Energy Harvesting

Natural Sciences Tripos Part III

MATERIALS SCIENCE

III

MATERIALS SCIENCE

M19: Energy Harvesting

Dr. Sohini Kar-Narayan

Michaelmas Term 2014 15

Name............................. College..........................

Michaelmas Term 2014-15

M19 Energy Harvesting

(Dr S Kar-Narayan)

This course covers the basic principles and recent advances in energy harvesting technologies

for small-power applications, including self-powered or autonomous systems.

Lecture 1: Overall scope and objectives. Introduction to energy harvesting for

autonomous systems; energy requirements and power sources. The role of materials in

energy harvesting.

Lectures 2 4: Photovoltaic (PV) energy harvesting. Evolution of PV materials and devices. Nanostructuring as a route to cheap and efficient PV technologies.

Lecture 5 7: Mechanical energy harvesting. Transduction mechanisms. Piezoelectric, electromagnetic and electrostatic generators. Nano-piezoelectric generators: materials,

performance and example devices. New-generation triboelectric nanogenerators.

Lecture 8. Thermoelectric energy harvesting. Basic thermoelectric theory.

Thermoelectric figures of merit. Novel nanostructured thermoelectric materials and

devices.

Lecture 9: Thermal energy harvesting using pyroelectric materials. Thermodynamic

cycles for pyroelectric energy harvesting. Nanostructured and micro-scale materials and

devices.

Lecture 10: Microbatteries. Thin film batteries for energy storage Materials for

high-energy density 2D and 3D batteries.

Lecture 11: Supercapacitors. Electrolyte and electrode materials. Fundamentals,

challenges and applications.

Lecture 12: Energy harvesting circuits and architectures. Power management

electronics. Relevant circuits and systems.

Recommended textbooks:

Energy Harvesting for Autonomous Systems - Stephen Beeby and Neil White, Artech House.

Energy Autonomous Micro and Nano Systems Marc Belleville and Cyril Condemine, Wiley.

Energy Materials Duncan Bruce, Dermot OHare, Richard Walton, Wiley.

Energy Harvesting Technologies Shashank Priya, Daniel Inman, Springer

Additionally, there are several useful resources available on the internet.

ENERGY HARVESTING Lecture 1

Introduction

Energy-autonomous systems

Power requirements

Ambient power sources

Energy storage

Role of Materials1

Introduction

Some energy harvesting applications:

- Portable electronics and wireless sensors- Bio-devices- Devices in remote or dangerous locations - Devices embedded in structures- Wearable electronics

Energy harvesting from ambient sources for self-powered micro/nanoelectronic devices.

Modern electronics is driven by the reduction in size of devices and the increase in functionality.

Traditional power sources such as batteries need replacing/recharging, do not scale easily with size.

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Smart Mater. Struct. 17 (2008) 043001

Bulky batteries

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Energy-autonomous systemsAdvances in low-power electronics and energy harvesting could make the powering of autonomous systems from ambient energy sources a reality.

Autonomous systems commonly refer to wireless sensor nodes (WSN) devices that can be deployed to monitor parameters of interest and to report these observations back, often to a central data collector known as a sink. [More generally, the term can be used to include small-power consumer electronics including mobile phones, iPods etc.]

Bare minimum requirement of a WSN: 8-bit microcontroller, radio transceiver, sensor and power supply, additional passive circuitry. Nodes have been conventionally battery-powered, but this limits their useful lifetime and the activities they can undertake.

Energy harvesting can eliminate the cost and inconvenience of replacing batteries, reduce waste and potentially enhance the energy-awareness of sensor nodes. 4

Power requirements

The multibillion dollar portable electronics market is an attractive arena for micro- and macro-scale energy harvesting when power requirements can be met. For e.g. the average mobile phone has a power consumption ~1 W during a call and ~ 10 mW in standby.

In situations where energy harvesting is incapable of delivering watts of power, it may permit a near-indefinite standby lifetime or even recharge the device when not in active use. Where battery replacement is not feasible, energy harvesting could potentially extend the battery life of such devices significantly or even indefinitely.

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Self-powered wireless sensors for IoT devices

The Internet of Things (IoT) is the interconnection of uniquely identifiable embedded computing devices within the existing Internet infrastructure. The interconnection of these embedded devices (including smart objects), is expected to usher in automation in nearly all fields, with applications in environmental/structural monitoring, resource management, smart city development etc.

Harvesting energy from ambient sources in our environment has generated tremendous interest as it offers a fundamental energy solution for small-power applications including, but not limited to, ubiquitous wireless sensor nodes, portable, flexible and wearable electronics, biomedical implants and structural monitoring devices. As an example, consider that the number of smart devices linking everyday objects via the internet is estimated to grow to 50 billion by the year 2020. Most of these Internet of Things devices will be extraordinarily small and in many cases embedded, and will wirelessly provide useful data that will make our lives easier, better and more energy-efficient. The only sustainable way to power them is usingambient energy harvesting that lasts through the product lifetime.

Power requirements and energy managementRadio-based communication technologies have achieved dominance for WSNs. A network of WSNs allows for individual nodes to cooperate and participate in routing data in the network. The challenging vision of smart dust (Kris Peter, Univ. California Berkley late 1990s) was for cubic millimetre-sized WSNs, including sensing, power, computation and communication hardware, distributed liberally throughout the environment, providing intelligence to everyday objects.

Reducing power requirements of WSNs is essential. Power can be conserved by sensing only when required and powering off as much as possible between measurements. The entire node architecture needs to be designed with the specific application at hand to maximise the efficiency.

For e.g. a high-speed processor may be more efficient than a low-speed one, but at a higher peak power usage. Slower low-power devices may be more suitable in certain cases. One must match the computing power to the actual application. A two-processor solution has been suggested whereby a small-power processor is used for low-intensity tasks and a larger processor is turned on to perform processor-intense functions and communication.

Intelligent energy management involves monitoring the availability of harvested energy from multiple ambient energy sources and enabling decisions to be made regarding how long a WSN should spend in the low-power sleep mode if the energy used in its active state is to be replenished. For e.g. during times of low ambient energy, a sleep duration may be set that will not allow fill replenishment of the energy source between cycles, but will keep the WSN at the minimum level of availability required. Once more energy is available, the WSN can replenish its energy source and have its availability increased by reducing the sleep time.

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Power requirements

The active current draw of sensor nodes is typically several orders of magnitude larger than the sleep current. Thus, overall power requirement of sensor nodes depends on the duty cycle (DC), i.e. the percentage of one period in which the node is active.

Energy reduction = (1 DC) (1 Psleep/Pactive)

Submilliwatt average operating powers mean that sensor nodes can potentially be powered for very long periods by batteries, or indefinitely using harvested energy. The rate of power generation from energy harvesting devices is typically insufficient to directly power the sensor node in its active mode, so it is necessary to store the energy in capacitors or rechargeable batteries.

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Regenerative energy harvesting

- Ambient sources could replace the need for batteries

- Sun, ubiquitous vibrations, waste heat etc.


Ambient power sources

Energy density comparison of natural energy sources10

Ambient power sources The SunThe Sun emits electromagnetic radiation as a black body at 5800 K. The energy distribution increases in intensity from the ultraviolet (UV) to the visible, with a maximum around 500 nm, tailing into the infrared (IR) and some in the radiowave, microwave, X-ray and gamma ray regions.

Sunlight is clean and abundant more energy strikes the Earth in 1 h (4.3 X 1020 J) than all the energy consumed on the planet in a year (4.1 X 1020 J) !! Solar energy conversion is thus a broad and rapidly exploding research field, spurred on by government incentives and lower production costs. 11

Ambient power sources The SunX-rays, gamma rays and UV radiation below 200 nm in wavelength are absorbed selectively by nitrogen and oxygen in the atmosphere, UV radiation between 200 nm and 300 nm is absorbed by ozone (O3) in the stratosphere, IR radiation above 700 nm is partially absorbed by CO2, O3 and water, and 30 % of the visible radiation (400 700 nm) is reflected back by the atmosphere or the Earths surface.

At high noon on a cloudless day, the surface of the Earth typically receives ~ 1000 W of solar power per square metre (I kW/m2). A cloudy day will provide ~100 W/m2 and ~5 W/m2 will be incident on most surfaces within a well-lit room.

Typical solar cells have efficiency values between 5 20 % under standard conditions and will often be much less efficient under low illumination levels.

The disadvantages of solar power include constraints in terms of positioning, period of operation, incompatibility with embedded/indoor devices.

Solar energy is commonly used in low-power electronic devices such as calculators. It is also often employed for isolated noncritical outdoor systems such as parking meters, weather stations and traffic information systems. It is less likely to be used in portable high-power systems. 12

Mechanical energy harvestingUbiquitous nature of vibrations makes mechanical energy harvesting a promising energy generation technology.

