Applied Physics. Department of Physics

ET1039 – Nanotechnology

ELECTROCHROMIC DEVICES

Iván López Lliberós

Norman Ferrer Valls

Academic year: 2014/2015

INDEX

INTRODUCTION ......................................................................... 4

INORGANIC, ORGANIC AND HYBRID MATERIALS ....................... 5

Inorganic semiconductors ....................................................................... 5

Organic semiconductors .......................................................................... 6

The bonding of sp2-hybridised carbon .....................................................................6

Small-molecule organic semiconductors .................................................................7

Polymer organic semiconductors ............................................................................8

Conjugated polymers ..............................................................................................8

Organic semiconductor application fields .............................................................12

ELECTROCHROMIC TECHNOLOGIES ......................................... 18

Liquid crystal display (LCD) .................................................................... 19

Light and Polarization ............................................................................................21

Polarized Light .......................................................................................................22

Design of liquid crystalline materials: ....................................................................23

Viologen functionalized with TiO2 ......................................................... 25

WO3 ....................................................................................................... 27

ELECTROCHROMIC APPLICATIONS ........................................... 30

Electrochromic window (“Smart window”) ........................................... 30

Smart glass or switchable glass ............................................................. 31

Sunglasses and Visors ............................................................................ 32

Electrochromic displays ......................................................................... 32

Rear-view car mirrors ............................................................................ 34

CONCLUSION ........................................................................... 35

BIBLIOGRAPHY......................................................................... 37

INTRODUCTION

Semiconductors form the basis of the most modern information processing devices. Electronic

devices such as diodes, bipolar junction transistor (commonly called BJT), and field effect

transistors lead modern electronic technology. Besides, optoelectronic devices such as laser

diodes, modulators and detectors drive the optical networks. Semiconductors are very useful

in electronic applications, as they have the ability to function like an electrical conductor or like

an insulator according different factors.

Since Alexander Volta discovered the first semiconductor materials in 1782, the investigations

focused on elements from group 12 to group 16 of periodic table. Elements like, Selenium,

Germanium or Silicon are the most used elements to manufacture the semiconductor

components that are used in all the electronic devices.

Nowadays, technological advances cause that new semiconductor materials are being

investigated and developed. It seems that they are the future in this matter and over time,

they will be more used in new devices and technological applications. For example, there is an

important progress about the applications based in organic, inorganic and hybrid materials.

The essay is going explain how organic, inorganic and hybrid materials work like

semiconductors. This let us to present the electrochromic materials and the main technologies

based on electrochromics, showing some real applications to round off.

Illustration 1. Essay following steps

Introduction

Inorganic, organic and hybrid materials

Electrochromic technologies

•Liquid crystal

•Liquid crystal display

•Viologen

•Viologen + TiO2

•WO3

Electrochromic applications

Conclusions

INORGANIC, ORGANIC AND HYBRID MATERIALS

Inorganic semiconductors

An inorganic semiconductor is a semiconductor made from a non-carbon based material, such

as silicon, gallium or arsenide. The most used inorganic materials since discovering of the

electrical conductivity properties of semiconductor materials have been Silicon and

Germanium, though chemical elements belonging between group 12 and group 16 of periodic

table also might be used in the manufacturing of different electronic devices.

Illustration 2. Inorganic semiconductors are used in all logic and memory chips

Next table let to see these elements and how many valence electrons they have.

Element Groups Valence electrons

Cd 12 2 e-

Al, Ga, B, In 13 3 e-

Si, C, Ge 14 4 e-

P, As, Sb 15 5 e-

Se, Te, S 16 6 e-

Table 1. Valence electrons of group 12-16 elements

The elements that were showed at Table 1 have the same behaviour that Silicon or

Germanium when they are in a blend of elements of group 12 and 13 with elements of

group15 and 16 respectively. For instance, they form compounds such as GaAs, PIn, AsGaAl,

TeCd, SeCd y SCd.

Despite of the fact that these elements have been used since their discovery until today (and

they are still used), there are great progresses on the developing of organic materials and

hybrid materials (organic + inorganic) which work such as semiconductors.

Conventionally semiconductors:

• Silicon

• Germanium

• Selenium

• Aluminium

Organic semiconductors

An organic compound or organic material is any member of a large class of gaseous, liquid, or

solid chemical compounds whose molecules contain carbon. Then, an organic semiconductor

is an organic material with semiconductor properties, that is, with an electrical conductivity

between that of insulators and that of metals.

First, it is necessary to clarify than organic materials are of nature insulator. However, some

single molecules, oligomers, and organic polymers can conduct electricity in some

circumstances.

The bonding of sp2-hybridised carbon

Organics semiconductors are based on the unusual properties of the carbon atom: Among

other configurations, it can form the so-called sp2-hybridisations where the sp2-orbitals form a

triangle within a plane and the pz-orbitals are in the plane perpendicular to it. A s-bond

between two carbons can then be formed by formation of an orbital overlap of two sp2-

orbitals. The energy difference between the occupied binding orbitals and the unoccupied

anti-binding orbitals is quite large and well beyond the visible spectral range. Correspondingly,

longer chains of bound carbon atoms would have a large gap between the highest occupied

molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), leading to

insulating properties.

