High performance red organic electroluminescent devices based on a trivalent iridium complex with stepwise energy levels

Abstract

In this work, electroluminescent (EL) devices with double light-emitting layers (EMLs) having stepwise energy levels were designed and fabricated to improve the EL performances of the red light-emitting trivalent iridium complex bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) [Ir(MDQ)₂(acac)]. To broaden the recombination zone and facilitate the balance of carriers on emitter molecules, the widely used p-type material 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TcTa) and bipolar material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) were chosen as host materials of EML1 and EML2, respectively due to their well matched energy levels. Interestingly, slight decomposition of Ir(MDQ)₂(acac) molecules was observed during the deposition of EML, which causes the rapidly decreased brightness at relatively high doping concentration. Finally, a high performance red EL device with maximum current efficiency of 44.76 cd A⁻¹, power efficiency of 40.19 lm W⁻¹, and external quantum efficiency (EQE) of 15.5% was obtained by optimizing the doping concentration of Ir(MDQ)₂(acac). Even at a high brightness of 1000 cd m⁻² (5.2 V), a current efficiency as high as 40.59 cd A⁻¹ (EQE = 14.4%) can still be retained by the same device.

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1. Introduction

Organic light-emitting diodes (OLEDs) have attracted great interest throughout the world due to their potential application in full-color flat-panel displays and solid-state lighting.^1–3 For commercial application, three primary colors of blue, green and red are basically required. While the efficiencies and brightness of green electroluminescent (EL) devices have already attained the requirements for practical applications,⁴ pure blue and red EL devices still present a challenge in terms of efficiency and color purity.^5–7 Phosphorescent transition metal complexes have been increasingly used in OLEDs because phosphors can harvest both singlet and triplet excitons thus realize a theoretical 100% internal quantum efficiency.^6,8,9 Significant enhancement on maximum EL efficiencies has been realized, but the roll-off of EL efficiency is quite severe in phosphorescent OLEDs and it detrimentally degrades the device working performance for practical applications particularly at high luminance.

In the past decades, many efficient red emitters and devices have been developed and reported. For example, Cheng et al. reported the typical red emitting iridium complex bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) [Ir(MDQ)₂(acac)], which displayed pure red emission peaked at about 608 nm and high quantum yield of 48% in dichloromethane. By doping Ir(MDQ)₂(acac) into the widely used host material 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl (CBP), red EL device with maximum current efficiency of 26.2 cd A⁻¹, power efficiency of 13.7 lm W⁻¹, and external quantum efficiency (EQE) of 12.4% was obtained.¹⁰ Later, Wang et al. obtained red OLED with the maximum current efficiency of 37 cd A⁻¹ and EQE of 24.8% by co-doping green emitting material bis(2-phenylpyridine)(acetylacetonate)iridium(III) [Ir(ppy)₂(acac)] and red emitting material [Ir(MDQ)₂(acac)] into CBP.¹¹ In addition, Ir(MDQ)₂(acac) was also utilized as red emitter to design high performance white OLEDs.^12,13 Although certain progresses have been realized, EL performances of these reported red devices are still not satisfactory compared with the theoretical values. Therefore, more efforts should be paid on the optimization of device structures to further improve the EL performances of the red emitting materials.

In this work, we aim to further improve the EL performances of the red-emitting iridium complex Ir(MDQ)₂(acac) (Fig. 1) by designing the well matched double light-emitting layers (EMLs) device structure, which has been demonstrated to be effective in balancing holes and electrons, confining exciton-formation zone as well as delaying the roll-off of efficiency.^14,15 By doping Ir(MDQ)₂(acac) into different host materials with stepwise energy levels, a series of EL devices with single or double EML(s) were fabricated and investigated. Although slight decomposition of Ir(MDQ)₂(acac) molecules was observed during the deposition of EML, high performance red EL device with the maximum current efficiency, power efficiency and EQE up to 44.76 cd A⁻¹, 40.19 lm W⁻¹ and 15.5%, respectively, was obtained by minutely optimizing the doping concentration. Even at high brightness of 1000 cd m⁻², EL efficiency as high as 40.59 cd A⁻¹ (EQE = 14.1%) can be retained. Our experimental results confirmed the efficacy of designing device structure with stepwise energy levels in realizing high performance EL devices.

Fig. 1 Proposed energy level diagram of the devices used in this work and the molecular structures of Ir(MDQ)₂(acac) and 26DCzPPy.

2. Experiment

All the organic materials used in this study were obtained commercially and used as received without further purification. Indium–tin-oxide (ITO) coated glass with a sheet resistance of 10 Ω sq⁻¹ was used as the anode substrate. Prior to film deposition, patterned ITO substrates were cleaned with detergent, rinsed in de-ionized water, and dried in an oven. All organic layers were deposited with the rate of 0.1 nm s⁻¹ under high vacuum (≤2.0 × 10⁻⁵ Pa). The EMLs were prepared by co-evaporating Ir(MDQ)₂(acac) and host material from two individual sources, and the doping concentration was modulated by controlling the evaporation rate of Ir(MDQ)₂(acac). MoO₃, LiF and Al were deposited in another vacuum chamber (≤8.0 × 10⁻⁵ Pa) with the rates of 0.01, 0.01, and 1.0 nm s⁻¹, respectively, without being exposed to the atmosphere. The thicknesses of these deposited layers and the evaporation rate of individual materials were monitored in vacuum with quartz crystal monitors. A shadow mask was used to define the cathode and to make ten 9 mm² devices on each substrate. Current density–voltage–brightness (J–V–B) characteristics were measured by using a programmable Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a silicon photodiode. The EL spectra were measured with a calibrated Hitachi F-7000 fluorescence spectrophotometer. The external quantum efficiency of EL device was calculated based on the photo energy measured by the photodiode, the EL spectrum, and the current pass through the device.

