Computer simulation and modeling of graded bandgap CuInSe2/CdS based solar cells

This paper proposes graded bandgap absorber material, Cu1-xAgxIn1-y-zGayAlxSe2(1-u-w)S2uTe2w (CIS*) multinary system, to improve the low open-circuit voltage (V-OC) seen in CuInSe2/CdS solar cells, without sacrificing the short-circuit current density (J(sc)). It also proposes a p-i-n model for the CuInSe2/CdS solar cell, where the intrinsic region is the graded bandgap CIS*. Reflecting surfaces are provided at the p - i and n - i interfaces to trap the light in the narrow intrinsic region for maximum generation of electron and hole pairs (EHP's). This optical confinement results in a 25-40% increase in the number of photons absorbed. An extensive numerical simulator was developed, which provides a 1-D self-consistent solution for Poisson's equation and the two continuity equations for electrons and holes. This simulator was used to generate J - V curves to delineate the effect of different grading profiles on cell performance. The effects of a uniform bandgap, normal grading, reverse grading, and a low bandgap notch have been considered. Having established the inherent advantages to these grading profiles an optimal doubly graded structure is proposed with grading between 1.5 eV and 1.3 eV regions which has V-oc = 0.86 V, eta = 17.9%, FF = 0.79 and J(sc) = 26.3 mA/cm(2) compared to 0.84 V, 14.9%, 0.76, and 23.3 mA/cm(2), respectively, for the highest efficiency 1.4-eV uniform bandgap cell. Replacing the thick CdS(2.42ev) layer assumed in our simulations with a aide gap semiconductor such as ZnO(3.35ev) increases all current densities by about 5 mA/cm(2), and increases the optimal calculated efficiency from 17.9% to roughly 21% for a doubly graded structure with a thickness of 1 mu m and bandgaps ranging from 1.3 eV to 1.5 eV.