MODULE 2: Materials for Memory and

Display Systems
Memory Devices: Introduction, Basic concepts of electronic memory, History of
organic/polymer electronic memory devices, Classification of electronic
memory devices, types of organic memory devices (organic molecules,
polymeric materials, organic-inorganic hybrid materials).
Display Systems: Photoactive and electroactive materials, Nanomaterials and
organic materials used in optoelectronic devices. Liquid crystals (LC’s) -
Introduction, classification, properties and application in Liquid Crystal
Displays (LCD’s). Properties and application of Organic Light Emitting Diodes
(OLED’s) and Quantum Light Emitting Diodes (QLED’s), Light emitting
electrochemical cells.
Memory devices
Introduction:
Memory devices are the electronic components or piece of hardware used to
store data and instructions temporarily or permanently.

There are different types of memory devices with unique characteristics and
uses example CD DVD USB and external hard disk

Memory devices are broadly classified into 2 categories

a. Volatile memory devices- it is a temporary memory; it loses its data or
content when power is removed. example ram random access memory
b. non-volatile memory - it is a permanent memory and retains all the data
when the power is removed. example ROM read only memory

Basic concept of electronic memory devices

 It is a type of storage technology uses semiconductor material

 Here the memory is stored in the form of binary code -zero and one.
zero and one can represent in many different ways by using different
techniques to store them in different memory devices
 They can be represented by voltage, magnetic field or even capacitor
change in the memory devices.
 The memory cells are arranged in rows and column, this memory array
determine how data is accessed
 The memory device may use only to read or both read as well as write
the stored data.

Classification of memory devices

There are 4 categories based on material used in it or from what it is made

up of.

a. Transistor type memory devices-

  transistor type memory devices they are the semiconductor devices
consist of 3 terminals for connecting electric circuit.
 it can produce strong output signals for week input signal and act as
amplifier
 this type of devices consists of billions of transistors in it and each
represent one bit of information which is zero and one.
 it is used for switching operation either for open or closed circuit when
the when the transistor is turned on it represents one (closed circuit) and
when the transistor is turned off it represents zero (open circuit)
example DRAM (dynamic), SRAM (static)

b. Capacitor type electronic memory devices

 it is a 2-terminal electric device that can store energy in the form of
electric charge.
 it consists of 2 metal plates that are separated by an insulating material
known as dielectric.
 the capacitor can be charged or discharged, this property of capacitor is
used in capacitor type memory devices
 the capacitor type memory devices consist of billions of capacitors each
capacitor represents one bit of information which may be zero or one.
while charging it represent bit one and at discharging it represent zero.
example ferroelectric random-access memory FeRAM

c. Resistor type memory devices

 register type electronic memory device the resistor is an electric
component that limits or regulates the flow of electric current in an
electric circuit
 the resistive switching device consists or made up of metal- insulator-
metal structure in which middle layer act as an active switching media
in which metal oxide is sandwiched between metal electrode
 Metal oxide act as switching active switching media
 In the process of charging and discharging resulting in 2 stable
resistance state that is low and high resistance state.
 the device also consists of billions of resistors each bit represents one
bit of information which is zero and one
 low resistance state is represented by one and high distance state is
represented by zero
 Example PRAM programmable RAM
d. charge transfer type memory devices
 charge transfer type electronic device stores data in the form of charge
packets that are transferred from one location to another.
 charge transfer complex consists of 2 molecules, electron acceptor and
electron donor that can self-attract through electrostatic force.
 conductivity in charge transfer depends on ionic bonding between
donor and acceptor component.
 the conductivity of charge transfer complex can be employed to design
molecular electronic devices which includes organometallic complexes,
carbon allotropes nanoparticles etc have been utilised for memory
applications.

Organic memory devices

Organic memory devices these are the memory devices which uses organic
semiconductor (P type and n type) and store data in different electric
conductivity state that is on and off.

