Chap2- 81

Solar Cell : DSSC nanostructure

• Porous interconnected structure• Surface area increased 1000 times when compared

to bulk materials• Crystals cause light-scattering and increase

efficiency, but also cause electron trapping• Thickness, shape, material all effect cell efficiency

Chap2- 82

Solar Cell : DSSC nanostructure• ZnO Nanostructures :

easy to fabricate various nanostructures

Chap2- 83

Solar Cell : DSSC nanostructure

a) Diagram of cell with nanowiresb) Image of nanowiresc) Comparison of cell performance for various shapes

and types of nanostructures

Chap2- 84

Solar Cell : electrolyte in a DSSC• Role of electrolyte in a DSSC

– Restore the original state of the dye by electron donation from the electrolyte

oxide

dye

electrolyte

e-

e-

e-

I- I3-

I3- I-

• Characteristics– Oxidation Reduction– Highly reusable – Redox potential lower than dye

Schematic of electron pathway in a DSSC system

HOMO

LUMOPhoto-excitation iodine (I-) and

triiodide (I3-) as a redox couple in a solvent

Chap2- 85

Solar Cell : electrolyte in a DSSCIdeal characteristics of the redox couple for the DSSC electrolyte

- Redox potential thermodynamically (energetically) favorable with respect to the redox potential of the dye to maximize cell voltage

- High solubility to the solvent to ensure high concentration of charge carriers in the electrolyte

- High diffusion coefficients in the used solvent to enable efficient mass transport

- Absence of significant spectral characteristics in the visible region to prevent absorption of incident light in the electrolyte

- High stability of both the reduced and oxidized forms of the couple to enable long operating life

- Highly reversible couple to facilitate fast electron transfer kinetics- Chemically inert toward all other components in the DSSC

Chap2- 86

Solar Cell : solvent in a DSSCIdeal characteristics of the solvent for the DSSC

- Liquid with low volatility at the operating temp. (-40~80˚C) to avoid freezing or expansion of the electrolyte, which would damage the cells

- Low viscosity to permit the rapid diffusion of charge carriers- A high dielectric constant to facilitate dissolution of the redox

couple- The sensitizing dye should not desorb into the solvent - resistant to decomposition over long periods of time - low cost and low toxicity

Chap2- 87

Solar Cell : DSSC operation

1. Dye electrons are excited by solar energy absorption.

2. They are injected into the conduction band of TiO2.

3. Get to counter-electrode (cathode) through the external circuit.

4. : Redox regeneration at the counter-electrode (oxidation).

5. : Dye regenerationreaction (reduction).

6. Potential used for external work:

--3 I32I e

e2II3 -3

-

redoxFext VEV

The voltage generated under illumination corresponds to the difference between the Fermi level of the electron in the solid and the redox potential of the electrolyte.

Chap2- 88

Solar Cell : Efficiency of DSSC

ISC : The short-circuit current, is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). VOC : The open-circuit voltage, is the maximum voltage available from a solar cell, and this occurs at zero current.FF : Fill factor

Chap2- 89

Solar Cell : DSSC operation

S: sensitizer C.E.: Counter electrode (cf. W.E. = working electrode)Chap2- 90

Solar Cell : Basics DSSC Layers

Electron energy levels

1. Electron injection from dye to conduction band2. Electron recombination with dye cation3. Dye regeneration from electrolyte4. Electron recombination with electrolyte5. Electron trapping in nanostructure

Chap2- 91

Solar Cell : DSSC operation

Chap2- 92

Solar Cell : DSSC modifications

• Replace organic electrolyte solution– Volatile, undergoes expansion and contraction– Gel electrolyte– Polymer electrolyte– Solid organic conductor– Inorganic semiconductor

• Replace ruthenium dye– Difficult to produce, environmentally dangerous– New organic dyes– Inorganic quantum dots

