Substrate-based GaAs solar cells having a dense Au/Cu front contact grid with 45% surface coverage were exposed to accelerated life testing at temperatures between 200 and 300 • C. TEM analysis of the front contacts was used to gain a better understanding of the degradation process. During accelerated life testing at 200 • C only intermixing of the Au and Cu in the front contact occurs, without any significant influence on the J-V curve of the cells, even after 1320h (55 days) of accelerated life testing. At temperatures ≥ 250 • C a recrystallization process occurs in which the metals of the contact and the GaAs front contact layer interact. Once the grainy recrystallized layer starts to approach the window, diffusion via grain boundaries to the window and into the active region of the solar cells occurs, causing a decrease in V oc due to enhanced non-radiative recombination via Cu trap levels introduced in the active region of the solar cell. To be a valid simulation of space conditions the accelerated life testing temperature should be < 250 • C in future experiments, in order to avoid recrystallization of the metals with the GaAs contact layer.

Experiments were performed in order to explore the potential of Ti and Ni as Cu diffusion barriers for III-V solar cells. Accelerated life testing with solar cells with 10 or 100nm thick Ti and Ni barriers at 300 • C indicates that 10nm thick barriers show a better performance compared to 100nm thick barriers, but further experiments at a lower temperature are required to validate this conclusion. Accelerated life testing at 250 • C with solar cells with various front contacts containing a Ti or Ni barrier shows that cells with Au/Ti/Cu, Ni/Cu and Au/Ni/Cu have a performance better than cells with Au/Cu contacts. The Au/Ti/Cu contact scheme appears to be most promising as cells with this contact scheme show degradation behaviour similar to that of cells with plain Au contacts.