Development of high band gap high efficiency photovoltaic device

Thin film solar cells based on wide gap semiconductors with energy band gap of 1.7 eV are of special interest with respect to the tandem cell structures. Top and bottom cells with respective band gap energies of 1.7 eV and 1.0 eV are the basic requirements for optimum tandem cell efficiency. CIGS with low Ga content having band gap of 1.0 eV meets the critical band gap requirement of the bottom cell and has already been developed to acceptable efficiency levels for this application. However, the development of high band gap top cell with 16--18% efficiency is essential to realize the ultimate 25% tandem efficiency.; Because of its direct bad gap of 1.72 eV, CdSe ideally fulfills basic requirement of being the absorber of the top cell in a tandem. It also offers the advantage of avoiding complex materials processing of other potential ternary compounds that meet the band gap requirement. In developing the top cell, inexpensive Close Space Sublimation process was established to deposit high electronic quality CdSe absorber on transparent substrates. Of various device structures evaluated in this study, SnO2/CdSe/ZnSe/Cu has shown the potential in achieving the eventual high efficiency of the top cell.{09}The current densities exceeding 17 mA/cm2 have been attained without correction for optical losses at Cu front contact.; Limited open circuit voltages of these devices (<350 mV) were found to be due to low effective contact energy of ZnSe/Cu layer. Use of Au in place of Cu resulted in substantial losses in current densities despite the improvement in Voc. Replacement of ZnSe by ZnTe was also found to be deleterious to Jsc's although achieved Voc's were considerably higher. These results have revealed the importance of achieving p-type conductivity in ZnSe for further improvement of the device performance. Doping studies with the use of excited nitrogen as a p-type dopant demonstrated the incorporation of sufficient nitrogen levels in ZnSe. However, the layers did not show any evidence of conductivity in ZnSe films suggesting the need of extensive investigation on further development of this layer.