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Non-doped phosphorescent white organic light-emitting devices with a quadruple-quantum-well structure
Physica B 407 (2012) 2753–2757
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Non-doped phosphorescent white organic light-emitting devices with a quadruple-quantum-well structure Juan Zhao, Junsheng Yu n, Lei Zhang, Jun Wang State Key Laboratory of Electronic Thin Film and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e i n f o
abstract
Article history: Received 13 February 2012 Received in revised form 7 April 2012 Accepted 9 April 2012 Available online 16 April 2012
Non-doped white organic light-emitting devices (WOLEDs) with a quadruple-quantum-well structure were fabricated. An alternate layer of ultrathin blue and yellow iridium complexes was employed as the potential well layer, while potential barrier layers (PBLs) were chosen to be 2,2’,2’’-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) or N,N’-dicarbazolyl-3,5-benzene (mCP) combined TPBi. On adjusting the PBLs for device performance comparison, the results showed that the device with all-TPBi PBLs exhibited a yellow emission with the color coordinates of (0.50,0.47) at a luminance of 1000 cd/ m2, while stable white emission with the color coordinates of (0.36,0.44) was observed in the device using mCP combined TPBi as the PBLs. Meanwhile, for the WOLED, with a reduced efficiency roll-off, a maximum luminance, luminous efficiency, and external quantum efficiency of 12,610 cd/m2, 10.2 cd/A, and 4.4%, respectively, were achieved. The performance improvement by the introduction of mCP PBL was ascribed to the well confined exciton and the reduced exciton quenching effect in the multiple emission regions. & 2012 Elsevier B.V. All rights reserved.
Keywords: White organic light-emitting device (WOLED) Phosphorescence Non-doped Quadruple-quantum-well structure Potential barrier layer
1. Introduction During the past few decades, organic light-emitting devices (OLEDs) have drawn extensive academic and industrial interest, especially white OLEDs (WOLEDs) that have potential application in displays and lighting sources. Among various configuration for OLED, the multiple-quantum-well (MQW) structure has been proved to be an effective approach for high device performance [1–3], by confining charge carriers and exciton within the multiple emitting layer (EML); thus the charge carrier recombination efficiency and exciton formation probability can be beneficially enhanced. However, the realization of constructing efficient MQW structures highly relies on the thickness of potential well layer (PWL), material choice of potential barrier layer (PBL), and so on [4]. Up to the present, several series of the MQW-devices have been fabricated by employment of fluorescent or phosphorescent emitters, using the host–guest doping or non-doping method [5,6]. It is well known that the fluorescence that can use only 25% singlet excitons for emission would largely limit device efficiency, while the phosphorescence can achieve a 100% internal quantum efficiency due to the ability of harvesting both singlet and triplet excitons [7]. Moreover, even though the host–guest doping system
n
Corresponding author. Tel.: þ86 28 83207157. E-mail address: [email protected] (J. Yu).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.04.021
has been widely utilized nowadays, this technique suffers from complex processability and poor reproducibility. In addition, the phase separation phenomenon in guest–host system is another potential problem [8], which can all be avoided in the technologically easy non-doping approach [9]. In view of the shortcomings aforementioned, considerable efforts for the MQW-devices have been devoted to explore the non-doping system by adopting the high-efficiency phosphors. Though monochrome phosphorescent devices with the MQW structure have been developed, the study of phosphorescent WOLED (PWOLED) based on the non-doped MQW is rarely reported. Recently, we reported a kind of device architecture for the non-doped PWOLED consisting of a doublequantum-well structure [10], using dual phosphors as the PWLs and N,N’-dicarbazolyl-3,5-benzene (mCP) as the PBLs. Particularly, attention was paid to optimize the thickness of ultrathin PWLs, while high efficiency and stable white emission was obtained. In addition, the fabrication process was greatly simplified and accurately controlled, providing a promising solution to lower the production cost. In this study, we introduced a simple process for the nondoped PWOLEDs with a quadruple-quantum-well (QQW) structure, while ultrathin blue and yellow iridium complexes were alternately formed as the PWLs. Meanwhile, the effect of material choice and thickness adjustment of the PBLs on device performance, which were composed of 2,2’,2’’-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) or mCP combined TPBi, was investigated.
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J. Zhao et al. / Physica B 407 (2012) 2753–2757
measurements were performed at room temperature in ambient environment.
