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

Electrical transport in ZnO and TiO2 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)

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)

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 resultCalculated from VASP

Ref.: APL 91,031914 (2007)

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

L Scattering region

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~G0exp(-βn) , n is number of ZnO cell

β=1.583 (relax only ZnO NW) =1.262 (relax include 1 layer Au)

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

Structure of TiO2 [010] nanowire

After optimization

]100[

]010[

]100[

]101[

Three simple Dye Molecules

catechol Cyanidin Fe(CN)64-

Red: Fe

Yellow: C

Grey: N

Band and orbital of dye-TiO2 NW

Band and orbital of dye-ZnO NW

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.