Interface Reactions in CdTe Solar Cell Processing

Abstract

AbstractCurrently, the best performing CdS/CdTe solar cells use a superstrate structure in which CdTe is deposited on a heated CdS/SnO2/Glass substrate. In the close-spaced-sublimation (CSS) process, substrate temperatures in the range 550°C to 620°C are common. Understanding how these high processing temperatures impact reactions at the CdS/CdTe interface in addition to reactions between previously deposited layers is critical. At the SnO2/CdS interface we have determined that SnO2 can be susceptible to reduction, particularly in H2 ambients. Room-temperature sputtered SnO2 shows the most susceptibility. In contrast, higher growth temperature chemical vapor deposited (CVD) SnO2 appears to be much more stable. Elimination of unstable SnO2 layers, and the substitution of thermal treatments for H2 anneals has produced total-area solar conversion efficiencies of 13.6% using non-optimized SnO2 substrates and chemical-bath deposited (CBD) CdS. Alloying and interdiffusion at the CdS/CdTe interface was studied using a new lift-off approach which allows enhanced compositional and structural analysis at the interface. Small-grained CdS, grown by a low-temperature CBD process, results in more CdTe1-x.Sx alloying (x=12–13%) relative to larger-grained CdS grown by high-temperature CSS (x7sim;2–3%). Interdiffusion of S and Te at the interface, measured with lift-off samples, appears to be inversely proportional to the amount of oxygen used during the CSS CdTe deposition. Our highest efficiency to date using CSS-grown CdS is 10.7% and was accomplished by eliminating oxygen during the CdTe deposition.