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SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
UNIT 3
FUNCTIONAL MATERIALS
Introduction to Conducting Polymers – Charge Transport Carriers: Exciton Formation in Organic Solar Cells and Organic Light Emitting Diodes (Principle and Working) – Conduction Mechanism in Polymers: Soliton, Polaron and Bipolaron Formation in Polyacetylene and Polyaniline Crystal Displays. – Liquid Crystals: Characteristic Features and Phases of Liquid Crystals – Liquid
3.1 INTRODUCTION TO CONDUCTING POLYMERS
1. What are conducting polymers? Conducting polymers are poly conjugated polymers, which are able to conduct electricity because of their conjugated π-bonding system. Examples: Polyacetylene; Polyaniline; Polythiophene, Polypyrrole 2. Classify conducting polymers.
Conducting Polymers I pnotrlyinmsiecraslly conducting E poxltyrimnseircsally conducting C haovnidnugc ctionngj (^) upgoaltyimoners D poolpyemde (^) rcsonducting
C filolendd upcotilvyem eelresment B polelynmdeedrs conducting
3. What is meant by doping of conjugated polymers? Doping of conjugated polymers involves partial oxidation (removal of electrons from the π π--bonding system of the polymerbonding system of the polymer backbone). backbone) or partial reduction (addition of electrons to Examples of p-type dopants: I 2 , Cl 2 , AsF 5 , BF 6 etc. Examples of n-type dopants: Na, K, Li, Tetra butyl ammonium etc. 4. Mention the various factors influencing the conductivity of polymers. The conductivity of conducting polymers increases with the length of conjugated polymer chain. The conductivity of conducting polymers increases with increasing in doping concentration. The conductivity of conducting polymers increases with increase of temperature.
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
5. List the various applications of conducting polymers. Conducting polymers are used in light emitting diodes, information storage devices, optical computers etc. Conducting polymers are used for making electronic display screens and optical fibers. Conducting polymers can be used as electrode materials for rechargeable batteries. Conducting polymers are used for making analytical sensors for pH, O 2 , NOx, SO 2 and glucose measurements. **3.2 CONDUCTION MECHANISM IN POLYMERS 3.2.1 Charge Transport Carriers (or) Conduction Mechanism in Polyacetylene
- How polyacetylene is prepared?** Polyacetylene is prepared by passing acetylene gas over the Zeigler-Natta catalyst. Zeigler-Natta catalyst is the coordination complex of tetra butoxy titanium, [Ti(OBu) 4 ], (organo metallic compound) and tri ethyl aluminium, [Et 3 Al]. n CH CH Ti(OBu) 784 o^ C/ Al(Et)^3 (CH CH)n The reaction forms copper coloured cis-polyacetylene at low temperature (‒78ºC). On raising the temperature of cis-polyacetylene film to 150ºC, a more stable silver coloured trans-polyacetylene is formed. 150 oC Conductivity 10^ Copper coloured Cis-PA-8^ - 10-7^ Scm^ -1^ Stable silver coloured Trans-PAConductivity 10-3^ - 10-2^ Scm- 2. Give the isomeric forms of polyacetylene. In cis-configuration of polyacetylene, the hydrogen atoms are bound on the same side of the carbon – carbon double bonds. C
H C
H C
H C
H C C^ C^ C H H H^ H In trans-configuration of polyacetylene, the hydrogen atoms are alternatively bound to opposite sides of the carbon – carbon double bonds.
C
H C
H C
H C
H
H (^) H (^) H H
C (^) C (^) C C
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
When a single bond of one domain [A] meets a single bond of another domain [B], a misfit called dangling bond is created with a radical defect, known as soliton.
Domain A Misfit
Radical defect: a single unpaierd electron, neutral soliton
Domain B The soliton is a radical defect which actually delocalized over ~15 CH units in the polymer chain. But for simplicity it is represented in the structure as if it resides on only one CH-unit. The formation of soliton is responsible for an intrinsic semi conductivity of 10 -^3 ‒ 10-^2 Scm˗1^ in trans-PA. Electronic behaviour (Conduction Mechanism) in Doped Polyacetylene (Formation of Polaron and Bipolaron) (i) P-type oxidative doping : When trans-PA is treated with a strong oxidizing agent like I 2 vapour, it removes an electron from the polymer chain and creates a positive charge with a radical site, known as polaron. When trans-PA is treated excess of I 2 vapour, it removes another electron forms a dication, known as bipolaron.
