Charge Generation Pathways in Organic Solar Cells: Assessing the

Review pubs.acs.org/CR

Charge Generation Pathways in Organic Solar Cells: Assessing the Contribution from the Electron Acceptor Dani M. Stoltzfus,* Jenny E. Donaghey, Ardalan Armin, Paul E. Shaw, Paul L. Burn,* and Paul Meredith Centre for Organic Photonics & Electronics, The University of Queensland, St Lucia, QLD 4072 Australia ABSTRACT: Photocurrent generation in organic bulk heterojunction (BHJ) solar cells is most commonly understood as a process which predominantly involves photoexcitation of the lower ionization potential species (donor) followed by electron transfer to the higher electron affinity material (acceptor) [i.e., photoinduced electron transfer (PET), which we term Channel I]. A mirror process also occurs in which photocurrent is generated through photoexcitation of the acceptor followed by hole transfer to the nonexcited donor or photoinduced hole transfer (PHT), which we term Channel II. The role of Channel II photocurrent generation has often been neglected due to overlap of the individual absorption spectra of the donor and acceptor materials that are commonly used. More recently Channel II charge generation has been explored for several reasons. First, many of the new high-efficiency polymeric donors are used as the minority component in bulk heterojunction blends, and therefore, the acceptor absorption is a significant fraction of the total; second, nonfullerene acceptors have been prepared, which through careful design, allow for spectral separation from the donor material, facilitating fundamental studies on charge generation. In this article, we review the methodologies for investigating the two charge generation channels. We also discuss the factors that affect charge generation via Channel I and II pathways, including energy levels of the materials involved, exciton diffusion, and other considerations. Finally, we take a comprehensive look at the nonfullerene acceptor literature and discuss what information about Channel I and Channel II can be obtained from the experiments conducted and what other experiments could be undertaken to provide further information about the operational efficiencies of Channels I and II.

CONTENTS 1. Introduction 2. Charge Generation Pathways: Channel I and Channel II 2.1. Exciton Generation and Decay 2.2. Donor−acceptor Heterojunctions and Charge Transfer 3. Experimental Methods for Probing Channel I/II 3.1. Device Quantum Efficiency Measurements 3.2. Photoluminescence Quenching Measurements 3.3. Transient Absorption Spectroscopy Measurements 4. Channel II Charge Generation in Organic Solar Cells 4.1. Categorizing Donor−Acceptor Blends by Their Absorption 5. Fullerene Systems 5.1. Conventional Bulk Heterojunctions 5.2. Low Donor Content Bulk Heterojunctions 6. Nonfullerene Systems 6.1. Small Molecule Acceptors 6.1.1. Phthalocyanines, Subphthalocyanines and Truxenones 6.1.2. Quinacridones, Fluoranthenes, and Rubicenes 6.1.3. Vinazenes 6.2. Nonpolymeric Macromolecules

6.2.1. Naphthalenediimides and Tetrabenzodifluoroanthene Diimides 6.2.2. Perylenediimides 6.2.3. Benzothiadiazoles 7. Conclusions Author Information Corresponding Authors Notes Biographies References

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1. INTRODUCTION Understanding the key properties that govern the performance of organic solar cells has led to their steadily improving device efficiencies and operating lifetimes. Organic solar cells are excitonic devices,1−3 and unlike inorganic semiconductors4−6 and organohalide lead perovskites,7−13 photoexcitation of organic semiconductors does not lead to substantial instantaneous free carrier generation.4 The reason for this is that the electron−hole pairs (excitons) formed in organic semiconductors experience strong Coulomb binding at room temperature due to their low dielectric constants, which leads to the spatial localization of the electron and hole wave functions. Crucially, the exciton binding energies in organic semiconductors are on

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Special Issue: Electronic Materials Received: February 17, 2016 Published: June 24, 2016

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DOI: 10.1021/acs.chemrev.6b00126 Chem. Rev. 2016, 116, 12920−12955

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Figure 1. (a and c): Photoinduced electron transfer (PET) or Channel I. (a) A simplified energy diagram of a donor−acceptor interface with different electron affinities [EA, often equated to the lowest unoccupied molecular orbital (LUMO) energy] and ionization potentials [IP, often equated to the highest occupied molecular orbital (HOMO) energy]. The electrodes (outer rectangles) are shown at short circuit. After photoexcitation occurs in the donor, the exciton (with Coulomb binding energy EDbinding) diffuses to a donor−acceptor interface, after which the electron can transfer into the acceptor LUMO with efficiency ηI (orange arrow). (b and c) Photoinduced hole transfer (PHT) or Channel II. (b) An exciton (with Coulomb energy EAbinding) in the acceptor dissociates at the interface with efficiency ηII. (c and d) The resulting interfacial CT state dissociates to generate free polarons with efficiency ηCS.

the order of 0.3 eV,14−19 which is substantially greater than kBT at room temperature, and hence, homojunction organic solar cells have very poor quantum efficiencies.20−31 The strategy to overcome the exciton binding energy and improve device efficiency has been to use a heterojunction, where the difference in electrochemical potential between two organic semiconductors provides the energy necessary to separate the exciton and drive charge transfer at the interface between the two materials. The material with the least positive oxidation potential or lowest ionization potential (IP) (often erroneously referred to as the HOMO energy) is generally termed the “donor”. The material with the least negative reduction or the highest electron affinity (EA) (also wrongly referred to as the LUMO energy) is termed the “acceptor”.32 There are two main ways that exciton dissociation can occur in heterojunction devices. First, the donor can be photoexcited with the excited electron being transferred to the higher electron affinity material (the acceptor). Second, the higher electron affinity material can be photoexcited with a ground-state electron being transferred from the donor to the acceptor material, equivalent to hole transfer from the acceptor to the donor. These two mechanisms have been coined Channels I and II, respectively.33 Historically the first donor−acceptor heterojunctions were planar32,34−43 which were followed by the development of solution processed bulk heterojunctions (BHJ), featuring a blend of donor and acceptor materials in a single layer,44−57 with the latter giving rise to the most efficient devices and used extensively thereafter. This review describes the charge generation process in solution processed bulk heterojunctions (and evaporated planar heterojunctions) comprised of donor−acceptor blends, with a particular focus on the materials combinations in which excitation of the acceptor gives rise to a component of charge generation by the Channel II mechanism. We will begin with a discussion of exciton separation via the Channel I and Channel II

processes and the factors that affect the efficiency. We will then describe the current experimental methods that can be employed to identify if Channel II is occurring and for evaluating the relative efficiency of Channel I and Channel II. As the organic solar cell community looks to further improve device performance through the design of novel donor and acceptor materials, it is important to consider the role Channel II charge generation can play, and therefore, in the final part of this review we focus on acceptor materials that have the potential for Channel II photocurrent generation.

2. CHARGE GENERATION PATHWAYS: CHANNEL I AND CHANNEL II 2.1. Exciton Generation and Decay

Singlet excitons are the primary excited state formed upon photoexcitation of organic semiconductors and are thus the primary species generated in an organic solar cell upon light absorption. Excitons have a finite lifetime, typically on the order of 1 ns58 for solution-based measurements, although it is generally shorter in the solid-state due to intermolecular interactions, before they decay radiatively or nonradiatively to the ground state. The probability of an exciton undergoing radiative decay is a key property of the material and is known as the photoluminescence quantum yield (PLQY). Singlet excitons may undergo intersystem crossing to triplet excitons, but this rate is typically very low unless the molecule incorporates a heavy atom, which due to spin−orbit coupling increases the intersystem crossing rate.58 The intersystem crossing in fullerenes has been reported to occur on the nanosecond time scale, which although fast is still slower than the exciton lifetime in most donor−acceptor BHJs.59 Hence, the primary species generated in an organic solar cell upon light absorption is generally assumed to be a singlet exciton. A critical property of excitons in organic semiconductors, that is highly relevant to the operation of solar cells, is that they are 12921


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Figure 2. (a) Different architectures for quantifying the light absorption in the active layer of an organic solar cell. One method is based on an active layer deposited on a glass substrate where the absorption is determined through UV−vis optical transmission. A more precise method is measurement of the total reflectance of the actual solar cell device that includes all of the relevant optical effects. (b) A flowchart showing the different approaches commonly used for IQE evaluation in thin film solar cells. Only the approach shown in the far right pathway results in a correct IQE spectrum, and the other protocols result in misleading IQE analysis. Reprinted from ref 64. Copyright 2014 American Chemical Society.

semiconductors the exciton diffusion length is typically on the order of 5−10 nm.61,62

able to migrate between chromophores by a process known as exciton diffusion. For molecules containing multiple chromophores, such as conjugated polymers, both intramolecular and intermolecular transfer is possible. For singlet excitons, diffusion occurs via a combination of Förster and Dexter energy transfer. Förster resonant energy transfer (FRET) is the incoherent nonradiative transfer of excitons between two chromophores via dipole−dipole coupling. The rate of energy transfer depends primarily on the PLQY of the donor chromophore, the extinction coefficient of the acceptor chromophore, and the relative orientation of both chromophores. FRET can be very efficient and allow excitons to transfer between chromophores several nanometers apart.60 In contrast, Dexter energy transfer is a shortrange process, mediated by the exchange of an electron, and therefore only enables the transfer of excitations between chromophores with overlapping wave functions. Determining the exciton diffusion coefficients or the diffusion lengths has been the subject of extensive research and for many organic

2.2. Donor−acceptor Heterojunctions and Charge Transfer

As described briefly in the Introduction, the efficient dissociation of excitons into free charges requires an interface between two semiconductor materials with sufficient energy offset between the excited state of one and the ground state of the other for a redox reaction to occur. In the seminal report on BHJs, the mechanism of exciton separation was described as occurring through photoexcitation of the material of low ionization potential (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV) followed by transfer of the excited electron to the high electron affinity material (a fullerene), forming a positive and negative charge on each, respectively.63 In chemical parlance, the process involves the exciton (or excited donor) being oxidized by the high electron affinity acceptor material (the fullerene). Given that the electron is transferred from one molecule to another, the molecule that loses the electron is commonly referred to as the donor, while the other 12922


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Figure 3. (a) Absorption spectra of PCDTBT and PC70BM on glass measured from the transmission and weighted for their volume fraction in a 1:4 blend. The absorption spectra of the 1:4 blend and neat PC70BM in a solar cell configuration (ITO/PEDOT:PSS/active layer/Sm/Al) measured from the device reflection. (b) EQE spectra of 1:4 and 1:20 blends and the device IQE of the1:4 solar cell. Reproduced from ref 69. Copyright 2015 American Chemical Society.

insignificant, it will reflect the charge generation quantum yield via each channel. It is important to note that this efficiency also includes the efficiency of the exciton harvesting process, which is determined by the exciton diffusion length and the morphology of the donor and acceptor phases. If the two channels are not operating with equal efficiencies then the IQE spectrum may be wavelength dependent; specifically, it will be higher at wavelengths that preferentially excite the more efficient channel. Similarly, if the donor and acceptor absorption spectra do not overlap, and the device exhibits a spectrally flat IQE, then both channels could be operating with equal efficiencies (assuming that exciton harvesting is equally efficient for both the donor and acceptor phases). Hence precise evaluation of device IQE is vital for probing the contributions of Channel I and Channel II to charge generation. Figure 2b shows the effect of using different approaches for evaluating the IQE which do or do not take into account either the cavity effect, scattering, and/or parasitic absorptions. For example, the differences between following pathway (i) and (ii) has led to a debate on the role of hot excitons in more efficient charge generation.66,67 Given the need to properly assess the IQE in an operational device, it is crucial to fully take into account interference effects as well as parasitic absorptions in the solar cell cavity.64 Burkhard and co-workers65 have introduced such an approach to quantify the IQE of thin organic solar cells, and the limitations of different approaches for IQE evaluation have been further discussed by Armin et al.64 As an example for the correct measurement of IQE we will consider the donor−acceptor system, poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2thienyl-2',1',3'-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC70BM), in which both components absorb light in the visible region and are spectrally overlapping. The correct approach begins with an analysis of the relative absorption spectra of a PCDTBT:PC70BM film (1:4 by weight; note: in the remainder of the review the ratios correspond to weight ratios) on glass (measured from transmission and weighted with their volume fraction in the blend) as well as the blend and PC70BM in a solar cell configuration measured from reflection (Figure 3a). It can be seen that the contribution of PCDTBT to the light absorption of the blend is much less than that of the fullerene as seen from the weighted absorptions on glass and also by comparison of the absorption spectra of PCDTBT:PC70BM and neat PC70BM films, which only have a minor difference near the 600 nm peak of PCDTBT. This suggests that PCDTBT:PC70BM solar cells are mostly Channel

that receives the electron is the acceptor (see Figure 1, panels a and c). We term the photoexcited electron transfer (PET) from the donor to the acceptor as the Channel I mechanism. The efficiency of the PET or the Channel I process is governed, to the first order, by the energy offset between the excited electron on the donor and the electron affinity of the acceptor in its ground state. It has also been established that photoexcitations generated on the acceptor moiety can contribute to the generated photocurrent through photoinduced hole transfer (PHT), which we have termed Channel II (Figure 1, panels b and c). From a chemical perspective the excited acceptor is simply oxidizing the donor, which is in the ground state. As with PET, the efficiency with which the PHT, or Channel II, process occurs is dependent on an energy offset but in this case between the oxidation potential of the donor in its ground state and the reduction potential of the excited acceptor. To first order, this can be approximated by comparing the ionization potentials of the two materials. It is important to note that it is generally assumed both the Channel I and II mechanisms yield the same charge-transfer (CT) state. Dissociation of the CT state then leads to separated charge carriers (Figure 1, panels c and d). Appreciation of the importance of the Channel II mechanism for charge generation has led to a new set of design criteria for materials available for organic solar cells, and this will be discussed in the next sections.

