CHEMPHYSCHEM MINIREVIEWS DOI: 10.1002/cphc.201301022

Free Carrier Generation in Organic Photovoltaic Bulk Heterojunctions of Conjugated Polymers with Molecular Acceptors: Planar versus Spherical Acceptors Alexandre M. Nardes,[a] Andrew J. Ferguson,[a] Pascal Wolfer,[b] Kurt Gui,[b] Paul L. Burn,[b] Paul Meredith,[b] and Nikos Kopidakis*[a] A comparative study of the photophysical performance of the prototypical fullerene derivative PC61BM with a planar smallmolecule acceptor in an organic photovoltaic device is presented. The small-molecule planar acceptor is 2-[{7-(9,9-di-npropyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile, termed K12. We discuss photoinduced free charge-carrier generation and transport in blends of PC61BM or K12 with poly(3-n-hexylthiophene) (P3HT), surveying literature results for P3HT:PC61BM and presenting new results on P3HT:K12. For both systems we also review previous work on film structure and correlate the structural and photophysical results. In both cases, a disordered mixed phase is formed between P3HT and the acceptor, although the photophysical properties of this mixed phase differ markedly for PC61BM and K12. In the case of PC61BM the mixed phase acts as a free carrier generation region that can efficiently shuttle carriers to the pure polymer and fullerene domains. As a result, the vast majority of excitons quenched in P3HT:PC61BM blends yield free carriers detected by the contactless time-resolved microwave conductivity (TRMC) method. In contrast, approximately 85 %

of the excitons quenched in P3HT:K12 do not result in free carriers over the nanosecond timescale of the TRMC experiment. We attribute this to poor electron-transport properties in the mixed P3HT:K12 phase. We propose that the observed differences can be traced to the respective shapes of PC61BM and K12: the three-dimensional nature of the fullerene cage facilitates coupling between PC61BM molecules irrespective of their relative orientation, whereas for K12 strong electronic coupling is only expected for molecules oriented with their p systems parallel to each other. Comparison between the eutectic compositions of the P3HT:PC61BM and P3HT:K12 shows that the former contains enough fullerene to form a percolation pathway for electrons, whereas the latter contains a sub-percolating volume fraction of the planar acceptor. Furthermore, the planar K12 co-assembles with P3HT into a disordered, glassy phase that partly accounts for the poor electron-transport properties, and may also enhance recombination due to the strong intermolecular interactions between the donor and the acceptor. The implication for the performance of organic photovoltaic devices with the two acceptors is also discussed.

1. Introduction Solution-processed organic photovoltaics (OPV) is currently one of the fastest-improving solar cell technologies, with the power conversion efficiency improving fivefold in just over a decade.[1, 2] The photoexcitation of an organic semiconductor results in a bound electron–hole pair (an exciton), with binding energy of the order of 200 meV,[3–5] so an energetic driving force must be provided to facilitate exciton dissociation to uncorrelated free charge carriers. This is accomplished by mixing two organic materials, an electron donor and an electron acceptor, which possess the appropriate energetics for efficient photoinduced electron transfer across the interface between the donor and the acceptor.[6, 7] In an efficient solution-pro[a] Dr. A. M. Nardes, Dr. A. J. Ferguson, Dr. N. Kopidakis Chemical and Materials Science Center National Renewable Energy Laboratory 15013 Denver West Parkway, Golden, CO 80401 (USA) E-mail: [email protected] [b] Dr. P. Wolfer, Dr. K. Gui, Dr. P. L. Burn, Dr. P. Meredith Centre for Organic Photonics & Electronics The University of Queensland Brisbane, QLD 4072 (Australia)

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

cessed OPV device the optimum film structure of the light-absorbing and photocurrent-generating active layer is achieved through self-organization of the individual components into a bulk heterojunction cast from a blend solution. The donor, typically a conjugated polymer, and the acceptor are blended in solution and deposited onto a substrate, and after solvent evaporation the ideal solid-state film structure of the binary mixture is envisioned as an interpenetrating network of polymer and acceptor phases.[6] This structure allows the free carriers to avoid recombination while they migrate through the independent percolation networks to the respective contacts, where they can be extracted as photocurrent.[6] The richness of organic structures that serve as building blocks for the synthesis of polymeric and molecular organic semiconductors means that OPV is not a single material technology. Over the history of the development of organic solar cells a vast array of polymers,[8] molecules,[9–11] macromolecules,[12] and even carbon nanotubes[13–17] and inorganic[18, 19] nanostructures of various shapes and sizes have been investigated as active materials, thus leading to different film structures of the active layer of the device. Despite this, the vast ChemPhysChem 2014, 15, 1539 – 1549

