Recent progress on non-fullerene acceptors for organic photovoltaics

Non-fullerene acceptors (NFAs) have attracted great interest as potential substitutes for fullerenes and their derivatives. With the advent of new building blocks and design strategy, remarkable progress has been achieved for NFAs during the past two years. Benefiting from the stronger absorption, more matched energy level, and rather lower energy loss, the power conversion efficiency of fullerene-free organic solar cells has surpassed 14% in 2018. In this review, the recent progress of NFAs including polymers and small-molecules is highlighted. We also summarize the design strategies of NFAs in terms of pairing with the corresponding polymer donor materials. The effect of the molecular structure and the optoelectronics properties of NFAs on device performance is discussed from their contributions on tuning light absorption, energy level, electron mobility, and morphology of the active layers in organic solar cells (OSCs) using polymer donating materials. The contributions on NFAs from several leading research groups are summarized. The challenges of developing better performance NFAs are suggested in the summary and perspectives.