Suppressed deprotonation enables a durable buried interface in tin-lead perovskite for all-perovskite tandem solar cells

Low-band-gap tin (Sn)-lead (Pb) perovskites are a critical component in all-perovskite tandem solar cells (APTSCs). Current state-of-the-art Sn-Pb perovskite devices exclusively use poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as the hole-transport layer (HTL) but suffer from undesired buried-interface degradation. Here, we show that the deprotonation of the –SO3H group in PSS is the root cause of the interface degradation due to its low acid dissociation constant (pKa), leading to acidic erosion and iodine volatilization in Sn-Pb perovskites. We identify that HTL featuring the carboxyl (–COOH) group with a higher pKa, such as poly[3-(4-carboxybutyl)thiophene-2,5-diyl] (P3CT), can suppress deprotonation and strengthen the interface, mitigating the buried-interface degradation. Motivated by established P3CT modification, we introduce Pb doping to P3CT to increase its work function and reduce interfacial energy loss. We fabricate APTSCs with a champion efficiency of 27.8% and an operational lifetime of over 1,000 h, with 97% retaining efficiency under maximum power point tracking.