a protective coating for solar cells , particularly solar cells positioned in space and which coating is a polyimide which is colorless , transparent , relatively non - brittle , has a high degree of thermal stability and readily transmits solar radiation without appreciable degradation . the coating is heat resistant and does not degrade significantly when exposed to ultraviolet radiation , and is highly effective in repelling low energy proton particles . in a preferred embodiment , the protective polymide coating is a polymer having the following recurring structural unit : ## str1 ##