What we plan to learn1. What is Electroluminescence?2. Basic principles and working .3. PPV(Polyparaphenylene vinylene)4. Applications5. Specific Example(OLED)
Electroluminescence• Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field.• Textbook defn. “ The property in which a material produces bright light of different colours when stimulated electronically is known as electroluminesence. The material which shows electroluminescence,is called as electroluminescent material. ”• Examples of electroluminescent materials: 1.Electroluminescent devices are fabricated using either organic or inorganic electroluminescent materials. The active materials are generally semiconductors of wide enough bandwidth to allow exit of the light. 2.The most typical inorganic thin-film EL (TFEL) is ZnS:Mn with yellow-orange emission. Examples of the range of EL material include: 3.Powdered zinc sulfide doped with copper (producing greenish light) or silver (producing bright blue light) 4.Thin-film zinc sulfide doped with manganese (producing orange-red color) 5.Naturally blue diamond, which includes a trace of boron that acts as a dopant. 6.Semiconductors containing Group III and Group V elements, such as indium phosphide (InP), gallium arsenide (GaAs), and gallium nitride (GaN). 7.Certain organic semiconductors, such as [Ru(bpy)3]2+(PF6-)2, where bpy is 2,2-bipyridine
PPV(Polyparaphenylene vinylene) • Poly(p-phenylene vinylene) (PPV, or polyph enylene vinylene) is a conducting polymer. • PPV is the only polymer of this type that has so far been successfully processed into a highly ordered crystalline poly(1,4-phenylene-1,2- thin film. ethenediyl) • PPV is easily synthesized in good purity and high molecular weight.
• Although insoluble in water, itsprecursors(a precursor is a compound thatparticipates in the chemical reaction that producesanother compound.) can be manipulated in aqueoussolution.•The small optical band gap and its bright yellowfluorescence makes PPV a candidate in many electronicapplications such as light-emitting diodes (LED) andphotovoltaic devices.• Moreover, PPV can be easily doped to form electricallyconductive materials.•Its physical and electronic properties can be altered bythe inclusion of functional side groups.
Manufacture of PPV • PPV can be synthesized by Wittig-type couplings between the bis(ylide) derived from an aromatic bisphosphonium salt and dialdehyde, especially 1,4-benzenedialdehyde. • Textbook method employs the use of a precursor polymer poly -n-octyl sulphinyl paraphenylene ethylene by heating it in vacuum. Elimination of n-octylthiol introduces the double bonds in the chain. reaction as follows :
Properties and Structure• PPV is a diamagnetic material and has a very low intrinsic electrical conductivity, on the order of 10-13 S/cm.• The electrical conductivity increases upon doping with iodine, ferric chloride, alkali metals, or acids. However, the stability of these doped materials is relatively low. In general, unaligned, unsubstituted PPV presents only moderate conductivity with doping.• Alkoxy-substituted PPVs are generally easier to oxidize than the parent PPV and hence have much higher conductivities. Longer side chains lower the conductivity and hinder interchain hopping of charge carriers.
Applications Of PPV• Due to its stability, processability, and electrical and optical properties, PPV has been considered for a wide variety of applications. In 1989 the first polymer-based light emitting diode (LED) was discovered using PPV as the emissive layer.• Polymers are speculated to have advantages over molecular materials in LEDs, such as ease of processing, reduced tendency for crystallization, and greater thermal and mechanical stability. Ever since the first breakthrough in 1989, a large number of PPV derivatives have been synthesized and used for LED applications.
•Although solid-state lasing has yet to be demonstrated inan organic LED, poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) has been proven to be apromising laser dye due to its high fluorescence efficiencyin solution.•Polyphenylene vinylene is capableof electroluminescence, leading to applications inpolymer-based organic light emitting diodes. PPVwas used as the emissive layer in the first polymerlight-emitting diodes.Devices based on PPV emityellow-green light, and derivatives of PPV obtainedby substitution are often used when light of adifferent color is required.
• . In presence of even a small amount of oxygen, singlet oxygen is formed during operation, by energy transfer from the excited polymer molecules to oxygen molecules. These oxygen radicals then attack the structure of the polymer, leading to its degradation. Special precautions therefore have to be kept during manufacturing of PPV in order to prevent oxygen contamination.• PPV is also used as an electron-donating material in organic solar cells. Although PPV- based devices suffer from poor absorption and photodegradation, PPV and PPV derivatives (especially MEH-PPV and MDMO-PPV) find frequent application in research cells.
OLED (organic light-emitting diode)• An OLED (organic light-emitting diode) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film oforganic compound which emits light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes. Generally, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as mobile phones, handheld games consoles and PDAs.• An OLED display works without a backlight. Thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions such as a dark room an OLED screen can achieve a higher contrast ratio than an LCD, whether the LCD uses cold cathode fluorescent lamps or LED backlight. Due to its low thermal conductivity, an OLED typically emits less light per area .
Principle and Working(OLED) • A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a substrate. The organic molecules are electrically conductive as a result of delocalization of pi electrons caused by conjugation over all or part of the molecule. • During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole.Schematic of a bilayer OLED: 1. Cathode (−), 2. Emissive This happens closer to the emissive layer, because in(Polymer)Layer, 3. Emission of radiation, 4. Conductive organic semiconductors holes are generallyLayer, 5. Anode (+) more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO.
Electroluminiscent polymers are thefuture of the semiconductor anddisplay unit industry