardipoles. In the following sections, we consider the extent towhich our studies have provided experimental evidence for theimportance of some of these parameters.

■ IMPACT OF FILM MORPHOLOGY AND MATERIALCRYSTALLINITY

There is now extensive evidence that, in addition to energetics,film morphology and material crystallinity play key roles in de-termining the efficiency of charge separation.34,40,85,110,116−120

In most text-book descriptions of polymer/fullerene bulkheterojunction solar cells, the film morphology is typicallydescribed as a two-component, bicontinuous blend comprisinga polymer phase and a fullerene phase. The function of suchfilms is then described as light absorption in one or bothphases, exciton diffusion to the polymer/fullerene interface,exciton dissociation at this D/A interface to yield electrons andholes, and transport of these charges through the two phases tothe device contacts. However, more detailed analyses areincreasingly questioning whether this pure polymer phase/purefullerene phase structural model is appropriate for the blendfilms typically employed in BHJ OSC, with increasing evidencefor the importance to device function of, for example, D/Amixing on a molecular scale, and the role of variations ofmaterial crystallinity within and between blends.40,85,117,121−128

The former consideration, the presence of molecularly mixedregions of the films, is important with regard to charge sep-aration, as it suggests that charge separation may be, at least inpart, a molecular rather than interfacial process. Molecularmixed regions can also function as recombination centers in thefilm due to the lack of spatial separation of donor and acceptormolecules. Variations in material crystallinity can impact chargeseparation by, for example, causing changes in wave functiondelocalization or material energetics (increased crystallinitytypically results in smaller electronic bandgaps). In this section,we consider some insights into these dependencies derivedparticularly from correlations with our transient optical studies.

Exciton Dynamics. Film morphology can have a majorimpact on the efficiency of exciton diffusion to D/A junctions.In this regard, a particularly important parameter for excitonsis their diffusion length; this is the average distance that anexciton can migrate during its lifetime. Typically this parameteris assessed by quantifying PLQ of bilayers as a function of alayer thickness using time-resolved or steady-state photo-luminescence spectroscopy.65,129−133 For example, measure-ments of this type have yielded exciton diffusion lengths of3−9 nm for P3HT and 5 nm for PC60BM.66,68,134−136 In gen-eral, achieving a high efficiency for diffusion of photogeneratedexcitons to the charge separation interface, ηdiff, requires do-main sizes smaller than these diffusion lengths. We note thatthe exciton diffusion length will depend not only on the excitondiffusion constant but also on its lifetime, with shorter excitonlifetimes likely to result in more severe exciton diffusion re-quirements.For many of the polymer/PCBM blends discussed in this

review, polymer photoluminescence quenching is remarkablyhigh (often >95%), indicating that polymer excitons do notneed to diffuse significant distances to reach a PCBM acceptor.Indeed, this high PLQ is indicative of the presence of significantmolecular mixing of PCBM into the donor polymer on thelength scale of the exciton wave function size (we note there iscurrently some discussion over the extent of exciton wavefunction delocalization at early times).85,137,138 Such molecularscale mixing is consistent with several recent reports of PCBMmiscibility with, and diffusion into, donor polymers domains, includ-ing, in particular. relatively amorphous polymers.122,124,128,139−143

More modest polymer PLQ, indicative of significant polymerexciton diffusion on length scales approaching the exciton dif-fusion lengths, is only observed for highly crystalline polymerssuch as P3HT and DPP-TT-T, consistent with the formation ofrelatively pure crystalline polymer domains for these materials.This photoluminescence quenching has been most extensivelystudied for P3HT, which has PLQ, and therefore exciton

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXH

dissociation yields, sensitive to changes in film morphology, forexample, thermal annealing, which increases phase segregationin the P3HT/PCBM blends.79,97,144

In general, polymer photoluminescence quenching followingblending with PCBM can derive from either electron or energytransfer to PCBM.67,145,146 In most cases, electron transferappears to be the dominant pathway. We have observed effi-cient energy transfer from donor polymer excitons to PCBM,but only for highly fluorescent, high bandgap fluorene-basedpolymers, where the strong fluorescence efficiency and goodspectral overlap between polymer emission and fullerene ab-sorption results in a sufficiently high energy transfer rate con-stant to enable efficient energy transfer.81,87 In this case, thisenergy transfer can result in a loss of charge photogeneration,as the resultant PCBM excitons can be too low in energy todrive efficient charge generation.In contrast to the typically very efficient quenching of poly-

mer photoluminescence in blend films, the quenching of fuller-ene photoluminescence is often less efficient.83,118,147−149 Thiscannot, in general, be attributed to unfavorable energetics, asoften the PCBM exciton has a higher energy than the polymerexciton. Rather, this efficiency loss appears to derive from thetendency of both PC60BM and PC70BM to form aggregates onthe length scale or larger than their exciton diffusion lengths, aswe have recently reported in a study of photocurrent generationfrom PC70BM excitons in blends with a low-bandgap DPP-based polymer.83 In this case, clear correlations were observedbetween photocurrent density, PC70BM photoluminescencequenching, and PC70BM domain size, indicative of exciton dif-fusion limitations. This result suggests that fullerene aggre-gation and its miscibility with the polymer are a key con-sideration for optimization of photocurrent generation fromfullerene excitons.Exciton Separation and Charge Carrier Mobility/

Delocalization. The molecular scale structure of the D/Ajunction can be expected to have a profound impact on the effi-ciency and dynamics of exciton separation. One considerationattracting particular interest is the potential for increased wavefunction delocalization and/or high carrier mobility to facilitateescape of charge carriers from the D/A junction (see proposedmodel in Figure 7). Several studies have reported both experi-mental and theoretical analyses of this issue.8,22,26−28,51,110,150

Our own studies comparing the charge separation efficiency ofpolymer/PCBM and polymer/PDI (perylene diimide) blendsare particularly relevant to this.40,86 We have observed thatreplacement of PCBM in blend films with crystalline PDIsresults in a large reduction in the energy offset dependence ofcharge separation (Figure 8). An analogous observation hasalso been made for polymer/CdS nanocrystals blends, where anincrease in CdS crystallinity was also observed to reduce theenergy offset dependence of charge separation.98 In both cases,the improved charge separation efficiency was assigned toincreases in electron delocalization/mobility in the acceptormaterial facilitating the escape of the charge carriers from theirmutual coulomb attraction. We have also observed thatdecreasing the fullerene content in the blend film increasesthe energy offset dependence of charge separation, attributed tomore finely dispersed PCBM molecules within the blendmaking it more difficult for photogenerated electrons and holesto move away from each other.80 Interestingly, we have notobserved a strong correlation between donor polymer holemobility/crystallinity and charge separation efficiency, at leastfor charge separation from polymer excitons,56,86,89 suggesting

that it is the mobility/delocalization of charge carrier that ismoving during exciton separation (e.g., the electron for LUMOto LUMO transfer), which is most important.

