Ternary spinel cadmium stannate, cadmium indate, and zinc stannate and binary tin oxide and indium oxide transparent conducting oxides as front contact materials for cadmium sulfide/cadmium tellurium photovoltaic devices

Transparent conducting oxides (TCO's) of Cd₂SnO ₄ (cadmium stannate), CdIn₂O₄ (cadmium indate), and Zn₂SnO₄ (zinc stannate) thin films were investigated from a materials and applications point of view through. All films were deposited by co-sputtering using either binary oxide or metallic (reactive sputtering) targets. The film properties were investigated as a function of film composition and stoichiometry. The effect of process parameters such as deposition temperatures, and post-deposition heat treatments on the structural and electro-optical properties of the films were also investigated extensively. All as-deposited films were found to be amorphous independent of substrate deposition temperature. The electro-optical and crystallographic properties were heavily dependant on the post deposition heat treatments. Cd₂SnO₄, Zn ₂SnO₄, and CdIn₂O₄ all produced highly transparent films with average transmission values (400–900 nm range) of 92%, 93%, and 90%, respectively. Cd₂SnO₄ and CdIn ₂O₄ were highly conductive with resistivity values as low as 2.01 × 10−4 Ω-cm and 2.90 × 10 −4 Ω-cm, respectively. Conversely, Zn₂SnO ₄ was not able to produce highly conductive films, with the lowest resistivity being 4.3 × 10−3 Ω-cm.; CdTe solar cells were fabricated using al the above materials as front contacts or as high-&rgr; layers in bi-layer structures. All cells were of the superstrate configuration: Low-&rgr; TCO/high-&rgr; TCO/CdS/CdTe/Back contact. Only the TCO layers were varied; the remainder of the device was held constant. In most cases the inclusion of a high-&rgr; TCO layer was found to improve solar cell performance, especially in regard to the open circuit voltage. Cd₂SnO₄ was the exception. The incorporation of Zn₂SnO₄ as a high-&rgr; layer enabled a greatest current collection from high energy wavelengths through an apparent thinning effect on the CdS. This increased the overall short circuit current density to values in excess of 24.9 mA/cm2. The standard device consisted of an all CVD SnO₂ bi-layer. When the high-&rgr; CVD SnO₂ was replaced by reactive sputtered SnO₂, device efficiencies of approximately 14.6% were obtained. This improvement was largely from the higher fill factors produced by this method, which were as high as 74%. All of the above mentioned TCO's produced devices with efficiencies of or in excess of 14%.