Thermodynamic and kinetic insights for regulating molecular orientation in nonfullerene all‐small‐molecule solar cells

Abstract The molecular orientation has a profound influence on the performance of organic solar cells. Both donor and acceptor adopt face‐on orientation guarantees efficient exciton dissociation and charge transport, which is a key to achieving high device performance. However, the molecules usually adopt edge‐on orientation in some blend systems, take small molecules based on an oligothiophene (DRCN5T): fused‐ring electron acceptor based on indacenodithieno[3,2‐b]‐thiophene core and thienyl side‐chains (ITIC‐Th) blend, for instance, the orientation of DRCN5T is edge‐on, which is detrimental to the photophysical process of the device. Herein, a solid additive strategy, that is, adding N2200 as a nucleus for DRCN5T, was proposed, which combined the nucleation process with molecular diffusivity, thus realizing the orientation transformation of DRCN5T from edge‐on to face‐on. Consequently, the device performance was significantly improved, and a clear relationship between molecular orientation and energy loss/biomolecular recombination was established. More importantly, this study revealed not only the thermodynamic factors, including the crystallinity of solid additive, the lattice matching degree, and miscibility between DRCN5T and solid additive but also the kinetic parameter, such as the diffusivity of DRCN5T are very important to efficiently regulate the molecular orientation. Overall, this study presents the in‐depth mechanism of orientation transformation via adding solid additives, which may provide a guideline for solid additive choices.