JOURNAL OF APPLIED PHYSICS 108, 113113 共2010兲
Highly efficient white organic light-emitting diodes based on fluorescent blue emitters Thomas C. Rosenow,a兲 Mauro Furno, Sebastian Reineke, Selina Olthof, Björn Lüssem, and Karl Leob兲 Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Straße 1, Dresden, Germany
共Received 11 August 2010; accepted 17 October 2010; published online 9 December 2010兲 Beside inorganic LEDs and fluorescent lamps, organic light-emitting diodes 共OLEDs兲 are evolving into a serious alternative to incandescent lamps. Up to now, it was assumed that all-phosphorescent OLEDs are required for reaching sufficiently high efficiencies. However, the stability of phosphorescent blue emitters is a major challenge. We present a novel approach to achieve highly efficient 共up to 90 lm/W at 1000 cd/ m2 using a macroextractor兲 white light emission from OLEDs. The here presented combination of a fluorescent blue and a phosphorescent red emitter simultaneously allows for a strong blue emission and efficient triplet transfer to the phosphor. The spectrum is extended in the green and yellow region by a full phosphorescent unit stacked on top of the triplet harvesting device. This superposition of four different emitters results in color coordinates close to illuminant A and a color rendering index of 80. Furthermore, color stability is given with respect to varying driving conditions and estimations of the electrical and optical efficiencies are provided. © 2010 American Institute of Physics. 关doi:10.1063/1.3516481兴 I. INTRODUCTION
The ban of the incandescent lamp in the European Union underpins the need for alternative white light sources. The benefits of organic light-emitting diodes 共OLEDs兲 are high efficiency and good color rendering properties. Furthermore, they are free of mercury and combine diffuse large area lighting with slim form factor. Although high efficiencies can be reached with full phosphorescent white OLEDs, color coordinates, color rendering index, and lifetime of such devices are limited by the commercially available phosphorescent blue emitters.1,2 Alternatively, fluorescent blue emitters are capable of reaching deep blue color coordinates and long lifetimes. However, because of spin statistics their internal quantum efficiency is limited to 25%, i.e., a quarter of the efficiency of phosphorescent emitters.3,4 New approaches, which increase the internal quantum efficiencies of fluorescent emitters up to 50% by triplet-triplet annihilation, are still not able to achieve lighting relevant efficiencies.5 However, the loss of triplet excitons on the blue emitter can be avoided by efficiently transferring otherwise lost triplets from the fluorescent to a phosphorescent emitter,6 using the fact that for warm white, about 25% of the photons need to be blue.7 For an efficient triplet transfer, the triplet state of the fluorescent emitter has to exceed the triplet state of the corresponding phosphorescent emitter. However, most fluorescent blue emitters do not satisfy this requirement for triplet transfer to phosphorescent green emitters. The reduction of singlet-triplet splitting may yield fluorescent emitters which allow for triplet harvesting to green phosphorescent emitters, but this is usually connected to an increased intersystem a兲
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crossing rate between the first excited singlet and triplet state. Thus, the total efficiency of a triplet harvesting OLED may remain high, but the blue emission is reduced. Here, we present a novel approach: we utilize a combination of a fluorescent blue and a phosphorescent red emitter, which simultaneously allows for a strong blue emission and efficient triplet transfer to the phosphor. In order to extend the spectrum in the green and yellow region without a loss of blue singlets, a full phosphorescent OLED is stacked on top of the triplet harvesting device. Both of the individual units have the ability to reach an internal quantum efficiency of 100%. They are connected by a charge generation layer consisting of a p/n-junction, which is known to become Ohmic if a very thin metal layer was sandwiched between the doped layers.8 This configuration is basically a series connection of two independent diodes: the quantum efficiency and driving voltage increase by a factor of two and the luminous efficacy remains comparable to the individual devices.9,10 Furthermore, the reduced current density of stacked OLEDs is essential if the stack is transferred to large areas. II. RESULTS
In the remainder of the text the stacked white OLEDs based on triplet harvesting are described. Before presenting the complete white OLEDs, we first discuss the single units with focus on operation principles. Additionally, we evaluate the internal quantum efficiency and the outcoupling efficiency for all fabricated devices to achieve further insight into the device physics. A. Triplet-harvesting unit
For triplet-harvesting the transfer of triplet excitons has to be optimized, whereas singlet exciton transfer has to be hindered at the same time. This can be accomplished by 108, 113113-1
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FIG. 1. 共Color online兲 共a兲 Diffusively harvesting triplets. Due to the primarily hole conducting character of the fluorescent blue emitter, excitons are formed close to the hole blocking layer. Whereas singlets recombine rapidly after creation, triplets diffuse to the red phosphorescent emitter and emit light. 共b兲 Patterned surface for enhanced light extraction from the substrate. By optimizing the cavity of the OLED for glass modes and applying this pyramid pattern, external quantum efficiencies can be strongly increased.
