Effects of copper diffusion in gallium arsenide solar cells for space applications

High efficiency, thin-film Epitaxial Lift-Off (ELO) III-V solar cells offer excellent characteristics for implementation in flexible solar panels for space applications. However, the current thin-film ELO solar cell design generally includes a copper handling and support foil. Copper diffusion has a potentially detrimental effect on the device performance and the challenging environment provided by space (high temperatures, electron and proton irradiation) might induce diffusion. It is shown that heat treatments induce copper diffusion. The open-circuit voltage (Voc) is the most affected solar cell parameter. The decrease in Voc can be explained by enhanced non-radiative recombination via Cu trap levels in the middle of the band gap. The decrease in Voc is found to be dependent on junction depth. In all Cu cells annealed at T≥300°C signs of Cu diffusion are present, which implies that a barrier layer inhibiting Cu diffusion is necessary. Electron radiation damage was found to have no influence on Cu diffusion.