Fabrication and Characterization of Single Crystal CdTe Solar Cells

CdTe as one of the II-VI semiconductors has become attractive over the last few decades as a result of commercial applications in thin-film solar photovoltaic technology. Although the current record CdTe solar cell efficiency is above 20%, it is still much lower than the maximum single junction theoretical efficiency of approximately 30%. Moreover, the impressive recent efficiency gains are mainly attributed to the short circuit current and fill-factor. Thus, improving open circuit voltage is required in order to achieve the high efficiency CdTe-based solar cells. Here, I will present fabrication and characterization of single crystal CdTe-based II-VI solar cells grown by molecular beam epitaxy (MBE) on Si substrates. I will begin by discussing my results related to material properties of indium tin oxide (ITO), arsenic-doped CdTe, and indium-doped CdTe. The results show that (a) temperature and doping are critical factors for both electrical and optical properties of ITO thin films as front contact materials of single crystal CdTe solar cells, (b) p-type carrier density greater than 10^16 cm^-3 can be achieved using arsenic acceptors in CdTe films, and (c) n-type carrier density close to 10^18 cm^-3 can be achieved using indium donors in CdTe films. Finally, I will present my studies of single crystal CdTe solar cell fabrication and performance based on the knowledge obtained from the material properties of ITO, arsenic-doped CdTe, and indium-doped CdTe. Relatively poor crystal quality of arsenic-doped absorber CdTe layers and indium diffusion issues during device fabrications are found to be correlated with the lower open circuit voltage and efficiency. Possible means of achieving high open circuit voltage will be discussed for future work.