Effects of pre-heat treatment of CdS on MOCVD CdTe/CdS solar cell performance

Preheat treatment of CdS/SnO₂/glass substrates prior to CdTe growth was investigated to improve the CdTe/CdS cell performance. The substrates were annealed in an H₂ atmosphere range of 300 to 450°C. N-i-p cells fabricated on these substrates showed a reduction of interface states with increasing annealing temperature. This may be associated with a reduction in oxygen concentration at the surface of CdS. Cells fabricated on substrates without any heat treatment had poor device performance with V_oc=0.55 V, fill factor (ff)=0.49, and an efficiency of 5.6%. Cell parameters improved with increasing annealing temperature, and the best values, V_oc=0.68 V, ff=0.59, and efficiency =~8%, were obtained at an annealing temperature of 450°C. In spite of the improved performance due to reduced interface states, thermal treatment also results in some detrimental effects, like a nonuniform Cd/Te ratio on the surface of CdS and a Cd-deficient CdS layer near the junction, which tend to limit the cell performance