• Replace TiO2 layer– SnO2– ZnO

Chap2- 93

Solar Cell : DSSC development history

• 1991– Nature paper by O'Regan and Grätzel– First suggestion of workable DSSC

• 2006– Use of nanowires and nanoparticles– Demonstrated good chemical and

thermal resistance

• 2007, 2008– Use of low-cost organic dyes and

solvent-free electrolyte solution investigated

Chap2- 94

Solar Cell : Advantage of DSSC• Ease of fabrication for large area from solution • Transparent• Conformal and flexible• Low cost of manufacturing

Type of cell Efficiency %(cell)

Efficiency %(module) Research and technology needs

Crystalline silicon 24 10-15 Higher production yields, lowering of cost and energy content

Multi-crystalline silicon 18 9-12 Lower manufacturing cost and

complexity

Amorphous silicon 13 7 Lower production costs, increase production volume and stability

Dye-sensitized nano-structured materials 10-11 7

Improve efficiency and high-temperature stability, scale up

production

Chap2- 95

Solar Cell : applications

Glass-based DSSC Module

Flexible DSSC Module

Chap2- 96

Solar Cell : applications

Chap2- 97

Solar Cell : DSSC summary

• Medium efficiency• Low cost

• Problems to be addressed:– Liquid electrolyte (freezing, expanding, volatility)– Poor performance in red region of light

Chap2- 98

Solar Cell : How to make DSSC better?

• Develop dyes to absorb more photons

• Create new electrolytes that provide higher voltages

• Develop DSSC that can last for 30 years

Chap2- 99

Solar Cell : How to make DSSC better?Where do we need to absorb the light?

DSSC absorb > 85% of visible light, but almost no light in the NIR

Chap2- 100

Solar Cell : How to make DSSC better?Design dyes that broadly absorb light

Chap2- 101

Solar Cell : How to make DSSC better?Design Near-Infrared Absorbing Dyes

• Can increase power conversion efficiency from 12% to 14% by absorbing light out to 900 nm

• Probably can’t make DSSC > 14% using liquid electrolytes

Chap2- 102

Solar Cell : How to make DSSC better?Increase the maximum voltage using plastic hole

- Potential to make > 20% efficient devices by replacing liquid electrolyte with plastic “solid-state” hole conductors

- Current “solid-state” DSSC are only 6.5% efficient

- Very promising research area

Chap2- 103

Electrochromic Dyes

- Electrochromism was discovered in 1968 by S.K. Deb and J.A. Chopoorian .

- Electrochromic material is able to reversibly change its color when it is placed in a different electronic state.

- So by absorbing an electron (the materials is reduced) or by ejecting one (the material is oxidized), the material is able to change its color.

Electrochhromic effect

Chap2- 104

Electrochromic Dyes

- Electrochromism is induced by the gain or loss of electrons. - Reversible change of color which occurs due to the

electrochemical redox reaction, oxidation and reduction- A redox reaction is triggered by weak electric current at a

voltage below one to a few volts. - Under the influence of electric current, electrochromic materials

change their compound from a colorless to a colored state, from one color to another or to a few different colors.

- The example of a multicolor electrochroimc compound is polyaniline which appears in yellow, dark blue or black color, depending on electrical potential.

Electrochromic compounds

Chap2- 105

Electrochromic Dyes

→ can be classified in different groups depending on their physical state at room temperature.

- Type I : soluble and remain in the solution during usage.ex. molecular dyes

- Type II : soluble in their neutral state and form a solid on the electrode after electron transfer

- Type III : solid and remain solid during usage.ex. metal oxide films (inorganic type), and conducting polymers

(organic type)

Classification of Electrochromes

Chap2- 106

Electrochromic Dyes

- Inorganic oxides of transition metals: tungsten (WO3), iridium, rhodium, ruthenium, magnesium, cobalt

- Prussian blue (Fe(III) hexacyanoferrate(II), which is an inorganic pigment for dyes, lacquers, printing inks

- Metallic phthalocyanines- Viologen – 4,4’-dipyridinium compounds (herbicides, paraquats)- Buckminsterfullerene (C60 – in the presence of alkaline metals

as antiions, thin fullerene films change their color from yellow-brown to silver-black

- Electroconductive conjugated polymers (polypyrrole, polyaniline, polythyophene, polyfurans, polyarbsols etc)

Classification of Electrochromes

Chap2- 107

Electrochromic Dyes

- The technology for making a working electrochromic cell is very similar to the technology used in LCD displays.

