Hot-spot generation model using electrical and thermal equivalent circuits for a copper indium gallium selenide photovoltaic module

Abstract

Hot-spot generation is critical to the reliability and performance of copper indium gallium selenide (CIGS) solar cells; however, its occurrence mechanism has not been fully elucidated. We conducted a partial shade stress test on three large-area CIGS modules (156 cm × 62.6 cm in size, with 102 cells) connected in series. Partial shade stress can cause irreversible and unpredictable damage to the modules, and we confirmed the generation of hot spots on the shaded cells via infrared imaging. To emulate numerically the internal electrical and thermal changes in the shaded cells, we present electrical and thermal equivalent circuits based on the material properties and layered structure of the CIGS cells and modules. Furthermore, we propose a hot-spot generation model describing the irreversible damage to CIGS cells via shunt resistance degradation because the dissipated electrical power exceeds the threshold power (Pth). The current–voltage characteristics simulated by the presented method are congruent with those measured under partial shade conditions. Partial shade induces high electrical power consumption in the shaded cells, up to ~34 mW/cm2 at a 5% shade ratio, ~1,500 times higher than that in the unshaded cells. The shunt resistance is degraded to ~18.5 Ω/mm2 (only ~0.1% of the initial normal value), and the Pth-value is estimated to be ~2.4 mW/cm2 at the center of the hot-spot cells. The presented method and Pth-parameter can be used to evaluate durability with respect to the effects of partial shade stress for CIGS and various other thin-film photovoltaic cells and modules.