Optical Modeling of Perovskite/Chalcopyrite Thin Film Tandem Solar Cells*

For high efficiency thin film solar cells, two-terminal tandem designs are under development using narrow and wide bandgap components.  The active component materials of recent interest for an all-thin-film tandem include the hybrid organic-inorganic perovskite (MA)PbI₃  (MA: CH₃NH₃⁺) as the wide bandgap component (E_g=1.61 eV) and the chalcopyrite CuInSe₂ (E_g=1.02 eV) as the narrow bandgap component.  For both material systems, bandgap variations are possible through alloying.  By using the mixed-halide perovskite MAPb(I_1-xBr_x)₃, band gaps over the range from 1.62 eV to 1.73 eV are possible for x increasing from 0 to 0.2.  This variation enables optimization of the performance of the tandem for a CuInSe₂ bottom cell.  In this study, we develop a realistic optical structure for the tandem solar cell and predict the external quantum efficiency spectra, total matched current collection, and inactive layer optical losses.  Such predictions require accurate optical spectra of all individual component layers in the form of their complex dielectric functions (ε₁, ε₂) which have been determined using spectroscopic ellipsometry measurements.