1. Introduction to organic light emitting devices (OLEDs)OLED - Organic Light emitting DiodeAn OLED is any light emitting diode (LED) which emissive electroluminescentlayer is composed of a film of organic compounds. It is made up of organicmaterial phenylene vinylene. Size of single OLED in a OLED Display is 5.4micro meter . OLEDs constitute a new and exciting emissive display technology. In general,the basic OLED structure consists of a stack of fluorescent organic layerssandwiched between a transparent conducting anode and metallic cathode.When an appropriate bias is applied to the device, holes are injected from theanode and electrons from the cathode; some of the recombination eventsbetween the holes and electrons result in electroluminescence (EL). It is Much faster response time .It is Consume significantly less energy . It is Able to display "True Black" picture .It has Wider viewing angle . It hasThinner display. It hasBetter contrast ratio. It is Safer for the environment. It Has potential to be mass produced inexpensively. OLEDs refresh almost 1,000 times faster then LCDs.
1.1 History of organic electroluminescence The first EL from a an organic molecule, anthracene, was reported by Popeand coworkers in 1963 . They reported EL from a thick anthracene crystal(10µm-5mm), when a bias of several hundred volts was applied across it. Theachievement did not stimulate much interest as the applied bias was very high.achieved bright blue EL from vacuum-deposited 0.6 m thick anthracene crystalfilms with an applied bias of less than 100V.The breakthrough was achieved by Tang and VanSlyke in 1987 , who made abilayer structure by thermally evaporating the small molecular weight organicmaterials, N, N-diphenyl-N, N-bis(3-methylphenyl) 1, 1-biphenyl-4, 4 diamine(TPD) and tris(8-hydroxyquinoline) aluminum (Alq3) to achieve a total thicknessof ~100 nm. They achieved a very bright green emitting OLED with a brightnesshigher than 1000 cd/m2 and an external quantum efficiency of ~1% when a lowbias of 10V was applied across the structure . Following this achievementAdachi et al  succeeded in fabricating stable multilayer devices by insertinghole and electron transport layers between the two electrodes. In 1989, Tang etal  developed a laser-dye doped Alq3 multilayer structure, in which thefluorescent efficiency was improved and the emission color varied from theoriginal green to the dopant emission color. Following the success of fabricating small molecular OLEDs, Burroughs et al discovered the first polymer LED (PLEDs) by spin coating a precursorpolymer of the luminescent poly-(para-phenlene vinylene) (PPV) onto a indiumtin oxide (ITO) coated glass. Compared to small molecular devices, polymer lightemitting devices (PLEDs) have several potential advantages, e.g. , fabrication byspin coating [9,10] or inkjet printing  from solutions and subsequent thermaltreatment.
Fluorescent emission of singlet excitons are the main mechanism of OLEDlight emission. As the probability of forming spin singlet states and spin tripletstates are 25% and 75% respectively, the ideal maximum fluorescent yield is,therefore, limited to 25% by spin statistics. To overcome this theoretical limit M.A. Baldo et al  fabricated and demonstrated phosphorescent OLEDs, bydoping phosphorescent molecules, where the EL is due to triplet emission, into afluorescent host layer.1.2 Advantages and disadvantages of OLEDs OLEDs are already commercialized and they are making ways to the displaymarkets. Currently OLEDs are used in low information displays with limited sizesuch as mobile phones, PDAs, MP3 players, digital cameras and some laptopcameras. The driving force behind this success is some advantages that theOLEDs enjoy.Advantages:Self-luminous- The efficiency of OLEDs is better than that of other displaytechnologies without the use of backlight, diffusers, and polarizers.Low cost and easy fabrication- Roll-to-roll manufacturing process, such as,inkjet printing and screen printing, are possible for polymer OLEDs.Color selectivity- There are abundant organic materials to produce blue to redlight.Lightweight, compact and thin devices-OLEDs are generally very thin,measuring only~100nmFlexibility- OLEDs can be easily fabricated on plastic substrates pavingthe way for flexible electronics.
High brightness and high resolution-OLEDs are very bright at low operatingvoltage (White OLEDs can be as bright as 150,000 cd/m 2)Wide viewing angle- OLED emission is lambartian and so the viewing angle isas high as 160 degreesFast response- OLEDs EL decay time is < 1us.Disadvantages: - Highly susceptible to degradation by oxygen and watermolecules. -Organic materials are very sensitive to oxygen and watermolecules which can degrade the device very fast . So the maindisadvantage of an OLED is the lifetime. With proper encapsulation, lifetimesexceeding 60,000 hours have been demonstrated. In our laboratory itself, wehave been able to increase the shelf life of green Alq 3 based OLEDs, from fewdays to almost a year. - Low glass transition temperature Tg for small moleculardevices (>70oC). So the operating temperature cannot exceed the glasstransition temperature. - Low mobility due to amorphous nature of the organicmolecules.
