Controlling Solution‐Phase Polymer Aggregation with Molecular Weight and Solvent Additives to Optimize Polymer‐Fullerene Bulk Heterojunction Solar Cells

The bulk heterojunction (BHJ) solar cell performance of many polymers depends on the polymer molecular weight (M _n) and the solvent additive(s) used for solution processing. However, the mechanism that causes these dependencies is not well understood. This work determines how M _n and solvent additives affect the performance of BHJ solar cells made with the polymer poly(di(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD). Low M _n PBDTTPD devices have exceedingly large fullerene‐rich domains, which cause extensive charge‐carrier recombination. Increasing the M _n of PBDTTPD decreases the size of these domains and significantly improves device performance. PBDTTPD aggregation in solution affects the size of the fullerene‐rich domains and this effect is linked to the dependency of PBDTTPD solubility on M _n. Due to its poor solubility high M _n PBDTTPD quickly forms a fibrillar polymer network during spin‐casting and this network acts as a template that prevents large‐scale phase separation. Furthermore, processing low M _n PBDTTPD devices with a solvent additive improves device performance by inducing polymer aggregation in solution and preventing large fullerene‐rich domains from forming. These findings highlight that polymer aggregation in solution plays a significant role in determining the morphology and performance of BHJ solar cells.     

Mixed Domains Enhance Charge Generation and Extraction in Bulk‐Heterojunction Solar Cells with Small‐Molecule Donors

The interplay between nanomorphology and efficiency of polymer‐fullerene bulk‐heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small‐molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2‐b:4,5‐b]dithiophene‐pyrido[3,4‐b]‐pyrazine BDT(PPTh₂)₂, namely SM1 and SM2, differing by their side‐chains, are examined as a function of solution additive composition. The results show that the additive 1,8‐diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM‐based BHJ solar cells compared with polymer‐fullerene devices. In polymer‐based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM‐based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes.     

Interplay Between Side Chain Pattern,Polymer Aggregation,and Charge Carrier Dynamics in PBDTTPD:PCBM Bulk‐Heterojunction Solar Cells

Poly(benzo[1,2‐b:4,5‐b′]dithiophene–alt–thieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) polymer donors with linear side‐chains yield bulk‐heterojunction (BHJ) solar cell power conversion efficiencies (PCEs) of about 4% with phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as the acceptor, while a PBDTTPD polymer with a combination of branched and linear substituents yields a doubling of the PCE to 8%. Using transient optical spectroscopy it is shown that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of nongeminate recombination and a lower fraction of sub‐nanosecond geminate recombination. In turn the yield of long‐lived charge carriers increases, resulting in a 33% increase in short circuit current (J _sc). In parallel, the two polymers show distinct grazing incidence X‐ray scattering spectra indicative of the presence of stacks with different orientation patterns in optimized thin‐film BHJ devices. Independent of the packing pattern the spectroscopic data also reveals the existence of polymer aggregates in the pristine polymer films as well as in both blends which trap excitons and hinder their dissociation.     

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.     

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.     

Influence of Phonon Scattering on Exciton and Charge Diffusion in Polymer‐Fullerene Solar Cells

Thermally activated transport and phonon scattering in P3HT:PCBM (poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenylC61‐butyric acid methyl ester) bulk heterojunction (BHJ) organic solar cells is studied via temperature‐dependent external‐quantum‐efficiency (EQE) spectroscopy. The hopping barriers for combined exciton and charge transport are balanced for the individual blended materials in a sample, which possesses a blending ratio and a morphology that give rise to a maximal power‐conversion efficiency. Increasing the PCBM weight fraction leads to a reduction of exciton hopping barriers in PCBM, while for P3HT exciton hopping barriers remain constant. This reduction of PCBM exciton hopping barriers is attributed to a higher PCBM crystallinity in the PCBM‐rich solar cell as compared to the BHJ with the optimized blending ratio. The morphology‐dependent difference in exciton hopping activation energies between P3HT and PCBM is attributed to a higher impact of phonon scattering in P3HT than in PCBM, as concluded from the much stronger decrease of P3HT‐related temperature‐dependent external quantum efficiencies above room temperature in the PCBM‐rich BHJ solar cell. All EQE data of P3HT:PCBM‐based BHJ solar cells is modeled consistently over a broad temperature range by a simple analytical expression involving temperature activation and phonon scattering, without the need to distinguish two separate hopping regimes.     

