Modeling of temperature profile, thermal runaway and hot spot in thin film solar cells

Abstract

Hot spot and thermal runaway are serious phenomena leading to the degradation of CdTe thin film solar cells. Here, we show that these issues are well related to temperature variation in the device structures mostly because of current flowing across transparent conducting oxide (TCO) layer or back contact of a CdTe device structure: glass/TCO/CdS/CdTe/graphene. Graphene nanolayer was chosen as the back contact because of its high thermal conductivity. We present a modeling of the temperature profile in CdTe thin film devices considering both uniform and nonuniform temperature distribution and current flowing across TCO layer. Temperature profile for hot spots at the edges of devices are modeled and compared to literature reports of both modelled and measured data. The model is based on the heat transfer equation (which uses thermal resistances) and in particular accounts for convection and conduction resistances by means of their ratio, the Biot number – a factor that could be optimized in the design of photovoltaic devices. Profiles were modelled taking into account both uniform and non-uniform temperature profiles for the glass, and currents flowing though the TCO. It is shown that the current flowing across the TCO layer can contribute to thermal runaway and its spreading to neighbouring areas. Overall the modelling data suggests that thin film solar devices could be designed to minimise hot spot runaway issues by taking into account the thickness and temperature dependence of the layers thermal conductivity, convection and conduction resistances. This can be extended to other solar cell structures or large scale modules.