3 / 2 I 2 I + 3 -
Bipolaron, Dication^ Polaron,^ Radical^ Cation
I 3 -
I 3 -
3 / 2 I 2
(ii) n-type reductive doping : When trans-PA is treated with a reducing agent like Na metal, it donates an electron to the polymer structure and a creates a negative with a radical, known as polaron. When trans-PA is treated excess of Na, it donates another electron forms a dianion, known as bipolaron.
Na Na+
Bipolaron, Dianion^ Polaron,^ Radical^ Anion
Na+
Na+
Na
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
(iii) Energy band diagram : At high doping level, the number of polarons / bipolarons is increased, which creates a new mid gap energy bands polaron and bipolaron bands. This new band decreases the energy gap between the valence band and conduction band and provides freedom for the flow of π- bonding electrons. Hence in doped PA the charged polarons / bipolarons are responsible for making the polymer conductive.
Valence band (VB)
Eg = 1.7eV
Conduction band (CB)
Valence band (VB)
Eg = 1.65eV
Conduction band (CB)
Valence band (VB)
Eg = 1.5eV
Conduction band (CB)
Cis-PA Trans-PA Doped Trans-PA
Soliton band Bipolaron bandPolaron band
Conductivity, 10-8-10-7^ Scm-1^ Conductivity, 10-3-10-2^ Scm-1^ Conductivity, 10^3 Scm-
150 oC I 2 / Na
3.2.2 Charge Transport Carriers (or) Conduction Mechanism in Polyaniline
1. How polyaniline is prepared? Polyaniline can easily be prepared by electrochemical oxidation of aniline by using ammonium peroxy disulphate or potassium iodate as oxidizing agent.
N
H NH 2 (NH^4 )^2 S^2 O^8
2. What is meant by protonic acid doping? Protonic acid doping involves the treatment of emeraldine based polyaniline with protonic acid (1M HCl) to give a fully protonated emeraldine hydrochloride salt. The emeraldine salt has an electrical conductivity of 3×10^2 S.cm‒1^ due to formation of bipolaron and polaron. 3. Draw the different oxidation states of polyaniline. Polyaniline has a distinct structure consisting of an alternating arrangement of benzene rings and nitrogen atoms. The nitrogen atoms exist as an amine or as imine. The benzene rings rich with amine linkage (-NH-) is known as reduced benzenoid amine while the benzene rings rich with imine linkage (-N=) is known as oxidized quinoid imine.
Reduced Benzenoid Amine
N N
H (^) H x N^ N 1 - x Oxidised Quinoid Imine n Depending upon the relative concentration of reduced benzenoid amine and oxidized quinoid imine in polymer material, polyaniline exists in three different oxidation states.
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
Formation of Bipolaron and Stable Polaron In the first step of protonic acid doping, the hydrogen of the acid attaches themselves to the quinoid imine nitrogen atoms to form bipolarons.
1 M HCl
N N
H (^) H N N Polyemeraldine
N N
H (^) H N+^ N+
H (^) H
Bipolaron, Highly unstab^ C le l^ Cl In the second step, the highly unstable bipolaron dissociates into a more stabl (the quinoid ring transforms into more stable benzenoid ring). The delocalization ofe polarons polarons in polymer structure magnifies the electrical of 3×10^2 Scm‒1.
N N
H (^) H
Polaron, Stable
N N
H (^) H
R deelsoocnaalinzceed (^) polfaorromn sl (^) atticoef
N N
H (^) H N N
H (^) H
.^ +^ ... + ..
