Electrical transport in ZnO and TiO

2

nanowires (for solar cell application)

Chun-Chung Su and Chao-Cheng Kaun Advanced Computation & Modeling Group

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.

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.

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)

[Fe(CN) 6 ] 4 Example (II) sensitization of TiO 2 nanoparticles Charge injection from the dye molecule to the conduction band of the nanoparticle.

JACS, 126, p15024 (2004)

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

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

Structure of Au-ZnO-Au systems

L

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

Scattering region Two optimized structures: 1.optimized ZnO NW + Au lead 2.optimized ZnO NW include 1 layer Au +Au lead

Transmission of Au-ZnO-Au system

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

Tunneling behavior of ZnO NW

G ~ G 0 exp(-β n ) , n is number of ZnO cell β=1.583 (relax only ZnO NW) =1.262 (relax include 1 layer Au)

The structure of TiO

2 TiO 2 nano-crystalline: mostly anatase (70%), then rutile (30%) ,when size 15~30 nm.

The primitive unit cell of TiO 2 group is I41/amd, tetragonal.

in the anatase structure ： the space Anatase

Structure of TiO

2

[010] nanowire

After optimization [ 100 ] [ 100 ] [ 101 ] [ 010 ]

Three simple Dye Molecules

catechol

Red: Fe Yellow: C Grey: N

Fe(CN) 6 4 Cyanidin

Band and orbital of dye-TiO 2 NW

Band and orbital of dye-ZnO NW

Future Work

Construct TiO 2 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.