Effective Ag Doping by He-Ne Laser Exposure to Improve the Electrical and the Optical Properties of CdTe Thin Films for Heterostructured Thin Film Solar Cells

The cadmium telluride (CdTe) thin film solar cell is one of the strongest candidates due to the optimum band gap energy (about 1.4 eV) for solar energy absorption, high light absorption capability and lower cost requirements for solar cell production. However, the maximum efficiency of a CdTe thin film solar cell still remains just 16.5% despite its excellent absorption coefficient; i.e., the electrical properties of CdTe thin film, including the resistivity, must be improved to enhance the energy conversion efficiency. Silver (Ag) was doped by using helium-neon (He-Ne) laser (632.8 nm) exposure into sputtering-deposited p-type CdTe thin films The resistivity of the Ag-doped CdTe thin films was reduced from 2.97 x 10(4) Omega-cm to the order of 5.16 x 10(-2) Omega-cm. The carrier concentration of CdTe thin films had increased to 1.6 x 10(18) cm(-3) after a 15-minute exposure to the He-Ne laser. The average absorbance value of CdTe thin films was improved from 1.81 to 3.01 by the doping of Ag due to impurity-scattering. These improved properties should contribute to the efficiency of the photovoltaic effect of the photogenerated charged carriers. The methodology in this study is very simple and effective to dope a multilayered thin film solar cell with a relatively short process time, no wet-process, and selective treatment.