This work is focused on the study of possible mechanisms affecting the electrical transport properties of ZnO thin films. The films were deposited using the reactive evaporation technique, obtaining transmittances greater than 80% and resistivities of the order of 8 × 10 -4 Ω cm without using extrinsic doping. This films are suitable for transparent front contact of solar cells.Measurements of resistivity and Hall coefficient, as a function of temperature, were performed on the films. The interpretation of these results was done with the help of a theoretical calculation of the carrier mobility as a function of the temperature. Several scattering mechanisms affecting the electrical transport in the temperature range studied (90 K -680 K) were found. The most important are processes occurring in the grain boundaries and interactions of free carriers with ionized impurities.
ZnO films with transmittances greater than 80% in the blue spectral region and electrical resistivities of about 8.5 × 10—4 Ωcm were deposited by reactive evaporation. Semiconductor thin films with these characteristics are very attractive to use as transparent contact in thin film solar cells. A parameter study revealed that the main deposition parameters affect significantly the opto‐electrical 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 of about 540 °C. Measurements of the Hall coefficient indicated that the changes in the electrical resistivity of the ZnO films induced by changes in the deposition parameters, are mainly due to changes in the free carrier density. On the other hand, it was found through AFM (Atomic Force Microscopy) measurements that the high values of blue transmittance obtained with ZnO films deposited at high O2 partial pressures are in part controlled by morphological effects.
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