Charge photogeneration indonor/acceptor organic solarcells

Safa ShoaeeTracey M. ClarkeMattias P. EngChun HuangStephen BarlowEva EspíldoraJuan Luis DelgadoBert CampoSeth R. MarderMartin HeeneyIain McCullochNazario MartínDirk VanderzandeJames R. Durrant

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Charge photogeneration in donor/acceptororganic solar cells

Safa Shoaee,a Tracey M. Clarke,a Mattias P. Eng,a Chun Huang,b

Stephen Barlow,b Eva Espíldora,e Juan Luis Delgado,e,f

Bert Campo,g Seth R. Marder,b Martin Heeney,c,d

Iain McCulloch,a,c Nazario Martín,e,f

Dirk Vanderzande,g and James R. DurrantaaImperial College London, Centre for Plastic Electronics,

Department of Chemistry, London, SW7 2AZ, United Kingdoms.shoaee06@imperial.ac.uk

bGeorgia Institute of Technology, School of Chemistry & Biochemistry and Center for OrganicPhotonics and Electronics, 901 Atlantic Drive, Atlanta, Georgia 30332-0400

cChilworth Science Park, Merck Chemicals, Southampton SO16 7QD, United KingdomdUniversity of London, Department of Materials, Queen Mary, EI 4NS, United Kingdom

eFacultad de Quimica Universidad Complutense,Departamento de Quimica Organica, E-28040 Madrid, Spain

fCiudad Universitaria de Cantoblanco, IMDEA-Nanociencia, Facultad de Ciencias,Módulo C-IX, 3a planta, E-28049 Madrid, Spain

gInstitute for Materials Research, Campus Diepenbeek, Division Chemistry,Agoralaan Gebouw D, BE 3590 Diepenbeek, Belgium

Abstract. We focus upon the role of interfacial energetics and morphology in influencing theseparation of CT states into dissociated charge carriers. In particular, we undertake transientoptical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended witha series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors. For the PDIfilm series, we observe a close correlation between the PDI electron affinity and the efficiency ofcharge separation. This correlation is discussed in the context of studies of charge photogenera-tion for other organic donor/acceptor blend films, including other polymers, blend compositions,and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). Furthermore, wecompare the charge recombination dynamics observed in films comprising P3HT blendedwith three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transientabsorption data indicate that replacement of PCBM with either of the pyrazolinofullerenederivatives results in a transition from nongeminate to monomolecular (geminate) recombinationdynamics. We show that this transition cannot be explained by a difference in interfacialenergetics. However, this transition does correlate with nanomorphology data that indicate thatboth pyrazolinofullerenes yield a much finer phase segregation with correspondingly smallerdomain sizes than observed with PCBM. Our results therefore provide clear evidence ofthe role of nanomorphology in determining the nature of recombination dynamics in suchdonor/acceptor blends. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI:10.1117/1.JPE.2.021001]

Keywords: charge photogeneration; nanomorphology; energy for charge separation.

Paper 11237SSP received Oct. 5, 2011; revised manuscript received Nov. 16, 2011; accepted forpublication Dec. 1, 2011; published online Mar. 12, 2012.

1 Introduction

Organic donor/acceptor blend films are attracting increasing interest for photovoltaic solarenergy conversion, with reported device efficiencies now exceeding 8%.1–3 The function of

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such devices is based on photoinduced charge separation at the donor/acceptor interface, asillustrated in Fig. 1.

Selecting the appropriate materials for bulk heterojunction organic solar cells is essential toproduce efficient devices. Here, we concentrate on electron-accepting materials. The range ofelectron acceptors that work well with common electron donors, namely polythiophenes andpoly(p-phenylene vinylenes), is surprisingly limited. The soluble C60 derivative, methanoful-lerene, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), has been studied extensively as anelectron acceptor and has produced some of the most efficient photovoltaic (PV) devices to date.In this study, we investigate some of the parameters affecting the efficiency of charge photo-generation by changing the electron transporter material from PCBM to perylene diimide (PDI)derivatives and other fullerene derivatives. The chemical structures of the materials are givenin Fig. 2.