Example: pentacene (P type), per fluoro pentacene (n type)

organic memory devices with organic P type semiconductors:

example pentacene

 Pentacene is an organic molecule consists of 5 linearly fused benzene

ring
 since pentacene is P type semiconductor which has deficiency of
electron and can readily accept electron from another
 using pentacene, memory device is constructed in which pentacene is
sandwiched between 2 electrodes cathode and anode
 pentacene molecule present in middle act as active layer in memory
device when the voltage is applied across the 2 electrode electrons flows
from the anode towards pentacene layer and the electron is accepted by
the pentacene molecule
  This flow of electron causes a change in electric property of the device in
terms of change in voltage or capacitance this change in voltage or
capacitance is used to represent the binary bit of information that is zero
or one
 the state of memory device can be read by the resulting current between
2 electrodes, when voltage is applied.

Organic memory devices with organic n type semiconductor:

example perfluoro pentacene

 It is an organic molecule consists of linearly fused benzene ring in which

the hydrogen of the benzene ring is replaced by fluorine.
 since perfluoro pentacene has an excess electron, it acts as n-type
semiconductor and can readily donates electron to another
 using perfluoro pentacene memory device is constructed in which
perfluoro pentacene is sandwiched between 2 electrodes cathode and
anode, act as active layer in a memory device
 when the voltage is applied across the electrodes the cathodes receive
electrons from perfluoro pentacene layer,
 this flow in electron causes the change the flow of electron causes the
change in electric property of the device, such as resistance or
capacitance.
 this change is used to represent the bit of information like zero and one
t
 The state of memory device can be there read by applying small voltage
between electrode and measuring the resulting current

Polymer used in memory devices

Polymer used in memory devices as polymer memory materials required
understanding between chemical and memory property, because volatile
memory cannot sustain 2 distinct state that is electron accepting and donating
ability without external power supply which causes the written data to
disappear, where non-volatile memory can sustain.

Polymer memories are Increasingly being used in memory devices due to their
ability to exhibit bi stable state and also unique property such as Flexibility,
low cost and compatibility.

In general, functional polyamide is used, this is incorporated with donor

triphenyl amine and acceptor phthalimide work was commonly used as
polymer backbone.
  In polymer used organic memory devices. Polymer unit is sandwiched
between 2 electrodes typically made up of metal like gold and
aluminium
 when a voltage is applied across the electrodes of the device electrons
are transform transferred from donor to acceptor in polymer unit to form
donor acceptor complex
 this creates a dipole moment within the material and electronic structure
is changed, hence there is a transition from low conducting to high
conducting state. thus, leads to store information.
 they are used in different types of memory devices volatile memory
DRAM and SRAM, in non-volatile memory devices ROM and flash devices

Organic memory devices with organic-inorganic hybrid materials

Generally, organic-inorganic hybrid materials are composed of

organic layers containing inorganic materials. Inorganic materials used
are allotropes of carbon like fullerenes, carbon nanotubes, graphene and
metal nanoparticles, semiconductor nanoparticles and inorganic
quantum dots (QDs).

1. Organic-carbon allotrope hybrid materials

Polymers containing electron donors such as thiophene, flourene,
carbazole and aniline derivatives can be combined with fullerenes to
obtain a charge transfer hybrid material with donor-acceptor ability
and electrical bistable states. Fullerenes exhibit high electron
withdrawing ability and can capture up to six electrons. They are
used on WORM memory effect devices.
2. Organic-inorganic nanocomposites
These are the hybrid electronic memory devices in which organic
polymer with appropriate functional group is clubbed with metal
nanoparticles, quantum dots and metal oxide nanoparticles. An
example is a composite of 8-hydroxyquinoline containing polymer
with gold nanoparticles sandwiched between two metal electrodes.
Bistable electronic transition states are observed when an electric
field is applied due to charge transfer between the gold nanoparticles
and 8-hydroxy- quinoline.

Display system
Display system it is the collection of hardware and software component, that
work together to generate and present visual information. such as text,
images or videos.
 Some special materials are used in display system like photoactive materials
and electroactive material.

