45317-G10
Functional Nanocrystal-Nanowire Composite Materials: Synthesis and Electron Transport Properties
The proposed research seeks to design and synthesize functional nanocrystal-nanowire composite materials, and to investigate their electron transport properties and applications as photoelectrodes in dye-sensitized solar cells. The advantage of the nanowire-nanocrystal hybrid architecture is to offer the capability of obtaining high surface area and fast electron transport simultaneously, which is desirable for improving the efficiency of dye-sensitized solar cells. In the past year, we have fabricated dye-sensitized solar cells based on the composites of anatase TiO2 nanoparticles and single crystalline anatase TiO2 nanowires. Three different composites were prepared with 5 wt%, 20 wt%, and 77 wt% nanowires, respectively. The performances of composite solar cells were compared with pure nanoparticle cells at a series of film thickness. With low nanowire concentrations (5 wt% and 20 wt%), the composite films maintain similar specific surface area as the pure nanoparticle films, while the composite cells show higher short-circuit current density and open-circuit voltage. An enhancement of power efficiency from 6.7 % for pure nanoparticle cells to 8.6% for the composite cell with 20 wt% nanowires has been achieved under 1 Sun AM1.5 illumination (100 mW/cm2). For the composite film with 77 wt% nanowires, the nanowires became the major phase. Their less compact packing resulted in significant decrease of the specific surface area, and thus the current density. However, with the increase of film thickness, the current density showed a continuous increase in the whole thickness range up to 17 μm, indicating the improved electron diffusion length due to the formed nanowire network. The nanowires also helped to preserve crack-free thick films. These results show that employing nanoparticle/nanowire composites represents a promising approach for further improving the performance of sensitized solar cells. In addition to our nanowire/nanoparticle composites, we have also started to investigate the application of complex oxide nanoparticles in dye-sensitized solar cells. Previous research has been limited to simple binary oxides including TiO2, ZnO, SnO2, Nb2O5, and In2O3. In contrast, the application of multication oxides has been rarely explored. To our best knowledge, the only reported ternary oxides are SrTiO3 and some doped binary oxides such as La3+- and Zr4+-doped CeO2. In comparison with simple binary oxides, multication oxides have more freedom to tune the materials' chemical and physical properties by altering the compositions. As an example, ZnO-In2O3 ternary compounds have been investigated as new n-type transparent conducting oxide (TCO) materials. By varying the relative Zn/In ratio, the bandgap energy, the work function and the electric resistivity of the ternary oxides can be readily tuned. The ternary oxides also show dramatically reduced acid etching rate in comparison with ZnO. Considering the availability of a wide range of mutication oxides and their tunable properties, it is, therefore, interesting to investigate their applications in DSSC. Potentially, new materials with better performance than anatase TiO2 could be found. This strategy is similar to the search of new TCO materials, which, historically, also started with simple binary oxide materials and then have been gradually expanded to many multi-component oxides. The first complex oxide that we have investigated is Zn2SnO4 nanoparticles. We have synthesized the nanoparticles by hydrothermal methods. Solar cells based on these nanoparticles have achieved an overall light-to-electricity efficiency as high as 3.8% under 1 sun AM 1.5 illumination. In comparison with ZnO and SnO2 as its simple component oxides, a Zn2SnO4 cell is more stable against acidic dyes than a ZnO cell, while it has much better performance than a SnO2 cell. Our results suggest that multication oxides, with the availability of a wide range of compositions and tunable properties, hold great promise as new electrode materials for dye-sensitized solar cells. This study has been published in J. Am. Chem. Soc. as a communication. We have further studied the band structure of Zn2SnO4 by photoelectrochemical methods (M. A. Alpuche-Aviles and Y. Wu. submitted to J. Am. Chem. Soc.). In MeCN solution, which simulates the DSC electrolyte, we demonstrate the dependence of the conduction band edge on the electrolyte composition. This opens the possibility of keep increasing open-circuit voltage, Voc, by using interfacial interactions to shift the conduction band edge, ECB, to higher energies. Moreover, cyclic voltammetry measurements show that the Zn2SnO4 has a much lower recombination rate near the ECB than anatase TiO2. These intriguing properties of Zn2SnO4- motivate us to systematically investigate the photoelectrochemical properties of other complex oxides. In summary, I would like to thank the support from ACS-PRF. This grant has a large impact to my career development. This is the 1st grant that I have received since I started my faculty career. It helped me build up my confidence in competing for other grants and carry out my research program on oxide nanomaterials.
