Host to Guest Energy Transfer Mechanism in Phosphorescent and

Sep 24, 2014 - The use of exciplex-forming hosts has recently emerged as an avenue to obtain very high efficiency with phosphorescent dopants in organ...
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Host to Guest Energy Transfer Mechanism in Phosphorescent and Fluorescent Organic Light-Emitting Devices Utilizing ExciplexForming Hosts Dong-Ying Zhou,†,‡ Hossein Zamani Siboni,‡ Qi Wang,‡ Liang-Sheng Liao,*,† and Hany Aziz*,‡ †

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, China ‡ Department of Electrical & Computer Engineering and Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *

ABSTRACT: The use of exciplex-forming hosts has recently emerged as an avenue to obtain very high efficiency with phosphorescent dopants in organic lightemitting devices (OLEDs). The exact electroluminescence (EL) mechanism, however, is still not clearly understood. In this work, we use time-resolved photoluminescence measurements and find that the EL mechanism is primarily based on efficient energy transfer from the exciplex to the emitter guest. By altering the distance between the guest and the exciplex-formation interface, we further uncover that this energy transfer occurs mostly by the Dexter mechanism. The results explain the high efficiency of phosphorescent OLEDs based on an exciplexforming host and shed light on the reasons behind the significant difference in the efficiency of OLEDs utilizing exciplex-forming hosts when used with phosphorescent versus fluorescent emitters.

1. INTRODUCTION Realizing high efficiency from organic light-emitting devices requires that the internal quantum efficiency (IQE) be maximized.1,2 To this end, phosphorescent OLEDs are being pursued. Due to the efficient intersystem crossing in these devices, light can be harvested from both singlet and triplet excitons, and therefore can, in theory, have a 100% IQE.3 Unfortunately, however, the efficiency of phosphorescent OLEDs is reduced by triplet−triplet annihilation (TTA) and triplet−polaron quenching (TPQ) at high driving currents.4,5 Very recently, the use of thermally activated delayed fluorescence (TADF) has emerged as an approach to break the 25% theoretical IQE limit of fluorescent OLEDs and thus can potentially realize high efficiency fluorescent OLEDs.6−8 The TADF is realized by efficient up-conversion from triplet to singlet exited states as a result of their small energy splitting (ΔETS). To achieve this effect, specially designed molecules containing electron-donating and -accepting groups, with steric hindrance in the connector between them, are utilized.9,10 As an alternative to the exquisite molecular design, the use of mixtures of electron donor and acceptor molecules is emerging as an easier approach for achieving TADF.11,12 In this approach, the two molecules (i.e., the donor and the acceptor) can act as an exciplex-forming host that can be then doped by emitter guest materials.13 Although, traditionally, the formation of exciplex species was generally undesired due to their low efficiency and broad emission, and thus their utilization was limited to white emission OLEDs,14 the possibility to obtain © 2014 American Chemical Society

high internal quantum efficiency from them via the upconversion of triplet to singlet excited states has stimulated interest in exciplex emitters recently. Very high efficiency blue, green, and yellow OLEDs based on the exciplex emission were reported recently utilizing this approach.12,15,16 Furthermore, due to their triplet harvesting features, such exciplex-forming hosts were found to be especially advantageous when used as hosts for phosphorescent emitter guests. Park et al. demonstrated a green phosphorescent OLED with a power efficiency of 124 lm/W using a mixture of 4,4′,4″-tris(Ncarbazolyl)-triphenylamine (TCTA) and bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM) as the exciplexforming host of bis(2-phenylpyridine)iridium(III)acetylacetonate [Ir(ppy)2(acac)].17 Seo et al. reported a red phosphorescent OLED with high external quantum efficiency through the exciplex-triplet energy transfer.18 Seino et al. utilized exciplex of di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) and 5′,5″-sulfonyl-di-1,1′,3′,1″-terphenyl (BTPS) to excite blue dopant, breaking the barrier of wide gap and high triplet energy.19 In previous studies of electroluminescence (EL) mechanism in state-of-the art exciplexforming host:guest systems, the dependence of emission spectra on doping concentration and the overlap of the exciplex emission with the guest absorption suggested energy Received: August 14, 2014 Revised: September 23, 2014 Published: September 24, 2014 24006

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Figure 1. (a) Molecular structures and (b) LUMO and HOMO energy levels of m-MTDATA, DCJTB, Ir(piq)3, and BPhen. (c) Energy levels of the singlet and the triple excited states of m-MTDATA, BPhen, and exciplex-forming host. The solid and dotted lines represent singlet and triplet energy levels, respectively.

