Abstract
The origins of parasitic heating for photovoltaic (PV) technologies based on silicon, perovskites, and their combination in monolithic tandems are investigated. To quantify heating losses, the cooling score (CS) as a new simple metric, representing the percentage of incident solar irradiance not contributing to module heating is introduced. This is a function of both the optical structure and power conversion efficiency (PCE) of the PV modules and allows a fair comparison between different technologies under identical performance-evaluation scenarios. Silicon single-junction devices have the lowest CS due to their low bandgap (causing significant carrier thermalization losses) and their use of light-trapping structures to increase their PCE which also undesirably increases parasitic absorption of sub-bandgap photons. Conversely, perovskite single-junction devices show the highest CS in all studied performance-evaluation scenarios thanks to their wider bandgap and high absorption coefficient, enabling absorption of all solar photons that may contribute to the photocurrent without requiring light-trapping structures. While perovskite/silicon tandems minimize thermalization losses, they also usually employ light-trapping structures in their bottom cell to increase their PCE, which lowers their CS. Through simulation and outdoor experiments, it is demonstrated that an efficient module-cooling environment may significantly suppress the detrimental effects associated with a low CS.