Optical modelling of thin film solar cells with textured interfaces using the effective medium approximation

Abstract

Simple methods for increasing the maximum achievable current density of amorphous silicon (a-Si:H) solar cells include bandgap and layer thickness optimisation, and light confinement strategies. The goal of the optical modelling work presented here has been to examine the nature and potential of these effects, in particular the optical enhancement resulting from the use of finely textured transparent conducting oxides. A computer program that combines coherent and incoherent optical theory has been used as a flexible tool for simulating the performance of any general thin film solar cell structure. An effective medium approximation has been used to model the optical effects of microroughness (texturing with correlation lengths smaller than the wavelength of light). This work suggests that effective interface grading due to microroughness does have a significant effect on the optical performance of a-Si:H solar cells, and that both enhancement and deterioration in the maximum achievable current density can be the outcome. Where both effective interface grading (microroughness) and larger scale texturing (macroroughness) are fully exploited, optical yields may be increased beyond their current level. This work emphasises the importance of characterising and controlling the interface morphology to optimise the short circuit current and maintain the open circuit voltage.