Hierarchical phase separation in all small-molecule organic solar cells

Solution processable small-molecule organic solar cells have progressed a lot in terms of donor and acceptor materials, device architectures, fabrication techniques, and optimization methodologies which have enabled credible performance gains. The hierarchical active layer morphology is one such strategy that has led to significant performance gains by overcoming the earlier systems’ shortcomings and providing an optimized active layer morphology with features well within the confines of the excitonic diffusion length (LD) (5–30 nm). Multi-length scale domains, lateral and vertical phase separation, and interconnected network-like charge transport pathways are some of the key morphological features that lead to enhanced open-circuit voltage, short circuit current, fill factor, and in turn, greater power conversion efficiencies. This has furthered our understanding of the relationship between morphology and the charge generation, transport, disassociation, and extraction processes. In this review, we summarize the efforts concentrated on achieving such optimized hierarchical morphologies in binary and ternary small molecule-based organic solar cells and provide insights into the relationship between performance and morphology.