2. Experimental The devices were fabricated on indium tin oxide (ITO)-coated glass substrates, which were consecutively cleaned in ultrasonic bath with detergent, acetone, de-ionized water, and ethanol. Prior to the deposition of organic layers, the ITO-coated substrates were treated by oxygen plasma for 5 min to increase the work function of the anode. Then, organic and metallic films were subsequently deposited by thermal evaporation under a pressure of 3 10 4 and 2 10 3 Pa, at a rate of 0.05–0.1 nm/s and 0.5–1 nm/s, respectively. The film thickness and deposition rates were monitored in situ by oscillating quartz thickness monitors. To study the effect of quadruple-quantum-well structure in PWOLED, the devices were fabricated with a structure of ITO/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (30 nm)/iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2’](picolinate) iridium (III) (FIrpic) (1 nm)/PBL1(X1)/ bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2’] iridium (acetylacetonate) [(tbt)2Ir(acac)] (0.5 nm)/PBL2(X2)/FIrpic(1 nm)/PBL3(X3)/ (tbt)2Ir(acac)(0.5 nm)/4,7-diphenyl-1,10-phenanthroline (Bphen) (40 nm)/Mg:Ag(200 nm). Therein, phosphorescent blue FIrpic and yellow (tbt)2Ir(acac) were alternately employed as the PWLs/EMLs, while the PBLs were chosen to be either TPBi or mCP combined TPBi. Thereby, the quadruple-quantum-well structure was formed by spatially separating the PWLs from the PBLs. TAPC and Bphen functioned as the hole transporting layer and electron transporting layer, respectively, while Mg:Ag alloy was used as a cathode. Meanwhile, in the device architecture depicted in Fig. 1, PBL1, PBL2, and PBL3 represented the PBL that was close to the anode side, in the middle, and close to the cathode, respectively, while PBL1 and PBL3 were kept to be TPBi. For device characterization, the three PBLs were consecutively adjusted, and five devices were given as follows. Device A: PBL2 stands for TPBi, X3¼4 nm. Device B: PBL2 stands for mCP, X3¼4 nm. Device C: PBL2 stands for mCP, X3¼4 nm. Device D: PBL2 stands for mCP, X3¼4 nm. Device E: PBL2 stands for mCP, X3¼6 nm.
X1¼4 nm, X2 ¼4 nm and X1¼4 nm, X2¼4 nm and X1¼6 nm, X2¼4 nm and X1¼ 6 nm, X2¼6 nm and X1¼6 nm, X2¼6 nm and
Luminance–current density–voltage characteristics were recorded with a Keithley 4200 source, while electroluminescent (EL) spectra and Commission Internationale de l’Eclairage (CIE) coordinates were measured with an OPT-2000 spectrophotometer. The external quantum efficiency (EQE) of the device was calculated with a computer program on the basis of previously reported theories in the literature [11]. All the
Fig. 1. Energy level diagram of the QQW-OLEDs.
3. Results and discussion Fig. 2 shows the normalized EL spectra of devices A–E at a practical luminance of 1000 cd/m2 for lighting. For all the devices, a peak at 470 nm with a shoulder at 495 nm arises from FIrpic emission, while a peak at 560 nm with a shoulder at 602 nm is from (tbt)2Ir(acac). Since there is a large difference of the highest occupied molecular orbital (HOMO) between PWLs and PBLs, together with a moderate difference of the lowest unoccupied molecular orbital (LUMO) levels, as shown in Fig. 1, it renders the blue/yellow PWLs shallow electron traps and deep hole traps. Hence, the charge carriers can be favorably confined in the triplet PWLs/EMLs, followed by direct charge recombination and exciton formation on the emitters, which is the so-called direct carrier trapping mechanism [12], contributing to excite both blue and yellow emissions. Apparently, in devices A–C, the yellow intensity overwhelms its blue counterpart, giving yellow-dominate emission, whereas white emission can be clearly observed in devices D and E. Then, the influence of PBLs on color emission of the devices is demonstrated. TPBi has been reported to serve as a suitable PBL material in the MQW-devices because of its high triplet energy (ET) (2.74 eV) and wide energy gap (Eg) [13], which can effectively confine the charge carrier and exciton in the PWLs. For device A with 4 nm ¨ TPBi PBLs, Forster energy transfer from FIrpic to (tbt)2Ir(acac) occurs through exciton diffusion across the PBL, owning to longer ¨ Forster energy transfer radius ( 4 nm) [5] than that of Dexter energy transfer ( 1 nm) [14]. Consequently, the yellow emission is strengthened. On the other hand, given the electron transporting property of TPBi PBLs, it is reasonably speculated that more excions are formed in the PWLs near the anode side. For this reason, the hole transporting material mCP with high ET (2.9 eV) and wide energy gap [15] is introduced as PBL2 in devices B–E, to facilitate hole transport into the PWLs situated at the cathode side, followed by recombination with local electrons. Therefore, the excitons could be distributed over the multiple PWLs/EMLs. Compared to device A, the blue intensity in device B is slightly boosted while the yellow intensity remains strong, due to the same reason as discussed above for device A. In the case of device C, the thickness of the PBL1 is enlarged to be 6 nm, so the energy
Fig. 2. Normalized EL spectra of the QQW-OLEDs at a luminance of 1000 cd/m2. Inset: the CIE coordinates of the devices in the luminance range 1000–10,000 cd/m2.
