Semitransparent organic solar cells based on all-low-bandgap donor and acceptor materials and their performance potential

Recent advances in organic solar cells (OSCs) based on large-bandgap donors and low-bandgap non-fullerene acceptors (NFAs) have increased the power conversion efficiency (PCE) of OSCs to ~18%. However, these state-of-the-art OSCs have strong absorption in the visible region, limiting their application in semitransparent organic solar cells (STOSCs). In this study, an all-low-bandgap system based on a low-bandgap polymer donor (PM2) and a low-bandgap NFA (Y6-BO), was introduced as the light-harvesting layer for STOCSs with absorption mainly localized in the near-infrared (NIR) spectrum from 600 to 900 nm. The corresponding opaque OSCs exhibited the highest PCE among reported all-low-bandgap OSC systems, and the corresponding STOSCs showed higher visible light transmittances (VLTs) and light utilization efficiencies (LUEs) than the reference devices based on state-of-the-art PM6:Y6-BO OSC system with broad range absorption from visible to NIR. Optical simulations predicted that the PM2:Y6-BO-based STOSCs have a greater potential to realize higher VLTs and PCEs and better PCE retention (PCEsemitransparent/PCEopaque) than those from the PM6:Y6-BO-based STOSCs. Guided by these simulations, PM2-based STOSCs with VLTs exceeding 40% and PCEs of ~6% were achieved. Further recombination analysis suggested that the PM2-based devices experienced more severe charge recombination and energy losses, indicating there is further room for PCE improvement by designing new all-low-bandgap systems. Overall, this work shows the great potential of all-low-bandgap systems in realizing STOSCs with high PCEs and VLTs, which is promising for the commercialization of OSCs as power-generating window applications.