Shunt screening, size effects and I/V analysis in thin-film photovoltaics

We present an analytical model that quantitatively describes the physics be hind shunting in thin film photovoltaics and predicts size-dependent effect s in the I/V characteristics of solar cells. The model consists of an array of microdiodes and a shunt in parallel between the two electrodes, one of which mimics the transparent conductive oxide and has a finite resistance. We introduce the concept of the screening length L, over which the shunt af fects the system electric potential. The nature of this screening is that t he system generates currents in response to the point perturbation caused b y the shunt. L is expressed explicitly in the terms of the system parameter s. We find the spatial distribution of the electric potential in the system and its I/V characteristics. The measured I/V characteristics depend on th e relationship between the cell size l and L, being markedly different for the cases of small (l <L) and large (l >L) cells. We introduce a new regime of the large photovoltaic cell where all the characteristics are calculate d analytically. Our model is verified both numerically and experimentally: good agreement is obtained. (C) 2001 American Institute of Physics.