J. Zhao et al. / Physica B 407 (2012) 2753–2757
transfer between blue and yellow PWLs close to the anode side can be partly prevented. Nevertheless, the weak blue intensity in device C suggests that the exciton formation zones have been positioned in the PWLs close to the cathode side, where the energy transfer still exists due to 4 nm-PBL, leading to the high yellow intensity. Increasing the thickness of PBL2 and PBL3 in devices D and E should further restrained the energy transfer between blue and yellow. As a consequence, white emission can be realized in devices D and E, which originate from comparable blue and yellow intensities in the spectra, as shown in Fig. 2. Meanwhile, it indicates that the excitons created in the PWLs are inhibited from escaping outside; thus, the recombination zones have been well confined in the multiple PWLs/EMLs. Additionally notice that the blue intensity in device E is moderately higher than that in device D, implying that the energy transfer is largely suppressed in the devices with thicker PBLs. Remarkably, as seen from the inset of Fig. 2, all the devices show excellent color stability in a broad luminance range of 1000–10,000 cd/m2. Reasons for the stable color emission are explained by: (1) effective confinement of charge carriers in the multiple EMLs because both HOMO and LUMO levels of the emitters are located between that of the PBLs, as seen from the device architecture of Fig. 1 and (2) well confined excitons in the multiple EMLs as the triplet energy of the PBLs is higher than that of the emitters. Besides, for the host-free EMLs, the un-expected exchange energy loss induced from host to dopant could be favorably eliminated. The results reveal that proper material choice and thickness adjustment of the PBLs are highly essential for the color emission of MQW-OLEDs. The luminance–current density–voltage (L–J–V) characteristics of devices A–E are displayed in Fig. 3. From the J–V characteristic, it is obvious that devices B and C incorporated with mCP as the middle PBL (PBL2) show slightly higher current density than that of device A with all-TPBi PBLs, which is ascribed to the higher carrier mobility of mCP [16], in contrast to that of TPBi [17]. Thereby, the charge carrier transport in devices B and C can be accelerated. With increasing the thickness of the PBLs in devices D and E, the current density was decreased due to the enlarged total thickness of the devices. As for the L–V characteristic, it shows that all the devices exhibit fairly low turn-on and operation voltages, while the luminance increases rapidly on raising the driving voltage. Besides, the maximum luminances of devices A, B, C, D and E achieved are 16,220 cd/m2, 27,010 cd/m2, 9,410 cd/m2, 12,610 cd/m2 and 8,120 cd/m2, respectively. More details are listed in Table 1. Fig. 4 shows the luminous efficiency (LE) versus the current density characteristics of the QQW-devices. At a current density
Fig. 3. Luminance–current density–voltage characteristics of the QQW-devices.
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of 20 mA/cm2, the LEs are 17.5 cd/A, 16.3 cd/A, 7.5 cd/A, 6.8 cd/A, and 5.7 cd/A for devices A, B, C, D, and E, respectively. It is worth pointing out that all the device efficiencies remain high over a wide range of current density, implying that the exciton quenching processes at high current density are effectively prevented and then the efficiency roll-off is reduced. As is well known, there are mainly two exciton quenching mechanisms in the phosphorescent EML, namely triplet–triplet annihilation (TTA) and triplet– polaron annihilation (TPA) [18], with the corresponding reaction equations [Eqs. (1) and (2) for TTA and Eqs. (3) and (4) for TPA] shown as follows: T1 þ T1 -T1 þ S0
ð1Þ
T1 þ T1 -S1 þ S0
ð2Þ
T1 þ e-S0 þe
ð3Þ
T1 þ h-S0 þ h
ð4Þ