However, in the sp2-hybridisation, the pz-orbitals form additionally p-bonds. These bonds have

much smaller energetic difference between the HOMO and LUMO, leading to strong

absorption in or near the visible spectral range and to semiconducting properties:

Figure 1. Scheme of the orbitals and bonds for two sp2-hybridised carbon atoms.

Small-molecule organic semiconductors

If carbon atoms form larger molecules, typically with benzene rings as the basic unit, the p-

bonds become delocalized and form a p-system, which often has the extensions of the

molecule. The gap between occupied and empty states in these p-systems becomes smaller

with increasing delocalization, leading to absorption and fluorescence in the visible. These

substances can be prepared as molecular single crystals. Due to the close coupling of the p -

systems of the molecules in these crystals, they show in a purified form remarkable transport

properties, including band transport up to room temperature with mobilities of 1-10 cm2/Vs.

Most of the molecules can also be easily evaporated to form polycrystalline (hopping transport

with mobilities typically around 10-3 cm2/Vs at 300K) or amorphous (hopping with mobilities

typically around 10-5 cm2/Vs at 300K) layers.

Figure 2. Scheme of a benzene ring (top) and the energy structure of small-molecule organics.

Polymer organic semiconductors

If a long chain of carbon atoms is formed, the p -bonds become delocalized along the chain

and form a one-dimensional electronic system. The 1D-band which results has considerable

band width (on the scale of an eV), i.e., we have a 1D semiconductor with a filled valence band

originating from the HOMOs and an empty conduction band originating from the LUMOs. The

transport properties of such polymers are usually determined by defects in the 1D-chains or by

hopping from chain to chain. Therefore, the samples do not show band transport, but

thermally activated hopping. Polymer organic semiconductors are usually deposited in wet

processes, like spin-coating or printing.

Figure 3. Scheme of a polymer subunit (top right) and the energy structure of polymer organics.

Conjugated polymers

Conjugated polymers offer the possibility to develop flexible and lightweight optoelectronic

applications thanks to their solubility and low-temperature processing. Both conductors and

semiconductors are available, enabling in principle an all-organic electronics when combined

with more traditional insulating plastics. This technology would be therefore substantially

based on carbon, an earth-abundant element. While being characterized by limited

performances with respect to other inorganic technologies, recent synthetic and processing

advancements clearly make conjugated polymers even stronger candidates for future large-

area, flexible electronics. Polymer conductors with conductivity values of a few 103 S/cm have

been demonstrated, and semiconductors with both p- and n-type carriers mobility exceeding

10 cm2 /Vs, with improved ambient stability are now available.

The development of suitable deposition techniques is crucial to fully exploit the potentiality of

conjugated polymers. Besides the development of printing tools for the controlled patterning

of polymer films, the possibility to deposit them in the form of micro- and nano-fibers is a very

attracting and emerging option. Extended wires of polymers offer unique systems

characterized by an improved mechanical strength, an increased surface-to-volume ratio, and

quasi 1-D dimensionality in the case of nano-fibers, where studying charge transport in a

confined system. Various electronic functionalities can be implemented into different fibers,

enabling to fabricate different devices and components. One of the possible tangible outcomes

of functional fibers is the development of “smart textiles”, where fabrics are equipped with

integrated electronic devices during their production, enabling a ubiquitous application of

wearable electronics.

Figure 4. Chemical structures of electrospun conjugated and insulating/supporting polymers.

There are different methods to synthesize in nanostructures such as nanotubes and

nanofibers. The following table shows a summary about the different synthesis methods to

obtain these nanostructures.

Figure 5. Synthesis methods of conducting polymer nanotubes and nanofibers.

The synthesis methods showed at Figure 6 are just an illustrative example about obtain this

organic semiconductors and they are not going to be explained in detail, since this matter is

out at this essay.

To sum up the topic of organic semiconductors, these are the main ideas:

1. Polymers are naturally insulated.

2. Through the interaction of alternating single and double bonds, is created a space

between the valence band and conduction band.

3. Dope a semiconductors is the fact to add or remove electrons in the polymer chain

through a redox reaction.

4. By means of doping effect, the polymers with inorganic semiconductors, it is possible

to achieve the creation of free carriers, which acting both oxidation or reduction

reaction and it makes that radical cations or anions are formed.

Figure 6. Doping process

5. Conjugated polymers with delocalized -electron systems behave as model organic

semiconductors

Figure 7. Effect of doping on polymers

Figure 7 shows the effect that has the doping in a polymer semiconductor and how anions and

cations are reorganized making that these materials can drive electricity.

This is the basis of the hybrid semiconductors (compounds of organic and inorganic materials),

which leads to the electrochromic technologies used to develop electrochromic devices.

Doping: achieves the creating of free carrier

Acting either oxidation or reduction

Radical cations or anions are formed

Organic semiconductor application fields

Nowadays, these materials have some important applications, overall on electronic devices

such as:

OSC Organic solar cell.

OLED Organic light emitting device.

OFET Organic field effect transistor.

Figure 8. Applications of organic materials

Organic solar cell

An organic solar cell or plastic solar cell is a type of polymer solar cell that uses organic

electronics, a branch of electronics that deals with conductive organic polymers or small

organic molecules, for light absorption and charge transport to produce electricity from

sunlight by the photovoltaic effect.