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Mechanical energy harvesting

Mechanical energy harvesting techniques-Electromagnetic energy harvesting

Production of electricity from the motionof a magnetic field relative to a conductive coil, which causes

current to flow in the coil

Well-established, but challenges in MEMS technology: assembly and alignment of sub-millimetre electromagnetic systems for implementation into small electronic devices is

difficult

-Electrostatic energy harvestingBased on the changing capacitance of vibration-dependent varactors.

Can be integrated into MEMS, but require input voltage/charge, issues with parasitic capacitances

- Piezoelectric energy harvestingBased on inherent piezoelectric properties of certain materials

Simplest means of scavenging power directly from ambient vibrations, well suited to MEMS, relatively easy to fabricate, no requirements for input voltage/charge or

additional complex circuitry and/or geometries

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Thermal energy harvesting

Thermal energy is ubiquitous and found in almost any environment, a large amount of which is unused.

In 1821, Thomas Johann Seebeck discovered that a thermal gradient formed between two dissimilar conductors produces a voltage. At the heart of the thermoelectric effect is the fact that a temperature gradient in a conducting material results in heat flow; this results in the diffusion of charge carriers. The flow of charge carriers between the hot and cold regions in turn creates a voltage difference. The Seebeck effect is thus the underlying principle of thermal energy harvesting.

Thermoelectric devices can be used to convert waste heat from automobiles or industry into useful electricity. They can also be integrated into autonomous systems to enhance their capability and lifetime by harvesting thermal energy from the environment. This can even be in the form of human body heat.

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Thermal energy harvestingTypically the average rate of heat generation for a human body is 100 W, and in normal environments, the temperature difference between the human body and ambient is ~ 5 10 K. The power output that can be harvested using a thermoelectric device is estimated to be ~ 20 50 W/cm2, i.e. 2 5 mW may be obtained with a realistic surface area of 0.1 m2. For e.g. self-powered energy harvesting aircraft seat with embedded WSN that report information such as occupancy, backrest, tray-table position etc. to a remote flight attendance panel.

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Schematic of a thermoelectric wristwatch. A miniature thermoelectric converter that consists of 2,268 pairs of Bi2Te3 thermocouples is mounted on the bottom case of the watch. It produces on average 25 W of electricity from a temperature difference of 23 K generated by body heat. The conversion efficiency is about 0.1%

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Thermal energy harvesting using pyroelectric materials

Pyroelectric materials are polar materials and exhibit a spontaneous polarization Ps in the absence of an applied electric field. Examples of polarisation include that of ionically bonded materials whereby the polarisation can be a consequence of the crystal structure, while in crystalline polymers with aligned molecular chains it can be due to the alignment of polarised covalent bonds.

The ability of small changes in temperature to produce a pyroelectric current has been exploited for infra-red imaging and motion detection by body heat. This small electric current can also be considered for energy harvesting applications.

Thermoelectric materials and systems generate electrical power from temperature gradients (dT/dx) while pyroelectric materials produce power from temperature fluctuations (dT/dt). Pyroelectric materials are of interest since under the correct conditions they have the potential to operate with a high thermodynamic efficiency and, compared to thermoelectrics, do not require bulky heat sinks to maintain a temperature gradient.

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Energy storage

There has been tremendous interest in the development of microenergy storage devices, in particular batteries and electrochemical capacitors, to be used in conjunction with one or more energy harvesters to provide permanent power to autonomous wireless systems.

Electrochimica Acta 45, 2483 (2000)

Development of micro-batteries and super-capacitors

Role of materials in energy harvesting technologiesThe development of energy harvesting technologies has been spurred by the development of materials in the respective fields. The ultimate success of energy harvesting technologies would depend on the development of novel materials and smart devices.

Photovoltaic technology:

Crystalline silicon This forms the basis of established PV technology. Si-based systems maje up ~ 90% of the current PV market. The raw materials are expensive and require energy-intensive processing (high purity Si wafers). Polycrystalline solar cells have lower efficiency due to the relative impurity of the Si and therefore require a larger area.

Thin-film technologies Advantages include ease of manufacture of large area at lower cost, wider range of applications, attractive appearance, possibility of assembling devices using flexible substrates. Most established thin-film technology use amorphous Si (a-Si). Others include Cadmium Telluride (CdTe) and Copper Indium (Gallium) Diselenide (CIGS). Multijunction devices with improved efficiencies.

Emergent technologies These are mostly driven by nanotechnology, including dye-sensitised solar cells, organic polymer solar cells and bulk-heterojunction solar cells.

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Role of materials in energy harvesting technologies

Mechanical energy harvesting technology:

(Piezoelectric energy harvesting) Traditional ceramics including Lead zirconium titanate and Barium titanate. Piezoelectric semiconductors e.g. Zinc oxide. Piezoelectric polymers, e.g. poly(vinylidene fluoride). Next generation nanogenerators driven by advances in nanotechnology

Thermal energy harvesting technology:

End of Lecture 1 21

Nanotechnology-driven advances!

ENERGY HARVESTING Lecture 2 Solar Energy Harvesting (Part 1)

Semiconductor Basics

Solar cell working principle

Performance parameters and requirements

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Solar Energy Harvesting

Electronic devices positioned outdoors, in rooms with windows or frequently used artificial light sources could benefit from photovoltaic (PV) technologies. The Sun can provide 1000 W/cm2 of optical power outdoors on a sunny day 100 W/cm2 on a cloudy day, and around 5 W/cm2 within a well-lit room.

Solar energy harvesting is based on PV cells which generate electric current when exposed to light. PV cells are based on semiconductor materials mostly.

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Semiconductor basicsA semiconductor has low concentrations of thermally generated intrinsic charge carriers at finite temperatures. It can be made n-type by adding donor atoms (e.g. phosphorus in silicon) , or p-type by adding acceptor impurities (e.g. boron in silicon).

When a junction is formed between n- and p-type semiconductors, the concentration gradient of charge carriers at the junction causes a net diffusion of electrons from n- type material to the p-type material and net diffusion of holes from p-type material to n-type material, leaving behind a charged depletion region on either side of the junction. This leads to the formation of an electrostatic field and a built-in voltage across the junction.

This built-in potential is used to separate photo-generated electron-hole pairs that are created when light is incident on a p-n junction 24

Semiconductor materials have bands where electrons are located and gaps in between where they are not. The highest energy band where electrons are located is the valence band (VB) and the lowest lying unoccupied band is the conduction band (CB). The gap in between these is called the band gap (Eg) where there are no energy states. When a photon of with sufficient energy (larger than Eg) is absorbed, the energy is transferred to an electron in the VB and it is promoted to a higher energy state in the CB, leaving behind a positively charged vacancy (hole) in the VB. The electron-hole pairs are separated and move in opposite directions until they are collected and the resulting current can be extracted to an external circuit to perform work.

Solar cell working principle

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Solar cell characteristicsIf electrical contacts are added to a p-n junction and a voltage V is applied, the device exhibits rectifying behaviour and the current passing through the device can be described by the ideal diode equation:

I = I0 [exp(qV / kT) -1]Where I0 is the reverse saturation current, k is the Boltzmann constant and T is the temperature

A solar cell is effectively an unbiased diode that is exposed to light. The injection of minority carriers due to absorption of photons adds to the drift current and this can be incorporated into the diode equation as an illumination current IL. Thus I-V characteristics of a solar cell is given by I = I0 [exp(qV / kT) -1] IL

Fill factor, f = VpIp/VocIscEnergy conversion efficiency = fVocIsc/Pi , where Pi is the incident light power 26

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Solar cell performance parameters

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Quantum EfficiencyThe "quantum efficiency" is the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell. The quantum efficiency may be given either as a function of wavelength or as energy. If all photons of a certain wavelength are absorbed and the resulting minority carriers are collected, then the quantum efficiency at that particular wavelength is unity. The quantum efficiency for photons with energy below the band gap is zero. A quantum efficiency curve for an ideal solar cell is shown below.

The "external" quantum efficiency (EQE) of a silicon solar cell (above) includes the effect of optical losses such as transmission and reflection. However, it is often useful to look at the quantum efficiency of the light left after the reflected and transmitted light has been lost. "Internal" quantum efficiency refers to the efficiency with which photons that are not reflected or transmitted out of the cell can generate collectable carriers.

