Alq3 Interface in an Organic Light

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Exciplex Formations at the HTL/Alq3 Interface in an Organic Light-Emitting Diode: Influence of the Electron-Hole Recombination Zone and Electric Field Naoki Matsumoto†,‡ and Chihaya Adachi*,‡ Tosoh Corporation, Nanyo Research Laboratory, 4560 Kaisei, Shunan, Yamaguchi 746-8501, Japan and Center for Future Chemistry, Kyushu UniVersity, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ReceiVed: December 23, 2009

In this work, we investigate the influence of weak intermolecular interactions, which have not previously been carefully considered, in hole transport material (HTM)/tris(8-hydroxyquinoline)aluminum (Alq3)-based organic light-emitting diodes (OLEDs). Although such weak interactions quench Alq3 fluorescence, no significant spectral shift is identified. Electroluminescence of OLEDs containing HTM:Alq3 codeposited (mixed) emitter is quenched by the formation of such exciplexes. In general, the electroluminescence quantum efficiency of OLEDs correlates closely with the photoluminescence quantum yields of HTM:Alq3 codeposited films. In contrast, in an OLED containing a layered structure of HTM/Alq3, exciplexes are less effective at quenching the electroluminescence of Alq3. Because exciplexes form only at the interface between the HTM and Alq3 layers in HTM/Alq3-based OLEDs, exciplex formation is affected not only by the electron donating nature of the HTM but also by the position of the electron-hole recombination zone and the application of an external electric field during OLED operation. 1. Introduction Multilayer structures composed of an emitting layer (EML) and charge transport layers have been the core focus of research in organic light emitting diodes (OLEDs) since the first efficient multilayered OLEDs were reported.1,2 Although multilayer structures efficiently confine charge carriers and molecular excitons, OLED characteristics are largely affected by the chemical and physical interactions at organic/organic interfaces. One of the significant interactions observed at organic/organic interfaces is the formation of a charge-transfer excited state complex known as an exciplex.3–10 Exciplexes are formed between two different molecules, such as an electron donor and an electron acceptor, where one molecule is in an excited state and the other is in its ground state. Exciplex formation can usually be detected by a red shift of the emission band compared with those of the intermolecular interaction-free components. In OLEDs consisting of an EML, a hole transport layer (HTL) and an electron transport layer (ETL), there is unavoidable exciplex formation at the ETL/EML or the HTL/EML interfaces because HTL and ETL usually have electron-donating and electron-accepting natures, respectively. Tris(8-hydroxyquinoline)aluminum (Alq3), a well-known green emitter in OLEDs, has an electron-accepting nature. In a double-layered device consisting of HTL and Alq3 layers, a remarkable change in emission color originating from exciplex formation at the HTL/Alq3 interface was observed when a strong electron-donating material such as 4,4′,4′′-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (m-MTDATA)11 was used as the HTL. In a previous work,12 we reported on the exciplex formation between Alq3 and triarylamine-based hole transport materials (HTMs) with various highest occupied molecular orbital (HOMO) levels. The intermolecular interaction between * To whom correspondence should be addressed. E-mail: adachi@ cstf.kyushu-u.ac.jp. † Tosoh Corporation, Nanyo Research Laboratory. ‡ Center for Future Chemistry, Kyushu University.

