Improved energy transfer in electrophosphorescent devicesD. F. O’Brien, M. A. Baldo, M. E. Thompson, and S. R. Forrest Citation: Applied Physics Letters 74, 442 (1999); doi: 10.1063/1.123055 View online: http://dx.doi.org/10.1063/1.123055 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/74/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Distinguishing triplet energy transfer and trap-assisted recombination in multi-color organic light-emitting diodewith an ultrathin phosphorescent emissive layer J. Appl. Phys. 115, 114504 (2014); 10.1063/1.4869056 Enhanced electrophosphorescence of copper complex based devices by codoping an iridium complex Appl. Phys. Lett. 90, 143505 (2007); 10.1063/1.2719238 Increased electrophosphorescent efficiency in organic light emitting diodes by using an exciton-collectingstructure J. Appl. Phys. 97, 044505 (2005); 10.1063/1.1853500 Polymer electrophosphorescent devices utilizing a ladder-type poly(para-phenylene) host J. Appl. Phys. 93, 4413 (2003); 10.1063/1.1562002 Energy transfer and device performance in phosphorescent dye doped polymer light emitting diodes J. Chem. Phys. 118, 2853 (2003); 10.1063/1.1535211

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Improved energy transfer in electrophosphorescent devicesD. F. O’Briena) and M. A. BaldoCenter for Photonics and Optoelectronic Materials (POEM), Department of Electrical Engineeringand the Princeton Materials Institute, Princeton University, Princeton, New Jersey 08544

M. E. ThompsonDepartment of Chemistry, University of Southern California, Los Angeles, California 90089

S. R. ForrestCenter for Photonics and Optoelectronic Materials (POEM), Department of Electrical Engineeringand the Princeton Materials Institute, Princeton University, Princeton, New Jersey 08544

~Received 30 September 1998; accepted for publication 7 November 1998!

External quantum efficiencies of up to~5.660.1!% at low brightness and~2.260.1!% at 100 cd/m2

are obtained from a red electrophosphorescent device containing the luminescent dye 2,3,7,8,12,13,17,18-octaethyl-21H23H-phorpine platinum~II ! ~PtOEP! doped in a 4,48-N,N8-dicarbazole-biphenyl~CBP! host. Due to weak overlap between excitonic states in PtOEP and CBP, efficiencylosses due to nonradiative recombination are low. However, energy transfer between the species isalso poor. In compensation, a thin layer of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline is used asa barrier to exciton diffusion in CBP, improving the energy transfer to PtOEP. This technique maybe applied to improve the efficiency of other electrophosphorescent devices. ©1999 AmericanInstitute of Physics.@S0003-6951~99!00403-9#

To date, efficiencies for organic light emitting devices~OLEDs! have been limited by the use of fluorescent emis-sive materials. This limit arises since fluorescence only in-volves singlet relaxation, thus eliminating the participationof the triplet exciton population. By employing a phospho-rescent dye where both singlet and triplet excited states par-ticipate, the OLEDinternal efficiency can, in principle, beincreased as high as 100%.1,2 Previous work on the red phos-phor 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine plati-num~II ! PtOEP in an aluminum tris ~8-hydroxy-quinoline! ~Alq3) host has moved some way towards realiz-ing the potential of electrophosphorescence in OLEDs withinternal quantum efficiencies at low brightness as high as23%, corresponding to an external quantum efficiency ofh54%.1 The efficiency of these devices decreased to 1.3% at100 cd/m2 and was ultimately limited by the low photolumi-nescent efficiency~;25%! of PtOEP in Alq3. In this work,we demonstrate that by altering the host material and thedevice structure it is possible to reduce nonradiative lossesand improveh to as high as~5.660.1!% at low brightnessand ~2.260.1!% at 100 cd/m2.

One notable difference between OLEDs based on fluo-rescence and phosphorescence is that phosphorescent effi-ciency decreases rapidly at high current densities. Longphosphorescent lifetimes cause saturation of emissive sites,and triplet-triplet annihilation may also result in significantefficiency losses. Such lifetimes are intrinsic to phosphores-cence, but may be reduced by stronger spin state mixing.Another principal difference is that energy transfer of tripletsfrom a conductive host to a luminescent guest molecule istypically slower than that of singlets; the long range dipole–dipole coupling~Forster transfer! which dominates singlet

energy transfer is forbidden for triplets by spin conservation.Rather, energy transfer occurs by the diffusion of excitons toneighboring molecules~Dexter transfer!, hence significantoverlap of donor and acceptor excitonic wavefunctions iscritical to energy transfer. Finally, triplet diffusion lengthsare long2 ~.1400 Å! compared to singlet diffusion lengths ofa few hundred angstroms.3 In this work, we exploit this latterproperty to improve energy transfer and thus OLED externalquantum efficiency.

