Optical Analysis of Planar Multicrystalline Perovskite Solar Cells

Organometal halide perovskites are attracting strong interest as light‐harvesting absorber materials in single‐ and multijunction solar cells. In order to advance the technology, careful optical design of the device architecture and elaborate analysis of optical losses are essential. In this work, a detailed optical analysis of semitransparent and opaque planar CH₃NH₃PbI₃ solar cells is reported. Using a combination of variable‐angle spectroscopic ellipsometry and spectrophotometry data, the complex refractive indices of all involved materials in the device architecture are accurately determined, taking the underlying layer stack explicitly into account. The optical properties of partial and complete layer stacks of solar cells, comprising CH₃NH₃PbI₃ films with different CH₃NH₃PbI₃ surface topography roughnesses, are simulated using the transfer‐matrix method. Very good agreement between simulated and experimental data is demonstrated. Sub‐bandgap absorption is observed in CH₃NH₃PbI₃ layer stacks, which is by means of a ray‐tracing model shown to be related to diffuse scattering at the multicrystalline CH₃NH₃PbI₃/air interface. Finally, the optical losses of all layers are discriminated for opaque and semitransparent CH₃NH₃PbI₃ solar cells and four‐terminal perovskite/Si tandem solar cells.