Domain Energetics, Material Crystallinity, and theSpatial Separation of Charges. As discussed above, there isincreasing evidence that, for many polymer/fullerene blends,there is extensive mixing of polymer and fullerene on a molec-ular scale, depending upon the details of film processing. This isparticularly the case where the polymer is highly miscible withPCBM, either through formation of a mixed cocrystal or theformation of amorphous, mixed regions of film.85,122,128,151−153

However, there is also strong evidence that molecularly mixedfilms, in the absence of any pure domains, exhibit very rapidcharge recombination, as charge carriers generated by excitonseparation are not spatially well separated.39,54,90,153−157 Wehave, for example, observed that replacing PCBM with a moremiscible fullerene in blends with P3HT results in appearance ofrapid (≤100 ns) recombination and, consequently, negligiblephotocurrent generation.40,154 Similarly, fast recombination hasbeen observed in films comprising covalently attached donorand acceptor.158 As such, it appears likely that molecularly mixedpolymer/fullerene domains, while being effective at enablingefficient exciton quenching, can also function as recombinationcenters for photogenerated charge carriers. In this regard, it isstriking that some of the highest device efficiencies reported todate have been achieved with relatively amorphous polymers (forexample PTB7) that show evidence for rather high miscibilitywith PCBM and, consequently, significant polymer/PCBMmixing on a molecular scale.2,4,5,122,151

We recently undertook a number of studies to address thisapparent contradictionnamely, that, for some blend films, thepresence of significant fractions of molecular mixed materialdoes not result in rapid electron/hole recombination and con-sequently poor device performance.40,85 The key to answeringthis conundrum appears to come from consideration of relativeenergetics of pure and mixed domains. In particular, we haveobserved that neat PCBM films exhibit an electron affinityapproximately 100 meV greater than PCBM dispersed in apolymer matrix.85 In films comprising both molecularly mixedpolymer/PCBM and pure PCBM domains, this energydifference provides an energy offset to localize electrons inthe pure PCBM domains, thereby spatially separating thecharges and enabling efficient charge collection, as illustrated inFigure 9c. A similar effect is likely for blend films exhibiting

Figure 8. Comparison of the ΔOD signal amplitudes of polymer/PCBM and polymer/PDI blend films. The substitution of PCBM withhighly crystalline PDI acceptors reduces the dependence of chargephotogeneration yields on energy offset driving charge separation.Adapted with permission from ref 86. Copyright 2010, AmericanChemical Society.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXI

significant fractions of pure, crystalline donor polymer; ingeneral, increased polymer crystallinity reduces its ionizationpotential, again providing an energy offset to stabilize chargeseparation.79

Appreciation that differences in carrier energetics within ablend film deriving from differences in material crystallinity/aggregation state can impact directly photovoltaic deviceperformance has important implications for materials design.For example, the requirement of relatively pure, aggregatedPCBM domains to stabilize charge separation is likely to be oneorigin of high PCBM weight fractions often required to achieveefficient device performance, and the reason this requirementis less pronounced for crystalline polymers (due to theirtypically lower immiscibility with PCBM). We have alsorecently shown that this additional energy offset requirementcan explain empirical observations that the low electron affinityacceptor ICBA works well with some crystalline donor poly-mers but not with amorphous donor polymers.40 ICBA appearsnot to exhibit a difference in electron affinity between neat andmolecularly dispersed films, attributed to its low tendency toaggregation/crystallization. As such, the energy offset betweenmixed and pure fullerene domains present in blends withPCBM is not present in blends with ICBA, such that thepresence of molecularly mixed domains are more likely to resultin rapid electron hole recombination (Figure 9b), as evidencedby observation of a rapid (100 ns) recombination decay phasein blends of amorphous donor polymers with ICBA.40,155 Thisis, however, avoided in blends with more crystalline donor poly-mers, which reduce the presence of molecular mixing, andresults in the formation of pure polymer domains capable ofstabilizing charge separation (Figure 9a); such blends do notexhibit this 100 ns recombination phase.A key consideration in the above discussion is that, for many

blend films, we need to consider two distinct interfaces thatdrive charge separation. In many cases, and particularly formore amorphous donor polymers, the initial exciton quenchingmay occur primarily in rather molecularly mixed domains,without the presence of a well-defined physical D/A interface.

However, the lifetime of charge carriers in such molecularmixed domains is relatively short (≤100 ns). As such, stabi-lization of this charge separation requires the presence ofrelatively pure domains, with a favorable energy offset betweenthe mixed and pure domains being essential to drive this sta-bilization. The presence of such mixed domains is likely to beless important for polymers that show lower miscibility withPCBM, in particular, for more crystalline polymers. However,even for these polymers, it has been suggested that the interfacebetween polymer and PCBM domains may be relativelyamorphous and molecularly mixed, with the higher bandgapin this interface region helping to drive and stabilize chargeseparation.85,119,121,159−161 Relating to this, recent studies havesuggested that relaxation into lower energy states resultingfrom local inhomogeneities may also be a key factor helpingto separate polarons away from the donor/acceptor inter-face.162,163

The presence of at least two charge separation interfaces, andrealization that crystalline and amorphous regions of the filmmay exhibit different energetics, clearly complicates bothenergetic and kinetic analyses of charge separation in blendfilms. We have, for example, suggested that photocurrentgeneration may be limited both by geminate recombination oftightly BPP (or CT) states and, for some systems that lacksecondary charge separation interfaces to stabilize the spatialseparation of charges, by recombination of more loosely boundpolaron pair states, as illustrated in Figure 9.86 However, ingeneral, experimental data on these issues are relatively limitedto date.

■ OTHER FACTORS INFLUENCING CHARGEGENERATION

In the previous sections, we focused on the roles of energeticsand film microstructure/crystallinity in determining the effi-ciency of charge photogeneration. In this section, we considerthe extent to which we observe empirical correlations betweenour ΔOD assay of charge generation and other potential

