Isomerization of Peripheral Functional Groups Refines Aggregation and Non-Radiative Energy Loss for Efficient Organic Photovoltaics

Side chain engineering plays an important role to modulate the aggregation of organic photovoltaic materials. However, exploration of the specific sites of side chains remains very limited. Herein, we attach two isomerized benzotriazoles (BTz-1 and BTz-2) into the terminal of linear alkyl chains, and elaborately explore the spatial and electronic effect of the overhanging groups on global behaviors of materials. This subtle difference brings about extensive distinctions of the resultant acceptors of YBTz-1 and YBTz-2. The asymmetric BTz-1 triggers rearrangement of electron clouds along the π-skeleton via spatial interactions, yielding a large dipole moment and greater aggregation of YBTz-1 with excessively phase-separated heterojunction textures. More importantly, the energy landscapes of charge transfer (CT) states are accordingly regulated, which ulteriorly impacts the excited states hybridization and non-radiative energy loss. Consequently, the D18:YBTz-2 binary devices afford an impressive efficiency of 19.1% with a low ΔEnr of 0.22 eV, outdistancing the D18:YBTz-1 with inferior efficiency of 14.7% and large ΔEnr of 0.30 eV. Moreover, the YBTz-2 greatly refines the D18:L8BO system, realizing an outstanding efficiency up to 19.9%. These results offer new insights into the meticulous side chain engineering, which are instructive to further advance the development of organic photovoltaics.