Transparent conductors for full spectrum photovoltaics

Low transparency of conventional transparent conducting oxides (TCOs) in the IR region (λ1 nm) due to absorption and reflection by the high concentration of free carrier restricts the applications of materials to photovoltaics that do not use the infrared part of the solar spectrum. In order to maintain low resistivity with reduced free carrier absorption, TCOs with high mobility are required. We have found that among metal oxides, CdO has all the essential requirements for low resistance and high infrared transmission: low electron scattering rates and high electron mobility. In this work we report the electrical and optical properties of CdO, undoped and doped with various n-type dopants, grown using pulsed laser deposition (PLD). We found that nominally undoped CdO films have Hall mobilities in the range of 50-150 cm²/Vs with carrier concentrations of 3×10²⁰-2×10¹⁹ cm³ corresponding to a high compensation ratio k=0.7. However, with appropriate intentional doping, ideal uncompensated CdO with extremely high conductivity (>10⁴ S/cm) and an excellent transmission window in the range from 400 to >1500 nm can be achieved. These properties make CdO an ideal TCO for photovoltaics with low band gap absorbers including Si and high efficiency multijunction cells.