Photo active materials absorb light and convert it into electric signal.
example photo sensors or photovoltaic cells
In photovoltaic cells, when the light falls on photo active material it knocks
and releases the electron, which generates electric current. this type of
materials is used in LCD to control the pixels in display system

Electroactive materials they are the materials that change their properties
such as size, shape or refractive index when exposed to electric fields they are
used in watches calculator or portable devices

Working

 In photoactive and electroactive materials absorbs and emits light in UV

to IR region. This allows the electron to jump from donor to acceptor to
generate and transfer charge carriers (electrons and holes)
 light emitting layer containing photoactive or electroactive material is
sandwiched between conducting layer act as electrode, adjacent to
electrode transparent subtract is placed to collect the light.
 when electric field is applied across the electrode, electrons move from
cathode and anode allows the movement of holes towards the light
emitting layer and create electron hole pair in light emitting layers
containing photoactive and electroactive materials with release of
energy.
 This energy is utilised to excite the electron in light emitting layer
containing photoactive or electroactive material. When electron
returning back, the excess energy is emitted in the form of Light.
 This light is extracted by transparent subtract which is used to control
the pixels of the screen

Optoelectronic device: Optoelectronic is a branch of electronic device that

deals with interaction between light and electric field. Optoelectronic device is
the electronic device that are capable of emitting, detecting or manipulating
the light. Example LED, photo diodes, solar cells and optical fibres
Optoelectrical devices contains special type of semiconductor that converts
light energy into electrical energy and electrical energy into light energy
The photon has energy larger than the energy gap of the semiconductor is
incident, the photon is get absorbed by the semiconductor and excites the
electron from valance band to conduction band in the semiconductor. when
the excited electron is returning back to valence band extra photon energy is
emitted in the form of light. Further light can be collected as electric current.
There are several molecules like nanomaterials, organic molecules, polymer
molecules etc have been used in optoelectrical devices.

Uses of nano material in optoelectronic devices

Nano materials are any substances in which at least one dimension is less
than 100 nanometres are called nano materials. Example: silicon nano
materials is one of the nano materials used in optoelectronic devices

Nano material used in optoelectronic devices have some properties

1.Quantum Confinement effect

2. Increased surface area to volume ratio

In the case of Silicon Nanocrystals, the quantum confinement effect is a major contributing
factor to their unique properties. As the size of the Silicon Nanocrystals decreases, the
electrons become confined in a much smaller space, resulting in a change in their energy
levels. This effect leads to the increase in the bandgap of Silicon Nanocrystals, which in turn
affects their optical and electronic properties.

Special properties of Silicon Nano crystals for optoelectronics

 SiNCs exhibit strong quantum confinement effect due to their small

size.
 SiNCs exhibit high photoluminescence quantum yield, which makes
them attractive for applications in light-emitting devices.
 Theybare biocompatible and can be easily integrated into
biological systems for bioimaging and biosensing applications.
 SiNCs are non-toxic and biocompatible, making them safe for
use in medical applications.

Applications:

 Silicon nanocrystals are being explored for energy storage

applications such as lithium-ion batteries and supercapacitors.
 Silicon nanocrystals are being explored for various catalytic
applications such as water splitting and CO2 reduction.
 Used for sensing applications such as gas sensing, biosensing, and
environmental sensing.
 SiNCs are used in neuromorphic computing and down-shifting in
photovoltaics.
Properties of organic materials [Light absorbing materials - Polythiophenes
(P3HT) in optoelectronic devices

