Synthesis and Optimization of Dispersion Conditions of Single-walled Carbon Nanotubes (SWCNTs) for Photovoltaic Application

Abstract

Photovoltaic performance of hybrid solar cells can be improved by introducing single-walled carbon nanotubes (SWCNTs) as an electron donor. SWCNTs increase the hole mobility in the hybrid solar cell by providing ballistic pathways for the hole transport. Hence, synthesis of CNTs of different features would allow researchers to make attempts to use them for various applications including solar cells. In order to exploit the advantageous characteristics of SWCNTs, they should be semiconductive (for solar cells) and well-dispersed (for most of the applications). As-synthesized SWCNTs are bundled due to the strong intertube van der Waals forces. Hence, achieving well-dispersed suspension of SWCNTs in water or organic solvents is challenging but important for various applications. At the same time, it is equally important to avoid any physical or chemical modification of pristine SWCNTs during dispersion, which may affect their properties. In this thesis, CNTs have been synthesized using Low Pressure Chemical Vapor Deposition (LPCVD) method at 700 ?C using acetylene as precursor gas with the annealed (at 500 ?C) Fe catalyst. Vertically aligned CNTs were observed with the catalyst annealing time in the range of 3 to 15 min. Raman spectra confirmed the presence of SWCNTs in the synthesized CNTs. Catalyst annealing time is found to be a key factor deciding the nature and features of the resulting CNTs as different growth is observed for different conditions. Vertically-aligned CNTs or CNFs have also been synthesized using Plasma Enhanced Chemical Vapor Deposition (PECVD) method employing Ni as the catalyst at 500 ?C using acetylene as precursor gas. The size of the CNTs/CNFs is in the range of 30 50 nm and the average height is as high as ?2.5?m. The other part of thesis work deals with optimization of dispersion conditions for commercial SWCNTs(7, 6) (CoMoCAT in aqueous surfactant solution or organic solvents towards photovoltaic application. Sodium dodecylbenzenesulphonate (SDBS) and sodium deoxycholate (DOC) were the surfactants used to achieve the best dispersion of SWCNTs in aqueous medium. On the other hand, an organic solvent such as 1,2-dichloroethane, dichlorobenzene or N,N-dimethyl formamide was used for identifying the optimum dispersion conditions in the absence of surfactants. UV-Vis-NIR absorption spectra of solution containing dispersed SWCNTs show well-resolved absorption peaks corresponding to semiconducting SWCNTs (Es 11, Es 22 and Es 33 electronic transitions) and metallic SWCNTs (Em 11). By analyzing the peak corresponding to Es 11 electronic transition of semiconducting SWCNTs, the resonant ratio (peak height to peak width) was calculated to determine the relative degree of dispersion. It has been found that the optimum surfactant concentration to achieve best dispersion for 0.25 mg/ml of SWCNTs is 9-10 mg/ml (SDBS) or 8-9 mg/ml (DOC). Optimum sonication time to disperse the SWCNTs in the organic solventsis found to be 4 h. SWCNTs were coated on glass or silica using SWCNTsdispersed solutions with surfactantfor atomic force microscopy (AFM) and scanning electron microscopy (SEM) characterizations. According to the AFM analysis, the diameter of the SWCNTs from the aqueous dispersion method lies between 0.6 and 2.5 nm whereas diameter of SWCNTs from organic dispersion method lies between 1.5 and 20 nm. Moreover, AFM images revealed the presence of well-dispersed nanotubes along with some amount of surfactant particles. The presence of surfactants implies that nanotubes may be covered with surfactants, which may be detrimental to solar cell performance in terms of the hole transport limiting. The advantage of using organic solvents over surfactant-aided dispersion in aqueous medium is discussed. Results proved that organic solvents provide pure individual SWCNTs after drying, which may possibly improve hole transport and solar cell device performance.

Date of Award	Jun 2013
Original language	American English
Supervisor	Amal Al Ghaferi (Supervisor)