Thin Film Group II-VI Solar Cells Based on Band-Offsets

Abstract

The amount of traditional energy sources are finite and the ecological impact of continuing to produce energy using fossil fuels will only exacerbate the carbon footprint. It is for these reasons that photovoltaic modules are becoming a larger and more necessary part the world's electricity production paradigm. Photovoltaic (PV) semiconductor modules are grouped into three categories. 'First generation' monocrystalline and polycrystalline silicon modules that consist of p-n junctions created via the addition of impurities known as dopants. Almost 85% of solar cells produced at this time are 'first generation' and it is the high production costs of silicon PV modules that motivated the search for new methods and materials to use as PV cells. 'Second generation' PV modules consist of semiconductor thin films. The 'second generation' PV modules in production at this time are copper indium gallium diselenide (CIGS), copper indium gallium (CIG), amorphous silicon (a-Si), and cadmium telluride (CdTe). The 'third generation' PV modules consist of dye-sensitized and organic materials. Thin films use less material, have less stringent production parameters and less waste, making thin films cost effective. In this investigation, solar cells were prepared using un-doped Group II-VI semiconductor thin films that exploit differences in band-offsets to form effective p-n heterojunctions as a viable low cost alternative to doping. The thin films were deposited by thermal evaporation upon glass substrates coated with indium tin oxide (ITO). A layer of aluminum formed the back contact. Various configurations of the solar cells were produced including: ITO/CdS/CdSe/Al, ITO/ZnTe/CdSe/Al, ITO/CdTe/CdSe/Al, ITO/ZnTe/CdTe/CdS/Al. The solar cells produced have been characterized to determine thin film internal resistances, quantum and 'wall-plug' efficiencies, as well as I-V and spectral response. The open circuit voltage, short circuit current density, fill factor, and efficiency of our best devices were 0.26 V, 4.6 mA, 27.5 and 0.4% respectively. Additional device optimization should be possible and should improve these results. Solar cell design based on band-offset is an effective method for predetermining likely PV structures, while future investigation using Group II-VI semiconductor nanowires and nanorods and employing epitaxial films are likely to enhance the efficiency.