White organic LEDs and their recent advancements

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INSTITUTE OF PHYSICS PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY

Semicond. Sci. Technol. 21 (2006) R35–R47 doi:10.1088/0268-1242/21/7/R01

TOPICAL REVIEW

White organic LEDs and their recentadvancementsAparna Misra, Pankaj Kumar, M N Kamalasananand Subhas Chandra

Polymeric & Soft Material Section, National Physical Laboratory, Dr K S Krishnan Road,New Delhi 110 012, India

E-mail: aparnamisra@mail.nplindia.ernet.in

Received 5 January 2006Published 25 April 2006Online at stacks.iop.org/SST/21/R35

AbstractWhite organic light emitting devices (WOLEDs) are being considered assubstitutes for conventional white light sources. They are efficientsolid-state lighting sources and their power efficiencies have surpassed thoseof the incandescent light sources, especially due to recent improvement indevice architectures, molecular engineering in synthesis of new materialsand the incorporation of electrophosphorescent emitters. This paper reviewsthe various approaches to achieve white light emission from organic lightemitting diodes (OLEDs), their advantages, disadvantages and recentprogress. The device architecture and problems related to various devicedesigns have been discussed.

1. Introduction

A tremendous amount of electricity is consumed every year allover the world. In terms of total primary energy consumption,lighting accounts for about 20% of all the electricity produced.Fluorescent tubes and incandescent lamps are the frequentlyused traditional light sources and account for about 40% ofthe total electrical energy consumed. Incandescent bulbsturn about 90% of the energy into heat, while fluorescentdo better by converting 70% of the energy they use intolight. Incandescent lamps and fluorescent tubes have typicalluminous efficiencies of 13–20 lm W−1 and 90 lm W−1,respectively [1]. So in turn to save the world one shouldstart getting rid of the light bulbs. Researchers have spentdecades to create novel semiconductors-based light emittingdiodes (LEDs) that do even better. Red, green, blue LEDs andother colours made from inorganic materials are already on themarket and their use is widespread in traffic lights, car taillightsand other small applications. Inorganic white LEDs are alsoin the market but they are too costly for general lighting use.Now a new competitor for light sources is coming onto themarket that is based on organic semiconductors.

In the past ten years it has been seen that organiclight emitting devices (OLEDs) have evolved into a trulypowerful display technology capable of rivalling liquid crystal

displays (LCDs). Since the first report on efficient greenelectroluminescence (EL) from tris (8-hydroxyquinoline)aluminium (Alq3) [2] in 1987 and from poly(p-phenylenevinylene) (PPV) [3] in 1990, the OLEDs have become the mostattractive display technology. They have ease of production,fast response time, high luminance, wide viewing angle, lowoperating voltage, light emission spanning the entire visiblespectrum and a promising application in large area flexible, fullcolour flat panel electronic displays. OLED displays are light,durable and efficient and can be built on flexible substratessuch as plastics and paper-thin surfaces, creating displays thatcan be bent or rolled. OLEDs are much more rugged andideal for portable applications compared to LCDs. UnlikeLCDs, OLEDs are self-emitting and require neither backlight,nor chemical shutters that must open and close, making themthinner and more compact.

Organic light emitting diodes are thin-film multilayerdevices in which active charge transport and light emittingmaterials are sandwiched between two thin film electrodes,of which at least one must be transparent to light. Generallyhigh work function (∼4.8 eV), low sheet resistant (∼20 �/��)and optically transparent indium tin oxide (ITO) is used as ananode, while the cathode is a low work function metal suchas Ca, Mg, Al or their alloys Mg:Ag, Li:Al. An organic layerwith good electron transport and hole blocking properties is

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Topical Review

Figure 1. Yearly progress in light emitting diode efficiency.

typically used between the cathode and the emissive layer.Similarly a hole transporting layer (HTL) and an electronblocking layer (EBL) are typically used between the anodeand the emissive layer. On the application of an externalbias electrons are injected from the cathode and holes areinjected from the anode. Under the influence of an externalfield the electrons and holes migrate in opposite directionsand are driven to the emissive zone where they form excitons,which decay radiatively to give out light. The nature andkinetics of migration of the excitons have been extensivelydiscussed in the literature [4].

White OLED (WOLED) technology is drawing mostattention due to its application for general solid-state lightingand flat panel display backlighting of LCDs. In the fabricationof full colour display all three primary colours have equalimportance but white light emission has drawn particularattention because any desired colour range can be achievedby filtering of white light [5, 6]. The first white OLED wasproduced by Kido and his colleagues in 1993. This devicecontained red, green and blue light emitting compounds thattogether produce white light. But there were some problemswith these devices such as their efficiency was less than1 lm W−1, they required large driving voltage and they burnedout quickly. But now the efficiency of these devices hasincreased very fast. The yearly progress in the efficiencies ofconventional LEDs, nitride LEDs and white OLEDs is shownin figure 1.

2. Characterization of white light sources

2.1. Colour quality

There are two critical parameters which define the colourquality of a white light source, the colour rendering index(CRI) and Commission Internationale d’Eclairage (CIE)chromaticity. For general illumination a light source shouldhave good colour rendering performance equivalent to thatof a black body source at 3000–6000 K, high-energyefficiency and Commission Internationale d’Eclairage (CIE-1931) chromaticity coordinates (x, y) close to the equal energywhite (EEW) (0.33, 0.33). CRI is a numerical measurement ofhow true colours look when illuminated with the light source.It is measured in 0–100 scales, with 100 representing truecolour reproduction. The colour of light is expressed by the

Figure 2. CIE (x, y) chromaticity diagram.

(This figure is in colour only in the electronic version)

Table 1. Chromaticity coordinates (CIE), colour correlatedtemperature (CCT) and colour rendering indices (CRI) for commonwhite light sources.

Chromaticitycoordinates

Light sources x y CCT (K) CRI

High pressure sodium lamp 0.519 0.417 2100 24Xenon lamp 0.324 0.324 5920 94Tungsten halogen lamp 0.448 0.407 2856 100Daylight 0.313 0.329 6500 90Fluorescent cool white lamp 0.375 0.367 4080 89Fluorescent warm white lamp 0.440 0.403 2940 72Incandescent bulb 0.448 0.408 2854 100

CIE colorimetry system. Any colour can be expressed bythe chromaticity coordinates x and y on the CIE chromaticitydiagram (figure 2). The boundaries of this horseshoe-shapeddiagram are the plots of monochromatic light, called spectrumloci, and all the colours in the visible spectrum fall within oron the boundary of this diagram. The arc near the centre ofthe diagram is called the Planckian locus, which is the plot ofthe coordinates of black body radiation at the temperaturesfrom 1000 K to 20 000 K, described as colour-correlatedtemperatures (CCT). The colours of most of the traditionallight sources fall in the region between 2850 and 6500 K ofblack body. A comparative study of various common whitelight sources is given in table 1 [7].

