ABSTRACTThe behavior of standard space photovoltaic assemblies in a high intensity, high temperature environment (HIHT) is addressed. Experimentally, an HIHT environment, typical for missions to the inner planets of the solar system such as Mercury, characterized by temperatures of 500K and 11 solar constant irradiance in the ultraviolet region below 400 nm, was simulated in a vacuum. Independently of the triple junction cell technology used, module degradation up to 20% in power was observed during several hundred hours of test. Electroluminescence analysis identified discrete top cell shunts close to the cell edge, in particular around the frontside contact pads. Cross‐sectional transmission electron microscopy performed on several degraded cells revealed an etched contact pad metallization/cap layer interface and more importantly, several 100‐nm large, oriented Cu3P inclusions at the shunted locations. A chemical degradation mechanism is proposed. Short wavelength ultraviolet light interacting with polysiloxanes used as module encapsulant produces hydrogen and methyl radicals. With these building blocks, an organic acid can be formed on external reaction surfaces such as the Ag busbars that simultaneously serve as a source of oxygen. Cu traces present in the Ag segregate to the surface and are transported by this acid to the contact pad of the cell in the liquid phase. An adapted cell design was developed to prevent this degradation mechanism believed to be of relevance for all HIHT space environments. A several hundred micrometer‐wide rim composed of the outermost cell area is electrically separated from the inner cell area and provides a barrier against environmental attack. None of the photovoltaic assemblies featuring this mesa cell design showed any fill factor‐induced power degradation any more. Copyright © 2011 John Wiley & Sons, Ltd.