Carbazole-Based Hole Transport Polymer for Methylammonium-Free Tin–Lead Perovskite Solar Cells with Enhanced Efficiency and Stability

As the most commonly used hole transport material (HTM) in tin–lead (Sn–Pb) perovskite solar cells (PSCs), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) limits the power conversion efficiency (PCE) and stability of the PSCs due to its acidic characteristics. Herein, an easily synthesized polymer HTM poly[(phenyl)imino[9-(2-ethylhexyl)carbazole]-2,7-diyl] (CzAn) with a shallow highest occupied molecular orbital (HOMO) level of −4.95 eV is used in a p-i-n structure, methylammonium-free, Sn–Pb PSC to replace PEDOT:PSS. Upon optimization using doping and surface engineering, high quality Sn–Pb PSCs could be successfully fabricated, boosting the PCE to 22.6% (stabilized PCE of 21.3%) compared with 21.2% for PEDOT:PSS. The perovskite films prepared on the modified CzAn HTM possess improved crystallinity, reduced trap-state density, and larger carrier mobility resulting in PSCs with greatly improved stability.