Alq3 and the HTMs correlated with the HOMO levels of the HTMs. From the analysis of the photoluminescence (PL) quantum yields and transient PL decay characteristics of HTM: Alq3 codeposited (mixed) films, we clarified the presence of weak exciplexes which quench Alq3 fluorescence without a significant spectral red shift. In the HTL/Alq3-based OLEDs, the formation of these weak exciplexes has not been discussed because such exciplexes are not directly observed in the electroluminescence (EL) spectra. The present work investigates the characteristics of such weak exciplexes at the HTL/Alq3 interface in OLEDs. In the layered structure of HTL/Alq3-based OLEDs, Alq3 molecules only contact with the HTM(s) at the interface between the HTL and Alq3 layer. The influences of the position of the electron-hole recombination zone and external electric fields on the exciplex formation at the HTL/Alq3 interface were investigated. We observed that the intermolecular interactions between the HTM and Alq3 excitons at the HTL/Alq3 interface were affected not only by the electron-donating nature of the HTM, but also by the position of the electron-hole recombination zone and the direction of the applied electric field. In double-layered HTL/ Alq3 OLEDs, the decrease in the EL efficiency originating from exciplex formation was suppressed by shifting the electron-hole recombination zone inside the Alq3 layer. 2. Experimental Section The chemical structures and HOMO and lowest unoccupied molecular orbital (LUMO) energy levels, of Alq3, 4,4′-bis(9carbazolyl)-1,1′-biphenyl (CBP), 9,9-bis(4-diphenylamino-1,1′biphenyl-4′-yl)-9H-fluorene (FL1), 4,4′-bis[1-naphthyl(phenyl)amino]-1,1′-biphenyl (NPD), 2,7-bis(diphenylamino)-9,9bis(1,1′-biphenyl-4-yl)-9H-fluorene (FL2) and 2,7-bis[di(4methylphenyl)amino]-9,9-bis(1,1′-biphenyl-4-yl)-9H-fluorene (FL3) used in this study are shown in Figure 1. All of the materials were purified by train sublimation. In the HTM:Alq3 codeposited film, NPD, FL2, and FL3 form exciplexes with Alq3.12

10.1021/jp9121062  2010 American Chemical Society Published on Web 02/24/2010

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Figure 1. Chemical structures and energy levels of Alq3 and hole transport materials.

The HOMO levels of all of the materials were estimated from the ionization potential energies (Ip) of their films. Ip values were measured by ultraviolet photoelectron spectroscopy (AC-3, Riken Keiki Co.). All of the values were obtained on the same spectrometer, ensuring that the comparison of the energy levels is consistent. The LUMO levels were estimated by subtracting the optical energy gaps (Eg) from the HOMO energies. The Eg values were determined from the energy at the onset of absorption in the UV spectra of the films. The optical characteristics of the organic films were evaluated from the PL spectra, absolute PL quantum yields (ΦPL) and fluorescence lifetimes (τf). The films were formed on quartz and silicon substrates by thermal vacuum deposition. The PL spectra of these films were recorded on a FP-6500-A-51 (Jasco Co.) spectrophotometer. The ΦPL values were measured with use of an absolute PL quantum yield measurement system (Hamamatsu C9920-02), and τf was measured with a streak camera (Hamamatsu C4334) in a vacuum using a N2 gas laser (MNL200, Laser Technik Berlin, λ ) 337 nm) as the excitation source. OLEDs were fabricated by conventional vacuum deposition of the organic layers and a cathode layer on a precleaned indium tin oxide (ITO)-coated glass substrate (ITO thickness ) 110 nm). The active area of the devices was 1 mm in diameter. The current density (J), voltage (V), and luminance (L) characteristics of the OLEDs were measured in an ambient atmosphere using a semiconductor parameter analyzer (Agilent E5273A) and an optical power meter (Newport 1930C). The EL spectra of the OLEDs were obtained on a multichannel spectrometer (Ocean Optics UBS2000). Devices for determining the effect of electric field on PL characteristics were fabricated as follows. HTL and Alq3 layers were formed on a precleaned glass substrate coated with a 110nm thick layer of ITO by thermal vacuum deposition. On top of the organic layers, a MgAg (10:1, 100 nm) layer and a Ag layer (10 nm) were deposited. Finally, glass encapsulation of the devices was performed in a dry glovebox. The active area of the devices was 2 × 2 mm2. The effects of an electric field on the PL characteristics of the devices were measured under an applied electric field, using a streak camera with a N2 gas laser as the excitation source. 3. Results and Discussion 3.1. EL Characteristics of HTM/Alq3-Based OLED. Two different OLED structures were fabricated (Figure 2): the structure of device type I was ITO/copper phthalocyanine (CuPc)(20 nm)/HTL(30 nm)/Alq3(50 nm)/MgAg(100 nm)/ Ag(10 nm) and the structure of device type II was ITO/CuPc(20 nm)/FL1(30 nm)/10 mol %-HTM:Alq3(20 nm)/2,9-dimethyl-

Figure 2. Schematic diagrams showing the structures of type I and II devices.