3. EXPERIMENTAL METHODS FOR PROBING CHANNEL I/II 3.1. Device Quantum Efficiency Measurements

The external quantum efficiency (EQE) of a device is often used to elucidate the photoresponse of the BHJ active layer.33 However, while comparing the absorption of the donor and acceptor materials with the EQE can give a first-order indication of whether Channel II is in play, since solar cells are weak optical cavities, it does not give an accurate representation of the magnitude of Channel I or II in photocurrent generation. Optical interference effects often distort the shape of the device EQE. Thus, to get a better understanding of the ratio of Channels I and II, the internal quantum efficiency (IQE) should be used. Since both Channel I and Channel II generally result in the formation of the same CT-state, any differences in IQE will be due to the differing efficiencies for the conversion of excitons to the CTstate. Provided that the IQE is measured at low light intensity (low excitation density) so that bimolecular recombination is 12923


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therefore both materials are simultaneously photoexcited. The most robust approach for evaluating whether a donor and acceptor are compatible is to use a low concentration of the quencher, on the order of 1%,74,76 as this will allow materials with overlapping absorption to be independently assessed, eliminate losses due to mutual absorption of the photoexcitation, and provide a better comparison with the neat film. A comparison of the PL quenching efficiency of [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and the nonfullerene acceptor K12 for the excited polymer poly(3-n-hexyl)thiophene (P3HT) is shown in Figure 5. Even at low concentrations very high quenching efficiencies (>80%) are to be expected if charge transfer is efficient.

II devices and the dominant photoactive component is the fullerene and not PCDTBT. Figure 3b shows the EQE spectra of 1:4 and 1:20 blends and only a minor spectral difference can be observed. This strengthens the argument for the dominance of Channel II in the 1:4 blend. The IQE of this system is also shown to be spectrally flat. This example highlights the difficult nature of understanding Channel I and Channel II photocurrent generation pathways in spectrally overlapping systems. 3.2. Photoluminescence Quenching Measurements

Photoluminescence (PL) intensity is proportional to the population of singlet excitons. Hence, comparisons of the PL intensity between a neat film of a material with a bulk heterojunction containing the same material and a quencher provides a qualitative measure of the quencher’s effectiveness. This technique has been widely used to qualitatively gauge whether a donor material is compatible with an acceptor (Channel I).69,70 However, the converse measurements for Channel II, where the capability of the donor to quench the PL of the acceptor is rarely performed.71,72 While such measurements are challenging on materials with very low PLQY values, such as fullerenes, many nonfullerene acceptors are sufficiently luminescent and such measurements are feasible. Although PL quenching measurements are experimentally simple, there are a few important considerations. Measurements can be performed in the steady-state, where the PL intensity is recorded, or in the time-domain, where the PL decay is recorded. Steady-state measurements are not quantitative unless an integrating sphere is used, as the PL from thin films is not isotropic and so small differences in the position of the sample can result in substantial changes in the measured PL intensity.73 Time-resolved measurements are quantitative as the PL decay kinetics are not affected by the positioning of the sample and as such can be used to estimate exciton diffusion lengths from the quenching efficiency (see Figure 4). Such measurements have

Figure 5. Photoluminescence quenching efficiency of P3HT with various weight ratios of K12 and PC60BM. Reprinted with permission from ref 76. Copyright 2011 John Wiley and Sons.

To conclude, it is important to draw attention to the fact that while PL measurements provide a means of assessing the effectiveness of one material to quench the singlet exciton population of another, they do not provide evidence as to whether a CT state or free charges are formed as a result. For example, depending on the relative concentrations and optical gaps, quenching can occur via energy transfer. Hence, it is important to consider PL quenching data in conjunction with device performance characterization or other spectroscopic measurements. 3.3. Transient Absorption Spectroscopy Measurements

Transient absorption spectroscopy (TAS) is a time-resolved pump−probe technique that allows the evolution of the excited state population to be monitored by detecting small changes in the absorption of a photoexcited sample. The use of ultrafast laser sources enables subpicosecond temporal resolution to be achieved, and thus it is a powerful and widely used technique for probing charge generation.69,77−79 As the absorption signals from different excited state species are broad and often overlap, analysis can be complex and thus TAS is best-suited for probing donor−acceptor systems where each component can be preferentially excited. Therefore, the excited state kinetics associated with Channel I and Channel II can be deconvolved. The example shown in Figure 6 is an early demonstration of charge generation in a bulk heterojunction via Channel I and Channel II following preferential excitation of the polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and PC60BM, respectively. The signal

Figure 4. PL quenching of PC70BM emission when varying the concentration of the 3,6-bis(5-((4-(dimethylamino)phenyl)ethynyl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione (DPP-NMe2) quencher from 0 to 5 wt %. The solid lines are fits to the experimental data using an exciton diffusion model where D represents diffusion co-efficient and RAD represents diffusion radius. Reprinted with permission from ref 74. Copyright 2013 Nature Publishing Group.

been used to show the quenching of singlet excitons in both fullerene74 and nonfullerene acceptors75 by the donor polymer. Lastly, the blend ratio used for the sample is important. While PL quenching measurements are often performed on films with the same blend ratio as the device, this can complicate the analysis if the absorption of the two components strongly overlap and 12924


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literature.72,80,83−85 That is, the drive to report the performance metrics of devices has meant that isolating the role of Channels I and II has not been a high priority and has been largely overlooked. 4.1. Categorizing Donor−Acceptor Blends by Their Absorption

The question that thus arises is how to identify when the Channel II mechanism is in play for materials that are defined as electron acceptors. In the following analysis and discussion, an acceptor is defined as having an electron affinity sufficient to oxidize the excited state of a donor. Given that for Channel II to occur, the acceptor must be photoexcited, then the absorption spectrum of each component in the blend provides first order information as to the extent by which the Channel I and Channel II mechanisms are in play. That is, identifying whether features in the photoresponse of the device match the acceptor absorption confirms whether or not it is contributing to the photocurrent. The photoresponse of an organic solar cell is often reported as the external quantum efficiency (EQE), and the validity of this measurement for this type of analysis has been discussed earlier. On the basis of the absorption of a donor−acceptor blend there are three scenarios which one can expect to come across when attempting to identify if the Channel II mechanism is occurring, which we term Category A, B, and C (Figure 7). Category A is when there is light absorption of the acceptor at a longer wavelength than that of the donor. Some spectral overlap may be present, but absorption of the acceptor should be distinguishable at longer wavelengths. If a photoresponse is observed in the region where only the acceptor absorbs then it can be unambiguously assigned to the Channel II charge generation pathway. Category B is when the absorption spectrum of the acceptor overlaps significantly with that of the donor material. Materials with a similar absorption edge will have a similar optical gap, and therefore, it follows that the offsets between the electron affinities and ionization potentials of the two materials should be similar and both Channel I and Channel II can be in play. It should be noted that there are numerous examples of this class of donor−acceptor combinations. Category C is when the absorption maximum of the acceptor sits in a low extinction coefficient “window” of the donor at shorter wavelengths than the absorption maximum of the donor. Again, spectral overlap between the two components may be present, but clear absorption by both components should be discernible. In the case of Category C, where the optical gap of the donor is smaller than that of the acceptor, it is reasonable to conclude that a photoresponse due to light absorption by the acceptor can occur by either (i) Channel II charge generation (the offset of the

Figure 6. Differential transmission dynamics from −3 to 8 ps of the MDMO-PPV/PC60BM (1:4 wt % ratio) bulk heterojunction at 970 nm after photoexcitation at 510 nm (■) and 660 nm (○). Reprinted with permission from ref 78. Copyright 2004 Springer.

resulting from Channel II generation is weaker due to the lower extinction coefficient of PC60BM.

4. CHANNEL II CHARGE GENERATION IN ORGANIC SOLAR CELLS While there were some indications in the early work on organic solar cell devices that the Channel II mechanism might play a role,78 the focus of organic solar cell materials development has been on new donor compounds that worked well with fullerene acceptors via the Channel I mechanism. Part of the reasoning for this was that C60-based fullerene acceptors have at best modest extinction coefficients in the solar harvesting window.80 As a consequence, the Channel II mechanism was not explored to any great extent. However, three factors have now changed this situation. First, many new narrow optical gap polymers are used as the minority component in bulk heterojunction blends with C70-based fullerenes, which have a greater extinction coefficient than C60-based fullerenes, meaning the acceptor has a more significant contribution to light absorption. Second, nonfullerene acceptors (which are the main focus of the latter half of this review) have been designed that possess high molar extinction coefficients and in some cases complementary absorption to the donor material, which in the latter case enables spectral disentanglement of the charge photogeneration pathways. Third, low donor content [typically 5% by weight (wt %) or less of the donor material in the bulk heterojunction] devices comprised of C70-based fullerenes have been shown to give power conversion efficiencies approaching those of conventional blend ratios.81,82 Despite the growing awareness of the importance of the Channel II photocurrent generation pathway, there are few examples of detailed analysis to be found in the

Figure 7. Schematic representation of Category A (left), B (middle), and C (right) scenarios, where D represents donor absorption, A represents acceptor absorption, EA represents electron affinity, and IP represents ionization potential. 12925


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Figure 8. (a) Chemical structure of YF25. (b) Thin film absorption spectrum of YF25 compared with that of PC60BM. (c) Thin film absorption profiles of neat P3HT, a blend of P3HT:PC60BM, and a blend of P3HT:YF25. (d) EQE spectra for devices containing P3HT:PC60BM and P3HT:YF25. (e) Photoconductance decay transients measured by TRMC for P3HT:YF25 (1:1.5) blend, pure P3HT and YF25 excited at 500 and 695 nm at a fixed photon flux of ∼1 × 1013 cm−2. ΔG is normalized by the absorbed photon flux I0FA and constants. The inset shows the intensity-normalized ΔG decays for the same data. (f) The maximum (or end of pulse) ΔG values, normalized the same way as panel (e), as a function of photon flux. Reprinted with permission from ref 33. Copyright 2013 John Wiley and Sons.

ionization potential has to be sufficient given the relative electron affinities and optical gaps of the two materials); (ii) FRET from the wider optical gap acceptor to the narrower optical gap donor followed by Channel I generation; or (iii) a combination of the two. Ultimately, the process that dominates will be determined by the relative rates for FRET and Channel II, which are related to the choice of donor and acceptor materials and the structure of the blend. It is therefore very difficult to predict which process is actually in play as both FRET and Channel II can be ultrafast (femtosecond timescale), although there are some factors that will favor one process over the other and should be considered. The requirements for efficient FRET from the acceptor to the donor are that the acceptor material has a high PLQY with the emission spectrum overlapping strongly with the absorption of

the donor material. Thus, acceptor materials that are at least moderately luminescent in neat films are more likely to transfer excitation to the donor material than charges via Channel II. The structure of the blend is also critical as large domains of the acceptor material are conducive toward FRET, which is more efficient across larger donor-acceptor separations than charge transfer. Conversely, finely mixed donor−acceptor blends will favor wave function overlap between the donor and acceptor molecules and therefore charge transfer. In the case of weakly luminescent acceptor materials, FRET is expected to be much less efficient and charge transfer via Channel II may dominate. Regardless of which category a donor−acceptor system falls in there are a number of properties that need to be known if the system is to be understood. 12926


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Figure 9. (a) Molecular structure of PDI and PBDTTT-CT. (b) Overlapping PL spectrum and absorption spectrum of PDI excimer (red) and PBDTTT-CT (black), respectively, with energy level alignment depicted in the inset. Reprinted from ref 91. Copyright 2015 American Chemical Society.

orientation, and vertical phase separation can all prove valuable for understanding the charge generation mechanism, particularly when analyzed in combination with the results from other techniques, such as spectroscopic data. Many papers in the field only provide a selection of this information, which is understandable given the considerable effort required, but it does hamper complete analysis of each blend and the evaluation of the relative contributions of Channel I and Channel II. Before moving onto a broader discussion of acceptor systems, we give examples of each of the categories (A, B, and C) and work that has been done to disentangle Channel I and II charge generation. Perhaps the first unambiguous report of a Category A system was by Fang et al., who used a simple EQE analysis to reveal how each of the components in a donor−acceptor BHJ contributed to free carrier generation.33 When the nonfullerene acceptor YF25 (Figure 8a) was blended with P3HT it was shown that the EQE spectrum (Figure 8d) had an EQE response in the region 650− 750 nm, which is outside the absorption window of P3HT (Figure 8c), consistent with a Category A system. By comparison of the long wavelength EQE response with the thin film absorption profiles of YF25 (Figure 8b) and P3HT and the knowledge of their relative IPs and EAs, it was shown that the photocurrent generated between 650 and 750 nm was due to photoexcitation of YF25 and subsequent hole transfer to P3HT, the Channel II mechanism. Further evidence for operation of the Channel II pathway was gathered from time-resolved microwave conductivity (TRMC) experiments that showed excitation of the P3HT:YF25 film at 695 nm, where only the acceptor absorbs (see Figure 8, panels e and f), led to an increase in photoconductance by a factor of 8 compared to the neat P3HT film (excited at 500 nm). It is also worthy to note that YF25 has a narrower optical gap than P3HT (i.e., there is no overlap between the YF25 photoluminescence spectrum and the P3HT absorption), therefore the possibility of FRET from YF25 to P3HT is insignificant. The archetypical BHJ blend of P3HT and C60 are a Category B system as the weak fullerene absorption overlaps that of P3HT. Most studies report that charge generation in P3HT:fullerene blends is by the Channel I mechanism.60,89 In principle, if P3HT is photoexcited then some excitons could undergo FRET to the fullerene leading to Channel II charge generation. To see whether this was the case, Rumbles et al. formed a three-layer film comprised of P3HT, a Channel I blocking layer, and C60.60 The blocking layer was designed to enable Channel II to occur, and its

