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Thermal Impact of Rear Insulation, Light Trapping, and Parasitic Absorption in Solar Modules

Reflection of sub-bandgap light has been argued to be the most effective path to lower temperature solar modules. This report compares GaAs modules with high sub-bandgap reflection to various Si modules under two experimental thermal configurations. At one sun, the GaAs modules operate ∼6 °C colder than both multicrystalline and monocrystalline Si within an open-rack configuration. This thermal advantage increases to ∼13 °C for the thermally insulated configuration. The experimental data are used to build a theoretical model, finding agreement with root-mean-square error between 1.4°C–2.8 °C. The model shows the main thermal advantage of these GaAs modules to be their high sub-bandgap reflection of 79%. Next, it is shown that the sub-bandgap reflection in modules with textured Si cells is fundamentally limited compared to values achieved by the planar GaAs modules, because of the amplification of parasitic absorption that occurs with light trapping. In Si modules, light trapping more than doubles the parasitic absorption of encapsulation layers, limiting the maximum sub-bandgap reflection to 63%. Higher values require thorough optimization of front, bulk, and rear layers, but could lower operating temperatures by up to 12 °C for insulated Si modules.