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CHEMPHYSCHEM MINIREVIEWS majority of efficient OPV devices have been composed of a conjugated polymer donor and a molecular electron acceptor, and more specifically the methanofullerene derivatives of C60 (PC61BM) and C70 (PC71BM).[6, 7] It has been proposed that the dimensionality of the fullerene electron acceptor (3D), which increases the entropic driving force for the free chargecarrier generation process[5, 20] and facilitates efficient chargecarrier transport in three dimensions, is a major contributing factor to its high performance. Among the large and rapidly growing selection of conjugated polymer donors for OPV, the material that stands out is regioregular poly(3-n-hexylthiophene) (RR-P3HT, termed P3HT hereafter). Although P3HT is no longer considered a high-efficiency OPV material,[2] it is the most studied system and it is still being used as a test bed for research on both the fundamental photophysics of OPV and for the development of device processing methodologies. The solid-state microstructure of P3HT depends on its molecular weight,[21, 22] and commercially available P3HT, with molecular weights in the range of 50–70 kg mol1,[23] forms a semicrystalline film with ordered P3HT domains (crystallites) interconnected with tie-chains that form an amorphous network,[21] estimated to occupy about half the volume of the film.[22] The importance of amorphous P3HT stems from two observations: 1) PC61BM is miscible in amorphous P3HT up to approximately 20 % by weight;[24–28] and 2) the density of hexyl side chains in P3HT does not allow the PC61BM to intercalate in the polymer crystallites.[29, 30] Indeed, recent work has shown that in reality the structure of the binary film can be more complex than the simple bicontinuous network described above. Structural studies have concluded that fullerene derivatives can be highly miscible in disordered polymers, or even within the amorphous regions of semicrystalline polymers.[24–26, 28] It was even demonstrated recently that fullerene molecules can intercalate into the ordered region of conjugated polymers provided the chemical and crystal structures of the polymer, as well as the specific size and shape of the fullerene derivative, are amenable to the formation of a bimolecular crystal.[29, 30] Currently, there is a significant effort to develop new fullerene derivatives,[31] in which the structures are tailored to optimize 1) the electronic properties and 2) the self-organization of the molecules in the bulk heterojunction. In parallel, a number of groups have devoted some effort to designing and synthesizing a variety of high-performance molecular acceptor classes,[32] in an effort to expand the pool of available molecular acceptors beyond fullerenes. These include polycyclic aromatic compounds, such as bis-tricyclic aromatic enes[33, 34] and perylene diimide derivatives,[35, 36] as well as symmetrical[9, 37–39] and asymmetrical[10, 11, 40] compounds employing electron-donating and electron-accepting moieties, designed to improve absorption of red/infrared photons. The burgeoning library of non-fullerene, small-molecule acceptors highlights the necessity for studies of the photophysical and electronic performance of these molecules. Herein, we discuss the photophysics of free carrier generation in a bulk heterojunction with a PC61BM acceptor as a function of the acceptor loading, and survey experimental results  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org that account for the success of fullerenes as acceptors in OPV. We then compare the photophysical performance with that of a new planar small-molecule acceptor 2-[{7-(9,9-di-n-propyl-9Hfluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (K12),[11] and discuss how the structure of the acceptor affects the photocarrier generation process. We note that although the fill factor and open-circuit voltage of OPV devices with a P3HT:K12 active layer are very similar to those obtained with P3HT:PC61BM, the short-circuit current density JSC is a factor of 4 lower in the P3HT:K12 case.[11] The chemical structures of PC61BM and K12 are shown in Figure 1. We demon-

Figure 1. Chemical structures of electron acceptors PC61BM and K12.

strate that the shape of the acceptor plays a pivotal role in determining the structure, and therefore performance, of the binary P3HT–acceptor film. In both cases, good mixing of the acceptor into the P3HT matrix is observed even at low acceptor concentrations in the blend; however, the structure and function of this mixed phase differs markedly between PC61BM and K12. In the case of PC61BM the mixed phase acts as a free carrier generation region, which can efficiently shuttle carriers to the pure polymer and fullerene domains. In contrast, the mixed phase with K12 exhibits poor electron-transport properties, which opens up a major loss channel for free charge carriers. We attribute these differences to the fact that PC61BM is able to form 3D percolation pathways in which the electronic coupling between molecules is sufficiently strong to shuttle carriers to the pure PC61BM domains. In contrast, the planar structure of K12 co-assembles with the semicrystalline P3HT to form a disordered glassy phase that contains a sub-percolating network of K12. Furthermore, the strong electronic coupling that creates fast electron transport can only be maintained in one dimension in the planar molecule, which further inhibits electron transport through the mixed phase, where K12 molecules are randomly oriented with respect to each other. This paper is structured as follows. In Section 2.1 we survey literature results on the optical spectroscopy and contactless photoconductivity of P3HT:PC61BM that show pure PC61BM clusters forming in P3HT even at PC61BM loadings as low as 1 % by weight.[41] We discuss the implications of this observation for our understanding of the film structure-dependent photophysics of polymer–fullerene bulk heterojunctions. In Section 2.2 we present new spectroscopic data using the planar small-molecule acceptor K12 and make a comparison with fullerenes, in an effort to understand how the molecular ChemPhysChem 2014, 15, 1539 – 1549

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CHEMPHYSCHEM MINIREVIEWS structure determines the performance of molecular acceptors in OPV. We summarize our findings and conclusions in Section 3.

2. Results and Discussion 2.1. Charge Generation in Polymer:Fullerene Bulk Heterojunctions The efficiency of free carrier generation per photon absorbed, f, in a P3HT:PC61BM bulk heterojunction is shown in Figure 2, as a function of PC61BM loading.[41] We use time-correlated

Figure 2. Free carrier yield per absorbed photon (f) and electron mobility (me) versus PC61BM loading for blends of P3HT with PC61BM. mh = hole mobility.

