Large Conduction Band Energy Offset Is Critical for High Fill Factors in Inorganic Perovskite Solar Cells

Although SnO2 has been reported to give high efficiencies of over 20% for organic-inorganic perovskite solar cells and has been frequently used in perovskite tandem solar cells, very few contributions have explored its feasibility in inorganic perovskite solar cells (IPSCs). Inorganic perovskites with a wide bandgap tunable from 1.7 to 2.0 eV are promising candidates for top cells in tandem structures; development of IPSCs based on SnO2 will greatly benefit their integration into tandem solar cells. We examined SnO2 in comparison to the prevalent TiO2. We found that although SnO2 had a good energy alignment with the inorganic perovskite and exhibited slower nonradiative recombination, the relatively low conduction band minimum energy offset restricted efficient charge extraction. In contrast, TiO2 that had a large energy offset of ∼400 meV led to a high fill factor of 78.7% and a state-of-the-art efficiency of 14.2% for IPSCs with a bandgap of 1.93 eV.