utilizing the different lifetimes of singlets and triplets. Because of a lifetime in the range of nanoseconds,11 singlets have a small diffusion length and will rapidly recombine close to where they are formed. For triplets, lifetimes in the order of microseconds can lead to larger diffusion lengths.12,13 Figure 1共a兲 illustrates how singlets and triplets can be separated by their difference in diffusion length. 4P-NPD is a highly efficient fluorescent blue bulk emitter with a hole mobility 0h = 6.6⫻ 10−4 cm2 / V s which is four orders of magnitude higher than its electron mobility 0e = 3.6 ⫻ 10−8 cm2 / V s.6 Hence, an OLED with 4P-NPD as emission layer has a very narrow recombination zone close to the hole blocking layer. If the thickness of the undoped 4P-NPD layer is larger than the extension of the recombination zone, the phosphorescent emitter can only be excited by exciton diffusion. Since the triplet diffusion length 共L = 11 nm13兲 in 4P-NPD is larger than the singlet diffusion length, quenching of blue singlets can be avoided, if the thickness of the undoped layer is larger than the singlet diffusion length.6 4PNPD features a triplet energy of 2.3 eV, which is sufficiently high for triplet transfer to Ir共MDQ兲2acac6. In order to prove this operation principle, the following devices on standard glass are discussed: 20 nm HTL/10 nm Spiro-TAD/10 nm 4P-NPD mixed with 8 wt % Ir共MDQ兲2acac/ 10 nm. . . 30 nm 4P-NPD/10 nm BPhen/30 nm BPhen: Cs and 100 nm of aluminum as cathode. For comparison, an OLED without the intrinsic 4P-NPD layer is also shown. The spectra in Fig. 2 are measured at a constant current density of j = 1.54 mA/ cm2. With decreasing thickness of the pure 4P-NPD layer, the diffusion path is shortened and more triplets are transferred to the red emitter 共Fig. 2兲. Hence, the external quantum efficiency at 1000 cd/ m2 increases from q = 4.9% for a thickness of 30 nm to q = 11.0% for a thickness of 10 nm. However, blue emission is not decreasing with increasing red emission, which proves that triplet harvesting is taking place. An optimization of the thicknesses and emitter concentrations with respect to spectral shape and efficiency results in the following OLED: 65 nm HTL/10 nm Spiro-TAD/5 nm
FIG. 2. 共Color online兲 Proof for triplet harvesting. Electroluminescence spectra taken at constant current density. Red emission increases with decreasing diffusion path length as more triplets reach the phosphorescent emitter. External quantum efficiency increases simultaneously. Because no singlets are quenched by the red emitter, blue emission intensity stays constant. Additionally, the external quantum efficiency of the optimized triplet harvesting OLED is plotted.
4P-NPD: 5 wt % Ir共MDQ兲2acac/ 5 nm 4P-NPD/10 nm BPhen/55 nm BPhen: Cs/100 nm Al. This optimized OLED reaches a luminous efficacy of v = 26 lm/ W and an external quantum efficiency of q = 16% at 1000 cd/ m2 共Fig. 2兲. In order to reach an overall brightness of 1000 cd/ m2, this unit will need to supply about 400 cd/ m2 in a stacked white OLED. The corresponding efficiencies are v = 29 lm/ W and q = 17%. Outcoupling efficiencies out equal to 23.8% and 15.7% are calculated for the red and blue components of the optimized OLED, respectively. The overall outcoupling efficiency of this device can be estimated to be out = 21.8%. By comparing the latter value and the measured q at 400 cd/ m2 共1.54 mA cm−2兲, we estimate an internal quantum efficiency for the triplet harvesting unit of i = 73%. An important advantage of this approach is the low color shift from 共x , y兲 = 共0.552, 0.333兲 at 208 cd/ m2 to 共x , y兲 = 共0.540, 0.325兲 at 1786 cd/ m2. This feature is a direct result of the transport properties of 4P-NPD which lead to a strong pinning of the recombination zone at the interface between emission and hole blocking layer. The limiting factor for color stability is triplet-triplet annihilation14,15 at higher brightness inducing a slight decrease of red emission. Because the highest triplet concentration is at the interface between 4P-NPD and BPhen, the annihilation process will mainly take place in the pure 4P-NPD layer. B. Mixed phosphorescent unit
A second unit is designed to fill the spectral gap between blue and red emission. This unit combines the emission of the green emitter Ir共ppy兲3 and the yellow emitter Ir共dhfpy兲2acac with photoluminescence maxima at 511 nm and 557 nm, respectively. Recently, we have shown that the
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FIG. 3. 共Color online兲 Green/yellow mixed phosphorescent unit. Spectrum and external quantum efficiency depending on Ir共dhfpy兲2acac concentration. The yellow emitter is positively influencing the charge carrier balance as indicated by enhanced green emission at low concentrations. With increasing concentration the transfer of triplets to Ir共dhfpy兲2acac is getting more likely.