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.     

Polymer Main‐Chain Substitution Effects on the Efficiency of Nonfullerene BHJ Solar Cells

“Nonfullerene” acceptors are proving effective in bulk heterojunction (BHJ) solar cells when paired with selected polymer donors. However, the principles that guide the selection of adequate polymer donors for high‐efficiency BHJ solar cells with nonfullerene acceptors remain a matter of some debate and, while polymer main‐chain substitutions may have a direct influence on the donor–acceptor interplay, those effects should be examined and correlated with BHJ device performance patterns. This report examines a set of wide‐bandgap polymer donor analogues composed of benzo[1,2‐b:4,5‐b′]dithiophene (BDT), and thienyl ([2H]T) or 3,4‐difluorothiophene ([2F]T) motifs, and their BHJ device performance pattern with the nonfullerene acceptor “ITIC”. Studies show that the fluorine‐ and ring‐substituted derivative PBDT(T)[2F]T largely outperforms its other two polymer donor counterparts, reaching power conversion efficiencies as high as 9.8%. Combining several characterization techniques, the gradual device performance improvements observed on swapping PBDT[2H]T for PBDT[2F]T, and then for PBDT(T)[2F]T, are found to result from (i) notably improved charge generation and collection efficiencies (estimated as ≈60%, 80%, and 90%, respectively), and (ii) reduced geminate recombination (being suppressed from ≈30%, 25% to 10%) and bimolecular recombination (inferred from recombination rate constant comparisons). These examinations will have broader implications for further studies on the optimization of BHJ solar cell efficiencies with polymer donors and a wider range of nonfullerene acceptors.     

Thermally Stable All‐Polymer Solar Cells with High Tolerance on Blend Ratios

Tuning the blend composition is an essential step to optimize the power conversion efficiency (PCE) of organic bulk heterojunction (BHJ) solar cells. PCEs from devices of unoptimized donor:acceptor (D:A) weight ratio are generally significantly lower than optimized devices. Here, two high‐performance organic nonfullerene BHJ blends PBDB‐T:ITIC and PBDB‐T:N2200 are adopted to investigate the effect of blend ratio on device performance. It is found that the PCEs of polymer‐polymer (PBDB‐T:N2200) blend are more tolerant to composition changes, relative to polymer‐molecule (PBDB‐T:ITIC) devices. In both systems, short‐circuit current density (J_sc) is tracked closely with PCE, indicating that exciton dissociation and transport strongly influence PCEs. With dilute acceptor concentrations, polymer‐polymer blends maintain high electron mobility relative to the polymer‐molecule blends, which explains the dramatic difference in PCEs between them as a function of D:A blend ratio. In addition, polymer‐polymer solar cells, especially at high D:A blend ratio, are stable (less than 5% relative loss) over 70 d under continuous heating at 80 °C in a glovebox without encapsulation. This work demonstrates that all‐polymer solar cells show advantage in operational lifetime under thermal stress and blend‐ratio resilience, which indicates their high potential for designing of stable and scalable solar cells.     

Charge Creation and Recombination in Multi‐Length Scale Polymer:Fullerene BHJ Solar Cell Morphologies

While the extremes in organic photovoltaic bulk heterojunction morphology (finely mixed or large pure domains) are easily understood and known to be unfavorable, efficient devices often exhibit a complex multi‐length scale, multi‐phase morphology. The impact of such multiple length scales and their respective purities and volume fractions on device performance remains unclear. Here, the average spatial composition variations, i.e., volume‐average purities, are quantified at multiple size scales to elucidate their effect on charge creation and recombination in a complex, multi‐length scale polymer:fullerene system (PBDTTPD:PC₇₁BM). The apparent domain size as observed in TEM is not a causative parameter. Instead, a linear relationship is found between average purity at length scales <50 nm and device fill‐factor. Our findings show that a high volume fraction of pure phases at the smallest length scales is required in multi‐length scale systems to aid charge creation and diminish recombination in polymer:fullerene solar cells.     

