Investigation of the structure-performance correlation in Y-series electron acceptors with different phenyl alkyl...

Y-Series non-fullerene acceptors (NFAs) make significant contributions to the booming efficiency of organic solar cells (OSCs). In particular, modifying inner side chains with phenyl groups affords an efficient and promising method to improve the efficiency and promote the large-area fabrication of OSCs. However, the relationship between the structure of the inner side chains and properties has rarely been investigated. In this work, three NFAs, Y6-PhC4, Y6-PhC6 and Y6-PhC8, are developed by introducing phenyl alkyl inner side chains with different lengths. It demonstrates that the length of the phenyl alkyl side chains has a negligible influence on the bandgap and energy levels of NFAs. However, the miscibility between NFAs and D18 declines along with the length of the phenyl alkyl inner side chains. Y6-PhC6 possesses the best crystallinity and D18:Y6-PhC6 blend films afford the finest topological morphology. Thus, the devices exhibit the tendency of first increasing and then decreasing in charge carrier dynamics, trap density and non-radiative recombination energy loss, which makes V_OC, J_SC and FF of OSCs show the same trend from D18:Y6-PhC4 and D18:Y6-PhC6 to D18:Y6-PhC8 pairs. Therefore, D18:Y6-PhC6 based devices achieve the highest power conversion efficiency (PCE) of 17.12%. Furthermore, D18:L8-BO:Y6-PhC6 based ternary OCSs are fabricated to afford an impressive PCE of 19.22%, benefiting from the superior properties of Y6-PhC6. This work manifests the importance of inner side chain engineering in regulating intrinsic properties and photovoltaic performance of Y-series NFAs.