In-situ optical imaging of carrier transport in multilayer solar cells

Abstract

The goal of this thesis is to explore the utility of in-situ optical imaging of charge transport imaging in multi junction solar cells. An in-situ measurement of a manufactured solar cell's key material parameters is difficult. Many sophisticated models may be used to predict performance of new cell arrangements and suggest next generation improvements. In parallel, an experimental view into a complex, multi layered, alloyed semiconductor device can provide important feedback for material growth and device fabrication. This body of work builds on the previous work of extracting estimated minority carrier diffusion lengths from multi junction solar cell materials. Indium Gallium Phosphide double heterostructures have been investigated previously with effective results. A technique to estimate electron diffusion length from a luminescent sample intensity distribution has been developed. This thesis investigates imaging transport and applying the diffusion length estimation directly in the triple junction device. Luminescence from individual layers was isolated using optical filters. The effect of varying temperature and applying bias during the imaging technique is investigated as well. A strong dependence of effective diffusion length on environmental temperature was measured. In addition, a weak dependence of effective diffusion length on bias was measured, with the effect slightly greater in the top, as compared to the central cell.