single-photon counting (TCSPC) to probe the exciton decay dynamics and flash-photolysis time-resolved microwave conductivity (TRMC) to measure the time-dependent density of free carriers after excitation of the sample with a laser pulse.[41, 42] This methodology does not require electrical contacts to extract photogenerated carriers and it is therefore applicable to unoptimized blend compositions. Although these samples would not produce functioning devices, their spectroscopic characterization is instructive, as discussed below. The free carrier generation yield per absorbed photon is 53 % at very low loading of PC61BM (1 % by weight),[41] which is an order of magnitude higher than that measured for the pure polymer.[22, 41, 43, 44] Thus, the PC61BM acceptor, even at low concentrations, is efficient in dissociating excitons to free carriers. As discussed above, TRMC does not require carrier collection at electrodes, which would be severely limited at PC61BM loadings below the percolation threshold (in general ca. 30 % by volume).[45] The efficiency for free carrier generation shown in Figure 2 is therefore an upper limit to the internal quantum efficiency of a device, since it is not influenced by the charge collection efficiency. At higher loading of PC61BM f increases monotonically, reaching 90 % for the device-optimized film with 50 % by weight PC61BM.[41] The efficient generation of free charges at low loading of PC61BM is not surprising: a 1 % by weight loading of homogeneously dispersed PC61BM molecules results in an average dis 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org tance of approximately 5 nm between fullerene molecules, which is smaller than the measured exciton diffusion length in P3HT.[46–49] Somewhat surprising, however, is the observation that even at 1 % PC61BM, the TRMC signal contains a substantial contribution from electrons in PCBM. Also shown in Figure 2 is the high-frequency (9 GHz) electron mobility measured by TRMC, as a function of PCBM loading.[41] The electron mobility in the bulk heterojunction with 1 % PC61BM is comparable to the hole mobility in P3HT,[22, 41, 50, 51] and as the PC61BM loading increases the electron mobility increases to become the dominant contribution to the sum of mobilities Sm [see Eq. (4) in the Experimental Section]. We have proposed that a measurable electron mobility in a P3HT:PC61BM bulk heterojunction film indicates that PC61BM is forming clusters in the polymer, since an electron in an isolated fullerene molecule can be considered trapped and therefore does not contribute to the absorption of microwaves.[41] In the Theoretical Section we validate this proposition and estimate the high-frequency (9 GHz) electron mobility in PC61BM as a function of the size of a PC61BM cluster. The conclusion from the calculation is that the high-frequency mobility of electrons in PC61BM clusters is limited by the size of the cluster below approximately 100 nm and that the mobilities reported in the literature[41, 52] correspond to cluster sizes of the order of 10 nm. The observation of cluster formation at PC61BM loadings as low as 1 % by weight is seemingly contradictory to the solubility of PC61BM in the amorphous region of a typical P3HT sample of approximately 20 % by weight.[24–28] However, the microwave conductivity data can be reconciled with the structural results by considering the dynamics of electron transport in fullerene domains as discussed below. The miscibility of PC61BM in amorphous P3HT indicates that for fullerene loading up to approximately 20 % by weight, the PC61BM disperses in the amorphous volume of the polymer “host”.[24, 25, 28] However, it is plausible that spatial restrictions still force some PC61BM to aggregate locally, thus causing the formation of clusters of PC61BM. Two possible mechanisms could then account for the TRMC data: 1) long-lived chargeseparated states originate only from exciton dissociation at the interface between the P3HT and a PC61BM cluster; or 2) electrons generated in an isolated fullerene in the mixed phase get transferred to PC61BM clusters within the 5 ns duration of the excitation pulse in the TRMC experiment. Both mechanisms take into account that the PC61BM cluster stabilizes the charge-separated state by delocalizing the electron,[53] in much the same way P3HT crystallites stabilize the hole in the polymer,[22] as discussed in more detail later. Figure 2 shows that the quantum yield for free carrier generation per absorbed photon reaches 0.9 as the PC61BM loading increases to the device-optimized 50 % by weight, while almost half of that PC61BM is still dispersed in the polymer. In the device-optimized blend composition the sum of the microwave mobilities is again dominated by the mobility of electrons in domains of pure PC61BM as discussed earlier. We therefore conclude that mechanism (2) above is occurring in the 1:1 P3HT:PC61BM bulk heterojunction (50 % by weight PC61BM): free electrons are generated throughout the sample, including ChemPhysChem 2014, 15, 1539 – 1549

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CHEMPHYSCHEM MINIREVIEWS at dispersed PC61BM in the mixed phase; however, the electrons we detect in the TRMC experiment reside almost exclusively in PC61BM clusters. This does not preclude free carrier generation by electron transfer from the exciton directly onto pure PC61BM domains, especially at high loading (higher than 20 % by weight). However, the TRMC results throughout the range of PC61BM loading from 1 to 50 % by weight indicate that the charge-separated state detected after the 5 ns laser pulse includes electrons in pure PC61BM domains, and that a significant fraction of these electrons originated from PC61BM in the mixed phase. This proposition implies that there is a driving force for the electrons to transition from the dispersed PC61BM to the pure PC61BM domains. There are two possible mechanisms driving this transition, one energetic and the other entropic, which have previously been proposed to drive carriers out of the ordered mixed phase in domains of PC61BM blended with poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2b]thiophene) (pBTTT).[54] Wavefunction delocalization in a PC61BM-only domain will lower the energy of the electron providing an energetic driving force favoring transition to PC61BM domains, and, importantly, also impeding the backtransfer of electrons from PC61BM domains to dispersed PC61BM in the mixed phase, in which recombination is more efficient. Delocalization of the carrier wavefunction has previously been proposed as a carrier stabilization mechanism in similar systems.[22, 53, 55, 56] Below we discuss electron transfer between fullerenes showing that delocalization can indeed occur within the timescale of the TRMC experiment. The change in entropy upon transition of an electron from PC61BM in the mixed phase to a PC61BM cluster arises from the realization that the number of states available to the electron at constant energy increases with increasing dimensionality of the system,[20] which in this case is highest for the fullerene cluster. Therefore, an increase in entropy is expected upon transfer of the electron to a threedimensional fullerene cluster favoring this transition.[20] TRMC measurements in another poly(thiophene) derivative, in which the mixed phase is ordered and has been studied in detail using X-ray diffraction,[29, 54, 57, 58] indeed showed that electrons generated in the mixed phase transfer to pure PC61BM clusters within the 5 ns duration of the excitation pulse.[54] Naturally, the same arguments regarding stabilization of the hole should apply: a hole in the mixed phase will transition to a P3HT crystallite; however, in that case the transition will be made easier since the disordered regions are linked to crystallites through tie-chains.[22] To further understand free carrier generation in P3HT:PC61BM we turn to the phase behavior of this system. The phase diagram of P3HT:PC61BM composites in the solid state shows a simple eutectic behavior. The eutectic point, that is, the blend composition for which donor and acceptor solidify simultaneously into a fine-grained microstructure, was identified at a PC61BM loading of approximately 35 % by weight.[59] This concentration is slightly above the percolation limit (ca. 30 %)[45] for randomly mixed PC61BM and also higher than the reported miscibility of PC61BM (20 %).[24, 25, 28] From these observations we conclude that the eutectic phase, which will always be present in P3HT:PC61BM films, already provides a pathway  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org for electrons to percolate out of that phase and it includes PC61BM domains formed by the PC61BM that is in excess of the solubility limit. Furthermore, the weight ratio of PC61BM in P3HT is approximately 50 % for optimized devices,[60] with the additional fullerene forming the macroscopic percolation path for electrons to reach the electrodes.[59] We can summarize carrier generation in P3HT:PC61BM with the diagram of Figure 3, in which the three phases of the system (pure P3HT, mixed P3HT–PC61BM, and pure PC61BM) are

Figure 3. Photophysical scheme for charge generation in donor (D)–acceptor (A) blends that contain a significant volume fraction of an intimately mixed phase. X represents formation of free charge carriers in the mixed phase, which must percolate to the pure donor and acceptor domains in an efficient bulk heterojunction.

