Comparison of the Morphology Development of Polymer–Fullerene and Polymer–Polymer Solar Cells during Solution‐Shearing Blade Coating

In this work, the detailed morphology studies of polymer poly(3‐hexylthiophene‐2,5‐diyl) (P3HT):fullerene(PCBM) and polymer(P3HT):polymer naphthalene diimide thiophene (PNDIT) solar cell are presented to understand the challenge for getting high performance all‐polymer solar cells. The in situ X‐ray scattering and optical interferometry and ex situ hard and soft X‐ray scattering and imaging techniques are used to characterize the bulk heterojunction (BHJ) ink during drying and in dried state. The crystallization of P3HT polymers in P3HT:PCBM bulk heterojunction shows very different behavior compared to that of P3HT:PNDIT BHJ due to different mobilities of P3HT in the donor:acceptor glass. Supplemented by the ex situ grazing incidence X‐ray diffraction and soft X‐ray scattering, PNDIT has a lower tendency to form a mixed phase with P3HT than PCBM, which may be the key to inhibit the donor polymer crystallization process, thus creating preferred small phase separation between the donor and acceptor polymer.     

Nanofiber‐Based Bulk‐Heterojunction Organic Solar Cells Using Coaxial Electrospinning

Nanofibers consisting of the bulk heterojunction organic photovoltaic (BHJ–OPV) electron donor–electron acceptor pair poly(3‐hexylthiophene):phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) are produced through a coaxial electrospinning process. While P3HT:PCBM blends are not directly electrospinnable, P3HT:PCBM‐containing fibers are produced in a coaxial fashion by utilizing polycaprolactone (PCL) as an electrospinnable sheath material. Pure P3HT:PCBM fibers are easily obtained after electrospinning by selectively removing the PCL sheath with cyclopentanone (average diameter 120 ± 30 nm). These fibers are then incorporated into the active layer of a BHJ–OPV device, which results in improved short‐circuit current densities, fill factors, and power‐conversion efficiencies (PCE) as compared to thin‐film devices of identical chemical composition. The best‐performing fiber‐based devices exhibit a PCE of 4.0%, while the best thin‐film devices have a PCE of 3.2%. This increase in device performance is attributed to the increased in‐plane alignment of P3HT polymer chains on the nanoscale, caused by the electrospun fibers, which leads to increased optical absorption and subsequent exciton generation. This methodology for improving device performance of BHJ–OPVs could also be implemented for other electron donor–electron acceptor systems, as nanofiber formation is largely independent of the PV material.     

Charge‐Carrier Mobility Requirements for Bulk Heterojunction Solar Cells with High Fill Factor and External Quantum Efficiency >90%

To increase the efficiency of bulk heterojunction (BHJ) solar cells beyond 15%, 300 nm thick devices with 0.8 fill factor (FF) and external quantum efficiency (EQE) >90% are likely needed. This work demonstrates that numerical device simulators are a powerful tool for investigating charge‐carrier transport in BHJ devices and are useful for rapidly determining what semiconductor pro­perties are needed to reach these performance milestones. The electron and hole mobility in a BHJ must be ≈10^?2 cm² V^?1 s^?1 in order to attain a 0.8 FF in a 300 nm thick device with the recombination rate constant of poly(3‐hexyl­thiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM). Thus, the hole mobility of donor polymers needs to increase from ≈10^?4 to ≈10^?2 cm² V^?1 s^?1 in order to significantly improve device performance. Furthermore, the charge‐carrier mobility required for high FF is directly proportional to the BHJ recombination rate constant, which demonstrates that decreasing the recombination rate constant could dramatically improve the efficiency of optically thick devices. These findings suggest that researchers should prioritize improving charge‐carrier mobility when synthesizing new materials for BHJ solar cells and highlight that they should aim to understand what factors affect the recombination rate constant in these devices.     