Morphology‐Dependent Trap Formation in High Performance Polymer Bulk Heterojunction Solar Cells

Bulk heterojunction solar cells (BHJs) based on poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) can have internal quantum efficiencies approaching 100% but require active layers that are too thin to absorb more than ～70% of the above band gap light. When the active layer thickness is increased so that the cell absorbs more light, the fill factor and open circuit voltage decrease rapidly, so that the overall power conversion efficiency decreases. We find that hole‐traps in the polymer, which we characterize using space‐charge limited current measurements, play an important role in the performance of PCDTBT‐based BHJs and may limit the active layer thickness. Recombination due to carrier trapping is not often considered in BHJs because it is not believed to be a dominant loss mechanism in the “fruit‐fly” P3HT system. Furthermore, we show that in contrast to P3HT, PCDTBT has only weak short‐range molecular order, and that annealing at temperatures above the glass transition decreases the order in the π–π stacking. The decrease in structural order is matched by the movement of hole‐traps deeper into the band gap, so that thermal annealing worsens hole transport in the polymer and reduces the efficiency of PCDTBT‐based BHJs. These findings suggest that P3HT is not prototypical of the new class of high efficiency polymers, and that further improvement of BHJ efficiencies will necessitate the study of high efficiency polymers with low structural order.     

Identifying the Nature of Charge Recombination in Organic Solar Cells from Charge‐Transfer State Electroluminescence

Charge‐transfer (CT) state electroluminescence is investigated in several polymer:fullerene bulk heterojunction solar cells. The ideality factor of the electroluminescence reveals that the CT emission in polymer:fullerene solar cells originates from free‐carrier bimolecular recombination at the donor‐acceptor interface, rather than a charge‐trap‐mediated process. The fingerprint of the presence of nonradiative trap‐assisted recombination, a voltage‐dependent CT electroluminescence quantum efficiency, is only observed for the P3HT:PCBM system, which is explained by a reduction of the competing bimolecular recombination rate. These results are in agreement with measurements of the illumination‐intensity dependence of the open‐circuit voltage.     

Charge Transport and Recombination in Low‐Bandgap Bulk Heterojunction Solar Cell using Bis‐adduct Fullerene

Charge transport and recombination are studied for organic solar cells fabricated using blends of polymer poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl] (Si‐PCPDTBT) with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (mono‐PCBM) and the bis‐adduct analogue of mono‐PCBM (bis‐PCBM). The photocurrent of Si‐PCPDTBT:bis‐PCBM devices shows a strong square root dependence on the effective applied voltage. From the relationship between the photocurrent and the light intensity, we found that the square‐root dependence of the photocurrent is governed by the mobility‐lifetime (μτ) product of charge carriers while space‐charge field effects are insignificant. The fill factor (FF) and short circuit current density (J_sc) of bis‐PCBM solar cells show a considerable increase with temperature as compared to mono‐PCBM solar cells. SCLC analysis of single carrier devices proofs that the mobility of both electrons and holes is significantly lowered when replacing mono‐PCBM with bis‐PCBM. The increased recombination in Si‐PCPDTBT:bis‐PCBM solar cells is therefore attributed to the low carrier mobilities, as the transient photovoltage measurements show that the carrier lifetime of devices are not significantly altered by using bis‐PCBM instead of mono‐PCBM.     

Recombination in Polymer:Fullerene Solar Cells with Open‐Circuit Voltages Approaching and Exceeding 1.0 V

Polymer:fullerene solar cells are demonstrated with power conversion efficiencies over 7% with blends of PBDTTPD and PC₆₁BM. These devices achieve open‐circuit voltages (V_oc) of 0.945 V and internal quantum efficiencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. V_oc’s above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fill factor are greatly reduced. In PBDTTPD blends with multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage‐dependent, steady state and time‐resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is inefficient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of –0.14 ± 0.06 eV is required for efficient hole transfer. These results suggest that the driving force for hole transfer may be too small for efficient current generation in polymer:fullerene solar cells with V_oc values above 1.0 V and that non‐fullerene acceptor materials with large optical gaps (>1.7 eV) may be required to achieve both near unity internal quantum efficiencies and values of V_oc exceeding 1.0 V.     

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.     

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.     

Influence of Thermal Annealing on PCDTBT:PCBM Composition Profiles

A variety of measurement techniques including photothermal deflection spectroscopy (PDS), auger electron spectroscopy (AES), (sub–bandgap) external quantum efficiency (EQE), and impedance spectroscopy are applied to poly[N‐900‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(40,70‐di‐2‐thienyl‐20,10,30‐benzothiadiazole (PCDTBT)/[6,6]‐phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) films and devices to probe the stability under thermal annealing. Upon annealing, solar cell performance is drastically decreased for temperatures higher than 140 °C. Detailed investigation indicate changes in polymer:fullerene interactions resulting in the formation of a polymer wetting layer upon annealing at temperatures higher than 140 °C. Upon device completion this wetting layer is located close to the metal electrode and therefore leads to an increase in recombination and a decrease in charge carrier extraction, providing an explanation for the reduced fill factor (FF) and power conversion efficiency (PCE).     

Efficient Charge Extraction in Thick Bulk Heterojunction Solar Cells through Infiltrated Diffusion Doping

A new strategy for improving the charge extraction in thick bulk heterojunction (BHJ) polymer solar cells (PSCs) is reported. By the deposition of a solution‐processed vanadium oxide (s‐VO_x) onto BHJ active layers, conductive charge‐transport channels are formed inside the active layer via a charge‐transfer doping reaction between the lone‐pair electrons of the sulfur atoms in the polymer and the Lewis‐acidic vanadium atoms of the s‐VO_x. Because the charge‐transport channels significantly reduce charge recombination in the BHJ films, high internal quantum efficiencies (IQEs) of over 80% are achieved in the thick inverted PSCs (≈420 nm). This finding represents a new strategy for improving the efficiency and feasibility of printable photovoltaic devices.     

How Photoinduced Crosslinking Under Operating Conditions Can Reduce PCDTBT‐Based Solar Cell Efficiency and then Stabilize It

Bulk heterojunction (BHJ) photovoltaic devices made of PCDTBT (poly[N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]) and PC₇₀BM ([6,6]‐phenyl‐C₇₀‐butyric acid methyl ester) are among the most efficient and stable devices studied so far. However, during a short regime called “burn‐in”, a significant decrease of power conversion efficiency was observed. A study of the photochemical mechanisms involved in the PCDTBT:PCBM active layer exposed to light in encapsulated systems is presented. It is found that the photochemical reactions resulting from the absorption of light by PCDTBT involve crosslinking between the 2,7 carbazole unit of PCDTBT and the fullerene unit of PCBM. Those reactions stabilize the BHJ by avoiding the formation of microsized PCBM crystals known to cause failure of BHJ solar cells. Using classical electron paramagnetic resonance spectroscopy (EPR) (without illumination), paramagnetic defects along the polymer chains have been detected. The kinetics of defects intensity show a burn‐in trend. The evolution of their relaxation times upon aging is in good agreement with a structural change (crosslinking) of the BHJ observed from the nanomechanical properties. Finally, light‐induced electron paramagnetic resonance (LEPR) measurements performed on aged samples revealed that electron transfer is not significantly affected upon aging, confirming thus the stabilization of the BHJ in solar cell operating conditions.     

Understanding Triplet Formation Pathways in Bulk Heterojunction Polymer:Fullerene Photovoltaic Devices

Triplet exciton (TE) formation pathways are systematically investigated in prototype bulk heterojunction (BHJ) “super yellow” poly(p‐phenylene vinylene) (SY‐PPV) solar cell devices with varying fullerene compositions using complementary optoelectrical and electrically detected magnetic resonance (EDMR) spectroscopies. It is shown that EDMR spectroscopy allows the unambiguous demonstration of fullerene triplet production in BHJ polymer:fullerene solar cells. EDMR triplet detection under selective photoexcitation of each blend component and of the interfacial charge transfer (CT) state reveals that low lying fullerene TEs are produced by direct intersystem crossing from singlet excitons (SEs). The direct CT‐TE recombination pathway, although energetically feasible, is kinetically suppressed in these devices. However, high energy CT states in the CT manifold can contribute to the population of the fullerene triplet state via a direct CT‐SE conversion. This undesirable energetic alignment could be one of the causes for the severe reduction in photocurrent observed when the open‐circuit voltage of polymer:fullerene solar cells is pushed to 1.0 V or beyond.