| Structures consisting of CuInSe2 films, deposited on molybdenum-coated glass substrates and covered by CdS or ZnO thin films, for application in solar cells, have been characterized by spectroscopic ellipsometry, atomic force microscopy, photoluminescence and photovoltaic response. Computer aided simulations of the photovoltaic operation of these structures have been made to reinforce and complement the results of the characterization studies.; The optical properties of CuInSe2, CdS and ZnO films over ranges of photon energies from 1.0 through 4.0 eV were determined. The measured optical bandgaps, 0.96 eV for CuInSe2, 2.35 eV for CdS and 3.33 eV for ZnO, are in good agreement with the values reported in the literature for bulk samples of those materials. In the case of CuInSe2, a second transition photon energy was detected at 1.02 eV. This transition was assumed to be due to gallium content in the CuInSe 2 film near the Mo interface.; Depth structure profiles of the films were generated. The one for as-deposited CuInSe2 was found to include a surface layer (ODC layer) about 230 nm thick which exhibited optical properties considerably different from those of the bulk of the film. In the case of US (as might be expected for films grown by the chemical bath deposition technique) it was found that the films consist of two layers: a very compact one, about 29 nm thick, at the bottom of the film; and a much thicker and porous one, about 127 nm thick, on top.; CdS/CuInSe2 and ZnO/CuInSe2 test cells were fabricated from substrates with as-deposited CuInSe2 and CuInSe2 from which a layer had been removed by etching. Test cells with CdS buffer layers on etched CuInSe2 exhibited degraded performance compared to cells fabricated on as-deposited CuInSe2. These results suggest that some interaction between US and the absorber material occurs in the case of etched absorbers. Thus, CdS may play a greater role than simply providing protection for the absorber layer during deposition of the TCO (top contact) layer.; Photoluminescence measurements from plain CuInSe2 films, compared with similar measurements taken for CdS- or ZnO-coated films, showed an increase in the photoluminescence emission in the later cases, indicative of a reduction of nonradiative recombination caused by a passivating effect of the buffer layers on the interface.; Numerical simulations of the operation of CuInSe2-based solar cells disclosed a mechanism by which the resistivity of the buffer layer may enhance the performance of the cells, and one that would promote inflected current-voltage characteristics. A comparison between the simulated performance of devices with experimentally determined results revealed the effects of a process-related connection between the resistivity of buffer layers and the electronic properties of the absorber, which have an impact on the conversion efficiencies of solar cells. |