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Solar cell requirements

Two major factors at play Absorption of light and Recombination of minority carriers

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Absorption of lightPhotons incident on the surface of a semiconductor will be either reflected from the top surface, will be absorbed in the material or, failing either of the above two processes, will be transmitted through the material. For photovoltaic devices, reflection and transmission are typically considered loss mechanisms as photons which are not absorbed do not generate power. If the photon is absorbed it has the possibility of exciting an electron from the valence band to the conduction band. A key factor in determining if a photon is absorbed or transmitted is the energy of the photon (Eph). Therefore, only if the photon has enough energy will the electron be excited into the conduction band from the valence band. Photons falling onto a semiconductor material can be divided into three groups based on their energy compared to that of the semiconductor band gap:

Eph < Eg : Photons with energy Eph less than the band gap energy EG interact only weakly with the semiconductor, passing through it as if it were transparent.

Eph = Eg : Photons have just enough energy to create an electron hole pair and are efficiently absorbed.

Eph > Eg : Photons with energy much greater than the band gap are strongly absorbed. However, for photovoltaic applications, the photon energy greater than the band gap is wasted as electrons quickly thermalize back down to the conduction band edges.

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The absorption coefficient, , determines how far into a material light of a particular wavelength can penetrate before it is absorbed. In a material with a low absorption coefficient, light is only poorly absorbed, and if the material is thin enough, it will appear transparent to that wavelength. The absorption coefficient depends on the material and also on the wavelength of lightwhich is being absorbed. Semiconductor materials have a sharp edge in their absorption coefficient, since light which has energy below the band gap does not have sufficient energy to excite an electron into the conduction band from the valence band. Consequently this light is not absorbed. The absorption coefficient for several semiconductor materials is shown below.

Absorption coefficient

(T = 300 K)

For photons which have an energy very close to that of the band gap, the absorption is relatively low since only those electronsdirectly at the valence band edge can interact with the photon to cause absorption. As the photon energy increases, a larger number of electrons can interact with the photon and result in the photon being absorbed.

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The absorption depth is given by the inverse of the absorption coefficient, and describes how deeply light penetrates into a semiconductor before being absorbed. (

Higher energy light is of a shorter wavelength and has a shorter absorption depth than lower energy light, which is not as readily absorbed, and has a greater absorption depth.

Absorption depth affects aspects of solar cell design, such as the thickness of the semiconductor material.

Light intensity at any point in the device e-x (x is distance into the material)

The absorption depth is a useful parameter which gives the distance into the material at which the light drops to about 36% of its original intensity, or alternately has dropped by a factor of 1/e. Since high energy light (short wavelength), such as blue light, has a large absorption coefficient, it is absorbed in a short distance (for silicon solar cells within a few microns) of the surface, while red light (lower energy, longer wavelength) is absorbed less strongly. Even after a few hundred microns, not all red light is absorbed in silicon.

Absorption depth and generation

The generation rate gives the number of electrons generated at each point in the device due to the absorption of photons. Generation is an important parameter in solar cell operation given by

Because the light used in photovoltaic applications contains many different wavelengths, many different generation rates must be taken into account when designing a solar cell.

nmcm-1

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Bulk recombinationAny electron which exists in the conduction band is in a meta-stable state and will eventually stabilize to a lower energy position in the valence band. When this occurs, it must move into an empty valence band state. Therefore, when the electron stabilizes back down into the valence band, it also effectively removes a hole. This process is called recombination. There are three basic types of recombination in the bulk of a single-crystal semiconductor.

Radiative recombination: An electron from the conduction band directly combines with a hole in the valence band and releases a photon. The emitted photon has an energy similar to the band gap and is therefore only weakly absorbed such that it can exit the piece of semiconductor. This recombination mechanism dominates in direct bandgap semiconductors. In radiative recombination.

Shockley-Read-Hall or SRH recombination involves recombination through defects in the crystal lattice. These defects can either be unintentionally introduced or deliberately added to the material, for example in doping the material.

Auger Recombination involves three carriers. An electron and a hole recombine, but rather than emitting the energy as heat or as a photon, the energy is given to a third carrier, an electron in the conduction band. This electron then thermalizes back down to the conduction band edge. This recombination mechanism is most important at high carrier concentrations caused by heavy doping.

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Minority carrier lifetime and diffusion lengthThe lifetime of minority carriers generated by incident light in a semiconductor is contingent upon the recombination rate R, which is dependent upon the concentration of minority carriers. The lifetime of the material takes into account the different types of recombination. Lifetime is an indicator of the efficiency of a solar cell, and thus is a key consideration in choosing materials for solar cells.

where is the minority carrier lifetime, n is the excess minority carriers concentration and R is the recombination rate.

Diffusion length is the average length a carrier moves between generation and recombination. Semiconductor materials that are heavily doped have greater recombination rates and consequently, have shorter diffusion lengths. Higher diffusion lengths are indicative of materials with longer lifetimes, and is therefore an important quality to consider with semiconductor materials.

where L is the diffusion length in meters, D is the diffusivity in m/s and is the lifetime in seconds.

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Areas of defect, such as at the surface of solar cells where the lattice is disrupted, recombination is very high.

Surface recombination is high in solar cells, but can be limited.

Understanding the impacts and the ways to limit surface recombination leads to better and more robust solar cell designs.

Surface recombination

The defects at a semiconductor surface are caused by the interruption to the periodicity of the crystal lattice, which causesdangling bonds at the semiconductor surface. The reduction of the number of dangling bonds, and hence surface recombination, is achieved by growing a layer on top of the semiconductor surface which ties up some of these dangling bonds. This reduction of dangling bonds is known as surface passivation.

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Collection probablilityThe "collection probability" describes the probability that a carrier generated by light absorption in a certain region of the device will be collected by the p-n junction and therefore contribute to the light-generated current, but probability depends onthe distance that a light-generated carrier must travel compared to the diffusion length. Collection probability also depends onthe surface properties of the device. The collection probability of carriers generated in the depletion region is unity as the electron-hole pair are quickly swept apart by the electric field and are collected. Away from the junction, the collection probability drops. If the carrier is generated more than a diffusion length away from the junction, then the collection probability of this carrier is quite low. Similarly, if the carrier is generated closer to a region such as a surface with higher recombination than the junction, then the carrier will recombine. The impact of surface passivation and diffusion length on collection probability is illustrated below.

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(May not need to be perfectly ordered)

Nanostructuring as a solution?

End of Lecture 2

LA absorption thicknessLC charge transport thickness


Efficiency of photovoltaic (PV) cells

Losses in PV cells

Overview of PV Materials and Technologies

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Shockley-Quesisser LimitShockleyQueisser limit or detailed balance limit refers to the calculation of the maximum theoretical efficiency of a solar cell made from a single pn junction. It was first calculated by William Shockley and Hans Queisser:William Shockley and Hans J. Queisser, "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells", Journal of Applied Physics, Volume 32, pp. 510-519 (1961).

The ShockleyQueisser limit is calculated by examining the amount of electrical energy that is extracted per incident photon.The calculation places maximum solar conversion efficiency around 33.7% assuming a single p-n junction with a band gap of 1.4 eV (under one sun). Therefore, an ideal solar cell with incident solar radiation will generate 337 Wm-2. When the solar radiation is modelled as 6000 K blackbody radiation the maximum efficiency occurs when the bandgap energy Eg=1.4 eV.

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Shockley-Quesisser Limit

Basic Assumptions

1. One semiconductor material (excluding doping materials) per solar cell.

2. One p-n junction per solar cell.

3. The sunlight is not concentrated - a "one sun" source.

4. All energy is converted to heat from photons greater than the band gap.

In order to overcome the S-Q limit, one must work around one or more of the critical assumptions listed above, i.e.

1. Use more than one semiconductor material in a cell.

2. Use more than one junction in a cell - "multijunction cells".

3. Concentrate the sunlight using concentrators

4. Combine a PV semiconductor with a heat based technology to harvest both forms of energy

Efficiency losses

Progress in Photovoltaics: Research and Applications 17, 320 (2009).41

Electrical losses

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IL

I

(n is the nonideality factor, between 1 and 2)

Equivalent circuit for nonideal solar cell

Electrical lossesBulk recombination: caused by defects within the semiconductor e.g. structural and intrinsic defects, extrinsic defects due to presence of impurity atoms. Source materials must be high-purity. For multicrystalline semiconductors, grain boundaries represent a significant source of recombination.

Biggest trade-off in a solar cell is the decision over thickness. Carrier collection is best for thin devices but photon collection is best for thick devices.

Surface recombination: caused due to defect chemistry at the surface and concentration of free carriers at surface. Efforts to reduce this effect include addition of a thin passivating material such as SiO2 to saturate dangling bonds and reduce surface defect density, field-effect passivation and creation of back-surface field.

Non-ideal diode behaviour: caused by non-uniform acceptor and donor impurity profiles, thickness variations.

Series resistance: arises due to the resistance of the bulk semiconductor material and contact resistances, and should be as small as possible.

