 Nano-cadmium telluride solar cells (CdTe) are thin-film solar cells based on heterojunctions of p-type CdTe and n-type CdS. A transparent conductive film (TCO) and a transparent high resistance transition (HRT) film are deposited on a glass substrate, then cadmium sulfide (n-type) and cadmium telluride (p-type) films are deposited by vapor transport deposition (VTD). The improvement of crystal structure and the activation of PN junction are accomplished through CdCl2 activation treatment. A metal electrode (Mo/Al/Cr) is further deposited by physical vapor deposition (PVD), and a glass /TCO /CdS /CdTe/Mo/Al/Cr battery structure is formed. Butyl and EVA tapes are attached to the base of battery, and then laminated and sealed by a laminator. At last a junction box is installed to complete the manufacture of a battery assembly.  Ideal forbidden band width CdTe has a forbidden band width of 1.5eV, where the spectral response matches the solar spectrum well. High optical absorption coefficient CdTe thin films have a high optical absorption coefficient of about 104cm-1, absorbing 95% of the visible light in the thickness of 1 μm. High theoretical conversion efficiency CdTe is the cubic sphalerite (ZnS) structure with a bond length of 2.806 Å and a lattice constant of 6.481 Å. CdTe is a direct band gap semiconductor with a band gap of 1.45~1.47eV and a light absorption coefficient of 105/cm-1. The theoretical photoelectric conversion efficiency of nano-cadmium telluride thin film solar cells is about 30%.  Superior temperature coefficient Generally, within a certain temperature range, the efficiency of the solar cell changes linearly with temperature. The increase of temperature causes the open circuit voltage to drop, the short circuit current rises slightly, and the battery conversion efficiency decreases. It can be seen from the figure to the left that CdTe has better temperature performance and can produce more electricity under high temperature conditions than normal poly-Si.  Low linear attenuation rate For the long-term efficiency decline (25 years), att present, the efficiency of crystalline silicon solar cells is attenuated by 20%, while the conversion efficiency of CdTe solar modules is attenuated by 12.5% according to the National Renewable Energy Laboratory.  Sensitive to weak light CdTe thin film is sensitive to weak light, can generate more electricity than other established PV technologies (c-Si, poly-Si), therefore, CdTe power station has a higher return on investment(ROI), especially in cloudy days and in areas with haze.  Minimum hazardous emission during production The partial replacement of thermal power generation with CdTe photovoltaic power generation has greatly reduced the emission of cadmium. Although Cd+ is toxic, the closed-loop production and zero-emission process of wastewater can ensure zero-pollution of cadmium. According to the histogram to the left, the emission of cadmium during the production of CdTe is equivalent to that of thermal power generation with natural gas which is the least cadmium emission among various power generation technologies.  Shortest energy payback period Energy payback period is the time span that a solar power system requires to produce as much energy as was needed for production of the system. The shorter the energy payback period is, the higher the social and economic benefits will be. Among numerous solar technologies, the cadmium telluride system has the smallest carbon footprint and the shortest energy payback period. Take 1KWp installed capacity for an example, the energy payback period of crystalline-silicon PV modules is about 1.19 years, while it takes less than 0.8 years for CdTe thin film modules to get energy paid back .  Strong environmental adaptability Crystalline-silicon PV modules are sealed with double-layer EVA and butyl rubber. After a long term usage, EVA will show the sign of yellowing. The thickness of crystalline-silicon battery is about 150μm, and EVA is more susceptible during packaging, which leads to its bad environmental adaptation. CdTe thin film modules use single-layer EVA to assemble, the thickness of film is only 5~7μm, and the package bonding is more secure, all of which makes the thin film modules less sensitive to the surrounding environment. The CdTe thin film modules are sealed with butyl rubber. Since the methyl groups in the molecular chains of the butyl sealant are densely packed, and the molecular chains are strongly intertwined, it is extremely difficult for gas to pass, which makes the thin film modules more safe and reliable. |