The four OLEDs are characterized by having m-MTDATA and BPhen either in adjacent layers or intermixed in the same layer, and thus are capable of forming exciplex species. The general structure is “ITO/MoO 3 (2.5 nm)/m-MTDATA(30 nm)/EML(10 nm)/BPhen(30 nm)/LiF(1 nm)/Al(60 nm)”. In devices A and B, the EML is m-MTDATA(5 nm)/BPhen(5 nm) bilayer and m-MTDATA:BPhen(10 nm) mixed layer with 1:1 volume ratio, respectively. Devices C and D contain a fluorescent dopant of 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and a phosphorescent dopant of tris[1-phenylisoquinolinato-C2,N] iridium (Ir(piq)3) in their emitting layers, respectively. In device C, the DCJTB is doped into the mixed host of mMTDATA:BPhen (1:1, 10 nm) at a concentration of 2% by volume. Whereas in device D, the Ir(piq)3 is doped into the mixed host of m-MTDATA:BPhen (1:1, 10 nm) at a concentration of 5% by volume. For comparison, we also fabricate and test devices E and F using a common host tris(8hydroxyquinolinato) aluminum(III) (Alq) for DCJTB and 1,3,5-tris(2-N-phenylbenzimidazolyl)benzeneare (TPBi) for Ir(piq)3, respectively, to be used as nonexciplex control. Device E has a structure of “ITO/MoO 3 (2.5 nm)/NPB(40 nm)/Alq:DCJTB(10 nm, 2% by volume)/Alq(30 nm)/LiF(1 nm)/Al(60 nm)”, and device F has a structure of “ITO/ MoO3(2.5 nm)/CBP(35 nm)/TPBi:Ir(piq)3(10 nm, 5% by volume)/TPBi(30 nm)/LiF(1 nm)/Al(60 nm)”, in which NPB is N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine and CBP is 4,4′-bis(9-carbazolyl)biphenyl. The structures of devices E and F have been typically studied in the published literature.21,22 The detailed layer structures of these devices are summarized in Table 1.

transfer occurred from exciplex to guest. However, the reason behind the higher efficiency obtained on using exciplex-forming hosts in comparison with common hosts (i.e., non-exciplexforming) is not clearly understood. In this work, we use photoluminescence (PL) (both steady state and time-resolved) and delayed electroluminescence (EL) measurements to study phosphorescent and fluorescent OLEDs using an exciplex-forming host. We find that in an exciplex-forming host:guest system, excitons are predominantly formed on the host and not on the guest. The efficiency enhancement when used in phosphorescent OLEDs is caused by efficient energy transfer from the exciplex to the dopant and lower triplet-polaron quenching effects. This energy transfer is found to occur mainly by Dexter mechanism. The results explain the high efficiency of phosphorescent OLEDs based on exciplex-forming hosts and shed light on the reasons behind the significant difference in the efficiency of OLEDs utilizing exciplex-forming hosts when used with fluorescent versus phosphorescent emitters.

2. EXPERIMENTAL SECTION In this work, we use 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)-triphenylamine (m-MTDATA) and 4,7-diphenyl-1,10phenanthroline (BPhen) as the hole-transporting material (HTM) and the electron-transporting material (ETM), respectively, to form the exciplex. The molecular structures of the materials are shown in Figure 1a. The OLEDs were fabricated on prepatterned indium tin oxide (ITO) glass substrates. Prior to film deposition, the ITO substrates were carefully cleaned in ultrasonic baths of acetone, isopropanol, sodium hydroxide aqueous solution, and deionized water in sequence. The films were deposited by vacuum thermal evaporation under the pressure of 4 × 10−6 Torr. After device fabrication, the electrical and the optical properties were measured under the protection of nitrogen gas. The EL characteristics were measured using a programmable power source meter (Agilent 4156C) and a photometer (Minolta Chroma CS-100). The EL spectra were obtained using a fiber spectrometer (Ocean Optics 2000). Time-resolved PL was collected using an Edinburgh Instruments FL920 spectrometer equipped with a pulsed excitation laser of 375 nm. The delayed EL characterization setup was as reported in our previous work.20

