Deposition process development for CSVT system and investigations into interfacial oxides within thin film cadmium selenide solar cells

Abstract

Cadmium Selenide (CdSe) is investigated in this work as a part of the global search for next generation materials for tandem junction solar cell viable active layers. A CSVT method for CdSe growth is developed as a means to consistently grow highly crystalline CdSe thin films of desired thickness. The specific method described was developed to grow CdSe thin films during a 15-minute deposition with a temperature-controlled rate of evaporation. Temperature-pressure-deposition rate correlations were made for rates as low as 4 nm/min and as high as 40 nm/minute. The described process grows highly crystalline wurtzite CdSe along the (200) plane. The second main aspect of this work revolves around the investigation of the role of oxides at cadmium interfaces within solar cell device architecture. Uncontrolled oxidation is hypothesized to be the hidden variable in previous device fabrication, which resulted in countless unexplainable deviations in film fabrications and subsequent device performance results. The presence of various species of oxides are experimentally detected. Theory regarding the explicit interactions at each cadmium containing layer is then developed. Finally, the impact of oxidation is experimentally observed through a series of intentional oxidations and electrical tests of device performance. Through this, cadmium oxide, selenium dioxide, and cadmium selenite are proven to be a major cause of unknown device performance variation.