Optical Properties of Organic Semiconductor Blends with Near‐Infrared Quantum‐Dot Sensitizers for Light Harvesting Applications

We report an optical investigation of conjugated polymer (P3HT)/fullerene (PCBM) semiconductor blends sensitized by near‐infrared absorbing quantum dots (PbS QDs). A systematic series of samples that include pristine, binary and ternary blends of the materials are studied using steady‐state absorption, photoluminescence (PL) and ultrafast transient absorption. Measurements show an enhancement of the absorption strength in the near‐infrared upon QD incorporation. PL quenching of the polymer and the QD exciton emission is observed and predominantly attributed to intermaterial photoinduced charge transfer processes. Pump‐probe experiments show photo‐excitations to relax via an initial ultrafast decay while longer‐lived photoinduced absorption is attributed to charge transfer exciton formation and found to depend on the relative ratio of QDs to P3HT:PCBM content. PL experiments and transient absorption measurements indicate that interfacial charge transfer processes occur more efficiently at the fullerene/polymer and fullerene/nanocrystal interfaces compared to polymer/nanocrystal interfaces. Thus the inclusion of the fullerene seems to facilitate exciton dissociation in such blends. The study discusses important and rather unexplored aspects of exciton recombination and charge transfer processes in ternary blend composites of organic semiconductors and near‐infrared quantum dots for applications in solution‐processed photodetectors and solar cells.     

The Importance of Fullerene Percolation in the Mixed Regions of Polymer–Fullerene Bulk Heterojunction Solar Cells

Most optimized donor‐acceptor (D‐A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ～80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) with 7.3% PCE and find that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge‐carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.     

Direct Evaluation of Intrinsic Optoelectronic Performance of Organic Photovoltaic Cells with Minimizing Impurity and Degradation Effects

A correlation between the photovoltaic performance and dynamics of transient photoconductivity is investigated by flash‐photolysis time‐resolved microwave conductivity (FP‐TRMC). This electrode‐less technique offers chances to mitigate barriers for direct, speedy, and robust evaluation of bulk heterojunction (BHJ) film. We examined the blend ratio, process (solvent and thermal annealing), and impurity (a metal complex of Pd) and degradation effects in BHJ films consisting of poly(3‐hexylthiophene) (P3HT) and methanofullerene (PCBM). The minimum charge carrier mobility of 0.22 cm²V^?1s^?1 was found in P3HT:PCBM = 1:1 film along with 3.26% power conversion efficiency. The revealed good correlation is not only applicable to process optimization, but also expected as a facile screening method to survey the potential of optoelectronic materials.     

Tail State‐Assisted Charge Injection and Recombination at the Electron‐Collecting Interface of P3HT:PCBM Bulk‐Heterojunction Polymer Solar Cells

The systematic insertion of thin films of P3HT and PCBM at the electron‐ and hole‐collecting interfaces, respectively, in bulk‐heterojunction polymer solar cells results in different extents of reduction in device characteristics, with the insertion of P3HT at the electron‐collecting interface being less disruptive to the output currents compared to the insertion of PCBM at the hole‐collecting interface. This asymmetry is attributed to differences in the tail state‐assisted charge injection and recombination at the active layer‐electrode interfaces. P3HT exhibits a higher density of tail states compared to PCBM; holes in these tail states can thus easily recombine with electrons at the electron‐collection interface during device operation. This process is subsequently compensated by the injection of holes from the cathode into these tail states, which collectively enables net current flow through the polymer solar cell. The study presented herein thus provides a plausible explanation for why preferential segregation of P3HT to the cathode interface is inconsequential to device characteristics in P3HT:PCBM bulk‐heterojunction solar cells.     

Domain Purity,Miscibility, and Molecular Orientation at Donor/Acceptor Interfaces in High Performance Organic Solar Cells: Paths to Further Improvement

