Non-fullerene acceptors with high crystallinity and photoluminescence quantum yield enable >20% efficiency organic solar cells

The rational design of non-fullerene acceptors (NFAs) with both high crystallinity and photoluminescence quantum yield (PLQY) is of crucial importance for achieving high-efficiency and low-energy-loss organic solar cells (OSCs). However, increasing the crystallinity of an NFA tends to decrease its PLQY, which results in a high non-radiative energy loss in OSCs. Here we demonstrate that the crystallinity and PLQY of NFAs can be fine-tuned by asymmetrically adapting the branching position of alkyl chains on the thiophene unit of the L8-BO acceptor. It was found that L8-BO-C₄, with 2-butyloctyl on one side and 4-butyldecyl on the other side, can simultaneously achieve high crystallinity and PLQY. A high efficiency of 20.42% (certified as 20.1%) with an open-circuit voltage of 0.894 V and a fill factor of 81.6% is achieved for the single-junction OSC. This work reveals how important the strategy of shifting the alkyl chain branching position is in developing high-performance NFAs for efficient OSCs.