The plastic used in organic solar cells has low production costs in high volumes. Combined with

the flexibility of organic molecules, organic solar cells are potentially cost-effective for

photovoltaic applications. Molecular engineering (changing the length and functional group of

polymers) can change the energy gap, which allows chemical change in these materials. The

Applications

OSC

(organic solar cells)

OLED

(organic light emitting devices)

ELECTROCHROMIC

OFET

(organic field effect

transistors)

optical absorption coefficient of organic molecules is high, so a large amount of light can be

absorbed with a small amount of materials.

Figure 9. Operation of organic sollar cell

1. Image above lets to see the processes occurring in a organic solar cell. Below these

lines, there are the description of the process step by step.

2. A photon is absorbed in the polymer, thus creating an exciton.

3. The nascent exciton dissociates into separated charge carriers. This is facilitated by the

presence of an electron acceptor, which accepts the electron while the hole remains

on the polymer chains.

4. The difference in work functions of the electrodes gives rise to an electric field which

drives the separated charge carriers towards electrodes.

5. The charge carriers are collected at the electrodes. If an electrical circuit is connected

to the electrodes, an electrical current flows through it.

On the one hand, the main disadvantages associated with organic photovoltaic cells are low

efficiency, low stability and low strength compared to inorganic photovoltaic cells. However,

this main disadvantage could be sorted adding some inorganic semiconductor materials, which

provide these devices of better rates of performance.

On the other hand, a good pro, which have the OSC, is the

flexibility such as it is possible to observe at following

figure.

Figure 10. Organic solar cell device

Organic light emitting device

Organic light-emitting diode

(OLED) displays are self-

luminous devices in which

organic thin films function as

the light emitting material.

Like conventional inorganic

light-emitting diodes (LEDs),

OLEDs require a low-drive

voltage to produce bright

visible light. Unlike discrete

LEDs, which have crystalline

origins, film-based OLED is an

amorphous area emitter

that can be lithographically

patterned to produce flat-

panel displays. In that regard, OLED technology is similar to that of liquid-crystal displays

(LCDs). In contrast to LCDs, however, OLED displays are self-luminous and do not require

backlights.

The device structure of OLED consists of several layers of organic materials sequentially

deposited on glass substrate, each layer having a specific purpose that serves to enhance

device quality and performance. The schematic representation of an ideal/standard OLED

device is shown at Figure 11.

The organic thin films used in OLED devices can be classed as small molecule or macro polymer

materials. Small molecule OLED films are applied using vapour sublimation in a vacuum

chamber. In the case of polymers, solvent coating techniques such as spin coating are often

used. Kodak focuses on small molecule technology, though we have development programs

focusing on polymer devices.

By doping the emissive layer with a small amount of highly fluorescent molecules in a Kodak-

proprietary method, researchers can enhance the electroluminescent efficiency and control

colour output.

Figure 11. Schematic illustration of multi layer structure of small

molecule based OLED

OLED offers numerous technology advantages over competing approaches for flat panel

display:

High brightness (>100 CD/M2)and contrast (>100:1).

Unlimited viewing angles.

Full color displays in small molecule device; polymer-based technology is still working

on full RGB.

Fast response in the microsecond range, better than 1000X faster than LCD

technology.

Wide operating temperature range temperature range (-80° C to 80° C) for quick

response displays in a variety of environments.

Low power consumption and operating voltage, maximizing battery life and minimizing

heat and electric interference in electronic devices.

Light weight, compact, and thin, reducing the size and weight of devices that use

displays.

Robust enough to use in portable devices such as cellular phones and personal digital

assistants (PDAs).

Economical.

Figure 12. OLED devices.

Organic field effect transistor

The organic transport material is separated from the gate electrode by an insulator and is

contacted by source and drain electrodes. On applying a negative gate bias Vg between source

and gate, holes accumulate at the interface between semiconductor and insulator. The

increased charge carrier density causes a highly conductive channel to open between the

source and the drain contact. The transistor is switched on and a current can be driven

between source and drain once a drain bias Vd is applied. By applying a Vg that does not allow

for charge carrier accumulation, the transistor can be switched off.

Figure 13. OFET structure

Electrochromic devices

Electrochromism is the reversibly ability of a material to change its colour by an

electrochemical oxidation or reduction reaction, caused by the application of an electrical

potential.

Electrochromic devices are constructed to modulate incident electromagnetic radiation via

transmission, absorption, or reflection of the light. This is done by the application of an electric

field across the materials in the device. Typically, an electrochromic device contains two

electrochromic materials separated by an electrolyte. The electrodes used in the devices are

dependent on the type of device - absorptive/transmissive or absorptive/reflective.

Transmissive Devices:

The absorptive/transmissive ECD operates typically by reversibly switching the electrochromic

materials between colored and bleached states. The colored and bleached states apply not

only in the visible region, but also in the NIR, mid-IR, and microwave regions of the spectrum.

In order for light to pass through the device, both working and counter electrodes are

transparent. Typical materials include indium-doped tin oxide (ITO), fluorine-doped tin oxide

(SnO2:F), PEDOT/PSS, and single-walled carbon nanotubes (SWNT). Our group has employed

both PEDOT/PSS and SWNT on plastic as transparent electrodes in flexible electrochromic

devices.