Figure 9. Illustration of charge separation in blend films comprising relatively an amorphous, intimately mixed polymer/fullerene phase as well asrelatively chemical pure, more crystalline phases. The crystalline/aggregated domains exhibit a smaller IP (for donor materials) and larger EA (foracceptor materials). The interface between the mixed and more crystalline domains thereby provides an energy offset that can stabilize the spatialseparation of charge carriers. The model is illustrated for blend films comprising a relatively crystalline donor polymer (a), relatively crystallineacceptor fullerene (c), or where neither polymer nor fullerene exhibit significant crystallinity (b). In part b, the absence of a well-defined interface toseparate spatially the polarons results in relatively rapid (≤100 ns) charge carrier recombination, preventing efficient photocurrent generation.Reprinted with permission from ref 40. Copyright 2010, Wiley-VCH Verlag GmbH & Co. KGaA.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXJ

parameters, including polymer hole mobility, macroscopicelectric fields, and population of triplet states.Hole mobility is one property of polymers that is often cited

in the literature in relation to good device performance. Ithas indeed been shown that excessively low hole mobilities canlimit device performance due to poor charge extraction andresultant space charge accumulation. However, relating to theefficiency of charge photogeneration, our investigations havenot observed any significant correlation between hole mobilityand charge separation from polymer excitons, at least in blendswith PCBM and with FET hole mobilities >10−4 cm2 V−1 s−1,89

in agreement with other studies.19 Similarly, we have not ob-served any correlation between charge photogeneration andpolymer crystallinity, beyond the impact of polymer crystal-linity upon ionization potential and miscibility with PCBM, aswe have discussed above and in more detail elsewhere.40 In thisregard, it should be noted that charge photogeneration frompolymer excitons corresponds to an electron rather than holetransfer process. As such, it is plausible that electron mobilitiesrather than hole mobilities might be the more relevant pa-rameter related to charge separation. Indeed, our comparison offullerene and PDI acceptors, and of blends as a function offullerene composition support this argument, as we discussabove. Hole mobilities, on the other hand, may be importantfor charge separation mediated via fullerene exciton dissociation(hole transfer); however, to the best of our knowledge, evi-dence for the importance of polymer hole mobility to thisprocess is not present to date.Another factor widely discussed in terms of its impact upon

charge separation is the macroscopic electric fields generatedby the difference in work function of the device electro-des.41,76,112,164−166 There is extensive evidence that these elec-tric fields play a key role in sweeping out photogeneratedcharge carriers, and thereby ensuring a high collection efficiencyηcoll. However, our own transient absorption data on devices asfunction of applied bias, as well as transient optoelectronicdevice analyses,16,74,84 have indicated that, for most photoactivelayers, the impact of these macroscopic electric fields on theefficiency of charge separation ηsep is relatively insignificant, atleast under normal device operating conditions within thefourth quadrant of the device JV curve.166 In some cases, wehave observed significant increases in the efficiency of chargeseparation between open circuit and short circuit and assignedthese to the effect of macroscopic electric fields; so far, we haveonly observed this field dependence in very amorphous blendfilms where the morphology of the active layer is most probablydominated by well-mixed polymer/fullerene phases.90,167 Thiscorrelation between field-dependent generation and blendcrystallinity/morphology has also been observed in elegantstudies employing time-delayed collection field measurementsby Neher et al.165 The origin of this correlation between fielddependence and crystallinity/morphology is unclear, but it isplausible that electric fields may be particularly helpful in spa-tially separating charges generated in molecular mixed domains,which would otherwise undergo relatively rapid recombination.There have been several studies in the literature, which con-

sider the role of low lying triplet states in accelerating chargerecombination in OSC.168−171 Both we and others have shownthat BPP state formation may result in high yields of polymer orfullerene triplet states, through intersystem crossing between singletand triplet BPP states, as illustrated in Figure 10.29,41,56,75,77,169,172

This polaron pair mediated triplet mechanism is analogousto that observed in for example photosynthetic reaction centers.

In addition, from spin statistics well established in the organiclight emitting diode field, nongeminate recombination ofdissociated charges is likely to lead to high yields of tripletsstates (Figure 10). However, our studies to date have indicated

that while triplet states may indeed be formed by suchrecombination processes when these triplet states lie energeti-cally below the relevant BPP or CS states, the presence of suchlow lying triplet states does not necessarily prevent charge gen-eration. For example, charge recombination in PTB7/PCBMblend films appears to result in efficient PTB7 triplet gen-eration, indicative of the presence of a lower energy tripletstate; yet under short-circuit conditions, photocurrent gen-eration can be remarkably efficient.75 Work on this topic isongoing and will be presented in more detail elsewhere.Finally, we note that there is currently extensive discussion in

the literature over whether interfacial BPP or CT states mayundergo thermally activated dissociation.76,150,173−175 Ourstudies indicate that low energy offsets result in low chargegeneration yields due to the presence of a competing loss path-way after exciton separation, which prevents efficient chargedissociation. The formation and recombination of Coulombi-cally bound polaron pairs are the most likely candidate for thisloss pathway. Whether these Coulombically bound states arethe same states as those involved in studies of ‘CT state’emission and sub-bandgap absorption is not fully resolved. Inthis context, it is important to note the presence of bothmolecular mixed and phase segregated domains in many blendfilms, as well as modulation to the energetic landscape fromvariations in material crystallinity, such that it is possible thatsuch ‘CT state’ absorption and electroluminescence studies maybe probing states formed at different interfaces to thoseassociated with photoinduced charge separation, as we havediscussed elsewhere.40 It has, for example, been noted thatstudies of CT state photoluminescence and electrolumines-cence often reveal rather distinct emission peaks,46 indicatingthe presence of different CT states in the film populated dif-ferently by optical or electrical generation and suggested to beassociated with CT states located within molecular mixeddomains and at domain interfaces, respectively.153 Also relevantto this discussion, Zhou et al. have considered a similar modelfor donor/acceptor pairs in solution and concluded that chargeseparation following sub-bandgap excitation in such systemscan derive primarily from direct excitation of loosely boundpolaron ion pairs.176

■ CONCLUSIONSThe focus of this review is on identifying materials designguidelines that may facilitate the development of new

Figure 10. Illustration of the charge separation and recombinationprocesses in organic solar cells, analogous to the model shown inFigure 7, but including the presence of triplet BPP and exciton states.Adapted with permission from ref 56. Copyright 2008, AmericanChemical Society.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXK

photoactive layer materials or processing procedures to max-imize the quantum efficiency of photoinduced charge sep-aration ηsep and thus photocurrent generation in OSC. Theseguidelines can be summarized as follows:

(1) A key determinant for high ηsep is a sufficiently largeenergy offset between the D and A material optical ban-dgaps, and the blend electronic bandgap IP−EA, ΔECS.However, a large value of ΔECS corresponds to a largeloss of energy per photon; therefore, a key challenge forOSC is to achieve a high quantum efficiency for chargeseparation with as small a value for ΔECS as possible. Em-pirically, we have observed that materials design strat-egies to achieve this include

a. the use of donor copolymers with a high degree of‘D−A’ character. This has proved to be remarkablyeffective at reducing the energy offset requirementfor charge separation and is likely to be a key factorbehind recent advances in OSC device efficiencies.

b. the use (for charge separation from polymer exci-tons) of acceptors with increased electron mobility/delocalization (e.g.: by increasing acceptor crys-tallinity). We note, however, that excessive accep-tor crystallinity may result in lower collectionefficiency.