Energetics: as previously emphasized, this review focuses on materials that have been designed as acceptors and hence their electron affinities are sufficient to allow Channel I to occur. It is however important to note that different methods are reported to measure the IP and EA of organic semiconductors and used to justify the cell performance. Often combinations of methods are used without an appreciation of the different assumptions and errors involved and with a level of precision that is unwarranted. There are a number of helpful publications and reviews that discuss these matters that we would draw to the attention of readers.86,87 Furthermore, knowledge of the reorganization energies and the quantum-mechanical transfer integrals associated with each Channel based on Marcus−Hush theory can provide valuable insights into the charge transfer process. It would be helpful to know the exciton binding energies of the donor and the acceptor, although determining these is not trivial. Light absorption properties: the absorption wavelengths are important to determine which category is in play. However, in addition to the wavelength range, the extinction coefficients of the two materials need to be known as well as the ratios of the two materials in the film as these, in conjunction with the film thickness, will give rise to the optical density. At this point, it should be noted that the use of weight percent or ratio can be misleading given the distinctly different molecular weights of the components. For example, the work by Ma et al. on dendritic donors with fullerene acceptors88 uses weight ratios in the normal range but from a molar ratio perspective these are really low donor content cells. Furthermore, as will be discussed later, most thin film organic solar cells are subject to weak cavity effects, which affect the absorption of the active layer. Optical spectroscopic properties: for both the donor and acceptor material, it is necessary to know their absorption and photoluminescence spectra and PLQY to ascertain whether FRET is likely to occur. Time-resolved photoluminescence measurements of the excited state lifetime can be used to provide further evidence of FRET as well as quantify the efficiency of exciton diffusion. Lastly, ultrafast studies using transient absorption spectroscopy are able to monitor the evolution of the excited state population in donor−acceptor blends and thus provide powerful insights into the operation of Channel I and Channel II. Film structure/morphology: the film structure of many BHJ films is complex and establishing the functional morphology of the donor−acceptor blend is not simple. Nonetheless, information with regard to domain sizes, chromophore 12927


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Figure 10. (a) Absorption spectra of PCPDTBT, PC70BM, and their 1:4 blend. The IQE spectrum for the PCPDTBT:PC70BM solar cell is also shown, derived from device reflectance measurements as well as transfer matrix calculations. (b) Absorption spectra of PC70BM, DPP-DTT, and their 1:3 blend, showing the separation of the donor and acceptor absorptions, indicating the two photocurrent channels can be disentangled. (c) Energy levels for PCPDTBT, DPP-DTT, and PC70BM. The IP values were determined by photoelectron spectroscopy in air (PESA), and the spectroscopically measured optical gaps were used to estimate the EAs. (d) IQE of the DPP-DTT:PC70BM (1:3) device measured and modeled as two parallel photodiodes. Reproduced from ref 68. Copyright 2014 American Chemical Society.

5. FULLERENE SYSTEMS

thickness was varied so that the efficiency of FRET also varied. Photoexcitation of the P3HT led to a photoresponse that could only have come from FRET to the C60 followed by a Channel II cascade through the blocking layer. Similar studies have also been undertaken on other donor:PC60BM blends, indicating that the Channel II mechanism might be more widely applicable than previously thought.89,90 The next most studied acceptors after the fullerenes are those based on perylene diimides (PDIs). PDIs have relatively wide optical gaps, and when used in combination with narrow optical gap donors Category C combinations are formed. PDIs are often luminescent, and so determining whether energy transfer or Channel II is occurring is a key question toward understanding how these systems function. A recent report by Singh et al. has specifically looked at the question of energy transfer versus charge transfer processes in PDI:PBDTTT-CT blends (Figure 9a).91 Due to the overlap of the PDI photoluminescence with the polymer absorption, the conditions are present to allow for FRET from the PDI to the polymer (Figure 9b). The authors varied the blend ratio and found that there was inefficient quenching of the PDI excimer, even though a substantial amount of polymer was present (10 wt %). The authors concluded that charge-transfer (via Channel II) was responsible for the PL quenching of the PDI excimer and that FRET was unlikely to occur due to the large PDI domain sizes. In all Category C systems, it is difficult to disentangle whether energy transfer or charge transfer is responsible for the acceptor response without carrying out such experiments; however, if the acceptor material is not luminescent, or weakly luminescent (as is the case with PC60BM) then it is likely that charge transfer is the dominant pathway.

5.1. Conventional Bulk Heterojunctions

Fullerene derivatives are the most widely used acceptors in polymer and nonpolymeric organic solar cells, and an extensive analysis of the Channel I and II mechanisms in all these materials is beyond the scope of this review. Nevertheless, fullerene-based systems have been the subject of extensive studies and therefore provide insightful exemplars on how to assess Channel II performance. We will describe two systems based on blends of the narrow optical gap polymers poly[2,6-(4,4-bis-{2-ethylhexyl}-4H-cyclopenta [2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3benzothiadiazole)] (PCPDTBT) and poly[3,6-bis(5-thiophen2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-2,2’-diyl-alt-thieno[3,2-b]thiophen-2,5-diyl] (DPP-DTT) blended with PC70BM as these provide examples of blends where the polymer and fullerene absorptions do not fully overlap [the polymer absorption extends into the nearinfrared (NIR) where the fullerene does not absorb], thus allowing the Channel I and Channel II processes to be probed and the factors that control their relative performance to be investigated. A comparison between the relative efficiencies of the two Channels is possible by considering their respective IQE spectra. Figure 10 (panels a and b) shows the relative absorption spectra of the polymers and fullerene (weighted based on their volume fraction in the corresponding blends) and the device IQE. The absorption spectra show there are distinct wavelength regions, where either the polymer or fullerene is the dominant absorber, that will populate Channel I and II, respectively. PCPDTBT:PC70BM exhibits a spectrally flat IQE, indicating that charge generation in the two channels is balanced (i.e., 12928


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Figure 11. (a) A cartoon depicting exciton diffusion in a small (left) and large (right) PC70BM aggregate. (b and c) Transmission electron microscopy (TEM) images of thin films of 1:2 blends of BTT-DPP (Mw = 22 kDa):PC70BM and BTT-DPP (Mw = 90 kDa):PC70BM. Dark gray features are assigned to PC70BM aggregates. As such, BTT-DPP-22:PC70BM with large PC70BM aggregates is subject to substantial exciton loss. (d) Kinetics recorded at probe wavelength of 1200 nm through transient absorption spectroscopy for BTT-DPP-90:PC70BM and neat BTT-DPP-90 normalized at 0.25 ps. Reprinted with permission from ref 92. Copyright 2014 Royal Society of Chemistry.

photocurrent generation (see cartoon in Figure 11a). Dimitrov et al.92 have shown that in order for a BHJ solar cell to operate efficiently, Channel II must be optimized through control of the size of the PC70BM aggregates. In their study, this was achieved through changing the molecular weight of the donor poly[(5decylbenzo[1,2-b:3,4-b':5,6-d'']trithiophene-2,8-diyl)-alt-co(3,6-bis{2-thienyl}-2,5-dihydro-2,5-di{2-octyldodecyl}pyrrolo[3,4c]pyrrolo-1,4-dione-5,5'-diyl)] (BTT-DPP) polymer (Figure 11, panels b and c). To probe the effect of the different morphologies they performed TAS on the blend, with an excitation wavelength of 470 nm employed to preferentially excite the fullerene and a probe wavelength of 1200 nm (corresponding to the BTT-DTT polarons). This experiment revealed that decreasing the domain size of the fullerene through increasing the molecular weight of the polymer resulted in an increase in polaron formation at time scales > 30 ps, which the authors concluded was due to the delayed arrival of PC70BM excitons at the interface via exciton diffusion (Figure 11d). They also noted that charge generation from BTT-DTT excitons (Channel I) was less efficient than from PC70BM excitons, which they suggested may be due to the very short singlet exciton lifetime and less favorable energetics for electron transfer. An additional consideration in donor−acceptor blends that contain PC70BM aggregates is that intersystem crossing of singlet to triplet excitons can occur over a time period of ∼1 ns.93 This process is a potential source of loss for a device as the lowest lying triplet state is lower in energy than the singlet state and has a higher binding energy due to the exchange interaction.94 Hence, triplet excitons in PC70BM aggregates could potentially reduce the efficiency of Channel II if they could no longer oxidize the ground state donor. Furthermore, triplet excitons on PC70BM are typically higher in energy than those of conjugated polymers and so transfer of the excitation to the polymer may occur instead of charge transfer,93 further reducing the efficiency of Channel II. Albert-Seifried et al. have also demonstrated that the polymer domain size can limit the efficiency of Channel I in a

photons absorbed by the PCPDTBT or the PC70BM are equally likely to result in free charge carriers). In contrast, the IQEs for the DPP-DTT:PC70BM devices are not flat and are wavelengthdependent, suggesting that photons absorbed by the polymer are less likely to result in free charges than those absorbed by the fullerene. As discussed earlier, for Channel I to be efficient, there needs to be sufficient offset between the EAs of the donor and acceptor to overcome the donor exciton binding energy. A similar condition is applicable to Channel II, where there needs to be sufficient offset in the IPs of the donor and excited acceptor to oxidize the ground state donor. These differences for PCPDTBT, DPP-DTT, and PC70BM are displayed in Figure 10c. In the case of DPP-DTT:PC70BM blends, the energy offset for Channel I is comparable to the exciton binding energy, which could explain why the two Channels are imbalanced, although it is important to note that differences in the exciton harvesting efficiency, caused by poor exciton diffusion and/or unfavorable morphology, could also be responsible. This results in a steplike IQE, which can be modeled as two parallel photodiodes operating over different wavelength ranges.68 The results from the DPP-DTT:PC70BM blend highlights the need to understand the molecular order and microstructure of the active layer film, often termed film morphology. BHJ solar cells require optimum domain sizes to achieve efficient charge separation, and the domain size that is optimum will be dependent upon the materials used. If the domains are too small in size then charge extraction becomes challenging and CT state recombination will dominate due to the increase in the interfacial area between the donor and acceptor.82 Conversely, if the size of the domains is too large, many excitons will decay prior to their arrival at the donor−acceptor interface. Hence, for balanced Channel I and Channel II, it is critical that the domain size of both materials in the blend is optimized. Importantly, if the domain size is appropriate for one material (either the donor or acceptor) while the other component is aggregated in large domains, this will result in imbalanced Channel I and Channel II 12929


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Figure 12. (a and b) Scanning electron microscopy (SEM) images of DPP-based polymer domains in blends with PC70BM without and with 1,8diiodooctane (DIO). The PC70BM has been removed using an orthogonal solvent. (c) EQE spectra of DPP-based polymer:PC70BM solar cells without and with DIO. Reprinted with permission from ref 95. Copyright 2014 Royal Society of Chemistry.

Figure 13. Current density−voltage curves under AM1.5G illumination for coevaporated (a) 1,1-bis-[4-bis(4-methylphenyl)aminophenyl]cyclohexane (TAPC):C60 and (b) MeO-TPD:C60” to “N,N,N,N-tetrakis(4-methoxyphenyl)benzidine (MeO-TPD):C60 with varied donor fraction. An increase in the Voc is observed in both cases. Reprinted from ref 82. Copyright 2014 John Wiley and Sons. (c) Shows the Voc change in four different systems in which either an increase or a decrease in the Voc is observed when the donor content is reduced from 50 wt % to 5 wt %, where SJ defines single junction. Reprinted with permission from ref 99. Copyright 2013 John Wiley and Sons.

