Morphology Modulating Vibrational Property Of Cu₂ZnSnS₄Nanostructures

The optoelectronic properties of Cu2ZnSnS4(CZTS) based thin film solar cells are stronglydependent on crystalline structure, lack of stoichiometric control, impurities, surface defects,secondary phases and morphology of CZTS, etc. These factors will affect the vibrational propertyof CZTS. In order to further improve the optoelectronic performance, therefore it is of vitalimportance to know more about the vibrational property of CZTS. There has been no report onmorphology modulating Raman spectroscopy about CZTS nanocrystals, to the best of ourknowledge. In this thesis, we prepare three morphologies of CZTS nanocrystals via three differentmethods and investigate the relationship between morphology and vibrational property of thoseCZTS nanostructures. We expect that the vibrational property of CZTS nanostructures can bemodulated via changing morphology. The main content is as follows:1. High quality CZTS nanoparticles have been synthesized via solvothermal route inethylenediamine using CuCl, ZnCl2, SnCl4·5H2O and S powers as reaction reagents. Theas-synthesized products are characterized by X-ray powder diffraction (XRD), X-rayphotoelectron spectrometer (XPS), field emission scanning electron microscope (FE-SEM),high-resolution transmission electron microscope (HRTEM) and electron diffraction pattern(SAED). The results confirm that CZTS nanoparticles display an agglomeration phenomenonwith average diameters of10-20nm.2. CZTS nanorods (NRs) have been prepared via a simple spin coating route using dimethylsulfoxide (DMSO) as a solvent. Firstly, a coating solution is prepared by dissolvingCu(CH3COO)2·H2O, ZnCl2, SnCl2·2H2O and thiourea into DMSO until a clear, light yellow solution is obtained. Subsequently, the precursor solution is spin-coated onto a molybdenumcoated soda-lime glass (Mo/SLG) substrate at700rpm for10-20s and then at1400rpm for20-30s. After coating, the sample is annealed at420oC under air. This coating and annealingprocesses are considered as one cycle. This cycle is repeated seven times to obtain CZTSNRs. The as-synthesized products are characterized by XRD, XPS, FE-SEM, HRTEM andSAED. The results show that most of CZTS NRs are nearly perpendicular to the substrateand some are upright to substrate at a certain angle. CZTS NRs have diameters from20to200nm and lengths from several hundred nanometers to about2m. In addition, theinfluences of annealing temperature, cycle time, precursor solution concentration androtational speed on the formation of CZTS NRs are investigated systematically.3. ZnO/CdS/CZTS heterostructure NR arrays have been successfully fabricated via asolution-based method. Based on our previous work, ZnO NR arrays are firstly prepared viahydrothermal route. Next, ZnO/CdS core/sheath NR arrays are synthesized via SuccessiveIonic Layer Adsorption and Reaction method. Then, the substrate with ZnO/CdS NR arrays isdipped into the precursor solution by dissolving Cu(CH3COO)2·H2O, ZnCl2, SnCl2·2H2O andthiourea into DMSO for a few minutes, and followed by annealing in N2atmosphere.Accordingly, ZnO/CdS/CZTS heterostructure NR arrays are successfully fabricated. Theas-synthesized products are characterized by XRD, XPS, FE-SEM, HRTEM and SAED. Theresults confirm that ZnO/CdS core/sheath NR arrays are covered by CZTS nanoparticleslayer of about10nm. The inter-planar spacing of0.317nm corresponds to the (112) plane ofCZTS. The corresponding SAED indicates that CZTS is polycrystalline, though the ZnO coreand CdS sheath are single crystalline.4. The morphology modulating vibrational property of CZTS nanostructures has beeninvestigated by Raman spectrum. All of room temperature Raman spectrum of CZTSnanoparticles, CZTS NR and ZnO/CdS/CZTS heterojunction NR arrays display one strongpeak, located at329,330and336cm-1, respectively, which can be assigned to the A mode ofCZTS. No other secondary phases or impurities can be observed, indicating the CZTSsamples are single phase. The frequency and the full widths at half-maximum (FWHM) of Amodes of three morphologies CZTS have a red shift and broadening in comparison with thatof CZTS thin film. The results demonstrate that the vibration of the A mode of CZTS is muchsensitive to the morphology than to the size, indicating that the vibrational property of CZTScan be modulated by its morphology. In order to explore the reason of the red-shift andbroadening of A mode, the relationship between A mode of CZTS nanoparticles, CZTS NRand ZnO/CdS/CZTS heterojunction NR arrays and temperature is studied. The resultsindicate that the red-shift of the A mode with increasing temperatures mainly results from the anharmonic coupling to other phonons. Combining with the phonon lifetime, the reason ofbroadening FWHM of A modes is:1) the3-phonon damping process at lower temperature (<280K), and2) the3-phonon and4-phonon damping processes play a combined role whentemperature is higher than280K. The Hall effect measurement indicates the as-preparedCZTS is p-type semiconductor.