(2) The generation of sufficiently long-lived charge carriersto enable efficient charge collection requires the spatialseparation of charges into different domains. This typi-cally requires the presence in the blend of reasonablypure domains of either donor or acceptor (or both). Forsuch pure domains to stabilize charge separation, theremust be an energy offset (of the order of 100 meV orgreater) driving one charge carrier from mixed domainsinto these pure domains. This energy offset can derivefrom differences in crystallinity/aggregation betweenpure and mixed domains.

(3) For most polymer/fullerene blends, the miscibility ofPCBM with many donor polymers results in efficientpolymer exciton quenching, with negligible exciton dif-fusion requirements. In contrast, PCBM exciton diffusionlimitations can result in significant photocurrent losses.We note that, for many OSC blends, and particularlythose with more amorphous donor polymers withPCBM, efficient device performance requires the for-mation of relatively pure PCBM domains, for reason 2above. This can be achieved by appropriate film pro-cessing (e.g., by use of cosolvent additives) or the addi-tion of excess PCBM to the blend. However, thisformation of aggregated PCBM domains can also reducethe efficiency of PCBM exciton utilization.

(4) Device macroscopic electric fields do not significantlyimpact upon photocurrent generation in most OSC(except at strong reverse biases), apart from in blendfilms employing highly amorphous donors. We also donot observe a significant correlation of charge separationefficiency with polymer hole mobility (for chargeseparation from polymer excitons).

Clearly these materials design guidelines are limited both inscope and detail. However, we hope that they provide someinsights that may aid synthetic chemists, materials scientists,and device physicists in their drive toward more efficient OSCmaterials and devices.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: j.durrant@imperial.ac.uk.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge the EPSRC projects EP/J500021/1and EP/G037515/1 for funding. We thank our manycolleagues and co-workers who have contributed to thesestudies over the last 8 years, and in particular Safa Shoaee, ElizaCollado Fregoso, and Yvonne Soon, who helped with thepreparation of some figures in this manuscript, and ChristianNielsen, Hugo Bronstein, Jenny Nelson, and Dieter Neher forhelpful discussions and comments. We also thank colleagues,and particularly Peter Wurfel for discussions regarding ΔECS vsΔGCS terminology.

■ REFERENCES(1) Bronstein, H.; Chen, Z. Y.; Ashraf, R. S.; Zhang, W. M.; Du, J. P.;Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.;Wienk, M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirringhaus, H.;Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272.(2) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.;Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3,297.(3) You, J.; Chen, C.-C.; Hong, Z.; Yoshimura, K.; Ohya, K.; Xu, R.;Ye, S.; Gao, J.; Li, G.; Yang, Y. Adv. Mater. 2013, DOI: 10.1002/adma.201300964.(4) Liang, Y. Y.; Xu, Z.; Xia, J. B.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.;Yu, L. P. Adv. Mater. 2010, 22, E135.(5) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y. W., H.; Chen, L.; Su,S.; Cao, Y. Adv. Mater. 2011, 23, 4636.(6) Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.;Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649.(7) Zhong, H.; Li, Z.; Deledalle, F.; Fregoso, E. C.; Shahid, M.; Fei,Z.; Nielsen, C. B.; Yaacobi-Gross, N.; Rossbauer, S.; Anthopoulos, T.D.; Durrant, J. R.; Heeney, M. J. Am. Chem. Soc. 2013, 135, 2040.(8) Li, W. W.; Furlan, A.; Hendriks, K. H.; Wienk, M. M.; Janssen, R.A. J. J. Am. Chem. Soc. 2013, 135, 5529.(9) Hendriks, K. H.; Li, W. W.; Wienk, M. M.; Janssen, R. A. J. Adv.Energy Mater. 2013, 3, 674.(10) Kirkpatrick, J.; Nielsen, C. B.; Zhang, W.; Bronstein, H.; Ashraf,R. S.; Heeney, M.; McCulloch, I. Adv. Energy Mater. 2012, 2, 260.(11) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.;Waldauf, C.; Heeger, A. J.; Brabec, C. L. Adv. Mater. 2006, 18, 789.(12) Peumans, P.; Forrest, S. R. Chem. Phys. Lett. 2004, 398, 27.(13) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736.(14) Bredas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Acc. Chem.Res. 2009, 42, 1691.(15) Wojcik, M.; Tachiya, M. J. Chem. Phys. 2009, 130, 104107.(16) Credgington, D.; Durrant, J. R. J. Phys. Chem. Lett. 2012, 3,1465.(17) Deibel, C.; Strobel, T.; Dyakonov, V. Adv. Mater. 2010, 22,4097.(18) Deibel, C.; Dyakonov, V. Rep. Prog. Phys. 2010, 73, 096401.(19) Chen, J. W.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709.(20) Janssen, R. A. J.; Nelson, J. Adv. Mater. 2013, 25, 1847.(21) Kirchartz, T.; Agostinelli, T.; Campoy-Quiles, M.; Gong, W.;Nelson, J. J. Phys. Chem. Lett. 2012, 3, 3470.(22) Credgington, D.; Hamilton, R.; Atienzar, P.; Nelson, J.; Durrant,J. R. Adv. Funct. Mater. 2011, 21, 2744.(23) Moule, A. J.; Bonekamp, J. B.; Meerholz, K. J. Appl. Phys. 2006,100, 094503.(24) Boix, P. P.; Guerrero, A.; Marchesi, L. F.; Garcia-Belmonte, G.;Bisquert, J. Adv. Energy Mater. 2011, 1, 1073.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXL

(25) Guerrero, A.; Marchesi, L. F.; Boix, P. P.; Bisquert, J.; Garcia-Belmonte, G. J. Phys. Chem. Lett. 2012, 3, 1386.(26) MacKenzie, R. C. I.; Kirchartz, T.; Dibb, G. F. A.; Nelson, J. J.Phys. Chem. C 2011, 115, 9806.(27) Foertig, A.; Rauh, J.; Dyakonov, V.; Deibel, C. Phys. Rev. B 2012,86, 115302.(28) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J.V. Phys. Rev. B 2010, 81, 125204.(29) Dyer-Smith, C.; Reynolds, L. X.; Bruno, A.; Bradley, D. D. C.;Haque, S. A.; Nelson, J. Adv. Funct. Mater. 2010, 20, 2701.(30) Bredas, J. -L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem.Rev. 2004, 104, 4971.(31) Halls, J. J. M.; Cornil, J.; dos Santos, D. A.; Silbey, R.; Hwang, D.H.; Holmes, A. B.; Bredas, J. -L.; Friend, R. H. Phys. Rev. B 1999, 60,5721.(32) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. Adv. Funct.Mater. 2009, 19, 1939.(33) Etzold, F.; Howard, I. A.; Mauer, R.; Meister, M.; Kim, T. D.;Lee, K. S.; Baek, N. S.; Laquai, F. J. Am. Chem. Soc. 2011, 133, 9469.(34) Howard, I. A.; Mauer, R.; Meister, M.; Laquai, F. J. Am. Chem.Soc. 2010, 132, 14866.(35) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H.M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.;Friend, R. H. Science 2012, 335, 1340.(36) Bakulin, A. A.; Dimitrov, S. D.; Rao, A.; Chow, P. C. Y.; Nielsen,C. B.; Schroeder, B. C.; McCulloch, I.; Bakker, H. J.; Durrant, J. R.;Friend, R. H. J. Phys. Chem. Lett. 2013, 4, 209.(37) Dimitrov, S. D.; Bakulin, A. A.; Nielsen, C. B.; Schroeder, B. C.;Du, J. P.; Bronstein, H.; McCulloch, I.; Friend, R. H.; Durrant, J. R. J.Am. Chem. Soc. 2012, 134, 18189.(38) Mingebach, M.; Walter, S.; Dyakonov, V.; Deibel, C. Appl. Phys.Lett. 2012, 100, 193302.(39) Morteani, A. C.; Sreearunothai, P.; Herz, L. M.; Friend, R. H.;Silva, C. Phys. Rev. Lett. 2004, 92, 247402.(40) Shoaee, S.; Subramaniyan, S.; Xin, H.; Keiderling, C.; Tuladhar,P. S.; Jamieson, F.; Jenekhe, S. A.; Durrant, J. R. Adv. Funct. Mater.2013, 23, 3286.(41) Offermans, T.; van Hal, P. A.; Meskers, S. C. J.; Koetse, M. M.;Janssen, R. A. J. Phys. Rev. B 2005, 72, 045213.(42) Piliego, C.; Loi, M. A. J. Mater. Chem. 2012, 22, 4141.(43) Arkhipov, V. I.; Emelianova, E. V.; Bassler, H. Phys. Rev. Lett.1999, 82, 1321.(44) Arkhipov, V. I.; Heremans, P.; Bassler, H. Appl. Phys. Lett. 2003,82, 4605.(45) Gulbinas, V.; Zaushitsyn, Y.; Sundstrom, V.; Hertel, D.; Bassler,H.; Yartsev, A. Phys. Rev. Lett. 2002, 89, 107401.(46) Tvingstedt, K.; Vandewal, K.; Zhang, F.; Inganas, O. J. Phys.Chem. C 2010, 114, 21824.(47) Loi, M. A.; Toffanin, S.; Muccini, M.; Forster, M.; Scherf, U.;Scharber, M. Adv. Funct. Mater. 2007, 17, 2111.(48) Scharber, M. C.; Lungenschmied, C.; Egelhaaf, H.-J.; Matt, G.;Bednorz, M.; Fromherz, T.; Gao, J.; Jarzab, D.; Loi, M. A. EnergyEnviron. Sci. 2011, 4, 5077.(49) Benson-Smith, J. J.; Goris, L.; Vandewal, K.; Haenen, K.; Manca,J. V.; Vanderzande, D.; Bradley, D. D. C.; Nelson, J. Adv. Funct. Mater.2007, 17, 451.(50) Faist, M. A.; Kirchartz, T.; Gong, W.; Ashraf, R. S.; McCulloch,I.; de Mello, J. C.; Ekins-Daukes, N. J.; Bradley, D. D. C.; Nelson, J. J.Am. Chem. Soc. 2012, 134, 685.(51) Muller, J. G.; Lupton, J. M.; Feldmann, J.; Lemmer, U.;Scharber, M. C.; Sariciftci, N. S.; Brabec, C. J.; Scherf, U. Phys. Rev. B2005, 72, 195208.(52) Bakulin, A. A.; Martyanov, D.; Paraschuk, D. Y.; van Loosdrecht,P. H. M.; Pshenichnikov, M. S. Chem. Phys. Lett. 2009, 482, 99.(53) Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo,M. A.; Manca, J. V.; Van Voorhis, T. J. Am. Chem. Soc. 2010, 132,11878.(54) Hwang, I.-W.; Moses, D.; Heeger, A. J. J. Phys. Chem. C 2008,112, 4350.