1.3 Basic OLED structure and operation The basic current structure of OLED has one low work function conductingtransparent anode, one or more organic layers and a cathode. Small molecularorganic materials are normally thermally evaporated and polymers are spincoated on a transparent ITO coated glass substrate to a thickness of about 100nm in case of small molecular OLEDs . The basic device structure and theequilibrium energy levels are shown in Figure 1.1. When a forward bias is appliedto this structure, holes (h+) are injected from the anode and the electrons (e -) areinjected from the cathode. These injected carriers recombine, form excitons andsome of them decay radiatively to give the EL. Thus, for injection EL thefundamental physical processes include carrier injection, transport,recombination and radiative exciton decay[3, 4]. Normal operating voltage isabout 2-20 V, corresponding to average electric field of 0.1-2 MV/cm, which isvery high compared to the typical fields ~10 kV/cm of inorganic semiconductordevices. The resistivity ρ of the devices ranges over eight order of magnitude,with very high values of 105-1013 Ω.cm in forward bias. In reverse bias, ρ is alsovery high (109 Ω.cm) [10,11]. The ITO, a wide band gap (Eg = 3.5-4.3 eV) semiconductor, is composedof indium oxide (In2O3) and a small amount of tin oxide (SnO2) (~5 wt%). MostOLEDs use ITO as the anode due to its relatively high work function and hightransparency (90%) to visible light . The conductance and transparency ofITO are mostly dependent on the film thickness and composition ratio of twocomponents. The resistivity of a 200 nm thick ITO is about 10 -3 Ω.cm withmobility μ ~ 10 cm2/Vs. With increased ITO thickness the conductance increases,but the transparency decreases. Another very important parameter is its workfunction (Φ0) or Fermi energy (Ef) relative to the organic materials . Becausethe highest occupied molecular orbital level (HOMO) energies of organicmaterials are typically EHOMO = 5-6 eV, a high Φ0 is needed for the anode toefficiently inject holes to the organic layers.
Fig.1.1. (a) Double layer device structure (b) equilibrium state energy level at V = 0 (c) energy level at a forward bias V = Vapp
For the cathode, low work function materials such as Ca(Φ0 ~3 eV), Mg (Φ0 ~3.7 eV), Al (Φ0 ~4.3 eV) are used to minimize the energybarrier for e- injection from Ef of the cathode to the lowest unoccupied molecularorbital (LUMO) level of organic materials. The problem of many low work function metals isextreme reactivity to oxygen and water, hence Ca and Mg should be protected byan additional layer, such as Al. Another way to minimize the barrier from electroninjection is to insert a very thin (~1 nm) insulating layer of LiF, CsF or AlOXbetween the top organic layer and the Al cathode. These buffer layers generate adipole layer and thus reduce the barrier for electron injection to the organiclayers. The hole and electron transport layers (HTL and ETL,respectively) are the layers favorable for hole and electrons respectively. Whenthe applied bias Vapp is less then the built in voltage Vbi, the injected current isnegligible and most of the current is caused by free carriers in the organic layersor leakage current. At high forward applied field the injected holes andelectrons hop from site to site through the organic layers. Some of the carriersmay accumulate in a specific area, called the charge accumulation zone, usuallyat the organic-organic interface of the multi-layer structure. If the density of electrons and holes are sufficientlyhigh, then the distance between them becomes sufficiently low for recombinationto radiative singlet excitons (SEs).
1.4 Carrier injectionIn organic materials, disorder, low bandwidth, electron phonon interactions andtemperature all work together to localize charge carriers. Thus, the primaryinjection event consists of a transition from an extended band-like state in themetal electrodes into a localized molecular polaronic state in the organicmaterial. The highly insulating nature
of most organic solids coupled with low charge carrier mobility resulting fromweak intermolecular interaction and disorder, make the standard thesemiconductor techniques inapplicable to study their electronic properties .Despite these difficulties, J. Kalinowski performed a thorough theoretical analysisof the mechanism of carrier injection. The possible mechanisms developed byvarious researchers over the time are briefly discussed in this section.1.4.1 Image force lowering  When carriers are injected from a metal electrode into the organic layers (Fig1.2), they encounter the injection barrier qΦm, which is the energy differencebetween the Fermi level Ef of the metal and the LUMO level ELUMO for electroninjection. Similarly, holes encounter a barrier, which is the difference between Efand EHOMO. Following the injection, many electrons remain on the surface ofthe organic layer at distance +x from the metal-organic interface. These electronsinduce equivalent hole charges in the metal layer at –x. The hole charges arereferred to as the image charges. As a result of these image charges, the newpotential of the metal-organic interface system becomes 2 q 2 −Ψ(x) = φm − χ − q qFx = φ − B qFx 16πεx 16πεx(1)ε = εr ε0We get the effective potential barrier height as:φB = (φm − χ ) − q3F / 4πε(2) χwhere φm is metal work function, is electron affinity, F is electric fieldand q iselectron charge.