- One way of making a working cell is by placing the electrochromic material between two transparent electrodes (preferentially Indium Tin Oxide, better known as ITO). The coloring of the EC-material results from changing the potential of the cell by charging the electrodes.

Working electrochromic cell preparation

Structure of ECD in off-state Structure of ECD in on-state Chap2- 108

Electrochromic Dyes

- An example of an electrochromic material that is able to turn blue is EV (Ethyl Viologen) and is one of the many electrochromic dyes, originating from the bipyridinium group.

- In most cases, the viologen molecules are symmetrical, so R' is equal to R''. For EV for instance, R' is equal to the ethylene group:

Molecular dyes

Chap2- 109

Electrochromic Dyes

- The coloring of EV from completely transparent to intense blue happens by absorption of an electron. This process is reversible.

- If the EV-molecule absorbs a second electron, EV turns pale blue. This reduction however is irreversible and definitely not wanted.

Molecular dyes : Viologen

Chap2- 110

Electrochromic DyesMolecular dyes : Viologen

- colorless bis-cationic molecules such as para-quat derivatives are reduced electrochemically to the colored radical cation.

- For electro chromic displays, the colored radical cation is deposited at an electrode.(derivatizationof quaternary group)

Chap2- 111

Electrochromic DyesMolecular dyes : Viologen

Chap2- 112

Electrochromic DyesMolecular dyes : Viologen

- These include symmetrical silanes, which can bond to the oxide lattice on the electrode surface,

- and a viologen with pyrrole side chain that undergoes anodic polymerization to form a film of the viologenbearing polypyrrole on the electrode.

- Polymeric bipyridilium salts have also been prepared for use in polymeric electrolytes.

- The quaternary groups in viologens can be derivatized to produce compounds capable of chemically bonding to a surface, especially electrode surfaces.

Chap2- 113

Electrochromic Dyes

- This reduction however is irreversible and definitely not wanted. The problem has been to overcome aging.

- Aging is the phenomenon whereby repeated switching of the display, which is absolutely necessary for practical display devices, causes the deposited molecules to eventually crystallize and thereby inhibit the reverse process.

Molecular dyes : Viologen

Chap2- 114

Electrochromic Dyes

- Thus diheptylviologen (1) the most commonly used viologen in experimental electro chromic display rigs, can stand only 5,000-10,000 oxidation-reduction cycles.

- By introducing steric hindrance and asymmetry into the molecules to inhibit crystallization, Barltrop and Jackson have improved the aging properties of viologens. One of the best compounds is (2) which gives at least 20,000 cycles without aging.

Molecular dyes : Viologen

Chap2- 115

Electrochromic Dyes

- Nippon Chemicals industries also has two patents on dyes for electro chromic displays . Nippon Chemical Industries also has two patents on dyes for electrochromic displays, although these cover the isomeric 2,2-bipyridyl system.

Molecular dyes : 2,2-bipyridyl system

N N

**

- It is fascinating that close analogues of powerful weed killers such as paraquatshould be finding use in a totally nonrelated, high technology applications.

Chap2- 116

Electrochromic Dyes

- IBM has also patented the use of bis-phthalocyanines in electro chromic displays.

Metal Phthalocyanine :

Chap2- 117

Electrochromic Dyes

- Many aromatic ring systems including aniline, pyrroles and thiophenes form extensively conjugated, electroactive polymers, which can be oxidized or reduced between an electrically neutral colorless state and a colored charged state, in the presence of a balancing counter ion often called doping.