Parallel resistance: caused by pin-holes that break through thin film p-n junctions, conductive paths that can be formed through grain boundaries, problems at the edges of devices. Should be as high as possible.

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Minimising surface recombination

Surface passivation: reduce dangling bonds Field-effect passivation: Since the passivating layer for silicon solar cells is usually an insulator, any region which has an

ohmic metal contact cannot be passivated using silicon dioxide. Instead, under the top contacts the effect of the surface recombination can be minimised by increasing the doping.

Back-surface field: Higher doped region at the rear surface of the solar cell. The interface between the high and low doped region behaves like a p-n junction and an electric field forms at the interface which introduces a barrier to minority carrier flow to the rear surface. The minority carrier concentration is thus maintained at higher levels in the bulk of the device and the back-surface field has a net effect of passivating the rear surface.

Optical lossesAny process that leads to photon loss will lead to a decrease in the current that can be generated by a solar cell.

Surface Reflection: Reflection from both top and rear surface of the cell leads to optical losses. Anti-reflection coatings or surface texturing help to mitigate this problem.

Top contact shading: Metals reflect light and shadow underlying device. Use of transparent conducting oxides (e.g. indium tin oxide) or thin metals reduces shading at the expense of increasing RS.

Incomplete absorption: Amount of light absorbed depends on amount of material, and hence thickness. Material cost of thick cells are higher plus the quality of the materials must be ensured. Light-trapping schemes for cheaper thin-film designs are required to increase effective optical path length.

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Optical lossesTop surface reflectance: The semiconductor surface itself will reflect a component of light depending on the refractive index of the semiconductor, the angle of incidence and the wavelength, thereby resulting in reduction in the short-circuit current. Thus antireflection schemes have to be employed to reduce reflectance. (Bare Si has a high surface relflectance of over 30%)

Antireflection techniques include:

Thin film coatings Destructive interference between light reflected from the interfaces created by adding one or more thin films to a surface minimizes reflectance at certain wavelengths. The refractive index and thicknesses of the layers must be carefully chosen for optimum reflection reduction over the required wavelength range.

Micron-scale texturing - Texturing with featured of dimensions above the wavelength of incident light reduces overall reflectance by forcing the light to undergo multiple reflections from the inclined walls of the features, with a portion of this light being coupled into the substrate at each reflection

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Anti-reflection coating

Anti-reflection coating consist of a thin layer of dielectric material, with a specially chosen thickness so that interference effects in the coating cause the wave reflected from the anti-reflection coating top surface to be out of phase with the wave reflected from the semiconductor surfaces. These out-of-phase reflected waves destructively interfere with one another, resulting in zero net reflected energy.

For a quarter wavelength anti-reflection coating of a transparent material with a refractive index n1 and light incident on the coating with a wavelength 0, the thickness d1 which causes minimum reflection is calculated by:

d1 = 0/4n1

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Subwavelength-scale texturing - Reflections occur at an interface between two materials because of a sudden change in refractive index. By texturing on the sub-wavelength scale at the interface, a more gradual change in refractive index can be introduced and so such reflections can be significantly reduced. The interface is effectively blurred resulting in a low reflectance for a broad range of wavelengths.

Texturing

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Planar reflector on the back of the device

Texturing with geometric features on front and back of the device

Light-trapping schemes

Light trapping using a randomised reflector on the rear of the cell. Light less than the critical angle escapes the cell but light greater than the critical angle is totally internally reflected inside the cell. In actual devices, the front surface is also textured using schemes such as the random pyramids mentioned earlier.

Module losses

Losses result when a solar cell is encapsulated into a module. Reflectance and absorption losses in encapsulant materials are low as they tend to have low refractive indices and are chosen for their transparency, e.g. glass.

Arrangement of single crystal silicon solar cells that are generally made from circular wafers gives rise to loss of efficiencydue to sub-optimal use of the surface region of the PV module. Cleaving or sawing to make square or hexagonal wafers is expensive and wasteful.

Series configuration of individual cells connected to form a module introduces losses. Although voltage is increased, the current is limited by the worst-performing cell in the array.

Careful placement of module is necessary to ensure all cells receive the same level of irradiance, for e.g. partial shading must be avoided.

Encapsulation affects heat dissipation, raising the operating temperature and reducing the open-circuit voltage. Severe overheating can cause cracks due to thermal expansion.

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C-Si solar cells High Price of Low Carbon?

Materials and technologiesCurrently, crystalline Si makes up 90% of the market share of PV cells

Materials and technologiesThere exists a large diversity of PV devices already in commercial production and many more being developed in labs.

Currently, most commercial PV devices are based on single crystal silcon (C-Si) having 14 17% efficiency. The cost of Si wafer preparation accounts for almost half the total cost of a C-Si-based PV module expensive! Multicrystalline silicon (mC-Si) is a cheaper alternative that can produce useful but less efficient PV cells (10 14% efficiency).

Textured AR surface

Metal top contact

Metal back contact

Schematic of a typical C-Si device (mC-Si devices use wavelength AR coatings)

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Materials and technologiesAmorphous hydrogenated silicon (a-Si:H) is a disordered network on silicon and hydrogen atoms. Effective Eg ~ 1.7 eV, so it cannot absorb photons with wavelength > 700 nm. But absorption of short-wavelength light in a-Si:H is good, hence it is suitable for indoor applications.

Staeblar-Wronski (SW) effect refers to the degradation of photocurrent in a-Si:H with exposure to light and is a serious limitation as cells can lose as much as 10% of their initial efficiency after a few months of use. Original photocurrent restorable with an anneal of 1500C. SW effect attributed to breaking of SiSi or Si-H bbonds within the random amorphous network leading to increase in midgap defect density and hence recombination current.

Can be deposited onto flexible polymer substrated due to low deposition temperatures. Efficiency in the range 8 10%. Devices typically used in pocket calculators have efficiency ~ 5%. Low cost and flexibility of a-Si:H makes it viable for low-power indoor applications

pin design

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Materials and technologiesMultijunction (MJ) devices consist of multiple p-n junctions having multiple bandgaps, each producing a current in response to a different wavelength of light, thus increasing overall conversion efficiency. Each subdevice must be current-matched, thus thickness of each layer must be carefully optimised.

Triple-junction GaInP/GaS/Ge device(Eg ~ 1.4 2.2 eV)

Efficiency of up to 32 % using concentrator cells. Very expensive technology.

MJ Si cell Efficiency of up to 16 %

Key consideration current matching!!54

Materials and technologiesCadmium Telluride (CdTe) has a direct bandgap of 1.45 eV and forms a good heterojunction with CdS (Eg =2.4 eV) High efficiency (10 16%) and low cost (< $1/W) makes CdTe technology a strong contender as the leading PV technology poised to displace C-Si and mC-Si technology. ButToxicity of Cd? Availability of Te?

CuInSe2 (CIS), has the highest absorption coefficient across the broadest spectral range. Slightly less than optimum Eg of 1.04 eV can be increased by the addition of gallium to form CuIn1-xGaxSe2 (CIGS). CIGS lab cells have the highest efficiency record of 19.4%. Disadvantage poor scalability, low yield and high production cost.

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Traditional Photovoltaic Materials

Materials and technologiesEmergent technologies Dye Sensitized Solar cells (DSSC) are bulk heterojunction devices fomed by complete intermixing of the two materials

that form the anode (n-type) and cathode (p-type). The anode is typically formed by nanospheres of TiO2photosensitized by dye, and the cathode is a liquid/solid electrolyte that completely surrounds the anode.

Absorption of light creates excited state in dye which injects an electron into the conduction band of TiO2. Electrons diffuse through successive nanospheres till they reach the top contact. In order to maintain neutrality, the dye molecule takes an electron from the electrolyte, forming a positively carged ion that diffuses through the electrolyte to the cathode.

Very efficient system limited by inability to absorb red and infrared photons. DSSC devices are cheap, can be manufactured by printing technologies rather than expensive semiconductor technologies.

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Materials and technologies Inoganic bulk heterojunction (BHJ) solar cells: use 3D junctions instead of planar junctions. Advanced designs include core-

shell nanowire structures, involving self-organized p-type nanowires surrounded by n-type layer creating an array of core-shell nanodevices that each for classic single crystal solar cells.

These devices are in early stages of development and have relatively low efficiency. But these nanodevices can act as standalone energy harvesting units, possibly to power single CMOS gates.

