Synthesis, Characterization And Photoelectrical Properties Of Tin Chalcogenide Semiconductor Nanomaterials

Compared with other semiconductor materials, tin chalcogenide semiconductor optoelectronic materials own many advantages, such as large absorption coefficients, adjustable band gap, earth-abundant, low-toxicity and unique photoelectric properties, and have garnered considerable interest around the world due to the applications in solar cells, lithium ion battery, photocatalysis and so on. Hence, the studies of tin chalcogenide semiconductor were carried out including their preparation method, characterization of phase structure, morphology and photoresponse properties and the photoelectric applications. The detailed discussion is presented in this paper as follows:In the first part, a phase-controlled synthesis of well-defined tin sulfides, including SnS, SnS2and SnS/SnS2heterostructured materials, was developed in a large scale by one-step pyrolysis method, where the oleylamine (OM) was used as a solvent, tin diethyldithiocarbamate (Sn-DEDTC) and carbon disulfide (CS2) as reactants, respectively. After the decomposition of the precursor at320℃for10min, the nanomaterials were obtained. Such a phase-controlled process was realized by varying the amount of CS? in the precursor, in which pure SnS (～1-2μm in width,～100nm in thickness) and SnS2(～200nm in length,～20nm in thickness) nanoparticles were obtained upon no CS2involved or CS2excessively existed, while SnS/SnS2heterostructured material formed by adding a suitable amount of CS2to the precursor.A possible mechanism of SnS, SnS2and SnS/SnS2heterostructures are also investigated by a series of temperature-dependent and time-dependent experiments. The formation processes of the materials via following steps:firstly, SnS nanoparticles (4-6nm) are formed at a low temperature (280℃) due to the decomposition of as-obtained precursor consisting of Sn, and then, nanoparticles aggregated into tetragonum nanoflake through a nucleation and growth process of SnS nanoflakes under pyrolytic conditions. Alternatively, SnS2nucleis are produced (260℃) and further grew into hexagonal SnS2nanoflakes upon the introduction of excessive CS2. Obviously, to minimize high-energy surfaces, the as-formed SnS or SnS2nanoparticles undergo a "nucleation-dissolution--recrystallization" process and then aggregated into a larger size, following the rule of Ostwald ripening. Furthermore, the suitable addition CS2could result in SnS/SnS2heterostructures.The typical UV-vis-NIR absorption spectra show that SnS and SnS2nanomaterials present a strong and broad UV-vis-NIR absorption band ranging from300to800nm and possess band gap (Eg) of1.42and2.04eV, respectively. Particularly, SnS/SnS2heterostructured materials show a higher intensive absorption comparing with SnS and SnS2within the scope of visible range. Furthermore, a "red shift" phenomenon can be observed, which might be attributed to the charge transfer between SnS and SnS2nanoflakes. The photocatalytic activities of as-prepared nanomaterials are evaluated by the photocatalytic degradation of RhB under the simulative solar light (300W) at room temperature. All of the samples show reversible adsorption abilities to RhB with small surface areas (11.9639m2/g. 7.1208m2/g,19.7104m2/g), implying that OM ligands are playing a certain role on it. When the illumination begins, SnS2nanomaterials can clear up the solution within90min, while SnS does not remove any RhB, and SnS/SnS2heterostructured nanlmaterials do not show enhanced photocatalytic performance on photodegrading organic dyes (just about90%) in despite of their excellent photoresponse property. There may be two reasones for this result:One is that the photoproduction electronics were prevented from transfering to organic dye with the presence of surface ligand. Second is that the aggregation of flakes may result in a small contact area between nanoflakes and organic dye molecules.In the second part, pure and high crystallinity Cu2ZnSnS4(CZTS) nanoparticles prepared by pyrolysis method, have been used for fabricating thin film solar cells combined with "NCs inks" and roll-to-roll printing techniques, where the oleylamine (OM) was used as a solvent,(CH3COO)2Cu·H2O,(CH3COO)2Zn·2H2O, SnC2O4and S as reactants, respectively. The typical UV-Vis-NIR absorption spectras show that both the Cu2ZnSnS4nanoparticles and films present a strong and broad UV-vis-NIR absorption band ranging from300to800nm and possess a band gap (E8) of-1.48eV. Researches show that Cu2ZnSnS4based solar cells with the structure of SLG/Mo/Cu2ZnSnS4/CdS/i-ZnO/ITO/Al-Ni have big positive spread current and small reverse current drift, suggesting that the typical characteristics of p-n heterojunction, i.e., one-way electrical conductivity. Photocurrent density/voltage characteristics of the resulting solar cells were measured under standard AM1.5illumination with a solar intensity of80mW/cm2. The as-fabricated cells exhibit open-circuit voltage (Voc) of160mV, short-circuit current density (Jsc) of1.2mA/cm2, and the fill factor (FF) of0.24, yielding an overall solar-to-electric energy conversion efficiency (η) of0.0576%. Further improvement of the conversion efficiency can be expected by optimizing the morphology, structure and composition of the Cu2ZnSnS4NCs or films, as well as the device fabrication technique.In the third part, excellent Cu2ZnSnS4thin film solar absorb materials are synthesized on substrates, by using in-situ growth method, where the pyridine was used as a solvent, Cu(C5H7O2)2, Zn(C5H7O2)2, SnC2O4and S as reactants, respectively. The compositions of the as-obtained films were monitored by ICP and EDS. proving that it can be tuned by changing the sulfurization conditions and the composition as Cu1.88Zn1.12Sn1.03S4.12close to the ideal Cu2ZnSnS4solar cell can be prepared in this easy method. UV-Vis-NIR absorption spectras indicate that the as-prepared Cu2ZnSnS4film absorb through the entire visible (300-800nm) and possess a band gap (Eg) of～1.44eV. And an excellent p-n heterojunction conductive property of the thin film solar cells was confirmed by the test of semiconductor properties. The as-fabricated device exhibits open-circuit voltage (Voc) of98mV, short-circuit current density (J(?)c) of0.315mA/cm2, and the fill factor (FF) of0.28, yielding and overall solar-to-electric energy conversion efficiency (η) of0.011%. Further improvement of the in-situ prepared method should be taken out to enhance the conversion efficiency of the devices in the future.