Synergistic effects of contamination and low energy space protons on solar cell current output

It is well known that solar cell coverglass materials are subject to darkening, or transmission degradation, due to interaction with protons. Our recent laboratory test results have shown that the transmission of coverglasses, once contaminated with organic molecular films, can be further degraded upon space proton irradiation (20-400 keV). The coverglass transmission loss occurs in the short wavelength region, thus multi-junction solar cells are expected to be particularly susceptible to such synergistic effects of contamination and proton irradiation when the top junction is the current limiting junction. In the previous work, AR/ITO coverglass materials, commonly used for space solar arrays, were photo-deposited with the model contaminant DC704. The contaminated coverglass samples were subsequently irradiated with a simulated 15-year geosynchronous orbit low energy proton radiation environment at 5-year increments. The progression of coverglass transmission change was characterized before and after each process. The measured coverglass transmission data were then convolved with the solar cell quantum efficiency and solar spectrum to determine the coverglass darkening effects on solar cell performance. Taking into account space proton radiation effects and the time dependent contaminant film accumulation process, our preliminary analysis indicates that, over a 15-year mission life, approximately 3.7 % solar cell current loss could be attributed to a beginning-of-life (BOL) contaminant film of 100 Å, with no additional on-orbit film growth. For a BOL film of 100 Å and additional film growth while on orbit, the end-of-life (EOL) solar cell current loss due to contamination is approximated at 5.5% for EOL 200 Å, and 7.3% for EOL 300 Å.