Table 1. Layer Structures of the OLEDs device

layer structure

A

ITO/MoO3(2.5 nm)/m-MTDATA(35 nm)/BPhen(35 nm)/LiF (1 nm)/Al(60 nm) ITO/MoO3(2.5 nm)/m-MTDATA(30 nm)/m-MTDATA:BPhen (10 nm)/BPhen(30 nm)/LiF(1 nm)/Al(60 nm) ITO/MoO3(2.5 nm)/m-MTDATA(30 nm)/mMTDATA:BPhen:DCJTB(10 nm, 2% by volume)/BPhen(30 nm) /LiF(1 nm)/Al(60 nm) ITO/MoO3(2.5 nm)/m-MTDATA(30 nm)/mMTDAT:BPhen:Ir(piq)3(10 nm, 5% by volume)/BPhen(30 nm) /LiF(1 nm)/Al(60 nm) ITO/MoO3(2.5 nm)/NPB(40 nm)/Alq:DCJTB(10 nm, 2% by volume)/Alq(30 nm)/LiF(1 nm)/Al(60 nm) ITO/MoO3(2.5 nm)/CBP(35 nm)/TPBi:Ir(piq)3(10 nm, 5% by volume)/TPBi(30 nm)/LiF(1 nm)/Al(60 nm)

B C D E

3. RESULTS AND DISCUSSION Four different OLEDs, all having the same general structure but different emitting layer (EML) configurations, are fabricated.

F

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The EL spectra of devices A−F are shown in Figure 2. Devices A and B exhibit yellow exciplex emission with an

Figure 2. Normalized EL spectra of devices A−F with different EMLs.

emission peak at 560 nm. The peak energy (∼2.2 eV) closely corresponds to the energy offset between the lowest unoccupied molecular orbital (LUMO) level of BPhen (∼2.9 eV) and the highest occupied molecular orbital (HOMO) level of m-MTDATA (∼5.1 eV), verifying the exciplex is formed through charge transfer from BPhen to m-MTDATA.23 This exciplex EL has been demonstrated as the delayed fluorescence arising from the easy reverse intersystem crossing process from triplet to singlet excited states due to their small energy splitting (ΔETS = 1.12 eV).24 The exciplex emission is observed regardless of whether the HTM and the ETM are present in separate adjacent layers (device A) or intermixed in one layer (device B). The fact that no emission of m-MTDATA or BPhen is observed proves that the injection of electrons from BPhen to m-MTDATA and of holes from m-MTDATA to BPhen is negligible, indicating that electrons reside solely in BPhen and holes in m-MTDATA. For m-MTDATA, the singlet and triplet energy is 3.1 and 2.67 eV, respectively.25 For BPhen, the singlet and triplet energy is 3.2 and 2.6 eV, respectively,26 as shown in Figure 1c. On the other hand, devices D and F show similar EL spectra with emission peak at 620 nm which corresponds to Ir(piq)3 emission. The absence of exciplex emission in devices D and F suggests that energy transfer from the exciplex to the guest Ir(piq)3 is very efficient or that the e− h recombination occurs on the guest directly in these devices. However, devices C and E show distinct EL spectra with emission peak at 596 and 620 nm, respectively. The blue shift in EL spectra of device C suggests energy transfer from the exciplex to the fluorescent emitter DCJTB is incomplete, resulting in additional emission from the exciplex in the DCJTB-doped m-MTDATA:BPhen emitting layer. The overlap of the emission bands of exciplex (i.e., 560 nm) and DCJTB (i.e., 620 nm) produces a spectrum with one emission peak. Current density versus voltage (J−V), luminance versus voltage (L−V), and current efficiency versus current density (CE−J) characteristics of the doped devices C−F are shown in Figure 3 (these EL characteristics of the nondoped devices A and B are shown in Supporting Information Figure S1). The current efficiency of device B is 7.8 cd/A at 10 mA/cm2, which can be further improved in the cases of optimizing the change balance in the device.24 Devices C and D, with the exciplexforming host, show lower driving voltage than devices E and F.

