Luke A. Rochford

Utilizing n-type vanadium oxide films as hole-extracting layers for small molecule organic photovoltaics

We report increased cell performance for boron subphthalocyanine chloride (SubPc)/fullerene (C60) organic photovoltaic (OPV) cells when thermally evaporated vanadium oxide (V2OX) thin films are incorporated as a hole-extracting layer at the indium-tin oxide (ITO)/SubPc interface. Ultra-violet photoemission spectroscopy (UPS) studies of the V2OX films reveal highly n-type character, with a large work function of 6.8 eV. This correlates well with recently reported data for other metal oxide hole-extracting layers, such as molybdenum oxide and tungsten oxide, in contrast to the p-type character previously reported for V2OX films. There is significant improvement in energy level alignment for hole-extraction when cells utilise the V2OX layer at the ITO/SubPc interface, resulting in substantial increases in open circuit voltage (VOC) and power conversion efficiency (ηp).

Understanding the multiple orientations of isolated superellipsoidal hematite particles at the oil–water interface

Non-spherical particles have the potential to adopt multiple orientations once adhered to a liquid–liquid interface. In this work we combine simulations and experiments to investigate the behaviour of an isolated microscopic hematite particle of superellipsoidal shape. We show that this microparticle can adopt one of three orientations when adhered to a hexadecane–water interface. Two of the orientations, and estimates for their relative populations, could be assigned to two thermodynamic minima on the energy landscape as generated through both free-energy minimization and particle trajectory simulations. The third orientation was found to correspond to a kinetically trapped state, existing on certain particle trajectories in a region of a negligible gradient in free energy. To underpin the simulations the individual orientation of a set of 100 isolated particles was explored by means of scanning electron microscopy (SEM) using the gel trapping technique as a tool. Atomic force microscopy (AFM) was additionally used to support the experimental findings. This is the first example of such a kinetic metastable state being observed for particles at liquid–liquid interfaces.

Exploring high temperature templating in non-planar phthalocyanine/copper iodide (111) bilayers

Elevated substrate temperature growth of phthalocyanine thin films is known to influence film morphology and increase crystallinity. Structural templating offers another method through which the structure of phthalocyanine films can be controlled. Here we combine the use of copper iodide (CuI) and elevated substrate temperatures and investigate their effect on the growth of a non-planar phthalocyanine system. Employing X-ray diffraction and atomic force microscopy we present detailed surface and crystal structure information. Vanadyl phthalocyanine (VOPc) is shown to adopt an edge-on orientation on CuI at ambient substrate temperatures, a behaviour in stark contrast to that of previously studied planar phthalocyanine molecules. Elevated substrate temperature is shown to result in changes in the surface morphology and structure demonstrating the versatility of the system. The crystal structure of VOPc was redetermined and used to infer the molecular orientation of the various VOPc/CuI bilayer structures.

Controlling templating effects at the organic/inorganic interface using (111) oriented copper iodide

Structural templating of organic semiconductors affords control of out-of-plane film structure and molecular orientation with respect to solid surfaces. Herein we use a prototypical copper iodide (CuI)/planar phthalocyanine system to produce detailed surface and crystal structure information using atomic force microscopy (AFM) and X-ray diffraction (XRD). The out-of-plane structure of the CuI layer was characterised and identified as the (111) plane of single crystal CuI. The dependance of surface morphology and grain size in the CuI (111) templating layer upon substrate temperature was demonstrated. The formation of a thin film of iron phthalocyanine (FePc) on this model layer was characterised at multiple points during growth, changes in the surface morphology were observed, and the crystal structure of the final film was used to infer the molecular orientation therein. These changes were elucidated using the re-determined single crystal structure of FePc which is also presented.

Magnetic properties of copper hexadecaphthalocyanine (F16CuPc) thin films and powders

The structural and magnetic properties of F16CuPc thin films and powder, including x-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometry, and theoretical modelling of exchange interactions are reported. Analysis of XRD from films, with thickness ranging between 100 and 160 nm, deposited onto Kapton and a perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) interlayer shows that the stacking angle (defined in the text) of the film is independent of the thickness, but that the texture is modified by both film thickness and substrate chemistry. The SQUID measurements suggest that all samples are paramagnetic, a result that is confirmed by our theoretical modelling including density functional theory calculations of one-dimensional molecular chains and Greens function perturbation theory calculations for a molecular dimer. By investigating theoretically a range of different geometries, we predict that the maximum possible exchange interaction between F16CuPc molecules is twice as large as that in unfluorinated copper-phthalocyanine (CuPc). This difference arises from the smaller intermolecular spacing in F16CuPc. Our density functional theory calculation for isolated F16CuPc molecule also shows that the energy levels of Kohn-Sham orbitals are rigidly shifted ∼1 eV lower in F16CuPc compared to CuPc without a significant modification of the intra-molecular spin physics, and that therefore the two molecules provide a suitable platform for independently varying magnetism and charge transport.