Figure 3. ηext of OLEDs and ΦPL of 10 mol %-HTM:Alq3 codeposited films as a function of the HOMO levels of the HTMs.

4,7-diphenyl-1,10-phenanthroline (BCP)(10 nm)/Alq3(20 nm)/ MgAg(100 nm)/Ag(10 nm). In each case, CuPc was used as a hole injection layer. FL1, NPD, FL2, and FL3 were used as different HTLs in type I devices. In the type II devices, BCP was the hole blocking layer, FL1, which has no intermolecular interaction with Alq3, was the HTL, and the EML was codeposited 10 mol %-HTM:Alq3. Figure 3 shows the maximum external EL quantum efficiencies (ηext) of the type I and II devices plotted against the HOMO level of the HTMs. The ΦPL values of the 10 mol %-HTM:Alq3 films are also plotted in Figure 3. The lower ΦPL values of the NPD, FL2 and FL3doped Alq3 films are attributed to exciplex formation.12 In type II devices that contain the HTM:Alq3, codeposited EML, NPD, FL2, and FL3 showed lower ηext than that of FL1 having no intermolecular interaction with Alq3. The EL spectra of all of the devices were identical (Figure 4). These results agree well with the PL characteristics of the HTM:Alq3 codeposited films. Therefore, the lower ηext of the OLEDs can be explained in terms of exciplex formation between the HTM and Alq3 in the HTM:Alq3 codeposited layer. The theoretical ηext values of the type II devices are also shown in Figure 3. These values were

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Figure 4. EL spectra of type I and II devices at a current density of 10 mA/cm2.

obtained by the following equation: ηext ) γηpηrΦPL, where γ is the probability of carrier recombination, ηp is the light outcoupling efficiency and ηr is the production efficiency of a singlet or triplet exciton. Assuming γ ) 1.0, ηp ) 0.20 and ηr ) 0.25, ηext is calculated to be 1.0% (FL1), 0.85% (NPD), 0.80% (FL2), and 0.45% (FL3) using the ΦPL values obtained for the 10 mol %-FL1, NPD, FL2 and FL3:Alq3 codeposited films. The experimental ηext of the type II devices that contain a 10 mol %-NPD, FL2, and FL3:Alq3 codeposited EML are lower than the theoretical values. This suggests that EL quenching originating from the formation of exciplexes between Alq3 and the HTMs are enhanced by other factors such as application of an external electric field. Compared to the type II devices, exciplex formation was less effective at decreasing the ηext of the type I devices. This tendency can be understood by considering the exciplex formation region in the bulk of the EMLs. In the type I devices, exciplexes are formed only at the interface between the HTL and Alq3 layers. However, in the type II devices, exciplexes can form in any part of the EML. Therefore, the production ratio of exciplexes to Alq3 excitons in type I devices would be lower than that in type II devices. It should be noted here that the decrease in ηext originating from exciplex formation was not observed for the NPD/Alq3-based type I OLED. In the following sections, the reason for the lack of exciplex formation at the NPD/Alq3 interface in the NPD/Alq3-based OLED is discussed with respect to the electron-hole recombination zone in the Alq3 layer and the effect of an applied electric field. 3.2. Electron-hole Recombination Zone of HTL/Alq3Based OLEDs. In the HTL/Alq3-based OLEDs, the electron-hole recombination zone is located on the Alq3 side close to the HTL/ Alq3 interface due to the electron transporting nature of Alq3 and the electron blocking nature of the HTMs.13,14 Because exciplexes are formed only at the interface between the HTL and Alq3 layer in the HTL/Alq3-based OLEDs, the effects of exciplex formation on EL efficiency would be markedly dependent on the concentration of Alq3 excitons produced at the HTL/Alq3 interface. The concentration of Alq3 excitons at the HTL/Alq3 interface is related to the location and width of the electron-hole recombination zone in the Alq3 layer. To investigate the influence of the HTM on the concentration of Alq3 excitons produced at the HTL/Alq3 interface, we analyzed the electron-hole recombination zone by doping a dye-sensitizer (rubrene, which has yellow fluorescence) locally in the Alq3 layer.13 The device structure was ITO/CuPc(20 nm)/FL-3(20

