Module VI

Electroluminescence (EL)

Electroluminescence-

Through electroluminescenceinnovation, the possibility for conversion of electrical energy

directly into light energy came into picture.

In 1950 the practical application of ELcame into existence.

Radiative Recombination is responsible for electroluminescence which is also known as

spontaneous emission. Less electrical current is responsible for emission of photons (light

particles) by phosphorescent substances in radiative recombination. EL applications comprises:

nightlights and automobile dashboard displays.

Other types of light generation include:

Incandescence happensdue to heat.

Chemiluminescence happens due to chemical reaction.

Sonoluminescence happens due to sound.

Mechanoluminescencether happens due to mechanical action.

Working Principle-

Electroluminescence (EL) is an optical as well as an electrical phenomenon where the material releases

light when either electric current flows in it or a strong electric field is applied across it.

For light production, alternating electric field is applied to a thin film of zinc sulphide doped with Cu.

Light is released due tothe charge carriers’ recombination at line-shaped inclusions of p-conductive CuS.

Different colors are produced by doping agent selection and operating frequencylevel.

EL is extensively used in light engineering due to uniform luminous area, flat design, and low power

consumption.

Two mechanisms through which electroluminescence can be producedin crystals-

Pure

Intrinsic and charge injection

The principal differences between the two mechanisms are that in the first, no net current passes through

the phosphor (electroluminescent material) and in the second, luminescence occurs during the passage of

an electric current.

In case of the intrinsic EL, electrons of the atoms are liberated in the conduction band through thermal

activation and the electric field.Severalelectrons of the conduction band are accelerated by the field till

they strike luminescent centres eventually ionizing them,meaning thatelectrons are ejected out from their

atoms.When recombination of electron occurs with an ionized atom at the centre, light is emitted in the

normal way.In order to maintain a continuous emission of light a constant voltage is applied, so as to

overcome the effect damping.

Charge injection can also be one of the causes for EL, for instance when acrystalassociateswith an

electrode to supply a flow of electrons or holes (electron extraction) for which a voltage is employed to a

p–n junction producing a flow of current that means electrons are flowing from the n-type material into

the p-type material. Electrons lose energy when they recombine with centres or positive holes

therebyemitting light.

Origin of Luminescent Light-

Charge carriers bearing opposite sign [holes (+) and electrons (-)] recombine to emit luminescent light. It

is essential to transport electrons to the higher energy level within the crystal lattice for production of

light. Thisdetaches them from lattice of the crystal. The emitted light wavelength is governed by the

defect level energy difference in agreementwith the equation E=h.v (Figure 1).

Figure 1ZnS EL band model. Emission of green light occurs when recombination of electron-hole

pair takes place.

In orderthat enough electrons are supplied to the higher energy band (i.e. conduction band), as much

donor defects as possible areintegrated into the lattice of host. When acceptor defects are introduced, then

capturing of free electrons are confirmed by the light emission. A place where electron recombines with

photon emission is known as recombination center which is also known as an accepter or capture defect.

The EL light emission is not dispersed uniformly over the entire crystal, instead it is restricted to the

lesser areas in the crystal in contrast to the above-described luminescence process.Luminous regions can

be viewed by using microscope thatis statistically dispersedthrough the whole crystal but thenwith

definitedesired directions (Figure 2).

Figure 2Dispersion of EL luminous stripes in ZnS crystals. (a) Parallel arrangement to the c axis in

a crystal surface, (b) Perpendicular arrangement to the c axis in a crystal surface.