2.2. Device efficiency

White emission from OLEDs can be achieved in both smallmolecules and the polymeric system. Both types of OLEDshave achieved similar operational lifetimes, therefore otherdevice characteristics such as efficiency, simplicity in thefabrication process and the ability to create good colour maydecide which one will rule the industry. The efficiencyof OLEDs is characterized by quantum efficiency, powerefficiency and luminous efficiency. Over the past several years,

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the power (ηp) and external quantum (ηext) efficiencies of whiteOLEDs have been steadily improving.

2.2.1. Quantum efficiency. The quantum efficiency of adevice can be characterized into internal and external quantumefficiencies.

2.2.1.1. Internal quantum efficiency (IQE). This is the totalnumber of photons generated inside the device per electron–hole pair injected into the device. It is represented by ηint.

For organic light emitting diodes the internal quantumefficiency in the case of fluorescent materials is given by [1]

ηint = γ ηs�f, (1)

where γ is the fraction of injected charges that produceexcitons and is called the charge balance factor, ηs is thefraction of singlet excitons called singlet exciton efficiencyand �f is the fraction of energy released from material as lightand called the quantum efficiency of fluorescence.

2.2.1.2. External quantum efficiency (EQE). This is the totalnumber of photons emitted from the device per electron–holepair injected into the device. It is represented by ηext.

The external quantum efficiency is related to the internalquantum efficiency and is given by [1]

ηext = Re ηint, (2)

where Re is the extraction or outcoupling efficiencyrepresenting the number of photons emitted from the deviceper number of photons generated in the device.

2.2.2. Power efficiency. The luminous efficacy or powerefficiency is the lumen output per input electrical power of thedevice. It is measured in lumen per watt (lm W−1) or candelaper ampere (cd A−1). It is represented by ηp.

In order to compete with the fluorescent lighting market,the efficiency of OLED sources should be 120 lm W−1. Tomeet the above requirement the OLED sources must havean electrical to optical power conversion efficiency of 34%.The aggressive projections are that by 2005 the efficiencyof white light OLEDs will exceed 30 lm W−1 and by 2008it will exceed 60 lm W−1 while by 2015 it will exceed100 lm W−1 with desirable lifetime and brightness and theywill start to replace indoor and outdoor lights. The firstwhite organic light emitting diodes for general lighting areexpected to hit the market in 2007. Due to spin statisticsthe efficiencies of OLEDs are limited, as only the singletsare responsible for light emission in EL in undoped devices.The recent developments in harvesting of triplet states, usingphosphorescent materials, led to an increase in the efficiencyand selectivity of colours. Electrophosphorescence achievedby doping an organometallic phosphor into a host has beensuccessfully used for generating the primary colours necessaryfor display applications [8–10]. Due to extensive work,the power efficiency of white organic light emitting devices(WOLEDs) has continuously increased over the past decadeand it has attained the level required for WOLEDs acceptanceinto the lighting market. Universal Display Corporation is aworld leader in developing and commercializing innovativeOLED technologies and materials for use in the electronic

flat panel display and other markets. Universal Displayis working with a network of world-class organizationsincluding Princeton University, the University of SouthernCalifornia, DuPont Displays, Samsung SDI Co., Seiko EpsonCorporation, Sony Corporation, Tohoku Pioneer Corporationand Toyota Industries Corporation. NOVALED GmbH,Dresden Germany, is another emerging company in the fieldof organic displays. NOVALED works in close cooperationwith Technical University Dresden and Fraunhofer DresdenInstitute IPMS. According to a press release on 10 February2005 from Dresden, Germany, NOVALED has set a newworld record for OLEDs by developing a green emittingOLED with an efficiency of 110 lm W−1. In August 2005at The International Society for Optical Engineering (SPIE)Symposia and Exhibition held in San Diego, CA, Muranoet al from NOVALED demonstrated efficient white OLEDsbased on an intentional doping of the charge carrier transportlayers and the usage of different state of the art emissionprinciples. They demonstrated white PIN-OLEDs based onphosphorescent, fluorescent emitters and stacked OLEDs.This intentional doping of the transport layer led to a very highpower efficiency of well above 20 lm W−1 at 1000 cd m−2 [11].The CRI properties of emitted light are very high, between 85and 95. In the same symposia Universal Display Corporation(UDC) announced the demonstration of a white OLED lightingpanel with a high power efficiency of 30 lm W−1 using thecompany’s phosphorescent OLED technology. This efficiencywas achieved at a colour temperature of 4000 K, which iscomparable to the colour temperature and power efficiency ofa cool fluorescent lamp. The colour-rendering index was >80across the measured colour temperatures because of the broadspectral output of the combined colours. D’Andrade et al[12] reported a quite large power efficiency of 42 lmW−1 for a white OLED that exceeds that of incandescentlamps. Therefore WOLEDs have great potential for energysaving and the replacement of traditional incandescent lightsources.

Generally based on spin statistics fluorescent organicmaterials exhibit 25% singlet and 75% triplet states in EL and100% singlet states in PL [8, 13]. In fluorescent materialstriplet energy states have a low emission quantum yieldand thus do not contribute to electroluminescence. Thismeans the quantum efficiency for EL can only be about 25%of the PL efficiency. But some organometallic complexes(phosphors) have a strong triplet emission quantum yield andprovide the possibility of a high efficiency EL device by usingthese materials. A research group from Princeton Universitydemonstrated the efficiency limitation breakthrough in OLEDsby energy transfer from fluorescent host to a phosphorescentguest material [8]. The phosphorescent dopants aredoped in host materials with a wide energy gap. Inelectrophosphorescence the energy from both the singlet andtriplet states of the fluorescent host is transferred to the tripletstate of the phosphorescent guest molecule or the charges aredirectly trapped to the phosphor triplet. This harvesting of bothsinglet and triplet states can result in 100% internal quantumefficiency [14]. But exciton–exciton quenching [15], polaron–exciton quenching [16] and exciton dissociation [17] mayreduce the internal quantum efficiency to much lower values.One of the best known phosphorescent dopants is iridium (III)

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bis [4, 6-di-fluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic)and a reasonably good external quantum efficiency of ∼10%has been reported [18]. An increment in the efficiency oflighting by a small amount has the potential to generatetremendous savings in both cost and energy. The developmentof WOLED panels is an important step in this direction.