DPP:PC70BM system.95 They reported that the short lifetime of the polymer excitons was an obstacle for Channel I charge

generation as the limited exciton diffusion length meant they could rarely reach the donor−acceptor interface. Through 12930


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Figure 14. Polymer donors used in conjunction with acceptors discussed in this review.

a PCE >3%.68,99 For example, it has been shown that BHJ films comprised of 5 wt % of P3HT blended with PC70BM with thicknesses ∼55 nm can deliver PCEs >3%.99 However, these devices lose the linearity of the photoresponse at relatively low light irradiance of ∼2 mW/cm2,99 indicating poor transport properties.100 This is not unexpected given the low concentration of hole-transport material (P3HT) in the blend. While the poor charge transport properties in these devices limits their operation to thin junctions, the high quantum efficiency indicates that charge photogeneration is efficient (>60%). Yang et al. have reported an EQE of ∼60% for a device comprised of 5 wt % of P3HT in PC70BM. Similar EQEs (∼60%) have been reported for other polymeric (e.g., 5 wt % of PCDTBT in PC70BM)68 (Figure 3) and for cothermally evaporated nonpolymeric (e.g., 5 wt % TAPC in C60) solar cells (Figure 13).82,99 Since the fullerene dominates the light absorption in these blends, the Channel II mechanism must be the dominant mechanism for charge carrier photogeneration. PC70BM and C60 are weakly luminescent with little or no spectral overlap of the emission with the absorption of the donor components, and hence, it is likely that FRET between the acceptor and the donor will be negligible. One striking difference between conventional and low donor solar cells is the value of the open circuit voltage (Voc). For low donor P3HT:PC70BM blends, Yang et al.99 have shown that the Voc increases from 0.58 V in a conventional 1:1 BHJ to 0.87 V when the P3HT content is reduced to 5 wt %. They have also reported three other donor:fullerene systems and compared 1:1

addition of 1,8-diiodooctane (DIO), the size of the polymer domain could be controlled (Figure 12, panels a and b) and recombination losses were minimized. Nonetheless, they were unable to fully address the imbalance between Channel I and Channel II, as evidenced in the EQE spectrum (Figure 12c). They noted that PC70BM outperformed the polymer and was responsible for generating almost two-thirds of the device photocurrent. These examples show how careful device IQE and spectroscopy measurements can elucidate the contributions of Channel I and Channel II toward photocurrent generation and also demonstrate that Channel II can be the primary source of photocurrent. 5.2. Low Donor Content Bulk Heterojunctions

In standard BHJ solar cells, the donor−acceptor ratio and active layer morphology need to be optimized such that charge generation and extraction occur efficiently.96,97 For many of the narrow optical gap polymers that have been developed, it has been found that the best performing devices contain a greater proportion of the fullerene by weight (polymer:fullerene ratios of 1:2 to 1:4) in the blend. While high fullerene content will inevitably benefit electron transport, the hole mobility can be compromised and charge extraction will be hindered due to the mobility imbalance. Under these circumstances, the thickness of the BHJ layer in most polymer:fullerence solar cells needs to be less than 100 nm to ensure efficient operation.98 There are now multiple reports showing that BHJs with a fullerene loading of more than 95 wt % can still give devices with 12931


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Table 1. Summary of Relevant Characteristics for Each Paper Discussed λmax(nm)

PLmax (nm)

Egopt (eV)

donor

EQE

IQE

device area(mm2)

active layer (nm)

PCE (%)

ref

YF25 a1 a2

575 574 451

725 − −

1.7 1.9 1.87

a3

423

−

2.04

a4 a5 a6 a7 a8 a9 a10 a11 a12 a13 a14 a15 a16 a17 a18 a19 a20 a21 a22

650 411 422 350 325 625 407 361 361 540 550 400 550 550 540 500 520 475 509

− 510 500 − 620 650 720

1.8 2.7 2.4 1.9 2.4 1.84 1.59 2.22 2.22 2.0 1.87 2.1 2.0 2.0 2.0 1.9 1.9 1.9 2.14

P3HT AlClPc SubPc ZnPc SubPc ZnPc P3HT P3HT P3HT P3HT D2 P3HT PSEHTT PTB7 PTB7 PBDTTT-CT PBDTTT-CT PBDTT-TT PffBT4T-2DT PffBT4T-2DT PBDTT-F-TT PBDTT-F-TT PBDTT-F-TT DPP-Py P3HT

X X X X X X X X X X X X X X X X X X X X X X X X X

− X − − − − − − − − − − − − − X − − − − − − − − −

− 3.3 13.4 13.4 13.4 13.4 5 15 15 10 − 17 4 4 4 − 6 9 5.9 5.9 5.9 3.14 3.14 3 4.5

80 13,15 30a, 65−70b 50a, 65−70b 30a, 50b 50a, 50b 110 − − − − 70 90 − − 100 100 130 120 120 90 85 85 104 80

1.43 0.19 1.0 0.7 0.6 0.1 1.57 1.61 2.4 3.05 1.1 0.48 1.80 3.08 1.42 3.2 6.1 6.05 5.4 6.3 5.53 3.54 0.63 2.4 4.11

84 111 114 114 114 114 122 125 126 83 133 135 138 140 140 71 148 150 149 149 147 153 153 72 69

650 750 − − 650 − 640 640 750 625

a Donor layer thickness in bilayer device. bAcceptor layer thickness in bilayer device. Molecule labels a1−a22 refer to the molecules discussed in section 6.

efficiently remains an open question. We also note that low donor solar cells have only been reported with fullerene acceptors, and it is curious to speculate whether such devices could also be made using nonfullerene acceptors. The breadth of nonfullerene acceptors available may provide the platform needed to fully understand the key operating principles behind low donor solar cells.

blends and 5 wt % donor devices in terms of the difference in the Voc. They find that while the Voc differs between the 1:1 blends and can be correlated to the IP of the donor and EA of the fullerene in the normal manner, the 5 wt % donor devices all had similar Vocs. Yang et al. proposed that this was due to the lowdonor devices operating as Schottky diodes, and therefore, the Voc is defined by the height of the Schottky barrier.99 However, their Mott−Schottky analysis gave equilibrium charge carrier densities on the order of 1017 cm−3 for PC70BM and (5 wt % P3HT:PC70BM blends), which is much higher than normally considered for organic semiconductors, which are often assumed to be undoped or slightly doped due to oxidation under ambient conditions.101,102 Kirchartz et al.103 have pointed out that Mott− Schottky analysis can be misleading when performed on thin diodes with a low doping concentration that are fully depleted of equilibrium charges. In particular, fitting the Schottky equation to the capacitance−voltage plot near Voc (flat band conditions) can lead to incorrect quantification of the doping concentration. Vandewal et al. have attributed the higher Voc of low donor content solar cells to the smaller interfacial area between the donor and the acceptor.82 They investigated coevaporated 1,1bis-[4-bis(4-methylphenyl)aminophenyl)cyclohexane (TAPC):C60 devices with donor concentrations of 1%, 5%, and 10%. Their results show that the CT state absorption (indicative of the interfacial area) decreases with reducing donor content. While the reduced interfacial area in the low donor content devices is still sufficient for an efficient charge separation, it results in lower charge carrier recombination and thereby the Voc increases. Their result, however, cannot explain the Voc reduction in the work of Yang et al. (Figure 13) in which the Voc seems to be stabilized at 0.87 V when a low donor fraction is used.99 For now, the full explanation of how low donor content cells work

6. NONFULLERENE SYSTEMS Having identified how to assess Channel II photocurrent generation using polymer:fullerene BHJ blends as an exemplar, the remainder of the review will focus on nonfullerene acceptors. There are an increasing number of examples of nonfullerene acceptors in the literature, and in each case, the materials combination will be defined by its Category and a comment will be made as to whether there is clear evidence of Channel II photocurrent generation. We note in this analysis that varying methods have been used to determine the IP and EA of the different materials, and hence, the values cannot always be considered precise or accurate and should only be used as a guide to determine whether the EA of a material is suitable for it to be considered an acceptor. The nonfullerene acceptor section of the review is separated into small molecule acceptors (defined as molecules that are less than 800 Da in molecular weight) and nonpolymeric macromolecules (those materials which have a molecular weight greater than 800 Da). Within each molecular weight range the materials are grouped together based upon structural similarities. For reference, we have included the chemical structures of all of the polymer donor materials in Figure 14, and Table 1 contains a summary of relevant characteristics for each of the papers discussed. 12932


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Figure 15. (a) Chemical structures of SubPc acceptors; (b) Absorption (dashed lines), EQE and IQE (solid lines) spectra of AlClPc with F12SubPc. Reprinted with permission from ref 111. Copyright 2009 John Wiley and Sons. (c) Chemical structures of the donors used in the study.

Figure 16. (a) Chemical structure of the two truxenone molecules. (b) Absorption profiles of the two truxenones in solution and film. (c) EQE and absorption spectra overlays for the two truxenone materials with either SubPc (top) or ZnPc (bottom). Reprinted with permission from ref 114. Copyright 2013 Royal Society of Chemistry.

6.1. Small Molecule Acceptors

However, it has been shown that when the macrocycle is peripherally fluorinated both phthalocyanines and metallophthalocyanines can exhibit n-type conductivity in a field-effect transistor configuration (FET), with an electron mobility of 5 × 10−3 cm2 V−1 s−1.105 The reason for the switch from p-type to n-

6.1.1. Phthalocyanines, Subphthalocyanines and Truxenones. Phthalocyanines (Pc) and metallophthalocyanines (MPc) are intensely colored dyes that in the context of organic optoelectronics are well-known for their p-type conductivity.104 12933


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Figure 17. (a) Dicyanovinyl substituted QAs used. (b) Comparison of the absorption profile of DCN-4CQA (□) and PC60BM (■) in the solid state. (c) P3HT:DCN-nCQA (1:1 w/w) blend films. (d) EQE spectra of P3HT:DCN-nCQA devices fabricated from chloroform:o-dichlorobenzene (1:1) and the reference P3HT:PC60BM device fabricated from o-dichlorobenzene. Reprinted with permission from ref 122. Copyright 2011 John Wiley and Sons.

B, and insufficient information is provided to enable determination of Channel I and Channel II photocurrent generation. However, the AlClPc/F12SubPc combination is a Category C system (Figure 15b). For the optimized device with an active area of 3.3 mm2 (ITO/AlClPc(13 nm)/F12SubPc(15 nm)/BCP/Al), where BCP is bathocuproine, a power conversion efficiency (PCE) of 0.19% was obtained. At wavelengths >600 nm, the absorption from the donor, AlClPc is dominant and the EQE response in this region is due to the Channel I mechanism. In the range of 400−600 nm, there is minimal absorption from AlClPc, but F12SubPc absorbs strongly. The IQE where the F12SubPc absorbs is higher than where the AlClPc absorbs, indicating that the F12SubPc makes a strong contribution to charge generation. SubPcs tend to show some PL112,113 (although not specifically mentioned in this publication) so FRET followed by PET cannot be ruled out, but the fact that the IQE in the region that AlClPc absorbs is lower suggests that Channel II is in play. That is, if efficient FRET occurred so that only Channel I took place then the IQE might be expected to be flat. Truxenone-based acceptor materials of the type shown in Figure 16a, produced by McCulloch and co-workers, are highly soluble in common organic solvents and highly absorptive across most of the UV−visible spectrum. The IP and EA of the two materials were estimated through electrochemical characterization, with the EA at approximately −4.1 eV (for both materials) and the IP in the range from −5.9 and −6.2 eV. PC60BM was also evaluated electrochemically as a comparison to give IP and EA values of −5.9 eV and −3.8 eV, respectively. The synthetic approach to these materials enables easy manipulation of the IP and EA, and therefore truxenones

type conductivity is attributed to the larger IP and EA associated with fluorination. Conveniently, fluorination of the macrocycles leaves the optical gap unchanged.106 A related class of macrocyclic materials are subphthalocyanines (SubPc),107 which consist of three diiminoisoindoline units (phthalocyanines contain four) arranged around a central boron atom. Subphthalocyanines have a strong absorption analogous to that of phthalocyanines, with absorption maxima typically centered at ∼600 nm. These materials have been shown to afford high-power conversion efficiencies when employed in a BHJ blend in an organic solar cell.108,109 Additionally, a peripherally fluorinated SubPc linked through its axial position to a ferrocene moiety has shown outstanding electron-accepting ability in solution.110 This characteristic combined with strong absorption and appropriate IP and EA means that fluorinated subphthalocyanines could potentially be excellent electron acceptors, with Torres et al. applying the strategy for determining if they would exhibit n-type conductivity in neat films and hence be used as electron acceptors in organic solar cells.111 The three SubPc acceptor materials (SubPc, F12SubPc, and F13SubPc, Figure 15a) exhibit absorption maxima at ∼600 nm, with onsets of absorption at ∼625 nm, giving them optical gaps of ∼2.0 eV (based on absorption onset). However, the IP and EA values for the materials were not provided. The three materials were used in combination with a variety of donors, including chloro[subnaphthalocyaninato]-boron(III) (SubNc), copper phthalocyanine (CuPc), chloroaluminum phthalocyanine (ClAlPc), and pentacene (Figure 15c) in bilayer devices. The majority of the donor−acceptor combinations have spectral overlap of the donor and acceptor, which makes them Category 12934


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Figure 18. (a) Chemical structures of FFI-1, Th-PhCHO, and Th-COOMe. (b) Absorption (solid line) and emission (dashed line) of FFI-1. (c) EQE spectra for blends of P3HT:FFI-1:1:1 (○), 1:2 (□), and 1:3 (●). (b and c) Reprinted with permission from ref 125. Copyright 2012 John Wiley and Sons. (d) Absorption profiles of Th-PhCHO (red) and Th-COOMe (black). (e) EQE spectra for 1:1 blends with P3HT (Th-PhCHO, red; ThCOOMe, blue). (d and e) Reprinted with permission from ref 126. Copyright 2014 Royal Society of Chemistry.

coincides with the absorption maxima of SubPc (also shown in Figure 16c). The EQE response in the range of 350−450 nm is outside the absorption range of SubPc and, therefore, is possibly due to Channel II photocurrent generated by a2 and a3. Moving to the ZnPc devices, ZnPc is less spectrally overlapped with a2 and a3 compared with SubPc, so EQE analysis is somewhat easier. In the region of 650−800 nm, the only absorbing species is ZnPc, so clearly this photocurrent is generated via the Channel I mechanism. In each case, the energy level offset between the IPs and EAs of the donor and acceptor materials are sufficient to allow for both PHT and PET to occur. However, FRET from the acceptor to the narrow optical gap donor may also be occurring. Without information with regard to the photophysical properties and extinction coefficient of the materials, and analysis of the IQE, it is not possible to draw clear conclusions about the efficiency of the two Channels. 6.1.2. Quinacridones, Fluoranthenes, and Rubicenes. Quinacridone (QA) and its derivatives form a family of compounds that are utilized in the high performance paint industry, as they possess excellent chemical and thermal stability. QAs also exhibit useful optoelectronic properties.115−121 In the

might have the potential to be a powerful class of nonfullerene acceptor materials.114 The absorption spectra of the two materials (Figure 16b) show maxima in the range of 300−500 nm, with minimal absorption past 600 nm, affording optical gaps of ∼1.9 eV for a2 and 2.0 eV for a3. The two materials were tested in inverted bilayer devices with either the donor SubPc or the narrow optical gap material ZnPc. The optimized device architectures were ITO/ZnO (10 nm)/a3 (65−70 nm) and a2 (50 nm)/SubPc (30 nm) or ZnPc (50 nm)/ MoO3 (5 nm)/Ag (120 nm). The acceptor materials were solution processed, and the donor layer was subsequently processed via vacuum deposition. The bilayer structure was chosen over the bulk heterojunction architecture to eliminate any potential morphological issues that could obscure the photovoltaic properties of the truxenones. For an active area of 13.4 mm2 maximum PCEs of 1.0% (SubPc/a2), 0.7% (ZnPc/a2), 0.6% (SubPc/a3), and 0.1% (ZnPc/a3) were achieved. The individual absorption spectra and EQE device spectra are shown in Figure 16c and depict a Category C type system. Considering the SubPc devices first, for both the a2 and a3 devices, the peak of the EQE response is centered just below 500 nm, which 12935