where T1, S0, S1, e, and h denote the first excited triplet state, the ground state, the first excited singlet state, the electron, and the hole, respectively. So, it can be learnt that the TTA as well as TPA processes are closely sensitive to the density of the triplet excitons or charge carriers in the EML. In the region of high current density, the numbers of the excitons and charge carriers are greatly enhanced, while effective suppression of the exciton quenching processes enables low efficiency roll-off. Considered less than one monolayer thick, the ultrathin non-doped layer of the emitter ( r1 nm) is discontinuous in atomic force microscopy images [19]. When sandwiched between two host-like neighboring layers, the ultrathin layer of the emitter forms a doping profile similar to the situation where the emitter is codoped with the two hosts simultaneously. Moreover, in the MQW-devices, the charge carriers and excitons are effectively dispersed in the multiple EMLs, while these emissive regions are periodically interrupted by multiple PBLs. Two things are responsible for these, i.e. small degree of aggregation and concentration quenching [20]. Therefore, the TTA and TPA processes as aforementioned can be restricted due to the low density of charge carriers and excitons in the multiple emissive regions, allowing for the reduced efficiency roll-off. On the other hand, the formation of excimer [21] could also be impeded. In addition, at a high current density, taking 80 mA/cm2 for example, the luminous efficiency of device B is 1.8 times higher than that of device A, while device D shows an enhancement of 1.3 times compared to that of device C. Thereby, it can be deduced that the incorporation and adjustment of the mCP PBL2 plays an important role in reducing the efficiency roll-off at the high current density, resulting from improved electron–hole balance in the PWLs caused by the enhanced electron mobility of the mCP PBL at a high electric field [16]. Fig. 5 depicts the external quantum efficiency–voltage (EQE–V) characteristics of the devices based on the QQW structure. It can be seen that the EQEs of devices A, D, and E are proportional to applied bias, while those of devices B and C slightly decrease at the high driving voltage. Consequently, the maximum EQE of the devices can be realized at high luminance while displaying low efficiency roll-off, which is an essential characteristic for both display and lighting applications. The results exhibit that the maximum EQEs of devices A, B, C, D, and E are 6.4%, 6.3%, 4.0%, 4.4%, and 2.1%, respectively. Compared to that of typical phosphorescent OLEDs based on the host–guest doping system, the LEs and EQEs of the QQW-devices are still relatively low. As is well known, most of the phosphorescent emitters suffer from a common problem, namely the concentration quenching effect in solid state caused by the molecule interactions [22].
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Table 1 Performance of the quadruple-quantum-well devices. Device
PBL1 (nm)
PBL2 (nm)
PBL3 (nm)
Voltagea (V)
Luminanceb (cd/m2)
LEb (cd/A)
EQEb (%)
CIEc (x,y)
A B C D E
TPBi(4) TPBi(4) TPBi(6) TPBi(6) TPBi(6)
TPBi(4) mCP(4) mCP(4) mCP(6) mCP(6)
TPBi(4) TPBi(4) TPBi(4) TPBi(4) TPBi(6)
3.4 3.3 3.4 3.7 3.5
16,220 27,010 9410 12,610 8120
18.8 18.7 7.6 10.2 5.8
6.4 6.3 4.0 4.4 2.1
(0.50,0.47) (0.49,0.48) (0.48,0.47) (0.36,0.44) (0.34,0.43)
a b c
At 1 cd/m2. Maximum value. At 1000 cd/m2.
utilizing ultrathin layers of blue and yellow iridium complexes as the PWLs.
4. Conclusions
Fig. 4. Luminous efficiency–current density characteristics of the QQW-devices.
In summary, we demonstrated the non-doped WOLEDs with a quadruple-quantum-well structure, in which ultrathin layers of the blue and yellow iridium complexes were alternately formed as the PWLs. The results showed that the device performance was greatly dependent on the material choice and thickness adjustment of the PBLs. For the device with all-TPBi PBLs, high performance was achieved with the sacrifice of white emission. In the case of the device employing mCP combined TPBi as the PBLs, stable white emission along with low efficiency roll-off was realized, attributed to well confined excitons in the multiple PWLs caused by high energy levels of the mCP PBL, accompanied by reduced exciton quenching and improved electron–hole balance. The described non-doped QQW concept indicated the feasibility of this strategy to obtain high-efficiency WOLEDs with excellent stability as well as simplified manufacturing.
Acknowledgments This work was supported by the National Science Foundation of China (NSFC; Grant nos. 61177032 and 61006036), the Foundation for Innovative Research Groups of the NSFC (Grant no. 61021061), the Fundamental Research Funds for the Central Universities (Grant nos. ZYGX2010Z004 and ZYGX2010J004), SRF for ROCS, SEM (Grant no. GGRYJJ08-05), and Doctoral Fund of Ministry of Education of China (Grant no. 20090185110020). References
Fig. 5. External quantum efficiency–current density characteristics of the QQW-devices.