BHJ Polymer/plastic solar cells: light generates excitons with subsequent separation of charges in the interface between an electron donor and acceptor blend within the devices active layer. Regions of each material in the device are separated by only several nanometers, a distance suited for carrier diffusion. BHJs require sensitive control over materials morphology on the nanoscale. A number of variables, are important including choice of materials, solvents, and the donor-acceptor weight ratio. (For e.g. poly-3-hexyl thiophene (P3HT), phenyl-C61-butyric acid methyl ester (PCBM) are used to form P3HT:PCBM solar cells) 58

End of Lecture 3


Evolution of PV materials and technologies

Recent advances

Future trends

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First-generation solar cells

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Second-generation solar cells

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Third-generation solar cells

Eex exciton energy, IPD - ionization potentialEAa electron affinity

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Organic Photovoltaics

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Device Working Principle from Light Absorption to Charge Collection

Organic Photovoltaics

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ExcitonsWhen incident photons hit electrons at the ground state, inorganic semiconductors generate free carriers. However, in organic semiconductors, excited electrons slightly relax (due to lower Coulomb screening) and then form an exciton, a bounded electron and hole pair.

To make an efficient organic photovoltaic cell, effective dissociation of excitons is a key issue because the binding energy of the exciton is large.

Solution?

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Dye-sensitized solar cells

67

Dye-sensitized solar cells

68

Bulk-heterojunction solar cells

69

Bulk-heterojunction solar cells

70

Nanoparticle hybrids

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Nanoparticle hybrid solar cells

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Quantum dot solar cells - basics

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Quantum dot solar cells - basics

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Quantum dot solar cells

Solar Energy Materials and Solar Cells, 117 (2013) 329 335

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Perovskite solar cells

NATURE MATERIALS | VOL 13 | SEPTEMBER 2014 |

Methylammonium lead iodide, CH3NH3PbI3

semiconducting pigment with a direct bandgap of 1.55 eV corresponding to an absorption onset of 800 nm which makes this material a good light absorber over the whole visible solar emission spectrum.

The excitons produced by light absorption have a weak binding energy of about 0.03 eV, which means that most of them dissociate very rapidly into free carriers at room temperature.

The electrons and holes produced in this material exhibit a small effective massresulting in high carrier mobilities.

Their recombination occurs on a timescale of hundreds of nanoseconds, resulting in long carrier-diffusion lengths that is, the average distance that can be covered by carriers before they recombine ranging between 100 nm and 1,000 nm

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Perovskite solar cellsThe extraordinary potential of hybrid perovskites in photovoltaic applications was only revealed less than 5 years ago by researchers working on liquid electrolyte-based dye-sensitized solar cells (DSSC)!

Reports on the use of tin or lead iodide perovskites in a solid-state version of the DSSC, set off the current meteoric rise ofperovskite solar cells (PSCs).

Burschka, J. et al. Nature 499, 316319 (2013).

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Perovskite solar cellsEvolution from a mesoscopic to a planar embodiment of the perovskite solar cell.

Problems:

Toxicity!Pb-compounds are harmful to the environment

Stability?CH3NH3PbI3 degrades in humid conditionsand forms PbI2 at higher temperatures dueto the loss of CH3NH3I. These instabilitiescould hamper outdoor applications.

End of Lecture 4

ENERGY HARVESTING Lecture 5 Mechanical Energy Harvesting (Part 1)

Inertial generator model

Electrostatic generators

Electromagnetic generators

Piezoelectric generators (Introduction)

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Mechanical energy harvesting

We will discuss the three main transduction mechanisms that are employed to extract mechanical energy from ambient environments and translate this into electrical energy, namely electrostatic, electromagnetic and piezoelectric transduction.

Kinetic energy is ubiquitously present in the environment. Useful vibration levels are found in many commonly used appliances such as refrigerators and washing machines, moving structures such as automobiles and airplanes, and civil structures such as buildings and bridges. Human movements are mainly characterized by low-frequency, high amplitude displacements.

Example of typical frequency spectrum of mains powered air compressor

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Mechanical energy harvesting energy flow

Commonly used transduction mechanisms: electrostatic, electromagnetic and piezoelectricRecent advance: Triboelectric transduction mechanism

Losses can be incurred, not just within the energy harvesting transducer, but at all stages in this process. Theeffectiveness of the transducer is not the only factor - performance can be dominated by losses in the transfer ofenergy across these system boundaries.

Mechanical energy harvesting simplified modelA simplified linear model based on an inertial generator can be used to estimate the behaviour and output power of a resonant mechanical harvester [Williams & Yates, Sensors and Actuators A 52, 8 (1996)]. An inertial generator can be attached to any moving body to generate electricity.

Consider a rigid box subjected to the environments vibrations y(t), with a mass m suspended by a spring k. The relative displacement of mass m with respect to its equilibrium position is represented by z(t) and its natural angular frequency n = (k/m)1/2. Part of the kinetic energy is lost in mechanical damping and the rest is converted to electricity which is modelled as an electrical damping. Together they are represented by the damping coefficient, cT.The external excitation is given by y(t) = Ysin(t). Assuming the mass of the vibrating source >> m, the differential equation of motion is described as:

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Mechanical energy harvesting Inertial generator

The steady-state solution is given by:

, where

The maximum energy is extracted when the excitation frequency equals the natural frequency,

[Williams & Yates, Sensors and Actuators A 52, 8 (1996)]

Power dissipated due to transduction and parasitic damping,

Therefore, maximum power generated is given by

,

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Mechanical energy harvesting Inertial generator

Substituting for excitation acceleration level, A = n2Y,

The value of maximum power is indeed finite, as reduction of the damping factor results in increased mass displacement which is ultimately limited by the size and geometry of the device.

The maximum power that can be extracted by the transduction mechanism can be predicted by accounting for the parasitic and transducer damping ratios and is given by:

Pe is maximized when parasitic damping equals transducer damping, p = e. Varying the level of damping provides a handle on bandwidth response of the generator, which may be useful in practical situations where frequency variations are common. Pe is proportional to mass, which should therefore be maximised subject to design constraints. For fixed A, Pe is inversely proportional to n, hence it is preferable to operate at the lowest fundamental frequency within the available vibration spectra. When a generator is coupled to an electrical circuit, losses on the circuit will limit the amount of useful electrical energy.

4

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Electromagnetic transductionElectromagnetic generators are based on Faradays law of electromagnetic induction. When an electric conductor is moved through a magnetic field, a potential difference, or electromotive force (emf), is induced between the ends of the conductor. The voltage induced in the conductor (V) is proportional to the time rate of change of the magnetic flux linkage () of that circuit:

In practice, the conductor is in the form of a coil of length l having N turns, and the magnetic field B is created by permanent magnets.

Damping coefficient arising from electromagnetic transduction,

Where, Rload, Rcoil and Lcoil are the load resistance, coil resistance and coil inductance respectively. Thus Rload can be used to adjust ce to match cp and therefore maximise power.

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Electromagnetic generators - microscale implementationMicroscale electromagnetic generators are fabricated with planar microcoils and deposited magnetic films. Both of these are inferior to their conventional macroscale counterparts.

While macroscale, high performance bulk magnets, and multi-turn and macroscale coils are readily available, there remain challenges for fabrication of MEMS scale systems due to the poor properties of planar magnets. The number of turns that can be achieved with planar coils are limited the assembly and alignment of wafer-scale (sub-millimeter) electromagnetic systems remain a challenge for implementation to MEMS devices.

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Novel electromagnetic generator devices


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Electrostatic generatorsThese types of generators consist of a variable capacitor whose two plates are electrically isolated from each other by air, a vacuum, or an insulator. External mechanical vibrations cause the gap between the plates to vary and hence the capacitance changes. In order to extract energy, the plates must be charged and the mechanical vibrations work against the electrostatic forces present in the device.

Capacitor Basics:

The capacitance of a capacitor is given by: C = Q/V, where Q is the charge and V is the voltage

It can also be expressed as C = A/d,

where is the dielectric permittivity of the materials between the plates, A is the area of the plates and d is the separation between the plates.

The stored energy is given by E = 0.5QV = 0.5CV2 = 0.5Q2 C

Voltage- and charge- constrained generators:

Energy gain:

Perpendicular force between plates:

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Electrostatic generatorsElectrostatic generators can be classified into three configurations:

Electrical damping coefficient for gap-varying and overlap-varying cases (Coulomb damping):

Ng is number of gaps, l is finger length, t is device thickness.

Note x here refers to the inertial mass displacement!

Note z here refers to the inertial mass displacement

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Novel electrostatic generator devices

Electrostatic generators are well-suited for MEMS devices but the main drawback is the need for input charge/voltage. Other problems include high output impedance and voltage, requirement of additional circuitry for signal processing, parasitic capacitances leading to reduced efficiencies and electrode shorting in wafer-scale applications.

Piezoelectric Energy Harvesting

Vibrations cause strain in piezoelectric materialCurrent generated from change in electric displacementCurrent extracted to power devices

Generating electrical energy from material strain.

