| Solar energy and waste heat can be converted into electricity by using solar cells and thermoelectric modules, respectively. Larger-scale commercialization of these deices are limited by their efficiencies and fabrication costs. The simplicity in fabrication process and availability of the materials are critical to the low-cost and high-performance devices. For some of I2-II-VI-VI4compounds, which are consisted of earth abundant elements, they have been proven to show promising applications in future thin film solar cells as light absorbance materials. However, the well-developed fabrication processes either generate high costs or have adverse effect on environment. Solution-based synthesis and processing of colloidal nanocrystals appears to be a low-cost approach. Furthermore, with intentionally introduced nanostructures, the final materials would benefit from enhanced performances, e.g. nanostructured bulk material will show depressed thermal conductivities and thus higher thermoelectric conversion efficiency can be expected. The syntheses of colloidal nanocrystals serve as the foundations to the above concept, and this is the major motivation of this thesis paper. The whole paper is organized as the following contents:1. Due to the promising applications in low-cost and high performance photovoltaic and thermoelectric devices, there is a booming development of syntheses of colloidal Ⅰ-Ⅲ-Ⅵ2(Ⅰ=Cu, Ag; Ⅲ=A1, Ga, In; Ⅵ=S, Se, Te) and Ⅰ2-Ⅱ-Ⅳ-Ⅵ4(Ⅰ=Cu, Ag; Ⅱ=Zn, Cd; Ⅳ=Si, Ge, Sn; Ⅵ=S, Se, Te) nanocrystals during the past ten years. In this section, we aim to comprehensively summarize recent developments of Ⅰ-Ⅲ-Ⅵ2, Ⅰ2-Ⅱ-Ⅳ-Ⅵ4nanocrystals, where we focus more on synthetic procedures towards controlled phases, compositions, morphologies and bandgaps. Combining with the already achieved theoretical understandings, we try to find some common features among different synthetic principles, which might be extended to synthesize other Ⅰ-Ⅲ-Ⅵ2and Ⅰ2-Ⅱ-Ⅳ-Ⅵ4nanocrystals with wanted phases, bandgaps and morphologies. We also highlight some of the latest advances in applications of these nanocrsytals in photovoltaic and thermoelectric devices.2. Wurtzite-derived Cu2ZnSn(S1-xSex)4alloys are studied for the first time through combining the theoretical calculations and the experimental characterizations. Ab initio calculations predict that wurtzite-derived Cu2ZnSnS4and Cu2ZnSnSe4are highly miscible, and the bandgaps of the mixed-anion alloys can be linearly tuned from1.0to1.5eV through changing the composition parameter x from0to1. A synthetic procedure for the wurtzite-derived Cu2ZnSn(S1-x Sex)4alloy nanocrystals with tunable compositions has been developed. A linear tunable bandgap range of0.5eV is observed in the synthesized alloy nanocrystals, which shows good agreement with the ab initio calculations.3. We report a colloidal approach for synthesizing non-branched but linearly arranged polytypic I-II-IV-VI nanocrystals, with a focus on polytypic non-stoichiometric Cu2ZnSnSxSe4-x nanocrystals. Each polytypic non-stoichiometric Cu2ZnSnSxSe4-x nanocrystal is consisted of two zinc blende-derived ends and one wurtzite-derived center part. The formation mechanism has been studied and the phase fraction can be tuned through adjusting the reaction temperature, which brings a new band-gap tuning approach to Cu2ZnSnSxSe4-x nanocrystals.4. We report a solution based synthesis of mono-dispersed Cu2CdSnSe4nanocrystals and a study on thermoelectric properties of this wide-band-gap dense materials compacted from nanocrystals for the first time. With the help of copper dopants and selenium vacancies generated during wet chemistry synthesis, a large increment in the power factor is observed and the dimensionless figure of merit ZT reaches a peak value of0.65at450℃.5. An up-scaled synthesis of non-stoichiometric Cu2ZnSnSe4colloidal nanocrystals has been demonstrated. The obtained non-stoichiometric Cu2ZnSnSe4pellets made from the colloidal nanoparticles show similar ZT values as the state-of-the-art Cu2ZnSnSe4-based bulk materials at similar temperatures.6. We discuss about some of our preliminary results and future plans, such as systematic study of other multinary chalcogenide nanocrystals with metastable phases, syntheses of other polytypic nanocrystals and how to improve the thermoelectric properties. |
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