Transparent conductive thin films of Cd2SnO4 obtained by the sol–gel technique and their use in a solar cell made with CdTe

• Cd2SnO4 thin films obtained by the sol–gel technique starting from two metallic salts.
• Cd2SnO4 thin films obtained by the sol–gel technique with high quality crystalline.
• Resistivity of 2×10−3 Ω cm obtained for Cd2SnO4 thin films annealed-vacuum at 550 °C.
• Cd2SnO4 thin films with high transmittance ~85% in the 500<λ<1500 nm range.
• Cd2SnO4 obtained by sol–gel and used as TCO in a solar cell of CdTe.

In this work Cd2SnO4 polycrystalline thin films with cubic phase were deposited on glass substrates by the sol–gel technique starting from the mixture of two simple precursor solutions of CdO and SnO2, obtained at room temperature. Both solutions were prepared using metallic salts, cadmium acetate and stannous chloride. The Sn atomic concentration percentages (X) in the precursor solution with respect to Cd (1−X) were 16, 25, 29, 33, 36 and 40. The films were sintered at 550 °C in air and, after that, exposed to two different annealing treatments, 96/4N2/H2 gas mixture and vacuum, at 350 °C and 550 °C, respectively. X-ray diffraction patterns of the films without annealing showed three types of films: i) for X<29 at%, the films were constituted of CdO+Cd2SnO4 crystals, ii) for X=29 at% and X=33 at%, Cd2SnO4 polycrystalline thin films were obtained, and iii) for X>33 at%, Cd2SnO4+ CdSnO3 crystals were formed. Cd2SnO4 films maintained the spinel-type crystalline structure of the Cd2SnO4 after the annealing processes. On the other hand, Electron Dispersion Spectroscopy measurements revealed that Cd2SnO4 films obtained at X=29 at%, without annealing, had a Cd/Sn ratio nearest to the one of the Cd2SnO4 compounds. With and without annealing Cd2SnO4 films showed high optical transmission ~85% in the 500 nm<λ<1500 nm range. A minimum resistivity value was around 2×10−3 Ω cm, which was obtained for Cd2SnO4 films (X=33 at%) annealed in vacuum at 550 °C, and a direct band gap energy value of 3.55 eV. As a test of the quality of these films as transparent electrodes, Au/Cu2Te/CdTe/CdS/Cd2SnO4/glass solar cells were obtained with average efficiency of 10.7%.