Carbon Nanomaterials and Their Application for Emerging Solar Cells

Abstract

The fact that only one–thousandth of the Sun’s energy incident on the Earth is equal to the entire world's current energy needs means direct conversion of this energy into electricity–photovoltaic (PV) energy–is now a mainstream renewable energy source. There are many types of emerging PV cells. Dye–sensitized solar cells (DSSCs) are an attractive potential source of renewable energy due to their eco–friendliness and ease of fabrication. However, in DSSCs, the rarity and high cost of some electrode materials (e.g. platinum) and the inefficient performance caused by slow electron transport, poor light–harvesting efficiency, and significant charge recombination present significant limitations. Over the past several years, carbon nanomaterials including carbon particles, carbon nanotubes and graphene have played important roles in addressing these issues. Although excellent progress has been made in the application of carbon materials in DSSCs, the exact role of nanocarbons in both the photoelectrode and counter electrode (CE) of DSSCs is still unclear. Organic–inorganic halides based perovskite solar cells (PSCs) have attracted a great deal of attention due to the extremely rapid increases in efficiencies observed over the past few years. Although the efficiencies of the PCS have exceeded 20%, they do have some disadvantages such as use of expensive electrode materials, the high temperature processing required during production and poor stability when in use. In this regard, it is no surprise that carbonaceous materials would have significant role in the development of PSCs as nanocarbons have been extensively studied in various energy related applications because of their fascinating properties, low cost and abundance. Research into the potential application of carbon nanomaterials in PSC is still at an early stage and a lot remains to be explored. This Ph.D. project focuses on the application and development of carbon nanomaterials for emerging PV devices such as DSSCs and PSCs. The following research has been included in this thesis: 1) A hybrid structure consisting of SnO₂ and reduced graphene oxide (SnO₂-RGO) was synthesized via a microwave-assisted method and has been employed as a photoanode in DSSCs, for the first time. It was found that the incorporation of RGO into the SnO₂ film not only enhances the electron transfer rate of the photoanode, but it also increases the adsorption of dye molecules into the film. Both these effects greatly enhance the DSSC performance. 2) As an alternative to platinum (Pt), a hybrid electrocatalyst based on sulfur-doped graphene with FeS₂ microspheres (SGN-FeS₂) was designed and used as a CE of DSSCs. Benefiting from the high conductivity of SGN and excellent electrocatalytic activity of FeS₂, the bifunctional hybrid electrocatalyst based device displays an efficiency of 8.1%, which was comparable to that (8.3%) of expensive Pt CE based DSSC and also exhibits excellent stability in ambient conditions. 3) Solution processed transparent conductive graphene films are utilized, for the first time, as an alternative to traditional transparent conducting oxide (TCO) electrodes at the electron collecting layer in perovskite solar cells (PSCs). By optimising the sheet resistance (Rs) and transparency of the films, maximum power conversion efficiency of 0.62% was obtained. The successful incorporation of graphene structures into both compact TiO₂ and mesoporous TiO₂ layers of the PSCs was also demonstrated. 4) The influence of CNTs on the PV performance of 1D titanium dioxide nanofiber (TiO₂ NF) photoelectrode perovskite solar cells (PSCs) was systematically explored. It was found that in addition to the significant enhancement in the efficiency of PSCs with SWCNTs, the incorporation of SWCNTs into TiO₂ NFs reduced the hysteresis effect and improved the stability of the PSC devices both under light and during storage in ambient conditions. 5) Significant enhancement in the power conversion efficiency (PCE) and stability (light- and long-term storage-stability) of perovskite solar cells (PSCs) by incorporating single-walled carbon nanotubes (SWCNTs) into the nanocrystalline TiO₂ photoelectrode was reported. The TiO2-SWCNTs photoelectrode based PSC device exhibited a PCE of up to 16.11%, while the cell fabricated without SWCNTs displayed an efficiency of 13.53%.