Effect of boron-doping on transparent conducting Al doped ZnO films for thin film solar cells

Al-doped ZnO (AZO) films with different boron (B) content have been prepared on glass substrates by co-sputtering of AZO (2wt. % Al₂O₃) and B-doped ZnO (BZO, 3wt. % B₂O₃) targets at room temperature. The B content in the B-doped AZO (BAZO) film was controlled by DC power on BZO target. X-ray diffraction analysis indicated that the B-doping improved the crystalline structure of AZO film. Hall measurement analysis showed that resistivity of BAZO films was lowered to 2.06×10^-3 Ω·cm, whereas that of AZO film was 5.68×10^-3 Ω·cm. Hall mobility of AZO film was improved with appropriate B-doping, and carrier concentration was increased with B-doping due to B activation as a donor. After heat treatment in atmospheric air, the resistivity of AZO film was increased from 5.68×10^-3 to 1.28×10^-2 Ω·cm, which resulted from oxygen chemisorptions on grain boundaries. However, resistivity of BAZO film with suitable B doping was significantly decreased from 2.14×10^-3 to 5.76×10^-4 Ω·cm. The electrical stability of BAZO films in oxidation atmosphere with high temperature is possibly attributed to passivating effect of grain boundaries by B-doping. When the as-deposited BAZO films were heat treated in vacuum, the resistivity of films was improved further to 4.8×10^-4 Ω·cm, which is ascribed to the dominant increase in mobility. BAZO films with better structural, electrical characteristics, and electrical stability than those of AZO film have been successively deposited by the appropriate B-doping to AZO film.