Solution Processed Nanocarbon-Based Materials for Use in Photovoltaic Systems

Author: Lachlan Larsen

Larsen, Lachlan, 2016 Solution Processed Nanocarbon-Based Materials for Use in Photovoltaic Systems, Flinders University, School of Chemical and Physical Sciences

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Abstract

The application of solution processed nanocarbon-based materials in a number of photovoltaic systems was investigated and the performance of these systems optimised. These systems included the use of solution processed single-walled carbon nanotubes (SWNTs) in the working electrodes of photoelectrochemical solar cells, along with surfactant-assisted exfoliated graphene and reduced graphene oxide (RGO) in solid state nanocarbon-based material-silicon solar cells. Vertically aligned arrays of SWNTs were chemically attached to fluorinated tin oxide (FTO) substrates. These arrays were subsequently sensitised by the addition of a layer of 7,14-bis[2-[tris(1-methylethyl)silyl]ethynyl]dibenzo[b,def]chrysene (TIPS-DBC), a polycyclic aromatic hydrocarbon used in organic photovoltaics. These new TIPS-DBC/SWNT/FTO electrodes were then used as the working electrodes in dye-sensitised solar cell- (DSC) like photoelectrochemical cells. Addition of the TIPS-DBC to the working electrodes significantly enhanced the photovoltaic performance of the cells, due to the photoresponse of the TIPS-DBC. The response of the cells was then tuned by controlling the concentration of TIPS-DBC solution used to deposit the TIPS-DBC layer, as well as by controlling the spin speed used during deposition. Following this procedure, the maximum power conversion efficiency (PCE) was found for these cells, outperforming previously reported DSCs using vertically aligned arrays of SWNTs as the main photoactive material. Aqueous dispersions of the non-ionic surfactant Tween-60 were found to be capable of producing surfactant-assisted exfoliated graphene dispersions containing few layer graphene. These dispersions were then vacuum filtered to make conductive films with highly reproducible optical transparencies and sheet resistances. By controlling the volume of dispersion filtered, the optical transparency of the films could be controlled, and two thickness regimes could be formed. In the thin regime, films were found to be non-continuous, leaving areas of exposed substrate, while in the thick regime continuous films with thicknesses on the order of hundreds of nanometres were formed. Both thin and thick films were then used to form graphene-silicon (G-Si) Schottky junction solar cells, which were found to be highly durable. The effect of graphene film thickness on the photovoltaic performance of G-Si Schottky junction solar cells was then investigated, along with the effects of thermal annealing of the graphene films. Chemical doping with the common dopants nitric acid and gold chloride was also investigated, with both found to have beneficial effects on cell PCE. Aqueous dispersions of graphene oxide (GO) were vacuum filtered to produce thin GO films. These films were then deposited onto silicon substrates and annealed at a range of temperatures under forming gas, to produce RGO-Si Schottky junction solar cells. The effect of annealing temperature on cell performance was investigated, along with previously unseen beneficial aging effects observed at higher temperatures. The effects of RGO film thickness were then investigated using the optimal annealing temperature. The effects of chemical doping using both thionyl chloride and gold chloride were investigated both at different annealing temperatures and different RGO film thicknesses and these effects were compared with those of chemical doping on G-Si Schottky junction solar cells. The effect of using different sources of GO for the RGO film production was also investigated.

Keywords: single-walled carbon nanotubes, graphene, graphene oxide, photovoltaics
Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2016
School: School of Chemical and Physical Sciences
Supervisor: Joe Shapter