Understanding the High Performance of over 15% Efficiency in Single-Junction Bulk Heterojunction Organic Solar Cells

The highly efficient single-junction bulk-heterojunction (BHJ) PM6:Y6 system can achieve high open-circuit voltages (V-OC) while maintaining exceptional fill-factor (FF) and short-circuit current (J(SC)) values. With a low energetic offset, the blend system is found to exhibit radiative and non-radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies reveal that the device shows moderate non-geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques are employed to understand the phase separation, long-range ordering, as well as donor:acceptor (D:A) inter- and intramolecular interactions at an atomic-level resolution. This is achieved using photo-conductive atomic force microscopy, grazing-incidence wide-angle X-ray scattering, and solid-state F-19 magic-angle-spinning NMR spectroscopy. The synergy of multifaceted characterization and device physics is used to uncover key insights, for the first time, on the structure-property relationships of this high-performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing reveals that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high J(SC) and FF to be retained despite the low energetic offset.