Bandgap Narrowing: From Molecular Design to Versatile Applications in Organic Solar Cells

Benefiting from Y-series small molecule acceptors (SMAs) with precisely engineered small bandgap ( E g ) values, organic solar cells (OSCs) have achieved an efficiency of over 21%. Despite such great progress, designing materials with even narrower E g still remains challenging but essential for practical applications of OSCs. In this Perspective, we summarize typical molecular tailoring strategies for narrowing the E g of SMAs and polymer donors (PDs). Then, the required E g values for specific applications in tandem solar cells (TSCs) and semitransparent OSCs (ST-OSCs) are clarified, with predicted efficiency limits provided synchronously. Finally, we outline several development trends of narrow-bandgap SMAs and PDs, looking forward to advancing the commercialization of TSCs and ST-OSCs by solving key problems.