Thermally Activated Sulfurization Scheme for CuInS2 Solar Cells
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Chalcopyrite Solar Cells are now being fabricated on flexible (polymer) substrates. However, CuInS2 solar cells have not been fabricated on the polymer substrates on the commercial scale due the sulfurization temperature which limits the use of polymer substrates during fabrication. To overcome this problem a thermally activated sulfurization process for the manufacture of CuInS2 solar cells from the Cu-In bilayers is described in this work. This sulfurization process can be employed for lower temperature for the formation of the absorber layer. For the sulfurization, the Cu-In bilayer with different compositions is prepared by thermal evaporation in which the In-rich film shows the presence of high CuIn2 phase while the Cu-rich film shows the presence of high Cu11In9 phase. The films were sulfurized in a home built reactor. In this reactor, the high molecular weight sulfur molecules are broken down into smaller rings at high temperature (600, 700 and 800oC) and then reacted with the Cu-In bilayer at lower temperature (500oC). The compositional analysis shows In loss during sulfurization which is expected due to the formation of In2S2 formation; and the EDS analysis shows the possible presence of Cu-S compound formation which are etched using the KCN solution. After etching the film shows more stoichiometric concentration of Cu, In and S elements. CuInS2 film is prepared. In5s4 compound also forms in the film during sulfurization process. The deposition method of CdS layer through chemical bath deposition has been discussed. The CdS layer forms the n-type layer for the p-n junction CuInS2 solar cell. The details of the setup are discussed and the film is characterized for its transmittance using UV-VIS spectrometer. 90% transmittance is observed for approximately 50nm thick CdS film on glass. Indium tin oxide layer is studied as the windows layer for the CuInS2 solar cell. Its properties are optimized using the sputtering equipment. Films with sheet resistance below 20 Ω/square have been prepared with the percent transmittance close to 90%.