Figure 14. Schematic of a transmissive ECD

Reflective Devices:

The reflective-type devices typically contain a reflective metal, such as gold, deposited onto an

ion permeable membrane. The electrochromic polymer is deposited onto this electrode and is

faced outward to allow incident light to reflect off the polymer/electrode surface. The counter

electrode is hidden behind the active electrode. Utilizing materials such as gold for the working

electrode, wavelengths far beyond the visible region can be modulated.

Figure 15. Schematic of a reflective ECD.

One advantage of the reflective-type devices is how the metal electrode material lends itself to

patterning on various substrates. The reflective metal can be deposited in defined patterns by

methods such as metal vapour deposition, line patterning, screen printing, inkjet printing, and

micro-contact printing. The combination of the wide variety of colours available in the

electrochromic polymers along with the various device designs allows for increasingly complex

displays to be constructed.

ELECTROCHROMIC TECHNOLOGIES

The addition of inorganic spherical nanoparticles to polymers allows the modification of the

polymers physical properties as well as the implementation of new features in the polymer

matrix. Hybrid materials are born here.

Then, the addition of conductive nanoparticles to polymers to obtain hybrid materials, has a

strong impact on the resulting composite dielectric properties. With respect to the aspired

integration of passive electronic devices, like resistors, capacitors and others, into the printed

circuit board (PCB), new composite materials have to be developed to meet the following

requirements:

Huge functionality like large capacitance values in case of integrated capacitors.

Process compatibility to industrial PCB-fabrication.

Abandonment of lead-containing materials

Low overall costs.

High reliability and extended life cycle.

Besides, the modification of the refractive index with coeval preservation of the transmittance

is one of the challenges for particle/matrix nanocomposites, and therefore reported quite

frequently in literature. Most of the research deals with TiO2 nanoparticles, embedded in an

organic matrix. This is because all TiO2 modifications exhibit an inherent high refractive index.

In some cases, research groups also use semiconducting nanoparticles as ZnS or PbS with

inherent high refractive indices. Due to the extreme specific surface area values of

nanoparticles with sizes below 10 nm, high nanoparticle contents cannot be expected.

Photoluminescence of nanocomposites is another interesting property reported for several

classical nanocomposite systems as well as for nanocomposite particles. Excitation and

emission of the composite differ significantly from the pure polymer. Interestingly already, a

particle loading of 1 wt % ZnO showed a significant luminescence. With increasing particle

concentration an increase in intensity was observed, but no influence on the emission

maximum. ZnO/polybutanediolmonoacrylate (PBDMA) nanocomposites showed an increase of

the excitation and the emission wavelengths with increasing particle sizes.

ZnO/polybutanediolmonoacrylate (PBDMA) nanocomposites showed an increase of the

excitation and the emission wavelengths with increasing particle sizes. Photoluminescence was

found in TiO2/PMMA nanocomposite particles with an emission maximum at 420 nm. A strong

luminescence was found in core/shell nanoparticles made of HfO2, ZrO2, Al2O3, or ZnO cores

and PMMA-shell, respectively. The luminescence of nonconducting oxide/polymer

nanoparticles was mainly attributed to the presence of carboxylate groups at the interface

ceramic/PMMA whereas ZnO as a semiconductor exhibited an inherent luminescence.

Coming up next, some technologies which are based on hybrid nanocompounds, are

explained.

Liquid crystal display (LCD)

The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich Reinitzer

observed that a material known as cholesteryl benzoate had two distinct melting points. In his

experiments, Reinitzer increased the temperature of a solid sample and watched the crystal

change into a hazy liquid. As he increased the temperature further, the material changed again

into a clear, transparent liquid. Because of this early work, Reinitzer is often credited with

discovering a new phase of matter – the liquid crystal phase.

Liquid crystal materials are unique in their properties and uses. As research into this field

continues and as new applications are developed, liquid crystals will play an important role in

modern technology.

Liquid crystal materials generally have several common characteristics. Among these are rod-

like molecular structures, rigidness of the long axis, and strong dipoles and/or easily polarizable

substituents. The distinguishing characteristic of the liquid crystalline state is the tendency of

the molecules (mesogens) to point along a common axis, called the director. This is in contrast

to molecules in the liquid phase, which have no intrinsic order. In the solid state, molecules are

highly ordered and have little translational freedom. The characteristic orientational order of

the liquid crystal state is between the traditional solid and liquid phases and this is the origin of

the term mesogenic state, used synonymously with liquid crystal state. Note the average

alignment of the molecules for each phase in the following diagram.

Figure 16. Alignment of the molecules for each phase.

It is sometimes difficult to determine whether a material is in a crystal or liquid crystal state.

Crystalline materials demonstrate long range periodic order in three dimensions. By definition,

an isotropic liquid has no orientational order. Substances that are not as ordered as a solid, yet

have some degree of alignment are properly called liquid crystals.

To quantify just how much order is present in a material, an order parameter (S) is defined.