Over 80% of light is lost to the internal absorptionand internal reflections due to high refractive indices of thematerials in the device. The internally reflected light is thenguided within the device. Some of the guided light escapes byscattering, the rest is absorbed or coupled out at the glass/ITOedges. One should include the light emitted through the surfaceand edge of the glass/ITO substrate when calculating the totalexternal efficiency. Detailed optical modelling [19] predictedthat the fraction of the light emitted in the forward directionis reduced by a factor of (4/3)n2, where n is the index ofrefraction of the emitter layer. Through a series of experimentsusing an integrating sphere, Cao et al [20] demonstratedthat the measured reduction factor is approximately 2–2.5,less than the theoretical value 2n2 ∼ 6. Forrest and co-workers found that the total external efficiency is larger bya factor of 1.7–2.3 than observed in the forward viewingdirection [12]. Thus poor light extraction is the most importantfactor which limits the external quantum efficiency of devicesand hence better outcoupling methods are to be developedto get higher efficiencies. Different outcoupling schemeshave been employed in WOLED [21–23]. Unless thereis a significant improvement in the outcoupling efficiency,fluorescent WOLEDs do not offer a significant advantage overthe expensive and inefficient incandescent light sources.

2.3. Lifetime

Another important aspect for an ideal illumination source is itslifetime. The lifetime may be defined as the average number ofhours of operation in which the initial light intensity drops to50%. The average lifetime of incandescent lamps is only 750–2500 h, while that of the fluorescent lamps is about 20 000 h.If an OLED display lasts for 5000 h and you use that in amobile phone (cell phone) that is on for 2 h per day, youwill get 7 years use from the display, which is several yearsmore than you will use the phone. Lifetime >20 000 h forseveral coloured OLEDs is repeatedly reported but today thetypical lifetime of white OLEDs is less than 5000 h and effortsare still required to make white OLEDs reach the fluorescenttubes for efficiency and lifetime. The OLED communitygenerally agrees that the first lifetime target for WOLEDsshould be 20 000 h with a maximum luminance loss of 20% at850 cd m−2.

Companies, governments and universities are pouringmoney into flexible display research and although flexibledisplays receive plenty of interest, it seems unlikely they willbe widely available within the next few years. A number ofcompanies are producing monochromatic OLEDs for mobilephones, car stereos and handheld TVs. The technology willnot only improve existing methods of illumination but willalso create entirely new lighting product possibilities. Thispaper is aimed at giving a review of the various techniquesemployed to produce white light from OLEDs during the pastdecade and their recent developments. The disadvantages andfacts to improve their performance have also been mentioned.

3. Approaches employed for white emission fromOLEDs

As discussed earlier, white light for illumination purposeshould have good colour rendering index (>75) and goodposition close to (0.33, 0.33) on the CIE-1931 diagram.Methods to generate white light from OLEDs can be broadlyclassified into two approaches.

(a) Wavelength conversion. Some blue or ultraviolet lightfrom one OLED is used to excite several phosphors,each of which emits a different colour and these differentcolours are mixed to make a white light with the broadestand richest wavelength spectrum. This technique is calleddown conversion by phosphors.

(b) Colour mixing. This method uses multiple emitters in asingle device and mixing of different lights produceswhite light. White light can be obtained by mixingtwo complementary colours (blue and orange) or threeprimary colours (red, green and blue). The typicaltechniques used for the production of white light bycolour mixing are (a) a multilayer structure consisting ofred, green and blue emissive layers, (b) Forster/Dexterenergy transfer, (c) exciplex/excimer charge transfer,(d) microcavity structure, (e) white emission by thehorizontally/vertically stacked pixels, (f) blending/

doping of different emitters into a single layer. Inthe colour mixing technique, no phosphors are used,and therefore the losses associated with the wavelengthconversion do not occur and this approach has the potentialfor the highest efficiency. These techniques are discussedin detail in this review.

Getting good quality white light does not require abreakthrough but achieving stable white colour is still an issueof research and development.

3.1. Colour mixing

3.1.1. Multilayer device structure. One of the approachesto achieve white light is a multilayer structure where simul-taneous emission of light from two or more separate emittinglayers with different emission colours results in white light.This technique is based on the consecutive deposition orco-evaporation of different emitting materials and control ofthe exciton recombination zone. This structure consists ofmany organic–organic interfaces leading to interface barriers,which may result in the inhibition of carrier injection andjoule heating. Therefore to minimize the charge injectionbarriers and joule heating at the organic/organic interfacesthe emissive materials are chosen in such a way that thehighest occupied molecular orbitals (HOMO) and the lowestunoccupied molecular orbitals (LUMO) of different adjacentemissive materials closely match with each other. Theemission from the device depends on the thickness andcomposition of each layer, and is to be precisely controlledto achieve colour balance. The exciton recombination zoneis controlled by inserting blocking layers that block onlyone type of carrier between the hole transporting layers(HTL) and electron transporting layers (ETL), so that therecombination takes place in two or three different layers[24–38]. It results in emission from different layers (figure 3).

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White

Figure 3. Schematic diagram of multilayer white OLED.

By controlling the recombination current within individualorganic layers, emission from red, green and blue lightemitting layers is balanced to obtain white light of the desiredcolour purity. Deshpande et al [24] achieved white lightemission by the sequential energy transfer between differentlayers. The device was fabricated in the configurationITO/α-NPD/αNPD:DCM2(0.6–8 wt%)/BCP/Alq3/Mg:Ag(20:1)/Ag. Here 4,4′ bis [N-(1-napthyl-N-phenyl-amino)]biphenyl (αNPD) was used as a hole injection layer,αNPD: DCM2 (2, 4-(dicyanomethylene)-2-methyl-6-[2-(2, 3, 6, 7-tetrahydro-1H, 5H benzo[I, j]quinolizin-8-yl)vinyl]-4H-pyran) layer was used as a hole transport layer (HTL) aswell as an emitting layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) layer was deposited for the purpose ofhole blocking, Alq3 was used as a green emitting electrontransporting layer (ETL) and Mg:Ag alloy followed by athick layer of Ag was deposited as the cathode. A maximumluminance of 13 500 cd m−2, a maximum external quantumefficiency>0.5%andanaveragepowerefficiencyof0.3 lmW−1

were reported for the above configuration. Recently Wu et al[39] reported white light emission from a dual emittinglayer OLED with and without blocking layers. The devicewith a blocking layer exhibited better performance with anexternal quantum efficiency of 3.86%. The emission colourof these devices strongly depends upon the thickness of theemissive layer and the applied voltage. The drawback ofthis technique is that it requires complex processing anda large amount of wasted organic materials resulting inrelatively high fabrication cost. The CIE coordinates are oftendependent upon the driving current due to shift of the excitonrecombination zone.

