Steve Dunn

Piezoelectric and ferroelectric materials and structures for energy harvesting applications

This review provides a detailed overview of the energy harvesting technologies associated with piezoelectric materials along with the closely related sub-classes of pyroelectrics and ferroelectrics. These properties are, in many cases, present in the same material, providing the intriguing prospect of a material that can harvest energy from multiple sources including vibration, thermal fluctuations and light. Piezoelectric materials are initially discussed in the context of harvesting mechanical energy from vibrations using inertial energy harvesting, which relies on the resistance of a mass to acceleration, and kinematic energy harvesting which directly couples the energy harvester to the relative movement of different parts of a source. Issues related to mode of operation, loss mechanisms and using non-linearity to enhance the operating frequency range are described along with the potential materials that could be employed for harvesting vibrations at elevated temperatures. In addition to inorganic piezoelectric materials, compliant piezoelectric materials are also discussed. Piezoelectric energy harvesting devices are complex multi-physics systems requiring advanced methodologies to maximise their performance. The research effort to develop optimisation methods for complex piezoelectric energy harvesters is then reviewed. The use of ferroelectric or multi-ferroic materials to convert light into chemical or electrical energy is then described in applications where the internal electric field can prevent electron–hole recombination or enhance chemical reactions at the ferroelectric surface. Finally, pyroelectric harvesting generates power from temperature fluctuations and this review covers the modes of pyroelectric harvesting such as simple resistive loading and Olsen cycles. Nano-scale pyroelectric systems and novel micro-electro-mechanical-systems designed to increase the operating frequency are discussed.

Light‐Emitting Diodes with Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals are promising luminophores for creating a new generation of electroluminescence devices. Research on semiconductor nanocrystal based light-emitting diodes (LEDs) has made remarkable advances in just one decade: the external quantum efficiency has improved by over two orders of magnitude and highly saturated color emission is now the norm. Although the device efficiencies are still more than an order of magnitude lower than those of the purely organic LEDs there are potential advantages associated with nanocrystal-based devices, such as a spectrally pure emission color, which will certainly merit future research. Further developments of nanocrystal-based LEDs will be improving material stability, understanding and controlling chemical and physical phenomena at the interfaces, and optimizing charge injection and charge transport.

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.

Fabrication and characterization of red-emitting electroluminescent devices based on thiol-stabilized semiconductor nanocrystals

Thiol-capped CdTe nanocrystals were used to fabricate light-emitting diodes, consisting of an emissive nanocrystal multilayer deposited layer by layer, sandwiched between indium tin oxide and aluminum electrodes. The emissive and electrical properties of devices with different numbers of nanocrystal layers were studied. The improved structural homogeneity of the nanocrystal multilayer allowed for stable and repeatable current- and electroluminescence-voltage characteristics. These indicate that both current and electroluminescence are electric-field dependent. Devices were operated under ambient conditions and a clear red light was detected. The best performing device shows a peak external efficiency of 0.51% and was measured at 0.35mA∕cm2 and 3.3V.

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.

Influence of ferroelectricity on the photoelectric effect of LiNbO3

A comparison between domain dependent photochemical and photoelectric cation reduction in LiNbO3 is presented. The reduction in photoelectric threshold for LiNbO3 due to the depolarization field allows UV irradiation to produce free electrons that can participate in photochemical reduction in silver nitrate. This is in addition to domain directed photophysics, where influences on the space charge layer due to the internal dipole of a ferroelectric determine the carrier at the surface. We show that the interaction of photoelectric and domain dependent influences is observed in LiNbO3 due to the low electron affinity (∼1–1.5 eV) and band bending (0.3–0.8 eV).

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.