Domain purity and interface structure are known to be critical for fullerene‐based bulk heterojunction (BHJ) solar cells, yet have been very difficult to study. Using novel soft X‐ray tools, we delineate the importance of these parameters by comparing high performance cells based on a novel naphtha[1,2‐c:5,6‐c]bis[1,2,5]thiadiazole (NT) material to cells based on a 2,1,3‐benzothiadiazole (BT) analogue. BT‐based devices exhibit ～15 nm, mixed domains that differ in composition by at most 22%, causing substantial bimolecular recombination. In contrast, NT‐based devices have more pure domains that are >80 nm in size, yet the polymer‐rich phase still contains at least 22% fullerene. Power conversion efficiency >6% is achieved for NT devices despite a domain size much larger than the nominal exciton diffusion length due to a favourable trade‐off in the mixed domain between exciton harvesting, charge transport, and bimolecular recombination. The miscibility of the fullerene with the NT and BT polymer is measured and correlated to the purity in devices. Importantly, polarized x‐ray scattering reveals preferential face‐on orientation of the NT polymer relative to the PCBM‐rich domains. Such ordering has previously not been observed in fullerene‐based solar cells and is shown here to be possibly a controlling or contributing factor to high performance.     

Plastic Solar Cells: Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend (Adv. Energy Mater. 1/2011)

Developing a better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than 10%. In the most extensively characterized system, the poly(3‐hexyl thiophene) (P3HT):[6,6]‐phenyl‐C₆₁‐butyric‐acid‐methyl‐ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphology during processes such as thermal annealing are not well understood. In this work, we use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. We find that PCBM aggregates and/or molecular species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the favorability of mixing in this system, opposing the belief that phase‐pure domains form in BHJs due to immiscibility of these two components.     

Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend

Developing a better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than 10%. In the most extensively characterized system, the poly(3‐hexyl thiophene) (P3HT):[6,6]‐phenyl‐C₆₁‐butyric‐acid‐methyl‐ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphology during processes such as thermal annealing are not well understood. In this work, we use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. We find that PCBM aggregates and/or molecular species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the favorability of mixing in this system, opposing the belief that phase‐pure domains form in BHJs due to immiscibility of these two components.     

Intra‐Molecular Donor–Acceptor Interaction Effects on Charge Dissociation,Charge Transport,and Charge Collection in Bulk‐Heterojunction Organic Solar Cells

This article reports experimental studies on internal charge dissociation, transport, and collection by using magnetic field effects of photocurrent (MFE_PC) and light‐assisted dielectric response (LADR) in highly‐efficient organic solar cells based on photovoltaic polymer PTB2 and PTB4 with intra‐molecular “donor–acceptor” interaction. The MFE_PC at low‐field (< 150 mT) indicates that intra‐molecular “donor‐acceptor” interaction generates charge dissociation in un‐doped PTB2 and PTB4 films, which is similar to that in lightly doped P3HT (Poly(3‐hexylthiophene)) with 5 wt% PCBM (1‐(3‐methyloxycarbonyl)‐propyl‐1‐phenyl (6,6) C₆₁). After PTB2 and PTB4 are mixed with PCBM to form bulk‐heterojunctions, the MFE_PC at high‐field (> 150 mT) reveals that the charge‐transfer complexes formed at PTB2:PCBM and PTB4:PCBM interfaces have much lower binding energies due to stronger electron‐withdrawing abilities, as compared to the P3HT:PCBM device, towards the generation of photocurrent. Furthermore, the light‐assisted dielectric response: LADR indicates that the PTB2:PCBM and PTB4:PCBM solar cells exhibit larger capacitances relative to P3HT:PCBM device under photoexcitation. This reflects that the PTB2:PCBM and PTB4:PCBM bulk heterojunctions have more effective charge transport and collection than the P3HT:PCBM counterpart. As a result, our experimental results indicate that intra‐molecular “donor‐acceptor” interaction plays an important role to enhance charge dissociation, transport, and collection in bulk‐heterojunction organic solar cells.     