FIG. 4. 共Color online兲 Spectra and spectral stability of the white OLEDs. Spectra are taken at constant current density of 1.54 mA/ cm2. The spectra of the blue/red triplet harvesting unit and the green/yellow phosphorescent unit combine to a well balanced white spectrum. The second plot illustrates the high spectral stability of the white OLED on standard glass, which is resulting from the spectral stability of the single units.
color point of OLEDs with two phosphorescent emitters mixed in a common matrix can be chosen freely without loss of efficiency.6 Here, the emitter concentrations are tuned to reach white color coordinates in combination with the tripletharvesting unit as discussed above. The architecture of this unit is: 50 nm HTL/10 nm Spiro-TAD/10 nm TCTA: 8 wt % Ir共ppy兲3: 1 wt % Ir共dhfpy兲2acac/ 10 nm TPBi/50 nm BPhen: Cs/100 nm Al. With the reduction from two to one emission layer, the total thickness of the intrinsic layers is reduced. The resulting lower driving voltage can lead to increased luminous efficacies. The Ir共dhfpy兲2acac has a positive influence on the charge-balance, which is reflected by an enhancement of green emission and quantum efficiency for low concentrations of Ir共dhfpy兲2acac 共Fig. 3兲. The decrease of green emission at higher concentration is correlated with enhanced triplet transfer to Ir共dhfpy兲2acac, but not with a decrease of quantum efficiency. External quantum efficiency and luminous efficacy at 1000 cd/ m2 increase from q = 12% to q = 16% and from v = 41 lm/ W to v = 58 lm/ W in the presence of 1 wt % of Ir共dhfpy兲2acac. According to optical simulations, out = 23% results for the OLED stack featuring 1 wt % of Ir共dhfpy兲2acac. An internal quantum efficiency close to 71% is estimated, when the device is operated at 1.54 mA cm−2. Because both emitters are evenly distributed in the emission layer, a spatial shift of the recombination zone does not have an influence on the spectral shape of emission. Hence, this device architecture features a very low color shift from 共x , y兲 = 共0.393, 0.572兲 at 458 cd/ m2 to 共x , y兲 = 共0.389, 0.575兲 at 4047 cd/ m2.
in between. The optimized stack is: 45 nm HTL 共HTL1兲/10 nm Spiro-TAD/5 nm 4P-NPD: 5 wt % Ir共MDQ兲2acac/ 5 nm 4P-NPD/10 nm BPhen/90 nm BPhen: Cs共ETL1兲/0.5 nm Al/85 nm HTL 共HTL2兲/10 nm Spiro-TAD/10nm TCTA: 8 wt % Ir共ppy兲3: 1 wt % Ir共dhfpy兲2acac/ 10 nmTPBi/ 60 nm BPhen: Cs共ETL2兲/100 nm Al. Figure 4 illustrates how the spectra of the individual units combine to a well balanced spectrum with warm white color coordinates of 共x , y兲 = 共0.505, 0.422兲 and a CRI of 77.6 at 1000 cd/ m2 共U = 6.2 V, j = 1.65 mA cm−2兲. The spectra of the white OLED are stable against varying brightness and the color coordinates exhibit only a slight shift from 共x , y兲 = 共0.506, 0.422兲 at 447 cd/ m2 to 共x , y兲 = 共0.491, 0.415兲 at 7058 cd/ m2 共Fig. 4兲. The luminous efficacy and the external quantum efficiency at 1000 cd/ m2 are v = 33 lm/ W 共Fig. 5兲 and q = 26%. From a calculated overall outcoupling efficiency out = 20.0%, obtained as the weighted sum of the efficiency values of the individual units in the stacked structure, we estimate an overall internal quantum efficiency of int = 130.0% at 1000 cd/ m2 for this device. Such a value, larger than 100%, indicates that the p/metal/n charge generation layer is effectively supplying charge carriers to the two emitting units. However, the internal quantum efficiency of this stacked device is considerably lower than the sum of the int values estimated for the single nonstacked units described in the previous sessions. Therefore, it is apparent that the efficiency in charge-to-photon conversion is reduced in the stacked white OLED. This can be ascribed to a nonideal behavior of the p/metal/n charge generation layer combined with a reduced Purcell factor for this second order optical cavity. Optical simulations show that dipole radiation in this device stack strongly couples to several waveguide modes with obvious dramatic penalties on its efficiency. As an example, waveguide losses are estimated to be as large as 45%
C. Four color white light
The units discussed above are combined to a four color white OLED, by stacking both units using a charge generation layer consisting of a p/n-junction with a thin metal layer
