Issues of thermal transient testing on photovoltaic modules

The thermal behavior of mass production terrestrial solar modules is a somehow neglected but very important issue of photovoltaics. Despite its significant influence on the power generation, the thermal properties of solar modules are mostly not taken into account in daily applications. In recent literature thermal transient testing was suggested as a possible solution to proper thermal characterization of photovoltaic devices. In this paper we give a short overview on the known issues of the thermal transient testing of photovoltaic devices and address the issue of the variation of the structure function at different heating currents. It is shown that for small current densities the increasing forward driving current (i.e. the heating current) can have a strong influence on the size of the active area that on the other hand determines the volume of the heat path. Due to this an increasing heating current results in the decrease of the Junction to ambient thermal resistance, with the thermal resistance converging to a minimum value at high heating currents. The phenomenon was investigated by Kelvin probe potential mapping, thermal imaging and thermal simulations which uniformly supported our explanation.