Shading-induced failure in thin-film photovoltaic modules: Electrothermal simulation with nonuniformities

Abstract

Finite element electrothermal modeling is employed to study shading-induced failure in monolithically integrated thin-film photovoltaic modules. A key element is spatial nonuniformity in current-voltage characteristics, which causes inhomogeneous current flow when part of a module is under reverse bias due to shading. Time-dependent calculations show that spots with lower reverse breakdown voltage experience greater current density and localized thermal runaway that can cause permanent damage, resulting in ohmic shunts. Such failure events lead to performance loss, especially when the module is in normal operating conditions and shunted currents lead to abnormally high module temperatures. Our simulations of temperature distributions, current-voltage characteristics, and electroluminescence are compared to data from the literature for copper indium gallium diselenide devices.

Characterization and modeling of reverse‐bias breakdown in Cu(In,Ga)Se2 photovoltaic devices

“…The device model designed by Nardone et al demonstrated that reverse-bias breakdown is likely to occur at regions with low reverse-bias breakdown voltage. 6 A significant spike in temperature around sites with reduced breakdown voltages was found under reverse bias, with higher temperatures occurring for encapsulated devices relative to unencapsulated devices. In the present work, we simulate partial shading conditions by applying reverse bias to unencapsulated devices in the laboratory to observe thermal-runaway breakdown events.…”

“…The rapid localized heating creates a large temperature gradient at which point heat conduction away from the shunt dominates, resulting in cooling of the shunt until stabilization is achieved. Additional discussion on these points can be found in Nardone et al…”

“…This configuration is used to model local variations in temperature as well as global device performance parameters. Further details of the model can be found in Nardone et al…”

“…In this work, we build on previous studies by Palmiotti et al and Nardone et al to further probe the structure of features before and after thermal‐runaway breakdown events and describe the experimental observations through a robust device model. Palmiotti et al demonstrated a process to identify sites where thermal‐runaway events are likely to occur without causing damage to the CIGS devices.…”

“…SEM imaging of these sites revealed that most are associated with either inclusions or voids in the CIGS absorber. The device model designed by Nardone et al demonstrated that reverse‐bias breakdown is likely to occur at regions with low reverse‐bias breakdown voltage . A significant spike in temperature around sites with reduced breakdown voltages was found under reverse bias, with higher temperatures occurring for encapsulated devices relative to unencapsulated devices.…”