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18a). This material was shown to have appropriate photophysical and electrochemical properties to be a replacement for PC60BM as a nonfullerene acceptor material in organic solar cells. The EA of FFI-1 was reported to be −3.5 eV, which matches the work function of the LiF/Al cathode and should lead to an increase in the open-circuit voltage of devices (compared with PC60BM). The absorption profile of FFI-1 is shown in Figure 18b. The onset of absorption is ∼475 nm, corresponding to an optical gap of ∼2.6 eV. Devices were fabricated using P3HT as the donor in a standard architecture (ITO/PEDOT:PSS/active layer/LiF/Al) leading to a Category C blend system. The hero device efficiency was 1.9% with a blend ratio of 1:2 P3HT:FFI-1, an active layer thickness of 90−95 nm and an active area of 15 mm2.125 The EQE spectrum in Figure 18c shows a photoresponse in the range of 350−650 nm. P3HT absorption is relatively weak at 400 nm, but the absorption of FFI-1 is strong, so the EQE response at 400 nm is likely to be due to absorption by FFI-1. Furthermore, the large offset in IP of 1.4 eV will also be beneficial in driving PHT (Channel II). However, the overlapping PL spectrum of FFI-1 with the P3HT absorption spectrum indicates that FRET from the acceptor to the donor could also be contributing to the acceptor photocurrent. Additionally, FFI-1 does not absorb past 475 nm, so any photocurrent generated at >450 nm is attributed to Channel I. Recently, further structural modifications to the FFI skeleton have been reported, where different electron-withdrawing groups were introduced to further alter the EA of the FFI core and attempt to improve the electron mobility of the material. The structurally modified materials a5 and a6 can be seen in Figure 18a.126 The EAs of the two new materials, Th-COOMe and ThPhCHO, were estimated through electrochemistry and were determined to be −3.4 and −3.3 eV, respectively, which were less than FFI-1 (−3.5 eV). The absorption spectra of thin films of ThCOOMe and Th-PhCHO were very similar (Figure 18d) with the maxima for Th-COOMe and Th-PhCHO at 411 and 422 nm, respectively. The onset of absorption for Th-PhCHO is sharp, at ∼460 nm, yielding an optical gap of ∼2.7 eV, whereas there is a significant tail to the absorption of Th-COOMe in the thin film, out to 520 nm, yielding an optical gap of ∼2.4 eV. The tailing of the absorption is likely an indication of aggregation in the thin film. BHJ devices with P3HT as the donor material were fabricated in an inverted structure (ITO/ZnO/D:A (1:1)/ MoO3/Ag) to yield hero devices with efficiencies of 2.4% (for Th-PhCHO) and 1.6% (for Th-COOMe). The active layer thicknesses were not reported for the devices, although the active area was reported to be 15 mm2. Through examination of the EQE spectra, the authors identify that both materials are absorbing photons to generate excitons126 but do not analyze the data further. The absorption of the acceptor is strongest at ∼400 nm, where P3HT absorption is weak, and this is where Channel II photocurrent can potentially be contributing. The large offset in energy between the donor and acceptor IPs (∼1 eV) should provide sufficient energy to give efficient Channel II, although the possibility of FRET must once again be considered as a competing pathway. Cyclopenta-fused polyaromatic hydrocarbons (CP-PAHs) also exhibit high EAs, which are well-suited to organic semiconductor applications as the one-electron reduction of these materials drives aromatization.127 Rubicene is one such CPPAH, a molecular fragment of C70, with such fragments often reported to have unusual optical and electronic properties.127 These materials have recently been demonstrated to function as n-type semiconductors.128−130 Chen et al. have studied the

solid state, QAs absorb intensely in the visible due to an intramolecular charge transfer (ICT) transition. It is possible to manipulate the energy of the ICT band through substitution of the carbonyl groups, for example with malononitrile, to form dicyanovinyl moieties (Figure 17a). The electron-withdrawing effect of the nitrile groups enhances and shifts the ICT transition to longer wavelengths, which is beneficial for solar cell applications. Functionalization of the ring nitrogen allows for the incorporation of solubilizing groups that then affords the QA derivatives sufficient solubility to be solution-processed. Wang and co-workers122 synthesized a range of dicyanovinyl QA derivatives (Figure 17a) and used these nonfullerene acceptor materials in BHJ devices with P3HT. First the authors were able to demonstrate that a P3HT:DCN-nCQA blend has a wider absorption window than a standard P3HT:PC60BM blend (300−800 nm compared with 300−650 nm, Figure 17c), which in principle means a device containing an active layer of P3HT:DCN-nCQA (1:1) should absorb more light than a comparable P3HT:PC60BM device. The wider absorption window of the P3HT:DCN-nCQA (1:1) blends can be understood when considering the absorption profile of a thin film of a DCN-nCQA material (Figure 17b) which has an absorption maximum at longer wavelengths than P3HT, resulting in a Category A system. The primary absorption band occurs at 550−720 nm, which is attributed to the ICT transition. The authors determined the IP of the materials using UV photoelectron spectroscopy and subsequently calculated the EA through addition of the optical gap to the IP (IP = from −5.8 to −5.9 eV). This calculation afforded EAs in the range from −4.0 to −4.1 eV, which are suitable for the material to act as an electron acceptor with P3HT. To probe whether the material would act as an acceptor, PL quenching experiments were conducted on a P3HT:DCN-nCQA blend. The experiments (not shown) confirmed that significant quenching of the P3HT PL occurred when DCN-nCQA was present in the film, which could be due to Channel I or FRET. An equivalent PL quenching experiment involving photoexcitation of the DCN-nCQA was not done. The combination of PL quenching, appropriate EAs and sufficient electron mobilities led to devices being fabricated in a standard architecture (ITO/PEDOT:PSS/P3HT:DCNnCQA (1:1)/LiF/Al). The hero device was achieved with the noctyl-derivatized material DCN-8CQA, which with a device area of 5 mm2 and an active layer thickness of 110 nm yielded a PCE of 1.6%. The absorption spectra of the P3HT:DCN-8CQA blend and the EQE spectrum can be seen in Figure 17 (panels c and d, respectively). The EQE response for the P3HT:DCN-8CQA device extends over the range of 400−700 nm, and the response >650 nm must be due to Channel II, as this is outside the absorption region of P3HT, in an analogous fashion to the P3HT:YF25 system discussed previously.33 Fluoranthenes are polycyclic aromatic hydrocarbons that consist of a benzene ring and a naphthalene ring fused by a 5membered ring. Historically known for being air pollutants resulting from combustion, fluoranthenes actually possess many qualities that are desirable for optoelectronic applications, including strong π−π interactions in the solid state, high thermal, and oxidative stability in air and high charge carrier mobilities.123 Pei et al. have reported a convenient synthetic route to fluoranthene-fused imides (FFI) and have subsequently demonstrated that the EAs of the materials can be tailored through placement of electron-withdrawing groups at key positions on the FFI scaffold.124 One of the first reports of an FFI utilized in organic solar cells was the material FFI-1 (Figure 12936


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Figure 19. (a) Chemical structure of the diindenorubicene. (b) Absorption profile of DIR-2EH in solution (pink), in film (purple), and a film of P3HT (orange). (c) EQE spectra of devices containing P3HT:PC60BM (dark red) and P3HT:DIR-2EH (1:4, red; 1:2, green; and 1:1, blue). Reprinted with permission from ref 83. Copyright 2015 Royal Society of Chemistry.

Figure 20. Structures of the donors D1 and D2 and acceptors A1 and A2.

the EQEs are given in Figure 19c) in a device with an active area of 10 mm2 (active layer thickness was not reported). The devices exhibited a very high Voc of 1.2 eV, which is the direct result of the lower EA of the DIR-2EH and is remarkably high for an organic solar cell. 6.1.3. Vinazenes. 2-Vinyl-4,5-dicyanoimidazoles, more commonly referred to as vinazenes, were first prepared by Rasmussen and co-workers,131 and their use in organic solar cells was first reported by Sellinger et al.132 who used them as solution processable nonfullerene acceptors in blends with P3HT. This was one of the first reports of a tunable class of materials that could be used to transport electrons. Much work has been published involving the use of this class of materials, most of which is beyond the scope of this review. However, one recent example of BHJ organic solar cells where vinazenes were used as the acceptor and a nonpolymeric diketopyrrollopyrrole macromolecule as the donor material by Nguyen et al.133 demonstrates that the combination of these two relatively simple materials can lead to moderate power conversion efficiencies as well as high open-circuit voltages. Two donor materials (D1 and D2, Figure 20) were studied along with two acceptor materials (A1 and A2, Figure 20), D1 was chosen due to its reputation to yield highperforming solar cells. D2 was chosen as a comparison as it has a higher EA than D1, which was believed would assist in electron

electrochemical properties of diindeno[1,2-g:1′,2′-s]rubicene (DIR-2EH, Figure 19a) to assess its IP and EA. The material exhibits two quasi-reversible reduction waves that were ascribed to the formation of two stabilized aromatic benzocyclipentadienyl anions.83 The EA was subsequently estimated to be approximately −3.3 eV, which is 0.5 eV less than PC60BM when studied under the same conditions. Figure 19b displays the absorption spectra of DIR-2EH in solution and thin film as well as the thin film absorption spectrum of P3HT. From this, we can assign the DIR-2EH:P3HT blend to a Category B system. Estimation of the optical gap from the onset of absorption in the film at ∼650 nm results in a gap of ∼2.0 eV. Furthermore, there is an intense absorption at 350 nm, which can be assigned to localized transitions, and a broad absorption in the region of 400−650 nm, corresponding to the delocalized π−π* transitions. To assess the electron-accepting ability of DIR-2EH, PL quenching experiments were conducted with P3HT. In solution measurements, there was a dramatic reduction in the PL of P3HT with increasing amounts of DIR-2EH, suggesting successful PET (Channel I) from P3HT to DIR-2EH. Devices containing blends of P3HT and DIR-2EH were fabricated in a standard architecture (ITO/PEDOT:PSS/P3HT:DIR-2EH/ Ca/Al). The best blend ratio was determined to be 1:4 P3HT:DIR-2EH, which afforded an efficiency of 3.1% (note 12937


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Figure 21. (a) Absorption spectra of as-cast (solid lines) and annealed (dashed lines) of D2 (red) Al (blue) and the blend (green). (b) Photoluminescence spectra of as-cast (solid lines) and annealed (dashed lines) of D2 (red), A1 (blue), and the blend (green). (c) Absorption spectra of as-cast (solid lines) and annealed (dashed lines) of A2 (blue) and the blend (green). (d) Photoluminescence spectra of as-cast (solid lines) and annealed (dashed lines) of A2 (blue) and the blend (green). (e) EQE spectra of an as-cast (blue) and annealed (80 °C, green; 110 °C, red; 140 °C black) 1:1 blend of D2:A1. (f) EQE spectra of an as-cast (blue) and annealed (80 °C, green; 110 °C, red; 140 °C black) 1:1 blend of D2:A2. Reproduced from ref 133. Copyright 2012 American Chemical Society.

transfer to the vinazene acceptors. The IPs of D1 and D2 were measured using ultraviolet photoemission spectroscopy and were determined to be −5.2 eV for both. The IP for A1 was measured with PESA and found to be −6.1 eV, with the IP of A2 being previously reported as −5.9 eV (estimated from cyclic voltammetry).133 The EAs of the materials were estimated through addition of the optical gap (taken from the absorption spectra) to the IP to yield EA values of −3.4, −3.2, and −3.5 eV for D1, D2, and A1, respectively. The EA of A2 has been previously reported133 to be −3.5 eV (estimated from cyclic voltammetry measurements). The authors conclude that the EAs of D1 and A1 are too similar, and there is insufficient driving force for efficient electron transfer from D1 to A1, evidenced in poorly performing devices. However, this was not the case for D2 and A2 where the offset in EAs is 0.3 eV, and these devices performed well. The fact that the absorption spectra of A1 and A2 overlap with that of D1 and D2 means that the blends are Category B. The absorption spectra for compounds D2 and A1 can be seen in Figure 21a. D2 exhibits strong absorption at 400 nm, as well as 550−650 nm, which was unaffected by thermal annealing, while

A1 has strong absorption at 350 and 450 nm. When the as-cast film of A1 was annealed, broadening of the absorption occurred out toward the NIR, which is indicative of light scattering and/or aggregation of the neat material. The PL spectra of the materials were also measured (Figure 21b), with D2 showing negligible emission in the as-cast and thermally annealed films, whereas A1 has significantly stronger emission which decreases after thermal annealing due to aggregation-induced quenching. The PL of the thin film blend D2:A1 is minimal (excitation wavelength not reported), suggesting that the two materials are intimately mixed, and efficient charge transfer is occurring. The absorption (Figure 21c) and emission (Figure 21d) spectra of A2 were also investigated, with the as-cast and annealed films showing almost identical absorption, suggesting this material is less aggregated than its counterpart A1. A similar PL quenching observation was reported for the blend of D2:A2, where the A2 emission is fully quenched, indicating efficient charge transfer from A2 to D2. Finally, based on the optical and electronic data, BHJ devices were fabricated with the following architecture: ITO/ PEDOT:PSS/D:A (1:1)/Al, to afford devices with a maximum efficiency of 1.1% (for D2:A2) and 0.8% (for D1:A1) (no device 12938


