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 (Eg) values, organic solar cells (OSCs) have achieved an efficiency of over 21%. Despite such great progress, designing materials with even narrower Eg still remains challenging but essential for practical applications of OSCs. In this Perspective, we summarize typical molecular tailoring strategies for narrowing the Eg of SMAs and polymer donors (PDs). Then, the required Eg 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.