A simple model for the photocurrent density of a graded band gap CIGS thin film solar cell

A simple model for the photocurrent density of a linearly graded band gap Cu(In,Ga)Se ₂ solar cell is presented. Both generation and recombination mechanisms in the space charge region and absorber region of the cell are considered. The carrier collection function and effective absorption coefficient are introduced in the calculations to obtain a more realistic model. The results show that photocurrent density of the graded band-gap solar cell is higher than that with a constant averaged band gap. There is an optimum for grading strength or band gap widening of the absorber region. Recombination current reduces the photocurrent density with a lower reduction in the absorber material than in the depletion region. For longer diffusion lengths (or greater values of carrier collection factor), a higher photocurrent density is obtained except where collection probability is already unity everywhere in the absorber.