The photoluminescent efficiency of PtOEP doped into ahost material is approximately proportional4 to the observedphosphorescent lifetime. Previous measurements of the life-time of PtOEP triplet excitons have demonstrated that thephotoluminescence efficiency of PtOEP in polystyrene5 issignificantly higher than the corresponding efficiency whendoped into Alq3. We speculate that the difference is due toweaker overlap of the excitonic states in PtOEP and polysty-rene molecules. This leads to a much lower probability forexciplex formation and reverse energy transfer from the dyeto the host, both of which can lead to nonradiative quenchingof PtOEP emission. It follows that improved electrophospho-rescent efficiencies may result from doping PtOEP into acharge transport material whose excitonic states only weaklyoverlap with states in PtOEP. Since PtOEP absorbs6 in thegreen at a wavelength ofl;530 nm, blue emitters~l;400–450 nm! are likely candidates for efficient host materials.

One such blue emissive material is 4,48-N,N8-dicarbazole-biphenyl~CBP!.7 The phosphorescent lifetime ina CBP host is;100 ms ~obtained by recording the lumines-cent transient decay with a streak camera!, or approximatelytwice that of an Alq3 host. Thus one might expect, providedthe effects of reduced energy transfer can be minimized, thatthe maximum quantum efficiency of a PtOEP/CBP deviceshould be double that of an optimized PTOEP/ Alq3 OLED.

The external quantum efficiencies of several devicesa!Present address: Department of Physics, Trinity College Dublin, Dublin 2,Ireland.

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 3 18 JANUARY 1999

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where PtOEP is doped at varying concentrations into Alq3

and CBP are shown in Fig. 1, with the device structure pre-sented in the inset. The OLEDs were grown by high vacuum(1026 Torr! thermal evaporation onto a cleaned glass sub-strate as described elsewhere.8 Devices were also made withan 80 Å thick 2,9-dimethyl-4, 7 diphenyl-1, 10-phenanthroline~bathocuproine, or BCP!9 layer inserted be-tween the luminescent and cap layers to improve device ef-ficiency, as discussed in the following. All measurementswere performed in air at room temperature.

Devices using CBP as the host without the BCP layerpossess poor external quantum efficiencies: devices with 6%~mol! PtOEP in a 250 Å thick CBP luminescent layer pos-sessh,1%. Here,h is measured by placing the substrateonto the surface of a silicon photodiode and measuring thered light scattered into the viewing~i.e., detection! direction.The efficiency improves somewhat as the doped region isextended and devices containing 6% PtOEP within a 400 Åthick CBP layer exhibit a maximumh of approximately 2%.The improvement indicates that the low quantum efficiencyof the 250 Å thick CBP devices is most likely due to poorenergy transfer in such a thin active layer. It is probable thatthis and the long lifetime of PtOEP in CBP are manifesta-tions of the same effect: weak overlap between excitonicstates in the molecular species leads to inefficient energytransfer.

Thus, high efficiency devices may be obtained by usingvery thick layers of CBP doped with PtOEP, thereby maxi-mizing the probability that triplets~with their long diffusionlengths! may encounter a PtOEP molecule prior to nonradi-ative recombination within the film or at the organic/cathodeinterface. However, since CBP is a relatively poor electronconductor, it is desirable to minimize its thickness to reducethe OLED operating voltage. Alternatively, energy transfercould be improved by increasing the doping concentration ofPtOEP, but this also increases nonradiative decay due toPtOEP-PtOEP interactions, i.e., ‘‘self-quenching.’’

By blocking the triplets from leaving the luminescentlayer, it should be possible to increase their residence time inthis region, thereby increasing the probability for energytransfer from CBP to the phosphorescent center. A materialsuitable for this purpose is BCP, which has previously beenused as a hole blocking layer in OLEDs.9 When placed be-

tween a doped HTL and an Alq3 ETL, it was found that lightemission originated from the HTL. This demonstrates thatBCP has a large ionization potential compared to Alq3,thereby blocking the passage of holes out of the HTL. Theseresults also suggest that the lowest unoccupied molecularorbital level of BCP freely allows the transport of electronsresulting in exciton formation in the HTL. From this, weinfer that exciton blocking in our devices can be achieved bycapping the PtOEP:CBP layer with 80 Å of BCP.

The current~I!–voltage~V! characteristics for Alq3 de-vices with and without the BCP layer are presented in Fig. 2.Distinct low and high current regimes of operations are ob-served as is typical of Alq3-based OLEDs.8 Introduction ofthe BCP cap increases the operating voltage of 10.6 V byonly ;0.7 V at 1 mA/cm2, indicating the presence of thisthin layer minimally affects the electron conduction proper-ties of the device.

The exciton blocking function of BCP is demonstratedby comparison of the spectra in Fig. 3 taken for PtOEP:CBPdevices with and without BCP. In Fig. 3~a!, the emissionspectra of devices without the BCP layer are shown for arange of current densities. At low currents emission is pre-dominantly from PtOEP, with a sharp emission peak atl5650 nm and no measurable luminescence from the PtOEP

FIG. 1. External quantum efficiencies of PtOEP/CBPand PtOEP/ Alq3 devices as a function of current withand without a BCP blocking layer. The top axis showsthe luminance of the device with a 80 Å thick BCPlayer and a 400 Å thick CBP luminescent layer dopedwith 6% PtOEP. Inset: Schematic cross section of thehigh efficiency OLED, consisting of a 450 Å thick4,48-bis@N-~1-naphyl!-N-phenyl-amino# biphenyl ~a-NPD! HTL, PtOEP doped ETL codeposited with eitherAlq3 or CBP acting as the host, an 80 Å thick layer ofBCP, and a further 200 Å thick cap layer of Alq3 toprevent nonradiative quenching of PtOEP excitons atthe cathode. Finally, a shadow mask with 1 mm diam-eter openings was used to define the cathode consistingof a 1000 Å thick layer of 25:1 Mg:Ag, with a 500 Åthick Ag cap.