One of the central issues in the development of efficient organic photovoltaic devices is todevelop an understanding of the relationship between molecular structure and photovoltaicdevice performance. This work primarily focuses on specific aspects of the materials’structure/efficiency of charge photogeneration at the donor/acceptor interface and the subsequent

Fig. 1 Energy level diagram of a donor/acceptor interface showing a simplified viewpoint ofphotoexcitation of an electron into the donor LUMO (with efficiency ηABS), followed by excitondissociation via electron transfer into the acceptor LUMO (ηDISSðEXÞ) and migration of the separatedcharges away from the interface (ηCOLL). Also illustrated is the potential for the electron transferinitially to generate a Coulombically bound CT state that also requires dissociation (ηDISSðCTÞ)before the free charge carriers can be collected.

Fig. 2 Molecular structures of perylene diimide derivatives (PDIX ), and pyrazolinofullerenederivative (C60-1 and C60-2).

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morphology in such solid films. We focus on the investigation of photophysics of photogener-ated charges by employing transient absorption spectroscopy (TAS) to monitor the subsequenteffect of energetics and nanomorphology in donor/acceptor blend films comprising of polythio-phene:perylene diimides and poly-3-hexylthiophene (P3HT):fullerene, which influence theobtained efficiency of photocurrent in devices.

The efficiency of charge photogeneration in organic donor/acceptor blend films has beenreported to depend on a range of factors, including film nanomorphology,4–11 the presence ofmacroscopic electric fields,12–14 the dielectric constant of the blend,4,15,16 charge-carrier mobi-lities,4,7,17–21 the energy difference driving charge separation (ΔGCS),

21–23 and the strength ofelectronic interactions at the donor/acceptor interface.24 There is increasing evidence that theefficiency of this charge-separation process can be determined by the presence of Coulombicallybound charge-transfer (CT) states at the donor/acceptor interface.4,12,13,22,23,25–30 Such studieshave typically concluded that the efficiency of dissociation of the CT state into free chargecarriers, illustrated as ηDISSðCTÞ in Fig. 1, is a crucial factor in determining the charge photoge-neration yield and thus device photocurrent densities. At present, there is little consensusconcerning the relative importance of the different factors potentially influencing charge photo-generation, and in particular how the efficiency of charge photogeneration can be related to, orpredicted from, specific materials properties.

We have previously conducted studies of the yield of the charge-photogeneration process for aseries of polythiophenes blended with PCBM.22,4 In this materials series, we observed a correla-tion between the energy levels of the polythiophene donors and the yield of dissociated chargecarriers, as quantified by the magnitude of photoinduced absorption of these charges (change inoptical density [ΔOD] measured at a time delay of 1 μs) determined by high-sensitivity TAS.More specifically, we observed a correlation between ΔOD and the energetic driving force forcharge separation, ΔGeff

CS, defined as ΔGeffCS ¼ ES − ðIPD − EAAÞ where ES is the energy of the

polymer singlet exciton, IPD the ionisation potential of the donor, and EAA the electron affinity ofthe acceptor. It was proposed that this correlation was consistent with a model in which efficientcharge separation was dependent upon the initially generated CT state possessing sufficientexcess thermal energy provided by ΔGeff

CS to overcome the Coulombic binding of this CTstate, resulting in a high value for ηDISSðCTÞ. This model is analogous to Onsager’s theory ofcharge separation, which is based upon the concept that the efficiency of separation of photo-generated geminate charge pairs is dependent upon the relative magnitudes of their thermalizationlength versus their Coulomb capture radius. It has been suggested that a large LUMO-LUMO(lowest unoccupied molecular orbital) offset (∼ΔGeff

CS) at the organic donor/acceptor interface willproduce a greater thermalization length,31 which in turn would increase the probability of separa-tion of the bound radical pairs into the fully dissociated charge carriers. The dependence of chargeseparation upon ΔGeff

CS observed by Okhita et al.22 has important implications for materialsdesign, as it suggested, at least for the polythiophene/PCBM series studied, that a relativelylarge energy loss (0.9 eV) was required to drive efficient charge separation.

We have subsequently extended our studies of charge photogeneration to other donor poly-mers,25,32 blend compositions,26,33 and the effect of thermal annealing.23 In these studies, furthercorrelations between ΔGCS and charge photogeneration were observed, as well as additionaleffects attributed to the influence of film nanomorphology and the CT character of the polymerexciton. In this paper, we focus upon the role of acceptor electron affinity in determining theefficiency of charge photogeneration, and specifically ηDISSðCTÞ. In particular, we study chargephotogeneration in films comprising 1∶1 blends of P3HT with a series of different perylenediimide derivatives (PDIx, Fig. 2) with varying electron affinities in order to investigate therole of the electron acceptor on ΔGCS and thus charge separation.

