Organic light-emitting diodes for lighting: High color quality by controlling energy transfer processes in host-guest-systems Caroline Weichsel, Sebastian Reineke, Mauro Furno, Björn Lüssem, and Karl Leo Citation: J. Appl. Phys. 111, 033102 (2012); doi: 10.1063/1.3679549 View online: http://dx.doi.org/10.1063/1.3679549 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i3 Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 111, 033102 (2012)
Organic light-emitting diodes for lighting: High color quality by controlling energy transfer processes in host-guest-systems Caroline Weichsel,a) Sebastian Reineke,b) Mauro Furno, Bjo¨rn Lu¨ssem, and Karl Leoc) Institut fu¨r Angewandte Photophysik, Technische Universita¨t Dresden, George-Ba¨hr-Str. 1, 01062 Dresden, Germany
(Received 30 September 2011; accepted 21 December 2011; published online 3 February 2012) Exciton generation and transfer processes in a multilayer organic light-emitting diode (OLED) are studied in order to realize OLEDs with warm white color coordinates and high color-rendering index (CRI). We investigate a host-guest-system containing four phosphorescent emitters and two matrix materials with different transport properties. We show, by time-resolved spectroscopy, that an energy back-transfer from the blue emitter to the matrix materials occurs, which can be used to transport excitons to the other emitter molecules. Furthermore, we investigate the excitonic and electronic transfer processes by designing suitable emission layer stacks. As a result, we obtain an OLED with Commission Internationale de lE´clairage (CIE) coordinates of (0.444;0.409), a CRI of 82, and a spectrum independent of the applied current. The OLED shows an external quantum C 2012 American Institute efficiency of 10% and a luminous efficacy of 17.4 lm=W at 1000 cd=m2. V of Physics. [doi:10.1063/1.3679549] I. INTRODUCTION
The lighting market is currently dominated by incandescent lamps and fluorescent tubes. Both are not perfect light sources, as incandescent lamps show only low efficiencies and fluorescent tubes often suffer from bad color coordinates and rendering. Organic light-emitting diodes (OLEDs) have the possibility to reach warm white color coordinates with high color rendering at high efficiencies, as the organic emitter molecules provide a broad emission spectrum.1 Although white OLEDs reaching efficiencies of fluorescent tubes have been presented,2 there is still a lack on efficient OLEDs with color coordinates close to the warm white point A and good color rendering. Additionally, the color coordinates should be constant over a wide luminance range to enable dimmable light sources. To achieve efficient white light-emitting OLEDs, an extensive knowledge of the processes leading to light emission, such as charge carrier recombination, exciton transfer processes in the host-guest-system, and relaxation of the excitons, are crucial. A color-rendering index (CRI) of 90 was already achieved using excimer and exciplex emission,3 which provide very broad emission spectra, but up to now suffer from low efficiencies. Recently, a CRI of 96 at 1000 cd=m2 was published using a hybrid approach with five emitters.4 However, the luminous efficacy was only 5.2 lm=W, the spectrum was not color stable, and no investigations on the excitonic processes were made. Using phosphorescent emitters having the possibility to reach an electron-to-photon conversion efficiency of 100%,5 CRI values over 80 have been reported, but mostly at color coordinates far from the warm white point A or with spectra heavily changing with the applied current.6,7 a)
Electronic mail:
[email protected]. URL: www.iapp.de. Present address: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. c) Electronic mail:
[email protected]. b)
0021-8979/2012/111(3)/033102/7/$30.00
In this work, we study the excitonic and electronic transfer processes in an efficient white OLED stack in order to realize warm white light emission with high color rendering. We investigate the multilayered, phosphorescent host-guestsystem2,7–9 by time-resolved spectroscopy and show that excitons are back-transferred from the emitter to the matrix. We investigate the influence of the position of the recombination zone on the emitted spectrum and the quantum efficiency and vary the thickness of thin interlayers. With these experiments, we gain a deep understanding of the exciton transfer pathways and the electronic processes in the emission layer. We show how to tune the emission color by adjusting the thicknesses of the emitting layers and intrinsic interlayers and finally present a warm white phosphorescent OLED with Commission Internationale de lE´clairage (CIE) coordinates of (0.444;0.409) and a CRI of 82, showing 10% external quantum efficiency and 17.4 lm=W at 1000 cd=m2. II. EXPERIMENTAL