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following architecture: ITO/PEDOT:PSS/P3HT:acceptor (70 nm)/Ca/Al with a device active area of 17 mm2, and all devices yielded a power conversion efficiency of ∼0.3% irrespective of the NDI used. The EQE spectra for all devices can be seen in Figure 23c. However, we concentrate in this discussion on the specific example highlighted in Figure 23d, that of P3HT:RF1 (1:2.5). The authors focused on determining if the acceptors were contributing to the photocurrent, and they note that the absorption spectra of P3HT and RF1 individually are not a superimposition of the absorption spectra of the P3HT:RF1 blend. But what is most interesting is that the normalized EQE clearly follows the P3HT absorption, except where the absorption from RF1 is strong, namely at 650 nm. Also, the IP values of the acceptor materials are sufficiently high to allow for efficient hole transfer to P3HT. This is an example of a Category A type system and is analogous to our previous evaluation of the P3HT:YF25 devices.84 Moving on from naphthalene diimides, Jenekhe and coworkers138 recently reported a new type of polycyclic system, tetraazabenzodifluoroanthene diimides (BFIs) (Figure 24), which consist of a tetraazaanthracene core fused to two naphthalene imide units. The design of this new system aims to achieve several things; first, to obtain a high electron affinity through a combination of imine nitrogen containing aromatic groups with dicarboxylic acid imide functionalities; second, to provide a large rigid π-conjugated system that should have good charge carrier mobility; and third, the potential to tune the optical and electronic properties through functionalization of the periphery of the BFI core or through quaternation of the imine nitrogens.138 The thin film absorption of the BFI materials can be seen in Figure 25a. All the BFI materials display a common high-energy absorption band centered at ∼380 nm, and for BFI-T2 and BFITM2 there is an additional low-energy ICT band in the region of 500−750 nm. Focusing on the material BFI-T2, the onset of absorption is at ∼750 nm, which leads to an optical gap of ∼1.6 eV for BFI-T2. The authors used electrochemical experiments to estimate the EAs and IPs of the new BFI materials. BFI-T2 shows two quasi-reversible reduction waves at −1.1 and −1.4 eV, giving an estimation of the EA as −3.5 eV. No oxidation waves were observed under the experimental conditions (up to 1.2 V), and the authors erroneously conclude that the material must have a very high IP, which they estimated to be −5.2 eV (through subtraction of the optical gap from the EA). However, this is not very high, especially for an electron-accepting material; PC60BM has an IP of −6.2 eV.33 In a preliminary investigation of these materials, BHJ solar cells were fabricated with the polymer poly[(4,4'-bis{3-[2-ethylhexyl]dithieno[3,2-b:",3'-d]silole}-2,6diyl-alt-{2,5-bis[3-(2-ethylhexyl)thiophen-2yl]thiazolo[5,4-d]thiazole})] (PSEHTT) with the architecture being ITO/ PEDOT:PSS/PSEHTT:BFI-T2(1:4)/LiF/Al. The hero device afforded an efficiency of 1.8%, where the active area was measured to be just 4 mm2 with an active layer thickness of 80 nm. The EQE spectrum for the hero device can be seen in Figure 25b. The photoresponse has an onset of ∼800 nm with peaks at 370 and 630 nm. The film absorption spectrum of the polymer PSEHTT exhibits two peaks centered on either side of 600 nm,139 therefore the EQE response at 630 nm will be due to Channel I. Similarly, the film absorption of BFI-TM2 displays a peak at ∼400 nm (Figure 25a), and the EQE peak at 370 nm will be due to the BFI-TM2 absorbing light. Upon analysis of the energy levels, it is clear that the IP of PSEHTT (−5.1 eV) is very close to that of the acceptors, providing little driving force for

areas or active layer thicknesses were reported). The EQE for the hero device containing D2 and A2 can be seen in Figure 21f (the EQE for D1:A1 is shown in Figure 21e for comparison). There is an EQE response in the range of 300−675 nm, with maxima at ∼350, ∼450, and ∼625 nm. The EQE features in the region of 600 nm fit well with the absorption profile of D2, and therefore in this region, photocurrent is likely to be generated via the Channel I mechanism. Similarly, the absorption of A2 is strong at ∼350 nm and ∼450 nm, and therefore, Channel II might provide photocurrent at these wavelengths. However, due to the PL properties of the acceptor, it is possible that FRET could play a part in the EQE response in this region. Further studies would be required in order to determine which mechanism was dominant. 6.2. Nonpolymeric Macromolecules

6.2.1. Naphthalenediimides and Tetrabenzodifluoroanthene Diimides. Naphthalene diimides (NDIs) are fluorescent redox-active planar materials that have relatively high EAs (comparable with PC60BM).134 Their ease of reduction can be attributed to the conjugated ring system, combined with peripheral electron-withdrawing carbonyl moieties, and as a result they are suitable candidates to be nonfullerene acceptors. The NDIs exhibit strong π−π interactions in neat films, therefore, the potential for high electron transport is good.134 The optical properties of NDIs are also easily tuned through substitution around the periphery.134 In a recent report by Sauvé et al., 2,6-dialkylaminonaphthalene diimides (Figure 22) were studied as nonfullerene acceptors

Figure 22. Structures of the dialkylamino NDI materials from Sauvé et al.

with P3HT as the donor.135 The absorption spectra of these NDIs can be seen in Figure 23 (panels a and b). All of the materials had solution absorption spectra similar to that already reported for 2,6-dialkylamino NDIs,136,137 with a strong absorption at 500−660 nm, which is a charge transfer band associated with the push−pull system between the donating alkylamino core substituents and the withdrawing imides, and a high energy band at 350−360 nm, which is due to the localized transitions in the naphthalene core. When moving from solution to thin film, the absorption peaks broadened slightly and redshifted, which could be an indication of molecular aggregation within the thin films. The two molecules had an optical gap of ∼1.9 eV, as calculated from the onset of absorption in solution, and electrochemistry revealed two chemically reversible reduction waves with E1/2s at −1.4 and −1.8 eV. From these measurements, the EA and IP were estimated to be −5.7 and −3.7 eV, respectively. BHJ devices were fabricated with the 12939


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Figure 23. (a) Solution (solid lines) and film (dashed lines) of RF1 (black), RF3 (blue), and RF5 (green). (b) Solution (solid lines) and film (dashed lines) of RF2 (black), RF4 (blue), and RF6 (green). (c) EQE spectra of P3HT:NDI (1:2.5) devices (RF1, black; RF2, red; RF3, blue; RF4, pink; RF5, green; and RF6, purple). (d) An overlay of the film absorption of P3HT (red), RF1 (blue), the blend (pink), and the EQE of the blend (black). Reproduced from ref 135. Copyright 2014 American Chemical Society.

semiconductors, and these materials have been shown to exhibit n-type properties in organic light-emitting diodes (OLEDs).141,142 Previously, these materials have only been suitable for processing by vacuum sublimation, and the report by Jenekhe discussed functionalized diphenylanthrazoline materials with alkylated naphthalene diimides to provide solubility and hence solution processability.140 The thin film absorption and emission properties of the two new materials BNIDPA-BO and BNIDPA-DT can be seen in Figure 25c. Both materials exhibit absorption maxima at 361 nm, which is 19 nm blue-shifted from the maxima in the solution absorption spectra, indicating these materials H-aggregate in the solid phase. There are also two long wavelength transitions at ∼500 nm, which were assigned to intermolecular interactions.140 The optical gaps of the materials were estimated from the onset of absorption and were 2.2 eV. The materials exhibit one peak in the PL spectra, centered at 615 and 627 nm for BNIDPA-DT and BNIDPA-BO, respectively. The authors estimated the IPs and EAs from cyclic voltammetry measurements and quote that both materials exhibit several unresolved multiple reduction waves in the region of −1.3−1.6 eV and, therefore, estimate the EA to be −3.6 eV (onset of reduction).140 No oxidation waves were seen in the range scanned, therefore, the IP was calculated by subtraction of the optical gap from the EA and was reported to be −5.8 eV. This value is much higher than what was previously reported for BFIT2 and BFI-TM2, which should improve the efficiency of the Channel II mechanism. To investigate the photovoltaic properties of these materials, inverted architecture devices were fabricated (ITO/ZnO/ donor:acceptor (1:4)/MoO3/Ag) with an unreported active layer thickness and an active area of 4 mm2. Three different polymers were tested; PSEHTT, poly[4,8-bis(5-{2-ethylhexyl}thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4{2-ethylhexyl}-3-fluorothieno[3,4-b]thiophene)-2-carboxylate2-6-diyl] (PBDTT-FTTE) and poly[(4,8-bis{2-ethylhexyloxy}benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)(3-fluoro-2-{[2-

Figure 24. Structures of the BFI-based materials and BNIDPA-based materials.

Channel II to occur, and in this case, energy transfer from BFITM2 to PSEHTT could well be occurring. Given that the long wavelength absorption of the BFI-TM2 is at a similar wavelength to the PSEHTT, the blend can be considered a Category B system. Although not discussed from their absorption spectra, BFI and BFI-CN2 would be Category C if the same donor polymer was used. Following the initial report on the BFI materials, Jenekhe and co-workers followed up with another new class of materials, BNIDPA (Figure 24).140 Diphenylanthrazolines have previously been utilized in the design of small molecule and polymer 12940


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Figure 25. (a) Thin film absorption profiles of the four BFI materials (BFI, black; BFI-T2, blue; BFI-TM2, pink; and BFI-CN2, green). (b) EQE spectrum of a 1:4 blend of PSEHTT:BFI-T2. (a and b) reprinted with permission from ref 138. Copyright 2013 John Wiley & Sons. (c) Thin film absorption (solid lines) and thin film photoluminescence (dashed lines) of BNIDPA-BO (black) and BNIDPA-DT (gray). (d) EQE spectra of a 1:3 blend of PTB7:BNIDPA-BO (black) and 1:3 blend of PTB7:BNIDPA-DT (gray). (c and d) reprinted with permission from ref 140. Copyright 2015 John Wiley and Sons.

blend morphologies with donors and increases the potential for isotropic charge transport. The following examples of PDI-based acceptors are all blended with donor polymers that have narrower optical gaps, resulting in Category C systems. As the majority of these three-dimensional PDI-based materials are photoluminescent, it is plausible that FRET from the PDI-based acceptor to the comparatively narrow gap polymer could compete with Channel II charge generation. Therefore, an EQE response in the acceptor region cannot unambiguously be assigned to Channel II. However, recent studies on a PDI: poly[(4,8-bis{2-ethylhexylthiophene-5-yl}benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)-alt-(2-{2'ethylhexanoyl}thieno[3,4-b]thiophen-4,6-diyl)](PBDTTTCT) system (see section 4) concluded that charge transfer and not FRET was responsible for PDI-excimer PL quenching.91 On the basis of this study, it is likely that FRET will not always be the dominant pathway in these systems. However, further PL studies on each system would be required to confirm whether this is the case, as morphology, and in particular acceptor domain sizes, can play a significant role in the efficiency of the FRET process.91 One of the first reports of a three-dimensional PDI-based molecule was the twisted TP (Figure 26a), by Narayan et al. in 2012.146 The absorption spectrum of TP can be seen in Figure 26b. TP exhibits a bimodal absorption pattern, with peaks at 506 and 545 nm. The onset of absorption is ∼600 nm, so the optical gap of the material was estimated to be 1.8 eV. The IP and EA of TP were estimated through a combination of optical absorption and cyclic voltammetry and were quoted as −5.9 and −4.1 eV, respectively. In order to implement a complementary absorption donor−acceptor pair, the narrow optical gap polymer PBDTTTCT was chosen. This polymer absorbs broadly in the range of 600−800 nm (Figure 26b), and the blend of the two materials exhibits excellent coverage over the entire visible spectrum (Figure 26b) and, hence, is a Category C system. Devices were fabricated in an inverted structure (ITO/ZnO/donor:acceptor

ethylhexyl]carbonyl}thieno[3,4-b]thiophenediyl)] (PTB7) (Figure 14). Irrespective of the polymer employed, all devices afforded efficiencies of ∼3.0%. An example of the EQE spectrum for one such device can be seen in Figure 25d. The PTB7:BNIDPA blends exhibit EQE peaks at ∼350, ∼500, and ∼700 nm. The peak at ∼350 nm is primarily due to absorption of the acceptor, as the absorption of PTB7 is low at this wavelength, making this a Category C system. The BNIDPA moiety has no absorption at 700 nm, therefore, the peak at 700 nm in the EQE can be assigned to Channel I generation. Energy level analysis confirms that the Channel II pathway is energetically viable with an IP offset between donor and acceptor of 0.7 eV. As this is a Category C system and these materials are luminescent, the possibility of FRET from the acceptor to the donor cannot be eliminated and must be kept in mind when evaluating the mechanism of charge generation.68 6.2.2. Perylenediimides. Perylenediimide (PDI) dyes were originally investigated as industrial paint pigments due to their excellent chemical, photo, thermal, and weather stability.143 They are intense absorbers with good electron-accepting properties and near unity PLQYs.144 Despite the promising properties of PDI dyes, they traditionally yielded poorly performing organic solar cells. This has been assigned to the strong intermolecular interactions that exist between molecules in the solid state, leading to large-scale phase separation, which is detrimental to organic solar cell performance.145 One way to overcome the inherent ability of PDIs to self-aggregate is to force them into a three-dimensional structure, thus hindering the formation of very large aggregates.146 Recent literature has shown that nonfullerene acceptors based on three-dimensional PDI structures have shown significant promise, consistently reaching device efficiencies in excess of 5% when blended with various narrow optical gap polymers.147−150 Increasing the dimensionality leads to the formation of amorphous materials, which form favorable 12941