2 Results and Discussion

We have previously demonstrated that high-sensitivity micro- to millisecond TAS can beemployed to monitor the yield of dissociated polarons in such donor/acceptor blendfilms.21,22 Power law (ΔOD α t−α) absorption transients (with 0.38 < α < 0.50) were observedfor all P3HT:PDIX films. While similar dynamics were observed for all films, the amplitude

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of these absorption transients varied significantly. For convenience, we quantify these differentsignal intensities by the magnitude of the PDI anion absorption at 700 nm34,35 at a time delay of1 μs (corresponding to the time delay used in our previous studies of polymer/PCBM blendfilms22 and approximating the typical timescale for charge collection in such donor/acceptorblend devices). The amplitudes of decay transients were observed to vary approximately linearly,with excitation density indicating that neither saturation effects nor nongeminate losses prior to1 μs significantly distorted this comparison. These ΔOD signal intensities are summarized inTable 1, normalized for variations (<� 35%) in the density of absorbed photons due to variationsin film optical density at the excitation wavelength. It is apparent that these changes in ΔODsignal intensities, and therefore yield of dissociated polarons, do not correlate with the variationin PL quenching observed between the blend films. As such, these variations in polaron yieldscannot be assigned primarily to changes in the efficiency of exciton dissociation ηDISSðEXCÞ.Rather, as we have concluded previously for other materials systems,22,23,25,26 the observationvariation in polaron yield is more reasonably assigned primarily to variations in geminate recom-bination losses on the pico-nanosecond timescales and thus to the variations in the efficiency ofdissociation of interfacial CT states into free charges, ηDISSðCTÞ.

By normalizing the ΔOD data to take account of the (small) variations in PL quenching,and thus ηDISSðEXCÞ, observed between the blend films, as detailed in Table 1 as ΔOD∕PLQ(photoluminescence quenching), ΔOD∕PLQ should be a direct indicator of the efficiency ofdissociation of initially generated interfacial polarons into dissociated charges, ηDISSðCTÞ. Figure 3shows a plot of the correlation between this assay of CT state dissociation and the energeticdriving force for charge separation ΔGeff

CS ¼ ES − ðIPD − EAAÞ. Estimates were performedusing the P3HT singlet energies for ES. The variation in ΔGeff

CS between the different blendfilms results solely from variations in the PDI electron affinities, as both ES and IPD correspondto the P3HT, which is invariant for this film series. We thus conclude that for this materials series,there is generally an excellent correlation between the LUMO energy level of the acceptor andthe yield of dissociated polarons.

The results presented herein build upon our previous studies, which have indicated that a keyparameter determining the efficiency of charge photogeneration in polymer/small molecule

Table 1 Properties of the P3HT:PDIx blends.

Blend (P3HT:PDIx )aEA (eV) bΔGeff

CS (eV) cΔOD dPLQ (%) ΔOD∕PLQ

PDIA 3.52 0.72 4.2 × 10−4 0.86 4.8 × 10−4

PDIB 3.52 0.72 5.1 × 10−4 0.92 4.7 × 10−4

PDIC 3.55 0.75 3.7 × 10−4 0.70 5.3 × 10−4

PDID 3.56 0.76 5.0 × 10−4 0.90 5.6 × 10−4

PDIE 3.59 0.79 5.1 × 10−4 0.83 6.2 × 10−4

PDIF 3.64 0.84 3.1 × 10−4 0.88 4.5 × 10−4

PDIG 3.69 0.89 6.3 × 10−4 0.78 8.1 × 10−4

PDIH 3.59 0.79 5.3 × 10−4 0.85 6.24 × 10−4

PDII 3.59 0.79 5.2 × 10−4 0.85 6.1 × 10−4

PDIJ 3.60 0.80 5.0 × 10−4 0.80 6.3 × 10−4

PDI2 3.50 0.70 2.9 × 10−4 0.70 4.1 × 10−4

aEstimated electron affinities evaluated by cyclic voltammetry from the peak value.bΔGCS estimated as ES − ðIP − EAÞ, where IP is the ionisation potential of the polymer evaluated by anambient ultraviolet photoelectron spectroscopy technique (4.8 eV for P3HT) and ES the singlet excitonenergy of the donor, measured from the intercept of the normalised absorption and emission spectra.

cΔOD evaluated from the amplitude of the transient absorbance power-law decay phase at 1 μs(λprobe ¼ 700 nm), after correction for the ground state absorbance at the excitation wavelength.dSteady-state PL quenching of the blend film relative to the corresponding pristine polymer film.

