Fabrication of Transparent Conducting Films for Solar Cell Applications

Abstract

In this research, transparent conducting films from In2O3 and ZnO groups were fabricated by ultrasonic spray pyrolysis for solar cell applications. The contribution of this thesis has been focused on the optical and electrical properties of these films. The project results can be divided in to five categories. In the first part, the effects of Sn doping on In2O3 films were investigated. All films showed cubic structure of In2O3 and showed high transmittance more than 80%. The conductivity of films was improved by Sn doping. However, the minimum resistivity was presented in 5 at.% Sn doping condition. This behavior may be due to the solubility limited of Sn4+ into In2O3 lattice. The second part, the ITO/metal/ITO multilayer films were fabricated for improving the electrical properties of ITO films. The In2O3 film doped with 5 at.% Sn doping was chosen as ITO layer and Au metal was employed for intermediated layer of multilayer films. The addition of Au layer enhanced the conductivity but reduced the transmittance of multilayer films. The ITO multilayer with 10 nm of Au layer had the maximum figure of merit or it has condition was the best performance of multilayer film among other samples. The effect Mg doping on band gap of the ZnO films has been investigated in third part. The starting solution of the prepared films was 0.2 M of Zn(CH3OO)22H2O with Mg doping in the range of 0-9 at.% . All films showed hexagonal wurtize structure of ZnO and highest transmittance of more than 80%. A shift of absorption edge in transmittance spectra of the prepared films occurred with addition Mg doping. These the shift behavior has significant effect on the increase of band gap in ZnO films. While Mg2+ ions have not changed conductivity in ZnO films due to the partial substitution of Zn2+ ion by the same valence Mg2+ ion. The 9 at.% Mg doped ZnO film deposited on glass substrate heated at 400°C showed the highest band gap of 3.364 eV. In the fourth part, Mg doped ZnO film was synthesized from lower concentration of 0.02 M of Zn(CH3OO)22H2O with Mg doping in the range of 0-20 at.%. All films showed hexagonal wurtizte structure of ZnO and high transmittance of the films was higher than 80%. Mg doped ZnO film from this concentration showed homogeneous film with small grain size and has wide band gap of 3.550 eV at 20 at.% Mg doping. While Mg2+ ions have not changed conductivity in ZnO films due to the partial substitution of Zn2+ ion by the same valence Mg2+ ion. In final part, effect of In on electrical properties of Mg doped ZnO films were investigated. All films showed hexagonal wurtize structure of ZnO film and high transmittance more than 80%. The band gap of these films decreased with addition of In doping. While the electrical properties of Mg doped ZnO films improved by In doping. However, the minimum resistivity was presented in 4 at.% In doping. This behavior could be resulted from the solubility limits of In3+ into ZnO lattice. The wide band gap of these conditions could be achieved at 3.403 eV, this range was suitable for application in solar cell.