Figure 5. EL spectra of rubrene-doped OLEDs: x ) 0, 5, 15, and 25 nm.

nm)/HTM(10 nm)/Alq3(x nm)/1 mol %-rubrene:Alq3(5 nm)/ Alq3(45-x nm)/MgAg /Ag, where FL1 (HOMO ) -5.7 eV), NPD (HOMO ) -5.5 eV), and FL3 (HOMO ) -5.3 eV) were used as a HTL, and x was 0, 5, 10, 15, 20, or 25 nm. The EL spectra of these OLEDs at a current density of 10 mA/cm2 (x ) 0, 5, 15, and 25 nm) are shown in Figure 5. The emission peaks of Alq3 and rubrene are 523 and 556 nm, respectively. The intensity of Alq3 emission changed gradually as x varied. In all of the devices, the ratio of emission intensity of rubrene to Alq3 was the highest when x was 0 nm. As x was increased, the rubrene-doped layer was shifted away from the HTL/Alq3 interface and the emission from Alq3 strengthened. These results primarily indicate that electron-hole recombination occurs close to the HTL/Alq3 interface, which is in good agreement with previous reports.13,14 The ratio of emission intensity of rubrene to Alq3 showed some dependence on the HOMO level of the HTM. When x was 0 nm, the ratio of emission intensity of rubrene to Alq3 for the OLED containing FL1 was slightly lower than that for the OLED containing NPD and FL3. In contrast, the ratio of emission intensity of rubrene to Alq3 in the OLED containing FL1 was slightly higher than that in the OLED containing NPD and FL3 when x was 5 to 25 nm. Figure. 6 shows the maximum ηext values of the OLEDs plotted against the doping position of rubrene (x). The maximum ηext values for the FL1, NPD, and FL3 devices gradually decreased with increasing x. This observation is supported by the fact that electron-hole recombination occurs close to the HTL/Alq3 interface and the ΦPL value of rubrene doped into an Alq3 matrix is higher than that of Alq3. Here, we estimate the width of the electron-hole recombination zone in these OLEDs using this experimental data. Assuming that the electron-hole recombination zone expands exponentially from the HTM/Alq3 interface to the cathode, ηext(x) of the rubrenedoped OLEDs can be expressed by the following:

[

ηext(x) ∝ ΦPL(Alq3) 1 -

d x exp L L

( )] + d x ΦPL(rubrene) exp L L

( )

where ΦPL(Alq3) and ΦPL(rubrene) are the ΦPL of an Alq3 neat film (20 ( 2%) and a 1 mol %-rubrene:Alq3 codeposited film (77

Exciplex Formations at the HTL/Alq3 Interface

Figure 6. Maximum external quantum efficiency of rubrene-doped OLEDs as a function of doping position (x).

SCHEME 1: Exciplex Formation and Emission Mechanisms under an Applied Electric Field

( 2%), respectively; d is the thickness of the rubrene-doped Alq3 layer (5 nm); and L is the width of the electron-hole recombination zone. The solid line in Figure 6 shows the best fit of the theoretical to the experimental ηext values obtained by varying the parameter L. Thus, we estimated L to be 24 ( 2 nm for FL1, 15 ( 2 nm for NPD and 11 ( 2 nm for FL3. A wider electron-hole recombination zone implies that holes are injected into the bulk of the Alq3 layer. The widest electron-hole recombination zone calculated for the FL1/Alq3-based OLED is most likely caused by efficient hole injection from the FL1 layer into the Alq3 layer because it has the lowest hole injection barrier at the HTL/Alq3 interface of the three devices. This result indicates that the width of the electron-hole recombination zone, i.e., the concentration of Alq3 excitons produced at the HTL/Alq3 interface, is dependent on the HOMO level of the HTMs. Therefore, in the NPD/Alq3-based OLED, the number of Alq3 excitons which can interact with the HTM should be fewer than that in the FL3/Alq3-based OLED because of the deep HOMO energy of NPD. We attribute high ηext value obtained for the NPD/Alq3-based OLED (see Figure 3) to a smaller contribution of exciplex formation to the EL efficiency. 3.3. the Effects of an External Electric Field on Exciplex Formation at the HTL/Alq3 Interface. There have been some reports that the exciplex formation process and exciplex