Polythiophenes are conjugated polymers, environmentally and thermally

stable material. Chemical structure of P3HT Poly (3-hexylthiophene) is a
polymer with chemical formula (C10H14S) n. Poly(3-hexylthiophene) or P3HT
is one of the most studied and widely used polythiophenes due to its
desirable properties, such as high charge carrier mobility, high absorption
coefficient in the visible region, and good solubility in common organic
solvents.
P3HT has a short alkyl group, specifically a hexyl group, attached to each
thiophene ring, which enhances the solubility of the polymer in organic
solvents.
Exhibit a high degree of crystallinity and a preferred orientation of the
polymer chains perpendicular to the substrate surface. This is due to the π-
π stacking interactions between the fully extended polymer chains, with a
distance of 0.33 nm between adjacent rings.
Properties:
 P3HT is a semiconducting polymer with high stability and exhibits
conductivity due to holes, therefore considered as p-type
semiconductor.
 P3HT has great capability as a light-absorbing material in organic
electronic devices.
 P3HT has a crystalline structure and good charge-transport
properties required for optoelectronics.
 P3HT has a direct-allowed optical transition with a fundamental
energy gap of 2.14 eV.
 The fundamental bandgap of P3HT is in the visible region at 490 nm,
corresponding to a π → π* transition, giving electron-hole pairs.
 P3HT indicates that an increase in the conductivity is associated with
an increase in the degree of crystallinity.
 Organic materials like P3HT have good charge carrier mobility, which
means that they can transport charges (electrons or holes)
effectively through the material.

Applications:
 used to fabricate organic field-effect transistors (OFETs)
 used as a hole-transporting layer in organic light-emitting diodes
(OLEDs) to improve their efficiency and stability.
 P3HTcomposite applications in energy storage, sensing, and catalysis.
 P3HT can also be used in the fabrication of biosensors
 Manufacture of smart windows

Properties of Light emitting materials – Poly [9- vinyl carbazole]

(PVK)]

Poly (N-vinyl carbazole) (PVK) is a highly processable polymer

that has been widely used as a hole-transport material (HTM) in planar
heterojunction perovskite solar cells.They have a high glass transition
temperature, which ensures thermal stability of the solar cell.
Properties:
 Luminescence: When excited with light or an electric field, PVK
emits light in the blue-violet range of the visible spectrum.
 Processability: PVK is a highly processable polymer that can be
easily spin-coated or printed to form thin films for use in
optoelectronic devices.
 Solubility: PVK is soluble in a wide range of solvents, which makes it
 easy to prepare solutions of the polymer for processing.
 Thermal stability: PVK has a high glass transition temperature,
which means that it can withstand high temperatures without
degrading.
Applications:
 used as a resist material in electron beam and X-ray lithography.
 PVK is used in the fabrication of light-emitting diodes and laser printers.
 PVK is used in the fabrication of organic solar cells when combined
with titanium dioxide (TiO2) on a glass substrate.
 PVK is used in the fabrication of solar cells when combined with
perovskite materials.
 PVK-Perovskite junctions are used in the fabrication of highly
efficient and stable planar heterojunction perovskite solar cells.
 Liquid crystals (LCs)
Introduction:

The liquid crystals are a unique state of matter between solid (crystalline) and liquid
(isotropic) phases. OR
Liquid crystals (LCs) are a state of matter that has properties of both liquids and crystals.
They are characterized by having an ordered molecular arrangement like a crystal but the
ability to flow like a liquid.
The unique properties of LCs make them ideal for use in various applications such as
LCDs.
Classification of Liquid Crystals:
i) Thermotropic liquid crystals
ii) Lyotropic liquid crystals

Thermotropic liquid crystal: A liquid crystal is said to be thermotropic if

molecular orientation is dependent on the temperature. By increasing the
temperature, the increase in energy and thereby movement of constituent
molecules, will induce phase changes.

