Star‐Shaped Oligomer Acceptors via Core Engineering for Ternary Organic Solar Cells With 20.8% Certified Efficiency

ABSTRACT Oligomerized non‐fullerene acceptors (o‐NFAs) have emerged as a promising material for constructing efficient and stable organic solar cells (OSCs). However, the precise role of their molecular geometry and surface properties in governing the vertical phase distribution for efficient charge collection near electrodes of OSCs remains underexplored. In this work, two trimerized star‐shaped oligomer acceptors, TAQ‐15F and TBQ‐15F, are synthesized with fused and non‐fused trithiophene‐benzene cores. We find although this core modulation barely affects the energy levels and absorption spectra, it substantially changes the surface free energy of these o‐NFAs and consequently influences their vertical phase distribution within the photoactive layer. With a more twisted conformation, TBQ‐15F allows its side chains to be ordered in out‐of‐plane and results in a much lower surface free energy compared to TAQ‐15F. When TBQ‐15F is incorporated into the classic D18/L8‐BO photoactive layer via layer‐by‐layer deposition, this reduced surface energy further correlates a more pronounced vertical stratification and gradient molecular distribution (acceptor‐rich near the cathode and donor‐rich near the anode) as revealed by angle‐dependent GIWAXS measurements and XPS depth profiling. Certified power conversion efficiency (PCE) of 20.8% with improved operational stability was achieved in the D18/L8‐BO+TBQ‐15F ternary OSCs.