Titanium dioxide (TiO2) thin films have a long history in silicon photovoltaics (PV) as antireflection (AR) coatings due to their excellent optical properties and low deposition cost. This work explores several novel areas where TiO2 thin films could be use to enhance silicon (Si) solar cell performance while reducing device fabrication costs. Amorphous, anatase and rutile TiO2 thin films are deposited using ultrasonic spraydeposition (USD) and chemical vapour deposition (CVD) systems, both designed and constructed by the author. Initial experiments confirmed that no degradation in the bulk minority carrier lifetime (????bulk) occurred during high-temperature processing, although the stability of the USD-deposited TiO2 films was dependent on the furnace ambient. A major disadvantage of TiO2 AR coatings is that they afford little surface passivation. In this work, a novel method of achieving excellent surface passivation on TiO2-coated silicon wafers is presented. This involved growing a 6 nm-thick SiO2 layer at the TiO2:Si interface by oxidising the wafer after TiO2 film deposition. The increase in surface passivation afforded by the interfacial SiO2 layer results in a decrease in the emitter dark saturation current density (J0e) by nearly two orders of magnitude to 4.7 ??? 7.7 ??~ 10???14 A/cm2. This demonstrates the compatibility of the TiO2/SiO2 stack with high-efficiency solar cells designs. By varying the film deposition and annealing conditions, TiO2 refractive indices in the range of 1.726 ??? 2.633 (at ???? = 600 nm) could be achieved. Subsequently, a double-layer antireflection (DLAR) coating was designed comprised of low and high TiO2 refractive index material. The best experimental weighted average reflectance (Rw) achieved was 6.5% on a planar silicon wafer in air. TiO2 DLAR coatings are ideally suited to multicrystalline silicon (mc-Si) wafers, which do not respond well to chemical texturing. Modelling performed for a glass and ethyl vinyl acetate (EVA) encapsulated buried-contact solar cell indicated that a TiO2 DLAR coating afforded a 7% increase in the short circuit current density, when compared to a standard, commercially-deposited TiO2 single-layer AR coating. Finally, it is demonstrated that chemical reactions with phosphorus prevent TiO2 from acting as a successful phosphorus diffusion barrier or dopant source. The applicability of TiO2 thin films to various silicon solar cell structures is discussed.
| Identifer | oai:union.ndltd.org:ADTP/258510 |
| Date | January 2002 |
| Creators | Richards, Bryce Sydney, Electrical Engineering & Telecommunications, Faculty of Engineering, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Electrical Engineering and Telecommunications |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Bryce Sydney Richards, http://unsworks.unsw.edu.au/copyright |
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