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.) 2+ −  Certain organic semiconductors, such as [Ru(bpy)3] (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.