Modeling and Measurement of Lumped Series Resistance with Varying Illumination and Current Condition of Low-Bandgap Solar Cells

Series resistance (RS) significantly impacts the fill factor (FF) in photovoltaic devices, especially at high current levels, by causing a voltage drop that reduces the voltage at the maximum power point. Thermophotovoltaic (TPV) cells, which use thermal radiation from high-temperature sources like furnaces, generate large photocurrents due to their proximity to the heat source. Understanding RS in TPV cells is essential for optimizing their performance and enhancing power conversion efficiency. This study examines the illumination and current dependence of RS in lowbandgap solar cells, specifically germanium heterojunction cells and commercial GaSb cells from JX Crystal, Inc. The current density-voltage (J-V) curves of these cells are measured under blackbody irradiation, and simulations are conducted using SCAPS-1D software at temperatures ranging from 800°C to 1200°C. RS values are extracted using three methods: analyzing the area under the J-V curve, determining the slope near the open-circuit voltage (VOC), and applying the double illumination method. The study explores trends in RS variation with blackbody temperature and finds that RS values decrease with the rise in blackbody temperature across these three methods.