All-Perovskite Tandem Solar Cells Approach 26.5% Efficiency by Employing Wide Bandgap Lead Perovskite Solar Cells with New Monomolecular Hole Transport Layer

Self-assembled molecules (SAMs) have been widely employed as hole transport layers (HTLs) that can improve the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, few SAMs are effective for wide band gap (WBG; Eg = 1.77 eV) PSCs. We found that [3-[4-(diphenylamino)phenyl]-9H-carbazol-9-yl]propylphosphonic acid (4dp3PACz) working as a monomolecular HTL is effective for WPG (FA0.8Cs0.2PbI1.8Br1.2) PSCs. The 4dp3PACz improved the quality of the perovskite film and reduced the defect density of the film, which reduced the nonradiative recombination and enhanced the carrier transport. 17.17% efficiency is reported. In addition, all-perovskite tandem solar cells consisting of the WBG PSC as the top cell gave 26.47% efficiency.