Titanium oxide nanotube arrays : template preparation, facile liquid-atomic layer deposition process (L-ALD) and potential in hybrid solar cells
Foong, Thelese Ru Bao
Date of Issue2010
School of Materials Science and Engineering
The ordered bulk heterojunction (BHJ) is a device structure that is widely expected to alleviate problems associated with exciton dissociation and charge transport that currently limit the efficiency of excitonic organic solar cells to ca. 7.4 %. This research focuses on the development and incorporation of electron accepting and transporting arrays of TiO2 nano-tubes in hybrid organic-inorganic solar cells, with regio-regular poly (3-hexylthiophene) (P3HT) constituting the photoactive donor material. Nano-porous anodic alumina (AAO) templates produced by anodizing Al thin films on substrates were a natural choice material to direct nano-array deposition as they offer good control over array dimensions. Although AAO templates have been exceptionally popular for nano-array fabrication, their use in opto-electronic applications has been restricted largely by the inability to stabilize Al anodization on transparent-conducting electrodes (commonly indium-tin-oxide (ITO) and fluorine-doped tin oxide (FTO)). Part of this report examines the root cause(s) for such instability and proposes the use of ultra-thin (0.2 – 0.5 nm) Ti inter-layers to passivate the Al-ITO interface, thereby stabilizing the AAO formation process. When thicker (2 – 10 nm) Ti layers were employed, thick barrier layers that prevent pore connectivity to the ITO coated glass substrate, were observed. The complex relationship between substrate properties and barrier layer formation was studied and is reported here. Next, a liquid-phase atomic layer deposition (“liquid-ALD” or LALD) procedure established to achieve TiO2 nano-tube arrays on ITO coated glass is detailed. Like its vapor-phase predecessor, LALD produces non-line-of-sight TiO2 coatings that conform to the shape of the AAO pore channels resulting in nano-tube arrays when the alumina template is removed. The wall thickness of the nano-tubes can be tuned by varying the number of cycles of TiO2 deposition. The key objective of evolving the LALD procedure was to circumvent vacuum conditions necessary to conventional ALD that are highly energy-consuming, facilitating a greener deposition route. The feasibility of doping the TiO2 films and nano-tubes by incorporating vanadia oxide precursors into LALD assembly thus tuning their band gaps and optical absorption properties is also discussed. Finally, device prototypes comprising P3HT and the LALD-derived arrays, with a champion efficiency of 0.3 % are reported. The nano-array dimensions were found to be ideal for exciton dissociation with strong photoluminescence quenching observed when P3HT was infiltrated into the arrays. Charge mobility studies suggest that the performance of devices with P3HT fully infiltrated was ultimately limited by the slow mobility of holes (1.5 x 10-5 cm2/Vs) in P3HT as a result of the polymer chains coiling under nano-confinement inside the nano-channels and inter-tube spaces of the arrays. Several research directions for further advancements are proposed.