shown. Process (i; kX) in Figure 3 represents the generation of free carriers in the mixed phase and process (ii; kCGb) is the generation of carriers directly into phase-separated polymer and fullerene. Here we propose that process (iii; kCGX) is the crucial one in this system, since it is sufficiently fast to transfer electrons efficiently from the mixed phase into PC61BM domains. Using pBTTT:PC61BM, for which the structure of the mixed phase is known, and the diffusion coefficient in bulk PC61BM clusters discussed in the Theoretical Section, we can estimate the timescale of electron diffusion through a one-dimensional “channel” of fullerenes of 10 nm length to be of the order of 1 ps, that is, three orders of magnitude shorter than the excitation pulse used in the TRMC experiment. This indicates that electrons have ample time to transition to clusters outside the domains of the mixed polymer and PC61BM phase, which have been reported to be approximately 10 nm.[61] The back-transfer of electrons from PC61BM domains to the mixed PC61BM phase, however, is much slower due to the energetic and entropic considerations mentioned earlier. We note that underlying this discussion is a characteristic time for recombination that is longer than about 1 ps, that is, that carriers “survive” the journey across the 10 nm mixed domain. The recombination rate coefficient has been measured, using TRMC, in poly(thiophene):PC61BM bulk heterojunctions to be approximately gR = 1011 cm3 s1.[41] Taking an effective carrier density n to be one electron per domain of volume (10 nm)3, the characteristic time for recombination is 1/gRn = 100 ns, that is, five ChemPhysChem 2014, 15, 1539 – 1549

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CHEMPHYSCHEM MINIREVIEWS orders of magnitude slower than the transit time of the electron through the 10 nm cluster. The lower recombination rate constant (by about 2–3 orders of magnitude)[41, 62] in P3HT:PC61BM compared to other OPV material systems favors the efficient collection of electrons from the mixed phase in this case. It should be emphasized that the recombination rate coefficient quoted above was estimated for uncorrelated electrons and holes in P3HT:PC61BM bulk heterojunctions, in which the carriers most likely reside in P3HT (holes) and PC61BM domains (electrons). It is plausible that were recombination measured for holes and electrons that both reside within a mixed P3HT and PC61BM phase—process (iv; gRX)—it would be higher, as is the case for blends of PC61BM with MDMO-PPV (poly[2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylenevinylene]).[63] Nonetheless, TRMC measurements in bulk heterojunctions of a poly(thiophene) derivative with PC61BM, in which the mixed phase was dominant (i.e. little or no PC61BM domain formation occurred), did not show substantially faster decay of the photoconductance transients compared to samples in which the polymer and the fullerene were phase-separated.[54] Hence, we do not necessarily expect gRX in the mixed phase to be orders of magnitude larger than for carriers separated into the pure donor and acceptor phases. The processes shown in Figure 3 will depend on the loading of PC61BM in the film: we expect that process (ii; kCGb) will be very limited at PC61BM loadings between 1 and 20 % by weight. However, process (iii; kCGX), which indicates transfer of electrons out of the mixed phase, will still take place, since we detect electrons in PC61BM clusters. In the dilute PC61BM loading case, the majority of the fullerene will be in the mixed phase,[24, 25] therefore most electrons have to transition through that phase to the pure PC61BM domains. Hence, process (i + iii; kX + kCGX) of Figure 3 is an essential step to the efficient (> 90 %) photogeneration of electrons at the higher PC61BM loading of the device-optimized P3HT:PC61BM blends and probably for high-performance OPV bulk heterojunctions in general. The full scheme of Figure 3, including process (ii; kCGb), will apply to the device-optimized blend, in which generation of free carriers occurs both in the mixed and the pure phases, but the electronic coupling between fullerenes as well as the dimensionality of the system result in the transition of electrons from the mixed to the pure PC61BM phase on timescales faster than recombination in the mixed phase. In the following section we will compare the fullerene case to a planar smallmolecule acceptor and show that the structure of the mixed phase in the latter case limits free carrier generation, as it cannot compete with recombination. 2.2. Replacing the Fullerene with a Planar Small-Molecule Acceptor In the following we use a planar solution-processable smallmolecule acceptor 2-[{7-(9,9-di-n-propyl-9H-fluoren-2yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile, termed K12,[11] and compare the photophysical properties of P3HT:K12 bulk heterojunctions to those of P3HT:PC61BM pre 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. a) Absorption spectra of films of P3HT, K12, and P3HT:K12 bulk heterojunctions of varying weight ratio. b) Example of the reconstruction of the absorption spectrum of a 50:50 P3HT:K12 film as a linear superposition of the P3HT and K12 components. c) Contributions of the individual components (P3HT and K12) to the bulk heterojunction absorptance at 500 and 620 nm.

sented in the previous section. An additional feature of the acceptor in this case, and an advantage of non-fullerene acceptors, is the strong light absorption by the acceptor in the blend, as shown in Figure 4 a. Decomposing the blend spectra to those of the individual components, shown in Figure 4 b, allows us to separate the contribution of each component to the absorptance (fraction of photons absorbed) spectrum at 500 and 620 nm, shown in Figure 4 c—the data presented here ChemPhysChem 2014, 15, 1539 – 1549

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CHEMPHYSCHEM MINIREVIEWS represent the fraction of photons absorbed by the individual components in the blend at the corresponding excitation wavelengths. Notably, the linear superposition of the two components does not reproduce the shape of the measured spectra perfectly. It has been demonstrated that the shape of the solid-state P3HT absorption spectrum is strongly dependent on the precise nature of the polymer chain ordering.[64] Although extending the approach applied here to include structure-dependent P3HT absorption would improve the quality of the fits, this is beyond the scope of the current study. In the following optical spectroscopy and TRMC measurements we will predominantly use 620 nm excitation, at which we are primarily photoexciting the P3HT, to probe photoinduced electron transfer from the polymer donor to K12 and compare the results to that using PC61BM as an acceptor. We also use 500 nm pulses to excite both components and evaluate free chargecarrier generation following excitation of the K12 acceptor. Another feature of P3HT:K12 bulk heterojunctions that sets them apart from P3HT:PC61BM is their crystallinity. Figure 5 shows X-ray diffraction data from P3HT:K12 films of different

www.chemphyschem.org tablish primary K12 domains and to optimize the optoelectronic performance of OPV devices.[11, 40] We used TCSPC of P3HT:K12 samples with varying K12 loadings to investigate the dynamics of exciton dissociation in these samples. The photoluminescence decays under 620 nm excitation are shown in Figure 6. The average exciton lifetimes in films with low K12 loading (1 and 5 % by weight) are much

Figure 6. Photoluminescence decays, normalized to the peak intensity, of P3HT:K12 bulk heterojunctions deposited onto quartz substrates.