N N
H (^) H N+^ N+
H (^) H
Bipolaron, Highly unstable
Energy band diagram At high doping level (~50% HCl) the polarons create a new mid gap energy band band) and are responsible for making the polymer highly conductive. (polaron
Valence band (VB)
Eg = 3eV
Conduction band (CB)
Valence band (VB)
Eg = 2.7eV
Conduction band (CB)
Undoped polyaniline (^) Protonated Emeraldine Salt
Polaron band
Bipolaron band
Conductivity, 10-8^ Scm-1^ Conductivity, 3x10^2 Scm-
Protonic Acid Doping
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
3.3 ORGANIC LIGHT EMITTING DIODES (OLEDs)
1. What is an organic light emitting diode (OLED)? It is a molecular electronic device capable of converting electrical signal into light signal. 2. Define electroluminescence. When an electric field is applied in forward direction, both the positive holes and electrons are injected simultaneously into the semiconducting polymer, where they recombine to form excited electron-hole pairs known as excitons. When the excited excitons (decay) come back to ground state, emit the light of definite wavelength. This phenomenon is known as electroluminescence. 3. What are excitons? The electron ( e‒ ) hole ( h+ ) pair bound by coulombic attraction is called as exciton. The binding energy of exciton is in the range of 0.1 – 0.5eV. 4. Give the merits and demerits of OLED. Merits of OLED O hence it is cost effective.LED can be processed at any substrate by inkjet printing or screen printing and, OLED have very high response time when compared to LCD and plasma screens. Demerits of OLED OLED have low lifespan (5 years with 8 hours per day usage). OLED may get disintegrated when exposed to moisture / water. 5. List the applications of OLED. Roll-up daily refreshable electronic newspapers; High-definition televisions; Multi or full colored cell phone displays; Aircraft cockpit instrumental panel etc. 6. Explain the principle, construction and working of OLED. Definition of OLED It is a molecular electronic device capable of converting electrical signal into light signal. Construction of OLED OLED consists of a thin layer of semiconducting polymer (100nm) sandwiched between a glass substrate coated with an ITO (Indium-Tin-Oxide) (200nm) and a thin layer of metal (Al, Ca, Mg) deposit (100nm).
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
- Poly (3,4- ethylenedioxythi ophene) [PEDOT] S
O O
650nm / Red
Ca 24%
Applications of OLED Roll-up daily refreshable electronic newspapers; High-definition televisions; Multi or full colored cell phone displays; Aircraft cockpit instrumental panel; Military uniforms. Advantages of OLED OLED can be processed at any substrate by inkjet printing or screen printing and, hence it is cost effective. OLED have very high response time when compared to LCD and plasma screens. Disadvantages of OLED OLED have low lifespan (5 years with 8 hours per day usage). OLED may get disintegrated when exposed to moisture / water. 3.4 ORGANIC SOLAR CELLS (OSC) ORGANIC PHOTO VOLTAICS (OPV)
1. What is an organic solar cell? O directly into electricity.rganic solar cell is a molecular electronic device that converts energy of sunlight 2. Mention the various steps involved in the energy in organic solar cell? conversion of light energy into electrical
The process of converting light energy in composed of four steps namely: to electric current in organic solar cells is
- Light absorption and exciton generation
- Exciton diffusion
- Exciton dissociation (Charge carrier generation)
- Electric current generation (Charge transport) 3. What are the advantages and limitations of organic solar cells? Advantages of organic solar cells: Simple fabrication process; Light weight, thin and flexibility; Easy solution processability for large scale production Disadvantages of organic solar cells: Low efficiency of conversion; Low stability; Low strength when compared to inorganic PV cells 4. List the various applications of organic solar cells. Portable power supply; Emergency power source; Toys, watches, calculators etc.
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
5. 6. Describe the construction and workingExplain the working principle and architecture of organic photo principle of organic solar cells ( voltaic OSC(OPV).).