X qΔφ -q2/16πεxqφm qφB Fx (-q2/16πεx)-qFxFig. 1.2. Image force of the barriers for electron injection at the metal-organicinterface. The energy barrier at the interface is lowered by an amount qΔφ fromqφm to qφB.Therefore, the barrier lowering is q3F∆φB = 4πε(3)The above treatment holds for neutral contact between metal and widegap intrinsicsemiconductors, which is the case for organic semiconductors.
The current voltage characteristics of OLED depend critically on the electronicstates at the metal-organic interface. Charge injection at low applied bias isprimarily due to thermal emission of charge carriers over the interface potentialbarrier when the barrier is not too high for thermal injection. Emtage and O’Dwyer solved drift-diffusion equation for the injection from metal into wide-gapintrinsic semiconductor, in which the depletion width is infinite without injection.Emtage and O’Dwyer derived that:(a) in the low field limit, E<< 4πεk2T2/q3The thermionic injection current density (J) over the barrier is given by q φBJ = N0qµE exp(− kT )(4)and(b) in the high field limit kT q φBJ = N0µ ( q) 1/2 (16πεqE3 )1 / 4 exp(− kT ) exp( f )1 / 2(5)Although not explicitly shown, the backflow current is present. The origin of thebackflow in wide bandgap organic semiconductors is disorder. The existence of disorder in organic semiconductors adds an obstacle to theinjected carriers. Due to disorder, a distribution of site energies is created, andcarriers injected occupy the molecular sites in contact with electrodes and also atthe low-energy end of the distribution. To move further into the organic materials,the carriers must overcome random energy barriers in addition to the imagepotential. For this reason, most injected carriers will backflow into the electrode atlow applied field strength. When the electric
field is increased, the efficiency of injection increment will be more significantthan in the case when only image force is considered. This thermal injectionprocess has been proved both by Monte Carlo simulation  and experiment.1.4.3 Field emission  Field emission is the process whereby carriers tunnel through a barrier in thepresence of a high electric field. When the barrier is triangular, the tunneling iscalled Fowler-Nordheim(FN) tunneling. When the forward field across the 100nmthin OLED is increased, the triangular energy barrier becomes shallower (Fig.1.1c) . It is typically ~2 nm wide at an applied field of 2 MV/cm, in which casethe width is sufficiently thin for tunneling. For a triangular barrier, the FN currentdensity is given byJFN = AF 2e- F 0 / F ,(6)where parameter A and F0 are related to the potential barrier and are given by 3 * 3 mq 2m φBA = 8πhm*φB , F0 = 8π 3qhThe barrier φB itself is a function of field F through the image-force loweringeffect. Typically, for low fields (<2 MV/cm), the thermionic current dominates; forhigh fields (>2 MV/cm), the tunneling current prevails .
1.5 Carrier transport in OLEDs Unlike inorganic semiconductors, the transport properties in OLEDs aredetermined by intersite hopping of charge carriers between localized states .If two molecules are
separated by a potential barrier, a carrier on one can move to the other either bytunneling through the barrier or by moving over the barrier via an activated state.The latter process is called hopping . The actual transit rate from one site toanother depends on their energy difference and on the distance between them.The carriers may hop to a site with a higher energy only upon absorbing aphonon of appropriate energy . This decreases the probability of transit to alocalized state with higher energy. The energetically allowed hops to a distantsite are limited also by the localized length . The energy states involved in thehopping transport of holes and electrons form narrow bands around the HOMOand LUMO levels. The width of these bands is determined by the intermolecularinteractions and by the level of disorder .1.5.1 Field dependent mobility In most organic semiconductors, the carrier mobility is a strong function ofapplied electric field, unlike inorganic semiconductors where mobility is, ingeneral, independent of the applied field. Over a reasonable range of fields, thetime-of-flight (TOF) measurement gives the mobility in organic semiconductorsas −∆ 1 1 0µ = µ0 exp( kT )exp[βF1 / 2 ( kT − kT0 )](7)or simply, ln S * F1/ 2 ,where S is a constant and F is electric field. The dependence of ln on F1/2 is of Poole-Frenkel type. The Poole-Frenkeleffect describes the electric-field assisted detrapping phenomenon. When a fieldis applied, the trap-potential in which a carrier is trapped will be deformed into anasymmetric shape. The situation is very similar to Schottky barrier lowering dueto image force with the