(55) Zhang, F. L.; Jespersen, K. G.; Bjorstrom, C.; Svensson, M.;Andersson, M. R.; Sundstrom, V.; Magnusson, K.; Moons, E.; Yartsev,A.; Inganas, O. Adv. Funct. Mater. 2006, 16, 667.(56) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang,W.; Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant,J. R. J. Am. Chem. Soc. 2008, 130, 3030.(57) Yin, C.; Kietzke, T.; Neher, D.; Hoerhold, H.-H. Appl. Phys. Lett.2007, 90, 092117.(58) Veldman, D.; Ipek, O.; Meskers, S. C. J.; Sweelssen, J.; Koetse,M. M.; Veenstra, S. C.; Kroon, J. M.; van Bavel, S. S.; Loos, J.; Janssen,R. A. J. J. Am. Chem. Soc. 2008, 130, 7721.(59) Kern, J.; Schwab, S.; Deibel, C.; Dyakonov, V. Phys. Status SolidiRRL 2011, 5, 364.(60) Armin, A.; Zhang, Y.; Burn, P. L.; Meredith, P.; Pivrikas, A. Nat.Mater. 2013, 12, 593.(61) Grancini, G.; Binda, M.; Criante, L.; Perissinotto, S.; Maiuri, M.;Fazzi, D.; Petrozza, A.; Egelhaaf, H. J.; Brida, D.; Cerullo, G.; Lanzani,G. Nat. Mater. 2013, 12, 594.(62) Scharber, M. Nat. Mater. 2013, 12, 594.(63) Wojcik, M.; Tachiya, M. J. Chem. Phys. 2009, 130, 104107.(64) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183.(65) Lewis, A. J.; Ruseckas, A.; Gaudin, O. P. M.; Webster, G. R.;Burn, P. L.; Samuel, I. D. W. Org. Electron. 2006, 7, 452.(66) Shaw, P. E.; Ruseckas, A.; Samuel, I. D. W. Adv. Mater. 2008, 20,3516.(67) Ward, A. J.; Ruseckas, A.; Samuel, I. D. W. J. Phys. Chem. C2012, 116, 23931.(68) Cook, S.; Furube, A.; Katoh, R.; Han, L. Y. Chem. Phys. Lett.2009, 478, 33.(69) Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.; Holmes,A. B. Appl. Phys. Lett. 1996, 68, 3120.(70) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science1995, 270, 1789.(71) Howard, I. A.; Laquai, F. d. r.; Keivanidis, P. E.; Friend, R. H.;Greenham, N. C. J. Phys. Chem. C 2009, 113, 21225.(72) Clarke, T. M.; Jamieson, F. C.; Durrant, J. R. J. Phys. Chem. C2009, 113, 20934.(73) Ohkita, H.; Ito, S. Polymer 2011, 52, 4397.(74) Shuttle, C. G.; O’Regan, B.; Ballantyne, A. M.; Nelson, J.;Bradley, D. D. C.; Durrant, J. R. Phys. Rev. B 2008, 78, 113201.(75) Soon, Y. W.; Cho, H.; Low, J.; Bronstein, H.; McCulloch, I.;Durrant, J. R. Chem. Commun. 2013, 49, 1291.(76) Jarzab, D.; Cordella, F.; Gao, J.; Scharber, M.; Egelhaaf, H. J.;Loi, M. A. Adv. Energy Mater. 2011, 1, 604.(77) Kyaw, A. K. K.; Wang, D. H.; Wynands, D.; Zhang, J.; Nguyen,T.-Q.; Bazan, G. C.; Heeger, A. J. Nano Lett. 2013, 13, 3796.(78) Clarke, T.; Ballantyne, A.; Jamieson, F.; Brabec, C.; Nelson, J.;Durrant, J. Chem. Commun. 2009, 89.(79) Clarke, T. M.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.;Durrant, J. R. Adv. Funct. Mater. 2008, 18, 4029.(80) Clarke, T. M.; Ballantyne, A. M.; Tierney, S.; Heeney, M.; Duffy,W.; McCulloch, I.; Nelson, J.; Durrant, J. R. J. Phys. Chem. C 2010,114, 8068.(81) Cook, S.; Ohkita, H.; Durrant, J. R.; Kim, Y.; Benson-Smith, J. J.;Nelson, J.; Bradley, D. D. C. Appl. Phys. Lett. 2006, 89, 101128.(82) Cook, S.; Ohkita, H.; Kim, Y.; Benson-Smith, J. J.; Bradley, D.D. C.; Durrant, J. R. Chem. Phys. Lett. 2007, 445, 276.(83) Dimitrov, S. D.; Nielsen, C. B.; Shoaee, S.; Tuladhar, P. S.; Du,J.; McCulloch, I.; Durrant, J. R. J. Phys. Chem. Lett. 2012, 3, 140.(84) Jamieson, F. C.; Agostinelli, T.; Azimi, H.; Nelson, J.; Durrant, J.R. J. Phys. Chem. Lett. 2010, 1, 3306.(85) Jamieson, F. C.; Domingo, E. B.; McCarthy-Ward, T.; Heeney,M.; Stingelin, N.; Durrant, J. R. Chem. Sci. 2012, 3, 485.(86) Shoaee, S.; Clarke, T. M.; Huang, C.; Barlow, S.; Marder, S. R.;Heeney, M.; McCulloch, I.; Durrant, J. R. J. Am. Chem. Soc. 2010, 132,12919.(87) Soon, Y. W.; Clarke, T. M.; Zhang, W.; Agostinelli, T.;Kirkpatrick, J.; Dyer-Smith, C.; McCulloch, I.; Nelson, J.; Durrant, J. R.Chem. Sci. 2011, 2, 1111.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXM

(88) Bronstein, H.; Collado-Fregoso, E.; Hadipour, A.; Soon, Y. W.;Huang, Z.; Dimitrov, S. D.; Ashraf, R. S.; Rand, B. P.; Watkins, S. E.;Tuladhar, P. S.; Meager, I.; Durrant, J. R.; McCulloch, I. Adv. Funct.Mater. 2013, DOI: 10.1002/adfm.201300287.(89) Clarke, T. M.; Ballantyne, A.; Shoaee, S.; Soon, Y. W.; Duffy,W.; Heeney, M.; McCulloch, I.; Nelson, J.; Durrant, J. R. Adv. Mater.2010, 22, 5287.(90) Credgington, D.; Jamieson, F. C.; Walker, B.; Thuc-Quyen, N.;Durrant, J. R. Adv. Mater. 2012, 24, 2135.(91) Nelson, J. Phys. Rev. B 2003, 67, 155209.(92) Shuttle, C. G.; Hamilton, R.; Nelson, J.; O’Regan, B. C.;Durrant, J. R. Adv. Funct. Mater. 2010, 20, 698.(93) Takacs, C. J.; Sun, Y.; Welch, G. C.; Perez, L. A.; Liu, X.; Wen,W.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2012, 134, 16597.(94) Pandey, L.; Risko, C.; Norton, J. E.; Bredas, J.-L. Macromolecules2012, 45, 6405.(95) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J.V. Nat. Mater. 2009, 8, 904.(96) Li, W.; Roelofs, W. S. C.; Wienk, M. M.; Janssen, R. A. J. J. Am.Chem. Soc. 2012, 134, 13787.(97) Kaake, L. G.; Jasieniak, J. J.; Bakus, R. C.; Welch, G. C.; Moses,D.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2012, 134, 19828.(98) Bansal, N.; Reynolds, L. X.; MacLachlan, A.; Lutz, T.; Ashraf, R.S.; Zhang, W.; Nielsen, C. B.; McCulloch, I.; Rebois, D. G.; Kirchartz,T.; Hill, M. S.; Molloy, K. C.; Nelson, J.; Haque, S. A. Sci. Rep. 2013, 3,1531.(99) Carsten, B.; Szarko, J. M.; Son, H. J.; Wang, W.; Lu, L.; He, F.;Rolczynski, B. S.; Lou, S. J.; Chen, L. X.; Yu, L. J. Am. Chem. Soc. 2011,133, 20468.(100) Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H.J.; Brida, D.; Cerullo, G.; Lanzani, G. Nat. Mater. 2013, 12, 29.(101) Jailaubekov, A. E.; Willard, A. P.; Tritsch, J. R.; Chan, W.-L.;Sai, N.; Gearba, R.; Kaake, L. G.; Williams, K. J.; Leung, K.; Rossky, P.J.; Zhu, X. Y. Nat. Mater. 2013, 12, 66.(102) Onsager, L. Phys. Rev. 1938, 54, 554.(103) Muntwiler, M.; Yang, Q.; Tisdale, W. A.; Zhu, X. Y. Phys. Rev.Lett. 2008, 101, 196403.(104) Zhu, X. Y.; Yang, Q.; Muntwiler, M. Acc. Chem. Res. 2009, 42,1779.(105) Scholes, G. D. ACS Nano 2008, 2, 523.(106) Pensack, R. D.; Asbury, J. B. J. Phys. Chem. Lett. 2010, 1, 2255.(107) Servaites, J. D.; Savoie, B. M.; Brink, J. B.; Marks, T. J.; Ratner,M. A. Energy Environ. Sci. 2012, 5, 8343.(108) Bakulin, A. A.; Hummelen, J. C.; Pshenichnikov, M. S.; vanLoosdrecht, P. H. M. Adv. Funct. Mater. 2010, 20, 1653.(109) Hertel, D.; Soh, E. V.; Bassler, H.; Rothberg, L. J. Chem. Phys.Lett. 2002, 361, 99.(110) Ayzner, A. L.; Doan, S. C.; de Villers, B. T.; Schwartz, B. J. J.Phys. Chem. Lett. 2012, 3, 2281.(111) Coffey, D. C.; Larson, B. W.; Hains, A. W.; Whitaker, J. B.;Kopidakis, N.; Boltalina, O. V.; Strauss, S. H.; Rumbles, G. J. Phys.Chem. C 2012, 116, 8916.(112) Zhou, Y.; Tvingstedt, K.; Zhang, F.; Du, C.; Ni, W.-X.;Andersson, M. R.; Inganas, O. Adv. Funct. Mater. 2009, 19, 3293.(113) Bakulin, A. A.; Martyanov, D.; Paraschuk, D. Y.; Loosdrecht, P.H. M. v.; Pshenichnikov, M. S. Chem. Phys. Lett. 2009, 482, 99.(114) Parkinson, P.; Lloyd-Hughes, J.; Johnston, M. B.; Herz, L. M.Phys. Rev. B 2008, 78, 115321.(115) van der Hofstad, T. G. J.; Di Nuzzo, D.; van den Berg, M.;Janssen, R. A. J.; Meskers, S. C. J. Adv. Energy Mater. 2012, 2, 1095.(116) Tsoi, W. C.; James, D. T.; Kim, J. S.; Nicholson, P. G.;Murphy, C. E.; Bradley, D. D. C.; Nelson, J.; Kim, J.-S. J. Am. Chem.Soc. 2011, 133, 9834.(117) Pfannmoeller, M.; Fluegge, H.; Benner, G.; Wacker, I.;Sommer, C.; Hanselmann, M.; Schmale, S.; Schmidt, H.; Hamprecht,F. A.; Rabe, T.; Kowalsky, W.; Schroeder, R. R. Nano Lett. 2011, 11,3099.