Definition of Organic Solar Cells (OSC) OSC cell is a molecular electronic device that converts energy of sunlight directly into electricity. Working Principle of Organic Solar Cells The process of converting light energy into electric current in organic solar cells is composed of four steps namely:
- Light Absorption and ExcitonExciton Diffusion Generation
- Exciton Dissociation (Charge Carrier Generation)
- Electric Current Generation (Charge Transport)
Photons of light
HOMO
AnodeITO
LUMO
HOMO
LUMO CathodeMetal
Donor [PPV/P3AT/PEDOT] Acceptor [C 60 ]
Donor-AcceptorInterface
Exciton
(^1 )
3 4
4
1. Light absorption and Exciton Generation 2. Exciton Diffusion3. Exciton Dissociation 4. Electric Current Generation 1. Light Absorption and Exciton Generation When the photons of sunlight strike the organic semiconductor, the electron moves from highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) energy levels. This phenomenon generates electron ( by coulombic attraction called as exciton. The binding energy of exciton is in the range e‒ ) hole ( h+ ) pair bound of 0.1 – 0.5eV. 2. Diffusion of Exciton This bound exciton diffuses towards the Donor-Acceptor interface in organic materials within 1ps and the diffusion length of excitons is limited to about 10-20 nm. 3. Exciton Dissociation and Charge Carrier Generation Because of strong electron affinity of acceptor material, an intermolecular electron transfer takes place from LUMO of donor material to LUMO of acceptor material. Further the energy level difference of d greater than binding energy of exciton (0.1onor material (1.1 – 0.5 eV), the dissociation of exciton takes eV) and acceptor (3 eV) is place with the time level of <100 fs.
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
Advantages of Organic Solar Cells Simple fabrication process Light weight, thin and flexibility Easy solution processability for large scale production Disadvantages of Organic Solar Cells Low efficiency of conversion Low stability Low strength when compared to inorganic PV cells Applications of Organic Solar Cells Portable power supply Emergency power source Toys, watches, calculators etc. 3.5 LIQUID CRYSTALS 3.5.1 Introduction to Liquid Crystals
1. What are liquid crystals? Liquid crystals are the intermediate mesophase of the long chain organic solid compounds having both fluidity like liquids and long-range order like solid crystals. Example: Cholesteryl benzoate. It undergoes phase transition at 145°C to liquid crystal and melts to isotropic clear liquid at 178°C. Cholesteryl benzoate (solid) Cholesteryl benzoate Cholesteryl benzoate (liquid crystal) (liquid)
145 oC 178 oC
2. Differentiate between mesogen and mesophases. Mesogen: Mesogen is an elongated, rigid rodlike or disclike molecule, which are the components of liquid crystalline materials. 4 - pentyl-4'-cyanobiphenyl Mesophase: Mesophase is an intermediate state of matter between liquid and solid. Molecules that demonstrate mesophases are called mesogens. 3. Differentiate solid crystals, true liquids and liquid crystals. Soli molecules. d crystals are characterized with the regular arrangement, long range arrangement of True liquids are characterized with completely random of arrangement of molecules and exhibit fluidity. Liquid crystals range order like solid crystals. are the intermediate mesophase having both fluidity like liquids and long-
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
**4. Show the diagrammatic representation of solid crystals, true liquids and liquid crystals.
- What are the characteristic features of liquid crystals?** Liquid crystals are elongated, rod-like molecular structure having strong dipole and/or easily polarizable substituents. The molecules can exhibit orientational order (molecules show regularity in their orientation) such that all the axes line up in a particular direction. The molecules can also exhibit positional order (molecules or groups of molecules can show translational symmetry). Liquid crystals exhibit anisotropic physical properties (optical, mechanical, electrical etc.). 6. Explain the structural features of calamitic liquid crystals. Calamitic mesogens usually follow the general structural formula:
where R' and R" are often flexible terminal units such that at least one R group is an alkyl chain which provides the stability to the mesophase; A, B, C, and D, are the rigid core flat segments (phenyl, cyclohexyl, heteroaromatics, and heterocycles) and [L] represents the linking units, such as CH=N, COO or N=N that can increase the length and flexibility of the molecule. Further, the presence of unsaturation increases the polarizability and maintain a linear shape suitable for mesophase formation. 3.5.2 Classification of Liquid Crystals
