Mathmatical model of a graded band gap CdSxTe1-X/CdTe solar cell.

A mathematical model describing the photovoltaic effect in the graded band gap
CdSxTe.x/CdTe thin film solar cell was derived. The modeled device consists of an ntype
graded band gap surface region divided in to 11 layer each one has uniform band gap extended from y=0 to y=D followed by a homojnction with a p-type semiconductor of uniform band gap. The model has been employed to calculate the performance parameters of the solar cell. Because of the complex nature of the analysis, all such calculations included some simplifications to get an analytical solution. A simple numerical model simulates the absorption, generation, and the distribution of photogenerated carriers and current photogeneraed by the solar spectrum. The model is extended to obtain the spectral response and the solar efficiency in the proposed solar cell. A computer program GRADCELL in basic language is designed, written, and applied to determine the carrier distribution, photocurrent, spectral response, and conversion efficiency of graded band gap solar cell. Then this model is used to optimize the design of the proposed n-CdSxTe. x/CdTe thin film solar cell by studying the effect of some device parameters such as the surface energy gap, junction energy gap, junction depth, and surface recombination velocity. The efficiency that is obtained from the modeled CdSxTe x /CdTe cell is 30.83 % at surface energy gap Ego= 2.42 eV, junction energy gap Egd = 1.5 eV and, the junction depth D= 1μm. From study the effect of the optimized parameters on the conversion efficiency of the graded band gap solar cell'. We conclude that the conversion efficiency is increases with increasing the surface energy gap and decreasing the junction energy gap, and increasing the junction depth until it reach the optimum value and decreasing above the optimum value. The efficiency that is obtained from the first optimized design CdSxTe. x/CdTe solar cell is η| = 38.95 % at the surface energy gap Ego= 2.42 eV, junction energy gap Egd= 1.5 eV, and the junction depth D= 0.6 μm. The efficiency that is obtained from the second optimized design CdSxTe_x/CdS.3Te 7 solar cell is η = 62.57 % at the surface energy gap Ego= 2.42 eV, junction energy gap Egd= 1.356 eV, and the junction depth D= 0.6 μm.