Small is Powerful: Recent Progress in Solution‐Processed Small Molecule Solar Cells

Over the last 5 years, research on the synthesis, device engineering, and device physics of solution‐processed small molecule solar cells (SMSCs) has rapidly expanded. Improvements in molecular design and emergent device processing techniques have helped solution‐processed SMSCs overcome earlier difficulties in controlling active layer morphology, such that many systems are now at—or approaching—10% power conversion efficiency. In this review, details of the highest performing blend systems are presented in order to identify key trends and provide perspective on current progress in the field. Among the best systems, a planarized molecular structure is prevalent, which can be achieved using large fused‐ring moieties, intermolecular non‐bonding interactions, and side chain engineering. To obtain efficient devices, the highest performing systems have been optimized through the careful combination of thermal and solvent annealing procedures. Even without additional processing, some systems have been able to obtain interconnected morphologies and efficient charge generation and charge transport. Ultimately, the design of more efficient materials also requires additional understanding of the device physics and loss mechanisms. After highlighting what is known to date on processes limiting device efficiency, an outlook on the most important challenges remaining to the field is provided. Bulk heterojunction photovoltaics based on small molecule donors have greatly improved their performance thanks to a growing multidisciplinary effort. This review presents recent advances in molecular design, control and characterization of bulk heterojunction film morphology, and understandings of charge generation and recombination in these devices. The review is concluded with a discussion of questions that need further research to push small molecule photovoltaics higher.