Figure 5. Semi-log plot of the X-ray diffractograms, as a function of K12 loading, of P3HT:K12 bulk heterojunctions deposited onto quartz substrates.

K12 loading, along with pure P3HT and pure K12 films for comparison. When the K12 loading is 20 % by weight or higher, strong diffraction from K12 domains is observed, which indicates a crystalline acceptor phase. The planar K12 acceptor has a higher propensity toward crystallization in the blend than PC61BM, in which crystalline domains of fullerene in blends with P3HT are not observed under normal processing conditions. Indeed, large domain formation in K12 results in a nonoptimal active layer structure in P3HT:K12 OPV devices.[11] We note that the planar structure of K12 also influences the P3HT microstructure. The thermal behavior of P3HT:K12 indicates the formation of a glassy phase in the composite film once the blend components have been brought into an intimately mixed state,[40] an effect not observed to the same extent with PC61BM.[59] Indeed, under specific processing conditions, P3HT:K12 blends feature a co-assembly of the two components to a glassy disordered phase at K12 loadings between approximately 20 and 50 % by weight.[40] As a consequence, higher acceptor concentrations (> 50 % by weight) are required to es 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

shorter (33–35 ps) than the exciton lifetime in pure P3HT (285 ps), which indicates high exciton quenching efficiency in these blends. Efficient exciton quenching and the absence of diffraction from acceptor domains indicates that the acceptor is well dispersed inside the P3HT at low (< 5 %) K12 loading. An estimated 50 % of the volume of the P3HT is amorphous,[22, 64] with a less spatially restrictive structure than ordered P3HT crystallites, so the K12 is most likely dispersed within the amorphous P3HT phase effectively turning it into a mixed P3HT:K12 phase. This is further corroborated by the observation of the P3HT diffraction peak remaining at the same 2q value after blending with K12, thus indicating that the structure of the P3HT crystallites remains intact for any K12 loading (Figure 5). Using the decay times measured for the bulk heterojunction tb and the decay time for the pure polymer tn we can quantify the fraction of quenched excitons in the blend QE as [Eq. (1)]:[41] QE ¼ 1 

tb tn

ð1Þ

The values for QE are summarized in Table 1: efficient exciton quenching is observed for all weight ratios of P3HT:K12. A decrease of QE is observed for the 20 % by weight K12 sample, which could be related to the onset of the formation of the glassy state at that concentration of acceptor;[40] however, further investigation is required to support this assertion. The photoconductance decays show two distinct profiles: the neat P3HT and the samples with 1 and 5 % K12 loading show a similar decay profile, whereas for the samples with ChemPhysChem 2014, 15, 1539 – 1549

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CHEMPHYSCHEM MINIREVIEWS

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Table 1. Photoluminescence lifetimes, exciton quenching efficiency, and free carrier generation yields. Sample[a]

t [ps]

QE[b]

f[c]

P3HT (100:0) P3HT:K12 (99:1) P3HT:K12 (95:5) P3HT:K12 (80:20) P3HT:K12 (50:50) P3HT:K12 (33:67)

285 35 33 88 77 65

– 0.88 0.88 0.70 0.73 0.77

0.03 0.05 0.09 0.11[d] 0.26[d] 0.19[d]

[a] Composition given in parentheses is the mass ratio. [b] Calculated from the photoluminescence lifetimes relative to the neat polymer, using Equation (1). [c] Calculated from the low-intensity (linear) yield-mobility product determined using TRMC, assuming only a contribution to the sum of mobilities from mobile holes in P3HT. [d] Upper limit of the free carrier generation yield per absorbed photon (see text).

higher K12 loading (> 20 %) the decays become slower. The transient photoconductance of neat P3HT has been discussed extensively in previous publications,[22, 41, 43] and the decay in samples of molecular weight in the 50 kg mol1 range (such as those used here) has been shown to arise from the equilibration of photogenerated holes with a large density of trapped holes that are present in the sample in the dark.[41, 50, 65–67] The observation that the photoconductance decays of the P3HT:K12 bulk heterojunctions with low loading ( 5 % by weight) follow the same profile as neat P3HT indicates that in these samples the photoconductance dynamics is dominated by the hole contribution [Eq. (4) in the Experimental Section]. We have verified that the decay profiles for the samples with low K12 loading are independent of light intensity (not shown), as is the case in neat P3HT.[41, 44] These observations suggest that at low loadings of K12 ( 5 % by weight) the electron mobility in K12 domains is well below the hole mobility in P3HT, mh,P3HT  0.014 cm2 (V s)1,[22, 41, 50, 51] which is consistent with molecular dispersion of K12 in the polymer host. However, as the weight ratio of K12 increases (> 20 % by weight), the profile of the photoconductance decay changes: samples show an excitation-dependent (dependence not shown) decay that, at the low excitation intensity employed for the data in Figure 7, is slower than that of pure P3HT. This behavior is indicative of increased electron mobility in K12 that now becomes a significant contributor to the photoconductance (cf. inhibited, intensity-dependent photoconductance decay dynamics in P3HT:PC61BM).[41] This is consistent with the phase behavior of P3HT:K12 composites, in which K12 dominates the blend microstructure in the hypoeutectic regime (with respect to P3HT), that is, at acceptor concentrations exceeding 20 % by weight, by forming extended ordered domains (Figure 5). We therefore observe a clear onset of the electron mobility contribution at the weight ratio at which the acceptor forms crystalline domains, in contrast to the PC61BM case, for which the electron contribution is significant at all acceptor loadings exceeding 1 % by weight. The formation of acceptor domains with crystalline order is one of the manifestations of the planar structure of K12. The other is the co-assembly of the planar acceptor with the semicrystalline P3HT, which is discussed below.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 7. a) Photoconductance transients of P3HT:K12 bulk heterojunctions, normalized by the absorbed photon flux (ca. 1012 photons cm2) and physical constants according to Equation (3) in the Experimental Section, measured by TRMC under excitation with 620 nm laser pulses. b) The same transients normalized to unity for better comparison of the photoconductance decay dynamics.

