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 (pK_a), leading to acidic erosion and iodine volatilization in Sn-Pb perovskites. We identify that HTL featuring the carboxyl (–COOH) group with a higher pK_a, 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.