The yield of light has been found to be maximum for Cu doped ZnS. Host lattice needs to be

oversaturated with Cu so that EL can occur. As the solubility of Cu2S exceeded, a separation of Cu2S

takes place. Cu2S starts depositing at the stacking faults in ZnS crystal structure. These stacking faulty

sites are too common in ZnS, which are created through common alteration of two types of sphere (cubic

and hexagonal) packing in the c-axis direction; a mechanism called as polytypia.The areas where copper

(I) sulfide deposits, have different conduction than the surrounding ZnS areas. As ZnS crystal is n-type,

and the line-shaped copper (I) sulfide depositions are n-type, pn-junctions are created which cause light

emission. During EL, light is emitted due to the recombination of charge carriers at the pn-junctions

formed at ZnS and copper (I) sulfide interface. When an alternating (ac) electric field is applied across the

electroluminescent crystal, a displacement current flows through it having positive and negative

halfwaves, leading to the emission of light during each halfwave at the interface between ZnS and copper

(I) sulfide. If a 50Hz signal is applied across the electroluminescent crystal, the crystal emits 100 flashes

of light in one second (Figure 3).

Figure 3Diagram showing emission of light at 50Hz.

Design-

The other name for EL cell is light panel as this light panelisplaneas well as it is built in the pattern of

rectangular panel whichis up to a few hundred square centimeters in area. A capacitor (flat in shape) is the

light panel, whoseelectrodes are appliedto a plate made up of glass in sandwich fashion (Figure 4). In

comparison to the traditionallight sources like point and line light sources, EL light source is the

leadinggenuineflatsource of light.

Figure 4 Electroluminescent light source.

The 40 μm layer thickness of powder phosphor is used in order to attain highest brightness on the other

hand, if thin layers are used then field breakdown occurs.

Electroluminescence Measurement System-

Figure 5Components of a typical EL measurement system.

Figure 6 Image of EL sample and holder.

ElectroluminescenceAnalysisSchemeconsist of-

Laser

Spectrometer

Lock-in amplifier

Optical Chopper

Sample Holder(PL/EL)

PMT/CCD

Fiber

Optical optics

Optical Table

Refrigerator (low temperature)

Sample Chamber

related accessories

Light Emitting Diode includesLuminescentResources-

Semiconductor Research: Laser: 325nm,442nm,758nm and so on.

The material used for excitation: Gallium Nitride, Zinc Oxide, Gallium Arsenide

Detectors-

EL has gained popularity due to cheapSibased Charge Coupled Device arrays. These are like the digital

cameras. However, CCD arrays have optimum sensitivity in the near-IR regionand they are also cooledfor

thermal noise reduction. Since these digital cameras are equipped with several detectors which has

numerous mega-pixels resolution of 2048 × 4096 pixels making it capable enough to achieve great

resolution images of whole modules. As Silicon has lower coefficient of absorption therefore detectors

based on Si has a biggest disadvantage that it shows insufficient response after 1000 nm. To overcome

this problem asubstitute detector used is Indium Gallium Arsenide based photodiodes arrays, which show

greater response in the range of 1000 to 1300 nm wavelength,allowingconsiderablyquickeracquisition of

data but with a major demerit ofhigh cost. Most likely resolution lies in a sub-megapixel range with 640 ×

512 pixels.

Figure 7Wavelength v/s quantumefficiencyplot of SiCharged Couple Devicedetectorsas well

asIndium Gallium Arsenide photodiode array.Because of the low resolution of the Si Charge

Couple Devicebecomes the reason for poor response in a desired range which is 1000 -1200 nm.

Image-

Figure 8Mono-crystalline Si wafer EL image. The given off light intensity is proportional to voltage

therefore poorly developed contact areasas well as inactive regions are indicated as blackregions.

Visual examination is unable to detect few problems such as micro-crack and printing problem.

The mainbenefitwhich is noticed above is that the capability of EL image as well aswhole solar cell or

module in a relatively short space of time. The output of light is increasedthrough the local voltage so that

areashaving poor contact appear darker.

Applications -

Generally, ELinstrumentscan beproduced from both organic as well as inorganic ELconstituents. These

electroluminescent dynamicsources are usually semiconductors having large bandwidth which can allow

light to transmit. A typical inorganic thin-film EL (TFEL) is ZnS-Mn exhibiting yellow-orange emission.

Applications of electroluminescence materials include:

EL spectra provide valuable information for analyzing IBSCs, because they show the dynamics of

carrier relaxation between various bands.

