Preparation And Characterization Of Materials For CZTS-Nanocrystal Thin-film Solar Cells

Cu₂ZnSnS₄（CZTS）, with an optimal direct band gap of ca.1.5eV and a large absorptioncoefficient over104cm-1, is one of the most promising materials as the absorber layers of thin-filmsolar cells. At present, the absorbing layer of the highest conversion efficiency CZTS photovoltaicdevice is prepared by vacuum deposition method. But this method imposes an additional cost onthe technology. And the synthesis of CZTS absorbing layer by colloidal chemistry method canreduce the cost effectively. So CZTS absorbing layer was prepared by the hot-injection method andthe sol-gel method in this thesis. The best preparation parameters were studied, the advantage anddisvatages of the two colloidal chemistry methods were also discussed. Then the preparation ofZnO:Al （AZO） window layer and ZnS buffer layer were investigated.In chapter one, the basic theory of solar cell was present. Then, the fabrication technology ofCZTS absorbing layer materials and its thin film solar cells were reviewed. At last, the maincontent, research ideas and significance of the dissertation were presented.In chapter two, CZTS nanoparticles were synthesized via the hot-injection processing, andcharacterized by various measurement methods, then nanoparticles thin films were prepared bydrop-casting and dip-casting. The two methods were compared, and the differences betweensintered film and unsintered film were also discussed. The X-ray diffraction spectra ofnanoparticles indicates that the samples synthesized under240°C,250°C and260°C are cubicCZTS nanocrystals, but a new wurtzite CZTS was obtained when the temperature raised to275°C.TEM pictures of nanoparticles show that the morphology of CZTS nanocrystals was spherical.These nanoparticles were well-dispersed, among them the nanocrystals synthesized under260°Cshow the best uniformity. But the nanocrystals synthesized under250°C show the nanoparticlesaggregation occurred. And the size of nanoparticles is ca.60nm. Raman scattering spectra furtherconfirm that the CZTS nanoparticles were obtained, since Raman spectra can distinguish thedifferences between ZnS, Cu₂SnS₃and CZTS effectively. The UV-Vis-NIR spectra demonstratedthat the CZTS nanocrystals have an optical band gap of1.5eV, which is optimal for photovoltaicapplication. SEM pictures show that nanoparticle films prepared by drop casting method wereporous, and these holes can be eliminated effectively by the dip casting method. The particle size of the film prepared by drop casting method grow big after sintering and the CZTS nanorodsappeared. EDS spectrum demonstrated that the sulfur of CZTS will lost after sintering.In chapter three, CZTS thin films were prepared by sol-gel method and their properties wereinvestigated. X-ray diffraction pattern shows that there were no clear peak can confirm the CZTSwere obtained. But the Raman spectrum demonstrated that the CZTS thin films were synthesized.The UV-Vis spectra indicated that the CZTS nanocrystals have an optical band gap of1.5eV. Thesurface SEM pictures of CZTS thin films show that the films were porous and this porous structurecould be reduced with the raise of sintering temperature. The cleavage SEM images of the CZTSthin films show that the films were composed by bulk polycrystal, and the thickness of the film is2.21μm.In chapter four, transparent conductive ZnO:Al （AZO） films were prepared by sol-gel methodand deposited on soda lime glass substrates via dip coating method, and the effect of different Al3+doping amounts and sintering temperature on these films were investigated. X-ray diffractionpattern shows that the peak of ZnO will decrease with the raise of doping amounts of Al3+. Ramanscattering spectra show two main peaks which all corresponded to the ZnO characteristic peaks.SEM pictures show that the size of particles which the films were composed will grow big with theraise of sintering temperature. The thickness of every layer prepared by dip coating method is40-50nm on average.In chapter five, the ZnS buffer layer of CZTS solar cell were prepared by chemical bathdeposition, and the optical properties of these films were investigated.Finally, results of the present work are summarized, and it is pointed out that somethingshould be done in the next stage.