To understand the origin of the free charge carriers in P3HT:K12 we turn to the dependence of the product of the yield for free carrier generation and the sum of the mobilities of electrons and holes, fSm [Eq. (3)], on K12 loading, shown in Figure 8. Under photoexcitation with 620 nm pulses (at which only the P3HT absorbs), addition of small amounts (up to 5 % by weight) of K12 to P3HT causes an increase of fSm by a factor of 3. As we discussed above, the dominant contribu-

Figure 8. The product of the yield for free carrier generation and the sum of the mobilities of electrons and holes, fSm, as a function of K12 loading. The data points correspond to a constant, low-intensity absorbed photon flux of approximately 1011 photons cm2, for excitation at 500 and 620 nm.

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CHEMPHYSCHEM MINIREVIEWS tion to Sm at low K12 loading is the hole mobility in P3HT, which allows us to use the known value for P3HT[22, 41, 50, 51] to estimate f; the results are listed in Table 1. The yield or free carrier generation per absorbed photon in pure P3HT is 0.03, consistent with previous reports,[41, 43] and it increases to approximately 0.09 for the bulk heterojunction with 5 % by weight loading of K12. At the same time, the efficiency of exciton quenching (measured by time-resolved photoluminescence on the same samples) is 0.88 (Table 1), which indicates that only a small number, of the order of 10 %, of the quenched excitons result in free carriers that survive to the nanosecond scales of the TRMC experiment. At K12 loadings exceeding 20 % the photoconductance decay profiles indicate a significant electron mobility contribution to fSm, as discussed above. However, noting that Sm  mh (the equality holding when me = 0) we can again use the hole mobility in P3HT to estimate an upper limit to f. The result, listed in Table 1, shows that at 20 % K12 loading no notable increase of f is observed, despite the appearance of crystalline K12 domains at this loading. Overall, the fSm product does not show an increase between the 5 and 20 % loadings of K12, which also indicates that the electrons in K12 at loadings over 20 % did not originate from the mixed phase: had that been the case we would predict a significant increase of fSm since electrons generated in the mixed phase would transition to K12 domains.[54] From the above discussion we conclude that the free carrier generation scheme in P3HT:K12 is subtly different from that for P3HT:PC61BM shown in Figure 3. In the case of P3HT:K12 exciton dissociation, processes (i + ii; kX + kCGb) occur within 40– 90 ps; however, the free carriers we observed with TRMC on the 5 ns scale are a much smaller fraction (10 %) of the excitons quenched. Contrary to the situation observed for P3HT:PC61BM, most excitons dissociated in the P3HT:K12 blend do not lead to long-lived (> 5 ns) free carrier generation. In the following we discuss possible structural origins of this observation. As P3HT:K12 is also a three-phase system we again use the scheme of Figure 3 for the photoinduced processes in this system. Free carriers are generated in the mixed phase, process (i; kX), and directly into the K12 domains, process (ii; kCGb). We propose that the modification to the scheme of Figure 3 in the case of P3HT:K12 is that efficient transfer of electrons from the mixed to the pure K12 phase is not observed, that is, the equivalent of process (iii; kCGX) of the scheme for P3HT:PC61BM is absent (or very slow) in the case of the K12 acceptor. Poor electron transport in the mixed P3HT:K12 phase can have a number of causes tracing back to the structure of that phase. First, the dimensionality of the acceptor might limit transport through the K12 network: K12 is a one-dimensional transporter, since it can only transport electrons efficiently along the p-stacking direction, contrary to PCBM which has a three-dimensional p system around the fullerene cage and can transport charge in three dimensions. This may have a strong impact in the disordered region, since strong electronic coupling between K12 molecules will only be permitted when they are oriented with their planes roughly parallel to  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org each other, whereas for PC61BM strong coupling is maintained irrespective of the relative molecular orientation. Second, the eutectic phase in the P3HT:K12 blend has been shown to be glassy (disordered), which arises from the competition of the planar small molecule and the semicrystalline polymer to form ordered domains.[40] In addition, the eutectic composition in P3HT:K12 includes a relatively low (ca. 22 %) weight fraction of K12,[40] which is below the percolation threshold of approximately 30 % of material that is required for electron transport. (We note that the density of P3HT and K12 is similar, hence weight ratios are approximately equal to volume ratios.) Therefore, electrons in the mixed phase simply do not find a percolation pathway out of that phase. By contrast, the eutectic composition in P3HT:PC61BM is approximately 35 % PC61BM, which is above the percolation limit. Finally, the co-assembly of the P3HT and the planar small molecule into a glassy phase[40] is an indication of strong interactions between the two components and is likely to promote recombination of charges in that phase, further limiting the free carriers we detect on the 5 ns timescale of TRMC to those generated through process (ii; kCGb). For K12 loading equal to or larger than 20 % it is clear that process (ii; kCGb) is the dominant free carrier generation process; however, process (i; kX) is still present because an intimately mixed glassy phase is also present.[40] The final aspect of the photophysics of the K12 acceptor is free charge generation by direct photoexcitation of the acceptor. In Figure 8 we also show the fSm product for excitation with 500 nm laser pulses, at which the K12 absorbs significantly. The comparison of the 620 and 500 nm excitation clearly demonstrates that photoexcitation of the acceptor is a significant channel for free carrier generation. Indeed, by using a narrower bandgap acceptor, for which the absorption band of the acceptor does not overlap with that of the P3HT and one can selectively excite the acceptor only, we have shown that excitation of the acceptor generates free charges efficiently.[10] This is a major advantage of non-fullerene acceptors as it allows one to create acceptor molecules with absorption complementary to the donor, thereby extending the absorption range of the blend and improving photon harvesting.[10] The difference in the free carrier generation yield f between P3HT:PC61BM and P3HT:K12 can explain the performance of the respective OPV devices. The short-circuit current density JSC of an optimized P3HT:K12 device under simulated AM 1.5 illumination at 1 sun is 2.4 mA cm2, which is a factor of approximately 4 lower than what is observed in P3HT:PC61BM devices.[11] As shown in Figure 4, for the device-optimized blend ratio of 33:67 P3HT:K12, a significant fraction of incident light is absorbed by the P3HT, in which case, as discussed earlier, the upper limit for f is 0.19 (Table 1). By contrast, for the device-optimized P3HT:PC61BM system, f = 0.9,[41] which results in a higher JSC output in this case.

