title: 100-fold improvement in carrier drift mobilities in alkanephosphonate-passivated monocrystalline TiO2nanowire arrays.
abstract: Single crystal rutile titania nanowires grown by solvothermal synthesis are actively being researched for use as electron transporting scaffolds in perovskite solar cells, in low detection limit ultraviolet photodetectors, in photoelectrochemical water-splitting, and in chemiresistive and electrochemical sensing. The electron drift mobility (μn) in solution-grown TiO2nanowires is very low due to a high density of deep traps, and reduces performance in optoelectronic devices. In this study, the effects of molecular passivation of the nanowire surface by octadecylphosphonic acid (ODPA), on carrier transport in TiO2nanowire ensembles, were investigated using transient space charge limited current measurements. Infrared spectroscopy indicated the formation of a highly ordered phosphonate monolayer with a high likelihood of bidentate binding of ODPA to the rutile surface. We report the hole drift mobility (μp) for the first time in unpassivated solvothermal rutile nanowires to be 8.2 × 10-5cm2V-1s-1and the use of ODPA passivation resulted in μpimproving by nearly two orders of magnitude to 7.1 × 10-3cm2V-1s-1. Likewise, ODPA passivation produced between a 2 and 3 order of magnitude improvement in μnfrom ∼10-5-10-6cm2V-1s-1to ∼10-3cm2V-1s-1. The bias dependence of the post-transit photocurrent decays in ODPA-passivated nanowires indicated that minority carriers were lost to trapping and/or monomolecular recombination for small values of bias (<5 V). Bimolecular recombination was indicated to be the dominant recombination mechanism at higher bias values.
authors.affiliation: Department of Electrical and Computer Engineering, University of Alberta, 9211-116 St, Edmonton, AB T6G 1H9, Canada.
authors.forename: A
authors.initials: A
authors.lastname: Mohammadpour
authors.forename: B D
authors.initials: BD
authors.lastname: Wiltshire
authors.forename: Y
authors.initials: Y
authors.lastname: Zhang
authors.forename: S
authors.initials: S
authors.lastname: Farsinezhad
authors.forename: A M
authors.initials: AM
authors.lastname: Askar
authors.forename: R
authors.initials: R
authors.lastname: Kisslinger
authors.forename: Y
authors.initials: Y
authors.lastname: Ren
authors.forename: P
authors.initials: P
authors.lastname: Kar
authors.forename: K
authors.initials: K
authors.lastname: Shankar