Transient absorption (TA) and photoluminescence spectroscopy have been performed on spray-deposited CuInS2 thin films. Sulfur and indium vacancies introduce electronic states in the bandgap located at 1.5 and 0.15 eV above the valence band, respectively. Deep donor and deep acceptor doublet states at 1.1 and 0.2 eV are assigned to copper/indium antisite defects. The excited-state dynamics, which are derived from TA experiments, show electronic coupling between indium antisite defects and indium vacancies. The Shockley, Read, and Hall recombination model has been modified to account for this coupling and to simulate the TA results. Furthermore, the lifetime of the 1.1 eV state is found to be 20−50 μs, which is related to the low photovoltage of CuInS2 based solar cells.
Spray deposition of CuInS 2 offers an attractive route towards industrial production of thin-film solar cells. With spray deposition it is possible to make nanocomposites of n-type TiO 2 and p-type CuInS 2 . Upon application of an In 2 S 3 buffer layer, solar cells can be made with efficiencies of ∼7%, being comparable to that of amorphous silicon. Rapid thermal annealing is not involved in the production of these solar cells. In order to further improve the performance, the concentration of electronic defect states in the bandgap must be reduced. Towards this end a detailed study has been undertaken to elucidate the role of associated point defects in the recombination of electron-hole pairs. Especially with transient absorption spectroscopy it is possible to make an accurate assessment of the fundamental electronic processes that are involved. We find electronic states in the bandgap related to the presence of anti-site defects. In addition, indium vacancies are also involved. State-to-state recombination occurs, indicating that the involved defects are associated. An electronic state located at 1.1 eV above the valence band, which is related to indium on a copper position, has a lifetime of about 20 μs at room temperature. The lower lying states related to copper on indium positions, and indium vacancies, are populated from this 1.1 eV state.
Time-of-Flight (TOF) measurements have been performed on n-type TiO2/p-type CuInS2 heterojunctions. The TiO2 film thickness has been varied between 200 and 400 nm, while the CuInS2 film thickness has been fixed at 500 nm. The TOF response can be accurately modeled, if the potential drop across the p-n heterojunction with a large density of interface states is properly accounted for. Also electron transport in a space-charge region for a not fully depleted semiconductor has to be considered. The electron mobility in TiO2 is found to be 10−2 cm2 V−1 s−1, independent of the TiO2 layer thickness. The interface-state densities are 5×1011, 2×1012, and 6×1012 eV−1 cm−2 for 200, 300, and 400 nm thick TiO2 films, respectively.
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