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blend films is the energetic driving force for charge separation ΔGeffCS. In particular, we have

extended our previous studies as a function of donor polymer ionisation potential and singletexciton energy22 to a study as a function of acceptor electron affinity. For the PDI acceptor seriesstudied herein, a remarkably good correlation is observed between the efficiency of chargephotogeneration, as determined by a transient absorption assay of the dissociated polarons,and ΔGeff

CS. We discuss first of all the charge-separation model that we have proposed to explainsuch energetic correlations and then go on to make a quantitative comparison of the correlationwe observe herein with those we have reported previously for polymer/PCBM blend films.

Figure 4 summarizes a kinetic model for charge photogeneration in donor/acceptor systems,proceeding via the formation of an interfacial bound polaron pair or CT state, as we and othershave proposed previously.22,23,34,36 We note that a similar model has previously been proposedto describe magnetic field and other effects on charge photogeneration in neat conjugatedpolymers.37,38 The model shown in Fig. 4 is based upon our experimental observation that

Fig. 3 Transient absorbance signal measured at 1 μs of various P3HT:PDIx (1∶1) blend filmsplotted against −ΔGeff

CS, estimated as Es − ðIPD − EAAÞ. The transient signal has been correctedfor variation in the optical absorbance at the excitation wavelength (500 nm) and the PL quench-ing. λprb ¼ 700 nm at 50 μJ cm−2.

Fig. 4 Energy diagram for charge formation in a donor/acceptor (D∕A) system via a bound(Dþ · · · A−) CT state. The initially formed bound CT state ðDþ · · · A−Þhot can undergo either ther-malization (k therm) or dissociation (kdiss) into the free charge carriers Dþ þ A−, which can thencontribute to the device current. Charges can recombine either by geminate (GR) or nongeminate(BR) recombination. We note that nongeminate recombination is likely to proceed via reformationof interfacial CT states.

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while polymer excitons are usually efficiently quenched at the donor/acceptor interfaces for allthe blend films considered herein, the final yield of dissociated charges is dependent upon theexciton energy ES relative to the energy of the dissociated polaron pairs (as defined byIPD − EAA). Following this model, the initial charge separation of the exciton results in the for-mation of a vibrationally excited (“hot”) CT state, analogous to the “crossing point” in Marcuselectron-transfer theory. The Coulombic attraction of these CT states results in a significantpotential energy barrier to their dissociation, as we discuss in detail elsewhere in terms of aCT state binding energy.39 The model is based upon the concept that the excess thermal energyof the initially formed hot CT state can enable the dissociation of the CT state into free charges.40

As such, the magnitude of the excess vibrational energy (which can be expected to be propor-tional toΔGeff

CS) can be expected to correlate with the efficiency of CT state dissociation ηDISSðCTÞ,consistent with the correlation we observe experimentally. Within this model, the key kineticcompetition in the photogeneration process is between vibrational relaxation (ktherm) of theinitially generated hot CT state and dissociation (kdiss) of this species. CT states that thermallyrelax prior charge dissociation are likely to undergo geminate recombination, with this mono-molecular process thus being the primary charge carrier loss pathway. We note that it may also bepossible for relaxed CT states to dissociate into free charges, although the importance of thispathway in the overall process of charge photogeneration is currently unclear.

The results reported in this paper provide further evidence that a key factor determining theefficiency of charge photogeneration in organic donor/acceptor blend films is the free energydifference driving charge separation, referred to herein as ΔGeff

CS. In particular we demonstratethat for a series of electron acceptors with differing electron affinities, there is a direct correlationbetween the acceptor LUMO level and the efficiency of charge dissociation. This dependence isnot correlated with the efficiency of exciton quenching at this interface (as measured by polymerexciton PLQ) but rather assigned to the dissociation of interfacial CT states being dependentupon the excess thermal energy of the initially generated CT state.

