The effects of Cd/Te mole ratio in the MOCVD growth ambient on CdTe solar cells

CdTe/CdS solar cells were fabricated by depositing CdTe films on glass/SnO₂/CdS substrates by MOCVD in different growth ambients with varying Te/Cd mole ratios in the range of 0.02 to 15. Short circuit current density (J_sc) showed a minimum at Te/Cd ratio of 0.1 and increased on both sides of this minimum giving rise to a “U” shaped curve. Open circuit voltage (V_oc) was highest for the Te-rich growth ambient and was low for the stoichiometric and the Cd-rich growth conditions. Such variations in J _sc and V_oc gave the best cell efficiency of 12% in this study at a Te/Cd ratio of six. The films grown under Te-rich conditions form p-type CdTe, but the Cd-rich growth conditions produce n-type CdTe which require 400°C/air anneal for the type conversion. Detailed measurements and analysis revealed a high degree of atomic interdiffusion at the interface when the CdTe films were grown in the Te-rich conditions. The enhanced interdiffusion reduces interface states which is further supported by higher effective lifetime in the Te-rich films, as well as rapid decrease in quantum efficiency under forward bias for the Cd-rich cells