3. Conclusion We have demonstrated that the shape of the acceptor plays a pivotal role in determining the structure, and therefore performance, of the binary P3HT–acceptor film. In both cases, ChemPhysChem 2014, 15, 1539 – 1549

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CHEMPHYSCHEM MINIREVIEWS a disordered mixed phase is formed between P3HT and the acceptor, although the performance of this mixed phase differs markedly for PC61BM and K12. In the case of PC61BM the mixed phase acts as a free carrier generation region, which can efficiently shuttle carriers to the pure polymer and fullerene domains. As a result, the vast majority of excitons quenched in P3HT:PC61BM yield free carriers detected by TRMC. In contrast, most (six out of seven) excitons quenched in P3HT:K12 do not result in free carriers over the nanosecond timescale of the TRMC experiment. We attribute this to poor electron-transport properties in the mixed P3HT:K12 phase. The differences between the PC61BM and the K12 acceptors can be traced to their respective shapes: the three-dimensional nature of the fullerene cage facilitates coupling between PC61BM molecules irrespective of their relative orientation, whereas for K12 strong electronic coupling is only expected for molecules oriented with their p systems parallel to each other. Comparison between the eutectic compositions of the P3HT:PC61BM and P3HT:K12 shows that the former contains enough fullerene to form a percolation pathway for electrons, whereas the latter contains a sub-percolating volume fraction of the planar acceptor. Furthermore, the planar K12 co-assembles with P3HT into a disordered, glassy phase that should exhibit poor electron-transport properties but also possibly enhance recombination due to the strong intermolecular interactions that drive co-assembly of the donor and the acceptor. We also show that significant carrier generation occurs by direct photoexcitation of the acceptor, which is a significant advantage of non-fullerene acceptors. As research into nonfullerene molecular acceptors grows, comparison between new classes of compounds will be useful to identify structural properties of the blend that promote efficient generation of long-lived charge carriers in these systems.

Experimental Section Materials and Sample Fabrication P3HT and PC61BM were used as received from Merck and Nano-C, respectively. K12 was synthesized and purified as reported previously.[68] Solutions of neat P3HT, 99:1, 95:5, 80:20, and 50:50 (by weight) P3HT:PC61BM (with total active material concentrations of 7.5 mg mL1) in chloroform were prepared and stirred overnight at 50 8C under a nitrogen atmosphere. The solutions were drop-cast onto clean, O2-plasma-treated quartz substrates and subsequently slow-dried in air. For P3HT:K12 films, stock solutions of P3HT (15 mg mL1) and K12 (15 mg mL1) in 1,2-dichlorobenzene were prepared and mixed by blending P3HT:K12 in various ratios. The bulk heterojunction was formed by spin-coating the blend solution (1000 rpm, 25 s) onto the quartz substrate, which was immediately covered with a Petri dish until dry (> 5 min later). The dried films were then baked at 65 8C for 10 min.

www.chemphyschem.org tube (Hamamatsu H6279), using the TCSPC technique.[69] The photoluminescence decays were analyzed using an established nonlinear least-squares iterative reconvolution procedure,[70] in which the finite width of the instrument response function was effectively deconvoluted from the measured data to give an overall temporal response of approximately 20 ps. Data were fitted to a sum of two or three exponentials and the quality of fit judged using stringent statistical procedures.[69]

Flash-Photolysis Time-Resolved Microwave Conductivity (TRMC) Photoinduced carrier dynamics were studied using TRMC, a contactless, pump–probe technique in which both the initial generation of mobile carriers and their eventual decay back to equilibrium are monitored through the time-resolved changes in absorbed microwave power by the sample.[41, 46, 71] The sample (active layer on a quartz substrate) was placed in an X-band microwave cavity terminated with a grating reflective to microwaves but transparent to the optical excitation that was used to generate carriers within the film. All films were excited with 3–5 ns laser pulses at the desired excitation wavelength (either 500 or 620 nm) from an optical parametric oscillator (Continuum Panther) pumped by the thirdharmonic (355 nm) from a Q-switched Nd:YAG laser (Continuum Powerlite). The transient change in microwave power, DP(t), was monitored and related to the transient change in photoconductance, DG(t), by [Eq. (2)]: DGðt Þ ¼ 

1 DPðt Þ K P

ð2Þ

in which K is a calibration factor determined experimentally from the resonance characteristics of the microwave cavity and the dielectric properties of the sample. The peak photoconductance, DGEOP, probed by TRMC can be related to the yield for free carrier generation f and the sum of the free carrier mobilities Sm by [Eq. (3)]: Sm ¼

DGEOP bqe I0 FA

ð3Þ

in which b relates to the dimension of the waveguide cross section, qe is the elementary electronic charge, I0 is the incident photon fluence, and FA is the fraction of absorbed photons. Importantly, the mobility probed by TRMC is the local mobility of free carriers (electrons and holes) probed by their absorption of the 9 GHz microwave beam: this high-frequency mobility is not necessarily directly related to the mobility determined in bulk (devicetype) measurements, in which the carrier has to traverse the entire device and the extracted bulk mobilities are typically limited by features such as grain boundaries. One of the complications of TRMC measurements is the difficulty in decoupling the yield–mobility product in Equation (3), which is related to the sum of the hole, mDh , and electron, mAe , mobilities in the donor and acceptor domains, respectively, by [Eq. (4)]: Sm ¼ mDh þ mAe

Time-Correlated Single-Photon Counting (TCSPC) Photoluminescence decays were recorded, after excitation at 620 nm with a 2 MHz train of pulses (pulse width:  6 ps fwhm; instrument response function:  160 ps fwhm) from a high-power fiber laser (Fianium WhiteLase Supercontinuum SC400-2) for emission at 750 nm, with a cooled photon counting photomultiplier  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ð4Þ

