2 Molecular design of non-fullerene acceptors (NFAs) is of vital importance for highefficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach as which may introduce undesirable steric hindrance that reduces charge transport in NFAs. Here we show the design and synthesis of a highly efficient NFA family by substituting the beta position of thiophene unit on Y6-based A-DAD-A backbone with branched alkyl chains. It was found that such modification of different alkyl chain length could completely change the molecular packing behavior of NFAs, leading to improved structure order and charge transport in thin films. Unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of branched alkyl chain topology in tuning the molecular packing and blend morphology that leads to improved organic photovoltaic performance.

a) E gap was determined from the intersection of the EQE edge and the local EQE maximum; b) V oc was calculated from the measured J-V curves; c) V oc rad was calculated from FTPS and EL measurements; d) EQE EL is the EL quantum efficiency of the fabricated devices; e) Exp. ΔE 3 is calculated with the Equation (ΔE 3 = −kTln(EQE EL )).

To date, organic solar cells (OSCs) with the development of photovoltaic materials have realized high power conversion efficiencies (PCEs) through the solution processing strategy with bulk heterojunction (BHJ) structure, but the BHJ morphology is difficult to control in large-scale fabrication of OSCs. Herein, we report an alternative film-forming technology known as layer-bylayer (LbL). As compared to its BHJ counterpart, LbL presents many unique advantages including controllable ''p-i-n'' morphology, good charge transport and extraction properties, and great universality. By using the LbL-bladed coating strategy, a high PCE of 16.35% was achieved in the PM6:Y6 OSCs. Notably, a large-area solar module of 11.52 cm 2 with a geometrical fill factor of over 90% exhibited an outstanding PCE of 11.86%, which represents the highest efficiency of large-area solar modules. The results may pave the way for the fabrication of the photoactive layer in the future industrial production of OSCs.

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A narrow-band-gap polymer acceptor (P A ), namely PYT, is reported, while a series of PYT polymer acceptors with controlled average molecular weight (M n ) values were synthesized for fine-tuning the molecular crystallinity and miscibility. When fabricated into all-PSCs with a polymer donor (P D ) PM6, we observed a clear M n dependence on device performance. The PM6:PYT M device gives a record-high PCE of 13.44% for the all-PSCs, resulting from the good P D -P A pair miscibility, suitable blend microstructure, and improved charge transport properties.