Surface-functionalized hole-selective monolayer for high efficiency single-junction wide-bandgap and monolithic tandem perovskite solar cells

Carbazole moiety-based 2PACz ([2-(9H-carbazol-9-yl)ethyl]phosphonic acid) self-assembled monolayers (SAMs) are excellent hole-selective contact (HSC) materials with abilities to excel the charge-transfer-dynamics of perovskite solar cells (PSCs). Herein, we report a facile but powerful method to functionalize the surface of 2PACz-SAM, by which reproducible, highly stable, high-efficiency wide-bandgap PSCs can be obtained. The 2PACz surface treatment with various donor number solvents improves assembly of 2PACz-SAM and leave residual surface-bound solvent molecules on 2PACz-SAM, which increases perovskite grain size, retards halide segregation, and accelerates hole extraction. The surface functionalization achieves a high power conversion efficiency (PCE) of 17.62% for a single-junction wide-bandgap (∼1.77 eV) PSC. We also demonstrate a monolithic all-perovskite tandem solar cell using surface-engineered HSC, showing high PCE of 24.66% with large open-circuit voltage of 2.008 V and high fill-factor of 81.45%. Our results suggest this simple approach can further improve the tandem device, when coupled with a high-performance narrow-bandgap sub-cell.