Another approach for white light emission frommultilayer OLEDs is the multiple quantum well structure[40] (figure 4), which includes two or more emissive layersseparated by blocking layers. Electrons and holes tunnelthrough the potential barriers of the blocking layers anddistribute uniformly in different wells and emit light. Matchingof the energy levels of different organic materials is not socritical in this system. Excitons are formed in different wellsand decay to produce different coloured lights in their ownwells. The confinement of charge carriers inside the quantumwell improves the probability of exciton formation and theydo not move to other zones or transfer their energy to thenext zone. But this approach is very complicated and requiresthe optimization of thicknesses of various light emitting andblocking layers. This multilayer architecture has relativelyhigh operating voltage due to the combined thickness of manylayers used.

ITO

Al

Blue

Red

Green

LUMO

HOMO

EBL

Figure 4. Schematic diagram of a multiple quantum well whiteOLED.

3.1.2. Host–guest or donor–acceptor system. By the dopingof a high energy emitting (higher band gap) host/donormolecule with low energy emitting (lower band gap)guest/acceptor molecule, the excitation energies can tunnelfrom higher energy host to lower energy guest molecules.Generally in such a system the dopant concentration is somaintained that the emission from the host is negligible andemission of the device is only from dopant sites, which willbe dominant. Such an emission is called emission due tothe complete energy transfer from host to guest. But anotheralternative to this system is the incomplete energy transfer,where device emission is from both the host and dopant sites.The emissions from both guest and host in a proper ratiocan produce white light. The host–guest system for whitelight generation can be either a single-doped or a multi-dopedsystem in a single layer [12] or a multilayer structure [41].But for white light the concentration ratio of dopants must beprecisely controlled to achieve stable white light and to avoidaggregate formation or luminance quenching. By adjustingthe doping concentration of host and guest materials one cancolour tune white OLEDs. The dopants can be fluorescentor phosphorescent in nature. The dopant site can be exciteddirectly or by energy/charge transfer from the host molecule.The energy transfer from host to guest can be either Forster orDexter type, while the charge transfer can be either excimer orexciplex type; the principles are discussed below.

3.1.2.1. Forster/Dexter energy transfer. The energy transfercan take place in two possible ways, either by Forster energytransfer or by Dexter energy transfer. These energy transfersare non-radiative. The Forster energy transfer involves dipole–dipole interaction between the donor and acceptor moleculeshaving a long-range separation of about ∼30–100 A. Theenergy transfer mechanism can be explained as

D∗ + A → X → D + A + hν,

where D∗, A, X, D and hν stand for excited donor, groundstate of acceptor, intermediate excited system, ground stateof donor and energy of emitted photon, respectively. HereX is the unexcited donor–excited acceptor couple. Basicallythis energy transfer is diffusion of excitons from donor toacceptor. Emission of the donor is absorbed by the acceptorand it is favoured by the spectral overlapping of donor emission

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Dexter Energy Transfer

Dexter Energy Transfer

Dexter Energy Transfer

Forster Energy Transfer..

Forster Energy Transfer..

Donor Acceptor Donor Acceptor

Donor Acceptor

Donor Acceptor Donor Acceptor

Singlet–Singlet Singlet–Singlet

Triplet–Triplet

Figure 5. Forster/Dexter energy transfer.

Wavelength (nm)

Inte

nsity

(a.

u.)

Figure 6. Spectral overlapping between emission of donor andabsorption of acceptor.

and acceptor absorption spectra. It allows only singlet–singlet transition at low acceptor concentrations and at amuch faster rate of <10−9 s, whereas Dexter energy transferinvolves electron exchange between host and guest moleculeshaving the short-range separation of ∼6–20 A. The Dexterenergy transfer allows both singlet-to-singlet and triplet-to-triplet energy transitions (figure 5) [42]. For Forster/Dexterenergy transfer the separation of the host–guest moleculesis of prime importance and it will correspond to an optimaldye concentration of ∼1% for fluorescent dyes and ∼10% forphosphorescent dyes. The fluorescent dyes can only utilizesinglet states via Forster energy transfer, while phosphorescentdyes can utilize singlet as well as triplet states via Dexterenergy transfer. Utilization of singlets as well as tripletsleads to a tremendous increase in the efficiency of the device,so Dexter energy transfer may result in efficient white lightemission.

For energy transfer it is necessary that the emissionspectrum of the host (donor) and the absorption spectrum ofthe guest (acceptor) must overlap (see figure 6). A lot of

effort has been made to achieve white light emission fromsmall molecules [41, 43–47] as well as from polymers [48–50, 51] using the Forster/Dexter energy transfer mechanism.Mazzeo et al [45] have fabricated OLED from a blendof N, N′-diphenyl-N, N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′diamine (TPD) (PL in the blue region) with a thiophene-based pentamer, (3,3′,4′′′,3′′′′-tetracyclehexyl-3, 4-dimethyl-2,2′:5′,2:5,2′′′;5′′′,2′′′′: quinquethiophene-1,1-oxide (T5oCx)(PL in the red region). The incomplete Forster energy transferwas from host (TPD) to guest (T5oCx) and as a result, theygot emission from both the molecules, which produced whitelight. This energy transfer was favoured by the overlapping ofthe strong emission spectra of TPD and absorption spectraof T5oCx. Wang et al [46] achieved a highly efficientwhite organic LED using two blue emitters with similarstructures 9,10-di-(2-naphthyl)-anthracene(ADN) and 9,10-di-(2-naphthyl)-2-terbutyl-anthracene (TADN) doped with(0.01–0.05%) yellow-orange emitting rubrene. The devicehad a maximum external quantum efficiency of 2.41%(5.59 cd A−1) and a maximum luminance of 20 100 cd m−2

at 14.6 V. The advantage of the similar structure of ADN andTADN is that it depresses the molecular aggregation, whichleads to better film morphology.

Recently Park et al [49] demonstrated white emissionfrom ITO/PVK/(PDHFPPV +MEHPPV)/Li:Al, ternarypolymer blended LED. Here poly(N-vinylcarbazole) (PVK)acts as an energy donor as well as electron blockerwhile poly[9,9-dihexyl-2,7-fluorene phenylenevinylene](PDHFPPV) + poly(2-methoxy-5-(2-ethylhexyloxyl)-1,4-phenylene vinylene) (MEHPPV) blend acts as an emittinglayer. In this bilayer system the spectral overlapping betweenthe emission of PVK and absorption of PDHFPPV andbetween the emission of PDHFPPV and absorption ofMEHPPV, meets the necessary condition for Forster energytransfer. The cascade energy transfer from PVK to PDHFPPVand then to MEHPPV and the emission from PDHFPPV andMEHPPV as a result produce whitish light emission. Atteret al [50] fabricated an efficient white PLED based on ablue emitting poly(9,9-bis(2-ethylhexyl)fluorine-2,7-diyl)endcapped with bis(4-methylphenyl)phenylamine (PF2/

6am4) doped with yellow-orange phosphor iridium [tri-fluorenyl] pyridine complex [Ir(Fl3Py)3]. The white lightemission was attributed to a strong Dexter energy transferfrom (PF2/6am4) to [Ir(Fl3Py)3] with a peak external quantumefficiency of 2.8% and a luminance of 16 000 cd m−2 at 5 V.The employment of phosphorescent materials in OLEDsreduces the injection current compared with fluorescentOLEDs, leading to the prevention from degradation of thedevices. But the deriving voltage is higher than theconventional organic EL devices, because large excitationenergy is required for carrier injection. But for most suchOLEDs the device quantum efficiencies drop rapidly withincreasing current density and consequently with thebrightness (figure 7) due to triplet–triplet annihilation at highcurrent densities. WOLED based on phosphorescent materialhad a maximum forward viewing power efficiency of 26 ±3 lm W−1 at low luminosity, decreasing to 11 ± 1 lm W−1

at 1000 cd m−2 [7, 12].