(118) Etzold, F.; Howard, I. A.; Forler, N.; Cho, D. M.; Meister, M.;Mangold, H.; Shu, J.; Hansen, M. R.; Muellen, K.; Laquai, F. J. Am.Chem. Soc. 2012, 134, 10569.(119) Paquin, F.; Latini, G.; Sakowicz, M.; Karsenti, P.-L.; Wang, L.;Beljonne, D.; Stingelin, N.; Silva, C. Phys. Rev. Lett. 2011, 106, 197401.(120) Burkhard, G. F.; Hoke, E. T.; Beiley, Z. M.; McGehee, M. D. J.Phys. Chem. C 2012, 116, 26674.(121) Rance, W. L.; Ferguson, A. J.; McCarthy-Ward, T.; Heeney,M.; Ginley, D. S.; Olson, D. C.; Rumbles, G.; Kopidakis, N. ACS Nano2011, 5, 5635.(122) Collins, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E.; McNeill, C.R.; Ade, H. Adv. Energy Mater. 2013, 3, 65.(123) Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.;Douglas, J. D.; Collins, B. A.; Tumbleston, J. R.; Graham, K. R.;Amassian, A.; Ade, H.; Frechet, J. M. J.; Toney, M. F.; McGehee, M.D. Adv. Energy Mater. 2013, 3, 364.(124) Collins, B. A.; Gann, E.; Guignard, L.; He, X.; McNeill, C. R.;Ade, H. J. Phys. Chem. Lett. 2010, 1, 3160.(125) Yin, W.; Dadmun, M. ACS Nano 2011, 5, 4756.(126) Hopkinson, P. E.; Staniec, P. A.; Pearson, A. J.; Dunbar, A. D.F.; Wang, T.; Ryan, A. J.; Jones, R. A. L.; Lidzey, D. G.; Donald, A. M.Macromolecules 2011, 44, 2908.(127) Rivnay, J.; Noriega, R.; Kline, R. J.; Salleo, A.; Toney, M. F.Phys. Rev. B 2011, 84, 045203.(128) Kaake, L. G.; Welch, G. C.; Moses, D.; Bazan, G. C.; Heeger,A. J. J. Phys. Chem. Lett. 2012, 3, 1253.(129) Theander, M.; Yartsev, A.; Zigmantas, D.; Sundstrom, V.;Mammo, W.; Andersson, M. R.; Inganas, O. Phys. Rev. B 2000, 61,12957.(130) Markov, D. E.; Amsterdam, E.; Blom, P. W. M.; Sieval, A. B.;Hummelen, J. C. J. Phys. Chem. A 2005, 109, 5266.(131) Haugeneder, A.; Neges, M.; Kallinger, C.; Spirkl, W.; Lemmer,U.; Feldmann, J.; Scherf, U.; Harth, E.; Gugel, A.; Mullen, K. Phys. Rev.B 1999, 59, 15346.(132) Burlakov, V. M.; Kawata, K.; Assender, H. E.; Briggs, G. A. D.;Ruseckas, A.; Samuel, I. D. W. Phys. Rev. B 2005, 72, 075206.(133) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D.E. Adv. Mater. 2007, 19, 1551.(134) Scheblykin, I. G.; Yartsev, A.; Pullerits, T.; Gulbinas, V.;Sundstroem, V. J. Phys. Chem. B 2007, 111, 6303.(135) Kroeze, J. E.; Savenije, T. J.; Vermeulen, M. J. W.; Warman, J.M. J. Phys. Chem. B 2003, 107, 7696.(136) Goh, C.; Scully, S. R.; McGehee, M. D. J. Appl. Phys. 2007,101, 114503.(137) Cowan, S. R.; Banerji, N.; Leong, W. L.; Heeger, A. J. Adv.Funct. Mater. 2012, 22, 1116.(138) Banerji, N. J. Mater. Chem. C 2013, 1, 3052.(139) Xin, H.; Guo, X.; Ren, G.; Watson, M. D.; Jenekhe, S. A. Adv.Energy Mater. 2012, 2, 575.(140) Kyaw, A. K. K.; Wang, D. H.; Tseng, H.-R.; Zhang, J.; Bazan,G. C.; Heeger, A. J. Appl. Phys. Lett. 2013, 102, 163308.(141) Walker, B.; Liu, J.; Kim, C.; Welch, G. C.; Park, J. K.; Lin, J.;Zalar, P.; Proctor, C. M.; Seo, J. H.; Bazan, G. C.; Thuc-Quyen, N.Energy Environ. Sci. 2013, 6, 952.(142) You, J.; Chen, C.-C.; Hong, Z.; Yoshimura, K.; Ohya, K.; Xu,R.; Ye, S.; Gao, J.; Li, G.; Yang, Y. Adv. Mater. 2013, 25, 3973.(143) Kaake, L. G.; Sun, Y.; Bazan, G. C.; Heeger, A. J. Appl. Phys.Lett. 2013, 102, 133302.(144) Zhokhavets, U.; Erb, T.; Hoppe, H.; Gobsch, G.; Sariciftci, N.S. Thin Solid Films 2006, 496, 679.(145) Coffey, D. C.; Ferguson, A. J.; Kopidakis, N.; Rumbles, G. ACSNano 2010, 4, 5437.(146) Lloyd, M. T.; Lim, Y. F.; Malliaras, G. G. Appl. Phys. Lett. 2008,92, 143308.(147) van Duren, J. K. J.; Yang, X. N.; Loos, J.; Bulle-Lieuwma, C. W.T.; Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. J. Adv. Funct. Mater.2004, 14, 425.(148) Burkhard, G. F.; Hoke, E. T.; Scully, S. R.; McGehee, M. D.Nano Lett. 2009, 9, 4037.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXN

(149) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.;Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed.2003, 42, 3371.(150) Murthy, D. H. K.; Gao, M.; Vermeulen, M. J. W.; Siebbeles, L.D. A.; Savenije, T. J. J. Phys. Chem. C 2012, 116, 9214.(151) Hammond, M. R.; Kline, R. J.; Herzing, A. A.; Richter, L. J.;Germack, D. S.; Ro, H.-W.; Soles, C. L.; Fischer, D. A.; Xu, T.; Yu, L.;Toney, M. F.; DeLongchamp, D. M. ACS Nano 2011, 5, 8248.(152) Ajuria, J.; Chavhan, S.; Tena-Zaera, R.; Chen, J. H.;Rondinone, A. J.; Sonar, P.; Dodabalapur, A.; Pacios, R. Org. Electron.2013, 14, 326.(153) Manca, M.; Piliego, C.; Wang, E.; Andersson, M. R.; Mura, A.;Loi, M. A. J. Mater. Chem. A 2013, 1, 7321.(154) Shoaee, S.; Eng, M. P.; Espildora, E.; Luis Delgado, J.; Campo,B.; Martin, N.; Vanderzande, D.; Durrant, J. R. Energy Environ. Sci.2010, 3, 971.(155) Faist, M. A.; Shoaee, S.; Tuladhar, S.; Dibb, G. F. A.; Foster, S.;Gong, W.; Kirchartz, T.; Bradley, D. D. C.; Durrant, J. R.; Nelson, J.Adv. Energy Mater. 2013, 3, 744.(156) Hodgkiss, J. M.; Campbell, A. R.; Marsh, R. A.; Rao, A.; Albert-Seifried, S.; Friend, R. H. Phys. Rev. Lett. 2010, 104, 177701.(157) Pal, S. K.; Kesti, T.; Maiti, M.; Zhang, F.; Inganas, O.;Hellstrom, S.; Andersson, M. R.; Oswald, F.; Langa, F.; Osterman, T.;Pascher, T.; Yartsev, A.; Sundstrom, V. J. Am. Chem. Soc. 2010, 132,12440.(158) Rand, B. P.; Cheyns, D.; Vasseur, K.; Giebink, N. C.; Mothy,S.; Yi, Y.; Coropceanu, V.; Beljonne, D.; Cornil, J.; Bredas, J.-L.;Genoe, J. Adv. Funct. Mater. 2012, 22, 2987.(159) Reid, O. G.; Malik, J. A. N.; Latini, G.; Dayal, S.; Kopidakis, N.;Silva, C.; Stingelin, N.; Rumbles, G. J. Polym. Sci., Part B: Polym. Phys.2012, 50, 27.(160) Savenije, T. J.; Kroeze, J. E.; Yang, X.; Loos, J. Thin Solid Films2006, 511, 2.(161) Skompska, M.; Szkurlat, A. Electrochim. Acta 2001, 46, 4007.(162) Vithanage, D. A.; Devizis, A.; Abramavicius, V.; Infahsaeng, Y.;Abramavicius, D.; MacKenzie, R. C. I.; Keivanidis, P. E.; Yartsev, A.;Hertel, D.; Nelson, J.; Sundstrom, V.; Gulbinas, V. Nat. Commun.2013, 4.(163) van Eersel, H.; Janssen, R. A. J.; Kemerink, M. Adv. Funct.Mater. 2012, 22, 2700.(164) Marsh, R. A.; Hodgkiss, J. M.; Friend, R. H. Adv. Mater. 2010,22, 3672.(165) Albrecht, S.; Schindler, W.; Kurpiers, J.; Kniepert, J.; Blakesley,J. C.; Dumsch, I.; Allard, S.; Fostiropoulos, K.; Scherf, U.; Neher, D. J.Phys. Chem. Lett. 2012, 3, 640.(166) Kniepert, J.; Schubert, M.; Blakesley, J. C.; Neher, D. J. Phys.Chem. Lett. 2011, 2, 700.(167) Dibb, G. F. A.; Jamieson, F. C.; Maurano, A.; Nelson, J.;Durrant, J. R. J. Phys. Chem. Lett. 2013, 4, 803.(168) Di Nuzzo, D.; Aguirre, A.; Shahid, M.; Gevaerts, V. S.;Meskers, S. C. J.; Janssen, R. A. J. Adv. Mater. 2010, 22, 4321.(169) Liedtke, M.; Sperlich, A.; Kraus, H.; Baumann, A.; Deibel, C.;Wirix, M. J. M.; Loos, J.; Cardona, C. M.; Dyakonov, V. J. Am. Chem.Soc. 2011, 133, 9088.(170) Schlenker, C. W.; Chen, K.-S.; Yip, H.-L.; Li, C.-Z.; Bradshaw,L. R.; Ochsenbein, S. T.; Ding, F.; Li, X. S.; Gamelin, D. R.; Jen, A. K.Y.; Ginger, D. S. J. Am. Chem. Soc. 2012, 134, 19661.(171) Rao, A.; Chow, P. C. Y.; Gelinas, S.; Schlenker, C. W.; Li, C.-Z.; Yip, H.-L.; Jen, A. K. Y.; Ginger, D. S.; Friend, R. H. Nature 2013,500, 435.(172) Yi, Y.; Coropceanu, V.; Bredas, J.-L. J. Am. Chem. Soc. 2009,131, 15777.(173) Pensack, R. D.; Guo, C.; Vakhshouri, K.; Gomez, E. D.;Asbury, J. B. J. Phys. Chem. C 2012, 116, 4824.(174) Pensack, R. D.; Asbury, J. B. J. Am. Chem. Soc. 2009, 131,15986.(175) Grzegorczyk, W. J.; Savenije, T. J.; Dykstra, T. E.; Piris, J.;Schins, J. M.; Siebbeles, L. D. A. J. Phys. Chem. C 2010, 114, 5182.

(176) Zhou, J. W.; Findley, B. R.; Braun, C. L.; Sutin, N. J. Chem.Phys. 2001, 114, 10448.

Chemistry of Materials Review

dx.doi.org/10.1021/cm402403z | Chem. Mater. XXXX, XXX, XXX−XXXO