However, one of the additional benefits of employing P3HT as the conjugated polymer donor is that it has also been extensively investigated by the TRMC technique,[41, 43, 44, 50] including a thorough = evaluation of the high-frequency (9 GHz) hole mobility: mP3HT h 0.014 cm2 (V s)1.[22, 41, 50, 51] If one assumes that the electron mobility in the acceptor is negligible one can use the P3HT hole mobility ChemPhysChem 2014, 15, 1539 – 1549

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and the yield–mobility product, fSm, calculated using TRMC to estimate an upper limit for the free carrier generation yield f. Note, however, that the contribution from electrons in the acceptor cannot always be ignored (for instance, electrons contribute to the measured photoconductance in blends of P3HT:PC61BM even at low loadings).[41]

Theoretical Section The frequency-dependent diffusion coefficient D(w) of an electron is usually described by the Kubo formula [Eq. (5)]:[72, 73] 1 DðwÞ ¼ w2 2

Z1

eiwt h½r ðt Þ  r ð0Þ2 idt

ð5Þ

1

in which h[r(t)r(0)]2i is the mean square displacement of the electron. To calculate the mean-square displacement we solved the three-dimensional diffusion equation inside a cube of size a, with reflecting boundary conditions at the sides of the cube. The frequency-dependent mobility m(w) can then be calculated by the Einstein relation [Eq. (6)]: q mðwÞ ¼ e DðwÞ kB T

ð6Þ

in which kB is the Boltzmann constant and T is the temperature. We note that at the typical excitation intensities used the photoinduced electron density is of the order of 1018 cm3, which is low enough so that in Equation (6) one can ignore corrections due to nonideal electron statistics.[74, 75] Our methodology is similar to that used by Prins et al. to calculate the chain-length-limited one-dimensional mobility along a polymer chain.[73] The crucial parameter in the calculation is the bulk diffusion coefficient D0 : that is, the diffusion coefficient of electrons in a fullerene sample large enough so that size effects can be ignored. Size effects depend on the frequency and as the frequency increases the real part of D(w) converges to D0.[73] The electron mobility in PC61BM domains obtained by a terahertz probe has been estimated to be approximately 20 cm2 (V s)1,[76] which is of the same order of magnitude as the terahertz electron mobility in C60—approximately 50 cm2 (V s)1.[77] Taking this value to be the bulk electron mobility, the electron mobility at our microwave frequency of 9 GHz is shown in Figure 9, as a function of the size of the PC61BM domain. The range of electron mobility in PC61BM, typically measured at microwave (9 GHz) frequencies, is approximately 0.01– 0.08 cm2 (V s)1,[27, 41, 52] which corresponds to PC61BM domain sizes of 10–14 nm. We emphasize that the purpose of this analysis is not to provide an accurate measurement of the size of PC61BM domains based on microwave conductivity data. Figure 9 shows that by using the terahertz electron mobility to represent the bulk value, the mobility measured at the 9 GHz microwave frequency depends strongly on the size of PC61BM domains. Furthermore, the measured values of the electron mobility in PC61BM domains correspond to reasonable domain sizes of the order of 10 nm. One might question our assumption that the terahertz mobility is equal to that of the bulk; however, using a higher value for the bulk electron mobility, 50 cm2 (V s)1, does not change the main conclusions. The electron mobility is still strongly dependent on size at 9 GHz, and the range of sizes that give mobilities comparable to measured values at the microwave frequencies employed here is shifted to slightly larger, but still reasonable, sizes (ca. 20 nm).  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 9. Real component of the high-frequency electron mobility calculated at 9 GHz, as a function of the size of the PC61BM domain. The bulk electron mobility is assumed to be 20 cm2/(V s) (see text for justification). Also highlighted is the range of electron mobilities obtained in PC61BM domains in P3HT:PC61BM bulk heterojunction, and the corresponding range of estimated domain sizes.

Acknowledgements We are grateful to Prof. Natalie Stingelin for useful discussions. The work carried out at NREL was supported by the Energy Frontier Research Center “Molecularly Engineered Energy Materials (MEEMs)” funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract Number DESC0001342:001. P.L.B. was a recipient of an Australian Research Council Federation Fellowship (Project number FF0668728) and is a University of Queensland Vice Chancellor’s Senior Research Fellow. P.M. is a University of Queensland Vice Chancellor’s Senior Research Fellow. P.W. would like to thank the Swiss National Science Foundation (SNSF) for an Advanced Researcher Fellowship (PA00P2 145395). The work carried out at UQ was funded by the University of Queensland (Strategic Initiative–Centre for Organic Photonics & Electronics), the Queensland Government (National and International Research Alliances Program–“Queensland Organic Solar Cell Alliance”). This work was performed in part at the Queensland node of the Australian National Fabrication Facility (ANFF-Q). Keywords: bulk heterojunctions · carrier transport · donor– acceptor systems · fullerenes · organic photovoltaics [1] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, Prog. Photovoltaics 2013, 21, 1 – 11. [2] A. J. Ferguson, W. A. Braunecker, D. C. Olson, N. Kopidakis, Organic Electronics: Emerging Concepts and Technologies, 1st ed. (Eds.: F. Cicoira, C. Santato), Wiley-VCH, Weinheim, 2013. [3] B. A. Gregg, M. C. Hanna, J. Appl. Phys. 2003, 93, 3605 – 3614. [4] B. A. Gregg, J. Phys. Chem. B 2003, 107, 4688 – 4698. [5] T. M. Clarke, J. R. Durrant, Chem. Rev. 2010, 110, 6736 – 6767. [6] B. C. Thompson, J. M. J. Frchet, Angew. Chem. 2008, 120, 62 – 82; Angew. Chem. Int. Ed. 2008, 47, 58 – 77. [7] C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia, S. P. Williams, Adv. Mater. 2010, 22, 3839 – 3856. [8] P.-L. T. Boudreault, A. Najari, M. Leclerc, Chem. Mater. 2011, 23, 456 – 469. [9] J. T. Bloking, X. Han, A. T. Higgs, J. P. Kastrop, L. Pandey, J. E. Norton, C. Risko, C. E. Chen, J.-L. Brdas, M. D. McGehee, A. Sellinger, Chem. Mater. 2011, 23, 5484 – 5490.

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