Traditionally, the order parameter is given as follows:

Where theta is the angle between the director and the long axis of each molecule. The

brackets denote an average over all of the molecules in the sample. In an isotropic liquid, the

average of the cosine terms is zero, and therefore the order parameter is equal to zero. For a

perfect crystal, the order parameter evaluates to one. Typical values for the order parameter

of a liquid crystal range between 0.3 and 0.9, with the exact value a function of temperature,

as a result of kinetic molecular motion. This is illustrated below for a liquid crystal material (to

be discussed in the next section).

The tendency of the liquid crystal molecules to point along the director leads to a condition

known as anisotropy. This term means that the properties of a material depend on the

direction in which they are measured. For example, it is easier to cut a piece of wood along the

grain than against it. The anisotropic nature of liquid crystals is responsible for the unique

optical properties exploited by scientists and engineers in a variety of applications.

The following parameters describe the liquid crystalline structure:

Positional Order

Orientational Order

Bond Orientational Order

Each of these parameters describes the extent to which the liquid crystal sample is ordered.

Positional order refers to the extent to which an average molecule or group of molecules

shows translational symmetry (as crystalline material shows). Orientational order, as discussed

above, represents a measure of the tendency of the molecules to align along the director on a

long-range basis. Bond Orientational Order describes a line joining the centers of nearest-

neighbor molecules without requiring a regular spacing along that line. Thus, a relatively long-

range order with respect to the line of centers but only short range positional order along that

line. (See discussion of hexatic phases in a text such as Chandrasekhar, Liquid Crystals)

Most liquid crystal compounds exhibit polymorphism, or a condition where more than one

phase is observed in the liquid crystalline state. The term mesophase is used to describe the

"subphases" of liquid crystal materials. Mesophases are formed by changing the amount of

order in the sample, either by imposing order in only one or two dimensions, or by allowing

the molecules to have a degree of translational motion. The following section describes the

mesophases of liquid crystals in greater detail.

Light and Polarization

Light can be represented as a transverse electromagnetic wave made up of mutually

perpendicular, fluctuating electric and magnetic fields. The left side of the following diagram

shows the electric field in the xy plane, the magnetic field in the xz plane and the propagation

of the wave in the x direction. The right half shows a line tracing out the electric field vector as

it propagates. Traditionally, only the electric field vector is dealt with because the magnetic

field component is essentially the same.

This sinusoidally varying electric field can be thought of as a length of rope held by two

children at opposite ends. The children begin to displace the ends in such a way that the rope

moves in a plane, either up and down, left and right, or at any angle in between.

Ordinary white light is made up of waves that fluctuate at all possible angles. Light is

considered to be "linearly polarized" when it contains waves that only fluctuate in one specific

plane. It is as if the rope is strung through a picket fence -- the wave can move up and down,

but motion is blocked in any other direction. A polarizer is a material that allows only light with

a specific angle of vibration to pass through. The direction of fluctuation passed by the

polarizer is called the "easy" axis.

If two polarizers are set up in series so that their optical axes are parallel, light passes through

both. However, if the axes are set up 90 degrees apart (crossed), the polarized light from the

first is extinguished by the second. As the angle rotates from 0 to 90 degrees, the amount of

light that is transmitted decreases. This effect is demonstrated in the following diagram. The

polarizers are parallel at the top and crossed at the bottom.

Polarized Light

Linear polarization is merely a special case of circularly polarized light. Consider two light

waves, one polarized in the YZ plane and the other in the XY plane. If the waves reach their

maximum and minimum points at the same time (they are in phase), their vector sum leads to

one wave, linearly polarized at 45 degrees. This is shown in the following diagram.

Similarly, if the two waves are 180 degrees out of phase, the resultant is linearly polarized at

45 degrees in the opposite sense.

If the two waves are 90 degrees out of phase (one is at an extremum and the other is at zero),

the resulting wave is circularly polarized. In effect, the resultant electric field vector from the

sum of the components rotates around the origin as the wave propagates. The following

diagram shows the sum of the electric field vectors for two such waves.

The most general case is when the phase difference is at an arbitrary angle (not necessarily 90

or 180 degrees.) This is called elliptical polarization because the electric field vector traces out

an ellipse (instead of a line or circle as before.)

Design of liquid crystalline materials:

A large number of chemical compounds are known to exhibit one or several liquid crystalline

phases. Despite significant differences in chemical composition, these molecules have some

common features in chemical and physical properties. There are three types of thermotropic

liquid crystals: discotics, bowlics and rod-shaped molecules. Discotics are flat disc-like

molecules consisting of a core of adjacent aromatic rings; the core in a bowlic is not flat but

like a rice bowl (a three-dimensional object).This allows for two dimensional columnar

ordering, for both discotics and bowlics. Rod-shaped molecules have an elongated, anisotropic

geometry, which allows for preferential alignment along one spatial direction.

The molecular shape should be relatively thin, flat or bowl-like, especially within rigid

molecular frameworks.

The molecular length should be at least 1.3 nm, consistent with the presence of long

alkyl group on many room-temperature liquid crystals.

The structure should not be branched or angular, except for the bowlics.

A low melting point is preferable in order to avoid metastable, monotropic liquid

crystalline phases. Low-temperature mesomorphic behavior in general is

technologically more useful, and alkyl terminal groups promote this.

