David Caffrey

Synthesis of nanocrystalline Cu deficient CuCrO2 – a high figure of merit p-type transparent semiconductor

The delafossite structured CuCrO2 system is well known as one of the best performing p-type transparent conducting oxides. In this paper the details of a low temperature facile growth method for CuCrO2 is described. The dependence of the growth on the precursors, the temperature and oxygen partial pressure are examined. The decomposition routes are critical to obtain the best performing films. The thermopower and electrical measurements indicate p-type films with conductivity ranging from 1–12 S cm−1 depending on the growth conditions. This p-type conductivity is retained despite the nanocrystallinity of the films. The figure of merit of these films can be as high as 350 μS, which is the best performing p-type TCO by solution methods to date. The optical properties are also investigated using ellipsometry and UV-Vis spectroscopy.

Spray pyrolysis growth of a high figure of merit, nano-crystalline, p-type transparent conducting material at low temperature

In this letter, we demonstrate a low temperature (≈345 °C) growth method for Cu deficient CuCrO2 performed by spray pyrolysis using metal-organic precursors and a simple air blast nozzle. Smooth films were grown on glass substrates with a highest conductivity of 12 S/cm. The most conductive samples retain transparencies above 55% resulting in a figure of merit as high as 350 μS, which is the best performing p-type transparent conducting material grown by solution methods to date. Remarkably, despite the nano-crystallinity of the films, properties comparable with crystalline CuCrO2 are observed. No postannealing of the films is required in contrast to previous reports on crystalline material. The low processing temperature of this method means that the material can be deposited on flexible substrates. As this is a solution based technique, it is more attractive to industry as physical vapour deposition methods are slow and costly in comparison.

Quantifying the Performance of P-Type Transparent Conducting Oxides by Experimental Methods

Screening for potential new materials with experimental and theoretical methods has led to the discovery of many promising candidate materials for p-type transparent conducting oxides. It is difficult to reliably assess a good p-type transparent conducting oxide (TCO) from limited information available at an early experimental stage. In this paper we discuss the influence of sample thickness on simple transmission measurements and how the sample thickness can skew the commonly used figure of merit of TCOs and their estimated band gap. We discuss this using copper-deficient CuCrO2 as an example, as it was already shown to be a good p-type TCO grown at low temperatures. We outline a modified figure of merit reducing thickness-dependent errors, as well as how modern ab initio screening methods can be used to augment experimental methods to assess new materials for potential applications as p-type TCOs, p-channel transparent thin film transistors, and selective contacts in solar cells.

Decoupling the refractive index from the electrical properties of transparent conducting oxides via periodic superlattices

We demonstrate an alternative approach to tuning the refractive index of materials. Current methodologies for tuning the refractive index of a material often result in undesirable changes to the structural or optoelectronic properties. By artificially layering a transparent conducting oxide with a lower refractive index material the overall film retains a desirable conductivity and mobility while acting optically as an effective medium with a modified refractive index. Calculations indicate that, with our refractive index change of 0.2, a significant reduction of reflective losses could be obtained by the utilisation of these structures in optoelectronic devices. Beyond this, periodic superlattice structures present a solution to decouple physical properties where the underlying electronic interaction is governed by different length scales.

Nitrogen grain-boundary passivation of In-doped ZnO transparent conducting oxide

D. Ali,1,2 M. Z. Butt,3 C. Coughlan,2 D. Caffrey,2,4 I. V. Shvets,2,4 and K. Fleischer2 1Department of Physics, GC University Lahore-54000, Pakistan 2School of Physics, Trinity College Dublin, The University of Dublin, College Green, Dublin 2, Ireland 3Centre for Advanced Studies in Physics, GC University Lahore-54000, Pakistan 4Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, The University of Dublin, College Green, Dublin 2, Ireland

Bending stability of Cu0.4CrO2—A transparent p-type conducting oxide for large area flexible electronics

The current best performing p-type transparent conducting oxides are typically highly crystalline materials, deposited at high temperatures, and hence incompatible with the drive to low cost flexible electronics. We investigated a nanocrystalline, copper deficient CuxCrO2, deposited at low temperatures upon a flexible polyimide substrate. The as-deposited film without post annealing has an electrical conductivity of 6Scm−1. We demonstrate that this p-type transparent oxide retains its excellent electrical conductivity under tensile strain, withstanding more than one thousand bending cycles without visible cracks or degradation in electrical properties. In contrast, compressive strain is shown to lead to an immediate reduction in conductivity which we attribute to a de-lamination of the thin film from the substrate.The current best performing p-type transparent conducting oxides are typically highly crystalline materials, deposited at high temperatures, and hence incompatible with the drive to low cost flexible electronics. We investigated a nanocrystalline, copper deficient CuxCrO2, deposited at low temperatures upon a flexible polyimide substrate. The as-deposited film without post annealing has an electrical conductivity of 6Scm−1. We demonstrate that this p-type transparent oxide retains its excellent electrical conductivity under tensile strain, withstanding more than one thousand bending cycles without visible cracks or degradation in electrical properties. In contrast, compressive strain is shown to lead to an immediate reduction in conductivity which we attribute to a de-lamination of the thin film from the substrate.