Simultaneous passivation of surface and bulk defects in all-perovskite tandem solar cells using bifunctional lithium salts

All-perovskite tandem solar cells have garnered considerable attention because of their potential to outperform single-junction cells. However, charge recombination losses within narrow-bandgap (NBG) perovskite subcells hamper the advancement of this technology. Herein, we introduce a lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), for modifying NBG perovskites. Interestingly, LiTFSI bifunctionally passivates the surface and bulk of NBG by dissociating into Li⁺ and TFSI⁻ ions. We found that TFSI⁻ passivates halide vacancies on the perovskite surface, reducing nonradiative recombination, while Li⁺ acts as an interstitial n-type dopant, mitigating the defects of NBG perovskites and potentially suppressing halide migration. Furthermore, the underlying mechanism of LiTFSI passivation was investigated through the density functional theory calculations. Accordingly, LiTFSI facilitates charge extraction and extends the charge carrier lifetime, resulting in an NBG device with power conversion efficiency (PCE) of 22.04% (certified PCE of 21.42%) and an exceptional fill factor of 81.92%. This enables the fabrication of all-perovskite tandem solar cells with PCEs of 27.47% and 26.27% for aperture areas of 0.0935 and 1.02 cm², respectively.