Polymeric electrochromes

- They can therefore be used as solid electrochromes in thin films

Chap2- 118

Electrochromic Dyes

- Polyaniline can exist in four different redox states;

yellow(305nm), green (740nm) and dark blue to black

Polymeric electrochromes

Chap2- 119

Electrochromic Dyes

- 3,4-substituted derivatives synthesized by the reaction of substituted monomer with FeCl3 in chloroform solution

Polymeric electrochromes

Chap2- 120

Electrochromic Dyes

- One of the big drawbacks associated with the use of many conducting polymers as electrochromic materials I is their low cycle life stability

- To overcome this, and other electrochromic properties, many composite materials have been studies.

- Mixtures with other optically complementary, conducting polymers inorganic electrochromes, such as tungsten trioxide and Prussian Blue, and color enhancing agents or redox indicator ex. inherently electrochromic indigo carmine

Polymeric electrochromes

Chap2- 121

Electrochromic Dyes

- Inorganic oxides exhibiting electrochromism include cobalt oxide, nickel oxide,molybdenum trioxide, vanadium oxide, tungsten trioxide and their mixtures

- The most important of these are those based on tungsten trioxide. (where M=lithium or hydrogen)

- Pure tungsten trioxide, WO3, is very pale yellow and practically colorless in thin films, while the reflected color of the reduction product, MxWVI

(1–x)WVxO3, is proportional to the charge

injected.

Inorganic oxides

Chap2- 122

Electrochromic Dyes

- Mechanistically the color is formed by an optical charge-transfer between metal centers in the solid-state lattice, e.g. in tungsten trioxide this involves partial reduction of the pale yellow WVI to the blue WV state. This reduction requires partial insertion of a balancing cation.

Inorganic oxides

Mechanism of colour formation in tungsten trioxide.

- The color changes from blue (x = 0.2), through purple (x = 0.6) and red (x = 0.7) to bronze (x = 0.8–1.0).

Chap2- 123

Electrochromic Dyes

- Potential electrochromes having redox behavior : carbazoles, methoxybiphenyls, fulorenones, benzoquinones, naphthalquinones and anthraquinones, tetracyanoquinodimethane, tetrathiafulvalence and pyrazolnes.

- Of particular interest are the 1,4-phenylenediamines, which form highly colored species on oxidation: Wurster’s salt.

- used in composite elctrochromic systems for car rear-view mirrors.

Other organic electrochromes

Chap2- 124

Electrochromic Dyes

- They change their color under the influence of electric current and are therefore suitable for application in electrochromicdevices(ECD) that are functioning as batteries.

- smart windows and mirrors (e.g. darkening a window to control the inlet of sun light), active optical filters (e.g. sunglasses), displays and computer data storage.

Application

A flexible, electrochromic display

Chap2- 125

Electrochromic DyesApplication

and always wondered why they couldn’t switch much faster, give more varied shades of darkness, also work indoors and inside automobiles.

→ Electrochromic sunglasses

Have you seen photochromic sunglasses worn by many people with prescriptions, which automatically darken when going outdoors,

One of the most promising approaches to achieving this goal involves a combination of dye sensitized solar cells and electrochromic cells

https://www.youtube.com/watch?v=yDA-Z0YauM0

Chap2- 126

Electrochromic DyesApplication : smart window

- The control of the solar gain within buildings occurring through the glazing units is a highly desirable objective

- It would be most attractive to be able to do this dynamically, i.e. change in response to environmental conditions, and this has led to the idea of ‘smart windows’.

https://www.youtube.com/watch?v=itLqUDlw2tk

Chap2- 127

Electrochromic DyesApplication : smart window

- The most widely studied systems for glazing units are those based on tungsten trioxide.

- The glass used in the glazing units is coated with a transparent conducting surface, e.g. indium tin oxide (ITO) or stannic fluorine oxide (SFO).

