Energy loss within organic solar cells (OSCs) is undesirable as it reduces cell efficiency 1-4. In particular, non-radiative recombination loss 3 and energetic disorder 5 , which are closely related to the tail states below the band edge and the overall photon energy loss, need to be minimized to improve cell performance. Here, we report how the use of a small-molecule acceptor with torsion-free molecular conformation can achieve a very low degree of energetic disorder and mitigate energy loss in OSCs. The resulting single-junction OSC has an energy loss due to non-radiative recombination of just 0.17 eV and a high power conversion efficiency of up to 16.54% (certified as 15.89% by the National Renewable Energy Laboratory). The findings take studies of organic photovoltaics deeper into a new regime, beyond the limits of energetic disorder and large energy offset for charge generation. Recent developments in organic/polymer bulk heterojunction solar cells (OSCs/PSCs) have led to tremendous advances in power conversion efficiency (PCE), with current leading certified efficiencies of over 15-16% for single-junction devices 6-9 and over 17% for multi-junction devices 10. However, the PCEs of OSCs still lag behind their inorganic semiconductors and perovskite counterparts, in most part due to the modest open-circuit voltage (V OC) imposed by the relatively large photon energy loss 1-4. The photon energy loss in solar cells, ΔE loss ¼ E g � qV OC I (q is the elementary charge, E g is the optical gap of the absorber; Fig. 1), represents the lower limit of energy loss during conversion of photon energy to electrical potential. So far, the best performing inorganic crystalline solar cells show a lower ΔE loss in gallium arsenide (0.32 eV) and crystalline silicon (0.38 eV) 11 , while most of the highly efficient perovskite solar cells have a ΔE loss in the range of 0.4−0.5 eV (ref. 4). Recently, there has been rapid progress in the reduction of photon energy loss in high-performance perovskite solar cells, leading to a record value of 0.34 eV (ref. 12). In contrast, state-of-the-art OSCs usually suffer from high ΔE loss in the range of 0.6−1.1 eV (ref. 13), which is much higher than the theoretical value of 0.25−0.30 eV predicted by Shockley-Queisser (SQ) theory 14. This is because of the relatively high radiative recombination loss due to absorption edge broadening effects 4 and the strong non-radiative recombination loss 1,2,15. Hence, it is clear that further improvement of V OC in OSCs requires a significant reduction of both radiative and non-radiative recombination loss. Although there is no intuitively simple approach to reduce the overall photon energy loss in OSCs, the energetic disorder 5 , which

A new acceptor–donor–acceptor-structured nonfullerene acceptor, 2,2′-((2Z,2′Z)-(((4,4,9,9-tetrakis­(4-hexylphenyl)-4,9-dihydro-s-indaceno­[1,2-b:5,6-b′]­dithiophene-2,7-diyl)­bis­(4-((2-ethylhexyl)­oxy)­thiophene-4,3-diyl))­bis­(methanylylidene))­bis­(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))­dimalononitrile (i-IEICO-4F), is designed and synthesized via main-chain substituting position modification of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)­dimalononitrile. Unlike its planar analogue IEICO-4F with strong absorption in the near-infrared region, i-IEICO-4F exhibits a twisted main-chain configuration, resulting in 164 nm blue shifts and leading to complementary absorption with the wide-bandgap polymer (J52). A high solution molar extinction coefficient of 2.41 × 105 M–1 cm–1, and sufficiently high energy of charge-transfer excitons of 1.15 eV in a J52:i-IEICO-4F blend were observed, in comparison with those of 2.26 × 105 M–1 cm–1 and 1.08 eV for IEICO-4F. A power conversion efficiency of 13.18% with an open-circuit voltage (0.849 V), a short-circuit current density of 22.86 mA cm–2, and a fill factor of 67.9% were recorded in J52:i-IEICO-4F-based polymer solar cells (PSCs), demonstrating that this main-chain twisted strategy can be a guideline that facilitates the development of new acceptors to maximize the efficiency in PSCs.

In recent years, a rapid increase in the power conversion efficiency above 10% in small-molecule-based organic solar cells (SM-OSCs) has been made possible. However, one of the key device parameters, fill factor (FF), which is mainly limited by comprehensive courses, including charge generation, recombination, transport, and extraction, still remains moderate. Here we demonstrate a record high FF of 78.35% in SM-OSCs obtained through dichloromethane solvent vapor annealing, which provides optimized phaseseparation morphology for efficient charge generation and facilitates charge transport and extraction at the same time. We use a combined charge dynamic measurement and current−voltage characteristic reconstruction to identify that geminate recombination loss that resulted from undesired film morphology is mainly responsible for low FF in the pristine devices. Even higher FF that is comparable with that of crystal silicon solar cells in organic solar cells is very likely with the presence of charge mobility around 5 × 10 −3 cm 2 V −1 s −1 and proper film morphology.

Efficient charge generation is a prerequisite to achieve high power conversion efficiency (PCE) in organic/polymer solar cells (OSCs/PSCs), which involves photoinduced electron transfer and/or hole transfer between the donor/acceptor interface upon photoexcitation. A high yield of charge from both processes usually requires sufficient energy offset between the donor and acceptor for charge separation, fast transport, and extraction for charge collection, as well as significant absorption complementation to maximize photon harvest. Here we demonstrate highly efficient PSCs with efficient dual photocurrent generation pathways from a blend of a polymer donor and two narrow-bandgap nonfullerene acceptors, with an outstanding certified PCE of 13.0% (verified as 12.5%) in PSCs with single-junction device architecture. The devices from these material systems show nonradiative recombination loss of ∼0.22–0.24 V, one of the smallest values for OSCs achieved so far and comparable to those of solar cells based on monocrystalline Si or metal-halide perovskites. This study highlights that dual charge generation pathways with high yield and strongly reduced voltage loss are indispensable for further increasing the PCE of OSCs.

In this study, we demonstrate that remarkably reduced open-circuit voltage in highly efficient organic solar cells (OSCs) from a blend of phenyl-C-butyric acid methyl ester and a recently developed conjugated small molecule (DPPEZnP-THD) upon solvent vapor annealing (SVA) is due to two independent sources: increased radiative recombination and increased nonradiative recombination. Through the measurements of electroluminescence due to the emission of the charge-transfer state and photovoltaic external quantum efficiency measurement, we can quantify that the open-circuit voltage losses in a device with SVA due to the radiative recombination and nonradiative recombination are 0.23 and 0.31 V, respectively, which are 0.04 and 0.07 V higher than those of the as-cast device. Despite of the reduced open-circuit voltage, the device with SVA exhibited enhanced dissociation of charge-transfer excitons, leading to an improved short-circuit current density and a remarkable power conversion efficiency (PCE) of 9.41%, one of the best for solution-processed OSCs based on small-molecule donor materials. Our study also clearly shows that removing the nonradiative recombination pathways and/or suppressing energetic disorder in the active layer would result in more long-lived charge carriers and enhanced open-circuit voltage, which are prerequisites for further improving the PCE.