Concentrating optical system optimization for 3- and 4-junction solar cells: impact of illumination profiles

Abstract

Optical component designs for concentrating photovoltaic systems with three different multijunction solar cells (MJSCs) are optimized to yield maximum system efficiencies under standard test conditions, specifically uniform illumination. Optimization uses an integrated optoelectrical approach with ray tracing of the optical train to generate an irradiance profile for input to the cell’s distributed circuit model. These cells, a three-junction lattice-matched (3JLM) solar cell, a three-junction lattice-mismatched inverted metamorphic (3JIMM) solar cell, and a four-junction lattice-matched (4JLM) solar cell, were individually designed for maximum efficiency at 1000×. The optical train introduces losses, modifies the spectrum, and produces a spatially nonuniform profile across the cell. We decouple spectral modification from spatial nonuniformity to separately determine their individual impacts on system efficiencies, finding the optimal set of optical design parameters for each case. Spectral modification yields modest loss penalties (from 1.0% to 1.6%, relative to the MJSC), but the impact of nonuniformity is more significant and cell dependent, with relative loss penalties of 1.1%, 3.8%, and 2.3%, for 3JLM, 3JIMM, and 4JLM, respectively. While spectral modification does not significantly impact design parameters, spatial nonuniformity does, with absolute losses of 1% and 3.4% if 3JIMM and 4JLM cells are used in a 3JLM optimized system, respectively.