Developing absorption-matched and encapsulation-free underwater organic photovoltaics with the acceptor-poor active layer

The underwater photovoltaic technique requires easily-tuned absorption range of photoactive layer and good water-resistant property. Organic solar cells (OSCs) based on the acceptor-poor active layer well satisfy the two demands. Unfortunately, the reduced acceptor content inevitably suppresses the photovoltaic performance by cutting off the electron transporting pathway. To alleviate the performance decay at this situation, a three-aspect strategy is proposed. First, polymer acceptors are used instead of small molecule acceptors to provide continuous channels for electron transport. Second, more extended chain conformation is introduced. Third, enhance acceptor crystallinity with proper domain purity to decrease the charge recombination. Obeying these rules, the steep power conversion efficiency (PCE) decay appears at a smaller donor:acceptor (D:A) ratio. Comparing at the same ratio of 1:0.4, the optimized all-polymer OSC has a PCE of 15.13 % which largely outperforms that of 11.23 % in the polymer-small molecule counterpart. Taking advantage of the water-resistant device structure, the all-polymer OSC displays PCEs of 13.41 % and 17.78 % under actual/simulated solar irradiation at 1 m and 20 m underwater depths. In addition, 53.6 % PCE retention is revealed after immersing the device in water for 96 h without encapsulation.