J. Phys. Chem. C, Vol. 114, No. 10, 2010 4655 emission properties are affected by an external electric field (F) in strong electron-donor and acceptor molecular systems.15–21 We also reported that PL quenching was enhanced by F in HTM:Alq3 codeposited films which form exciplexes.12 The exciplex formation and emission mechanisms under an applied electric field, including the generation of charge carriers, are shown in Scheme 1. In the scheme, Alq3* represents an excited state of Alq3. Alq3* forms an encounter complex (HTM · · · Alq3*) in the presence of a HTM molecule (A), and then a radical-ion pair (HTM+ · · · Alq3-) is produced through intermolecular electron transfer from the HTM to Alq3* (B). Relaxation of the radical-ion pair to an exciplex state (HTM+Alq3-)* (C) occurs in competition with dissociation into free carriers: HTM+ + Alq3- (D). The processes of intermolecular electron transfer from the HTM to Alq3* and the dissociation of the radical-ion pairs are affected by F. We investigated the effects of an applied electric field on the intermolecular electron transfer from HTM to Alq3* at the HTL/Alq3 interface by focusing on the direction of the electric field. In well-ordered organic films composed of electron-donor and acceptor molecules that were stacked by a LangmuirBlodgett technique, it has been clarified that the rate of photoinduced electron transfer from electron-donor to acceptor molecules is dependent on the direction of F.22–24 For the HTL/ Alq3-based OLEDs, the direction of the applied electric field is perpendicular to the HTL/Alq3 interface when the OLED is operating. Assuming that an exciplex forms via intermolecular electron transfer from an HTM molecule to Alq3* in the HTL/ Alq3-based OLEDs, the rate of exciplex formation at the HTL/ Alq3 interface should be changed by the direction of applied electric field. To investigate this, we fabricated three different types of device structures (see Figure 7): the structure of device type III was ITO(cathode)/Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/ Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/ CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(10 nm)/ MgAg/Ag(anode); the structure of device type IV was ITO(anode)/Mg(2 nm)/Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/ Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/ CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(10 nm)/ MoO3(10 nm)/MgAg/Ag(cathode); and the structure of device type V was ITO(cathode)/10 mol %-HTM:Alq3(50 nm)/MgAg/ Ag(anode). In each case, CBP was used as an inert layer as it has no intermolecular interaction with Alq3. CBP, NPD, FL2, and FL3 were used as the HTM. In the type IV devices, the thin Mg layer and the MoO3 layer block the injection of holes and electrons into the organic layer, respectively. We confirmed that the current density of the devices was less than 1 µA/cm2 and that no EL was observed under an applied forward bias (∼1.5 MV/cm). In the type III and IV devices, the opposite direction of electric field can be applied at the HTM/Alq3 interface: the field directions in the type III and IV devices are

Figure 7. Schematic diagrams of the structures of type III, IV, and V devices.

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Figure 8. PL spectra of organic films composed of Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(10 nm) (a) and 10 mol %-HTM:Alq3(50 nm) (b).