Fig. 5 Plot showing the correlation between our transient absorbance assay of the yield of dis-sociated charges and the energy difference driving charge photogeneration for six different donor/acceptor materials series evaluated from the amplitude of the transient absorbance power lawdecay at 100 ns (data determined at 1 μs and 100 ns give essentially indistinguishable plots).ΔOD signal amplitude has been normalized to account for differences in film absorption at theexcitation wavelength, excitation energy density (with all points being collected at excitation den-sities low enough to avoid saturation effects) and any variations in nongeminate losses between100 ns and 1 μs prior to the ΔOD measurement time delay of 1 μs. Data for the P3HT:PDIx ,polymer:PDID , and polymer:PDI2 series are reported herein and include correction for PL quench-ing efficiency, as detailed above. These two data series employed a 700-nm probe wavelength tomonitor the PDI anion absorption; all other data series probed the NIR polymer polaron absorptionmaximum, typically at 980 nm. The data employing the 700-nm probe have been normalized toaccount for the relative high PDI anion extinction coefficient relative to typical polymer polarons.Data for the polymer:PDI2 are taken from Ref. 29, with the additional correction for PL quenchingefficiency. Data for the three PCBM-based series have been reported previously.4–6 The electronaffinities of the PDIs have been normalized against that of PCBM at 3.7 eV.

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More generally, we have now observed clear correlations between charge photogenerationyield and ΔGeff

CS for six different donor/acceptor materials series, as shown in Fig. 5, providingclear evidence that this energetic dependence is a key factor determining charge photogenerationefficiency for many organic blend films. We further find that differences in charge photogenera-tion between materials series cannot attributed to this energetic dependence. Rather, suchdifferences between materials series are indicators of some of the other factors also influencingcharge photogeneration in such blend films, including film nanomorphology, the CT character ofthe donor polymer, and the electron mobility of the acceptor. As such, we are starting to developa series of materials design rules for charge photogeneration at organic donor/acceptor interfacesthat can be employed to aid materials and blend nanomorphology design for increasing theperformance of organic solar cells.

Numerous studies provide general evidence indicating the importance of CT states indetermining the yield of charge photogeneration. The majority of the studies have focused uponcomparative studies as a function of nanomorphology. We therefore turn now to consideringthis issue. The observation of efficient photoinduced electron transfer from a semiconductingpolymer to a fullerene in blend films has motivated extensive studies of the application of suchfilms for photovoltaic solar energy conversion.41–43 For such blend films, the formation of aninterpenetrating donor/acceptor network, with nanoscale phase separation (the bulk heterojunc-tion approach), is essential to enable efficient separation of photogenerated excitons at the poly-mer/fullerene interface.42 Following this charge photogeneration, two alternative recombinationpathways can compete with charge collection and photocurrent generation: geminate recombi-nation of the initially generated charges at the polymer/fullerene interface44–49 and nongeminaterecombination of dissociated charges during their transport to the device electrodes.50,51 We havepreviously shown that the extent of geminate recombination losses can be strongly dependentupon the charge separation energetics at the donor/acceptor interface.22 Furthermore, severalstudies have addressed the potential importance of nanomorphology, and in particular domainsize, in influencing the importance of geminate versus nongeminate recombination losses.52–54 Inthis study, we focus upon the correlation between blend nanomorphology and recombinationdynamics and show a transition between geminate and nongeminate recombination of photo-generated charges correlated with a change in blend nanomorphology.

For the study reported herein, we employ films of polythiophenes blended with threedifferent C60 derivatives. Our control films are based upon films of PCBM blended withP3HT. Data for these control films are compared with blend films employing two alternativepyrazolinofullerenes: C60-1 and C60-2, illustrated in Fig. 2.

Electrochemical studies determined that C60-1 and C60-2 are both stronger electron acceptors(less negative reduction potentials) than PCBM.55 As a consequence, the energetic driving forcefor charge separation will be larger for C60-1 and C60-2 compared with PCBM. Following ourprevious22 and above studies, we therefore expect, in terms of energetic considerations alone,replacement of PCBM with either C60-1 or C60-2 to result in lower geminate recombinationlosses, due to the initially formed charges having more thermal energy available to overcome

Fig. 6 PL quenching of blends of C60-1, C60-2, and PCBM together with 50 wt% of P3HT, excitedat 550 nm.

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their Coulomb attraction. Thus, we might expect more dissociated charges (less geminaterecombination) when using C60-1 and C60-2 as electron acceptors rather than PCBM.