Consequently, the host–guest doping system has become a universal method for solving this problem. Here, the presented QQW structure has advantages of simplifying the fabrication process and guaranteeing device reproducibility, together with realizing low efficiency roll-off and excellent color stability. On the other hand, the results suggest there is still a large room for performance enhancement through further device optimization, and more studies are required on these MQW-devices. To our best knowledge, this study is the first by applying quadruple-quantum-well structure to the non-doped PWOLEDs,
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Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Non-doped phosphorescent white organic light-emitting devices with a quadruple-quantum-well structure Juan Zhao, Junsheng Yu n, Lei Zhang, Jun Wang State Key Laboratory of Electronic Thin Film and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e i n f o
abstract
Article history: Received 13 February 2012 Received in revised form 7 April 2012 Accepted 9 April 2012 Available online 16 April 2012
Non-doped white organic light-emitting devices (WOLEDs) with a quadruple-quantum-well structure were fabricated. An alternate layer of ultrathin blue and yellow iridium complexes was employed as the potential well layer, while potential barrier layers (PBLs) were chosen to be 2,2’,2’’-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) or N,N’-dicarbazolyl-3,5-benzene (mCP) combined TPBi. On adjusting the PBLs for device performance comparison, the results showed that the device with all-TPBi PBLs exhibited a yellow emission with the color coordinates of (0.50,0.47) at a luminance of 1000 cd/ m2, while stable white emission with the color coordinates of (0.36,0.44) was observed in the device using mCP combined TPBi as the PBLs. Meanwhile, for the WOLED, with a reduced efficiency roll-off, a maximum luminance, luminous efficiency, and external quantum efficiency of 12,610 cd/m2, 10.2 cd/A, and 4.4%, respectively, were achieved. The performance improvement by the introduction of mCP PBL was ascribed to the well confined exciton and the reduced exciton quenching effect in the multiple emission regions. & 2012 Elsevier B.V. All rights reserved.
Keywords: White organic light-emitting device (WOLED) Phosphorescence Non-doped Quadruple-quantum-well structure Potential barrier layer
1. Introduction During the past few decades, organic light-emitting devices (OLEDs) have drawn extensive academic and industrial interest, especially white OLEDs (WOLEDs) that have potential application in displays and lighting sources. Among various configuration for OLED, the multiple-quantum-well (MQW) structure has been proved to be an effective approach for high device performance [1–3], by confining charge carriers and exciton within the multiple emitting layer (EML); thus the charge carrier recombination efficiency and exciton formation probability can be beneficially enhanced. However, the realization of constructing efficient MQW structures highly relies on the thickness of potential well layer (PWL), material choice of potential barrier layer (PBL), and so on [4]. Up to the present, several series of the MQW-devices have been fabricated by employment of fluorescent or phosphorescent emitters, using the host–guest doping or non-doping method [5,6]. It is well known that the fluorescence that can use only 25% singlet excitons for emission would largely limit device efficiency, while the phosphorescence can achieve a 100% internal quantum efficiency due to the ability of harvesting both singlet and triplet excitons [7]. Moreover, even though the host–guest doping system
n
Corresponding author. Tel.: þ86 28 83207157. E-mail address: [email protected] (J. Yu).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.04.021
has been widely utilized nowadays, this technique suffers from complex processability and poor reproducibility. In addition, the phase separation phenomenon in guest–host system is another potential problem [8], which can all be avoided in the technologically easy non-doping approach [9]. In view of the shortcomings aforementioned, considerable efforts for the MQW-devices have been devoted to explore the non-doping system by adopting the high-efficiency phosphors. Though monochrome phosphorescent devices with the MQW structure have been developed, the study of phosphorescent WOLED (PWOLED) based on the non-doped MQW is rarely reported. Recently, we reported a kind of device architecture for the non-doped PWOLED consisting of a doublequantum-well structure [10], using dual phosphors as the PWLs and N,N’-dicarbazolyl-3,5-benzene (mCP) as the PBLs. Particularly, attention was paid to optimize the thickness of ultrathin PWLs, while high efficiency and stable white emission was obtained. In addition, the fabrication process was greatly simplified and accurately controlled, providing a promising solution to lower the production cost. In this study, we introduced a simple process for the nondoped PWOLEDs with a quadruple-quantum-well (QQW) structure, while ultrathin blue and yellow iridium complexes were alternately formed as the PWLs. Meanwhile, the effect of material choice and thickness adjustment of the PBLs on device performance, which were composed of 2,2’,2’’-(1,3,5-benzenetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) or mCP combined TPBi, was investigated.
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J. Zhao et al. / Physica B 407 (2012) 2753–2757
measurements were performed at room temperature in ambient environment.
