Recent Advances in Squaraine Dyes for Bulk-Heterojunction Organic Solar Cells

Squaraine (SQ) dyes are an important class of electron-donating (donors or p-type) semiconductors for organic solar cells (OSC) due to their facile synthetic access, broad optical absorption with high oscillator strengths, and chemical robustness. Blending them with compatible electron-acceptors (acceptors or n-type) yields OSC devices known as bulk-heterojunction (BHJ) small molecule donor organic solar cells (SMD-OSCs). Through extensive research on materials design, synthesis, characterization, and device optimization over the past ˝ve years, SMD-OSCs employing SQ-based structures have achieved remarkable increases in device power conversion e˚ciency (PCE), now approaching 8%. Although these PCEs have not yet equaled the performance of state- of-the art donor polymers and some other SMD semiconductors, SQ-based OSC progress highlights successful and generalizable strategies for small molecule solar cells that should lead to future advances. In this review, recent developments in SQ-based OSCs are discussed and analyzed.