Figure 3. (a) J−V−L and (b) CE−J characteristics of devices C−F.

According to the L−V characteristics (Figure 3a), low turn-on voltage (defined as voltage at 1 cd/m2) of devices C and D is in the range of 2.3−2.5 V. Such low turn-on voltage is typical of exciplex-based OLEDs in which holes and electrons on the HOMO of the HTM and the LUMO of the ETM, respectively, recombine bimolecularly.23 Clearly, the devices demonstrate different current efficiency (Figure 3b). Compared to device F using TPBi as the host of Ir(piq)3, device D using the exciplexforming host shows a remarkable improvement in current efficiency. For instance, at the current density of 20 mA/cm2, the current efficiencies of devices D and F are 10.4 cd/A and 6.5 cd/A, respectively. However, in the cases of DCJTB as dopant, device C using the exciplex-forming host shows a lower current efficiency than device E with the Alq host. The current efficiencies of devices C and E are 2.2 cd/A and 3.5 cd/A, respectively, at the current density of 20 mA/cm2. In order to investigate if energy transfer from the exciplex to the dopant occurs and whether it plays a role in the efficiency enhancement, time-resolved PL measurements are conducted. Figure 4 shows PL versus time following excitation by a pulsed laser (λ = 375 nm, pulsed width 71 ps) collected from the mixed m-MTDATA:BPhen and the Ir(piq) 3 -doped mMTDATA:BPhen layers. The PL transients are measured at the exciplex PL peak, i.e., 545 nm. The small blue shift in PL versus EL can be attributed to optical interference effects due to the presence of the reflective cathode in the case of the OLEDs (i.e., Figure 2) which are known to slightly alter the peak position.27 As can be seen from Figure 4, the PL decay rate from m-MTDATA:BPhen layer is very slow (∼2 μs). The slow24008

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the Ir(piq)3 into this mixed m-MTDATA:BPhen layer results in a shorter exciplex lifetime, indicating that efficient energy transfer from the exciplex occurs, more likely from its triplet states, to the Ir(piq)3. For an investigation of the exact energy transfer mechanism from the exciplex to the guest, a second batch of devices with a structure of ITO/MoO3(2.5 nm)/m-MTDATA(30 nm)/mMTDATA:Ir(piq)3(10 nm, 5% by volume)/m-MTDATA(x nm)/BPhen(35 nm)/LiF(1 nm)/Al(60 nm) is fabricated (schematic diagrams shown in Figure 5a). In these devices, an m-MTDATA layer of various thickness is inserted between the Ir(piq)3-doped m-MTDATA emitting layer and the BPhen layer to act as a spacer. Its thickness is changed from 0 to 10 nm. The J−V−L characteristics of these devices are shown in Figure 5b. The gradual increase of driving voltage at higher currents is caused by the increasing thickness of the mMTDATA spacer layer. The turn-on voltage of luminance is very low, again proving that e−h recombination occurs bimolecularly (holes on m-MTDATA and electrons on BPhen), producing exciplex species directly. It should be pointed out that as Ir(piq)3 has a close HOMO energy level to that of m-MTDATA (HOMO energy levels shown in Figure 1b), it does not act as a charge trap in the doped layer. In the normalized spectra of Figure 5d, the emission peaks at 620 and 560 nm correspond to emission of Ir(piq)3 and the exciplex, respectively. In the absence of the m-MTDATA spacer layer (i.e., 0 nm), the emission is exclusively from Ir(piq)3. Increasing

