Introduction

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Power production with zero greenhouse gas emission is desirable economically and environmentally. Direct photovoltaic conversion of sunlight into electricity is therefore one highly attractive alternative energy source. Although silicon solar cells have gained considerable market share and commercial success, high production costs still limits their commercial viability. This is why we are exploring the development of low cost alternative photovoltaics in the Applied Nanotechnology Group (QUT Strategic Collaborative Grant).

The proposed photovoltaic device

Device fabrication

The devices are made from a mixture of poly(3-hexylthiophene) (P3HT) and carbon nanotubes (CNTs). The OPVs are made by sandwiching the π-conjugated polymer composite between two different metal contacts

However, to date our results (as well as those of other groups - e.g. E. Kymakis and G.A.J. Amaratunga, Appl. Phys. Lett. 80 ,112-115 (2002)) have not been satisfactory, with efficiency spanning from 0.04% up to 0.1%.

Poor order in the microscopic arrangement, oxidation and pollution of the compound, presence of impurities or low grade of carbon nanotubes purity can all be possible causes of these low efficiencies.

Our purpose is to find the problems that are at the base of the low efficiency of this mixture, trying to obtain a molecular scale description of the system.

Current studies

Our studies aim at:

• unveil the microscopic mechanisms that control
  • the light-electricity conversion
  • the carriers transport in these materials.
• find the correct fabrication procedures to improve
  • the ordering of the mixture by wrapping polymers around nanotubes
  • the overall electrical conductivity
  • the solar cell efficiency

To this purpose we undertake:

• In house growth and purification of Multi Walled Carbon Nanotubes (MWNT)
• Purification of purchased Single Walled Carbon Nanotubes (SWNT)
• Spectroscopic measurements at the nanotube /polymer interface.
• Scanning Tunneling Microscopy studies of P3HT on graphite and wrapped on nanotubes - (QUT | ePrints Archive)
• Thermogravimetric analysis of the nanotubes and of the mixture

High resolution STM images of a nanotube wrapped by P3HT

Further studies

Further work to be performed include:

• Detailed STM studies of the nanotube-polymer structure.
• Current Voltage characterization in Ultra High Vacuum by Scanning Tunneling Microscopy (In collaboration with INRS-Varennes Canada)
• Doping of nanotubes (In collaboration with Louisiana State University)
• Photoluminescence study at Low Temperature.
• Visible-Ultra Violet spectroscopy.
• Construction and test of a solar cell under controlled atmosphere.

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