FIG. 2. Current–voltage characteristics of the devices with and without an80 Å BCP layer. There is only a;0.7 V increase in voltage at 1 mA/cm2,for devices with BCP, suggesting that this blocking layer does not signifi-cantly alter the conduction properties of the device.

443Appl. Phys. Lett., Vol. 74, No. 3, 18 January 1999 O’Brien et al.

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singlet state atl5580 nm.10 The red Commission Internatio-nale de L’Eclairage chromaticity coordinates of~0.7, 0.3! areidentical to those obtained with an Alq3 host. As the currentincreases, green emission characteristic of the Alq3 cap layerat l5520 nm is observed. The Alq3 emission is absent indevices containing BCP@Fig. 3~b!# due to the exciton block-ing effects of this layer. The lack of Alq3 emission also sug-gests that no holes reach the Alq3 cap. Therefore, BCP mayalso block holes in addition to confining excitons within theCBP layer.

As expected, the benefits of the BCP barrier are mostnoticeable for the thinner~250 Å! CBP layers, where energytransfer from CBP to PtOEP is particularly inefficient. It isless useful for devices employing PtOEP in an Alq3 hostwhere previous studies1 have shown that under optimumconditions, energy transfer is nearly complete with up to;90% of excitons transferred to the dye. Note that the effi-ciency of the 250 Å thick CBP device begins to decrease ata lower current density than that of the 400 Å thick CBPdevice ~Fig. 1! due to the lower number of luminescentPtOEP sites in the thinner device.

The BCP barrier layer substantially improves the quan-tum efficiency in CBP doped devices toh5~2.260.1!% at100 cd/m2, which compares with a best result of 1.3% inPtOEP/Alq3 devices.1 Note that the Alq3 devices in Fig. 1 aremore lightly doped with PtOEP and possessh,1% at 100cd/m2. This is due to the weaker energy transfer observed ata lower doping densities, but in this work, poor transfer isrequired to illustrate the effect of the BCP layer. The peakh5~5.660.1!% is equivalent to an internal quantum effi-ciency of ;32%.11 This improvement is less thanh;8%implied by the lifetime data, suggesting that some triplet ex-citons are quenched at the CBP/BCP interface or leakthrough to the cathode where additional nonradiative recom-bination occurs. In addition, since the phosphorescent life-time in CBP is roughly twice that in Alq3, saturation ofPtOEP sites in the CBP devices occurs at a concomitantlylower current density. However, the increased susceptibility

of PtOEP in CBP to saturation is overcome by much weakerbimolecular quenching in these devices.

Unlike most red dyes12 with peak emission around 650nm, PtOEP emission is color saturated and does not extendsignificantly into the infrared, thereby maximizing its lumi-nosity. In comparison with saturated reds such as the Eucomplexes,13–15 PtOEP possesses quantum efficiencies thatare higher by at least an order of magnitude. Unsaturatedreds such as DCM2 and its variants12 possess comparablequantum efficiencies to PtOeP~e.g., 1%–3%! but they typi-cally emit in the orange; and further red-shifting by increas-ing the DCM2 concentration causes the quantum efficiencyto decrease to~<1%! along with a considerable increase ininfrared emission.16

Evidently, the CBP host reduces bimolecular quenchingand lengthens the lifetime of PtOEP triple excitons. Both ofthese effects are consistent with weaker interactions betweenCBP and PtOEP. We therefore propose that a general tech-nique for obtaining high efficiency from phosphorescentemitters is to employ a host chosen such that the phospho-rescence lifetime is as long as possible and to use a blockinglayer such as BCP to keep triplets within the luminescentregion.

In conclusion, we have demonstrated saturated red elec-trophosphorescent devices employing exciton blocking withexternal efficiencies of~5.660.1!% at low brightness~;1cd/m2) and ~2.260.1!% at 100 cd/m2. However, significantpotential for improvement remains given that the maximuminternal quantum efficiency of;32% is still roughly a factorof three lower than the theoretical limit.

This work was funded by Universal Display Corpora-tion, DARPA, AFOSR, and NSF.

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FIG. 3. Emission spectra of CBP-based electroluminescent devices with andwithout a BCP exciton blocking layer. The spectra of the device withoutBCP are shown in~a! to exhibit significant Alq3 emission at a peak wave-length ofl;520 nm. The effect of the BCP layer is clearly shown in~b!,where no Alq3 emission is observed.

444 Appl. Phys. Lett., Vol. 74, No. 3, 18 January 1999 O’Brien et al.

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