(Invited) Role of Domain Purity in Non-Fullerene Acceptor Based Organic Solar Cells

Abstract

The field of non-fullerene organic solar cells (OSC) has experienced rapid development during the past few years, mainly driven by the development of novel non-fullerene acceptors and matching donor semiconductors with device efficiencies approaching 15%. However, organic solar cell material development has progressed via a trial-and-error approach with limited understanding of the materials’ structure–property relationships and the underlying device physics of non-fullerene devices. In addition, the availability of hundreds of donor and acceptor semiconductors creates a large pool of possible donor–acceptor combinations, which poses a daunting challenge for rationally screening material combinations. Resonant soft X-ray scattering (RSoXS) has proven to be a powerful technique to measure domain purity in OSC active layer blends. In polymer-fullerene systems, RSoXS has shown that higher average domain purity is generally correlated to device performance. A simple interpretation of RSoXS purity measurements is complicated by the consideration that real morphologies may be composed of three or more phases, with mixed amorphous regions in addition to relatively pure, aggregated donor and acceptor domains. Whilst charges are understood to migrate to the aggregated domains due to a favorable electronic landscape, these charges might be effectively trapped in aggregates if there is insufficient number of percolating pathways in the amorphous mixed regions. Conversely, if the mixed regions have a composition much beyond the percolation threshold or if the volume faction of the mixed regions is large, then recombination is enhanced. Therefore, understanding the role of domain purity over different length scales and controlling the mixed amorphous phase for maximum charge creation and minimum recombination remains a principal challenge for non-fullerene OSCs. We will summarize several of our RSoXS studies that compare non-fullerene acceptor based organic solar cells having different primary chemical structures of the constituents and with significant processing variations. Our analysis suggests that mixed regions are often detrimental to good performance in these systems, typically having a pronounced negative impact on FF. Optimizing performance continues to be a delicate balance of possibly competing factors and the volume fraction of the ideal morphology remains to be determined. However, the miscibility of blend components, which might be probed using near-edge X-ray absorption spectroscopy as a screening tool, can guide expectations of achievable phase purity and ultimately the achievable performance of the active layer.