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Piezoelectric Materials

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Piezoelectric MaterialsPiezoelectricity relates to the electric displacement D (surface charge per unit area) induced in a material by an applied stress T. In three dimensions, stress is described by two vectors, applied force Fi and the normal to the area upon which the force acts Aj. Stress, F/A, is thus a second rank tensor Tij whose diagonal elements represent normal stress and off-diagonal elements represent shear stress. Electric displacement and electric field are vectors Di and Ej respectively, and the permittivity, ij=D/E, is represented by a second rank tensor. Taking T and E as independent variables, D is specified by

where d is the piezoelectric constant of the material and is represented by a third rank tensor. Strain S is a second rank tensor specified by

where dt is the transpose of d, and s is elastic compliance. The above equations are the piezoelectric constitutive equations.

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Piezoelectric Generator

96

Piezoelectric constants in typical energy-harvesting modes:

Piezoelectric damping coefficient,

Piezoelectric constants

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Piezoelectric materials

End of Lecture 5

Energy harvesting performance is directly related to the piezoelectric coefficients, but the applied stress or strain is also an important factor. This is why the coupling between the mechanical source and the piezoelectric material is a critical factor in determining the energy harvesting performance. The energy output also depends on the ability of the piezoelectric material to sustain an applied force or to repeatedly undergo a recoverable strain.


Piezoelectric generators (Continued)

Ceramics versus polymers

Nano-piezoelectric generators

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Piezoelectric generator

The domains in a ceramic element are aligned by exposing the element to a strong, DC electric field, usually at a temperatureslightly below the Curie temperature. This is referred to as the poling process. After the poling treatment, domains most nearly aligned with the electric field expand at the expense of domains that are not aligned with the field,and the element expands in the direction of the field. When the electric field is removed most of the dipoles are locked intoa configuration of near alignment

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Piezoelectric generatorPiezoelectric vibration harvesters exploit piezoelectric properties of materials. In certain cases, the strain in the piezoelectric material is created by the inertia of a suspended mass undergoing acceleration, rather than being directly deformed by the source. There are many ways of achieving this coupling, but perhaps one of the most common is the piezoelectric cantilever

The cantilever is clamped at one end (the root) to the vibration source. A mass is fixed to the other end. When the base accelerates, the inertia of the tip mass bends the cantilever. Simple bending a piezoelectric element creates equal and opposite strains on the inside and outside of the bend. These cancel, so no net current is generated. To be effective as a generator it is necessary to move the piezoelectric layer away from the neutral axis. This is usually accomplished either by fixing the piezoelectric material to a non-piezoelectric elastic layer, or by joining two piezoelectric layers poled in opposite directions. These are referred to as unimorph of bimorph configurations as shown in the figure above.

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Piezoelectric structures

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Novel piezoelectric generator devices

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Piezoelectric generatorPractical considerations

Piezoelectric generators are usually operated at or close to resonance, where the amplitude of the oscillation is only limited by the losses from the mechanical system resulting from the energy harvested as well as internal and external losses due to friction, internal electrical losses and air damping. This means that the most effective energy harvester does not necessarily employ the material with the highest piezoelectric coefficients.

For example, lead zirconium titanate (PZT) is obtainable in a range of compositions from hard materials which have low losses but small piezoelectric coupling, through to soft materials with much higher piezoelectric coupling, but also much higher losses. In some cases, hard materials with much smaller piezoelectric coefficients can produce larger power output than soft materials. However, this depends on the magnitude of the electrical power harvested compared to other sources of loss i.e. the efficiency, and for many systems non-harvested losses dominate.

The maximum oscillation amplitude in a resonant device is determined by the losses. An efficient device could produce very large amplitude oscillations resulting in damage to the device. This means that a practical constraint of the power output of a resonant energy harvester may well be determined by material strength and reliability considerations rather than piezoelectric coefficients.

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Piezoelectric ceramics vs. polymers

Crossley, Whiter & Kar-Narayan, Materials Science and Technology 2014 VOL 30 NO 13a 1615

Piezoelectric Polymers

- Flexible and light- Ease of fabrication- Cost-effective- Lead free

Poly-vinlyidene fluoride (PVDF)105

Polyamides (nylons)

The ferroelectric phase of PVDF

stretching

poling annealing annealing poling

poling

The ferroelectric phase can be stabilised by addition of TrFE

P(VDF-TrFE) 106

Piezoelectric Polymers

C. Chang et al, Direct-write piezoelectric polymeric nanogenerator with high energy conversion efficiency," Nano Letters, vol. 10, no. 2, pp. 726-731, 2010.

Effect of geometry

Piezoelectric nanogenerators (NGs) -BackgroundZnO nanogenerators Wang et. al , Science 2006

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NG based on ZnO arrays

Performance degrades on exposure to air.Surface passivation required

Adv. Mater. 2012, 24, 110114

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NG based on PZT nanowire arrays

Xu et. al, Nat. Communications 2010

Epitaxially grown PZT nanowires on Nb-doped STO by hydrothermal decomposition, high annealing temperature 110

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NG based on BaTiO3 nanowire arrays

The BaTiO3 nanowire arrays were grown on Ti substrate using a two-step hydrothermal synthesis method.Adv. Energy Mater. 2014, 4, 1301660

112


Crossley, Whiter & Kar-Narayan, Materials Science and Technology 2014 VOL 30 NO 13a 1615

Polymer nanogenerators

FlexibleRobustCheapEasy to fabricateLead-freeBiocompatibleAcoustic impedance matchingLow piezoelectric constant

Polymers versus Ceramics?

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Electrospinning> 10 kV

When a sufficiently high voltage is applied to a liquid droplet, the body of the liquid becomes charged, and electrostatic repulsion counteracts the surface tension and the droplet is stretched; at a critical point a stream of liquid erupts from the surface.

Applied Physics Letters 93 (2008) 123111.

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Electrospinning

Electrospinning is a relatively complex fabrication process requiring high voltages (550 kV) and specialized equipment. The associated high electric fields and stretching forces result in poled nanowires, however this fabrication process often suffers from poor control over nanowire size-distribution and alignment, and is yet to be conveniently and cost-effectively scaled up.

Nano Letters 3 (2003) 11671171.

Template Wetting

Simple Scalable Versatile

After forming nanowires inside template:

Can coat template surfaces in metal to make simple devices

Can dissolve template to release nanowires into solution

Can form nanotubes or nanowires Can change dimensions with different

templates

Template-grown P(VDF-TrFE) Nanowires

117R. Whiter, V. Narayan & S. Kar-Narayan, Advanced Energy Materials (2014)

Energy harvesting experiments

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Lighting an LED with a polymer nanogenerator

R. Whiter, V. Narayan & S. Kar-Narayan, Advanced Energy Materials (2014)119End of Lecture 6


Triboelectric Generators

Operating configurations

Outlook

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Triboelectric generatorThe triboelectric effect describes a contact-induced electrification in which a material becomes electrically charged after it is contacted with a different material through friction.

Tribolelectric generator - a device that converts mechanical energy into electricity using the coupling effects between triboelectrification and electrostatic induction through the contact separation or relative sliding between two materials that have opposite tribo-polarity.

Triboelectric generators represent a novel mechanical energy harvesting mechanism and were first demonstrated by ZL Wangs group in Georgia Tech in 2012. The field has since witnessed tremendous growth.

Z.L.Wang, ACSNano 2013

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Fundamentals of Tribolelectrification The triboelectric effect is a contact-induced electrification in which a material becomes electrically charged after it

is contacted with a different material through friction. As an example, triboelectric effect is a general cause of commonly experienced static charging. The sign of the charges to be carried by a material depends on its relative polarity in comparison to the material to which it will contact.

While the effect has been known for many centuries, the mechanism behind tribolelectrification is still not entirely clear. It is generally believed that, after two different materials come into contact, a chemical bond is formed between some parts of the two surfaces, called adhesion, and charges move from one material to the other to equalize their electrochemical potential. The transferred charges can be electrons or may be ions/molecules. When separated, some of the bonded atoms have a tendency to keep extra electrons and some a tendency to give them away, possibly producing triboelectric charges on surfaces.

Materials that usually have strong triboelectrification effect are more likely to be insulators, thus enabling them to capture the transferred charges and retain them for an extended period of time. The build-up of electrostatic charges are usually considered to be a negative effect in our daily life and technology developments.

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Choice of materials

Triboelectric Series for some common materials following a tendency to easily lose electrons (Positive) and to gain Electrons (Negative)

Z.L.Wang, ACSNano 2013

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Traditional triboelectric generators

Van de Graaf generator (1880)

Wimhurst machine (1929)

Both machines use the accumulated static charges generated by triboelectrification; the tribo-charges are transferred from a rotating belt to a metal brush by the corona discharging (e.g., the electric-field-induced arcing of air); once the accumulated charge density reaches a critical value, discharging over two opposite electrodesOccurs.

