In this thesis, nanostructures based on II-VI semiconductor ZnO and its derivant Cu₂ZnSnS₄ (CZTS) are studied for solar energy application. The morphologies, crystal structures, growth mechanisms of these materials have been investigated by scanning electron microscopy (SEM) and transmission electron microcopy (TEM). Great efforts have been devoted to their solar performances such as photoelectric conversion efficiency, optical property and charge transport.

Through a facile direct precipitation method, a new kind of ZnO hierarchical flowers (HFs) were synthesized by interlaced single crystalline ZnO nanosheets. The size of HFs, thickness and packing density of the nanosheets and HF inner porosity can be tuned by changing the synthesis conditions. The HFs containing porous in...[ Read more ]

In this thesis, nanostructures based on II-VI semiconductor ZnO and its derivant Cu₂ZnSnS₄ (CZTS) are studied for solar energy application. The morphologies, crystal structures, growth mechanisms of these materials have been investigated by scanning electron microscopy (SEM) and transmission electron microcopy (TEM). Great efforts have been devoted to their solar performances such as photoelectric conversion efficiency, optical property and charge transport.

Through a facile direct precipitation method, a new kind of ZnO hierarchical flowers (HFs) were synthesized by interlaced single crystalline ZnO nanosheets. The size of HFs, thickness and packing density of the nanosheets and HF inner porosity can be tuned by changing the synthesis conditions. The HFs containing porous inner structures showed an excellent performance as the photoanode material in quasi-solid (using polymer gel electrolytes) dye-sensitized solar cells (DSSCs) because of their superior optical and electrical properties. Especially, the electron diffusion coefficient of HF-based photoanode is nearly one order of magnitude higher than that in nanoparticle (NP)-based photoanode. Eventually, the high current density (10.26 mA cm^-2) and efficiency (4.93%) have been obtained from the HF-based DSSCs.

Based on the direct precipitation method, the structures of the ZnO HFs are improved by introduction of ultrasound irradiation, which can promote nucleation and accelerate diffusion in aqueous solution. The nanosheets on the HFs are not only interlaced and monocrystalline, but also axially oriented, porous and ultrathin. Through in-situ sampling and observation we interpret the HF formation processes using a new mechanism based on oriented attachment and reconstruction. The relationships between synthetic conditions and HF structures are then established. Structural improvements reveal that the specific area of the novel HFs as well as their performances on light-capturing and electron transport have been largely improved compared with those prepared through direct precipitation. Finally, high photoelectric conversion efficiencies of up to 6.42% were achieved. These results established a new record for quasi-solid ZnO-based DSSCs.

An asymmetric panel-like ZnO hierarchical architecture (PHA) has also been synthesized by the direct precipitation method. The two sides of PHA are differently constructed by densely interconnected, mono-crystalline and ultrathin ZnO nanosheets. By mixing these PHAs with ZnO NPs, we developed an effective and feasible strategy to improve the electrical transport and photovoltaic performance of the composite photoanodes in DSSCs. These highly crystallized and interconnected ZnO nanosheets on PHA largely minimized the total grain boundaries within the composite photoanodes and thus served as rapid pathways for transport and effective collection of free electrons. These novel composite photoanodes achieved high conversion efficiencies up to 5.59% for ZnO-based quasi-solid DSSCs.

Through a hot injection method, we have successfully synthesized zinc blende CZTS nanocrystals (cubic crystal system and space group F4̅3m) with randomly arranged cations of Cu, Zn and Sn. We observe an interesting solid-solid phase transformation of these nanocrystals at low temperature due to the cation rearrangements. The XRD and HRTEM studies indicate that the resulted CZTS phase is the Kesterite type (tetragonal and space group I4̅) with periodical arrangement of cations. When fabricated into thin film CZT(S, Se) solar cells, they show high short circuit current of 27.6 and 16.7 mA cm^-2 based on ITO and graphene electrode respectively, suggesting the great potential for this nanocrystal based CZTS solar cells.