Effect of π-Bridge Ring Fusion on the Wide-Bandgap Donor and Its Application for High-Efficiency All Small-Molecule Tandem Solar Cells

Abstract

The fused thiophene derivative as a π-bridged moiety affects optical/electrochemical properties, intermolecular packing, and charge carrier transporting behavior in a small molecule (SM) donor. In this work, we developed wide-bandgap SM donors, SM1 to SM4, containing different π-extended conjugation bridges and successfully utilized them for all small molecule organic solar cells (ASM OSCs). By replacing the π-bridge from nonfused to fully fused, light absorption and frontier molecular orbital properties were modulated due to effectively changing the intermolecular interaction with different fused π-lengths. The dithieno[3,2-b:2′,3′-d]thiophene bridge-based SM4 shows superior light absorption and planar π-bridge geometry. When incorporating PC71BM as an acceptor, ASM OSC with SM4 achieves a remarkable VOC of 0.97 V and FF of 75%. Moreover, SM4 was applied as the bottom cell on the ASM tandem device because SM4 exhibits strong short-wavelength light absorption and efficient charge transport/collection, attaining the highest efficiency of 17.25% reported for ASM OSCs in the literature so far. These results demonstrate that solution-processed SM4 is an excellent candidate, and fused π-bridge engineering optimizes the intermolecular interaction and physicochemical properties for ASM single- and double-junction OSCs. © 2024 American Chemical Society.