PL data were collected to evaluate the efficiency of exciton quenching for the three blendfilms. As shown in Fig. 6, for all blend films studied herein, we observe quenching of the P3HTpolymer PL by inclusion of 50 weight percent (wt%) of the fullerene to be >90%, indicative ofefficient exciton diffusion to and quenching at the polymer/fullerene interface. However, as wehave shown previously, PL quenching alone is not a reliable indicator of the generation of fullydissociated charges, due to the potential of the initially formed radical pairs to undergo geminaterecombination rather than dissociation into separated charges.22 To address this issue, weemployed TAS to monitor the charge separation yields and recombination dynamics.56

TAS of all the blend films shows the formation of P3HTþ radical cations with a typicalabsorption around 900 to 1000 nm. The spectra of P3HT:C60 is consistent with literatureexhibiting spectral features characteristic of radical ion formation (e.g., P3HTþ absorptionmaximum at ∼950 nm57–60 and C60- absorption at ∼1000 nm61,62). Figure 7 shows typicaldecay dynamics of the photoinduced absorption, monitored at the P3HTþ absorption maximum(950 nm). These decay dynamics were all oxygen independent, consistent with their assignmentto polaron rather than triplet absorption. The control P3HT:PCBM film exhibited a strong, long-lived ΔOD signal, obeying power law (ΔOD α t−0.7, corresponding to a straight line on the log/log plot shown), typical of those we have reported previously, and assigned, as previously, tonongeminate recombination. In contrast, both the P3HT:C60-1 and P3HT:C60-2 blend films exhi-bit a relatively weak, much shorter-lived transient signal. Moreover, these faster decay dynamicsare not power laws, but rather monoexponentials, as evidenced by the straight line on the log/linear insert plot. This monoexponential (first order) behavior, and the much faster decaydynamics, are clear evidence that the decay dynamics of P3HT:C60-1 and P3HT:C60-2 filmsare dominated by monomolecular (geminate) rather than nongeminate recombination.

The recombination dynamics of the blend films were further investigated by excitationintensity studies. Typical data for a P3HT:C60-1 blend film is shown in Fig. 8. As expected fora first-order process, the signal amplitude increases linearly with excitation density, while thedecay dynamics are invariant. In contrast, the absorption transients for the P3HT:PCBM films(Fig. 8), showed a sublinear increase in amplitude, and faster decay dynamics, with increasingexcitation density, consistent with their assignment to nongeminate recombination.

We thus conclude that replacing PCBM in P3HT/PCBM blend films with either C60-1 orC60-2 results in a transition from nongeminate to geminate recombination dynamics. More

Fig. 7 Transient decay on log/log plot: blends of C60-1 (red), C60-2 (blue) obeying monoexponen-tial and PCBM blend (black) as control film exhibiting a straight light with power law characteristics.Inset: Same blend films data on log-linear plot with monoexponential fits. Excited at 533 nm,probed at 970 nm at 60 μJ · cm−2, N2 atmosphere.

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specifically, P3HT:C60-1 and P3HT:C60-2 blend films exhibit much smoother blend filmsindicative of a finer blend nanomorphology, reducing the blend domain size, and thus impedingthe dissociation of the initially generated radicals pairs to dissociate into spatially uncorrelatedcharges.

This is consistent with our atomic force microscope (AFM) micrographs (Fig. 9) of the threeblends, showing a remarkably smooth image for P3HT:C60-1 and P3HT:C60-2 blend films,indicating a high degree of blending and interpenetration of the P3HT and fullerene in thisblend film compared with the rougher AFM image, indicative of significant phase segregationfor the P3HT:PCBM control. The finer nanomorphology of the P3HT:C60-1 and P3HT:C60-2

Fig. 8 Excitation intensity dependence of (top) P3HT:C60-1 and (bottom) P3HT:PCBM blend filmunder N2 atmosphere. Excited at 533 nm and probed at 970 nm.

Fig. 9 Tapping mode atomic force microscopy images of P3HT:C60-1 (left), P3HT:C60-2 (middle),and P3HT:PCBM (right) blend films.

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films are consistent with C60-1’s and C60-2’s reduced tendency to crystallize; previous studieshave identified PCBM crystallization as a key driver for P3HT:PCBM phase segregation.