Impact of Hole Transport Layer Surface Properties on the Morphology of a Polymer‐Fullerene Bulk Heterojunction

Investigations on the impact of interfacial modification on organic optoelectronic device performance often attribute the improved device performance to the optoelectronic properties of the modifier. A critical assumption of such conclusions is that the organic active layer deposited on top of the modified surface (interface) remains unaltered. Here the validity of this assumption is investigated by examining the impact of substrate surface properties on the morphology of poly(3‐hexylthiophene):1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐[6,6]C₆₁ (P3HT:PCBM) bulk‐heterojunction (BHJ). A set of four nickel oxide and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layers (HTL) with contrasting surface properties and performance in organic photovoltaic (OPV) devices is studied. Differences in vertical composition variation and structural morphologies are observed across the samples, but only in the near‐interface region of <～20 nm. Near‐interface differences in morphology are most closely correlated with surface polarity and surface roughness of the HTL. Surface polarity is more influenced by surface composition than surface roughness and crystal structure. These findings corroborate the previously mentioned conclusions that the differences in device performance observed in solar cells employing these HTLs are dominated by the electronic properties of the HTL/organic photoactive active layer interface and not by unintentional alteration in the BHJ active layer morphology.     

Mapping Spatially Resolved Charge Collection Probability within P3HT:PCBM Bulk Heterojunction Photovoltaics

Material properties in polymer and fullerene bulk heterojunctions (BHJs) such as donor to acceptor volume fraction, morphology, and molecular orientation critically influence light absorption, exciton dissociation, charge transport, and recombination, all of which are crucial device properties in organic photovoltaics (OPV). Spatial variation of BHJ properties normal to the substrate, caused by phase segregation, can thereby create corresponding spatial variations in the OPVs optoelectronic properties. Here, normally incident and wave‐guided optical modes are used to selectively excite localized regions within an inverted poly(3‐hexythiophene‐2,5‐diyl) and phenyl‐C61‐butyric acid methyl ester BHJ OPV and corresponding internal quantum efficiencies are measured to study the spatial‐dependent charge carrier collection probability within the BHJ. An electron‐limited charge collection profile is observed for a thick (920 nm) BHJ due to fullerene‐poor regions as a result of phase segregation. As the thickness of the BHJ is reduced (100 nm), charge transport is seen to be unaffected by the phase segregation. This has the potential to be a versatile non‐destructive characterization technique for measuring the spatially varying charge collection probability in thin film photovoltaics and will help enable optimum device design and characterization.     

Spontaneous Charge Transfer and Dipole Formation at the Interface Between P3HT and PCBM

In the pursuit of developing new materials for more efficient bulk‐heterojunction solar cells, the blend poly (3‐hexylthiophene):[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT:PCBM blend comes from efficient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these, hindering the development of new material systems. In this report UV photoelectron spectroscopy measurements on both regiorandom‐ (rra) and regioregular‐ (rr) P3HT are presented, and the results are interpreted using the Integer Charge Transfer model. The results suggest that spontaneous charge transfer from P3HT to PCBM occurs after heat treatment of P3HT:PCBM blends. The resulting formation of an interfacial dipole creates an extra barrier at the interface explaining the reduced (non‐)geminate recombination with increased charge generation in heat treated rr‐P3HT:PCBM blends. Extensive photoinduced absorption measurements using both above‐ and below‐bandgap excitation light are presented, in good agreement with the suggested dipole formation.     

Selective Dye Loading at the Heterojunction in Polymer/Fullerene Solar Cells

Selective dye loading at the polymer/fullerene interface was studied for ternary blend bulk heterojunction solar cells, consisting of regioregular poly(3‐hexylthiophene) (RR‐P3HT), a fullerene derivative (PCBM), and a silicon phthalocyanine derivative (SiPc) as a light‐harvesting dye. The photocurrent density and power conversion efficiency of the ternary blend solar cells were most improved by loading SiPc with a content of 4.8 wt%. The absorption and surface energy measurements suggested that SiPc is located in the disordered P3HT domains at the RR‐P3HT/PCBM interface rather than in the PCBM and crystal P3HT domains. From the peak wavelength of SiPc absorption, the local concentration of SiPc ([SiPc]_Local) was estimated for the RR‐P3HT:PCBM:SiPc ternary blends. Even for amorphous films of regiorandom P3HT (RRa‐P3HT) blended with PCBM and SiPc, [SiPc]_Local was higher than the original content, suggesting dye segregation into the RRa‐P3HT/PCBM interface. For RR‐P3HT:PCBM:SiPc blends, [SiPc]_Local increased with the increase in the P3HT crystallinity. Such interfacial segregation of dye molecules in ternary blend films can be rationally explained in terms of the surface energy of each component and the crystallization of P3HT being enhanced by annealing. Notably, the solvent annealing effectively segregated dye molecules into the interface without the formation of PCBM clusters.     

