Recently, non-fullerene n-type organic semiconductors have attracted significant attention as acceptors in organic photovoltaics (OPVs) due to their great potential to realize high-power conversion efficiencies. The rational design of the central fused ring unit of these acceptor molecules is crucial to maximize device performance. Here, we report a new class of non-fullerene acceptor, Y6, that employs a ladder-type electron-deficient-core-based central fused ring (dithienothiophen[3.2-b]-pyrrolobenzothiadiazole) with a benzothiadiazole (BT) core to fine-tune its absorption and electron affinity. OPVs made from Y6 in conventional and inverted architectures each exhibited a high efficiency of 15.7%, measured in two separate labs. Inverted device structures were certified at Enli Tech Laboratory demonstrated an efficiency of 14.9%. We further observed that the Y6-based devices maintain a high efficiency of 13.6% with an active layer thickness of 300 nm. The electron-deficient-core-based fused ring reported in this work opens a new door in the molecular design of high-performance acceptors for OPVs.

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 non-fullerene acceptor, named N3, was developed by using a 3 rd -position branched alkyl chain on the pyrrole motif of the molecule, which yielded better performance than the state-of-the-art acceptor Y6. Ternary devices were fabricated, achieving a power conversion efficiency of 16.74% in the lab and a certified efficiency of 16.42% by Newport.

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π–π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.

Organic solar cells are currently experiencing a second golden age thanks to the development of novel non‐fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high‐performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near‐unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier.