Low-temperature electron-transporting materials for perovskite solar cells: Fundamentals, progress, and outlook

• Fundamental theory and design guideline of low-temperature ETMs are discussed for economical, efficient, and stable perovskite solar cells and modules. • Progress in the effective regulation strategies is assessed from small scale toward industrial scale. • Cryo-process induced stability hazards and service mechanism of low-temperature ETMs are scrutinized. • Outlook of key technical challenges and potential pathways are proposed to push the performance bottleneck. Perovskite solar cells (PSCs) have unprecedentedly rapid emerged as a promising next-generation clean-energy-harvesting technology. Compelling market advantages over existing photovoltaic devices include material properties and low-temperature processes. Throughout the development of PSCs, the electron-transporting materials (ETMs) as an integral part, has played a distinctive role in photon-to-electron conversion and device stability. With the accelerating commercialization, the market demand puts forward new criterion for the evolution of ETMs to suitable for flexible, tandem cells and large-scale module development. However, guidelines for low-temperature ETMs critical for industrial applications are still missing. In this review, we discuss the fundamental theory and propose the design guideline of low-temperature ETMs for economical, efficient and stable PSCs. Then, the notable progress of low-temperature ETMs is outlined with emphasis focused on the various low-temperature processing routes. Further, we emphatically reviewed the optimization strategies of low-temperature ETMs for champion solar cells, and extend to large-scale ETMs covering their strategic importance, current status, and development tendency. Beyond that, the service mechanism of low-temperature ETMs are systematically summarized. We conclude with an outlook of critical targets, the key technical challenges of low-temperature ETMs, and outline potential pathways to push the performance bottleneck of low-temperature perovskite cells and modules.