Reduced mixed phase interface causes increased charge transfer state separation in polymer solar cells

Investigations into organic solar cells have shown that molecular mixing within domains and interfaces significantly impacts device performance. However, studies often use performance metrics that blur fundamental structure-function mechanisms – in particular, the role of the mixed phase in charge generation versus charge extraction. Here, we present a study based on time-delayed collection field to separately quantify each fundamental step in the charge generation process. Additionally, we utilize a novel resonant X-ray scattering analysis to quantify the state of the nanostructure as it exists within real devices. We find that in a model semicrystalline system, decreasing the mixed interface between donor and acceptor domains has minimal impact on charge transfer (CT) state generation while greatly enhances CT state separation. With both a high correlation (>99%) and a granular quantification of all dynamics, we eliminate competing processes in working devices to determine a causal relationship between the mixed phase volume and the efficiency of charge separation. Our results here support that a reduction in the interfacial mixed phase establishes a steep energy gradient between pure phases to aid charge generation in organic solar cells.