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optical modes in the OLED stack. In case of the pyramid pattern, the efficiencies of the OLED are v = 47.5 lm/ W and q = 41.6% and the enhancement factor compared with the flat OLED on standard glass is only 1.6. Hence, a large amount of the substrate modes are lost, emphasizing the need for improved outcoupling patterns. III. CONCLUSION
FIG. 5. 共Color online兲 Efficiencies of the white OLEDs. Comparing the flat devices, the luminous efficacy decreases when the stack is transferred from standard glass to high-refractive-index glass. By carefully choosing an outcoupling structure for the substrate surface, more light can be extracted than is lost in the first place 共Ref. 1兲. A luminous efficacy of 90 lm/W at 1000 cd/ m2 can be achieved with a hemisphere attached to the substrate.
for the triplet harvesting unit. By transferring this OLED to a glass substrate with increased refractive index, waveguide effects can be considerably weakened or entirely avoided and nearly all light can be coupled into the substrate. By further optimizing the cavity of the OLED for glass modes and by carefully choosing an outcoupling structure for the substrate surface, light extraction can be strongly increased.1,16,17 First, a quadratic pyramid pattern 关Fig. 2共b兲兴 is applied, which is scalable to large areas.1 Additionally, a high-refractive-index hemisphere is used to harvest all the light coupled into the substrate and thus assess the potential of improvement for this OLED. In order to reduce the absorption of the cathode, silver is used instead of aluminum. The optimized thickness of each transport layer is: HTL1 = 40 nm, ETL1 = 90 nm, HTL2 = 65 nm, and ETL2 = 75 nm. The corresponding spectrum measured in forward direction is plotted in Fig. 4. The color coordinates are 共x , y兲 = 共0.462, 0.429兲 and the CRI is 80.1 at 1000 cd/ m2 共U = 6.4 V, j = 2.34 mA cm−2兲. The efficiencies of the device on high refractive index glass with different surface patterns attached are summarized in Fig. 5. As mentioned above, the cavity of this OLED is optimized for glass modes. Therefore, the efficiency of the flat device is reduced in comparison to the OLED on standard glass 共v = 21.8 lm/ W, q = 19.0%兲. The measurement with the hemisphere demonstrates, how much light can be extracted from the device. A luminous efficacy of 90.5 lm/W and an external quantum efficiency of 75.8% is achieved at 1000 cd/ m2, which is an increase of extracted photons by a factor of 2.9 compared with the flat OLED on standard glass. According to optical simulations, the overall outcoupling efficiency is out = 60.7% when including the extraction efficiency of the hemisphere. The average internal quantum efficiency is estimated to be equal to 125.0%, marginally lower than the value estimated for the OLED device on standard glass. This indicates that the two devices exhibit almost identical electrical behavior and confirms that the attached hemisphere is very effective in extracting virtually all available
Our results show that it is possible to overcome the efficiency restrictions induced by fluorescent blue emitters in white OLEDs. By combining the concept of triplet harvesting with stacked OLED structures, white devices with excellent color quality and high efficiencies are achieved. Further research has to address emitter molecules with higher quantum efficiencies, improved outcoupling structures, and charge generation layers. IV. METHODS