Figure 4. Time-resolved PL of the exciplex emission detected at 545 nm from the m-MTDATA:BPhen (10 nm) and mMTDATA:BPhen:Ir(piq)3 (10 nm, 5% by volume) layers. The inset shows the normalized steady state PL spectra of m-MTDATA:BPhen and m-MTDATA:BPhen:Ir(piq)3 layer excitation by 375 nm and the enlarged PL spectra of the m-MTDATA:BPhen:Ir(piq)3 layer in the range 490−570 nm.

decaying photoluminescence at 545 nm is consistent with it mostly arising from delayed fluorescence due to reverse intersystem crossing of triplet states to singlet states.24 Doping

Figure 5. (a) Schematic diagram, (b) J−V−L characteristics, (c) CE-J characteristics, and (c) normalized EL spectra of Ir(piq)3 devices with various thicknesses of the m-MTDATA spacer layer. 24009

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transfer, the total spin multiplicity of the system is conserved; energy transfer from exciplex singlet and triplet states will produce singlets and triplets on the acceptor molecule, respectively. For a phosphorescent guest, any singlets that are formed due to energy transfer from the exciplex are quickly converted into triplets due to the efficient intersystem crossing on the phosphorescent guest material. Therefore, both singlet and triplet exciplex species on the host produce triplet excitons on the guest, that can decay radiatively. In the case of a fluorescent guest, the energy transfer occurs through the Dexter mechanism as well, again producing singlet and triplet excitons on the guest material. However, as in this case, intersystem crossing is inefficient, and the decay of triplet states occurs by nonradiative transitions; only singlet excitons will produce photons. This explains the significantly lower OLED efficiency that can be obtained when using fluorescent guests, as opposed to phosphorescent guests, in exciplex-forming hosts. In the previous argument, it is presumed that e−h recombination happens mostly on the host rather than on the guest. In order to verify if this is indeed the case, delayed EL measurements of devices A, B, and D are carried out.20 In this measurement technique, devices are driven with a forward bias pulse of 7.5 V and a pulse width of 0.5 ms, which is long enough to promote prompt EL to its steady state intensity. To avoid collection of prompt EL signal, optical chopper opens around 0.1−0.3 ms after the end of the forward bias to collect delayed EL. In general, the delayed EL can arise from one or several of the following processes: (i) direct recombination of detrapped charges at the end of forward bias; (ii) slow migration of host excitons to the proximity of guest molecules and subsequent host to guest energy transfer; and (iii) triplet− triplet annihilation due to the fusion of host triplet excitons.28−31 Figure 7 shows time-resolved delayed EL

the m-MTDATA thickness results in a gradual decrease in Ir(piq)3 emission and an increase in yellow emission from the exciplex. As can be seen, the red emission decreases very quickly as the thickness of the spacer increases, and becomes undetectable as the thickness reaches 4 nm, leaving only the yellow emission. Due to the unipolar hole transporting nature and low LUMO value (∼2.0 eV) of m-MTDATA, e−h recombination within the Ir(piq)3-doped m-MTDATA layer is insignificant. We should also note that the shape of the EL spectra of these devices does not change with driving voltage (see Figure S2 in the Supporting Information), indicating that electron tunnelling from the BPhen layer into the Ir(piq)3doped m-MTDATA layer across the m-MTDATA spacer is also insignificant. Therefore, e−h recombination occurs across the m-MTDATA/BPhen interface producing exciplex, and the energy is transferred from the exciplex to the Ir(piq)3. The red emission is therefore the result of energy transfer from the exciplex. As the thickness of the spacer gradually increases, and the Ir(piq)3-doped layer becomes increasingly far from the exciton formation region, energy transfer from the exciplex to Ir(piq)3 becomes inefficient. After 4 nm, there is nearly no energy transfer from the exciplex of m-MTDATA/BPhen bilayer to the Ir(piq)3. The very short energy transfer range suggests it occurs via Dexter energy transfer. In general, the exciplex species will include both singlet and triplet states. Therefore, in order to differentiate between the roles of the two types of exciplex states, we also fabricate and test devices of the same structure but with a fluorescent material DCJTB instead of Ir(piq)3. The device structure is ITO/MoO3(2.5 nm)/m-MTDATA(30 nm)/mMTDATA:DCJTB(10 nm, 2% by volume)/m-MTDATA(x nm)/BPhen(35 nm)/LiF(1 nm)/Al(60 nm). Again here, increasing the thickness of the m-MTDATA spacer layer leads to a change in the emission color from red (DCJTB) to yellow (exciplex). The maximum distance for efficient energy transfer from the exciplex species (at the m-MTDATA/BPhen interface) to the fluorescent material, beyond which emission from the fluorescent dopant becomes undetectable, is again on the order of a few nanometers (∼1−2 nm) as shown in Figure 6. This suggests that the mechanism of energy transfer from the exciplex species to the fluorescent material is the same as in the case of the phosphorescent material. Therefore, we can see from the results that the main energy transfer mechanism is the Dexter type. As in Dexter energy