The organic light-emitting diodes are prepared on precleaned glass substrates coated with a 90-nm prepatterned indium tin oxide (ITO) layer as anode. All organic layers and the 100-nm-thick aluminum cathode are deposited by thermal evaporation in a UHV chamber (Kurt J. Lesker Co.) with a base pressure of 10–8 mbar. The sample size is 6.49 mm2. Single layers for optical measurements are prepared on precleaned quartz substrates. All samples are encapsulated with glass lids under nitrogen atmosphere directly after preparation. We use the pin-concept, i.e., intrinsic emission and blocking layers sandwiched between p- and n-doped transport layers.10,11 All materials are commercially purchased as stated below and further purified by high-vacuum gradient sublimation. The hole transport layer (HTL) consists of 60 nm N,N,N0 ,N0 -tetrakis-(4-methoxyphenyl)-benzidine [MeO-TPD, Sensient] doped with 4 wt. % 2,20 -(perfluoronaphthalene-2,6diylidene)dimalononitrile [F6TCNNQ, Novaled AG], and the
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electron transport layer (ETL) consists of 40 nm Cs-doped 4,7-diphenyl-1,10-phenanthroline [BPhen, abcr GmbH & Co. KG]. For hole and electron blocking layers (HBL and EBL, respectively), 10 nm of the adjacent matrix materials are used. The emission layer (EML) design, as well as the emitter molecules and matrix materials, will be introduced in Sec. III. Current density, voltage, and luminance of the OLEDs are measured in an automated measurement setup using a Keithley SM2400 source-measure unit, a calibrated silicon photo- diode, and a calibrated CAS140CT spectrometer from Instrument Systems GmbH. External quantum efficiency and luminous efficacy are calculated assuming Lambertian emission characteristics. Time-resolved photoluminescence measurements are carried out to investigate the host-guestsystem. Here, 100 nm thin films of the material are excited by a nitrogen laser MNL200 from Lasertechnik Berlin with 1.3 ns pulses at a wavelength of 337.1 nm. The signal passes a color filter to suppress the excitation wavelength and is recorded using a PDA10A-EC silicon gain detector from Thorlabs connected to an Infinium 54815A oscilloscope from Hewlett Packard. III. STRUCTURE OF THE EMISSION LAYER
The emission layer of our white OLEDs consists of two matrix materials with different transport properties, i.e., one material conducting holes and the other electrons. Using this so-called double-emission layer concept,12 we achieve a defined charge carrier recombination zone at the interface between the two matrix materials (cf. Figure 1). The overall width of the emission layer is kept at around 20 nm, with only slight variations to tune the emitted color by adjusting the thicknesses of the emitting layers. To generate white light with high color quality, materials which cover all parts of the visible spectrum are needed. Here, we use four different phosphorescent emitter molecules providing a broad emission spectrum: the blue emitting iridium(III) bis(40 ,60 - difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate [FIr6, Lumtec], the green-emitting fac-tris(2phenylpyridine) iridium [Ir(ppy)3, Sensient], the yellowemitting bis(2-(9,9-dihexylfluorenyl)-1-pyridine)(acetylacetonate)iridium(III) [Ir(dhfpy)2(acac), American Dye Source,
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Inc.] and the red-emitting iridium(III)bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) [Ir(MDQ)2(acac), American Dye Source, Inc.]. All materials are highly efficient with internal quantum efficiencies of more than 75%.13–16 As phosphorescent Ir(III)-complexes suffer from concentration quenching when evaporated as a neat layer,17 all materials are diluted into host materials having triplet energy levels higher than the guest molecules to provide efficient exothermic energy transfer14 (cf. Table I). The blue emitter FIr6 has the highest triplet energy level with 2.72 eV. We use 4,40 ,400 -tris (N-carbazolyl)-triphenylamine [TCTA, Sensient] and 2(diphenylphosphoryl)spirofluorene [SPPO1, Lumtec]18,21 as hole and electron transporting matrix materials, respectively. Both materials provide high triplet levels with energies of approximately 2.8 eV. TCTA is known as a highly efficient hole-transporting matrix material,12 but electron-transporting matrix materials with high triplet energies are still rare.18,19 Very common is the use of the ultrawide bandgap organosilicon compound p-bis(triphenylsilyl)benzene [UGH2],20 but its large energy barriers lead to high voltages, which cause a decrease in luminous efficacy. SPPO1 with HOMO and LUMO values of –6.5 eV and –2.8 eV,18 respectively, can provide better electric properties. The four phosphorescent emitter materials are doped into the matrix materials, as depicted in Fig. 1. Since the blue emitter has the highest triplet energy, it is doped at a concentration of 20 wt. % into the center of the EML. Excitons created at the interface between the two matrix materials can be transferred to the blue-emitting molecules first. The exciton transfer paths from FIr6 to the other emitters will be intensively investigated in Secs. IV A-C. To avoid exciton quenching on FIr6 by other emitter molecules, which provide lower triplet energy levels, thin interlayers of pure matrix material are introduced on both sides of the blue emitter.23 Their functionality will be explained in Sec. IV. Ir(MDQ)2(acac) is doped with 10 wt. % into TCTA and Ir(ppy)3 and Ir(dhfpy)2(acac) are doped both into SPPO1 with 8 and 1 wt. %, respectively. IV. ENERGY TRANSFER PROCESSES A. Host-guest-system
To properly study the transfer mechanisms in our white OLEDs, we investigate the energy transfer from host to guest molecules by time-dependent photoluminescence measurements first. Figure 2 shows the transient signal of 100-nmthick layers of 20 wt. % FIr6-doped TCTA and SPPO1. Both host-guest-systems show a biexponential decay. The decay TABLE I. Triplet energy levels of the used host and guest molecules.
FIG. 1. (Color online) Schematic of the emission layer. Filled squares correspond to host materials, while hatched squares indicate guest molecules. Solid lines represent the HOMO and LUMO values and dashed lines the triplet energies of the used materials.
Material
Triplet energy [eV]
TCTA SPPO1 FIr6 Ir(ppy)3 Ir(dhfpy)2(acac) Ir(MDQ)2(acac)
2.8 (Ref. 2) 2.8 (Ref. 21) 2.72 (Ref. 20) 2.4 (Ref. 22) 2.21 (Ref. 22) 2.0 (Ref. 22)
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FIG. 2. (Color online) Photoluminescent transient decay of solid mixed films comprising the blue-emitter FIr6 doped into the host materials TCTA and SPPO1 with 20 wt. %. The transients are shifted in intensity for better visibility. The deviations from monoexponential decay (dotted lines) indicate an energy back-transfer to the matrix materials.
times for FIr6 doped into SPPO1 are 2.6 ls and 6.1 ls, while in TCTA, 2.2 ls and 3.4 ls are obtained. This deviation from monoexponential decay can be attributed to an energy back-transfer from the guest to the host molecules.2,24,25 For TCTA, the difference between the two decay times is much smaller than for the SPPO1:FIr6-system. As the triplet energy levels of both materials are nearly identical, we suggest that the triplet excitons reside shorter on TCTA than on SPPO1, probably caused by a shorter intrinsic exciton lifetime of the matrix. The shorter decay time in TCTA is therefore due to a much faster relaxation of the excited states, either back to FIr6 or non-radiatively into the ground state of the matrix. The longer lifetime of back-transferred excitons on SPPO1 leads to a larger diffusion length of excitons on SPPO1 molecules than on TCTA. The energy transfer from the host materials to the green-, yellow-, and red-emitting molecules is exothermic, as these molecules have much lower triplet energy levels than FIr6 (cf. Table I). In blue-emitting OLEDs with FIr6-doped SPPO1 or TCTA as an emission layer, the energy backtransfer would cause a loss channel and, therefore, reduce the internal quantum efficiency. In white OLEDs instead, the energy back-transferred onto the matrix can still be used by transferring it to other emitter molecules. This is comparable to the so-called triplet harvesting concept, where excited triplet states from fluorescent molecules are transferred to phosphorescent dopants, which provide lower triplet energy levels.15,22 Making use of this concept, internal quantum efficiencies up to 100% can be reached. Besides, doping of emitter molecules into matrix materials with resonant energy levels leads to a voltage reduction and, hence, an increase in power efficiency.2,26
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FIG. 3. (Color online) Schematic of the emission layer for the sample series investigating a shift of the recombination zone. Filled squares indicate the matrix materials, while hatched squares indicate the guest molecules. The width of the squares corresponds to the layer thicknesses. By doping FIr6 either in TCTA, SPPO1, or both host materials, a shift of the recombination zone over the blue-emitting molecules can be obtained.