An extended, structurally rigid, highly anisotropic shape seems to be the main criterion

for liquid crystalline behavior, and as a result many liquid crystalline materials are

based on benzene rings.

The most common application of liquid crystal technology is liquid crystal displays (LCDs.). This

field has grown into a multi-billion dollar industry, and many significant scientific and

engineering discoveries have been made.

Figure 17. Liquid crystal display

Figure 18. Schematic liquid crystal device

Viologen functionalized with TiO2

Viologen display functionalized with TiO2: an electrochromic display based on an

electrolyte consisting of an aqueous solution of a dipositively charged organic salt,

containing a colourless cation that undergoes a one-electron reduction process to produce

a purple radical cation, upon application of a negative potential to the electrode.

The device is made up of 5 layers, whose composition is:

Electrochromic device based on dye-modified semiconductor electrodes has much

improved switching response and enhanced contrast, especially for displays such as

electronic paper and billboards. The key part of these devices is a working electrode

composed of a nanocrystalline semiconductor modified with electrochromophoric

molecular species, such as redox active viologen derivatives. TiO2 nanostructures have

been successfully used for this purpose, resulting in high-performance electrochromic

devices.

Figure 20. Photos of the mesoporous TiO2 film EC cell before (a) and after (b) applying −2.5 V, and UV-visible

absorption spectra (c) of the same samples before (dashed line) and after (solid line) applying −2.5 V.

Figure 19. Schemtaic illustration of a composition layer of TiO2 electrochromic device

It has been synthesized viologen modified mesoporous TiO2 film with hexagonally close-

packed mesopores and channel walls made of 8–10 nm anatase nanostructures. The film

exhibited similar switching speed and reversibility as nanocrystalline titania but better

contrast. The higher contrast can be attributed to a contiguous pathway of well-connected

anatase nanocrystallites arranged into a well-defined mesoporous architecture, which

results in a greater volume density of loaded viologen molecules. Weng, who is an

investigator in the electrocomic material field, demonstrated a high-speed passive-matrix

EC display using a leuco modified mesoporous TiO2 electrode with vertical porosity. The

device exhibited better background whiteness, which improves readability and reduces

eyestrain. Imaging and erasing can be carried out by applying a potential of ±3.0 V. The

thickness of the mesoporous layer critically affects the contrast of the displayed image.

Upon writing, the clear images can remain for a few minutes without becoming blurry

because the vertical pores of the electrode can support effective diffusion of leuco dyes

perpendicular to the electrode and prevent the diffusion of the dye around the electrode.

The leuco modified mesoporous TiO2 electrode shows potential for the realization of a full-

color reflective display for use in e-papers.

Figure 21. Illustration for (a) imaging using a leuco dye on a mesoporous TiO2 electrode with vertical pores and

(b) imaging process for a passive-matrix electrochromic display.

WO3

Most of the electrochromic investigations on WO3 were focused on its amorphous films.

Compared with the amorphous structure, crystalline WO3 is much more stable due to the

denser structure and slower dissolution rate in electrolytes. However, crystalline WO3 bulk

material usually has slow switching response. To improve the switching response,

nanocrystalline WO3 were applied in electrochromic materials and devices in recent years.

It has been synthesized crystalline WO3 nanoparticles and nanorods by hot-wire chemical

vapor deposition and fabricated an electrochromic film by electrophoresis deposition. The

fabricated crystalline WO3 nanoparticle film has greater charge density for H+ ions

intercalation, which is attributed to a larger active surface area and the low density of the

films. Compared with amorphous WO3 film, the crystalline WO3 nanoparticle film exhibited

better cycling stability in H2SO4 solution.

Figure 22. cyclic voltammetry (CV) curves of the EC film showing the high cycling stability.

The synthesized WO3 nanorods were assembled on the surface of the film, showing

tunable coloration (green, green-blue, and blue colors) at different voltages high stability

(Figure 8) both in LiClO4 and H2SO4 electrolytes, comparable contrast and switching

coloration/bleaching responses. The assembly of WO3 nanorods allows sufficient Li+ ions to

be intercalated into WO3 nanorods, resulting in a high contrast. The displayed colors of the

WO3 nanorod film can be tuned by changing the applied voltages to control the amount of

intercalated Li+ ions into WO3 nanorods. The crystalline structure of the synthesized WO3

nanorods greatly enhances the stability of the EC film. The coarse surfaces of nanorods

and interstices among nanorods increase the specific surface area of the film and

accelerate the intercalation/deintercalation of Li+ ions, resulting in fast

coloration/bleaching switching. The WO3 nanorod film can be used in H2SO4 electrolyte

due to the crystalline structure. Compared with the EC switching characteristics of WO3

nanorod film in lithium-based electrolyte, faster response, much higher charge density,

comparable coloration efficiency and contrast were achieved in H2SO4 electrolyte.

Switching characteristics of the WO3 nanorod film, UV-visible spectra at different voltages

and in situ coloration/bleaching characteristic showing the switching responses.

Figure 25. Characteristics of WO3 and different colorations

Figure 23. Transmittance spectra measured at

different voltages.

Figure 24. CV curves of the hydrothermally

grown WO3 thin film.

