Modelling an optimised thin film solar cell

Abstract

A phenomenological model has been developed to simulate the efficiency of thin film solar cells. The model uses key equations for p-n heterojunctions and includes radiative recombination, Auger recombination, Shockley-Read-Hall recombination and surface recombination losses. This framework is appropriate for final year undergraduate and Masters students. Key solar cell phenomena are related to an equivalent circuit enabling the maximum conversion efficiency to be determined under standard AM1.5 solar illumination. The underlying physical basis of the model is presented together with algorithms to allow numerical simulation of a solar cell under the full range of operating conditions. The simulation accounts for optical losses within the device and uses a shunt resistance to account for recombination losses. Solar cells can be optimised for efficiency, or other operating characteristics, by adjusting layer thicknesses and doping levels. The model is used to investigate an emerging solar technology: thin-film p-n heterojunction Cu2ZnSnS4/CdS solar cells. An optimised solar cell is found to have an overall PV conversion efficiency of (10±1)%, however significant uncertainties on the values of some Cu2ZnSnS4 material properties mean that the trends predicted by the model are a more useful output from the simulation as these can be related to underlying physical phenomena in a solar cell. A region of maximum efficiency is found for absorber layer thicknesses of the order of microns. The range of CdS thicknesses for which this region is maximised is found when the n-type doping concentration of the CdS is maximised. An abrupt drop in efficiency is found when the CdS doping concentration is less than the doping in the Cu2ZnSnS4. Varying device and material parameters provides physical insights into the operation of solar cell devices. Strategies for managing optical losses and carrier losses can be tested leading to the identification of designs optimised for high efficiency thin film solar cells. The model can be used for other thin film PV technologies by inputting material properties.