3.1.2.2. Exciplex/excimer charge transfer. In terms ofreducing the number of dopants and structural heterogeneities

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Power Density (W/cm2)

Pow

er e

ffici

ency

(Im

/W)

Current Density (mA/cm2)

10

1

0.1

0.0110-3

10-5 10-4 10-3 10-2 10-1 100

10-2 10-1 100 101 102

Figure 7. Lighting (circle) and display relevant (square) powerefficiencies versus current and power densities. Proposed energylevel diagram of WOLED showing the HOMOs and LUMOs of thematerials used.

in multilayer architecture, exciplex/excimer charge transferis an alternative phenomenon. Unlike the multiple dopantsystem, there are no complicated intermolecular interactions,which make colour balancing difficult. It involves the host–guest system where either inter or intramolecular chargetransfer excited states are generated and decay radiativelyat lower energy than the molecular excited states. Theterm ‘exciplex’, strictly used, refers to excited species madeby the combination of two non-identical moieties, atomsor molecules. Excited species that do not fall into thiscategory are known as excimer [52, 53]. White lightemission based on the excimer charge transfer in fluorescentand phosphorescent small molecules has been reported [53,54]. Only a single species is responsible for both blue light(monomer) and orange-red light (excimer) emission. So theparticular approach of excimer is the possibility of reductionof differential colour ageing in single dopant excimer devices.The performance of white light emission based on fluorescentexcimer was not good for lighting applications but in contrast,WOLED based on the phosphorescent excimer charge transferhad efficient broad white light emission [53]. A maximumexternal quantum efficiency of 4.0 ± 0.4% corresponding to9.2 ± 0.9 cd A−1 and a maximum luminance of 31 000 ±3000 cd m−2 at 16.6 V demonstrated an efficient and brightelectrophosphorescent WOLED based on excimer emissioncompared to fluorescent WOLED based on the same principle[53].

In the excimer formation the wavefunction of excitedstates extends over the molecules and the molecules are boundtogether only in the excited state but not in the ground state.This absence of the bound ground state provides a way forefficient charge transfer from higher energy host to lowerenergy guest. The charge transfer mechanism can also beexplained as

D∗ + A → X → D + A + hν,

where D∗, A, X, D and hν stand for excited donor, groundstate of acceptor, intermediate excited system, ground state

Figure 8. (a) Exciton emission in PVK and (b) electroplex emissionacross PVK and BCP interface.

of donor and energy of emitted photon, respectively. Here Xis the charge transfer exciplex/excimer complex. The chargetransfer takes place at the interface of the charge transport layerand the emitting layer [52, 55–58], because of the mismatchedelectronic structure of the two molecules (exciplex) andwavefunction overlapping (excimer). The charge transferexcitations occur at energies close to those of excitationslocalized at the donor and acceptor molecules [59]. The chargetransfer mechanism occurs due to the interaction between theexcited states of one molecule with the ground state of theother molecule (see figure 8), resulting in a radiative electron–hole recombination. The exciplex formation is favoured bya large difference between the HOMOs and LUMOs of theemitter and the charge transport layer. Because of this largedifference the injection of the charge carriers from transportlayer to the emitter layer and from the emitter layer to thetransport layer will be difficult and there will be accumulationof the carriers at the interface. Now the indirect recombinationfrom LUMO of the transport layer to HOMO of the emitterlayer is more favoured. The energy of the exciplex is alwaysless than the energy of the excited single molecules and itsemission is very broad. Attention has been given to achievewhite light by exciplex formation in devices based on smallmolecules [45, 56–58, 60, 61] as well as on polymers [43].It has been proposed that exciplex formation supports thespectral broadening in electroluminescence [43, 60]. RecentlySingh et al [61] demonstrated white emission from an organicLED based on a simple bilayer structure consisting of TPDand zinc banzothiazole. The electroluminescence had aspectral width of approximately 260 nm and a deconvolutionstudy of PL and EL spectra revealed that as much as 60%of the broad EL emission originates from multiple exciplexformation. Similarly Cheng et al [62] reported saturatedwhite light emission from a blend of tris(5-mesitylsulfone-8-hydroxyquinolato)aluminium {Al(MSq)3} and TPD throughexciplex charge transfer. The device exhibited a maximumbrightness of 26 cd m−2 at 15.2 V and a maximum efficiencyof 597 µW A−1 at 12.5 V. Exciplexes generally have a smallradiative recombination rate constant and hence also often alower quantum efficiency so it is important to choose suitablehost and guest molecules in order to get a highly efficientdevice.

