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 similar to 90 degrees between ZR1 and Y6 molecules at the interface, an angle close to 0 degrees 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.

Automated summary

The bulk heterojunction in organic solar cells is where charge separation and recombination occur, with molecular orientation playing a key role in solar cell efficiency. Polarization-selective transient absorption spectroscopy reveals differences in molecular orientations between different solar cell systems, providing important insights into key photophysical steps in energy conversion.