Self-assembled 1D/3D heterojunction enables all-inorganic perovskite 4-terminal tandem solar cells with 21.54% certified efficiency

All-inorganic perovskite solar cells (PSCs) have emerged as a prominent research focus because the high thermal/photo stability they can offer is critical to commercialization of the burgeoning photovoltaic (PV) technology. However, there remain issues pertaining to the susceptibility of the all-inorganic perovskites to surface degradation from moisture ingress under ambient conditions and the suboptimal PV efficiency that still lags substantially behind that of their organic-inorganic hybrid counterparts. To address these challenges, this work employs an in situ self-assembly strategy to construct a 1D/3D perovskite heterojunction on top of the all-inorganic perovskite using tetrabutylammonium trifluoromethanesulfonate (TTFS). While typical ammonium salts only provide a cationic barrier or weak passivation, the TTFS-based design uniquely synergizes a hydrophobic cationic barrier with strong anionic passivation, and concurrently creates fast electron extraction channels through a nanostructured interface. This approach overcomes the conventional trade-off between stability and efficiency. By exploiting it to optimize a semi-transparent wide-band PSC for 4-terminal (4-T) tandem devices, a certified power conversion efficiency (PCE) of 17.10% was achieved together with an ultrahigh operational stability (one-sun illumination with maximum power point tracking in air atmosphere, T ₉₈  > 2000 h). When it is combined with a narrow-band all-inorganic PSC in the 4-T tandem configuration, a certified efficiency of 21.54% was obtained, which is the highest reported for this type of tandem cells. Through synergistic optimization of interface stabilization and tandem optoelectronic management, this work provides valuable insights for developing efficient and stable all-inorganic perovskite tandem solar cells.