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3.1.3. White emission by a single emissive layer structure.The fabrication process and the device operation of whiteorganic OLEDs from the techniques discussed above are verycomplex and many parameters need to be optimized for goodcolour rendering and high luminescence efficiency. These de-vices have high operating voltage because of the thick profiledue to the several stacked organic layers used to perform aparticular function. But in WOLEDs the device profile mustbe thin to ensure low voltage operation. These complexitiescan be resolved by single layer emission. Single layer whitelight emitting devices consist of only one active organic layer.White emission from a single layer consisting of a blue emitterdoped with different dyes or blending two or more polymershas been reported [45, 48, 50, 51, 63–68]. The mostimportant benefit of OLEDs with only one emission zone overthe others is the fact that high emission colour stability canbe achieved. But the approach of white emission by two orthree different light emitting dopants in a single layer has itsown problem that different rates of energy transfer betweendopants may lead to colour imbalance. Some fraction of thehighest energy (blue) will readily transfer energy to the greenand red emitters and the green emitter can transfer energy tothe red emitter. If the three emitters are at equal concentrationsthe red emitter will dominate the spectrum. So the dopingratio must be blue > green > red at a very carefully balancedratio. Recently Shao et al [65] demonstrated the achievementof highly colour stable WOLED using a single emissive layercontaining a uniformly doped host. To avoid the difficulties inthe precise control of dopants by thermal co-evaporation,the host α-naphthylphenylbiphenyl diamine (α-NPD) wasuniformly doped by the fused organic solid solution methodprior to the deposition with 4,4′-bis(2,2-diphenylethen-1-yl)biphenyl (DPVBi) for the blue emission, and 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7,-tetra methyl-1H,5H,11H benzopyrano[6,7,8-ij] quinolizin-11-one (C545T)for the green emission, 5,6,11,12 tetraphenylnaphthacene(rubrene) for the yellow emission and 4-(dicyanomethylene)-2-tertbutyl-6-(1,1,7,7 -teramethyljulolidyl -9 –enyl)-4H-pyan(DCJTB) for the red emission. The correct weight ratioof NPD, DPVBi, rubrene, DCJTB and C545T for stablewhite light emission was 100:5.81:0.342:0.304:0.394. Theexcitons generated from the blue dopant easily transfertheir energy to other dopants. But the energy transferfrom host to guest exhibits energy losses which can beavoided by the process of direct triplet exciton formationin the phosphorescent dyes. This leads to reduction in theoperating voltage and hence increases the power efficiency.Recently D’Andrade et al [12] reported white light emissionfrom a single emissive layer WOLED. The emissive layercontained three organometallic phosphorescent dopants:tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] for green lightemission, iridium (III)bis(2-phenylquinolyl-N, C2′)(acetylacetonate) [PQIr] for red light emission andiridium(III)bis(4′, 6′-difluorophenylpyridinato) tetrakis(1-pyr-azolyl)borate [FIr6] providing blue light emissionsimultaneously codoped into wide energy gapp-bis(triphenylsilyly)benzene (UGH2) host (figure 7).The triplet doped WOLED exhibited a peak power efficiencyof 42 lm W−1 with a colour rendering index ∼80 and amaximum external quantum efficiency of 12%.

It is not necessary to use dyes for white light emissionbut blends of polymers can also be used [48, 69, 70].Recently Gong et al [69] achieved WOLED by using a blendof conjugated polymers {PFO-ETM and PFO-F (1%)} andorganometallic complex [Ir(HFP)3] as an emissive layer. Thedevice exhibited 10 000 cd m−2 at 25 V. The emission ofwhite light can be understood as the electrons and holes arerecombined by two processes: direct recombination on themain chain (PFO-ETM) to produce blue and green emissionin parallel with electron and hole trapping on the fluorenoneunits and on the Ir(HFP)3 followed by radiative recombinationwith green light from PFO-F (1%) and red light from thetriplet excited states of Ir(HFP)3. As a result the mixture ofthese primary colours gives white light. The devices hada colour-correlated temperature (CCT) value of ∼4500 K,which is very close to that of sunlight (∼4700 K) at a solaraltitude of 22◦ and a CRI value of 86. Both CCT andCRI values were insensitive to applied voltage and currentdensity. But a minor shift in the dopant or polymer ratio willsignificantly affect the quality of colour. This problem canbe solved if a single material is used as an emissive layerand the material has chromophores emitting in the differentvisible regions. Research is in progress on the development ofwhite OLEDs based on a single molecule as emissive material[68, 71, 72]. Recently Mazzeo et al [73] reported a brightsingle layer white OLED by spin coating a single emittingmolecule 3,5 dimethyl 2,6-bis (dimesitylboryl)-dithieno[3,2-b:2′,3′-d]thiophene. The white emission was achieved by thesuperposition of intrinsic blue-green light emission of a singlemolecule with a red shifted emission due to the formation ofcross-linked dimers. Bright white electroluminescence wasobtained with a maximum luminance of 3800 cd m−2 at 18 Vand an external quantum efficiency of 0.35%. RecentlyTu et al [74] reported a successful development of aWOLED by using a single polymer: polyfluorene derivativeswith 1,8-naphthalimide chromophores chemically doped onpolyfluorene backbones. Optimization of the relative contentof 1,8-naphthalimide derivatives in the polymer resulted inpure white-light electroluminescence from a single polymer.The performance of the ITO/PEDOT:PSS/polymer/Ca/Aldevice is given in table 2. The external quantum efficiencyof the single emissive WOLEDs is significantly affected bythe thickness of emissive and transport layers. Better deviceefficiency requires the optimization of these layers for balancedcharge recombination within the emissive layer.

3.1.4. Microcavity structure. The microcavity is nothing buta system consisting of a pair of highly reflecting mirrors havingseparation of the order of a micron. In 1994 Dodabalapur andhis co-workers at Bell laboratories constructed EL devices bysandwiching Alq and an inert material between two reflectingsurfaces forming a microcavity. This structure conformsto the physics of a Fabry–Perot cavity. In conventionalstructures, light is wasted since it leaks in all directions.But in a microcavity light emerges only from one end ofthe cavity and the structure is more efficient. By varyingthe thickness of the layer, undesirable light can be filteredout and the emission of light can be obtained at any desiredwavelength. Since a microcavity LED is more efficient anduses less current, it lasts longer. The microcavity structure

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Table 2. Performance of white OLEDs through various approaches.

Efficiency Power efficiencyDevice CIE (x, y) Luminance (cd m−2) (cd A−1) EQE (%) (lm W−1) Reference

Multilayer 0.29, 0.33 74 000 (13 V) 11.0 4.6 6.0 (13 V) [25]0.34, 0.35 24 700 (15 V) – 1.93 1.93 (6.5 V) [26]0.35, 0.36 18 000 (14.5 V) 18 12 10 [27]0.32, 0.33 17 000 (22 V) 3.05 – 1.92 (5 V) [28]0.32, 0.38 15 800 (12.5 V) 1.14 0.82 0.32 [29]0.29, 0.33 13 600 (5.9 V) 12.3 – 4.4 [38]0.33, 0.33 13 500 (18.2 V) – 0.5 0.30 [24]0.33, 0.33 14 000 (12.6 V) – – 1.34 [90]0.30, 0.41 35 538 9.9 3.86 2.69 [39]0.28, 0.28 10 855 (19 V) – – 1.31 [91]

Forster/Dexter 0.37, 0.40 31 000 (14 V) 11 5.2 – [92]0.32, 0.34 21 000 (15 V) – – (11 V) [47]0.31, 0.34 20 100 (14.6 V) 5.59 2.41 – [46]0.32, 0.38 18 100 (13.2 V) – 1.9 2.1 (7.4 V) [43]0.35, 0.36 18 000 (14.5 V) 18 12 10 [15]0.34, 0.36 16 000 (5 V) 4.57 2.8 – [50]0.27, 0.24 1 076 (15 V) 1.35 – – [93]

