Co-Sputtered Sn-Doped ZnO Thin Film n-Type Layers for Incorporation into CdTe Based Photovoltaics

Tin dioxide (SnO
2
) and zinc oxide (ZnO) could be used to replace cadmium sulfide (CdS) in cadmium telluride (CdTe) photovoltaic cells. CdS undergoes parasitic absorbance due to a low bandgap and does not have a favourable band alignment. SnO
2
and ZnO were co-sputtered to form a Sn-doped ZnO film. The bandgap of the film is tunable depending on the amount of SnO
2
present within the structure. This was analysed by depositing SnO
2
at varying power densities. The co-sputtered film at 2 % oxygen in argon process gas, substrate temperature 500°C and a deposition power density of 2.97 W/cm
²
for SnO
2
and ZnO at 3.96 W/cm
²
had the most promising window layer characteristics with high transmission, a bandgap of 3.82 eV, a high carrier concentration of 2.56 ×10
¹⁷
cm
⁻³
and low resistivity. However when full devices were made with CdTe it was seen the sample with the highest resistivity (10
⁶
) and lowest carrier concentration (10
¹²
) made the most efficient device with an efficiency of 6.2 %