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Figure 26. (a) Chemical structure of TP. (b) Absorption spectra of pristine TP (blue), pristine PBDTTT-CT (red), and a 1:1 blend of PBDTTTCT:TP (black). (c) Steady-state PL spectra of films excited at 543 nm showing emission maxima at 650 and 820 nm for pristine TP (blue), pristine PBDTTT-CT (red), and quench >99% (black) by blending PBDTTT-CT and TP in a 1:1 weight ratio (inset: schematic of hole transfer mechanism). (d) Steady-state PL spectra of films excited at 632 nm, showing an emission maximum at 820 nm for pristine PBDTTT-CT (red) which quenches to 99% (black) by blending PBDTTT-CT and TP in a 1:1 weight ratio (inset: schematic of electron transfer). (e) EQE spectra of PBDTTT-CT:TP at different weight ratios of 1:1 (black), 1:2 (red), and 1:3 (blue). (f) IQE spectra for blends of PBDTTT-CT:TP at different weight ratios of 1:1 (black), 1:2 (red), and 1:3 (blue). Reprinted with permission from ref 71. Copyright 2014 Royal Society of Chemistry.

blend was first excited at 543 nm to preferentially excite the TP and the PL spectrum obtained indicated that >99% of the TP excitons were quenched by the polymer, and they concluded that the Channel II process was highly efficient. Although, it is important to note that PL quenching measurements are not quantitative unless performed with an integrating sphere. However, we note that the authors did not comment on the luminescence of TP and the occurrence of FRET. Next the blend film was excited at 632 nm to selectively excite PBDTTT-CT, and the PL spectrum revealed >99% of the polymer excitons were quenched by TP, demonstrating that Channel I was a highly efficient process for the blend. The authors then examined the EQE spectra to assign Channel I or Channel II photocurrent generation. The relevant EQE spectra can be seen in Figure 26e for different blend ratios. The shape of the EQE spectra are the same irrespective of the blend ratio, with the peak in the range of 400−600 nm being assigned to Channel II from TP to PBDTTT-CT and the peak between 600 and 800 nm being due to Channel I. Additionally, the IQE spectra (Figure 26f) were

1:1/MoO3/Ag), with an active layer thickness of 100 nm (device area not reported). Efficiencies up to 2.8% were achieved for the blend of PBDTTT-CT:TP (1:1). The EQE spectrum for the 1:1 blend can be seen in Figure 26e. There are two clear regions in the EQE, one ranging from 400 to 600 nm and the other from 600 to 800 nm. Given the distinct difference in the absorption spectra of each material in the blend, the EQE response from 400 to 600 nm is from Channel II generation and/or FRET followed by Channel I, whereas the EQE response from 600 to 800 nm is Channel I. Further studies on TP have shown its potential to reach PCEs of 3.2% when used in an inverted device structure and blended with PBDTTT-CT.71 The charge dynamics of this system have been probed in an attempt to explain the origin of this enhanced performance, and in turn more evidence for the Channel II pathway was presented. First the authors investigated the PL quenching of blends of PBDTTT-CT and TP to ascertain whether both Channel II and I were occurring; the results are depicted in Figure 26 (panels c and d, respectively). The 1:1 12942


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Figure 27. (a) Chemical structure of the acceptor. (b) Solution absorption spectra of Bis-PDI-T-EG (●), its monomer (solid line), and polymer PBDTTT-CT (△). Reprinted with permission from ref 151. Copyright 2013 John Wiley and Sons. (c) EQE spectra of PBDTTT-CT:Bis-PDI-T-EG devices (thick black line, maxima ∼40%). Reprinted with permission from ref 148. Copyright 2015 John Wiley and Sons. (d) EQE spectra of the best cells obtained under four conditions (green line, maxima ∼60%). Reprinted with permission from ref 148. Copyright 2015 American Chemical Society.

reported for the blends, although the authors acknowledge that spectra were not corrected for interference effects. The IQE spectra are not flat, with slightly higher efficiencies observed for TP excitation, suggesting that Channel II is more efficient than Channel I. However, as stated above, FRET followed by Channel I could also be taking place, although analysis of the energy level offsets showed that the difference in IPs being larger than the difference in EAs suggested that Channel II could be more energetically favored. Recently Yao and co-workers reported a PDI dimer (Bis-PDIT-EG, Figure 27a) representing an alternative method for achieving a three-dimensional system by linking two PDI units with a central thiophene via the bay positions.151 Bis-PDI-T-EG exhibits a broader absorption than its monomeric counterpart (Figure 27b, monomer not shown) with an absorption peak centered at 550 nm and a smaller peak at 325 nm. The absorption of PBDTTT-CT is also provided, which shows its absorption maximum is at longer wavelengths than the acceptor, giving rise to a Category C system. The authors estimated the IP and EA energies from cyclic voltammetry experiments and calculated the values to be −5.7 and −3.8 eV, respectively, for Bis-PDI-T-EG and −5.1 and −3.3 eV, respectively, for the polymer. Photovoltaic devices were prepared in a conventional architecture (ITO/PEDOT:PSS/donor:acceptor 1:1 Ca/Al) with active layer thickness of 100 nm and a device area of 6 mm2. With the addition of DIO to the processing solvent, efficiencies of up to 3.9% were achieved. The EQE spectrum for the best device can be seen in Figure 27c. The thick solid line represents the PBDTTT-CT:Bis-PDI-T-EG device with 5% added DIO to the processing solution. Through comparison of the absorption spectra of the polymer with the device EQE, it can be seen that the polymer has strong absorption between 650 and 800 nm, where the acceptor has minimal absorption, therefore Channel I clearly occurs at these wavelengths. Conversely, Bis-PDI-T-EG has strong absorption between 400 and 600 nm where the polymer absorption is minimal, and so the majority of the photocurrent generated in this region will be by Channel II or

FRET followed by Channel I, i.e., the blend is a Category C system. Through the analysis of the energy level offsets, it can be confirmed that both Channel I and II pathways are energetically allowed. Following the first report by Yao on Bis-PDI-T-EG, a second report followed describing the improvement of the device efficiencies through a careful analysis of the spin-coating conditions and the concentration of DIO in the active layer solution.148 The optimization resulted in enhanced photocurrent generation as evidenced by comparing the EQE spectra in Figure 27 (panels c and d), where the maximum EQE increased from ∼40% to just under 60% and an overall improvement in device efficiency with a PCE of 6% being recorded. The shape of the EQE spectra remained unchanged. Another method to create three-dimensional PDI structures is to directly fuse two PDI moieties at the bay positions to form a twisted PDI material or “helical PDI” as shown in Figure 28a.150 Helical PDI has strong absorption in the visible spectrum, with one peak centered at ∼400 nm and a multipeaked feature from 450 to 600 nm (Figure 28b). Helical PDI had previously been reported to have a high electron mobility, and an EA of approximately −4.0 eV.152 For these reasons, it was chosen as an electron acceptor for BHJ devices with the narrow optical gap polymers PTB7 and poly[4,8-bis(5-{2-ethylhexyl}thiophen-2yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-{2-ethylhexyl}-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl] (PBDTT-TT). Devices were fabricated in both a conventional architecture (ITO/PEDOT:PSS/PTB7:helical PDI/BCP/Al) and an inverted architecture (ITO/ZnO/PTB7:helical PDI/ MoOx/Al), with the active layer thickness and device area for both devices being 90 nm and 9 mm2, respectively. For the PTB7 devices the best material ratio was 3:7, and the inverted structure outperformed the conventional structure with efficiencies of 3.5% and 4.5%, respectively. Addition of solvent additives DIO or 1-chloronaphthalene (CN) improved the active layer morphology and subsequently improved the device efficiency up to 5.2% when 1 v/v% of either were added to the active layer solution prior to casting and up to 5.9% when both were added. Overall, 12943


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Figure 28. (a) Chemical structure of helical PDI. (b) Film absorption spectra of PTB7 (red), PBDTT-TT (blue), and helical PDI (1, black), normalized at the low-energy peaks. (c) EQE spectra (symbols) of PTB7:1 (black) and PBDTT-TT:1 (red) devices with 1% DIO and 1% CN and absorption spectra (solid lines) for the PTB7:1 (black) and PBDTT-TT:1 (red) blend films (3:7 D:A). (d) Differential transmission spectra as a function of probe wavelength with pumping at 390 nm for neat helical-PDI at 0.25 and 400 ps. (e) Differential transmission spectra as a function of probe wavelength with pumping at 670 nm for neat helical PDI, neat PTB7 at 0.25 ps, and a blend of helical PDI and PTB7 and 0.25 and 6 ps. (f) As for (e) but pumped at 390 nm. Reproduced from ref 150. Copyright 2014 American Chemical Society.

were assigned to excited state absorption of helical PDI polarons, although this assignment is unusual given that the state featured a lifetime of 800 ps (See Figure 28d). Next the photophysics of a 3:7 blend of PTB7:helical PDI was investigated, the results of which can be seen in Figure 28 (panels e and f). At a pump wavelength of 670 nm, the PTB7 is selectively excited and the negative signals at 625 and 680 nm result from the ground-state bleach of PTB7. New features at 515 and 555 nm are present in the blend film, which are attributed to ground-state bleaching of helical PDI. The ground-state bleaching of helical-PDI when only PTB7 is excited is consistent with Channel I from PTB7 to helical PDI. Similar results were observed for the preferential excitation of helical PDI at 390 nm. Initially (0.25 ps), both PTB7 and helical PDI exhibit ground-state bleaching, but the bleach signal in the region of 600−720 nm continues to increase over 6 ps, consistent with the transfer of excitation from helical PDI to PTB7 via FRET or conversion of excitons into polarons via Channel II or FRET-mediated Channel I. Thus, the measurements thus far do not unequivocally show Channel II is the dominant pathway at shorter wavelengths. Another twisted PDI material, reported by Yan et al., consists of two PDI moieties linked at the bay positions through a spirobifluorene.149 The SF-PDI2 material (Figure 29b) was compared with diPDI (Figure 29a) with two narrow optical gap

the same trends were observed for the PBDTT-TT devices as the PTB7 devices (active layer was increased to 130 nm) and the hero device with PBDTT-TT reached an efficiency of just over 6%. An overlay of the absorption of PTB7:helical PDI and PBDTT-TT:helical PDI blends and the EQE spectra can be seen in Figure 28c. The authors note that the sharp and strong absorption of the helical PDI allows for disentanglement of the contributions from Channel I and Channel II. They state that it is clear that between 300 and 600 nm, the major source of photocurrent will be coming from Channel II, as both PTB7 and PBDTT-TT have minimal absorption in this range (by comparison to helical PDI). However, past 600 nm, where helical PDI does not absorb, the photocurrent must all be generated via Channel I. However, these are also Category C systems and as with the previous PDI-based acceptor, FRET can also play a role in photocurrent generation and further study was necessary to confirm the role of Channel II. To provide further evidence of the charge generation mechanisms, the authors performed TAS on the blends. First, experiments were conducted on a neat film of helical PDI and the differential transmission spectra are displayed in Figure 28d. The film was excited at 390 nm, and the negative peaks at 480, 516, and 556 nm are the result of ground-state bleaching. The positive signals below 470 nm and above 570 nm at later times (400 ps) 12944


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Figure 29. (a) Chemical structure of diPDI. (b) Chemical structure of SF-PDI2. (c) Normalized film absorption spectra of SF-PDI2 (red), diPDI (dark red), PffBT4T-2DT (green), and PTB7-Th (blue). (d) EQE spectra of devices tested; PffBT4T-2DT:SF-PDI2 (red), PffBT4T-2DT:diPDI (dark red), and PTB7:SF-PDI2 (blue). Reprinted with permission from ref 149. Copyright 2015 Royal Society of Chemistry.

