COMPUTER-SIMULATION AND MODELING OF GRADED BANDGAP CUINSE2 CDS BASED SOLAR-CELLS/

This paper proposes graded bandgap absorber material, Cu1-xAgxIn1-y-zG ayAlxSe2(1-u-w)S2uTe2w (CIS) multinary system, to improve the low ope n-circuit voltage (V-OC) seen in CuInSe2/CdS solar cells, without sacr ificing 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 re gion for maximum generation of electron and hole pairs (EHP's). This o ptical confinement results in a 25-40% increase in the number of photo ns absorbed. An extensive numerical simulator was developed, which pro vides a 1-D self-consistent solution for Poisson's equation and the tw o continuity equations for electrons and holes. This simulator was use d to generate J - V curves to delineate the effect of different gradin g profiles on cell performance. The effects of a uniform bandgap, norm al grading, reverse grading, and a low bandgap notch have been conside red. Having established the inherent advantages to these grading profi les an optimal doubly graded structure is proposed with grading betwee n 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 2 3.3 mA/cm(2), respectively, for the highest efficiency 1.4-eV uniform bandgap cell. Replacing the thick CdS(2.42ev) layer assumed in our sim ulations with a aide gap semiconductor such as ZnO(3.35ev) increases a ll current densities by about 5 mA/cm(2), and increases the optimal ca lculated efficiency from 17.9% to roughly 21% for a doubly graded stru cture with a thickness of 1 mu m and bandgaps ranging from 1.3 eV to 1 .5 eV.