Solubilisation of carbon nanotubes with a non-conjugated polymer for device applications

Thesis

Carbon nanotubes (CNT) solubilisation represents a major challenge for their use on a large scale in electronic devices, as CNT's tendency to aggregate prevents an easy and uniform deposition of CNT films. This thesis demonstrate a novel way to solubilise CNTs using the non-conjugated polymer ethylene-vinyl acetate (EVA). The characterisation of the optical and electrical properties of the resulting EVA-CNT nano-hybrids is analysed, and their performance as a holes extraction layer in photovoltaic devices discussed.

First, it is shown that EVA can solubilise single- and multi-walled carbon nanotubes, and by photoluminescence excitation mapping is demonstrated that the polymer creates a strong binding with the CNT inducing a mechanical stress. Raman spectroscopy studies show a non-selective wrapping process, with a tendency to wrap more metallic CNTs. By atomic force microscopy it is confirmed that the nanotubes are individually wrapped. This causes the screening of the aggregation forces in solution and enables the production of uniform and aggregate-free dispersions.

These dispersions allow the fabrication of uniform thin films by spray-coating, conducting and semi-transparent, with conductivities of up to 34 S/cm, two higher of magnitudes ever reached for this combination of materials. The films have been the subject of percolation studies, showing that the films with increasing density show an increase in conductivity which can be well fitted by the expected behaviour for a 2D percolation model.

Finally, to test the electronic properties of such produced CNT films in devices, they have been used as hole transporting layers in perovskite solar cells. It is shown that despite the insulating polymer wrapping the nanotubes, it is possible to use them in conjunction with undoped 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD) to efficiently extract charges from the perovskite layer, obtaining solar cells with a power conversion efficiency of 16.8% with EVA-SWCNTs. and 17.3% with EVA-MWCNTs. With further optimisation it is expected that they can replace CNT wrapped with conjugated polymers, offering favourable optical properties at lower cost.

Authors: Mazzotta, G

• DOI: 10.5287/ora-00kg1vjxm
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: CNT polymer wrapping; conductive films; carbon nanotubes; perovskite solar cells; non-conjugated polymer
• Subjects: Condensed matter