a semiconductor - based optoelectronic device such as a solar cell has an n - type layer and a p - type layer , together forming a p - n junction . contact regions are formed on the device , with light - receiving regions between contact regions . a window layer is formed over the n - type layer or the p - type layer at the light - receiving region , the window layer promoting reduced carrier recombination at the surface of the n - type or p - type layer , and / or reflection of minority carriers in the n - type or p - type layer towards the p - n junction . the device has a window protection layer formed over the window layer , the window protection layer providing protection from degradation of the window layer during manufacture and / or operation of the device . for gaas - based devices the window layer may be al0 . 9ga0 . 1as and the window protection layer may be gaas . additionally , an alas etch stop layer may be provided over the window protection layer .