Increased charge transfer state separation via reduced mixed phase interface in polymer solar cells

Abstract

Investigations into bulk heterojunction organic solar cells have demonstrated that molecular mixing within domains and at interfaces significantly impacts device performance. However, these studies often use broad 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 new analysis based on time-delayed collection field that separately quantifies each fundamental step in the charge generation process. Additionally, we present a novel resonant X-ray scattering analysis to quantify the state of the three-phase nanostructure (phase volumes and their compositions) as it exists in the devices. We find that in a model semicrystalline system, decreasing the mixed phase interface between pure donor and pure acceptor domains has little effect on the efficiency of charge transfer (CT) state formation but instead dramatically increases the efficiency of CT state separation. While charge extraction efficiencies are affected as well, this has only a minor impact on device performance. With both structure and properties quantitatively resolved for the first time on the exact same devices, we determine a simultaneous >99% (anti)correlation between (mixed)pure phase volume and charge separation efficiency, with values fitting well to an exponential saturation model. This result plus our ability to eliminate other possible contributing factors provides strong evidence of a causal relationship that reducing interfacial mixed phases to establish a steep energy gradient between pure phases aids in charge generation and extraction in organic solar cells.