from an HTM layer to an Alq3 layer and from an Alq3 layer to an HTM layer, respectively. The applied field direction of the type III devices is the same as that of a typical OLED. The PL spectra of organic films composed of Alq3(5 nm)/ HTM(3 nm)/CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/ Alq3(5 nm)/HTM(3 nm)/CBP(2 nm)/Alq3(5 nm)/HTM(3 nm)/ CBP(2 nm)/Alq3(10 nm) and 10 mol %-HTM:Alq3(50 nm) deposited on quartz substrates are shown in Figure 8, parts a and b, respectively. Alq3 emission was predominantly observed in the multilayered films because of the efficient energy transfer from an excited state of the HTM to the ground state of Alq3 and effective confinement of the singlet exciton energy of Alq3 by the HTMs. Therefore, intermolecular interactions between the HTMs and Alq3* at the HTM/Alq3 interface could be investigated in the type III and IV devices. Figure 9 shows the normalized PL intensity (I/I0) versus electric field strength of the type III, IV, and V devices. The I/I0 of the type III, IV, and V devices is composed of a neat Alq3 film (that is, all of the HTMs were replaced with Alq3) decreased slightly as the field strength exceeds 1.0 MV/cm. This is attributed to the separation of charge carriers from the excited states.25,26 The PL quenching characteristics of the type III, IV, and V devices containing CBP as the HTM were similar to those of the devices composed of the neat Alq3 film because CBP has no intermolecular interactions with Alq3. However, the I/I0 of NPD, FL2, and FL3, which all form an exciplex with Alq3, decreased rapidly as the field strength increased. Compared to the type IV and V devices, the decreases in the I/I0 of NPD, FL2 and FL3 were more gradual in the type III devices, indicating that the direction of the electric field affects the rate of PL quenching. The quenching of Alq3 fluorescence essentially results from the formation of radical-ion pairs (Scheme 1). Radiative decay of Alq3* competes with the formation of radical-ion pairs. We estimated the quenching rate constant (kefq) of Alq3* originating from the formation of radical-ion pairs in the presence of F using PL lifetimes. The PL lifetimes of the devices in the absence and presence of F can be expressed by τ0 ) 1/(kr + knr + kq) and τF ) 1/(kr + knr + kq + kefq), respectively. Here, kr and knr represent the radiative and the nonradiative rate constants of Alq3, respectively. kq represents the quenching rate constant of Alq3* in the absence of F. The values of kefq can be obtained from the following equation using PL lifetimes: kefq ) 1/τF 1/τ0. The dependence of kefq of the devices containing NPD, FL2, and FL3 as the HTM on F are shown in Figure 10. The values of kefq increased with increasing F, indicating that

formation of radical-ion pairs is enhanced by F. The dependence of kefq on F was also affected by the structure of the device. Compared to the type III devices, the type IV devices showed a rapid increase in kefq with increasing F. This result indicates that field direction affects the formation of radical-ion pairs at the HTM/Alq3 interface. The formation of radical-ion pairs occurs through electron transfer from a ground state of a HTM molecule to an excited state of Alq3. Therefore, the electron transfer process was enhanced effectively by F in the type IV devices (where the direction of the electric field is from an Alq3 layer to an HTM layer). Because the HTM and Alq3 contact randomly in the type V devices, the dependence of kefq on F in the type V devices was similar to that of the type IV devices. Here, we note that the rather large kefq of the type V device containing FL3 indicates the presence of a stronger charge dissociation process in the mixed film of FL3 and Alq3 than that in the type IV layered device structure. We believe that differences in local molecular aggregation could enhance kefq. The results found for the effects of an applied electric field on the formation of radical-ion pairs between HTM and Alq3* agree well with the OLED characteristics presented earlier (see Figure 3). The experimental ηext values of the type II devices containing codeposited HTM:Alq3 EMLs are lower than the theoretical ones, indicating that quenching of Alq3 emission originating from the formation of radical-ion pairs is efficiently enhanced by F. For the type I devices, the direction of the applied field at the HTL/Alq3 interface is the same as that of the type III devices. Therefore, in the HTL/Alq3-based OLEDs, the applied electric field is less effective at enhancing the formation of radical-ion pairs at the HTL/Alq3 interface while the OLED is operating. 4. Conclusions We investigated exciplex formations that show without significant spectral red shifts in HTL/Alq3-based OLEDs by using HTMs with various HOMO levels (-5.3 to -5.7 eV). The ηext values of these OLEDs were dependent on the HOMO levels of the HTMs and correlated with the ΦPL values of the HTM:Alq3 codeposited films. Decreases in ηext originating from exciplex formation were observed in the OLEDs containing FL2 (-5.4 eV) and FL3 (-5.3 eV) as the HTL. In contrast, for the OLEDs containing NPD (-5.5 eV), the obtained ηext value was the same as that for the intermolecular interaction-free OLED containing FL1 (-5.7 eV), even though NPD apparently forms exciplexes with Alq3 in an NPD:Alq3 codeposited film.

