Carbon Nanotubes for Photovoltaic Devices

Author: Mark Alexander Bissett

Bissett, Mark Alexander, 2011 Carbon Nanotubes for Photovoltaic Devices, Flinders University, School of Chemical and Physical Sciences

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Abstract

The aim of this work was to investigate how carbon nanotubes can be applied in the development of novel photovoltaic devices. This has been done by taking an existing system of vertically aligning single-walled carbon nanotubes on oxide surfaces and adapting it to solar cell design. Once the ability to construct solar cells from CNT functionalised electrodes was demonstrated, work then focused on improving the performance of these cells. Initially arrays of vertically aligned SWCNT were used as the working electrode in a DSSC type cell architecture. These CNT solar cells were then characterised by photovoltaic testing. The arrays themselves were investigated using electrochemistry and Raman spectroscopy. It was found that the vertically aligned single walled carbon nanotube arrays were capable of producing a prompt, response times less than 200ms, and stable photocurrent of ~13[mu]A.cm-2 and a voltage of 42mV when exposed to 100mW.cm-2 of light. This photoresponse changed with the number of nanotubes attached to the surface and the treatment time used to process the CNTs before attachment. Multi-walled carbon nanotube arrays were also created and analysed and found to be inferior to the SWCNT arrays due to their metallic band structure. To improve upon the response of the SWCNT arrays they were then chemically modified to increase the cell's performance. This will be done firstly by further functionalising the CNT arrays with chromophores such N3 dye and ruthenium tetraphenyl porphyrin molecules. Attachment of these redox active molecules was verified by electrochemistry and the surface concentration and electron transfer rates compared to literature and found to be in good agreement. Photovoltaic testing indicated that N3 dye attachment lead to an increased photocurrent density (~17[mu]A.cm-2) but a reduced voltage (26mV) when compared to the unmodified array, in agreement with similar work in the literature. This response could also be modified by altering the attachment of nanotubes to the surface thus altering the resultant dye concentration, with 2 hours of CNT attachment found to produce the maximum dye concentration. Functionalisation was then progressed from simple molecules to PAMAM-type dendrons that were grown from the SWCNT array acting as a core. These dendrons were analysed using electrochemistry, Raman spectroscopy and photovoltaic testing and found to be able to increase performance over the unmodified array by ~70% for the 2nd generation dendron. The two methods of chemical modification were then combined with the dendrons being grown from the SWCNT array and then N3 dye attached to the amine terminated chains. This produced an increased performance over the unmodified dendron with a current density of ~15[mu]A.cm-2 whilst maintaining a voltage of 45mV. To further increase the density of carbon nanotubes on the surface growth of CNTs was undertaken using chemical vapour deposition and then the resultant performance compared to the chemically attached arrays. Growth of nanotubes was undertaken using both thermal and plasma-enhanced procedures. Thermal CVD was found to produce predominantly MWCNT whilst PECVD was able to produce SWCNT. It was found upon comparison of the CVD growth procedure to the covalent attachment that the chemical attachment provided for superior electron transfer kinetics despite lower nanotube coverage. This equated into a superior photoresponse. It was also found that the grown SWCNT were superior to the grown MWCNT, in agreement with previous results which suggested that SWCNT are needed to produce photocurrent due to their semi-conducting nature.

Keywords: carbon nanotubes,single-walled,multi-walled,SWCNT,MWCNT,solar cell,photovoltaic,nanotechnology
Subject: Nanotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2011
School: School of Chemical and Physical Sciences
Supervisor: Prof. Joe Shapter