sion layer of the investigated sample series. The 20-nm-thick EML is composed of the following layers: 6 nm TCTA:Ir(MDQ)2(acac) — 2 nm TCTA — either R1) 4 nm SPPO1:FIr6, R2) 2 nm TCTA:FIr6 and 2 nm SPPO1:FIr6, or R3) 4 nm TCTA:FIr6 — 2 nm SPPO1 and 6 nm SPPO1:Ir(ppy)3:Ir(dhfpy)2(acac). Within this series, the recombination zone is shifted from the interface between FIr6 and the TCTA interlayer across the blue emission layer to the interface between FIr6 and the SPPO1 interlayer. Current density-voltage characteristics of the devices are plotted in Fig. 4(a). As all samples show the same electrical behavior, it can be concluded that the charge carrier balance is not affected and that the recombination zone is only shifted due to the different host structure. Figure 4(b) shows the spectral radiance of the three samples at a constant current density of 46.2 mA=cm2, corresponding to a brightness of around 7000 to 12 000 cd=m2. The contribution of the emitters to the measured spectra of the three devices is calculated by making use of the dipole model, as presented in Ref.
B. Shifting the recombination zone
The position of the recombination zone has a strong impact on the exciton distribution within our multilayer emission stack. In this section, we study its influence on the emitted spectrum and the obtained external quantum efficiency. With these experiments, we gain a deeper understanding on the exciton generation and radiative and nonradiative relaxation. Figure 3 shows a schematic of the emis-
FIG. 4. (Color online) (a) Current density-voltage characteristics of the sample series under a shift of the recombination zone. The current density remains constant. (b) Spectral radiance of the three samples at 46.2 mA=cm2. The recombination zone shift leads to changes in the ratio between red and green-yellow emission, as well as increases in the FIr6 emission for sample R2. Inset: CIE coordinates of the devices and the warm white point A for comparison.
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TABLE II. Calculated photon contribution wi of each emitter in % to the spectrum in Fig. 4(b). Sample no.
FIr6
Ir(ppy)3
Ir(dhfpy)2(acac)
Ir(MDQ)2(acac)
R1 R2 R3
15.5 21.5 16.3
13.0 21.1 30.2
7.1 12.9 16.3
64.4 44.5 37.1
27. For each emitter, we calculate the spectral outcoupling efficiency in the forward direction n0,i(k), i.e., the spectrally resolved ratio between the number of photons emitted in the direction perpendicular to the substrate to the total number of photons internally radiated. With calculated n0,i(k) spectra, we derive, then, the relative contribution of each emitter wi by fitting the measured white spectrum with the function X wi n0;i ðkÞ; (1) SðkÞ ¼ i
X
wi ¼ 1:
(2)
i
According to the above equations, the effect of eventual variations in outcoupling efficiency in different regions of the visible spectrum is accounted for by the terms n0,i(k), whereas the weights wi quantify the relative fraction of excitons decaying radiatively in each emitting layer.15 Table II summarizes the calculated contributions of each emitter for the devices R1, R2, and R3. Here, the position of the recombination zone has a large influence. By shifting it from TCTA to the SPPO1 interlayer, the intensity of the red emission decreases, while the intensity of the green and yellow emission increases. Besides, an increase in blue emission is visible for sample R2, where the recombination zone lies in the center of the blue emitter, which is attributed to the higher exciton density within the blue emission layer. Sample R2 shows, with 10.1% at 1000 cd=m2, the lowest external quantum efficiency (cf. Figure 5(a)). We suggest that this is due to the increased blue portion in the spectrum, as the back-transfer of energy from FIr6 to TCTA and SPPO1 will cause a decreased photoluminescence quantum efficiency, when not harvested by other emitter molecules.
