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A study of electronic transport properties of hydrogenated amorphous silicon (a-Si:H)

Subaer, (1995) A study of electronic transport properties of hydrogenated amorphous silicon (a-Si:H). PhD thesis, Murdoch University.

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Abstract

Hydrogenated amorphous silicon (a-Si:H) is gaining increasing use in photovoltaic solar cells and other electronic devices. The purpose of present study was to experimentally investigate the optical and electronic transport properties of a-Si:H thin films and devices. These must be understood in order to optimise the performance of a-Si:H semiconductor devices.

The a-Si:H materials were prepared by using a radio frequency plasma enhanced chemical vapour deposition (PECVD) technique. Measurements indicated that our a-Si:H films exhibited a low defect density, high optical absorption coefficient and good photoconductivity, all features of high quality material. In doped (p-type and ntype) a-Si:H films, two transport mechanisms dominate conduction in different temperature regimes. At high temperature, the conduction is due to electrons excited into extended states above the conduction band. At low temperature, the conduction takes place by the hopping of carriers in the band tail states.

The metastable conductivity of doped a-Si:H was investigated. The results indicated that the thermally induced defect structure of a-Si:H reaches a thermal equilibrium above a well-defined temperature (TE) (about 110°C for n-type and 85°C for p-type) . This phenomenon is explained in relation to the motion of bonded hydrogen and its interaction with dopant atoms.

I-V measurements were made to investigate the photovoltaic performance of single junction p-i-n a-Si:H solar cells. Dark and Illuminated I-V curves were measured as a function of temperature to examine the internal parameters of the cells including the diode quality and the dark saturation current.

The stability of the cells was investigated by subjecting the cells to light soaking and annealing cycles. The drop in efficiency of the cells was found to be the result of a reduction in the short circuit current (Isc) . The open circuit voltage (Voc) and fill factor (FF) remained relatively constant. The initial efficiency of the cells was recovered by annealing the cells at about 150 °C for 30 minutes.

Because of the importance of the current in achieving and maintaining high efficiency, two techniques were used to investigate the behaviour of the current carriers photo-induced open-circuit voltage decay (OCVD) and transient photocurrent measurements. Both techniques were applied over a range of temperatures from room temperature to about -60 °C. The results provided valuable information about the temperature dependence of the carrier trapping process and recombination including the deep-trapping process which increases as a result of light soaking.

As a result of this study, the electronic transport properties of a-Si:H are better understood and this may lead the way to the production of devices with an improved performance.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Physical Sciences, Engineering and Technology
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Cornish, John
URI: http://researchrepository.murdoch.edu.au/id/eprint/51559
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