Recently, it has been prepared crystalline plate-like WO3 nanostructures on fluorine-doped

tin oxide (FTO) coated glass by a crystal-seed-assisted hydrothermal method. The

hydrothermally grown film is well adhesive to the substrate, which is attributed to the use

of crystal seeds. The film, consisting of crystalline WO3 nanostructures, exhibited tunable

transmittance modulation under different voltages, high cycling stability, and high optical

contrast.

According to the above results, we draw a conclusion that crystalline WO3 nanostructures

improve the cycling stability (due to the crystalline structure) of the EC materials and

devices without degrading the switching responses, contrast and coloration efficiency (due

to the nanoscale structure). This is important progress towards the practical application of

high-performance EC devices.

Figure 26. An electrochromic display based on the hydrothermally grown WO3 thin film.

ELECTROCHROMIC APPLICATIONS

The Electrochromic device (ECD) controls the optical properties such as optical transmission,

absorption, reflectance, and/or emittance in a continual but reversible manner on application

of a voltage. This property enables the ECD to be used for applications like smart-window,

electrochromic mirror, and electrochromic display devices.

There are proposed applications of electrochromic materials including their use in controllable

light-reflective or light-transmissive devices for optical information and storage, sunglasses,

protective eyewear for the military, controllable aircraft canopies, glare-reduction systems for

offices and “smart Windows” for use in cars and in buildings.

Some of these applications are going to be named and discussed at this chapter.

Electrochromic window (“Smart window”)

“Smart window” is a window that changes transmittance due to a stimulus. They are also

called “self darkening windows”. Such windows are used in state of art buildings and aircrafts.

They are also introduced in luxury high speed railways. Use of smart electrochromic windows

is motivated by both economic and environmental factors. These windows regulate sunlight

and outgoing heat and therefore reduce the cost of air conditioning by providing better

temperature regulation. Low air conditioning requirement decreases energy usage and hence

more environment friendly. In a smart window the whole glass may be electrochemically

colored or it may be used to support a layer of

electrochrome. Such windows are also used in

roofs of cars, helmets and goggles. For large

screens in corporate offices, microprocessors

are used to adjust to best possible

illumination. In other applications, it can be

used as top floor light source utilizing solar

radiations. Electrochromic windows have been

installed in ‘Dreamliner’ series of aircrafts by

Boeing at a total expense of about $50 million. These windows offer a larger surface area as

the amount of light entering the aircraft can be regulated. Similarly, these can be used for

cockpit canopies in fighter jets.

Figure 27. example of smart car window

Smart glass or switchable glass

Smart glass or switchable glass (also smart windows or switchable windows in those

applications) is glass or glazing whose light transmission properties are altered when voltage,

light or heat is applied. Generally, the glass changes from translucent to transparent, changing

from blocking some (or all) wavelengths of light to letting light pass through.

Smart glass technologies include electrochromic, photochromic, thermochromic, suspended

particle, micro-blind and liquid crystal devices.

When installed in the envelope of buildings, smart glass creates climate adaptive building

shells, with the ability to save costs for heating, air-conditioning and lighting and avoid the cost

of installing and maintaining motorized light screens or blinds or curtains. Most smart glass

blocks ultraviolet light, reducing fabric fading; for SPD-type smart glass, this is achieved in

conjunction with low emissivity coatings.

Critical aspects of smart glass include material costs, installation costs, electricity costs and

durability, as well as functional features such as the speed of control, possibilities for dimming,

and the degree of transparency.

Figure 28. Smart glass sample.

Sunglasses and Visors

Conventional photochromic glasses darken automatically when exposed to bright illumination.

However, electrochromic sunglasses may be fine tuned to a variable opacity. Nikon were

pioneers in this field and were subsequently followed by Dannely.

Electrochromic displays

Various electrochromes operating in either re- flecting or transmitting modes are used as

displays. Tungsten trioxide was the most commonly used material for such applications, but it

is fast being replaced by polymers. Conventional ECDs require a light source for illumination

and are therefore called passive devices. This is a possible disadvantage compared to self

illuminating light emitting diode (LED) and cathode ray tube (CRT) displays. Apart from these,

other display technologies that are widely used are liquid crystal display (LCD) which is non

emissive and plasma which is emissive. Demand of high quality displays is increasing every year

worldwide. Such displays are required for monitors, television, tablets, cell phones and

advertising hoardings.

Figure 29. The principles of four different applications of electrochromic devices.

Total revenue associated with this industry has gone up by 10 times during the period 1994 to

2007. CRTs have been replaced by LCD and LED displays because they were heavier and more

cumbersome requiring a large electron gun behind the screen. Research and market feasibility

studies are carried out to use electrochromic devices in flat panel displays like television,

VDUs, data boards at stations etc. Also, they find applications in smaller electronic devices like

watches, calculators, mobile phones and tablets. One of the first such application was in a

watch developed by NTera, which replaced hour and minute hands by electrochromic sectors

on a circle which changed color to show the time of day. To be commercially viable, ECDs must

be able to compete with LCD displays. LCDs are fabricated at very low costs due to large

production volumes. LCD displays are sometimes so cheap that they can be used as disposable

displays. The power consumption in ECDs is lower than LCDs. Many ECD displays once formed,

can maintain the configuration with little or no power supply due to ‘memory effect’. Further,

ECD displays maybe made as large as required by increasing the size of electrodes or using

multiple electrodes in conjugation. Apart from being a flat display, it may be used in other

shapes like bent or curved. Comparatively, large area LCDs are expensive and CRTs with large

area require a huge electron gun and hence infeasible to use. The downside of conventional

electrochromes is that they have long response time and therefore not suitable for

applications like televisions. Further, they may undergo degradation after some electrochromic

cycles. For immobile coloring, the typical response time obtainable is 20s, rendering them

useless for applications like television.

