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.