Transport imaging of multi-junction and CIGS solar cell materials

Abstract

Multi-junction solar cells are an emerging technology that improves the conversion rate of solar energy. Indium Gallium Phosphide (InGaP) is commonly used as the top cell in multi-junction cells grown on Germanium (Ge) or Gallium Arsenide (GaAs) substrates. To design more efficient solar cells using InGaP, it is important to characterize its transport parameters, particularly the minority charge carrier mobility, diffusion length and lifetime as a function of doping and material growth conditions. In this work, transport imaging was performed on a set of InGaP heterostructures (with differing thicknesses, doping levels and minority carrier types) to determine their minority carrier diffusion length. These measurements, together with an independent set of time-resolved photoluminescence (TRPL) lifetime data, were used to calculate the minority carrier mobility values. For the shortest diffusion lengths, experimental limitations were encountered involving the finite carrier generation volume. Simulations were performed to explore the potential of modeling the convolution of diffusion behavior with a finite generation region to address these limitations. Transport imaging was also performed on a set of Copper Indium Gallium Selenide (CIGS) materials. Polycrystalline CIGS represents an alternative to the expensive single-crystal InGaP. These initial experiments identified the challenges of applying transport imaging to polycrystalline materials.