Joe Briscoe

A self-powered ZnO-nanorod/CuSCN UV photodetector exhibiting rapid response.

These photodetectors typically require an external bias as the driving force to prevent the recombination of photogenerated electron-hole pairs. Over recent years, it has been realized that for functional nanosystems (where multiple nanoscale-devices are integrated) to be a possibility, these devices must be selfsuffi cient; that is, to function independently of external power sources that typically create the bulk of a system. It has been common in the literature to call devices of this type “selfpowered” although it should be noted that almost all previously published devices demonstrate a measurable dark current at the nominal zero-volt, or short-circuit, position. [ 2–5 ] We ascribe this to the measurement systems used, where a small bias exists, even when set to apply zero volts. ZnO is an increasingly well-studied material for optoelectronic devices due to its wide bandgap ( ≈ 3.3eV), its high exciton binding energy ( ≈ 60 meV), and the chemical and radiation hardness of the material. [ 6 ] ZnO has been considered in a range of optoelectronic and electronic devices that include photovoltaics, [ 7 , 8 ] fi eld-effect transistors, [ 9 ] and energy-harvesting devices. [ 10 , 11 ] In recent years, there has been growing interest in using nanostructured ZnO as a UV photodetector. [ 12–15 ]

Biomass‐Derived Carbon Quantum Dot Sensitizers for Solid‐State Nanostructured Solar Cells

New hybrid materials consisting of ZnO nanorods sensitized with three different biomass-derived carbon quantum dots (CQDs) were synthesized, characterized, and used for the first time to build solid-state nanostructured solar cells. The performance of the devices was dependent on the functional groups found on the CQDs. The highest efficiency was obtained using a layer-by-layer coating of two different types of CQDs.

Non-volatile electrically-driven repeatable magnetization reversal with no applied magnetic field.

Repeatable magnetization reversal under purely electrical control remains the outstanding goal in magnetoelectrics. Here we use magnetic force microscopy to study a commercially manufactured multilayer capacitor that displays strain-mediated coupling between magnetostrictive Ni electrodes and piezoelectric BaTiO(3)-based dielectric layers. In an electrode exposed by polishing approximately normal to the layers, we find a perpendicularly magnetized feature that exhibits non-volatile electrically driven repeatable magnetization reversal with no applied magnetic field. Using micromagnetic modelling, we interpret this nominally full magnetization reversal in terms of a dynamic precession that is triggered by strain from voltage-driven ferroelectric switching that is fast and reversible. The anisotropy field responsible for the perpendicular magnetization is reversed by the electrically driven magnetic switching, which is, therefore, repeatable. Our demonstration of non-volatile magnetic switching via volatile ferroelectric switching may inspire the design of fatigue-free devices for electric-write magnetic-read data storage.

Measurement techniques for piezoelectric nanogenerators

Electromechanical energy harvesting converts mechanical energy from the environment, such as vibration or human activity, into electrical energy that can be used to power a low power electronic device. Nanostructured piezoelectric energy harvesting devices, often termed nanogenerators, have rapidly increased in measured output over recent years. With these improvements nanogenerators have the potential to compete with more traditional micro- or macroscopic energy harvesting devices based on piezoelectric ceramics such as lead zirconate titanate (PZT), polymers such as polyvinylidene fluoride (PVDF) or electrostatic, electret or electromagnetic kinetic energy harvesters. Power output from a nanogenerator is most commonly measured through open-circuit voltage and/or short-circuit current, where power may be estimated from the product of these values. Here we show that such measures do not provide a complete picture of the output of these devices, and can be misleading when attempting to compare alternative designs. In order to compare the power output from a nanogenerator, techniques must be improved in line with those used for more established technologies. We compare ZnO nanorod/poly(methyl methacrylate) (PMMA) and ZnO nanorod/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) devices, and show that despite an open-circuit voltage nearly three times lower the ZnO/PEDOT:PSS device generates 150 times more power on an optimum load. In addition, it is shown that the peak voltage and current output can be increased by straining the device more rapidly and therefore time-averaged power, or time-integrated measures of output such as total energy or total charge should be calculated. Finally, the internal impedance of the devices is characterised to develop an understanding of their behaviour and shows a much higher internal resistance but lower capacitive impedance for the ZnO/PMMA device. It is hoped that by following more rigorous testing procedures the performance of nanostructured piezoelectric devices can be compared more realistically to other energy harvesting technologies and improvements can be rapidly driven by a more complete understanding of their behaviour.

Acoustic Enhancement of Polymer/ZnO Nanorod Photovoltaic Device Performance

Acoustic vibrations are shown to enhance the photovoltaic efficiency of a P3HT/ZnO nanorod solar cell by up to 45%, correlated to a three-fold increase in charge carrier lifetime. This is assigned to the generation of piezoelectric dipoles in the ZnO nanorods, indicating that the efficiency of solar cells may be enhanced in the presence of ambient vibrations by the use of piezoelectric materials.

