Isomerization of peripheral functional groups refines aggregation and non-radiative energy loss for efficient organic photovoltaics

Abstract

Side chain engineering plays an important role in modulating 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) to the terminals of linear alkyl chains, and explore in detail the spatial and electronic effects of the overhanging groups on the global behavior of the materials. This subtle difference brings about extensive distinctions in the resultant acceptors of YBTz-1 and YBTz-2. Asymmetric BTz-1 triggers the 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 regulated accordingly, which subsequently have an impact on the hybridization of excited states and non-radiative energy loss. Consequently, D18:YBTz-2 binary devices afford an impressive efficiency of 19.1% with a low ΔE_nr of 0.22 eV, outperforming D18:YBTz-1 with an inferior efficiency of 14.7% and a large ΔE_nr of 0.30 eV. Moreover, YBTz-2 greatly refines the D18:L8BO system, realizing an outstanding efficiency of up to 19.9%. These results offer new insights into meticulous side chain engineering, which are instructive for further advancing the development of organic photovoltaics.