Exciplex/excimer – 31 000 (16.6 V) 9.2 4 4.4 [53]0.31, 0.35 2 000 (22 V) – – 0.58 (9 V) [56]0.36, 0.44 – 17 6.4 12.2 [94]

Down conversion – 1 080 (5.5 V) 6.57 – 3.76 [87]phosphorMicrocavity 0.32, 0.40 – 12.8 – – [80]Single layer/molecule 0.32, 0.36 11 900 3.8 1.5 2 [74]

0.31, 0.33 42 000 (4 V) 6.11 – 2.92 [64]0.29, 0.33 15 000 (12.8 V) 2.5 – – [65]0.25, 0.35 11 100 5.3 – 2.8 (6 V) [72]0.43, 0.45 1 000 (5 V) – 12 42 [12]0.30, 0.34 930 (15 V) – 0.34 – [68]0.24, 0.36 10 190 (8 V) – – 0.89 [95]

Stacking of 0.32, 0.38 40 000 (26 V) 11.6 – – [85]different devices

can be employed for the optical optimization of coloursand white OLEDs by microcavity is an example of opticaloptimization. A microcavity resonator is one of the mosteffective ways of enhancing the luminance and brightness ofmonochromatic OLEDs [75, 76]. Spectral narrowing andintensity enhancement of spontaneous emission in OLEDsby microcavity has been reported [77]. However it isincapable of providing white OLEDs because its emissionis monochromatic. Dodabalapur et al [78] demonstrated thecontrol of the emission of OLEDs by multimode resonantcavities such that the thickness of the cavity is greater thanthe single mode cavity devices so that it has several resonantmodes within the emission spectrum of the material. Bymixing of the colours from different modes white emissioncan be achieved. In the microcavity-structured devices theemitting layer is embedded between two metallic mirrorsor a metallic mirror and a partially reflecting bottom mirrorcomposed of a distributed Bragg reflector (DBR) [79]. Itleads to strong modulation of the emission spectrum. TheDBR consists of a periodic structure of two materialswith large index difference, which offers tunable reflectivityover a certain wavelength region. During the operationof the device, standing waves are generated, and theirwavelength depends upon the length and refractive index ofthe cavity. Shiga et al [80] employed a modified Fabry–Perotresonator cavity which had two different cavities of differentlengths (see figure 9, the terms MM, DM, EML and FL

(a) (b)

Figure 9. Concept of (a) usual cavity and (b) multiwavelengthresonant cavity (MWRC).

represent metallic mirror, dielectric mirror, emission layer andfilter layer, respectively), and the combined emission fromthe cavities produced white light. One disadvantage of thisapproach is that the colour coordinates change with viewingangle and this angular dependence of the emission limits themicrocavity applications for white OLEDs.

3.1.5. White emission by horizontally/vertically stackedpixels. This technology is similar to liquid crystal flat paneldisplays. Here the pixels of the three principal coloursare patterned separately either horizontally or vertically andaddressed independently [81–83] (see figure 10). In the

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White

Red OLED

Green OLED

Blue OLED

Green BlueRed

(a) (b)

Figure 10. Schematic diagram of (a) horizontally and (b) vertically stacked OLED.

horizontally stacked pattern the individual colour emittingpixels are deposited either in the form of dots, squares, circles,thin lines or very thin strips. As a result of mixing ofthese colours any desired range of colours can be producedin the same pane. As each colour component is addressedindividually, the differential colour ageing can be mitigated bychanging the current through the components. Each pixel canbe optimized to operate at a minimum operating voltage andfor highest efficiency. Also by reducing the size of the pixelsthe lifetime of the device can be controlled to the maximum.

Stacked OLED can be a good candidate as a light sourcebecause double or even triple current efficiency can be obtainedas compared to the single emitter device. Matsumoto et al[84] in IDMC’03 reported a stacked OLED by connectingindividual blue and red devices in series to achieve pinkemission. It is accepted that various hues can be obtainedby this kind of stacked architecture. It is anticipated thatstacked white OLED can produce higher brightness andefficiency than those of conventional WOLED. Recently Sunet al [85] reported an efficient stacked WOLED where theyused a novel anode cathode layer (ACL) for connecting oneblue phosphorescent and a red phosphorescent emissive unit.This ACL layer was used as a middle electrode and ELcharacteristics of two individual emissive units were alsostudied. By biasing the two emissive units in a proper ratiowhite emission was obtained. They reported a maximumluminescence of 40 000 cd m−2 at 26 V with CIE coordinatesof (0.32, 0.38). The luminescence efficiency was 11.6 cd A−1

at 28 mA cm−2.Kido found that the luminance of the stacked device with N

units is usually N times as high as that of the single unit device.This strategy seems to be attractive for getting highly efficientWOLED. Recently Chang et al [86] fabricated two types ofstaked/tandem WOLEDs containing an interconnecting layerof Mg:Alq3/WO3 and one control white emitting device forcomparison. In these devices white emission was obtainedby mixing complementary blue and yellow colours. Device 1was obtained by connecting blue and yellow devices in series,while device 2 stacked two white emitting devices with thesame blue and yellow dopants as used in device 1. Device 2shows better performance compared to device 1 and the controldevice. An interesting amplification effect was observedin device 2 such that it exhibited the highest efficiency of22 cd A−1, which was almost three times that of the controldevice. This was due to the microcavity effect, which enhancesthe amount of light emitted in the forward direction. So by

just connecting two devices higher efficiency can be achieved.It was found that the driving voltage increases with increasingnumber of active units. Device 2 was the least stable, whilethe control device showed the longest half-life. This was dueto the fact that device 2 suffered more driving power that thecontrol and device 1. The thermal breakdown process maybe present in these stacked devices due to non-ohmic contactof the interconnecting layers. However the half-life of device2 at 100 cd m−2 was projected to be greater than 80 000 h.In these stacked devices the emissive intensity and colourwere dependent on the viewing angle. This viewing angledependence of emissive intensity and colour was attributed tothe microcavity effect. So it is important to have a good opticaldesign for the stacked devices. However the devices made bystacking two white emitting OLEDs could produce deviceswith high efficiency, good colour reliability and sufficientoperating stability.

