Reducing Trap Density and Enhancing Carrier Transport in Methylammonium-Free Ideal-Band-Gap Tin–Lead Perovskite Solar Cells Using Phenethylhydrazine Hydrochloride

Abstract Tin–lead (Sn–Pb) perovskite solar cells (PSCs) with band gaps around 1.4 eV have attracted significant interest due to their great potential for obtaining high power conversion efficiencies (PCEs) as their band gaps are near the optimum band gap value predicted by the Shockley–Queisser (SQ) limit for single-junction solar cells. Methylammonium (MA)-free Sn–Pb PSCs are of particular interest due to their better intrinsic thermal stability relative to MA-based counterparts. However, the facile oxidation of Sn2+ to Sn4+ results in defect formation and nonradiative recombination, ultimately leading to poor photovoltaic performance. Herein, we report a hydrazine-based additive, phenethylhydrazine hydrochloride (PEHHCl), for enhancing the photovoltaic performance of MA-free Sn–Pb PSCs possessing a band gap of 1.41 eV. The hydrazine functionality of the PEHHCl effectively suppresses Sn2+ oxidation while simultaneously passivating grain boundary defects by coordinating with undercoordinated Sn and Pb sites, thus resulting in improved crystallinity, lower energetic disorder, optimized energy-level alignment, and reduced trap densities. Consequently, the PEHHCl-incorporated perovskite films demonstrated reduced trap-assisted nonradiative recombination, faster charge-carrier transport time, longer charge-carrier recombination times, longer charge-carrier diffusion lengths, and higher recombination resistance. As a result of these improvements, the PEHHCl-incorporated MA-free PSC demonstrated a champion PCE of 19.59% with a stabilized PCE of 19.55%, placing it among some of the high-performing MA-free Sn–Pb PSCs reported for the band gap range of around 1.4 eV.