Electrical transport in ZnO and TiO 2 nanowires ( for solar cell application)

1. Electrical transport in ZnO and TiO2 nanowires (for solar cell application) Chun-Chung Su andChao-Cheng Kaun Advanced Computation & Modeling Group

2. Introduction (I) • Titanium dioxide and zinc oxide are cheap, nontoxic and photostable semiconductor：widely used in the photovoltaic devices and photo catalysis. • The electron-donating molecules such as catechol and ascorbic acid absorbed on the surface could affect the solid surface state and lower the optical absorption threshold to visible sunlight. • The dye-sensitized solar cell (DSSC), often called the “Gratzel cell” is a promising route toward harvesting solar energy.

3. Introduction (II) • 1D nanowires provide benefits in two respects: 1.) due to high length-to-diameter ratio and a total length reaching hundreds of micrometers, visible light scattering and absorption are much enhanced 2.) the 1D geometry facilitate rapid, diffusion-free electron transport to the electrodes • Despite intensive studies on surface-based and nanoparticle-based DSSCs, the mechanism and electronic coupling between a nanowire and a dye molecule has hardly been addressed.

4. Example (I)Nanowire dye-sensitized solar cells • Traditional nanoparticle film are replaced by the crystalline ZnO nanowires (NWs). • The direct electrical pathways provided by the NWs ensure the rapid collection of carriers generated throughout the device. Nature Material, 4, p455 (2005)

5. Example (II)[Fe(CN)6]4- sensitization of TiO2 nanoparticles • Charge injection from the dye molecule to the conduction band of the nanoparticle. JACS, 126, p15024 (2004)

6. The structure of ZnO • The primitive unit cell of ZnO is the wurtzite phase：the space group is P63mc , hexagonal. • ZnO nanowires can grow along the [0001] direction, having hexagonal or triangular cross section. • Choose (n=26, triangular) as our system. • Wire diameter is about 6 angstrom. Side view Top view

7. Band structure and Transmission of ZnO NW Calculated from VASP Ref.: APL 91,031914 (2007) Our result

8. Scattering region L Structure of Au-ZnO-Au systems • Length of ZnO unit : 1cell : L=0.591nm 2cell : L=1.135nm 3cell : L=1.668nm 4cell : L=2.205nm 5cell : L=2.783nm 6cell : L=3.281nm • Two optimized structures: 1.optimized ZnO NW + Au lead 2.optimized ZnO NW include 1 layer Au +Au lead

9. Transmission of Au-ZnO-Au system • An obvious transmission peak appear in Au-ZnO-Au system, ex: 3cell and 4cell systems.

10. Tunneling behavior of ZnO NW • G~G0exp(-βn) , n is number of ZnO cell • β=1.583 (relax only ZnO NW) =1.262 (relax include 1 layer Au)

11. The structure of TiO2 • TiO2 nano-crystalline: mostly anatase (70%), then rutile (30%) ,when size 15~30 nm. • The primitive unit cell of TiO2 in the anatase structure：the space group is I41/amd, tetragonal. Anatase

12. Structure of TiO2 [010] nanowire After optimization

13. Three simple Dye Molecules Red: Fe Yellow: C Grey: N catechol Fe(CN)64- Cyanidin

14. Band and orbital of dye-TiO2 NW

15. Band and orbital of dye-ZnO NW

16. Future Work • Construct TiO2 NW and ZnO NW sensitized by different dye molecules. • Construct NW-dye/NW junction system and NW-dye/metal junction system. • Investigate the transport properties and electronic coupling between the nanowire and molecules.