3.2. Wavelength conversion

3.2.1. Down conversion by phosphors. In several methodsused for white emission from organic LEDs, a difficulty ofcolour stability due to differential ageing of various speciesis generally faced. White emission by down conversionphosphors may be an alternative method, which should befar more robust to colour shifting as EL intensity decreaseswith age. In this technology a blue emitting OLED is coupledwith one or more down conversion phosphor layers, in whichone of the phosphor layers contains inorganic light scatteringparticles. On the backside of the blue OLED, the phosphorsare coated. Some portion of the blue light is scattered andgoes through the phosphors without down conversion but therest is used to excite the phosphors, each of which emits adifferent colour. These different colours are mixed with someof the blue light leaking through to make white light withthe broadest and richest wavelength spectrum. Here only theblue emitter conducts the charge and is the only site whichis directly excited. Once the excitons are generated theyexcite other phosphors to produce balanced white emission.As the blue light emission decreases with age, light from othercoupled phosphors should decrease proportionately becausetheir relative intensities are directly related to those of theblue light emitter. Thus there is no differential colourageing in the down conversion technique. Duggal et al [87]reported white light emission from a blue OLED coupledwith a down conversion phosphor system. On the backside

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Figure 11. Schematic diagram of white light emission by downconversion.

of the blue OLED, the down converting orange and thered organic phosphor, namely perylene orange and perylenered dispersed in poly (methacrylate) (PMMA), were coated,followed by a layer of inorganic light scattering phosphor,namely Y(Gd)AG:Ce dispersed in polydimethyl siloxanesilicon. The quantum efficiency of photoluminescence dyesin PMMA was found to be >98% and the quantum yield ofY(Gd)AG:Ce was 86%. During the operation of the device asmall portion of the blue light is scattered and goes throughthe phosphors without down conversion, but the phosphorlayers absorb the rest of the blue light and emit accordingto their intrinsic property. The mixing of all the emittedcolours produces white light. The colour tunability can beachieved by changing the concentration and thickness of thephosphor layers. A schematic diagram of the down conversionphosphors mechanism is shown in figure 11. White emissionfrom down conversion can also be obtained by coupling UVlight with red, green and blue phosphors. Here UV light excitesseveral phosphors, each of which emits a different colour, as aresult of mixing these colours white is obtained. The techniquehas colour stability but the losses associated with wavelengthconversion are the main drawbacks of this technique.

Junji Kido, an OLED expert at Yamagata University inJapan and his colleagues have worked through numerousgenerations of devices, steadily improving their efficiency,lifetime and operating characteristics. One of the biggestchanges pioneered by Kido and Forrest’s groups has beenthe switching from fluorescent materials to phosphorescentmaterials. Phosphorescent dyes can convert both singletand triplet excitons into light, making the devices potentiallymuch more efficient. At a recent meeting of the MaterialsResearch Society (2005 MRS Fall Meeting, 28 November–2 December, Boston, MA, USA), researchers from Japan,Germany and United States reported steady progress in turningthin organic films into high efficiency lights. In the meetingKido reported that he and his colleagues have synthesizednovel phosphorescent compounds and used them to make highefficiency white OLEDs. They reported a new blue OLEDbased on phosphorescent compound FIrpic, with a record-breaking efficiency of 42 lm W−1 at 100 cd m−2. Theysurrounded the FIrpic containing layer with layers of othercompounds that allow triplet excitons to reside within theemissive layer. This creates an energetic well in the lightemitting material so that excitons cannot get out and they decayin the presence of blue phosphor to give off blue light. To getwhite light they added a yellow phosphor to their blue light-emitting layer, converting some of the emitted light to yellow,which combined with the blue to give off white light. The light

photons emitted from OLEDs reflect off the glass–air interfaceand bounce back inside the device, where many of them arereabsorbed and generate heat instead of light. Kido and Forrestsay that they can boost the efficiency of their OLEDs by addinga specialized antireflective coating to the outside of the glass.Kido reported that an antireflective coating to their best whiteOLED enhanced the efficiency from 36 lm W−1 to 57 lm W−1.Kido is a part of a large Japanese consortium that is creatingindustrial scale machines to manufacture large white OLEDspanels at high speed and low cost. Working with Matsushitaand other companies, the group has already produced 30 ×30 cm2 panels and expects to begin selling products withintwo years.

The performance of white OLEDs through variousapproaches with their CIE coordinates is summarized intable 2.

4. Conclusions

The review indicates that great efforts are necessary toimprove the luminous efficiency of white OLEDs to competewith existing illumination sources. Charge imbalance inthe emissive layers is one of the main reasons for lowluminous efficiency and high operating voltage. The chargeinjection, thickness and transport properties of materials arevery important to maximize the exciton recombination in theemissive layer and in the enhancement of device efficiency.Differential colour ageing is another issue of concern forwhite OLEDs. Different emitters have different colour ageingand the quality of white would deteriorate with operation andcolour would shift towards the most stable colour. Howeverthe problem can be solved by designing the emitters in sucha way that they age with the same rate and the ageing affectsthe device uniformly or by designing a single white lightemitter. Attention should be paid to single layer emittingdevices so that advantage can be exploited in terms of largearea, low thickness, low cost and flexible display fabrication.The surface area of today’s devices is only of the order of afew square inches. To be useful for illumination purposes,display panels should have a surface area of several squarefeet. Non-uniformity in these devices is another hurdle toovercome. Cost is another important factor for WOLEDsto be acceptable in the general illumination market, but therealization of a target cost through the vacuum-depositeddevice architecture is difficult. Therefore focus should begiven to solution processable electrophosphorescent WOLEDsby spin coating or inkjet printing, where cost is significantlylower than with vacuum technologies. But solution processedWOLED architecture meets the problems of imbalancecharge injection and polymer impurity and maximum powerefficiency remains below ∼15 lm W−1, the required value forlighting purposes. Another approach of dendrimers may beinteresting for solution processable WOLEDs. Dendrimersare hybrids of both small molecules and polymers [88, 89].Unlike light emitting polymers, dendrimers can incorporatethe best features of small molecules such as highly efficientphosphorescent cores, at the same time also being solutionprocessable. The processibility of dendrimers by spin coatingand inkjet printing may lead to the roll to roll manufacturingtechnique giving the opportunity for the mass production of

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efficient low-cost WOLEDs. But to date we do not knowany WOLED reported that incorporates dendrimers. Indeed,as a minimum, the total cost of ownership of the WOLED,in the lighting market, throughout its operational lifetimemust present a significant improvement over incandescent lightsources. For illumination purposes the lifetime of WOLEDswith desired luminance needs to be increased by more thanone order of magnitude. Several parameters, which are to beunderstood and optimized, provide opportunity for researchin this field. Although WOLEDs are facing significantchallenges, still the technology will meet the desired goalsand expectations. With OLED lighting, it is possible to surpassfluorescent tubes; whether it will be cheap enough and stableenough is still an issue. Research in this field is vibrant andexpanding and WOLEDs certainly have a bright future.

Acknowledgments

Financial support by the Ministry of Information andCommunication Technology, Government of India, CSIR(India) Network project no. CMM-0010 and CSIR (India)Junior Research Fellowship are gratefully acknowledged. Theauthors would like to thank Professor Vikram Kumar, DirectorNPL for his keen interest in this work. Thanks are also dueto Dr Anil Kumar Gupta and Dr S S Bawa for their help andcooperation.

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