Figure 7. Time-resolved delayed EL intensity of devices A, B, and D with and without the reverse bias voltage during the measurement. The dotted line and the solid line are measured without and with the reverse voltage, respectively. The applied reverse bias voltage is −8 V with a pulse time of 0.5 ms.

intensity of devices A, B, and D. As can be seen from the figure, device B with the m-MTDATA:BPhen mixed layer has a higher delayed EL intensity than device A with the mMTDATA/BPhen bilayer, verifying that the exciplex concentration in the mixed layer device (i.e., device B) is higher than in the bilayer device (i.e., device A) mainly due to the increased ETM/HTM interfacial area. Interestingly, device D with the

Figure 6. Normalized EL spectra of DCJTB devices with various thicknesses of the m-MTDATA spacer layer. 24010

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characteristics of the DCJTB-doped m-MTDATA devices with various thicknesses of the m-MTDATA spacer layer. This material is available free of charge via the Internet at http:// pubs.acs.org.

doped phosphorescent Ir(piq)3 emitter shows higher delayed EL intensity in comparison to device B which is due to the harvest of triplet excitons by Ir(piq)3. In order to test if the delayed EL emission in device D is the product of charge trapping on the guest and thus from e−h recombination on the guest directly (as opposed to e−h recombination on the host), a 0.5 ms reverse bias pulse of −8 V is applied during the delayed EL signal collection. In our previous work, it was shown that direct charge trapping and exciton formation on the guest typically manifests itself with the appearance of spike at the beginning of the reverse bias. The appearance of spike is an indication of the increased e−h recombination of the detrapped charges under the influence of the reverse bias.31 As can be seen from the figure, the delayed EL signal in device D does not show a noticeable spike at the beginning of the reverse bias suggesting that direct exciton formation on the guest plays an insignificant role on the device emission. On the other hand, the delayed EL intensity in device D decreases at the beginning of the reverse bias and rebounds at the end of the reverse bias, a behavior that is also observed in device B, and is mainly due to electric field-induced quenching of exciplex singlets. This temporary reduction in the delayed EL intensity during the reverse bias pulse and the recovery at the end of the bias is an indication of TTA of exciplex triplets. It should be pointed out that guest−guest TTA in the millisecond time frame is unlikely, and therefore, TTA is mainly due to the fusion of long-lived exciplex triplets indicating a presence of high concentration exciplex in the EML. Therefore, the results suggest that e−h recombination occurs on the exciplex-forming host even when a guest material is doped into it. This is then followed by energy transfer to the guest. It should be noted that although the delayed EL signals of devices B and D show a recovery at the end of the reverse bias, this recovery is not complete, contrary to devices with conventional host:guest systems.32 The observed incomplete recovery at the end of the reverse bias could be due to electric field induced quenching of exciplex triplets. Because in the exciplex-forming host the singlet−triplet energy splitting is small, the binding energy of triplet excitons is not significantly higher than that of singlet excitons, we can therefore expect them to be similarly susceptible to dissociation by high enough electric fields similar to their singlet counterparts. This may explain the incomplete recovery of delayed EL at the end of the reverse bias pulse observed in this case.

