Unraveling the Role of Non‐Fullerene Acceptor with High Dielectric Constant in Organic Solar Cells

Increasing the relative dielectric constant is a constant pursuit of organic semiconductors, but it often leads to multiple changes in device characteristics, hindering the establishment of a reliable relationship between dielectric constant and photovoltaic performance. Herein, a new non‐fullerene acceptor named BTP‐OE is reported by replacing the branched alkyl chains on Y6‐BO with branched oligoethylene oxide chains. This replacement successfully increases the relative dielectric constant from 3.28 to 4.62. To surprise, BTP‐OE offers consistently lower device performance relative to Y6‐BO in organic solar cells (16.27% vs 17.44%) due to the losses in open‐circuit voltage and fill factor. Further investigations unravel that BTP‐OE has resulted in reduced electron mobility, increased trap density, enhanced first order recombination, and enlarged energetic disorder. These results demonstrate the complex relationship between dielectric constant and device performance, which provide valuable implications for the development of organic semiconductors with high dielectric constant for photovoltaic application. A non‐fullerene acceptor with enhanced dielectric constant is developed by replacing alkyl chains with branched oligoethylene oxide chains, which deteriorate solar cell performance to surprise. Further investigation unravels that this is caused by the increased trap density, enlarged energetic disorder, and broadened density of states distribution. These observations provide significant implications for material design toward more efficient solar cells.