FIG. 5. (Color online) (a) External quantum efficiency and (b) luminous efficacy in dependence of the luminance under variation of the recombination zone. Both efficiencies are heavily dependent on the ratio of the color contributions from each emitter material.
Furthermore, the external quantum efficiency and luminous efficacy of sample R3 is higher compared to sample R1 (cf. Figure 5(b)), which is correlated to a more greenish spectrum and to the high sensitivity of the human eye in the green part of the visible spectrum. In comparison to the SPPO1 interlayer, we suspect that, in the TCTA interlayer, some of the excitons are quenched non-radiatively, because of the shorter lifetime on TCTA molecules (cf. Sec. IV A). Instead, excitons can be transported in the SPPO1 interlayer over longer distances, which decreases the probability of non-radiative quenching. The inset of Fig. 4(a) contains the CIE coordinates of the three samples. All spectra show only weak blue emission and, therefore, do not reach the warm white point A. C. Role of the interlayers
In this section, the role of the thin interlayers (IL) between the emitters is discussed. By varying their thickness, it is possible to study the transfer mechanisms taking place between different emitting molecules. Therefore, two sample series are produced, varying the TCTA and SPPO1 interlayer thickness from 1.8 nm to 2.4 nm in steps of 0.2 nm (cf. Figure 6). The layer stack corresponds to sample R2 of the previous sample series, but without Ir(dhfpy)2(acac). The current transport is not influenced by thickness variations in the sub-nanometer regime, and j-V-characteristics of the samples remain constant. Besides, the optical outcoupling is hardly influenced. Figure 7 shows the spectral radiance of all samples measured at a constant current density of 15.4 mA=cm2. With increasing TCTA IL thickness, the ratio between Ir(ppy)3 and Ir(MDQ)2(acac) emission shifts to a more greenish spectrum. Additionally, a slight increase in blue emission is observed for 2.0 nm IL in comparison to 1.8 nm IL. Increasing SPPO1 IL thickness instead shifts the spectrum slightly to the red, but its influence is weaker than for the TCTA IL. Only a decrease in Ir(ppy)3 emission is observed. This can be explained as follows: The interlayers control both Fo¨rster- and Dexter-type energy transfer, as depicted in Fig. 6. Fo¨rster energy transfer is based on dipole
FIG. 6. (Color online) Schematic of the transport channels from FIr6 to the matrix and emitter molecules. The height of the squares indicates the triplet energy levels. Fo¨rster and Dexter transfer are abbreviated with F and D, respectively. From FIr6, a back-transfer to the matrix materials is possible. This enables Dexter-transfer to Ir(ppy)3 on SPPO1 molecules. The transport on TCTA molecules is less pronounced, because of a shorter exciton lifetime. Instead, direct Fo¨rster and Dexter transfer from FIr6 to Ir(ppy)3 and Ir(MDQ)2(acac) are suppressed by the corresponding interlayers.
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FIG. 7. (Color online) Spectral radiance for a variation of the interlayer thickness of (a) TCTA IL and (b) SPPO1 IL measured at 15.4 mA=cm2. With an increase of the TCTA IL, the red emission decreases and the green emission increases. With increasing thickness of the SPPO1 IL, only slight changes in the green emission appear.