2. Experimental The devices were fabricated on indium tin oxide (ITO)-coated glass substrates, which were consecutively cleaned in ultrasonic bath with detergent, acetone, de-ionized water, and ethanol. Prior to the deposition of organic layers, the ITO-coated substrates were treated by oxygen plasma for 5 min to increase the work function of the anode. Then, organic and metallic films were subsequently deposited by thermal evaporation under a pressure of 3 10 4 and 2 10 3 Pa, at a rate of 0.05–0.1 nm/s and 0.5–1 nm/s, respectively. The film thickness and deposition rates were monitored in situ by oscillating quartz thickness monitors. To study the effect of quadruple-quantum-well structure in PWOLED, the devices were fabricated with a structure of ITO/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (30 nm)/iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2’](picolinate) iridium (III) (FIrpic) (1 nm)/PBL1(X1)/ bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2’] iridium (acetylacetonate) [(tbt)2Ir(acac)] (0.5 nm)/PBL2(X2)/FIrpic(1 nm)/PBL3(X3)/ (tbt)2Ir(acac)(0.5 nm)/4,7-diphenyl-1,10-phenanthroline (Bphen) (40 nm)/Mg:Ag(200 nm). Therein, phosphorescent blue FIrpic and yellow (tbt)2Ir(acac) were alternately employed as the PWLs/EMLs, while the PBLs were chosen to be either TPBi or mCP combined TPBi. Thereby, the quadruple-quantum-well structure was formed by spatially separating the PWLs from the PBLs. TAPC and Bphen functioned as the hole transporting layer and electron transporting layer, respectively, while Mg:Ag alloy was used as a cathode. Meanwhile, in the device architecture depicted in Fig. 1, PBL1, PBL2, and PBL3 represented the PBL that was close to the anode side, in the middle, and close to the cathode, respectively, while PBL1 and PBL3 were kept to be TPBi. For device characterization, the three PBLs were consecutively adjusted, and five devices were given as follows. Device A: PBL2 stands for TPBi, X3¼4 nm. Device B: PBL2 stands for mCP, X3¼4 nm. Device C: PBL2 stands for mCP, X3¼4 nm. Device D: PBL2 stands for mCP, X3¼4 nm. Device E: PBL2 stands for mCP, X3¼6 nm.
X1¼4 nm, X2 ¼4 nm and X1¼4 nm, X2¼4 nm and X1¼6 nm, X2¼4 nm and X1¼ 6 nm, X2¼6 nm and X1¼6 nm, X2¼6 nm and
Luminance–current density–voltage characteristics were recorded with a Keithley 4200 source, while electroluminescent (EL) spectra and Commission Internationale de l’Eclairage (CIE) coordinates were measured with an OPT-2000 spectrophotometer. The external quantum efficiency (EQE) of the device was calculated with a computer program on the basis of previously reported theories in the literature [11]. All the
Fig. 1. Energy level diagram of the QQW-OLEDs.
3. Results and discussion Fig. 2 shows the normalized EL spectra of devices A–E at a practical luminance of 1000 cd/m2 for lighting. For all the devices, a peak at 470 nm with a shoulder at 495 nm arises from FIrpic emission, while a peak at 560 nm with a shoulder at 602 nm is from (tbt)2Ir(acac). Since there is a large difference of the highest occupied molecular orbital (HOMO) between PWLs and PBLs, together with a moderate difference of the lowest unoccupied molecular orbital (LUMO) levels, as shown in Fig. 1, it renders the blue/yellow PWLs shallow electron traps and deep hole traps. Hence, the charge carriers can be favorably confined in the triplet PWLs/EMLs, followed by direct charge recombination and exciton formation on the emitters, which is the so-called direct carrier trapping mechanism [12], contributing to excite both blue and yellow emissions. Apparently, in devices A–C, the yellow intensity overwhelms its blue counterpart, giving yellow-dominate emission, whereas white emission can be clearly observed in devices D and E. Then, the influence of PBLs on color emission of the devices is demonstrated. TPBi has been reported to serve as a suitable PBL material in the MQW-devices because of its high triplet energy (ET) (2.74 eV) and wide energy gap (Eg) [13], which can effectively confine the charge carrier and exciton in the PWLs. For device A with 4 nm ¨ TPBi PBLs, Forster energy transfer from FIrpic to (tbt)2Ir(acac) occurs through exciton diffusion across the PBL, owning to longer ¨ Forster energy transfer radius ( 4 nm) [5] than that of Dexter energy transfer ( 1 nm) [14]. Consequently, the yellow emission is strengthened. On the other hand, given the electron transporting property of TPBi PBLs, it is reasonably speculated that more excions are formed in the PWLs near the anode side. For this reason, the hole transporting material mCP with high ET (2.9 eV) and wide energy gap [15] is introduced as PBL2 in devices B–E, to facilitate hole transport into the PWLs situated at the cathode side, followed by recombination with local electrons. Therefore, the excitons could be distributed over the multiple PWLs/EMLs. Compared to device A, the blue intensity in device B is slightly boosted while the yellow intensity remains strong, due to the same reason as discussed above for device A. In the case of device C, the thickness of the PBL1 is enlarged to be 6 nm, so the energy
Fig. 2. Normalized EL spectra of the QQW-OLEDs at a luminance of 1000 cd/m2. Inset: the CIE coordinates of the devices in the luminance range 1000–10,000 cd/m2.