Figure 30. Transparent and tactil smart display.

In electronics containing liquid elements, ions have faster diffusion coefficients and therefore

offer faster response time. Such devices include alphanumeric and digit display boards. To

produce a multipixel image, an array of electrodes is used. Typical resolution sizes are 100

pixels per inch. In large ECD displays, areas of patchy coloring may be formed due to uneven

distribution of ions. Viscosity of electrolyte, electrical potential and dimensions of display are

optimized to obtain the best possible display characteristics.

Rear-view car mirrors

Electrochromic reflecting surfaces are employed as self darkening mirrors that regulate

reflections of flashing light from following vehicles at night so that a driver can see them

without discomfort. It is important that such mirrors continue to reflect when no vehicle is

following i.e. dark state. Gentex night vision system is the best selling self darkening mirror

with a market share of 90-95%. Gentex Corporation are a firm based in Zeeland and Michigan

and they supply WO3 based mirrors for luxury cars like Audi, Ford, Fiat, Hyundai, Mitsubishi,

Porsche, Toyota and Nissan. These mirrors undergo thorough testing procedures for durability

in variety of driving conditions.

Figure 31. Example of electrochromic rear-view car mirror application

CONCLUSION

Electrochromic materials have the property of a change, evocation, or bleaching of color as

effected either by an electrontransfer (redox) process or by a sufficient electrochemical

potential. Usually materials are considered as being electrochromic when they showed marked

visible color changes. The field of electrochromism is rapidly expanding both innovel systems

or applications. Recent interest in electrochromic devices for multispectral energy modulation

by reflectance and absorbance has extended the working definition. Electrochromic devices

are now being studied for modulation of radiation in the near infrared, thermal infrared and

microwave regions and color can mean response of detectors of these wavelengths, not just

the human eye.

Since the discovery of EC materials, extensive investigations have been carried out to improve

their EC properties, including enhancement of the switching responses; contrast; coloration

efficiency and cycling stability; as well as EC device fabrications and applications. ZnO

nanowire/nanorod arrays modified by EC materials, mesoporous and tubular EC materials

significantly enhance the switching responses of EC devices, for potential application in fast-

switching displays and e-papers. Crystalline WO3 nanoparticles, nanorods and crystalline

mesoporous WO3 exhibit enhanced EC stability and contrast, especially in acidic electrolytes.

Soluble PB and its analogues Ni-PBA, Co-PBA nanoinks exhibit blue, yellow and red colors,

promising the possibility of achieving full-color displays. Due to the solubility of the nanoinks,

micro-fabrication processes, such as spin-coating, ink-jet printing and photolithography, can be

used in the fabrication of PB-based EC devices, which is cost-efficient, facile and easily scaled-

up. Switchable mirrors are the most effective energy-saving EC devices in all kinds of smart

windows. However, the metal alloys are easily oxidized, resulting in a poor long-term stability.

Further efforts are needed to accelerate practical applications of EC devices. For large-area EC

devices, the difficulties may be in slow responses, poor stability and high cost. To overcome

these difficulties, the synthesis of new nanostructured EC materials needs to be developed.

The ideal nanostructures for EC materials may include ultrathin crystalline nanorods,

nanowires or nanotubes, crystalline mesoporous structures and quantum dots. These

nanostructures with large specific surface areas, are expected to possess fast and stable EC

switching. Low-cost synthesis routes for these EC nanostructures are favorable. The coating

process for EC nanostructures also needs to develop to obtain well-adhesive EC films on

transparent conductive substrates. The combination of different kinds of EC materials is also

worthwhile, for example, to exhibit multi-colors and to enhance the coloration modulation and

stability. Complimentary EC devices need further development to enhance the contrast,

coloration efficiency and stability of EC devices. Inorganic-organic hybrid EC devices are also

promising for practical application, for example in e-papers. Besides the developments of EC

materials and devices, transparent conductive substrates with better conductivity, solid-state

electrolytes, advanced sealing and packaging technologies, are also critical and deserve

intensive investigations for the practical applications of EC materials and devices. Gallium

doped ZnO (GZO), antimony doped tin oxide (ATO), carbon nanotubes or graphene coated

glass are promising substitutes to replace ITO. Low-cost processes for the fabrication of EC

devices must be developed to realize their practical applications, which need the cooperation

of researchers and technicians to solve both scientific and technical challenges.

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MATERIALS FOR ELECTROCHROMIC DEVICES Materials, Applications, Devices and

Future. CHE463: ELECTRONIC, POLYMERIC & CERAMIC MATERIALS & PROCESSING

Instructor: Dr. Raju Kumar Gupta Department of Chemical Engineering Indian Institute

of Technology, Kanpu.