Thermal Load in Thinned Ge-Based Multijunction Space Solar Cells

Low energy photons absorbed in a solar cell can contribute to the heating of the device, which negatively affects performance. This is especially important in germanium, as it presents high free-carrier absorption. Thinning of Ge substrates is a technique which aims to reduce weight and improve performance of multijunction solar cells. Additionally, it presents the opportunity to reflect low energy photons via a rear side infrared reflector. We perform optical simulations and show that thinning an InGaP/GaAs/Ge solar cell from 140 to 40 μm with a gold back reflector can reduce the cooling power requirements by 9.6% for a highly doped substrate, and 7% for a lightly doped one. In terms of temperature, this represents a reduction of 8 °C and 6 °C, respectively.