Temperature-Dependent Measurements of an Inverted Metamorphic Multijunction (IMM) Solar Cell

The inverted metamorphic multijunction (IMM) solar cell has demonstrated efficiencies as high as 40.8% at 25°C and 326 suns concentration. The actual operating temperature in a commercial module, however, is likely to be as much as 50-70°C hotter, reaching as high as 100°C. In order to be able to evaluate the cell performance under these real-world operating conditions, we have measured the open-circuit voltage, short-circuit current density and efficiency at temperatures up to 125°C and concentrations up to 1000 suns, as well as the temperature coefficients of these parameters. Spectral response and one-sun current-voltage characteristics were measured by carefully adjusting the incident spectrum to selectively current-limit the different subcells. Concentrator measurements were taken on a pulsed solar simulator to minimize any additional heating due to the high intensity illumination. We compare our measured values to predictions based on detailed models of various triple junction solar cells. By choosing the optimum bandgaps for high temperature operation, the IMM can potentially result in greater energy production and lower temperature sensitivity under real operating conditions than a Ge-based solar cell.