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Operating principle of triboelectric nanogenerator (TENG)

Vertical Contact-Separation Mode-Based TENG: Dielectric-to-Dielectric Case

Nano Lett. 2012, 12, 49604965.

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Operating principle of triboelectric nanogenerator (TENG)

The operating principle of the TENG for the case of dielectric-to-dielectric in contact mode can be described by the coupling of contact charging and electrostatic induction.

If we define electric potential of the bottom electrode (UBE) to be zero, electric potential of the top electrode (UTE) can be calculated by

where is the triboelectric charge density, 0 is the vacuum permittivity, and d0 is the interlayer distance at a given state.

The net effect is that induced charges accumulate with positive sign on the top electrode and negative sign on the bottom electrode (see figure on previous slide). The induced charge density (0) when the generator is fully released is given by:

where rk and rp are the relative permittivity of Kapton and PMMA, respectively, and d1 and d2 are the thicknesses of the Kapton film and the PMMA layer. The maximum value of max is obtained by substituting d3 for d in the equation above.

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TENG performance

Nano Energy 2013, 2, 491497

128

TENG configurations

Vertical Contact-Separation Mode-Based TENG: Metal-to-Dielectric Case

If is the charge density of the PTFE surface, 1 is the charge density of the Cu surface that is contacted with PTFE, and 2 is charge density of the Ag upper surface, then

where d1 and rp are the thickness and permittivity of PTFE, respectively, and charge Q is stable for a relatively long time on the PTFE surface; thus 1 is dictated by the gap distance d2. The working mechanism of theTENG is similar to a variable-capacitance generator except that the charges are self-generated triboelectric charges rather than an external power source.

Nano Energy 2013, 2, 491497

129

Effect of nanostructuringMicro- or nanopatterns can be generated on surfaces to enhance the contact area and the effectiveness of the triboelectrification

Nano Lett. 2012, 12, 63396346.

130

TENG configurations

Lateral Sliding-Mode-Based TENG: Dielectric-on- Dielectric Case

There are two basic friction processes: normal contact and lateral sliding. A periodic change in the contact area between two surfaces leads to a lateral separation of the charge centers, which creates a voltage drop for driving the flow of electrons in the external load

Nano Lett. 2013, 13, 22262233.

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TENG configurations

Lateral Sliding-Mode-Based TENG: Metal -on- Dielectric Case

Linear gratings with uniform period are fabricated onboth sliding surfaces. The rows of grating units have the same size as the intervals in between, with all rows being electrically connected at both ends by two buses. The grating patterns on both sliding surfaces are identical so that they can match well with each other when aligned. Although the grating design reduces the total contact area by half, thus seemingly sacrificing half of the triboelectric charges, it increases the percentage of the mismatched area to 100% for a displacement of only a grating unit length rather than the entire length of the TENG so that it dramatically increases the transport efficiency of the induced charges.

Nano Lett. 2013, 13, 22822289.

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TENG configurations

Rotation mode based TENG

The working principle of the disk TENG is based on the triboelectrification and the relative-rotation induced cyclic in-plane charge separation between Al and Kapton.

In the relative rotation, the Al surface and Kapton surface slide relative to each other, so that the electrons will be injected from the Al foil to the inner surface of the Kapton film, leaving net positive charges on the Al foil and net negative charges on the Kapton film.

Nano Lett. 2013, 13, 29162923

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TENG configurations

Single-electrode TENG in contact-separation mode

Single-electrode TENGs may be more practical and feasible in several mechanical energy harvesting scenarios, e.g. rotating tyres, body contact etc.

Yang et. al, Adv. Mat. 2013

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TENG configurations

Single-electrode TENG in sliding mode

ACS Nano 2013, 7, 73427351.

135

TENG as self-powered sensor

Pressure sensorTouch sensor

Chemical sensorAngew. Chem., Int. Ed. 2013, 52, 50655069

ACS Nano 2013, 10.1021/nn403838y

Nano Lett. 2012, 12, 31093114

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Outlook

Triboelectric generators are generating significant interest in the mechanical energy harvesting community due to the relative simplicity of the materials and processing involved, and the promising output performance

Surface modification may be key to performance optimisation

These types of devices tend to have high output impedence, thus impedence matching may be an issue and additional power conditioning circuitry will be required.

A major issue is the presence of static charges which may be detrimental for electronic devices in the vicinity of the nanogenerator.

The field is very new and research in the coming years should strengthen our understanding of the tribolelectric effect in order to fully harness its potential in mechanical energy harvesting.

End of Lecture 7

ENERGY HARVESTING Lecture 8 Thermal Energy Harvesting (Part 1)

Thermoelectric devices

Figure of merit and materials selection

Nanostructured thermoelectric materials

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Thermal energy harvestingThere are several sources of heat in our environment, most of which goes to waste. For e.g. waste heat from vehicle exhausts and radiators, cooling water of steel plants and other industrial processes, heat generated in computers, and temperature difference between the surface and the bottom of oceans. Temperature differences are even present between human bodies and the ambient.

Thermoelectric devices, which are capable of converting heat into electricity, have potential for thermal energy harvesting. Thermoelectric devices can help to improve energy efficiency and reduce CO2 emissions of fossil fuel systems through waste heat recovery. They can also be integrated into autonomous systems to enhance the capability and lifetime of self-powered electronic devices by harvesting thermal energy from their environment, or even charging wireless sensors and mobile devices from human body heat.

Thermoelectric generators are designed on the principle of the Seebeck effect, and scale down easily with size. They have the potential for being green, sustainable, maintenance-free with virtually infinite power for wireless devices.

Micropelt Thermogenerator Chip MPG-D751

139

Thermoelectric effectsThermoelectricity refers to a class of phenomena in which a temperature difference creates an electric potential/current creates a temperature difference.

Seebeck Effect is the generation of a voltage across a material as a result of a temperature difference and is the principle used in thermoelectric generators.

Peltier Effect is the opposite generating a temperature difference as a result of an applied voltage/current. This is used for refrigerant free cooling.

Thomson effect refers to heat being absorbed or produced when current flows in material with a certain temperature gradient. The heat is proportional to both the electric current and the temperature gradient. This is known as Thomson effect.

Seebeck effect Peltier effect Thomson effect

140

Thermoelectric effectsMathematically, the Seebeck effect can be described as , where is the Seebeck coefficient (units V/K)

Typically, metals have Seebeck coefficients up to few tens of mV/K, semiconductors can have Seebeck coefficients 1-2 orders larger in magnitude. A thermoelectric device operated in the Seebeck mode converts heat into electricity and is a generator

The Peltier effect can be described by the following: the rate at which heat is removed from one junction to another junction is given by , where I is the current flowing in the circuit and is the Peltier coefficient (unit V). A thermoelectric device operated in the Peltier mode pumps heat from one junction to another and can be used as a refrigerator, e.g. during camping, as portable coolers, for cooling electronic components and small instruments.

The Thompson effect can be mathematically described as , where is the Thomson coefficent (unit V/K).

The three thermoelectric coefficients are not independent of each other, but are related by the Kelvin relationships:

The first equation describes the relationship between the Seebeck and Peltier coefficients. This indicates that the materials that are suitable for thermoelectric power generation are also suitable for thermoelectric refrigeration.

141

Thermoelectric generatorsIn a thermoelectric material there are free electrons or holes which carry both charge and heat.

To a first approximation, the electrons and holes in a thermoelectric semiconductor behave like a gas of charged particles. If anormal (uncharged) gas is placed in a box within a temperature gradient, where one side is cold and the other is hot, the gasmolecules at the hot end will move faster than those at the cold end. The faster hot molecules will diffuse further than the cold molecules and so there will be a net build up of molecules (higher density) at the cold end. The density gradient will drive the molecules to diffuse back to the hot end. In the steady state, the effect of the density gradient will exactly counteract the effect of the temperature gradient so there is no net flow of molecules.

If the molecules are charged, the build-up of charge at the cold end will also produce a repulsive electrostatic force (and therefore electric potential) to push the charges back to the hot end. If the free charges are positive (the material is p-type), positive charge will build up on the cold which will have a positive potential. Similarly, negative free charges (n-type material) will produce a negative potential at the cold end.

142

Thermoelectric generatorsThe simplest thermoelectric power generator consists of a thermocouple, comprising a p-type and n-type materialconnected electrically in series and thermally in parallel.

A thermoelectric module consists of a number of the basic building blocks connected electrically in series but thermally in parallel and sandwiched between two ceramic plates to maximize voltage output.

