Photoelectric properties of highly conductive samarium-doped cadmium telluride thin films for photovoltaic applications

Cadmium telluride (CdTe) is one of the most important materials for photovoltaic applications. CdTe-based solar cells have significantly high light-to-current conversion efficiency. However, further improvement of the efficiency is affected by two major problems: high electrical resistivity of CdTe and the difficulty to form ohmic contacts with this material. Extrinsic doping has been employed to rectify these problems; in particular, samarium is a rare-earth element that is characterized by its excellent electrical conductivity, high valence, and low oxidation affinity. In this work, thermally evaporated samarium-doped CdTe thin films were deposited with a samarium concentration of 0–6.2 at%. After doping, there were drastic changes in the photoelectric properties of the films, as demonstrated by: (i) conversion of CdTe from p- to n-type material, (ii) reduction of electrical resistivity by eight orders achieving the lowest value of 7.6 × 10⁻² Ω⋅cm, (iii) formation of ohmic contacts to CdTe, and (iv) reduction of the optical bandgap, which enhanced the absorption of solar radiation.