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

A two-dimensional model was developed to simulate the optoelectronic characteristics of indium-gallium-nitride (InξGa1 − ξN), 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 InξGa1 − ξN wafer, and a metallic periodically corrugated backreflector. The ratio of indium to gallium in the wafer varies periodically throughout the thickness of absorbing layer of the solar cell. Thus, the resulting InξGa1 − ξN 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 InξGa1 − ξN aids in the separation of the excited electron–hole pairs, whereas the periodicities of optical properties and the backreflector 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. Midband 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.