Optimization of nonhomogeneous indium-gallium-nitride Schottky-barrier thin-film solar cells

Abstract

A two-dimensional model was developed to simulate the optoelectronic characteristics of indium-gallium-nitride (InGaN), thin-film, Schottky-barrier solar cells. The solar cells comprise a window, designed to reduce the reflection of incident light, Schottky-barrier and ohmic front electrodes, an n-doped InGaN wafer, and a metallic periodically corrugated back-reflector (PCBR). The ratio of indium to gallium in the wafer varies periodically throughout the thickness of the absorbing layer of the solar cell. Thus, the resulting InGaN wafer's optical and electrical properties are made to vary periodically. This material nonhomogeneity could be physically achieved by varying the fractional composition of indium and gallium during deposition. Empirical models for indium nitride and gallium nitride were combined using Vegard's law to determine the optical and electrical constitutive properties of the alloy. The nonhomogeneity of the electrical properties of the InGaN aids in the separation of the excited electron-hole pairs, while the periodicities of optical properties and the back-reflector enable the incident light to couple to multiple guided wave modes. The profile of the resulting charge-carrier-generation rate when the solar cell is illuminated by the AM1.5G spectrum was calculated using the rigorous coupled-wave approach. The steady-state drift-diffusion equations were solved using COMSOL, which employs finite-volume methods, to calculate the current density as a function of the voltage. Mid-band Shockley-Read-Hall, Auger, and radiative recombination rates were taken to be the dominant methods of recombination. The model was used to study the effects of the solar-cell geometry and the shape of the periodic material nonhomogeneity on efficiency. The solar-cell efficiency was optimized using the differential evolution algorithm.