Revealing the Effect of Solvent Additive Selectivity on Morphology and Formation Kinetics in Printed Non‐fullerene Organic Solar Cells at Ambient Conditions

Abstract Solvent additives enable the efficient modification of the morphology to improve the power conversion efficiency (PCE) of organic solar cells. However, the impact of solvent additive selectivity on the film morphology and formation kinetics is still unclarified. Herein, this work investigates two solvent additives, 1‐chloronaphthalene (1‐CN) and tetralin, characterized by their varying selectivity for the polymer donor (PBDB‐T‐2F) and the non‐fullerene small molecule acceptor (BTP‐C3‐4F). Specifically, 1‐CN exhibits superior solubility for BTP‐C3‐4F over PBDB‐T‐2F, whereas tetralin shows the opposite trend. The blend films with and without solvent additives are fabricated with the slot‐die coating at ambient conditions. Both solvent additives can promote larger phase separation and increase the size of crystals of the selectively dissolved component. In situ grazing‐incidence wide‐angle X‐ray scattering and UV–vis absorption spectra during printing unveil two distinct kinetic processes induced by 1‐CN and tetralin, leading to large‐sized crystals. 1‐CN can prolong the liquid‐solid phase separation to provide sufficient time for the BTP‐C3‐4F crystal growth but suppress the crystal growth of PBDB‐T‐2F. Tetralin can swell PBDB‐T‐2F and break down BTP‐C3‐4F crystals at the same time. Upon thermal annealing, the oversized crystals triggered by both solvent additives can be optimized to an appropriate size, resulting in an enhanced PCE.