Solar Cell Applications of π-Conjugated Molecules

Because organic molecules possessing extended π-conjugation show strong absorption in the visible region of the solar spectrum, they have been used as dyes in solid-state photovoltaic devices known as organic solar cells (OSCs). The power-conversion efficiency of tandem OSCs already reportedly exceeds 17% by processing state-of-the-art polymer donors and non-fullerene acceptors from their mixed solution, but this efficiency is still far below that of silicon-based solar cells (∼25%). Deep understanding of the fundamental aspects of OSC operation, such as the mechanism for free-charge generation and the interfacial electronic and morphological properties, is now recognized as being vital to establishing well-defined guidelines for designing material interfaces and device structures and for improving the performance of OSCs. In this chapter, we give a brief historical review of solar cells and the operational mechanism of OSCs, as well as recent developments of new donors, acceptors, and interface modifiers. We then give a thorough explanation of the electronic structure of photoactive π-conjugated molecules and their interfaces used in OSCs based on recent studies, with links to OSC performance.