Stability of Sub-Micron-Thick CdTe Solar Cells

Sputtered CdS/CdTe cells with only 0.75 μm of CdTe have reached AM1.5 efficiencies over 12.5%. But the use of very thin absorber layers of CdTe raises questions about the possible impact on long-term stability when the back contact is very close to the main junction. In this study, we have performed accelerated life testing (ALT) on unencapsulated CdTe dot cells with absorber thickness ranging from 0.7 to 2.1 μm. After 900 h of ALT at 85°C under continuous one-sun illumination, with open circuit biasing and no encapsulation, we find that any decrease in stability as CdTe thickness decreases is within the ~10% statistical uncertainty shown by the sample sets of more than 20 cells each. Cells of all thicknesses exhibited some decrease in performance under these stress conditions, and open-circuit voltage appears to be the key factor in decreased efficiency. These changes in performance under ALT at 85°C are found to be consistent with a projected field lifetime of about 40 years in typical conditions. Secondary ion mass spectroscopy depth profiles of several elements including Cu showed no evidence of ALT-driven diffusion in these sputtered CdTe cells.