coupling between two molecules. For various iridium compounds, the Fo¨rster radius is only about 2 nm.17 In the Dexter type, the energy transfer takes place by an exchange of electrons between the interacting molecules instead.28 This transfer is only possible for short distances, as its rate decreases exponentially with the distance of the molecules. Regarding the SPPO1 interlayer, direct Fo¨rster and Dexter energy type transfer from FIr6 to Ir(ppy)3 molecules can be suppressed with increasing IL thickness. However, back-transferred excitons can still diffuse across the interlayer by successive Dexter transfer between adjacent matrix molecules. Here, the energy back-transfer from guest to host leads to enhanced exciton transfer to the green-emitting molecules. The transport over this channel is less dependent on the IL thickness than direct Fo¨rster or Dexter transfer between the dopants, where an energy barrier has to be crossed. Therefore, the overall spectral changes with increasing SPPO1 IL thickness are small (cf. Figure 7(b)). Similar to SPPO1, the TCTA IL controls both Fo¨rster and Dexter energy transfer from FIr6 to Ir(MDQ)2(acac). However, as excitons back-transferred to TCTA will relax within a shorter time, a diffusion of excitons on TCTA toward the red emitter is hindered. Therefore, the spectral changes under IL variation are more pronounced than for the SPPO1 IL. As the FIr6 peak remains constant for interlayers thicker than 1.8 nm, we conclude that 2 nm interlayers are sufficient to suppress Dexter-type energy transfer from FIr6 to Ir(MDQ)2(acac). We suggest that the increased green emission follows from a higher exciton density on FIr6, which is caused by the fast relaxation of excited TCTA states. These excitons can then be back-transferred from FIr6 to SPPO1 and, hence, be transported to the green-emitting molecules. Additionally, it has to be taken into account that layers of around 2 nm are not fully closed, which means that the possibility for an energy transfer is minimized, but not prevented. Figure 8 shows the external quantum efficiency of the devices. With increased TCTA IL, the efficiency slightly decreases from 13.7% with 1.8 nm IL to 13.0% with 2.4 nm IL. Due to the fact that excitons on TCTA interlayer will partly relax before reaching a quenching dopant, it is most likely that the increased IL thickness leads to an enhanced non-radiative recombination. On SPPO1 instead, the exciton
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FIG. 8. (Color online) External quantum efficiency of the samples under variations of the thickness of (a) TCTA and (b) SPPO1 interlayer. With thicker interlayers, the quantum efficiency decreases, due to an enhanced non-radiative recombination in this layer.
lifetime is longer and the probability for excitons reaching an emitter molecule is enhanced, which reduces nonradiative recombination. Therefore, the decrease in efficiency with increasing IL thickness is less pronounced. The TCTA IL thickness should be minimized, as nonradiative quenching inside this interlayer leads to decreasing efficiencies. For further color adjustments toward the warm white point A, the recombination zone should be placed further apart from the green emission layer and the blue emission layer thickness should be enlarged, as the spectrum still contains too much green and too little blue emission at this point. V. EFFICIENT WARM WHITE OLEDS
Using the detailed knowledge on exciton generation and transfer processes, we can tune the color of our OLED stack to reach warm white color point A. Figure 9 shows the stack of our final sample series. Following the results from Secs. IV, we leave out the TCTA IL. Additionally, the TCTA blocker is exchanged by 2,20 ,7,70 -tetrakis-(N,N-diphenylamino)-9,90 spirobifluorene [Spiro-TAD, Lumtec] to enhance hole conduction and to increase luminous efficacy. The SPPO1 blocker is reduced to only 5 nm thickness, because SPPO1 hinders the electron transport. The thicknesses x and y of EML1 and
FIG. 9. (Color online) Layer structure of the final sample series. The thicknesses x and y of the layers EML1 and EML2 have been varied between 1 and 4 nm and 2 and 5 nm, respectively, in steps of 1 nm.
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FIG. 10. (Color online) CIE coordinates of the final sample series at approximately 1000 cd=m2. Same symbols indicate equal EML1 thickness and same colors indicate equal EML2 thickness. With increasing the TCTA:FIr6 layer thickness, the CIE coordinate x decreases, while an increase of the SPPO1:FIr6 layer thickness leads to a decreasing CIE y coordinate. All devices emit colors close to the Planckian locus (black line), and the sample with EML1 = EML2 = 4 nm comes closest to the warm white point A (encircled data point). Luminous efficacy, external quantum efficiency, and the CRI at 1000 cd=m2 of that sample and of the device with EML1 = EML2 = 2 nm (dashed encircled data point) are mentioned.
