Preparation of a CuInS2 Nanoparticle Ink and Application in a Selenization-Free, Solution-Processed Superstrate Solar Cell

We report a study on chalcopyrite solar cells fabricated by low-cost, nonvacuum, and selenization-free methods. Super-strate-type CuInS2 (CIS) thin-film solar cells were prepared by sequential ink deposition. The CIS film was formed from a stable low-carbon ink, which was synthesized at low temperature (<20 degrees C). The CIS nanoparticle ink was prepared with n-butylamine and acetic acid as the solvent and stabilizer, respectively. The viscous and stable ink that formed through the dispersion of the final nanoparticles in N,N-dimethylformamide (DMF) could be deposited readily onto the substrate. The major features of the obtained ink are the small amount of impurity phases and negligible carbon residue (ca. 2.8%). The obtained ink was used for the fabrication of superstrate solar-cell devices with a glass/FTO/TiO2/In2S3/CIS/carbon (FTO = fluorine-doped tin oxide) structure. The performance of the solar cells was improved by optimizing different synthetic and geometric parameters, including the sulfur source, reaction time, reaction temperature, buffer-layer thickness and second annealing temperature. The best cells were obtained by using inks with thiourea as the sulfur source, and a high current density of 21 mA/cm(2) and 5.2% efficiency were obtained. This current density is the highest among those reported for solution-based superstrate CIS solar cells and is comparable to those of solar cells fabricated with a selenization step at high temperature. The ink is stable, and the efficiency of the device fabricated with ink aged for 100 d is ca. 12% less than that of the device fabricated with fresh ink.