Elsevier

Carbon

Volume 50, Issue 7, June 2012, Pages 2431-2441
Carbon

Comparison of carbon nanotube modified electrodes for photovoltaic devices

https://doi.org/10.1016/j.carbon.2012.01.065Get rights and content

Abstract

Substrates with four different nanotube modifications have been prepared and their electron transport properties measured. Two modification techniques were compared; covalent chemical attachment of both single and multi-walled carbon nanotubes to transparent conductive (fluorine doped tin oxide) glass surfaces and chemical vapour deposition (CVD) growth of both single and multi-walled carbon nanotubes on highly doped conductive silicon wafers. These carbon nanotube modified substrates were investigated using scanning electron microscopy and substrates with nanotubes grown via CVD have a much higher density of nanotubes than substrates prepared using chemical attachment. Raman spectroscopy was used to verify that nanotube growth or attachment was successful. The covalent chemical attachment of nanotubes was found to increase substrate electron transfer substantially compared to that observed for the bare substrate. Nanotube growth also enhanced substrate conductivity but the effect is smaller than that observed for covalent attachment, despite a lower nanotube density in the attachment case. In both modification techniques, attachment and growth, single-walled carbon nanotubes were found to have superior electron transfer properties. Finally, solar cells were constructed from the nanotube modified substrates and the photoresponse from the different substrates was compared showing that chemically attached single-walled nanotubes led to the highest power generation.

Introduction

Carbon nanotubes (CNTs) are unique materials that hold promise for applications in the electronics industry. This is due to the high conductivity found in CNTs, caused by the delocalised π electrons allowing for ballistic conduction along the tube length [1]. As well as being excellent conductors, carbon nanotubes also possess a complex density of states, allowing them to exist as both metallic conductors and semi-conductors for single-walled carbon nanotubes (SWCNT) and predominantly metallic conductors in the case of multi-walled carbon nanotubes (MWCNT) depending on chirality and tube diameter. One application that makes use of both the conductivity and complex electronic properties of CNTs is photovoltaics. There have been several publications about the response of carbon nanotubes, both SWCNTs [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and MWCNTs [12], [13], to visible light. These have shown that CNTs by themselves, without any further chemical functionalisation, can be used to generate a photovoltaic response. The general consensus in the literature is that photocurrent generation is caused by electronic transitions between van Hove singularities present in CNTs which then leads to exciton creation upon illumination [6], [12]. SWCNT are generally thought to be the most applicable because the semiconducting nature allows for electron–hole separation, whilst metallic MWCNT provide routes for easy recombination reducing the generated photocurrent [14], although it has been recently shown that MWCNT are excellent hole carriers and may be able to provide charge separation with further chemical modification [15]. This previous work on the photoresponse of CNTs has used varied methods for producing the CNT working electrode, ranging from individual nanotubes [4], [5], [7], [15], growth of CNTs by chemical vapour deposition [13], deposition from solution [9], [16], deposition by electrophoresis [6], [8], [17], [18] and chemical attachment of nanotube arrays to electrodes [11], [19]. Previously, it has been shown that chemical attachment of CNTs to surfaces provides enhanced electron transport when used in electrochemistry compared to unmodified electrode surfaces [20], [21], [22], [23], [24]. This enhanced electron transfer has also been shown to be dependent upon the orientation of the CNTs used, either vertically or horizontally aligned [25]. The use of chemical attachment to create CNT arrays is expected to be advantageous for applications in electronic devices due to the increased electron transfer rates and increased conductivity provided by a direct covalent bond, as opposed to a physisorption process present in grown CNT arrays.

We have previously performed extensive work on creating chemically attached vertically aligned SWCNT arrays on substrates for use as electrochemical sensors [22], [23], [26], [27], field emission sources [28] and photovoltaics [11], [29]. The work presented here allows a direct comparison between chemically attached CNTs, both multi and single-walled, and grown CNTs, again both single and multi-walled for applications such as electrochemical sensors and photovoltaic devices.

Section snippets

SWCNT modified FTO

All chemicals were purchased from Sigma–Aldrich and used as received unless otherwise described. To produce SWCNT modified electrodes a previously published method was used, and shown in Fig. 1 [11]. It is described here in brief. Firstly, purified low functionality single-walled carbon nanotubes (P2-SWCNT, Carbon Solutions, Inc., USA) were sonicated at 0 °C in a 3:1 (v/v) mixture of H2SO4:HNO3 for 8 h. The resulting functionalized nanotube solution was then filtered through a 0.4 μm membrane

Scanning electron microscopy

The morphology of the CNT arrays formed on each sample was analysed using SEM. Fig. 3A shows a top down SEM image of the CNTs grown by tCVD on silicon, showing a dense coverage, the inset shows the side view showing a forest of vertical CNTs with a height of around 20–30 μm, whilst Fig. 3B shows the PECVD modified surface. In Fig. 3B no discernible structures can be seen and other techniques, such as Raman microscopy, must be used to determine the presence of CNTs as is discussed in Section 3.2.

Conclusions

The work presented here has allowed for a direct comparison between chemically attached carbon nanotube arrays and those grown with chemical vapour deposition and also compares single-walled to multi-walled carbon nanotubes. Using Raman spectroscopy it was possible to first identify and verify each nanotube modification had occurred successfully. Cyclic voltammetry results showed that all of the modification of surfaces with carbon nanotubes provided an increased electrode performance over the

Acknowledgement

This work was supported by the Australian Microscopy and Microanalysis Research Facility (AMMRF).

References (44)

  • E. Laviron

    General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems

    J Electroanal Chem

    (1979)
  • E. Laviron

    The use of linear potential sweep voltammetry and of a.c. voltammetry for the study of the surface electrochemical reaction of strongly adsorbed systems and of redox modified electrodes

    J Electroanal Chem

    (1979)
  • E. Laviron

    Adsorption, autoinhibition and autocatalysis in polarography and in linear potential sweep voltammetry

    J Electroanal Chem

    (1974)
  • R. Saito et al.

    Physical properties of carbon nanotubes

    (1998)
  • P. Avouris et al.

    Carbon-nanotube photonics and optoelectronics

    Nat Photonics

    (2008)
  • Y. Zhang et al.

    Elastic response of carbon nanotube bundles to visible light

    Phys Rev Lett

    (1999)
  • M. Freitag et al.

    Photoconductivity of single carbon nanotubes

    Nano Lett

    (2003)
  • K. Balasubramanian et al.

    Photoelectronic transport imaging of individual semiconducting carbon nanotubes

    Appl Phys Lett

    (2004)
  • S. Barazzouk et al.

    Single-wall carbon nanotube films for photocurrent generation. A prompt response to visible-light irradiation

    J Phys Chem B

    (2004)
  • J.U. Lee

    Photovoltaic effect in ideal carbon nanotube diodes

    Appl Phys Lett

    (2005)
  • A. Kongkanand et al.

    Single wall carbon nanotube scaffolds for photoelectrochemical solar cells. Capture and transport of photogenerated electrons

    Nano Lett

    (2007)
  • Y. Shi et al.

    Photoresponse in self-assembled films of carbon nanotubes

    J Phys Chem C

    (2008)
  • Cited by (0)

    View full text