J. Zhao et al. / Physica B 407 (2012) 2753–2757
transfer between blue and yellow PWLs close to the anode side can be partly prevented. Nevertheless, the weak blue intensity in device C suggests that the exciton formation zones have been positioned in the PWLs close to the cathode side, where the energy transfer still exists due to 4 nm-PBL, leading to the high yellow intensity. Increasing the thickness of PBL2 and PBL3 in devices D and E should further restrained the energy transfer between blue and yellow. As a consequence, white emission can be realized in devices D and E, which originate from comparable blue and yellow intensities in the spectra, as shown in Fig. 2. Meanwhile, it indicates that the excitons created in the PWLs are inhibited from escaping outside; thus, the recombination zones have been well confined in the multiple PWLs/EMLs. Additionally notice that the blue intensity in device E is moderately higher than that in device D, implying that the energy transfer is largely suppressed in the devices with thicker PBLs. Remarkably, as seen from the inset of Fig. 2, all the devices show excellent color stability in a broad luminance range of 1000–10,000 cd/m2. Reasons for the stable color emission are explained by: (1) effective confinement of charge carriers in the multiple EMLs because both HOMO and LUMO levels of the emitters are located between that of the PBLs, as seen from the device architecture of Fig. 1 and (2) well confined excitons in the multiple EMLs as the triplet energy of the PBLs is higher than that of the emitters. Besides, for the host-free EMLs, the un-expected exchange energy loss induced from host to dopant could be favorably eliminated. The results reveal that proper material choice and thickness adjustment of the PBLs are highly essential for the color emission of MQW-OLEDs. The luminance–current density–voltage (L–J–V) characteristics of devices A–E are displayed in Fig. 3. From the J–V characteristic, it is obvious that devices B and C incorporated with mCP as the middle PBL (PBL2) show slightly higher current density than that of device A with all-TPBi PBLs, which is ascribed to the higher carrier mobility of mCP [16], in contrast to that of TPBi [17]. Thereby, the charge carrier transport in devices B and C can be accelerated. With increasing the thickness of the PBLs in devices D and E, the current density was decreased due to the enlarged total thickness of the devices. As for the L–V characteristic, it shows that all the devices exhibit fairly low turn-on and operation voltages, while the luminance increases rapidly on raising the driving voltage. Besides, the maximum luminances of devices A, B, C, D and E achieved are 16,220 cd/m2, 27,010 cd/m2, 9,410 cd/m2, 12,610 cd/m2 and 8,120 cd/m2, respectively. More details are listed in Table 1. Fig. 4 shows the luminous efficiency (LE) versus the current density characteristics of the QQW-devices. At a current density
Fig. 3. Luminance–current density–voltage characteristics of the QQW-devices.
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of 20 mA/cm2, the LEs are 17.5 cd/A, 16.3 cd/A, 7.5 cd/A, 6.8 cd/A, and 5.7 cd/A for devices A, B, C, D, and E, respectively. It is worth pointing out that all the device efficiencies remain high over a wide range of current density, implying that the exciton quenching processes at high current density are effectively prevented and then the efficiency roll-off is reduced. As is well known, there are mainly two exciton quenching mechanisms in the phosphorescent EML, namely triplet–triplet annihilation (TTA) and triplet– polaron annihilation (TPA) [18], with the corresponding reaction equations [Eqs. (1) and (2) for TTA and Eqs. (3) and (4) for TPA] shown as follows: T1 þ T1 -T1 þ S0
ð1Þ
T1 þ T1 -S1 þ S0
ð2Þ
T1 þ e-S0 þe
ð3Þ
T1 þ h-S0 þ h
ð4Þ
where T1, S0, S1, e, and h denote the first excited triplet state, the ground state, the first excited singlet state, the electron, and the hole, respectively. So, it can be learnt that the TTA as well as TPA processes are closely sensitive to the density of the triplet excitons or charge carriers in the EML. In the region of high current density, the numbers of the excitons and charge carriers are greatly enhanced, while effective suppression of the exciton quenching processes enables low efficiency roll-off. Considered less than one monolayer thick, the ultrathin non-doped layer of the emitter ( r1 nm) is discontinuous in atomic force microscopy images [19]. When sandwiched between two host-like neighboring layers, the ultrathin layer of the emitter forms a doping profile similar to the situation where the emitter is codoped with the two hosts simultaneously. Moreover, in the MQW-devices, the charge carriers and excitons are effectively dispersed