143

Power outputA thermoelectric generator can be viewed as a thermal battery. The electromotive force of this thermal battery is the Seebeck voltage Vo = npT. See equivalent circuit below:

Power delivered to the load, , where s = RL/R

Maximum power output is obtained at R = RL,

For given np and R values, the maximum power output of a thermoelectric generator increases parabolically with an increase in temperature difference. But, in practice, the Pmax T plot deviates slightly from the parabolic relationship becauseboth np and R change slightly with temperature.

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Energy conversion efficiency

We have already seen that . , we can work out the heat flux at the hot junction by considering the

Peltier heat, Joule heat and heat conduction in the material.

It can be shown that the maximum conversion efficiency is given by:

=

Where Z = np2/(RK) is the thermoelectric figure of merit and

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Energy conversion efficiency

Thermoelectric generators suffer from poor efficiency values. There is tremendous ongoing effort to improve the thermoelectric figure of merit, and hence efficiency through clever materials engineering.

146

For a given temperature difference between the hot and cold ends, depends on Z of the material

For a single material with electrical conductivity and thermal conductivity , the thermoelectric figure of merit can be simplified to an expression that involves only fundamental properties of the material, independent of geometry.

A good thermoelectric materials must thus have a large Seebeck coefficient to produce a large voltage for a given temperature difference, large electrical conductivity to minimize Joule heating, and low thermal conductivity to retain the heat at the hot junction. i.e. thermal transfer needs to be dominated by electrons NOT phonons. ( = el + ph)

There has to be a compromise between , and in order to maximise Z.

Thermoelectric figure of merit

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Optimum carrier concentration values in the range 1023 1026 m-3 heavily doped semiconductors

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Z varies with temperature, its unit being K-1. A dimensionless figure of merit, ZT, is more commonly used. Currently, all established thermoelectric materials have a maximum ZT ~ 1. As of now, Bi2Te3 alloys have ZT ~1 at room temperature, making them the preferred choice for most thermoelectric generators, and indeed coolers.

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The evolution of thermoelectric materials

The discovery of new and exciting thermoelectric materials has spurred the development of novel thermoelectric generators

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Improving ZT

The higher the electrical conductivity, the higher the value of el , according to Wiedemann-Franz law.

Our best bet is therefore to try and minimize ph . Key strategies include:

- Alloying : create point defects, vacancies, or rattling structures to scatter phonons.

- Complex crystals to separate the phonon-glass from the electron-crystal: In the expression for ZT, thermal conductivity and electrical conductivity compete. It has been proposed that in order to improve ZT, phonons responsible for thermal conductivity must experience the material as they would in a glass, i.e. experiencing a high degree of phonon scattering which lowers the thermal conductivity, while electrons must experience the material as a crystal, i.e. experiencing very little scattering meaning high electrical conductivity.

Phonon-gas, electron-crystal (PGEC) behaviour

- Multiphase nanocomposites that scatter phonons at the interfaces of nanostructured materials.

- Reduced dimensionality: The Seebeck coefficient heavily depends od the band structure, specifically the number of sub-bands that contribute and the density of states in each band. Reduced dimensionality, e.g. in quantum wells/dots, decreases the number of bands and creates a more distribution of density of states in-plane, enhancing the thermoelectric properties of the system.

= el + ph

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PGEC materials Skutterudites and ClathratesSkutterudites such as CoSb3 and clathrates such as Sr3Ga16Ge30 have an open cage-like structure. When atoms are placed into the interstitial voids or cages of these materials, the lattice thermal conductivity can be substantially reduced, retaining at the same time good electrical properties.

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Nanostructured chalcogenides

(Bi2Te3 has the same structure)

Nanostructured superlattices comprising alternating Bi2Te3 and Sb2Te3 layers produce a device in which there is good electrical conductivity but poor thermal conductivity perpendicular to the layers. The result is an enhanced ZT (approximately 2.4) at room temperature for p-type devices.

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Strongly correlated materials - cobaltites

Cobaltites are generating significant interest due to their promising thermoelectric properties. They exhibit low thermal conductivity due to their layered structure.

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Quantum dot superlattices PbTe/PbSeTe systemQuantum dot (0D) formation due to lattice mismatch of PbTe and PbSeTe. Significant reduction in phonon thermal conductivity is observed in these structures due to confinement effects and scattering at interfaces. ZT close to 4!!

However, these technologies are still quite young and suffer from problems such as scalability, cost, reproducibility, toxicity and availability of constituent materials.

End of Lecture 8

ENERGY HARVESTING Lecture 9 Thermal Energy Harvesting (Part2)

Pyroelectric Materials

Thermodynamic Cycles

Pyro-Piezo Hybrid nanogenerators

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Thermoelectric materials and systems generate electrical power from temperature gradients (dT/dx), while pyroelectric materials produce power from temperature fluctuations (dT/dt)and have some similarities to the way in which piezoelectric harvesters convert mechanical oscillations (dS/dt) into electricity.

Thermal energy harvesting using pyroelectric materials

Under the correct conditions, pyroelectric materials have the potential to operate with a high thermodynamic efficiency and, compared to thermoelectrics, do not require bulky heat sinks to maintain a temperature gradient

Since temperature oscillations are often slow, efforts to transform a temperature gradient into a time variable temperature include the use of cyclic pumping.The power consumed by the pumping process can be a relatively small fraction of the harvested energy (

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Pyroelectric materials

Examples of Pyroelectric materials

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The generated current due to a pyroelectric charge Q is given by:

The ability of small changes in temperature to produce a pyroelectric current has been exploited for infra-red imaging and motion detection by body heat.

To maximise the pyroelectric current, the pyroelectric material should have a large surface area, large pyroelectric coefficient and a high rate of temperature change.

In general, ferroelectric materials have larger pyroelectric (and piezoelectric) coefficients compared to non-ferroelectric materials. However if a ferroelectric material is heated beyond the Curie temperature (TC) it undergoes a phase transition where the spontaneous polarization and both the pyroelectric (and piezoelectric) behaviour vanish.

While the loss of piezoelectric properties above the Curie temperature is a disadvantage for vibration harvesters, the phase transition at the Curie temperature has attracted some interest for pyroelectric harvesting since the material has the potential to discharge a large amount of electrical energy as the level of polarisation falls to zero

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Pyroelectric coefficientsA temperature change alters the degree of polarisation in a pyroelectric materials and leads to an electric current. The primary pyroelectric effect is relevant to the condition of a perfectly clamped material under constant strainwith a homogenous heat distribution without an external field bias.

Since thermal expansion induces a strain that alters the electric displacement via the piezoelectric effect, in many cases of measurement and energy harvesting, a secondary pyroelectric effect is present.

Using tensor notation the primary pyroelectric coefficient at constant strain (px), i.e. in the clamped condition, is related to the pyroelectric coefficient at constant stress (p

) by

For thin-film materials, substrate clamping can reduce the pyroelectric response to a small value compared to its unclamped value. Thus, it may be beneficial to replace thin films with a nanorod geometry that is less likely to suffer from substrate clamping.

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Pyroelectric coefficients

Tertiary pyroelectricity, due to non-uniform heating, is also possible since non-uniform heating generates shear stresses that result in polarization through the piezoelectric effect. In this case the current generated is dependent on the magnitude of the temperature gradient.

Secondary and tertiary effects are therefore potential routes for enhancing thermal harvesting along with heat transfer enhancement or materials selection or development, which will now be described. For example, coupling a pyroelectric to an external structure which undergoes large thermal deformations is also a potentialapproach to enhance harvested energy.

For thin-film materials, substrate clamping can reduce the pyroelectric response to a small value compared to its unclamped value. Thus, it may be beneficial to replace thin films with a nanorod geometry that is less likely to suffer from substrate clamping.

Nano Lett., 2012, 12, 28332838

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Pyroelectric materials how much energy?

Pyroelectric charge,

Equivalent capacitance

Therefore, (as, Q = CV)

Amount of energy stored in the material at the end of the temperature change and is expressed as:

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Figures of meritTo maximise the pyroelectric current generated for a given energy input, the FOM is:

The above figures of merit are often used for selection of materials for heat and infra-red detection, but these are not to be confused with energy harvesting applications where generated energy or power is a key criterion as well as the overall efficiency of the conversion of thermal energy to electrical energy. For pyroelectric energy harvesting, an electro-thermalcoupling factor has been defined to estimate the effectiveness of thermal harvesting:

where Thot is the maximum working temperature.Another figure of merit widely used for materials selection and design is

Note that these static definitions of figures of merit do not take into account the transient nature of heat transfer and dielectric losses.

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Comparison of pyroelectric materials

Bowen et. al, Energy and Enviromental Scie

Energy Harvesting

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