1. Classify liquid crystals.
(Mesophase formation istemperature dependent) concentration dependent)(Mesophase formation is
Liquid Crystals Thermotropic Lyotropic
Rod-like Molecules Disc-like Molecules (Discotics) Nematic (^) Smectic Ordinary Nematic Twisted Nematic(Chloesteric)
PerpendicularArrangement Tilted LongAxis
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
The optical observations indicated different macro-structures of smectic phases. They are known as A, B and C types of smectic liquid crystals. (i) Smectic-A Mesophase [Example: Ethyl p-(p’-phenylbenzalamino) benzoate] Smectic-A is a phase in which the molecules are parallel to one another and are arranged in layers with the long axes perpendicular to the layer plane. (ii) Smectic-C Mesophase [Example: p-n-octyloxy benzoic acid] Smectic C is a smectic A like structure in which the long axes of the molecules of a tilted average angle differing from 90° with respect to the plane of the layer in a "bookshelf” arrangement.
3. Cholesteric Mesophase [Example: Cholesterylnonanoate] The cholesteric (or chiral nematic) is composed of nematic mesogenic molecules containing a chiral center which produces intermolecular forces that favour alignment between molecules at a slight angle to one another.
This twist may be right-handed or left-handed and depends on the molecular conformation. A cholesteric liquid crystal can rotate the direction of linearly polarized light. It is roughly 1000 times stronger than the activity of ordinary optically active substance such as quartz and hence it is optically active. An important characteristic of the cholesteric mesophase is the pitch (chiral pitch). The chiral pitch refers to the distance over which the liquid crystal molecule undergoes a full 360- degree twist. 3.5.3 Liquid Crystal Display (LCD)
1. Define LCD. LCD is an acronym used for L iquid C rystal D isplay. It uses twisted nematic type liquid crystals to display the characters, images, videos and animations by simply altering the light traveling through it. 2. What are the merits and demerits of LCD? Merits of LCD: The heat generated during operation is less as compared to CRT and LED display; The power consumption by an LCD is very less in comparison to other display devices; The overall cost of the device is low.
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
Demerits of LCD: It needs an external source of light for displaying the image; Its operating temperature range is limited that lies in between 0 to 60°C; The image visibility relies on light intensity.
3. List the various applications of LCD? Liquid crystals are used as temperature sensors. Liquid crystal screens are used in high-definition televisions, personal computer, electronic and electrical measuring devices etc. Liquid crystalline substances are used to locate the veins arteries, infec tumours. tions and Liquid crystals can be used to find the point potential of failure in electrical circuits. 4. Give a detailed account on liquid crystal display. Definition of LCD LCD is an acronym used for L iquid C rystal D isplay. It uses twisted nematic type liquid crystals to display the characters, images, videos and animations by simply altering the light traveling through it.
Construction of LCD For making LCD, a thin layer twisted nematic liquid crystal material with gap spacer of thickness 10 – 20 μm is sealed between two transparent glass sheets coated on their inside faces with indium tin oxide (ITO) (with a typical concentration of 90% In 2 O 3 and 10% SnO 2 ) to act as conducting electrodes. The electrodes are used to control the electric fie crystal molecules. Hence, the liquid crystal cell acts as a light switch in LCD.ld on the direction of alignment of liquid
In LCD, a suitable light source is used for emitting unpolarized light. An LCD consisting of a vertical polarizer and horizontal will pass only vertical components of the lights and horizontal components will polarizer. The vertical polarizer be absorbed by it. Similarly, the horizontal polarizer will pass only horizontal components of lights and vertical components are absorbed.
SCYA1101: Engineering Chemistry UNIT 3 : Functional Materials
5. Compare LCD and OLED.
S. No. LCD OLED
- LCD is liquid crystal display which modulates light and produces an image on the screen.
O whichLED is emits organic light light emitting diode, during forward biasing.
- Low power consumption (microwatts). High power consumption (milliwatts).
- Response time is in microseconds. Response time is in nanoseconds.
- Intensity of light cannot be controlled. Intensity of light can be controlled.
- Suitable for large area display. Not suitable for large area display.
- Cost is very low. Cost is high.