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Figure 9. Normalized PL intensity (I/I0) versus applied electric field for type III (a), IV (b), and V (c) devices.

Figure 10. PL quenching rate constant (kefq) versus applied electric field for devices containing: NPD (a), FL2 (b), and FL3 (c).

We investigated the influence of the width of the electron-hole recombination zone and the application of an external electric field on the exciplex formation at the HTL/Alq3 interface. The electron-hole recombination zone is located in the Alq3 layer close to the HTL/Alq3 interface and its width is dependent on the height of the hole injection barrier at the HTL/Alq3 interface. The estimated widths of the electron-hole recombination zones were 24 ( 2 nm for FL1, 15 ( 2 nm for NPD, and 11 ( 2 nm for FL3. Therefore, in the NPD/Alq3-based OLED, the number of Alq3 excitons which could interact with the HTM was fewer than that in the FL2 and FL3/Alq3-based OLEDs because of the deep HOMO energy of NPD. We attribute the high ηext value obtained for the NPD/Alq3-based OLED to the smaller effect of exciplex formation on the EL efficiency. Exciplex formation

in the OLEDs was also affected by an applied electric field. Quenching of Alq3 excitons originating from the formation of radical-ion pairs was effectively enhanced by applying an electric field when the HTM and Alq3 were randomly contacting. In contrast, at a well-ordered HTL/Alq3 interface, the rate of formation of radical-ion pairs was dependent on the direction of the applied electric field. An applied electric field, which has a direction that is the same as that within the OLED, is less effective at enhancing the formation of radical-ion pairs. In HTL/ Alq3-based OLEDs, exciplex formation at the HTL/Alq3 interface was affected not only by the electron donating nature of the HTM but also by the width of the electron-hole recombination zone and external electric fields applied during OLED operation.

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Acknowledgment. This work was supported by a Grant-inAid for the Global COE Program,“Science for Future Molecular Systems” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References and Notes (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Adachi, C.; Tokito, S.; Tsutsui, T.; Saito, S. Jpn. J. Appl. Phys. 1988, 27, L269. (3) Tamoto, N.; Adachi, C.; Nagai, K. Chem. Mater. 1997, 9, 1077. (4) Wang, J. F.; Kawabe, Y.; Shaheen, S. E.; Morrell, M. M.; Jabbour, G. E.; Lee, P. A.; Anderson, J.; Armstrong, N. R.; Kippelen, B.; Mash, E. A.; Peyghambarian, N. AdV. Mater. 1998, 10, 230. (5) Noda, T.; Ogawa, H.; Shirota, Y. AdV. Mater. 1999, 11, 283. (6) Cocchi, M.; Virgili, D.; Giro, G.; Fattori, V.; Di Marco, P.; Kalinowski, J.; Shirota, Y. Appl. Phys. Lett. 2002, 80, 2401. (7) Palilis, L. C.; Ma¨kinen, A. J.; Uchida, M.; Kafafi, Z. H. Appl. Phys. Lett. 2003, 82, 2209. (8) Li, F.; Chen, Z.; Wei, W.; Cao, H.; Gong, Q.; Teng, F.; Qian, L.; Wang, Y. J. Phys. D. 2004, 37, 1613. (9) Li, G.; Kim, C. H.; Zhou, Z.; Shinarb, J.; Okumoto, K.; Shirota, Y. Appl. Phys. Lett. 2006, 88, 253505. (10) Su, W. M.; Li, W. L.; Xin, Q.; Su, Z. S.; Chu, B.; Bi, D. F.; He, H.; Niu, J. H. Appl. Phys. Lett. 2007, 91, 43508. (11) Itano, K.; Ogawa, H.; Shirota, Y. Appl. Phys. Lett. 1998, 72, 636.

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