Tin oxide stability effects—their identification, dependence on processing and impacts on CdTe/CdS solar cell performance

Abstract

High efficiency polycrystalline thin film CdTe solar cells involve the growth of CdTe films on CdS/SnO2/glass substrates. The CdS layer in such a structure is commonly reported to benefit from a brief hydrogen anneal prior to the deposition of the CdTe film. In this paper, we show that the SnO2 layer can be susceptible to reduction in H2 and that the degree of susceptibility is dependent on the type of SnO2 used. Chemical vapor deposited (CVD) SnO2/glass substrates (Solarex Corp.) show the most resistance to reduction while room-temperature sputtered SnO2 films show the least resistance. When annealed under reducing conditions, Sn from the SnO2 reacts with S-containing impurities and oxygen in as-grown chemical bath deposited (CBD) CdS films to form SnS. Cd-containing impurities are more volatile resulting in a loss of Cd relative to S in films annealed in H2. These films appear dark due to the presence of SnS, a grayish-black impurity, in the CdS and possibly SnO in the SnO2. In normal CSS CdTe deposition processes where H2 annealing is followed by further heating to deposition temperatures in either He or He:O2 ambient, S loss occurs at temperatures exceeding the H2 anneal. If oxygen is absent, CdS films undergo loss of both Sn and S due to evaporation of the SnS. When O2 is present, SnS converts to SnO2 allowing for only the evaporation of sulfur. In this fashion, Sn levels on the CdS surface immediately prior to the deposition of CdTe, can be affected not only by the temperature of the H2 anneal, but also by the oxygen present during the CdTe deposition step. Modifications to the CdS/CdTe device fabrication process including the use of more stable tin oxide layers (CVD-grown) and lower temperature H2 anneals yield devices with higher open circuit voltage, fill-factors, and total-area efficiencies. Room-temperature sputtered tin oxide can be strengthened against reduction by annealing at 550 °C in 400 torr O2 prior to the CdS deposition step.