Anthony J. Morfa

Nanostructured copper oxide semiconductors: a perspective on materials, synthesis methods and applications

The oxides of copper (CuxO) are fascinating materials due to their remarkable optical, electrical, thermal and magnetic properties. Nanostructuring of CuxO can further enhance the performance of this important functional material and provide it with unique properties that do not exist in its bulk form. Three distinctly different phases of CuxO, mainly CuO, Cu2O and Cu4O3, can be prepared by numerous synthesis techniques including, vapour deposition and liquid phase chemical methods. In this article, we present a review of nanostructured CuxO focusing on their material properties, methods of synthesis and an overview of various applications that have been associated with nanostructured CuxO.

Enhancing the current density of electrodeposited ZnO–Cu2O solar cells by engineering their heterointerfaces

Using ZnO seed layers, an efficient approach for enhancing the heterointerface quality of electrodeposited ZnO–Cu2O solar cells is devised. We introduce a sputtered ZnO seed layer followed by the sequential electrodeposition of ZnO and Cu2O films. The seed layer is employed to control the growth and crystallinity and to augment the surface area of the electrodeposited ZnO films, thereby tuning the quality of the ZnO–Cu2O heterointerface. Additionally, the seed layer also assists in forming high quality ZnO films, with no pin-holes, in a high pH electrolyte solution. X-ray electron diffraction patterns, scanning electron and atomic force microscopy images, as well as photovoltaic measurements, clearly demonstrate that the incorporation of certain seed layers results in the alteration of the heterointerface quality, a change in the heterojunction area and the crystallinity of the films near the junction, which influence the current density of photovoltaic devices.

Transparent metal electrodes from ordered nanosphere arrays

We show that perforated metal electrode arrays, fabricated using nanosphere lithography, provide a viable alternative to conductive metal oxides as transparent electrode materials. The inter-aperture spacing is tuned by varying etching times in an oxygen plasma, and the effect of inter-aperture “wire” thickness on the optical and electronic properties of perforated silver films is shown. Optical transmission is limited by reflection and surface plasmons, and for these results do not exceed 73%. Electrical sheet resistance is shown to be as low as 3 Ω ◻−1 for thermally evaporated silver films. The performance of organic photovoltaic devices comprised of a P3HT:PCBM bulk heterojunction deposited onto perforated metal arrays is shown to be limited by optical transmission, and a simple model is presented to overcome these limitations.

Understanding Anisotropic Plasma Etching of Two-Dimensional Polystyrene Opals for Advanced Materials Fabrication

Anisotropic deformation of polystyrene particles in an oxygenated (O2/Ar) plasma is observed for radio frequency (rf) plasma and inductively coupled plasma (ICP). A facile model based on a ratio of completely isotropic and completely anisotropic etching is presented to describe the anisotropy of the etching process and is implemented to determine the height of the spheroid-shaped polystyrene particles. In our systems, we find the plasma etching to be 54% isotropic in the rf plasma and 79% isotropic in the ICP. With this model, the maximum material deposition thickness for nanofabrication with plasma-etched nanosphere lithography or colloid lithography can be predicted. Moreover, the etching of polystyrene particles in an oxygenated plasma is investigated versus the etching time, gas flow, gas composition, temperature, substrate material, and particle size. The results of this study allow precise shape tuning during the fabrication of nanostructured surfaces with size-dependent properties for bionic, medical, and photonic applications.

Investigation of Line Width Narrowing and Spectral Jumps of Single Stable Defect Centers in ZnO at Cryogenic Temperature.

Finding new solid state defect centers in novel host materials is crucial for realizing integrated hybrid quantum photonic devices. We present a preparation method for defect centers with photostable bright single photon emission in zinc oxide, a material with promising properties in terms of processability, availability, and applications. A detailed optical study reveals a complex dynamic of intensity fluctuations at room temperature. Measurements at cryogenic temperatures show very sharp (<60 GHz) zero phonon lines (ZPLs) at 580 nm to  620 nm (≈ 2.0 eV) with frozen out fast fluctuations. Remaining discrete jumps of the ZPL, which depend on the excitation power, are observed. The low temperature results will narrow down speculations on the origin of visible-near-infrared (NIR) wavelength defect emission in zinc oxide and provide a basis for improved theoretical models.

Nanoscale Imaging of Exciton Transport in Organic Photovoltaic Semiconductors by Tip-Enhanced Tunneling Luminescence

In organic solar cells, the efficiency of the exciton transport and dissociation across donor-acceptor (D/A) interfaces is controlled by the nanoscale distribution of the donor and acceptor phases. The observation of photoluminescence quenching is often used as confirmation for efficient exciton dissociation but provides no information on the nanoscopic nature of the exciton transport. Here we demonstrate nanoscale imaging of the exciton transport in films consisting of the conjugated polymer poly(3-hexylthiophene) (P3HT, electron donor) blended with the C60 derivative 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM, electron acceptor) by a tunneling luminescence spectroscopy based on atomic force microscopy. The excitonic luminescence is significantly enhanced when the conjugated polymer is coupled to the plasmon excitation at the tip (tip-enhanced luminescence). This effect allows one to dramatically improve the detection efficiency of the excitonic luminescence and, consequently, resolve individual domains of the conjugated polymer in which the exciton will recombine before dissociation at the D/A interface. Under thermal annealing conditions promoting the segregation of the donor and acceptor phases, a clear increase of the luminescence is seen from polymer-rich regions, consistent with domains of dimensions much larger than the exciton diffusion length. The described scanning luminescence microscopy can thus be applied to the optimization of the blends used in solar cells.

Short-term metal/organic interface stability investigations of organic photovoltaic devices

As organic photovoltaic (OPV) devices have begun to move toward initial applications, issues of their stability are becoming increasingly important. The de facto standard OPV devices are made from a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and these serve as a test bed for lifetime testing. Delamination, oxidation, and chemical interactions at the metal electrode/organic interface have been posited as degradation pathways in organic electronic devices.[1,2] Here, two short-term experiments were employed to evaluate the stability of this interface in the light and dark. Full devices and unprotected organic surfaces were separately examined, and both were found to be stable in air over the course of 10s of minutes while in the dark. While unprotected devices were stable (less than 20% loss in efficiency) in air for 100 minutes under constant 1-sun illumination, unprotected organic surfaces were not, and good devices could not be made on them subsequently.