Chap2- 128

Electrochromic DyesApplication : smart window

- The electrochromic cell is composed of an active electrode layer of tungsten trioxide (WO3) coated onto the conducting surface of one of the glass sheets, and a counter electrode of the nonstoichiometric lithium vanadium oxide (LixV2O5) or lithium nickel oxide (LixNiO2) laid down on the conducting surface of the other glass sheet.

- In between these two there is a thicker layer of a lithium ion polymer electrolyte

Chap2- 129

Electrochromic DyesApplication : smart window

- e.g. insertion of 12 mC cm–2 produces a sky blue color, which reduces the visible light transmission by a factor of 4. The color of the window cell is erased on reversing the applied charge.

- On applying a potential of around 1.5 V to the cell, lithium ions are discharged and dissolved at the WO3 surface, the lithium being supplied from the counter electrode of LixV2O5.

- The colorless WO3 becomes blue colored (LixWO3) on lithium ion insertion, the precise depth of color depending on the degree of insertion;

Chap2- 130

Electrochromic DyesApplication : smart window from combined DSSC and EC cell

- The dye absorbs light, becomes excited and injects electrons into the TiO2electrode.

- The electrons travel into the transparent WO3 film and then, to balance the charge, lithium ions from the electrolyte solution insert into the WO3 and in so doing create the colored species as described above.

- If the light source is removed then the cell is bleached back to its original color.

- However, if the cell is disconnected form the circuit before the light is removed it remains colored, because the electrons cannot escape from the reduced WO3film.

Chap2- 131

Electrochromic DyesApplication : Displays

- The big difference between devising electrochromic systems for displays and glazing is the need to construct arrays of smaller cells rather than one large one. and hence the need for multiplexing.

- Since coloration in EC cells is caused by a chemical change rather than by a light-emitting effect or interference, the color remains even when the current is turned off.

- This has led to the suggestion that the memory effect could be used in low power large area information billboards.

Chap2- 132

Electrochromic DyesApplication : Displays - Recently, Grätzel’s work on

nanocrystalline TiO2 has been extended into the displays area: an electrochromic molecule is linked to the surface of the colorless TiO2 semiconductor on conducting glass.

- On applying a negative potential, electrons are injected into the conduction band of the semi-conductor and the absorbed molecule is reduced and changes color.

- Applying a positive potential reverses the process. The nanocrystalline TiO2layer (5 μm) is highly porous and able to absorb several hundred monolayers of the electrochromic molecules, enabling the system to produce deep colors.

Chap2- 133

Electrochromic DyesApplication : Displays

- Using phosphonated viologens, color absorbance changes of more than 2 have been achieved in 0.1 to 0.5 s.

- To construct closed cells for use as display units, a counter electrode of zinc was coated

- The process is reversed by applying a voltage of 1–2 V. - The system needs rapid ion migration in the electrolyte so that the charge can

be compensated, and polar solvents of low viscosity polar solvents, e.g. acetonitrile (switching time of 100 ms) : 150 000 oxidation–reduction cycles

with a white reflector of microcrystalline TiO2 or ZrO2 .

- In operation, short-circuiting of the two electrodes causes electrons to flow from the zinc electrode, it being oxidized to Zn2+, to the viologens on the nanocrystalline electrode.

Chap2- 134

Electrochromic DyesApplication : electrochromic iris (June 19, 2014)

Future Smartphone Cameras may use a Micro Electrochromic Iris made from Smart Glass Eliminating the use of Actuators

Chap2- 135

Electrochromic DyesApplication : electrochromic iris (June 19, 2014)

- In the human eye, the iris controls the diameter of the pupil and subsequently the amount of light that reaches the retina.

- The purpose of an iris, or aperture stop, in a camera is exactly the same; it controls the amount of light that reaches a camera's sensors, which affects the overall focus of the image.

- Traditionally, cameras have contained a set of overlapping blades that are mechanically moved to change the size of the hole—or aperture—through which light enters. However, with the rising popularity of small, compact and lightweight consumer devices that are integrated with cameras, it has been almost impossible to miniaturize these mechanical systems.