Figure 30. (a) Chemical structure of PDI4. (b) Normalized absorption spectra of TPE-PDI4 in solution (▼) and film (▲). (c) EQE spectrum of the PBDTT-F-TT:TPE-PDI4 device (■). Reprinted with permission from ref 147. Copyright 2015 John Wiley and Sons.

polymers poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt(3,3'''-di{2-octyldodecyl}-2,2';5',2'';5'',2'''-quaterthiophen-5,5'''diyl)] (PffT4T-2DT) and poly[4,8-bis(5-{2-ethylhexyl}thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4{2-ethylhexyl}-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate2-6-diyl] (PTB7-Th). The absorption spectra of the two new PDI materials as well as the two polymer materials studied can be seen in Figure 29c. The absorption spectra of SF-PDI2 and diPDI are almost identical, with the main feature at ∼525 nm, and a high energy peak at ∼300 nm. The absorption of the polymer materials as well as the PDIs, in this Category C system, are as

expected. The authors report that the IPs were estimated from cyclic voltammetry experiments, the optical gaps from the onset of absorption, and subsequently the EAs from the difference between the IP and the optical gap. For SF-PDI2, the values were −5.9, −3.8, and 2.1 eV for the IP, EA, and optical gap, respectively. For diPDI, the values were −6.0, −4.0, and 2.0 eV. Devices were fabricated in an inverted structure [ITO/ZnO/ donor:acceptor (1:1.4)/V2O5/Al] with active layer thicknesses of 120 nm and a device area of 5.9 mm2. The best performing combination was PffBT4T:SF-PDI2 with a device efficiency of 6.3%, and replacement of SF-PDI2 with diPDI decreased the 12945


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Figure 31. (a) Structure of Tetra-PDI. (b) Structure of Mono-PDI. (c) Solution and film absorption spectra of Tetra-PDI (solution, black; film, red) and Mono-PDI (solution, blue; film, green). (d) Photoluminescence spectra of Tetra-PDI (black), PBDTT-F-TT (blue), PBDTT-F-TT:Tetra-PDI 1:1.2, irradiated at 457 nm (red) and PBDTT-F-TT:Tetra-PDI 1:1.2 irradiated at 650 nm (green). (e) EQE spectra of blends of PBDTT-F-TT:Tetra-PDI (blue), PBDTT-F-TT:Tetra-PDI, 3% chloronaphthalene (red), and PBDTT-F-TT:Mono-PDI (black). (f) Film absorption spectra of Tetra-PDI (black), PBDTT-F-TT (red), and PBFTT-F-TT:Tetra-PDI 1:1.2. Reprinted with permission from ref 153. Copyright 2015 John Wiley and Sons.

efficiency to 5.4%. The EQE spectra for the hero devices can be seen in Figure 29d. As has been seen with all the other narrow optical gap polymer:PDI combinations discussed previously, the EQE response in the range of 600−800 nm is due to Channel I current generation, whereas at wavelengths 550 nm) significantly lower in efficiency (∼17%). The large energy offset in IP between the donor and acceptor are in agreement with these findings. However, an evaluation of the IQE would be necessary to confirm this analysis. Furthermore, as BT(TTI-

n12)2 is luminescent, FRET to the donor must also be considered. However, at the ratios used, this should be efficient, and the fact that it is not indicates that Channel II and not FRET is occurring in this Category C blend (Figure 33c). Film morphology is a significant contributor to the efficiency of these devices, as evidenced by grazing-incidence X-ray diffraction (GIXD) experiments (not shown), which confirmed the presence of ordered BT(TTI-n12)2 domains. The McCulloch group reported a similar style of BTcontaining acceptor material, FBR (Figure 34a) that consists of a rigid fluorene core flanked by high electron affinity BT and rhodanine moieties.69 The absorption profile of FBR in solution and thin film is depicted in Figure 34b (with comparison to PC60BM). In solution, FBR exhibits a peak in absorption at 488 nm that red shifts to 509 nm in the thin film. This red shift is an indication of aggregation of FBR in the solid state. The electrochemical properties of FBR were examined to extract 12949


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understanding of how a device works and indeed in the design of the next generation of materials. However, as is evident from this review, much of the published work does not provide sufficient information to enable elucidation of the different charge generation pathways, and this lack of knowledge could well hinder the advancement of the field. Thus, in the review, we have provided guidance as to the level of characterization that needs to be undertaken. Given the new knowledge, the power to molecularly engineer new donors and acceptors, the future of organic solar cells is promising in spite of the current interest in organic−inorganic hybrid materials.

values for the optical gap, EA and IP, and these values were 2.1, −3.6, and −5.7 eV, respectively. Additionally, the optical gap was also measured from the onset of absorption and also gave a value of 2.1 eV. FBR was studied with P3HT as the donor to form a Category B system. PL quenching experiments were conducted to ensure that P3HT excitons were quenched by FBR (Figure 34c). For the blend of P3HT:FBR both P3HT (Figure 34d) and FBR (not shown) PL is strongly quenched, indicating that both Channel I and II could be efficient in this system. As FBR is luminescent and its PL will partially overlap the absorption spectrum of P3HT, FRET is also going to be occurring in this blend. Thermal annealing of the blend had a minimal impact of the efficiency of PL quenching. To further investigate the charge generation process, the blend was studied with TAS (Figure 34, panels d and e). Figure 34d displays the transient data at a probe wavelength of 725 nm, which corresponds to the P3HT PL maximum. At this wavelength, there is initially a negative signal, probably due to stimulated emission from singlet excitons, which rapidly decays to leave a positive P3HT polaron signal. For the P3HT:PC60BM blend, the authors describe the transient data as biphasic, with an instrument response limited fast phase, as well as a slower phase.69 The fast phase has previously been assigned to charge separation from P3HT excitons that occur close to the PC60BM interface, whereas the slower phase is assigned to diffusion limited charge separation from excitons generated deep within pure P3HT domains. For the P3HT:FBR blend, the rise kinetics are significantly faster than for the P3HT:PC60BM blend, indicating a more intimate mixing of P3HT:FBR compared with P3HT:PC60BM. The authors suggest that this is supported by the PL quenching experiments and state that there is more efficient quenching of the P3HT excitons by FBR compared with PC60BM. While polaron generation is faster and more efficient in the P3HT:FBR blends, it also exhibits faster recombination, which is apparent from Figure 34d between 100 and 2000 ps. The authors speculate that this is due to geminate recombination losses. On a longer time scale (Figure 34e), the P3HT:FBR blend exhibits faster decay dynamics, which is indicative of faster nongeminate recombination. Devices were fabricated in an inverted structure (ITO/ZnO/P3HT:acceptor (1:1)/MoO3/Ag) with an active layer thickness of 80 nm and a device area of 4.5 mm2. The hero device reached an efficiency of 4.1%, and it should be noted that thicker active layers led to lower efficiencies, which the authors noted was consistent with the faster nongeminate recombination observed with TAS. The EQE spectrum for the hero device can be seen in Figure 34f (alongside the P3HT:PC60BM device). In this Category B system, in spite of the reported analysis, very little information about Channel I and Channel II can be obtained from either the EQE spectrum or the TAS experiments, again demonstrating the difficulty in disentangling the charge generation mechanisms in Category B systems.

AUTHOR INFORMATION Corresponding Authors

*Email: [email protected]. *Email: [email protected]. Notes

The authors declare no competing financial interest. Biographies Dani M. Stoltzfus received her Ph.D. in chemistry at Flinders University (Australia) in 2007. From 2007 to 2010, she was a postdoctoral fellow at the University of Auckland (New Zealand). From 2011−2012, she was a postdoctoral fellow at The University of Texas at Austin (USA). She is currently a postdoctoral research fellow at the Centre of Organic Photonics & Electronics at the University of Queensland (Australia). Her current research interests include the synthesis and application of optoelectronic materials for solar cells and photodetectors. Jenny E. Donaghey obtained her Ph.D. from Imperial College London (U.K.) in 2012, specializing in the synthesis of organic semiconducting polymers for use in organic photovoltaics and field effect transistors. From 2012 to 2014, she was a postdoctoral research associate working within the Centre for Plastic Electronics at the same institution. Since 2014, she has been working as a postdoctoral research fellow at the Centre of Organic Photonics & Electronics at the University of Queensland, focusing on the synthesis of high dielectric constant organic semiconductors for organic photovoltaics. Ardalan Armin obtained his Ph.D. in 2015 at the University of Queensland, studying electro-optical properties, charge transport, and photophysics of disordered semiconductors. After graduation, he was appointed as a postdoctoral research fellow at the University of Queensland in the Centre for Organic Photonics & Electronics as part of the Australian Centre for Advanced Photovoltaics working on organic semiconductor and organohalide perovskite photodetectors and solar cells. Paul E. Shaw received his Ph.D. in Physics from the University of St. Andrews (UK) in 2009 on the topic of exciton diffusion in conjugated polymers. Since then, he has worked as a postdoctoral research fellow at The University of Queensland in the Centre for Organic Photonics & Electronics and currently holds an Advance Queensland Research Fellowship. His research interests lie in the application of optical spectroscopy to understand the properties of organic semiconductors and the development of sensing technology based on fluorescent compounds.

7. CONCLUSIONS The improvement in the efficiency of organic solar cells has come through the design of new materials and device architectures. While there have been some broad design principles, much of the improvement has relied on a degree of empirical methodology. In particular, it has only recently been appreciated that photoinduced hole transfer can play as important of a role as photoinduced electron transfer in the photocurrent generation process (Channel II versus Channel I). Indeed, in some cases, it could well be the dominant pathway. Identifying which pathway is involved and/or dominant is important if we are to have a full

Paul L. Burn is a Fellow of the Australian Academy of Science and Royal Society of Chemistry. He is currently a Professor of Chemistry and the Head of the Centre for Organic Photonics & Electronics and holds a Vice Chancellor’s Research Focused Fellowship at The University of Queensland, Australia. He received his Ph.D. from the University of Sydney before moving to Cambridge University, during which he held the Dow Research Fellow at Christ’s College, Cambridge, England. In 12950


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(17) Campbell, I. H.; Hagler, T. W.; Smith, D. L.; Ferraris, J. P. Direct measurement of conjugated polymer electronic excitation energies using metal/polymer/metal structures. Phys. Rev. Lett. 1996, 76, 1900−1903. (18) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999. (19) Popovic, Z. D.; Hor, A.-m.; Loutfy, R. O. A study of carrier generation mechanism in benzimidazole perylene/tetraphenyldiamine thin film structures. Chem. Phys. 1988, 127, 451−457. (20) Arkhipov, V. I.; Bässler, H. Exciton dissociation and charge photogeneration in pristine and doped conjugated polymers. Phys. Status Solidi A 2004, 201, 1152−1187. (21) Chamberlain, G. A. Organic solar cells: A review. Sol. Cells 1983, 8, 47−83. (22) Cozzens, R. F. Electrical properties of polymers; Academic Press: New York, 1982. (23) Skotheim, T. A. Handbook of conducting polymers; Dekker: New York, 1986; Vol. 1 & 2. (24) Margolis, J. M. Conductive polymers and plastics; Chapman & Hall: New York, 1989. (25) Heeger, A. J. Semiconducting and metallic polymers: The fourth generation of polymeric materials. J. Phys. Chem. B 2001, 105, 8475− 8491. (26) Weinberger, B. R.; Akhtar, M.; Gau, S. C. Polyacetylene photovoltaic devices. Synth. Met. 1982, 4, 187−197. (27) Glenis, S.; Tourillon, G.; Garnier, F. Influence of the doping on the photovoltaic properties of thin films of poly-3-methylthiophene. Thin Solid Films 1986, 139, 221−231. (28) Marks, R. N.; Halls, J. J. M.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. The photovoltaic response in poly(p-phenylene vinylene) thin-film devices. J. Phys.: Condens. Matter 1994, 6, 1379−1394. (29) Antoniadis, H.; Hsieh, B. R.; Abkowitz, M. A.; Jenekhe, S. A.; Stolka, M. Photovoltaic and photoconductive properties of aluminum/ poly(p-phenylene vinylene) interfaces. Synth. Met. 1994, 62, 265−271. (30) Frankevich, E.; Zakhidov, A.; Yoshino, K.; Maruyama, Y.; Yakushi, K. Photoconductivity of poly(2,5-diheptyloxy-p-phenylene vinylene) in the air atmosphere: Magnetic-field effect and mechanism of generation and recombination of charge carriers. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 53, 4498−4508. (31) Yu, G.; Zhang, C.; Heeger, A. J. Dual−function semiconducting polymer devices: Light−emitting and photodetecting diodes. Appl. Phys. Lett. 1994, 64, 1540−1542. (32) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Semiconducting polymers (as donors) and buckminsterfullerene (as acceptor): photoinduced electron transfer and heterojunction devices. Synth. Met. 1993, 59, 333−352. (33) Fang, Y.; Pandey, A. K.; Nardes, A. M.; Kopidakis, N.; Burn, P. L.; Meredith, P. A narrow optical gap small molecule acceptor for organic solar cells. Adv. Energy Mater. 2013, 3, 54−59. (34) Tang, C. W. Two−layer organic photovoltaic cell. Appl. Phys. Lett. 1986, 48, 183−185. (35) Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Exciton diffusion and dissociation in a poly(p−phenylenevinylene)/ C60 heterojunction photovoltaic cell. Appl. Phys. Lett. 1996, 68, 3120− 3122. (36) Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V. I.; Heeger, A. J.; Stucky, G.; Wudl, F. Semiconducting polymer−buckminsterfullerene heterojunctions: Diodes, photodiodes, and photovoltaic cells. Appl. Phys. Lett. 1993, 62, 585−587. (37) Halls, J. J. M.; Friend, R. H. The photovoltaic effect in a poly(pphenylenevinylene)/perylene heterojunction. Synth. Met. 1997, 85, 1307−1308. (38) Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Exciton dissociation at a poly(p-phenylenevinylene)/C60 heterojunction. Synth. Met. 1996, 77, 277−280. (39) Pettersson, L. A. A.; Roman, L. S.; Inganäs, O. Modeling photocurrent action spectra of photovoltaic devices based on organic thin films. J. Appl. Phys. 1999, 86, 487−496.

1992, he moved to the University of Oxford and then in 2007 to The University of Queensland as an Australian Research Council Federation Fellow. His research focuses on the development of organic (opto)electronic materials and their application in technologies such as thin film photovoltaic devices, organic light-emitting diodes for displays and lighting, photodiodes, and sensors. Paul Meredith is a Professor of materials physics at the University of Queensland, School of Mathematics and Physics, and an Australian Research Council Discovery Outstanding Researcher Award Fellow. He received his Ph.D. in Optoelectronics from Heriot-Watt University, Edinburgh, in 1993, and after a postdoctoral position at Cambridge University and a period of industrial research with Proctor and Gamble, he joined the University of Queensland in 2001. His research interests span sustainable advanced materials with a focus on the physics of disordered semiconductors and conductors for applications such as bioelectronics and next generation optoelectronics.

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