Numerical Analysis of Pb-Free Perovskite Absorber Materials: Prospects and Challenges

Optoelectronic properties of organic-inorganic halide perovskites are exceptional with solar cells showing efficiency comparable with conventional photovoltaic technologies. However, with issues of material stability and toxicity of Pb, it is important to understand if Pb can be replaced while maintaining the high power conversion efficiencies of (FA,MA,Cs)Pb(I,Br)(3). Herein, practical efficiency limits of Pb and Pb-free perovskite absorbers are analyzed using a 1D simulator for n-i-p or p-i-n device structures. SCAPS-1D baseline models for perovskite absorber materials with and without Pb are developed to numerically reproduce the experimental current density-voltage (JV) and external quantum efficiency (EQE) of champion devices from literature. From these baseline models, the efficiency limits are determined based on optimizing the interface band alignments, reduction in midgap defect density, increased absorption coefficient, and no parasitic losses. SCAPS-1D simulations suggest that 1) theoretically determined efficiency limit of Cs2PtI6 perovskites is comparable with (FA,MA,Cs)Pb(I,Br)(3) perovskites, 2) FA(4)GeSbCl(12) is a promising photoabsorber; and 3) for efficient photoconversion with Sn-, Ge-, Ti-, or Ag-based compounds, a reduction of defect density and increase in absorption coefficient is needed.