| The bandgap value of ZnIn2S4 was determined to be 2.4-2.8eV by UV-Vis absorption measurement. Powder X-ray diffraction and SEM-EDS were also employed for the analysis of composition and lattice structure. Multiple dyes were tested to sensitize ZnIn2S4, among which MB (methylene blue) and DTTC (diethylthiatricarbocyanine) work the best. The absorption peak of MB is too close to the ZnIn2S4 band gap, therefore DTTC was chosen for sensitization. The Langmuir isotherm for DTTC dye absorption was measured to further investigate the interaction between dye molecules and crystal surface.;Cu2ZnSnS4 (CZTS) is a promising candidate for low-cost thin film solar cells, due to its bandgap of 1.4-1.5eV, which falls in the range of optimal bandgap value for a solar cell. Cu2ZnSnS 4 also has a large absorption coefficient of greater than 10 4 cm-1. All constituents of Cu2ZnSnS 4 are earth abundant and non-toxic. The efficiency achieved for a Cu 2ZnSnS4 thin film solar cell has increased from 0.66% in 1996 to 9.8% in 2010 by scientists at IBM. Cu2ZnSnS4 has been called a forgiving material as high conversion efficiency has been reached in a relatively short time. In previous research, zinc-rich and copper-poor Cu2ZnSnS4 films were found to have higher conversion efficiencies than the stoichiometric films. However very little research has been conducted on the solid-state chemistry of the material. As crystals are more ideal systems than thin films (no grain boundaries, fewer impurities), basic research on lattice structure vs. composition trends can be more easily conducted on crystals. We have used the vertical Bridgeman crystal growth method to prepare Cu2ZnSnS4 crystals. The semiconducting properties of the obtained crystals were investigated using photoelectrochemical methods. The quantum yields for photo-generated carrier collection as well as the spectral dependence of the quantum yields was measured. Powder X-ray diffraction, SEM-EDS, and Raman spectroscopy were also employed for the analysis of composition and lattice structure. In addition photoelectrochemical techniques were used to characterize the semiconducting properties of flux grown Cu 2ZnSnS4 crystals obtained from our collaborators at DuPont. |