ITO 共thickness= 90 nm兲 precoated and structured standard glass n = 1.51 is used as substrate. For outcoupling enhancement glass 共SCHOTT AG: N-SF11兲 with an increased refractive index of 1.78 is used. The active area is 6.49 mm2 in the case of OLEDs on standard glass and 6.00 mm2 for the glass with enhanced refractive index. Using an index matching immersion oil 共n = 1.78, obtained from Cargille Laboratories兲, different outcoupling enhancement structures are attached to the glass. The quadratic pyramid pattern is made of N-LAF21 glass from SCHOTT AG with a refractive index of n = 1.788 and has a periodicity of 0.5 mm. For details about the pattern please refer to Fig. 1共b兲. The hemisphere has a diameter of 14.8 mm and a refractive index of 1.78. Samples are produced by thermal deposition at a base pressure of 10−8 mbar in a single-chamber tool. Encapsulation is done by the use of an additional glass and epoxy resin in nitrogen atmosphere. The following organic materials are used in this work, the corresponding abbreviations used to describe the OLED-stacks are stated in parenthesis: N , N , N⬘ , N⬘-tetrakis共4-methoxyphenyl兲-benzidine electrically doped with the commercially available NDP-2 as hole injection and transport layer 共HTL兲; 2 , 2⬘ , 7 , 7⬘-tetrakis共N, N-diphenylamino兲-9 , 9⬘-spirobifluorene 共Spiro-TAD兲 as electron and exciton blocking layer; the fluorescent blue emitter N , N⬘-di-1-naphthalenyl-N , N⬘-diphenyl-关1 , 1⬘ : 4⬘ , 1⬙ : 4⬙ , 1-Quaterphenyl兴-4 , 4-diamine 共4P-NPD兲; the phosphorescent emitters for green fac-tris共2-phenylpyridine兲 iridium共III兲 共Ir共ppy兲3兲, yellow bis共2-共9,9-dihexylfluorenyl兲-1pyridine兲 共acetylacetonate兲 iridium共III兲 共Ir共dhfpy兲2acac兲 and red emission bis共2-methyldibenzo-关f,h兴 quinoxaline兲 共acetylacetonate兲 iridium共III兲 共Ir共MDQ兲2acac兲; the wide gap matrix material 4 , 4⬘ , 4⬙ tris共N-carbazolyl兲-triphenylamine 共TCTA兲; the hole blocking material 2 , 2⬘2⬙-共1,3,5-benzenetriyl兲tris关1-phenyl-1H-benzimidazole兴 共TPBi兲, and the electron transport material 4,7-diphenyl-1,10-phenanthroline 共BPhen兲. For improved electron injection and transport BPhen is sometimes electrically doped with cesium 共BPhen:Cs兲. A spectrometer CAS140CT-153 from the Instrument Systems GmbH calibrated for absolute intensity, a Siphotodiode, and a source-measurement unit SMU2400
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共Keithley兲 are used for measurement of the luminance emitted perpendicular to the substrate and current-voltage characteristics. The integrated quantities luminous efficacy and external quantum efficiency are determined by goniometer measurement in the case of diodes on standard glass. Here, an USB-4000 mini-spectrometer 共Ocean Optics, Inc.兲 is employed. In the case of the outcoupling enhanced diodes on high-n glass, an Ulbricht-sphere connected to a CAS140CT153 spectrometer 共Instrument Systems GmbH兲 is used. Optical optimization of the device stacks and calculations of the outcoupling efficiencies were performed by using a numerical algorithm based on the point dipole model and the transfer matrix method. Details of the optical modeling approach are given in Ref. 18. The internal quantum efficiency int of the OLEDs, defined as the ratio of the total number of internally generated photons to the number of injected charges, was estimated as int = q / out, where q denotes the measured external quantum efficiency and out the calculated outcoupling efficiency. For the multi-emitter OLED devices, the outcoupling efficiency of each emitting unit was calculated separately and then averaged by using appropriate weighting factors wi extracted from the measured electroluminescence spectra of the devices. For the tripletharvesting unit, the weighting factors were extracted from the spectra in Fig. 2. The extracted values were wb = 0.22 and wr = 0.78 for the blue and red emitter, respectively. For the mixed phosphorescent unit, an effective electroluminescence spectrum for the composite green/yellow emitter was extracted by fitting the spectra in Fig. 3. The weighting factors used in the simulations of all white OLED devices, extracted from Fig. 4, were wb = 0.11, wr = 0.39, and wgy = 0.5 for the red, blue, and green/yellow emitting units, respectively. The intensity matrix formalism18,19 was finally applied to evaluate light extraction by application of the high-index hemisphere, yielding an overall photon extraction efficiency equal to 97.0%.
ACKNOWLEDGMENTS
The work leading to these results has received funding from the European Community’s Seventh Framework Programme under Grant Agreement No. FP7-224122 共OLED100.eu兲, and by the BMBF under Contract No. 13N 8855, project acronym “R2flex.” We also thank Novaled AG, Dresden, for providing the hole-transport layer dopant NDP-2. 1
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