LEDs.

Backlights

Liquid crystal displays

Night lamps

Electroluminescent lighting

MaterialsScience(LargeBandwidth Semiconductor Luminescence Properties Testing)

Used in Biology(pigment like chlorophyll,Carotenoids)

Medicinal Biology (Fluorescentdetection of Metastasis）

EnvironmentalObservation

EL constituents’ examples -

By utilization of organic or inorganic EL resources electroluminescent devices are

fabricated.Significantly wide bandwidth based semiconductor active materials are commonly used for

allowing light to exit.The most characteristic inorganic thin-film electroluminescent (TFEL) is Zinc

Sulphide: Manganese with yellow-orange emission.

Examples of the range of EL material include:

Doping of powder ZnS with Cu (generate green light) or with Ag (generate bright blue light).

ZnS thin-film doped with Mn (generating orange-red color).

Natural blue color diamond contains a sign of Bwhichbehaves as a dopant.

Semiconductors containing Group III and Group V elements, such as indium phosphide.

(InP), gallium arsenide (GaAs), and gallium nitride (GaN) (Light-emitting diodes.)

Certain organic semiconductors, such as [Ru(bpy)3]2+(PF6−)2, where bpy is 2,2'-bipyridine.

Merits-

Less watt requirement

It has Long life

External circuit is not required (for limiting current no ballast is required, that can be plugged

straightly insidethe AC power and utilize its intrinsic resistivity to self-regulate power)

This can be fabricated into flat flexible panels, narrow strings, and other small shapes

Waterproof computer monitors can be manufacturedwhich are more durable and light weight as

compared to LCDs or Plasma screens.

Not directional like LCDs when used as a computer monitor but appearsnicefromevery angle.

Displays of electroluminescent devices can bear a remarkabletemperature range i.e. -60° C to 95°

C thatmonitors of LCD cannot do.

De-merits-

Because of less lumen O/P of phosphors, it is not applied for common lighting of

enormousregions.

Poor lux intensityfor eachpower rating, but then the lamp is not utilized for greatlux intensity

output in any case.

Lumen output is decreasedwith time, even thoughinnovativeequipmentis better in comparison

to older phosphors at this point.

Work is being done upon the durability of electroluminescent sheets (Flexible flat in nature)

as wear out when they are flexed.

Around 60-600 volts of electricity is used by lamps.

Typicallya converter is required by electroluminescent device when utilizedby DC sources

for example on watches (to generateAC power of higher frequency which is audible range).

Review your learning:

MCQs

1) Electro-luminescence occurs in:

a) Electrical conductors

b) Electrical insulators

c) p-n junctions

d) all

2) Fluorescence occurs within:

a) 10-5 s.

b) 10-5 ms.

c) 10-5 μs.

d) 10-5ns.

3) Luminescence is because of

a) Photons emitted while excited electrons drops down

b) Knocking out of electrons by photons

c) Photons stimulated by photons

d) All

4) Visible light’s wavelength range:

a) 0.39 – 0.77 mm

b) 0.39 – 0.77 μm

c) 0.39 – 0.77 nm

d) 0.39 – 0.77 cm

5) Juan takes off his sweater and sees sparks of light. He is observing which of the

following?

a) Electroluminescence

b) Bioluminescence

c) Phosphorescence

d) Chemiluminescence

Long type questions:

1) Give the working principle of Electroluminescence?

2) Give few applications of EL?

References

1) https://www.britannica.com/science/electroluminescence

2) http://www.tech-faq.com/what-is-electroluminescence.html

3) https://en.wikipedia.org/wiki/Electroluminescence

4) http://www.edisontechcenter.org/electroluminescent.html

5) http://www.zolix.com.cn/en/prodcon_466_471_682.html

6) http://www.pveducation.org/pvcdrom/characterisation/electroluminescence

7) Degenhardt, Heinz. "Principles and applications of electroluminescence." Naturwissenschaften

63.12 (1976): 544-549.