EML2 have been varied: x = 1, 2, 3, and 4 nm and y = 2, 3, 4, and 5 nm, leading to 16 different devices built within a 4 4matrix. With this method, the recombination zone is shifted, and the overall thickness of FIr6 containing layers is varied. The spectral changes and, hence, changes in the CIE color coordinates are depicted in Fig. 10. With increasing TCTA:FIr6 (EML1) layer thickness, the red emission decreases, while blue and green emissions increase. However, increasing the SPPO1:FIr6 (EML2) layer thickness leads to less green and yellow, but enhanced blue emission. The device with EML1 = EML2 = 4 nm shows the best CIE color coordinates with (0.444;0.409) at 1000 cd=m2, differing from warm white point A by only (–0.004;0.002). Besides very good color coordinates, a CRI of 82 is reached, which makes the OLED suitable for lighting applications. Figure 11 depicts the spectrum of this device. Ir(ppy)3 shows very low contribution to the spectrum due to the fact that FIr6 has a pronounced shoulder in the green emission region. Therefore, blue emitters with emission at even lower wave-
FIG. 11. (Color online) Normalized spectral radiance of the OLED at 1000 cd=m2 with CIE coordinates of (0.444;0.409) and a CRI of 82. The inset shows the spectrum at different current densities from 0.77 mA=cm2 up to 77 mA=cm2 (increasing current in the direction of the arrows). The spectrum is nearly current stable.
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length are needed. With increasing green emission, not only the CRI could be enhanced, but also a much higher power efficiency could be obtained. This OLED reaches 10.0% external quantum efficiency and 17.4 lm=W at 1000 cd=m2. In comparison, the device with 2 nm TCTA:FIr6 and 2 nm SPPO1:FIr6 reaches 12.4% and 22.8 lm=W, which can not only be attributed to the higher green contribution to the spectrum, but also to an enhanced conversion of electrons into photons, which is attributed to the thinner FIr6 layer. As already mentioned, we suspect that SPPO1 hinders the electron transport and that, therefore, the charge carrier balance in our stack is not unity, which reduces the external quantum efficiency.29 The inset of Fig. 11 shows the spectrum of the device with EML1 = EML2 = 4 nm for different current densities from 0.77 mA=cm2 up to 77 mA=cm2, covering a luminance range from 100 cd=m2 up to 10 000 cd=m2. The spectrum is very color stable, with a color shift of only (0.025;–0.011) within the measured luminance range, which is due to a stable position of the recombination zone in the doubleemission layer device. VI. CONCLUSIONS
We have discussed a method to control the exciton generation and transfer between different emitter materials by studying the exciton transfer processes inside the emission layer of a highly efficient white OLED stack. We found that FIr6 promotes an energy back-transfer to the TCTA and SPPO1 host. While the back-transfer on TCTA leads to an enhanced nonradiative recombination, the excitation of SPPO1 could be used by transferring it to Ir(ppy)3. Furthermore, we showed that thin interlayers have the possibility to suppress Fo¨rster and Dexter energy transfer. Nevertheless, Dexter energy transfer over the matrix is possible if the energy levels of the interlayer material are not much higher than those of the emitter molecules and if the triplet lifetimes of the interlayers are high. Using the detailed knowledge on the exciton transfer processes, we presented an OLED stack reaching CIE color coordinates of (0.444;0.409), lying very close to the warm white point A with a high color rendering of 82, fulfilling the color requirements for lighting technologies. At 1000 cd=m2, this OLED reaches 10.0% external quantum efficiency and 17.4 lm=W luminous efficacy. We want to point out that more greenish spectra have the possibility to reach higher efficiencies. By choosing two matrix materials with different conduction properties, our OLED stack reaches high color stability, which is necessary for dimming light. To obtain higher efficiencies with equal or even better color rendering, efficient phosphorescent blue emitters with a spectrum at even lower wavelengths have to be developed.30,31 Besides, there is still a need for further research on electrons conducting matrix materials with high triplet energies and better mobilities to obtain a charge carrier balance of one.32 ACKNOWLEDGMENTS
The work leading to these results has received funding by the European Community’s Seventh Framework Program [Grant No. FP7-224122 (OLED100.eu)].
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