The collection probability and spectral response in isotype heterolayers of tandem solar cells

An analytical model for the position-dependent collection probability in uniformly doped one-dimensional layers with abrupt compositional and bandgap changes is presented. The collection probability is derived from coupled system of diffusion equations for low-level injection of photo-generated minority carriers in a stack of isotype heterolayers. Collection probability maintained continuity across isotype heterojunctions despite discontinuities of excess minority carrier concentration. An estimate for window and BSF passivation showed that the effective surface recombination velocity exponentially depended on the energygap difference, and linearly depended on increased doping, reduced mobility and increased thickness of sub-diffusion length passivation layers. Analytical expressions for the photo-generated current, and the internal quantum efficiency from each heterolayer were developed, and applied to the analysis of reported spectral response of a dual junction GaInP/GaAs tandem solar cell. Calculated internal quantum efficiencies closely matched reported experimental results, with the exception of sub-band absorption due to sub-bandgap deficiencies in the optical models and photon recycling. Calculated spectral response showed that upper AlInP2 window, quasi-neutral emitter, and SCR layers dominated collection of photo-generated carriers in the top GaInP2 cell, whereas, the base dominated collection of photo-generated carriers in the bottom GaAs cell. Results show that augmenting Hovel’s three layers (emitter, SCR, and base) analysis with the response from the top window layer should be sufficient to capture the spectral response of solar cells with thin passivation layers.