Organic photovoltaic cells utilising ZnO electron extraction layers produced through thermal conversion of ZnSe

In this work, a thin ZnSe layer was deposited in a vacuum and then thermally annealed in air to provide an efficient electron extraction layer for an inverted organic photovoltaic (OPV) cell. Annealing the ZnSe film at 450 °C (ZnSe(450 °C)) increased the device performance and gave an efficiency of 2.83%. X-ray photoelectron spectroscopy (XPS) measurements show that the increased device performance upon annealing at 450 °C is due to the thermal conversion of ZnSe to ZnO. ZnO has a wider band gap than ZnSe, which allows for more light to reach the photoactive layer. The electronic structures of the treated ZnSe films were explored by ultraviolet photoemission spectroscopy (UPS) which showed that the ZnSe(450 °C) films had a Fermi level close to the conduction band edge, allowing for efficient electron extraction compared to the energetic barrier for extraction formed at the ZnSe(RT)/organic interface.

Copper light-catching electrodes for organic photovoltaics

Optically thin copper films with a random array of sub-optical wavelength apertures couple strongly with light in the wavelength range 600–800 nm due to excitation of surface plasmonic resonances. Herein we show that this trapped light can be used to excite electronic transitions in a nearby strongly absorbing organic semiconductor before the plasmonic excitations dissipate their energy as heat into the metal. This energy transfer process is demonstrated using model small molecule and polymer photovoltaic devices (based on chloro-aluminium phthalocyanine:C60 and PCE-10:PC70BM heterojunctions respectively) in conjunction with a nano-hole copper electrode formed by thermal annealing an optically thin Cu film supported on polyethylene terephthalate. The efficiency of this process is shown to be highest for wavelengths in the range 650–750 nm, which is part of the solar spectrum that is weakly absorbed by todays high performance organic photovoltaic devices, and so these findings demonstrate that this type of electrode could prove useful as a low cost light catching element in high performance organic photovoltaics.

Processing Solvent-Dependent Electronic and Structural Properties of Cesium Lead Triiodide Thin Films

Cesium lead triiodide (CsPbI3) is an attractive material for photovoltaic applications due to its appropriate band gap, strong optical absorption, and high thermal stability. However, the perovskite phase suffers from moisture induced structural instability. Previous studies have utilized a range of solvent systems to establish the role of solvent choice in structural instabilities. Despite this, effects of different solvents on the electronic structure of this material have not been compared. We report substantial chemical and compositional differences in thin films of CsPbI3 prepared from a range of solvent systems. We confirm via X-ray diffraction thin films formed from DMF, DMSO, and a mixture of these solvent systems share the same crystal structure. However, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and low energy ion scattering measurements reveal significant differences between films processed via different solvent systems. Our findings reveal the critical impact solvents have upon compositional stoichiometry and thin-film morphology.

The influence of polar (0001) zinc oxide (ZnO) on the structure and morphology of vanadyl phthalocyanine (VOPc)

Metal oxide thin films are increasingly utilized in small molecular organic photovoltaic devices to facilitate electron transport and injection. Despite this there is little understanding of the influence these layers have on the structure of adjacent organic semiconductor layers. Here we use both O- and Zn-terminated (0001) single crystal zinc oxide (ZnO) as a model system to investigate the effect of a metal oxide surface on the growth of a molecular semiconductor, vanadyl phthalocyanine (VOPc). The surface reconstructions of these model surfaces are determined and the properties of thin films of VOPc deposited atop are investigated. The nature of the bulk truncation of the surface is found to have pronounced effects on both the morphology and crystal structure of these molecular films. This work highlights the importance of considering the effects of the chemical composition and surface termination of metal oxide films on the structure of adjacent molecular semiconductor films.

Fabrication of calcium phosphate microcapsules using emulsion droplets stabilized with branched copolymers as templates

We report on a versatile and time-efficient method to fabricate calcium phosphate (CaP) microcapsules by utilizing oil-in-water emulsion droplets stabilized with synthetic branched copolymer (BCP) as templates. The BCP was designed to provide a suitable architecture and functionality to produce stable emulsion droplets, and to permit the mineralization of CaP at the surface of the oil droplet when incubated in a solution containing calcium and phosphate ions. The CaP shells of the microcapsules were established to be calcium deficient hydroxyapatite with incorporated chlorine and carbonate species. These capsule walls were made fluorescent by decoration with a fluorescein-bisphosphonate conjugate.