Modeling of the effects of charge transport on voltage-dependent photocurrent in ultrathin CdTe solar cells

Abstract

An analytical model is developed to study the current–voltage characteristics of CdTe thin film solar cells by incorporating exponential photon absorption, carrier trapping, carrier drift, and diffusion in the photon absorber layer. An analytical expression for the external voltage-dependent photocurrent is derived, considering partial depletion of the absorber layer at operating terminal voltage. The overall load current is calculated considering the actual solar spectrum and dark current components. The analytical model is verified by Taurus Medici simulation and published experimental results for different absorber layer thicknesses. A good agreement of the analytical and numerical models with experimental data is ensured by reducing carrier lifetimes in the thinner absorber layers. The hole lifetime is reduced drastically by decreasing the width of the absorber layer from 1.1 to 0.5 μm, and the cell efficiency depends critically on the transport properties of the holes. The results of this paper indicate that the improvement of the charge transport properties in submicron CdTe solar cells is extremely important to increase the power conversion efficiency.