| Efficient supply of energy resources to the development of economy.has become a bottleneck of modernized world. In other words, an increasingly modern society relies on steady supply of energy. Currently, approximately85%of the global energy consumption is met by burning fossil fuel. However, the depletion of non-renewable fossil fuel is a well-known global challenge. What’s more, the burning of fossil fuel leads to global warming and other environmental problems. To address this issue, alternative, renewable means of energy supply have been studied. Among them, solar energy as an abundant, free energy source stands out and has therefore attracted intense attention for many years.To better utilize solar energy, we not only need to come up with methods to convert it into other energy forms with high efficiencies, but also need to devise technologies that can store solar energy for off-hour usage.In this dissertation, various methods are explored to realize the functional-preparation of nanostructured CZTS or Si semiconductor materials and modulate novel properties, accompanied by understanding the structural and functional correlations of separate constituents. A series of multi-functional nanosructure unit are successfully synthesized, and their physical or chemical properties are systematically studied.1. Band-gap Tunable (Cu2Sn)x/3Zn1-xS Nanoparticles for Solar CellsThere are several important demands for further photovoltaic materials. Firstly, toxic elements such as cadmium should be avoided so that the materials are environmentally benign; Secondly, materials should be comprised of abundant and inexpensive elements and prepared via cost-effective routes to reduce the cost of solar cells; furthermore, tunable band gaps are also required for materials to maximize solar absorption, so that the resulting solar cells would fully use the energy of photons and have remarkable energy conversion efficiencies.To meet the requirement, here we report the synthesis of composition-adjustable (Cu2Sn)x/3Zn1-xS (0<x<0.75) nanoparticles by a facil solvothermal method. Judging from the HRTEM and XRD analyses, the prepared nanoparticles have the same cubic structure. Absorption spectra of the nanoparticles in toluene exhibit distinct onsets changing from350nm to1100nm by adjusting the chemical composition, which correspond to tunable band gaps from3.48eV to1.23eV.By applying the as prepared nanoparticles to quantum dot sensitized solar cells (QDSSCs), considerable potential in photoelectric conversion is revealed from their implementation in QDSSCs.2. Porous Copper Zinc Tin Sulfide Thin Film as Photocathode for Double Junction Photoelectrochemical Solar CellDye sensitized solar cells (DSSCs), are believed as one of the promising cost-effective alternatives to conventional solar cells. Traditional DSSCs are single junction solar cells, which are fabricated with a dye sensitized n-type semiconductor photoanode (n-DSSC) or a dye sensitized p-type semiconductor photocathode (p-DSSC) and a counter electrode. Recently, p-DSSCs and n-DSSCs are combined to fabricate double junction solar cells (pn-DSSCs) with a theoretical efficiency limitation well beyond that of single-junction DSSCs. N-DSSC using dye sensitized n-type TiO2(DS-TiO2) as photoanode has obtained a considerable efficiency. However, it is a great challenge choosing suitable dye and p-type semiconductor for the photocathode. Nanostructured p-type semiconductor thin film with narrow band gap might meet the demand of photocathode for double junction solar cells. Traditional p-type semiconductors, such as CdTe and CuIn1-xGaxSe2(CIGS), however, are not amenable for eventual production due to the scarcity and cost of Te, In, and Ga. Composed of earth abundant elements, Copper Zinc Tin Sulfide (CZTS) has recently been made a focus of attention in photovoltaic field. Actually, this p-type semiconductor exhibits a large absorption coefficient(>104cm-1) and a suitable direct band gap (Eg=1.4-1.5eV) which could match the range of solar irradiation.We convert the traditional DSSCs to double junction photoelectrochemical cells using porous CZTS thin films. Firstly, we successfully synthesized porous CZTS thin films on flexible Mo foils via a facile template-free solvothermal approach. Composed of interconnected CZTS nanocrystallites with a diameter of20-40nm, the film exhibits high surface area and porosity compared with traditional CZTS film. As a result, there shows quite low reflectivity in the full spectra at a range of400-1600nm, indicating effective light trapping. Then we fabricated photoelectrochemical cells with double junctions using the porous CZTS thin film:dye sensitized n-type TiO2(DS-TiO2) as the photoanode and porous CZTS film as the photocathode. The solar cell shows increased short circuit current (22%) and conversion efficiency (7%) compared with solar cells using traditional Pt counter electrodes. External quantum efficiencies (EQEs) of the solar cells prove that the porous CZTS act as a light absorber as well as a counter electrode. Actually, this study successfully provided a new photocathode for low-cost and high-efficiency double junction photoelectrochemical cells.3. Solar Hydrogen Generation by Si Nanowires with Atomic Layer Deposition Pt Nanoparticle CatalystsAs a material candidate for next generation’s solar energy conversion applications, Si nanowires (SiNWs) are easy to prepare, more tolerant to impurities than planar Si, and have attracted significant research attention. Because the conduction band edge of Si (～4.05V vs. vacuum) is more negative than the water reduction potential (-4.51V vs. vacuum at pH0), there has been particular interest in using Si for the production of solar hydrogen. One important challenge in doing so is the application of hydrogen evolution reaction (HER) catalysts, which are necessary as the exchange current density of Si for H2production is insufficient to sustain a photocurrent that matches the solar flux without the need for large overpotentials.Existing catalyst deposition techniques are typically not optimized for high aspect ratio structures like SiNWs. They are either limited by their line-of-sight nature (e.g., evaporation) or diffusion in solution (e.g., electrochemical or electroless deposition) and tend to produce a non-uniform catalyst distribution, with most catalyst particles aggregated on the tips of the NWs, where the space is most accessible for catalyst deposition. Such a catalyst profile is not ideal for solar H2production. In order for most photoelectrons whicharegenerated away from the tips of the NWs to reach the catalyst sites, a long charge diffusion distance is required. This requirement dictates that one needs extremely high purity and high quality crystals for high performance, leading to high cost of material preparation.Here we show that such a goal can be achieved by growing Pt nanoparticles using atomic layer deposition (ALD). Pt was used for this body of work because it is a HER catalyst with the highest activities. Indeed, the resulting SiNW/Pt materials enable high performance H2O reduction reactions. With a conformal catalyst coverage, the strategy also allows us to understand the design advantages and caveats of SiNWs for H2O reduction applications. |
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