Optical characterization of ITO films on fluorescent borate glasses for high efficiency solar cells

Solar cells with a cover glass have a poor response in the blue and near UV spectral range due to absorption in the glass and the front contact layer. The efficiency of these solar cells can be improved by replacing the top glass by a spectral converter, i.e. a frequency down-converter, without modifying the solar cell itself. Potential candidates are fluorescent, samarium-doped borate glasses. They convert the incident violet and blue part of the solar spectrum to visible red light, which is more efficiently absorbed by the solar cell. The optical properties of borate glasses with different network modifiers are investigated. Transmission spectra of undoped sodium, barium, and lead borate glass reveal significant differences in the absorption edge. Optical simulations were performed to determine the complex refractive indices of the glasses. In addition, the different borate glasses were used as substrates for deposition of a conductive indium tin oxide (ITO) film. The deposition is done by radio frequency magnetron sputtering. The layer thickness is subsequently analyzed by profilometry and scanning electron microscopy (SEM) of cross-section samples. Again, optical simulations provide the refractive indices of the ITO films. Apart from the optical parameters, the electrical ones such as resistivity and I-V characteristics are determined. Moreover, the influence of different sputter parameters, i.e. gas pressure, oxygen flow, and film thickness on the optical and electrical properties of the deposited films is investigated.