Properties of ZnO thin films prepared by reactive evaporation

Non-doped ZnO thin films with suitable characteristics to be used as transport contact and as buffer layer in solar cells, were prepared by reactive evaporation. Through a parameter study, it was found that the main deposition parameters affect the optoelectrical properties of the ZnO films, being the oxygen partial pressure the parameter which most affects both, the transmittance and the conductivity. Actually, high-conductive ZnO films with blue transmittances greater than 80% are routinely prepared by using oxygen partial pressures greater than 0.2 mbar and evaporation temperatures of Zn about 540°C. AFM measurements revealed that the high values of blue transmittance obtained with ZnO film deposited at high O₂ – partial pressure are in part controlled by morphological effects. From Hall coefficient and conductivity measurements it was found that the conductivity is dominated by the resulting carrier concentration which is controlled by oxygen vacancies. The conditions to prepare in situ the double low ρ-ZnO/high ρ-ZnO bilayer structure regularly used in the fabrication of CdS-free, thin films solar cells, were also found through this study.