Probing molecular orientation at bulk heterojunctions by polarization-selective transient absorption spectroscopy

A bulk heterojunction in organic solar cells is where charge separation and recombination occur. Molecular orientation at the interface is one of the key factors that dictate solar cell efficiency. Although X-ray scattering-based methods can determine donor/acceptor domain orientations between an anisotropic phase and an isotropic fullerene-based phase, the rise of non-fullerene solar cells presents a new challenge in delineating local molecular directions at the interface between two anisotropic donor/acceptor domains. Here, we determine interfacial molecular orientations of three high-efficiency small molecule solar cells (ZR1:Y6, B1:BO-4Cl, and BTR:BO-4Cl) using polarization-selective transient absorption spectroscopy. The polarization anisotropy of charge separation dynamics indicates an angle of ∼90° between ZR1 and Y6 molecules at the interface, an angle close to 0° between B1 and BO-4Cl, and random orientations between BTR and BO-4Cl. These observations provide complementary information to X-ray scattering measurements and highlight polarization-selective transient absorption spectroscopy as a tool to probe interfacial structure and dynamics of key photophysical steps in energy conversion.