Enhanced quantum dot deposition on ZnO nanorods for photovoltaics through layer-by-layer processing

ZnO nanorods are coated with a conformal film of CdTe nanoparticles using a layer-by-layer (LbL) process. By increasing the number of CdTe layers the absorption of incident light increases at wavelengths lower than the absorption onset of the nanoparticles (650 nm). At the absorption peak of the nanoparticles, 50 layers of nanoparticles coated onto ZnO nanorods absorb 80% of the incident light. Annealing of the ZnO–CdTe composites leads to a small red-shift in this absorption onset, but does not destroy the quantum confinement of the nanoparticles. Solar cells are produced by filling the CdTe-coated nanorods with CuSCN or PEDOT:PSS, and tested under AM 1.5 illumination. Annealing of the LbL films is essential to reduce the series resistance of the cell. Annealed cells generate an open-circuit voltage of 120 mV and a short-circuit current density of 0.19 mA cm−2.

Influence of anneal atmosphere on ZnO-nanorod photoluminescent and morphological properties with self-powered photodetector performance

ZnO nanorods synthesised using an aqueous pH 11 solution are shown to exhibit surface-sensitive morphology post-annealing in oxygen, air, and nitrogen as shown by scanning electron microscopy and transmission electron microscopy analysis. Raman analysis confirms the nanorods were nitrogen-doped and that nitrogen incorporation takes place during the synthesis procedure in the form of N-Hx. A strong green photoluminescence is observed post-annealing for all samples, the intensity of which is dependent on the atmosphere of anneal. This luminescence is linked to zinc vacancies as recent reports have indicated that these defects are energetically favoured with the annealing conditions used herein. ZnO-nanorod/CuSCN diodes are fabricated to examine the effect of material properties on photodetector device performance. The devices exhibit a photocurrent at zero bias, creating a self-powered photodetector. A photocurrent response of 30 μA (at 6 mW cm−2 irradiance) is measured, with a rise time of ∼25 ns, and sens...

Extremely thin absorber solar cells based on nanostructured semiconductors

Abstract Extremely thin absorber (eta) solar cells aim to combine the advantages of using very thin, easily and cheaply produced absorber layers on nanostructured substrates with the stability of all-solid-state solar cells using inorganic absorber layers. The concept of using nanostructured substrates originated from the dye-sensitised solar cell, where having a very high surface area allows the use of very thin layers of dye while still absorbing sufficient sunlight. However, both the dye and liquid electrolyte used in these devices demonstrated poor stability, and efforts were made to replace them with very thin inorganic absorber layers and solid state hole collectors respectively. The combination of these concepts – a nanostructured substrate coated with a very thin inorganic absorber and completed with a solid state hole collector – is known as an eta solar cell. This review summarises the development of both the inorganic absorbers and solid state hole collectors in porous TiO2 and ZnO nanorod based cells, focusing on the material properties and growth/deposition methods. Future possibilities for eta solar cells are discussed, including utilisation of a wider range of materials, synthesis methods and novel materials such as quantum dots to produce tuned band gap and multijunction solar cells.

Surface passivation effect by fluorine plasma treatment on ZnO for efficiency and lifetime improvement of inverted polymer solar cells

Zinc oxide (ZnO) is an important material for polymer solar cells (PSCs) where the characteristics of the interface can dominate both the efficiency and lifetime of the device. In this work we study the effect of fluorine (SF6) plasma surface treatment of ZnO films on the performance of PSCs with an inverted structure. The interaction between fluorine species present in the SF6 plasma and the ZnO surface is also investigated in detail. We provide fundamental insights into the passivation effect of fluorine by analyzing our experimental results and theoretical calculations and we propose a mechanism according to which a fluorine atom substitutes an oxygen atom or occupies an oxygen vacancy site eliminating an electron trap while it may also attract hydrogen atoms thus favoring hydrogen doping. These multiple fluorine roles can reduce both the recombination losses and the electron extraction barrier at the ZnO/fullerene interface improving the selectivity of the cathode contact. Therefore, the fabricated devices using the fluorine plasma treated ZnO show high efficiency and stable characteristics, irrespective of the donor : acceptor combinations in the photoactive blend. Inverted polymer solar cells, consisting of the P3HT:PC71BM blend, exhibited increased lifetime and high power conversion efficiency (PCE) of 4.6%, while the ones with the PCDTBT:PC71BM blend exhibited a PCE of 6.9%. Our champion devices with the PTB7:PC71BM blends reached a high PCE of 8.0% and simultaneously showed exceptional environmental stability when using the fluorine passivated ZnO cathode interlayers.

In situ antimony doping of solution-grown ZnO nanorods

ZnO nanorods are doped with Sb during the aqueous chemical synthesis by addition of Sb acetate dissolved in ethylene glycol.