Thermotropic liquid crystals have been classified into the following

types:

a) Nematic (or thread-like liquid crystals): The molecules move either

sideways or up and down. Increase in temperature decreases the
degree of orientation. Example: p-azoxyphenetole

b) Smectic (or soap-like liquid crystals): The molecules in smectic crystals

are oriented parallel to each other as in the nematic phase but in layers.
 c) Cholesteric liquid crystals: Molecules in successive layers are slightly
twisted and form helical pattern.
d) d) Discotic liquid crystal: Molecules are arranged in a column arranged
with disclike structure.

ii) Lyotropic liquid crystals: The orientational behaviour of Lyotropic

crystals is a function of concentration and solvent. These molecules are
amphiphilic – they have both hydrophilic and hydrophobic ends in their
molecules. At low concentrations, molecules are randomly arranged and
higher concentration produces a definite pattern heads out and tail in.
Soaps and detergents form Lyotropic crystals when they combine with
water

Lyotropic LCs

Properties of liquid crystals (LCs) are:

 Anisotropy: LCs exhibit anisotropy, which means that their
properties depend on the direction in which they are measured.
 Optical properties: LCs have unique optical properties, including
birefringence, which makes them useful in displays and other
optical applications.
 Polarizability: LCs are polarizable, which means that they can
respond to an electric field.
 Viscosity: LCs have a relatively high viscosity, which makes them
resistant to flow.
 Phase transitions: LCs undergo phase transitions between different
ordered states this leads to change in optical properties.
 Chemical stability: LCs are generally chemically stable, which
makes them suitable for use in a variety of applications.

Applications:
1. LCD displays in consumer electronics and computers.
2. Temperature sensors and thermometers.
3. Optical shutters and filters.
4. Electro-optic modulators in telecommunications.
5. Optical data storage and holography.
6. Alignment layers in LCD manufacturing.
7. Biosensors and bio-imaging in medical applications.

Liquid Crystal displays (LCDs)

A liquid crystal display (LCD) is a flat panel display technology
that uses liquid crystals to produce images. LCDs are commonly used
in electronic devices such as televisions, computer monitors, and
mobile phones.
Application in Liquid Crystal displays (LCDs)
Liquid-crystal displays (LCDs) consist of multiple layers. Light
produced by a light-emitting diode passes through polarizing filters,
colour filters, and a liquid-crystal layer to produce an image.
Liquid Crystal Displays (LCDs) have a wide range of applications
across various industries. Here are some key uses:
 Consumer Electronics: LCDs are commonly used in televisions,
computer monitors, smartphones, and tablets due to their energy
efficiency and high-quality display.
 Aviation: Aircraft cockpit displays rely on LCD technology for clear
and reliable instrumentation.
 Medical Equipment: Many medical imaging devices and
diagnostic tools incorporate LCD screens for precise visual
representation.
 Automobiles: Modern car dashboards and infotainment systems
use LCDs for navigation and control displays.
 Industrial Instruments: Laboratory equipment and measuring
instruments often feature LCD screens for data visualization.
 Digital Cameras & Watches: LCDs are used in digital cameras for
image previews and in wristwatches for displaying time and other
functions

Organic Light Emitting Diodes (OLED’s)

“OLEDs are thin film devices consisting of organic layers
sandwiched between two electrodes and capable of emitting light
when an electric field is applied. OLEDs operate by converting
electrical current into light via an organic emitter”

Properties:
 Thinness and flexibility: OLEDs are very thin and flexible,
which makes them suitable for use in curved or flexible
displays.
 High contrast: OLEDs have a high contrast ratio, which means
that they can produce deep blacks and bright whites,
resulting in images with vivid and rich colors.
 Fast response time: OLEDs have a fast response time, which
means that they can switch on and off quickly, resulting in
smooth and seamless motion in video content.
 Wide viewing angle: OLEDs have a wide viewing angle, which
means that the image quality is maintained even when
viewed from different angles
 Energy efficiency: OLEDs are energy efficient, as they do not
require a backlight like traditional LCD displays, resulting in
lower power consumption.
 Self-emissive: OLEDs are self-emissive, which means that
they do not require a separate light source, resulting in a
thinner display.
 Long lifespan: OLEDs have a long lifespan, as they do not
contain a backlight that can degrade over time, resulting in a
longer-lasting display.
Applications

Organic Light Emitting Diodes (OLEDs) have a wide range of applications

due to their unique properties, including high contrast, energy efficiency,
thinness, and flexibility. Here are some of the applications of OLED:

1. Televisions and displays: OLED displays are used in televisions,

monitors, smartphones, and other electronic devices.

