Surface charge transfer doping of narrow-bandgap Sn–Pb perovskites for high-performance tandem solar cells

Abstract

This study reports on the surface charge transfer doping of narrow-bandgap Sn/Pb binary metal perovskites to controllably tune the carrier concentration and electrical properties, and therefore reduce the energy disorder. The excess holes on the Sn/Pb binary metal perovskite surface can be compensated by the applied benzyl viologen (BV in short) molecules to reduce the self-p-doping hole density. Consequently, the doping level of the BV-modified Sn–Pb perovskite films was carefully altered from heavy p-type to less p-type with up-shifted Fermi level and considerably reduced the surface energetic disorder, which effectively suppressed the nonradiative recombination at the perovskite/electron transport layer interface. The perovskite solar cells (PSCs) using narrow-bandgap FA_0.7MA_0.3Sn_0.5Pb_0.5I₃ exhibited dramatically enhanced photovoltaic performance with an optimized power conversion efficiency (PCE) up to 22.58%, thus representing one of the highest PCE among Sn/Pb binary metal PSCs reported so far. Furthermore, combining the narrow-bandgap (1.25 eV) FA_0.7MA_0.3Sn_0.5Pb_0.5I₃ bottom sub-cell with a wide-bandgap (1.78 eV) Cs_0.2FA_0.8PbI_1.8Br_1.2 top sub-cell, the best performing 2-terminal all-perovskite tandem solar cell achieved a prominent PCE of 26.33%. Moreover, the optimized tandem device exhibited excellent long-term stability by retaining 88.3% of the initial PCE following 1000 h of operation at the maximum power point under 1-sun illumination.