Organic:inorganic heterojunctions for photovoltaic devices.

VALE, Alistair Douglas John. (2005). Organic:inorganic heterojunctions for photovoltaic devices. Doctoral, Sheffield Hallam University (United Kingdom)..

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Abstract

Thin films of TiO[2] with a thickness around 200nm were produced by DC reactive sputtering at room temperature. A variety of types of bandgap including direct allowed, direct forbidden, indirect allowed, and indirect forbidden were observed, with good fits of the Tauc model, between annealed and unannealed samples, and within the same sample. Planar conductivity was measured at 8.7x10[-3]O[-1]m[-1], falling to 7.4x10[-3]O[-1]m[-1] after annealing. Refractive index and extinction coefficient spectra were obtained via the Swanepoel technique and were found to be around 2.2 and 3x10[-3] respectively, in the region of weak absorption, with little change after annealing. Surface details in the micro and nano regimes showed flat films with few defects. XRD showed the Rutile phase dominating, and XPS showed correct stoichiometry. AFM showed the grain size to be in the range of 64nm rising to 82nm after annealing. TiO[2] films with an extended surface area were made using Degussa P25 and were deposited by the new technique of flood printing developed for this research. 2D profiles of thin films of P25 were measured at different concentrations and the thickness of the highly rough films was in the range of 5 mum to 15 mum rising with increasing concentration. AFM also showed films that were highly rough in the nanoscale. Thin films of CuPc and Rose Bengal were deposited on differently coated substrates; F:TO, TiO[2], and bare glass. The absorption spectra was seen to vary significantly with substrate, in particular, the 530nm peak of Rose Bengal was seen to diminish greatly when the dye was deposited on TiO[2]. Heterojunction photovoltaic devices based on dye layers adsorbed on semiconducting layers were produced in fifteen different designs with each design based on another to observe the effect of single parameters changes. Such changes included different thickness of porous TiO[2] layer, and whether a conducting polymer layer is applied. The devices were built up on F:TO coated glass slides and electrical contacts were made by DC sputtering platinum fingers on the top. The best device design gave a fill factor of 35.5%, and an overall conversion efficiency of 4.2x10[-3]%. Devices were thought to be greatly affected by series resistance from the top contact being applied to a highly rough surface. The top contact on different structures were electrically characterised and were found to have resistances in the region of kO, which manifests as a high series resistance in the final solar cells. This is one of the reasons for the low efficiency. The I(V) spectra of all the devices were fitted to a diode model using an algorithm developed for this research. The model fit gave reverse saturation currents in the range of 3.2x10[-9] A to 7.5x10[-3]A. Ideality factor varied from 3.8 to 80, series resistance varied from 15O to 500O, and parallel resistance varied from 117O to 778kO. An experimental setup capable of measuring the lifetime of photogenerated carriers in the dye films was developed that could produce pulses with a cut off time in the nano-seconds time regime, but can be simply improved to give cut off times in the sub nano-second regime, and only costs around £200 for the optical modulator.

Item Type: Thesis (Doctoral)
Additional Information: Thesis (Ph.D.)--Sheffield Hallam University (United Kingdom), 2005.
Research Institute, Centre or Group: Sheffield Hallam Doctoral Theses
Depositing User: EPrints Services
Date Deposited: 10 Apr 2018 17:22
Last Modified: 10 Apr 2018 17:22
URI: http://shura.shu.ac.uk/id/eprint/20472

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