Several theoretical modeling and device performance analyses have previously proposed acorrelation between nanomorphology and geminate recombination in polymer/PCBM blendfilms.54,63,64 For example, studies as a function of annealing and PCBM composition havebeen interpreted in terms of changes in domain size influencing the efficiency of dissociationof the initially generated geminate radical pairs.52,53,65 However, the data we report herein are adirect observation of a transition in the observed decay dynamics from monomolecular to non-geminate induced by such a change in nanomorphology. At present we are unclear whether thegeminate recombination we observe herein on a 100-ns timescale corresponds to the recombi-nation of Coulombically bound radical pairs, or rather the recombination of polaron pairs thathave escaped their Coulomb attraction but remain physically confined by the physical size oftheir respective domains. In this regard, we note that the initial amplitude of the polaron signals(measured at ∼10 ns) is significantly smaller for the P3HT:C60-1 and P3HT:C60-2 blendsrelative to P3HT:PCBM, indicative of the presence of an additional, faster decay phase for theP3HT:C60-1 and P3HT:C60-2 blends. We note that picosecond timescale geminate recombina-tion has been previously reported in nonannealed P3HT:PCBM blend films.66 On the other hand,geminate recombination dynamics of Coulombically bound, emissive CT states in polymer/polymer blends (sometimes referred to as “exciplexes”) have been reported to occur on the∼50-ns timescale.52,67,68 Notwithstanding these uncertainties, it is clear from the data shownherein that blend nanomorphology can have a substantial role in minimizing geminate recom-bination in the organic polymer/fullerene blend films.

3 Concluding Remarks

The results reported in this paper provide further evidence that a key factor determining theefficiency of charge photogeneration in organic donor/acceptor blend films is the free energydifference driving charge separation, referred to herein as ΔGeff

CS. In particular, we demonstratethat for a series of electron acceptors with differing electron affinities, there is a direct correlationbetween the acceptor LUMO level and the efficiency of charge dissociation. This dependence isnot correlated with the efficiency of exciton quenching at this interface (as measured by polymerexciton photoluminescence quenching) but rather assigned to the dissociation of interfacial CTstates being dependent upon the excess thermal energy of the initially generated CT state.

More generally, we have now observed clear correlations between charge photogenerationyield and ΔGeff

CS for six different donor:acceptor materials series, as shown in Fig. 5, providingclear evidence that this energetic dependence is a key factor determining charge photogenerationefficiency for many organic blend films. Moreover, we further find that differences in chargephotogeneration between materials series cannot be attributed to this energetic dependence.Rather, such differences between materials series are indicators of some of the other factorsalso influencing charge photogeneration in such blend films, including the CT character ofthe donor polymer, the electron mobility of the acceptor, and the film nanomorphology asreported above; replacement of PCBM in P3HT:PCBM blend films with either C60-1 orC60-2 results in a transition from nongeminate to geminate recombination dynamics. As we haveargued above, it appears likely that this transition results from the pronounced change in nano-morphology between the blend films. This study therefore demonstrates that nanomorphology iskey not only to determining the efficiency of exciton quenching and charge transport in organicbulk heterojunction solar cells, but is furthermore important in enabling the dissociation of theinitially generated radical pairs.

As such, we are starting to develop a series of materials design rules for charge photogenera-tion at organic donor/acceptor interfaces that can be employed to aid materials and blendnanomorphology design for increasing the performance of organic solar cells.

Acknowledgments

The authors are grateful to Solvay SA and the EPSRC from the UK Office of Science and Tech-nology. Work was also supported financially by the Science and Technology Center Program of

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the National Science Foundation (DMR-0120967) and by the Office of Naval Research(N00014-04-1-0120). Dr. Keivanidis and Professor Nelson provided helpful discussions, andJohn Seddon allowed the use of his light microscope. MPE acknowledges the Knut andAlice Wallenberg foundation for funding. This work has been supported by the EuropeanScience Foundation (05-SONS-FP-021 SOHYD), the Spanish Ministry of Education andScience (CT2008-00795/BQU), Consolider-Ingenio (2010C-07-25200, Nanociencia Molecu-lar), and Comunidad de Madrid (P-PPQ-000225-0505). JLD thanks the Ministry of Scienceand Innovation for a Ramón y Cajal Fellowship, cofinanced by the EU Social Funds. Theresearch of BC is funded by a PhD grant of the Institute for the Promotion of Innovation throughScience and Technology in Flanders (IWT-Vlaanderen). Supporting information is availableonline from Wiley InterScience or from the author.

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Biography and photographs of authors are not available.

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