Thermally Activated Sulfurization Scheme for CuInS2 Solar Cells
Abstract
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%.