Chap2- 136

Electrochromic DyesApplication : electrochromic iris (June 19, 2014)

- a small, low-powered camera component made from a "smart glass" material has been created in order to replace older styled mechanical actuators by a group of researchers in Europe, with the hope of inspiring the next generation of smartphone cameras.

- the newly developed micro-iris is an electro-chemical equivalent to the bulky, mechanical blades that are usually found in cameras and has very low power consumption, making it an ideal component for a wide-range of camera-integrated consumer devices.

Chap2- 137

Electrochromic DyesApplication : electrochromic iris (June 19, 2014)

- The report noted that the electrochromic iris consists of an electrochemical cell made of two sandwiched glass substrates each carrying a thin film electrochromic material on an underlying transparent electrode : (a) Cross section and (b) exploded view of the

electrochromic iris

*PEDOT : poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, a conjugated polymer and carries positive charges (=PEDOT:PSS)

Chap2- 138

Electrochromic Dyes

- Low power consumption:Recent EC displays are bistable and are able to work in reflection. This means that when you switch off the power, the color remains. You can even lower the power consumption by the addition of high reflective molecules, like Titania. This way, you don't need a backlight, so you are able to use the display as an electronic book.

- Cheap: The raw materials are cheap. - Switching is quite fast:

This is the time needed to change color, starting from the transparent state used to be about 5 seconds. This however can be solved by using porous electrodes. Typical switching times are now of the order of 200 milliseconds

Advantages of electrochromic devices

Chap2- 139

Electrochromic Dyes

- Integration of colors without color filters:Researchers have developed a molecular dye that can display red, green or blue, depending upon the applied voltage. This implies that you wouldn't need to use color filters, which diminish the brightness of the display.

- Easy transformation to make them in large amounts:Recent studies show that existing LCD-manufacturers could easily transform their assembling machines to assemble EC displays.

Advantages of electrochromic devices

Chap2- 140

Electrochromic Dyes

- These dyes used in electrochromic printing are the same as the colorless leuco dyes used in thermal printing except that they have been acylated.

- The dyes are incorporated into a moist paper, together with sodium bromide; electrical oxidation converts the bromide ion to bromine and the latter oxidizes the leuco dye to the dye proper.

Electrochromic Printing

Leuco dye

+

Br-

Leuco dye

+Br2

Dye

+Br-

- e

Chap2- 141

Electrochromic Dyes

- Dyes are available which cover the complete gamut of shades from yellow (1) to red (2) to blue (3).

Electrochromic Printing

Blue

S

N

N(CH3)2

O

(H3C)2N

HO3S

O

N

H3C O

OH

Yellow

Chap2- 142

Electrochromic Dyes

- The leuco dyes are converted into basic dyes such as methylene blue. Consequently they will have poor light fastness on paper. Therefore , electrochromic printing will have to compete at the cheap end of the market i.e., that dominated by thermal and pressure sensitive printing.

- It is difficult to see any great merit in electro chromic printing, since the acryl leuco dyes will be more expensive than the conventional leuco dyes used in thermal printing, a moist paper is required, and the fastness properties will be low.

Electrochromic Printing

Chap2- 143

Chromism

- The phenomenon of reversible color change is known as chromism.

- It is based on the phenomena that generate the change of the electron density of substances, especially π- or d-electron state, or the change in the arrangement of the substance supramolecular structure.

- Independently of the factors that trigger reversible color change, several kinds of chromism are known :

Color change mechanism

Chap2- 144

Chromism

- Photochromism (induced by sunlight or UV rays)- Thermochromism (induced by changes in temperature)- Solvatochromism (polarity of the solvent)- Hygrochromism (moisture)- ionochromism (ions)- Halochromism (pH value)- Acidochromism (acids)- Chemochromism (specific chemical agents like dangerous gases,

warfare agents, etc)- Electrochromism (electricity)- Piezochromism (pressure)- Mechanochromism (deformation of substances)

Classifications: