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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

The maximum open-circuit voltage of a solar cell can be evaluated in terms of its ability to emit light. We herein verify the reciprocity relation between the electroluminescence spectrum and subband-gap quantum efficiency spectrum for several photovoltaic technologies at different stages of commercial development, including inorganic, organic, and a type of methyl-ammonium lead-halide CH 3 NH 3 PbI 3−x Cl x perovskite solar cells. Based on the detailed balance theory and reciprocity relations between light emission and light absorption, voltage losses at open circuit are quantified and assigned to specific mechanisms, namely, absorption edge broadening and nonradiative recombination. The voltage loss due to nonradiative recombination is low for inorganic solar cells (0.04-0.21 V), while for organic solar cell devices it is larger but surprisingly uniform, with values of 0.34-0.44 V for a range of material combinations. We show that, in CH 3 NH 3 PbI 3−x Cl x perovskite solar cells that exhibit hysteresis, the loss to nonradiative recombination varies substantially with voltage scan conditions. We then show that for different solar cell technologies there is a roughly linear relation between the power conversion efficiency and the voltage loss due to nonradiative recombination.