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

TLDR

Perovskite solar cells have attracted attention for their high efficiencies and facile fabrication, with Pb‑based and Sn‑based perovskites being the two main material families. This study investigates the performance of alloyed perovskite solid solutions CH₃NH₃Sn₁₋ₓPbₓI₃. By exploiting the non‑linear band‑gap behavior of mixed Pb/Sn perovskites, which yields gaps below 1.3 eV and extends absorption to ~1050 nm, the authors fabricated solution‑processed devices with a spiro‑OMeTAD/LiTFSI/pyridinium hole‑transport layer and varied the Sn/Pb ratio. The CH₃NH₃Sn₀.₅Pb₀.₅I₃ composition exhibited the widest absorption and the highest short‑circuit photocurrent density of ~20 mA cm⁻² under 100 mW cm⁻² illumination.

Abstract

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-x)Pb(x)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(0.5)Pb(0.5)I3 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)).