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Corresponding Authors

*Phone: 0086-512-65880945. E-mail: [email protected]. *Phone: 001-519-8884567x36848. E-mail: h2aziz@uwaterloo. ca. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Natural Science Foundation of China (No. 61177016), the Nature Science & Engineering Research Council of Canada (NSERC), and Canada Foundation of Innovation (CFI). This is also a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and by the SUN-WIN Collaboration Program of Suzhou Industrial Park, as well as by the Fund for Excellent Creative Research Teams of Jiangsu Higher Education Institutions.

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REFERENCES

(1) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% Internal Phosphorescence Efficiency in an Organic LightEmitting Device. J. Appl. Phys. 2001, 90, 5048−5051. (2) Reineke, S.; Gregor Schwartz, F. L.; Seidler, N.; Walzer, K.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234−238. (3) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. High-Efficiency Fluorescent Organic Light-Emitting Devices Using a Phosphorescent Sensitizer. Nature 2000, 403, 750−753. (4) Reineke, S.; Walzer, K.; Leo, K. Triplet-Exciton Quenching in Organic Phosphorescent Light-Emitting Diodes with Ir-Based Emitters. Phys. Rev. B 2007, 75, 125328. (5) Song, D.; Zhao, S.; Luo, Y.; Aziz, H. Causes of Efficiency Roll-Off in Phosphorescent Organic Light Emitting Devices: Triplet-Triplet Annihilation Versus Triplet-Polaron Quenching. Appl. Phys. Lett. 2010, 97, 243304. (6) Endo, A.; Ogasawara, M.; Takahashi, A.; Yokoyama, D.; Kato, Y.; Adachi, C. Thermally Activated Delayed Fluorescence from Sn4+Porphyrin Complexes and Their Application to Organic Light Emitting Diodes-A Novel Mechanism for Electroluminescence. Adv. Mater. 2009, 21, 4802−4806. (7) Deaton, J. C.; Switalski, S. C.; Kondakov, D. Y.; Young, R. H.; Pawlik, T. D.; Giesen, D. J.; Harkins, S. B.; Miller, A. J. M.; Mickenberg, S. F.; Peters, J. C. E-Type Delayed Fluorescence of a Phosphine-Supported Cu2(μ-NAr2)2 Diamond Core: Harvesting Singlet and Triplet Excitons in OLEDs∥. J. Am. Chem. Soc. 2010, 132, 9499−9508. (8) Lee, S. Y.; Yasuda, T.; Nomura, H.; Adachi, C. High-Efficiency Organic Light-Emitting Diodes Utilizing Thermally Activated Delayed Fluorescence from Triazine-Based Donor-Acceptor Hybrid Molecules. Appl. Phys. Lett. 2012, 101, 093306. (9) Méhes, G.; Nomura, H.; Zhang, Q.; Nakagawa, T.; Adachi, C. Enhanced Electroluminescence Efficiency in a Spiro-Acridine Derivative through Thermally Activated Delayed Fluorescence. Angew. Chem., Int. Ed. 2012, 51, 11311−11315. (10) Nakagawa, T.; Ku, S. Y.; Wong, K. T.; Adachi, C. Electroluminescence Based on Thermally Activated Delayed Fluorescence Generated by a Spirobifluorene Donor−Acceptor Structure. Chem. Commun. 2012, 48, 9580−9582.

4. CONCLUSION In summary, it is found that, in the exciplex-forming host:guest system, excitons are directly formed mostly on the host and not on the guest. The efficiency enhancement of the phosphorescent OLED is attributed to efficient energy transfer from the exciplex to the dopant and the reduced triplet-polaron quenching effect. This energy transfer occurs mainly by Dexter mechanism. The results explain the high efficiency of phosphorescent OLEDs based on an exciplex-forming host and shed light on the reasons behind the significant difference in the efficiency of OLEDs utilizing exciplex-forming hosts when used with phosphorescent versus fluorescent emitters.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

EL characteristics of devices A and B, EL spectra of the Ir(piq)3-doped m-MTDATA devices with the 2 and 4 nm mMTDATA spacer layer at various driving voltages, and EL 24011

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp508228z | J. Phys. Chem. C 2014, 118, 24006−24012