Anomalous Band Gap Behavior in Mixed Sn and Pb Perovskites Enables Broadening of Absorption Spectrum in Solar Cells

Perovskite-based solar cells have recently been catapulted to the cutting edge of thin-film photovoltaic research and development because of their promise for high-power conversion efficiencies and ease of fabrication. Two types of generic perovskites compounds have been used in cell fabrication: either Pb- or Sn-based. Here, we describe the performance of perovskite solar cells based on alloyed perovskite solid solutions of methylammonium tin iodide and its lead analogue (CH₃NH₃Sn_1–<i>x</i>Pb_<i>x</i>I₃). We exploit the fact that, the energy band gaps of the mixed Pb/Sn compounds do not follow a linear trend (the Vegard’s law) in between these two extremes of 1.55 and 1.35 eV, respectively, but have narrower bandgap (<1.3 eV), thus extending the light absorption into the near-infrared (∼1,050 nm). A series of solution-processed solid-state photovoltaic devices using a mixture of organic spiro-OMeTAD/lithium bis­(trifluoromethylsulfonyl)­imide/pyridinium additives as hole transport layer were fabricated and studied as a function of Sn to Pb ratio. Our results show that CH₃NH₃Sn_0.5Pb_0.5I₃ has the broadest light absorption and highest short-circuit photocurrent density ∼20 mA cm^–2 (obtained under simulated full sunlight of 100 mW cm^–2)