Improved Power Conversion Efficiency of P3HT:PCBM Organic Solar Cells by Strong Spin–Orbit Coupling‐Induced Delayed Fluorescence

Solution‐processed organic bulk heterojunction solar cells based on poly(3‐hexylthiophene) (P3HT) blended with [6,6]‐phenyl‐C₆₀‐butyric acid methyl ester are doped with different concentrations of iron (II,III) oxide nanoparticles (Fe₃O₄). The power conversion efficiency of the devices doped at low concentrations is improved up to 11%. The improvement finds its origin in a lower recombination current, which is a consequence of an increased effective exciton lifetime according to the J–V characteristics and the optoelectronical analysis of the films. The increase in performance cannot be attributed to changes in morphology or crystallinity according to grazing‐incidence X‐ray scattering experiments. The evolution of the solar cell short‐circuit current at low doping concentrations is related to variations in the arrangement of the crystalline regions of P3HT. For high doping concentrations (above 1.0 wt%) the performance of the solar cell decays rapidly, ascribed to the increased leakage currents in the device caused by the presence of nanoparticles.     

In Situ Morphology Studies of the Mechanism for Solution Additive Effects on the Formation of Bulk Heterojunction Films

The most successful active film morphology in organic photovoltaics is the bulk heterojunction (BHJ). The performance of a BHJ arises from a complex interplay of the spatial organization of the segregated donor and acceptor phases and the local order/quality of the respective phases. These critical morphological features develop dynamically during film formation, and it has become common practice to control them by the introduction of processing additives. Here, in situ grazing incidence X‐ray diffraction (GIXD) and grazing incidence small angle X‐ray scattering (GISAXS) studies of the development of order in BHJ films formed from the donor polymer poly(3‐hexylthiophene) and acceptor phenyl‐C61‐butyric acid methyl ester under the influence of two common additives, 1,8‐octanedithiol and 1‐chloronaphthalene, are reported. By comparing optical aggregation to crystallization and using GISAXS to determine the number and nature of phases present during drying, two common mechanisms by which the additives increase P3HT crystallinity are identified. Additives accelerate the appearance of pre‐crystalline nuclei by controlling solvent quality and allow for extended crystal growth by delaying the onset of PCBM‐induced vitrification. The glass transition effects vary system‐to‐system and may be correlated to the number and composition of phases present during drying.     

Trade‐Off between Trap Filling,Trap Creation,and Charge Recombination Results in Performance Increase at Ultralow Doping Levels in Bulk Heterojunction Solar Cells

Doping of organic bulk heterojunction solar cells has the potential to improve their power conversion efficiency (PCE). Deconvoluting the effect of doping on charge transport, recombination, and energetic disorder remains challenging. It is demonstrated that molecular doping has two competing effects: on one hand, dopant ions create additional traps while on the other hand free dopant‐induced charges fill deep states possibly leading to V _OC and mobility increases. It is shown that molar dopant concentrations as low as a few parts per million can improve the PCE of organic bulk heterojunctions. Higher concentrations degrade the performance of the cells. In doped cells where PCE is observed to increase, such improvement cannot be attributed to better charge transport. Instead, the V _OC increase in unannealed P3HT:PCBM cells upon doping is indeed due to trap filling, while for annealed P3HT:PCBM cells the change in V _OC is related to morphology changes and dopant segregation. In PCDTBT:PC70BM cells, the enhanced PCE upon doping is explained by changes in the thickness of the active layer. This study highlights the complexity of bulk doping in organic solar cells due to the generally low doping efficiency and the constraint on doping concentrations to avoid carrier recombination and adverse morphology changes.     

Plasmonic Back Reflectors: A Small Molecule Non‐fullerene Electron Acceptor for Organic Solar Cells

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.     

A Small Molecule Non‐fullerene Electron Acceptor for Organic Solar Cells

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.