in the multiple EMLs, while these emissive regions are periodically interrupted by multiple PBLs. Two things are responsible for these, i.e. small degree of aggregation and concentration quenching [20]. Therefore, the TTA and TPA processes as aforementioned can be restricted due to the low density of charge carriers and excitons in the multiple emissive regions, allowing for the reduced efficiency roll-off. On the other hand, the formation of excimer [21] could also be impeded. In addition, at a high current density, taking 80 mA/cm2 for example, the luminous efficiency of device B is 1.8 times higher than that of device A, while device D shows an enhancement of 1.3 times compared to that of device C. Thereby, it can be deduced that the incorporation and adjustment of the mCP PBL2 plays an important role in reducing the efficiency roll-off at the high current density, resulting from improved electron–hole balance in the PWLs caused by the enhanced electron mobility of the mCP PBL at a high electric field [16]. Fig. 5 depicts the external quantum efficiency–voltage (EQE–V) characteristics of the devices based on the QQW structure. It can be seen that the EQEs of devices A, D, and E are proportional to applied bias, while those of devices B and C slightly decrease at the high driving voltage. Consequently, the maximum EQE of the devices can be realized at high luminance while displaying low efficiency roll-off, which is an essential characteristic for both display and lighting applications. The results exhibit that the maximum EQEs of devices A, B, C, D, and E are 6.4%, 6.3%, 4.0%, 4.4%, and 2.1%, respectively. Compared to that of typical phosphorescent OLEDs based on the host–guest doping system, the LEs and EQEs of the QQW-devices are still relatively low. As is well known, most of the phosphorescent emitters suffer from a common problem, namely the concentration quenching effect in solid state caused by the molecule interactions [22].
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Table 1 Performance of the quadruple-quantum-well devices. Device
PBL1 (nm)
PBL2 (nm)
PBL3 (nm)
Voltagea (V)
Luminanceb (cd/m2)
LEb (cd/A)
EQEb (%)
CIEc (x,y)
A B C D E
TPBi(4) TPBi(4) TPBi(6) TPBi(6) TPBi(6)
TPBi(4) mCP(4) mCP(4) mCP(6) mCP(6)
TPBi(4) TPBi(4) TPBi(4) TPBi(4) TPBi(6)
3.4 3.3 3.4 3.7 3.5
16,220 27,010 9410 12,610 8120
18.8 18.7 7.6 10.2 5.8
6.4 6.3 4.0 4.4 2.1
(0.50,0.47) (0.49,0.48) (0.48,0.47) (0.36,0.44) (0.34,0.43)
a b c
At 1 cd/m2. Maximum value. At 1000 cd/m2.
utilizing ultrathin layers of blue and yellow iridium complexes as the PWLs.
4. Conclusions
Fig. 4. Luminous efficiency–current density characteristics of the QQW-devices.
In summary, we demonstrated the non-doped WOLEDs with a quadruple-quantum-well structure, in which ultrathin layers of the blue and yellow iridium complexes were alternately formed as the PWLs. The results showed that the device performance was greatly dependent on the material choice and thickness adjustment of the PBLs. For the device with all-TPBi PBLs, high performance was achieved with the sacrifice of white emission. In the case of the device employing mCP combined TPBi as the PBLs, stable white emission along with low efficiency roll-off was realized, attributed to well confined excitons in the multiple PWLs caused by high energy levels of the mCP PBL, accompanied by reduced exciton quenching and improved electron–hole balance. The described non-doped QQW concept indicated the feasibility of this strategy to obtain high-efficiency WOLEDs with excellent stability as well as simplified manufacturing.
Acknowledgments This work was supported by the National Science Foundation of China (NSFC; Grant nos. 61177032 and 61006036), the Foundation for Innovative Research Groups of the NSFC (Grant no. 61021061), the Fundamental Research Funds for the Central Universities (Grant nos. ZYGX2010Z004 and ZYGX2010J004), SRF for ROCS, SEM (Grant no. GGRYJJ08-05), and Doctoral Fund of Ministry of Education of China (Grant no. 20090185110020). References
Fig. 5. External quantum efficiency–current density characteristics of the QQW-devices.
Consequently, the host–guest doping system has become a universal method for solving this problem. Here, the presented QQW structure has advantages of simplifying the fabrication process and guaranteeing device reproducibility, together with realizing low efficiency roll-off and excellent color stability. On the other hand, the results suggest there is still a large room for performance enhancement through further device optimization, and more studies are required on these MQW-devices. To our best knowledge, this study is the first by applying quadruple-quantum-well structure to the non-doped PWOLEDs,
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