Manipulation of the spacing between the donor and acceptor by gradual side-chain modification to reduce energy loss of the organic solar cells

As one of the simple but effective molecular modify strategies, the side chain engineering based on siloxane-terminated alkyl side chain has been proved to be a promising design strategy for facilitating the formation of optimal film morphology, and further improving the photovoltaic performance of organic solar cells (OSCs). Herein, two polymer donors named 2FP-1EH-1Si and 2FP-2Si with symmetrical and asymmetrical siloxane-terminated alkyl side chain substitution were designed and synthesized. The results demonstrate that compared with the alkyl side chain substituted polymer 2FP-2EH, the introduction of siloxane-terminated alkyl side chain not only regulates the miscibility between the polymer donor and small molecular acceptor (SMA), but also affects the spacing between the donor (D) and the acceptor (A) molecules. As a result, the asymmetrical and symmetrical siloxane-terminated alkyl side chain substituted polymer 2FP-1EH-1Si and 2FP-2Si exhibit lower surface energy, stronger self-aggregation ability and higher hole mobility. More importantly, the OSCs based on 2FP-1EH-1Si:IT-4F and 2FP-2Si:IT-4F exhibited gradually decreased energy loss, which is suppressed by controlling the D-A spacing, along with the increased open-circuit voltage (VOC) from 0.77 V to 0.80 V and then to 0.86 V. This work further reveals the regulation mechanism of siloxane-terminated alkyl chain on the photovoltaic performance of active layer materials for efficient OSCs with enhanced stability.