2. Lighting: OLEDs can also be used as a source of lighting in various

applications, including automotive lighting, street lighting, and
architectural lighting.

3. Wearable devices: The thin and flexible nature of OLEDs makes them
suitable for use in wearable devices, such as smartwatches and fitness
trackers.

4. Automotive: OLEDs can be used in automotive applications, such as

dashboard displays, interior lighting, and taillights.

5. Medical: OLEDs can be used in medical applications, such as in surgical

lighting and medical imaging. They offer bright and highly accurate
lighting options that can help improve medical procedures and diagnosis.

Quantum Light Emitting Diodes (QLED’s): Quantum dot light-emitting diodes

(QLEDs) are a type of electroluminescent device that utilize quantum dots (cadmium
selenide or zinc sulphide nano crystals) to produce high-performance displays. QLEDs
have several advantages over traditional display technologies.

Properties:
 Wide viewing angles: which means that the colours and image
quality remain consistent even when viewed from different angles.
 Environmentally friendly: QLEDs are free from harmful substances
such as mercury and lead, making them more environmentally
friendly than traditional LCD displays.
 Accurate and vibrant colours: QLEDs use quantum dots to emit
light of a specific colour when excited by a light source or electrical
current, resulting in highly accurate and vibrant colours.
 Energy-efficient: QLEDs are more energy-efficient than traditional
LCD displays because they require less backlighting.
 High contrast: QLED displays have high contrast ratios, which
 means that the difference between the darkest and brightest areas
of the display is greater, resulting in more detailed and lifelike
images.
 Long lifespan: QLEDs have a longer lifespan than traditional LCD
displays because they do not suffer from the same issues of
backlight burnout or colour fading over time.
 Fast response times: means that they can display fast-moving
images without motion blur or ghosting.
 Flexibility: QLEDs can be made on flexible substrates, allowing for
the creation of flexible displays that can be bent or curved.

Applications:

1. Used in ultrafast and energy-efficient communication systems.

2. QLEDs can be utilized in quantum cryptography to secure
communication channels.
3. Additionally, QLEDs can be employed in medical imaging
applications such as MRI machines to generate high-resolution and
accurate images.
4. Lighting: QLEDs can also be used as a source of lighting in various
applications, including automotive lighting, street lighting, and
architectural lighting. They offer energy-efficient and highly
customizable lighting options.
5. QLEDs can be utilized in quantum sensing applications for high-
precision measurements.
6. Televisions and displays: QLED displays are commonly used in
televisions, monitors, smartphones, and other electronic devices.
They offer superior image quality and colour accuracy compared to
traditional LCD displays.
7. QLED displays can be used in advertising displays, such as digital
bill boards and signage.

Light emitting electro chemical cells

They are simple electroluminescent devices containing emissive

layer having mobile ions sandwiched between two electrodes. The
operating mechanism involves ionic and electronic transport.

Properties

It is a single layer device prepared by solution-based technologies.

It is less depended on work function of electrode i.e. the electrode can be
made up of same type of material.
Low voltage vis sufficient to operate.
They can be printed by using cheaper printing processes

Application

LECs have several promising applications, particularly in the field of

flexible and printable electronics.

 Display Technology: LECs can be used in low-cost, flexible

displays for consumer electronics.

 Wearable Devices: Their ability to be printed and stretched

makes them ideal for wearable lighting applications.

 Signage & Advertising: LECs can be used in illuminated signs

and billboards due to their efficient electroluminescence.

 Medical & Bioelectronics: They have potential applications in

medical imaging and bioelectronic devices.

 Automotive Lighting: LECs can be integrated into vehicle

interiors and exteriors for efficient lighting solutions.