3. Result and discussion

Device structure and energy level diagram of the designed OLEDs are depicted in Fig. 1. In this case, 3 nm MoO₃ film was deposited upon indium–tin-oxide (ITO) layer as anode modification layer to improve the injection and transport of holes from anode into organic layers. Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) was used as hole transport/electron block layer (HTL/EBL) due to its high hole mobility (1 × 10⁻² cm² V⁻¹ s⁻¹) and high-lying LUMO level (−1.8 eV),¹⁶ while 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) was used as hole block/electron transport layer (HBL/ETL) due to its low-lying HOMO level (−6.7 eV) and high electron mobility (1 × 10⁻³ cm² V⁻¹ s⁻¹).¹⁷ Ir(MDQ)₂(acac) was chosen as emitter due to its pure red emission and matched energy levels.¹⁰ p-Type material 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TcTa) and bipolar material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) were chosen as host materials of EML1 and EML2, respectively.¹⁸

Theoretically speaking, the stepwise HOMO levels of TAPC (−5.5 eV), TcTa (−5.7 eV), and 26DCzPPy (−6.1 eV) are beneficial for the injection and transport of holes,^16,18,19 while the stepwise LUMO levels of TmPyPB (−2.7 eV), 26DCzPPy (−2.6 eV), and TcTa (−2.4 eV) are beneficial for the injection and transport of electrons.^17–19 Therefore, balanced distribution of carriers (holes and electrons) and wide recombination zone could be expected. More importantly, the LUMO level of TAPC is 0.6 eV higher than that of TcTa while the HOMO level of TmPyPB is 0.6 eV lower than that of 26DCzPPy; thus, holes and electrons are well confined within EMLs.²⁰ Since the HOMO and LUMO levels of Ir(MDQ)₂(acac) (−5.4 and −2.8 eV, respectively) are within those of TcTa and 26DCzPPy,^10,18 carrier trapping is conceived to be the dominant EL mechanism of these devices.^20,21 In addition, the triplet energies of TcTa (2.83 eV) and 26DCzPPy (2.71 eV) are higher than that of Ir(MDQ)₂(acac) (2.0 eV), which effectively avoid the reverse energy transfer from dopant to host molecules.^12,22,23

Firstly, five single-EML devices with the structure of ITO/MoO₃ (3 nm)/TAPC (40 nm)/Ir(MDQ)₂(acac) (x wt%):26DCzPPy (10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) were fabricated and examined by adjusting x to be 1.8, 2.0, 2.2, 2.4 and 3.0, respectively. Fig. 2 depicted the doping concentration dependence of EL efficiency. Brightness–current density–voltage characteristics of these single-EMLs devices were depicted in the inset of Fig. 2. The key performances of these five devices were summarized in Table 1, where the turn-on voltage (V_turn-on) was defined as the voltage on which the brightness of 1 cd m⁻² was obtained. The 2.0 wt% doped single-EML device displayed the maximum current efficiency of 35.20 cd A⁻¹ (EQE = 12.3%) and power efficiency of 24.39 lm W⁻¹, while the 1.8 wt% doped single-EML device displayed the maximum brightness of 63 152 cd m⁻². As shown in Fig. 2 and Table 1, EL brightness decreased rapidly with increasing doping concentration of Ir(MDQ)₂(acac), which can be attributed to the aggregation of Ir(MDQ)₂(acac) molecules. In addition, during the deposition of EML, vacuum degree of the evaporation chamber decreased significantly, which indicates the slight decomposition of Ir(MDQ)₂(acac) molecules at high temperature; to some degree, the rapidly decreased brightness with increasing doping concentration can be partially attributed to the slight decomposition of Ir(MDQ)₂(acac) molecules.

Fig. 2 EL efficiency–current density (η–J) characteristics of the single-EML devices with Ir(MDQ)₂(acac) at different doping concentrations. Inset: brightness–current density–voltage (B–J–V) characteristics of the single-EML devices with Ir(MDQ)₂(acac) at different doping concentrations.

Table 1 Key performance parameters of the single-EML devices with Ir(MDQ)₂(acac) at different doping concentrations

Device	Vturn-on (V)	B^a (cd m^−2)	ηc^b (EQE^c) (cd A^−1)	ηp^d (lm W^−1)	ηc^e (cd A^−1) (EQE^f) (1000 cd m^−2)	CIEx,y^g
a The data for maximum brightness (B).b Maximum current efficiency (ηc).c Maximum external quantum efficiency (EQE).d Maximum power efficiency (ηp).e Current efficiency (ηc) at the certain brightness of 1000 cd m^−2.f External quantum efficiency (EQE) at the certain brightness of 1000 cd m^−2.g Commission Internationale de l'Eclairage coordinates (CIEx,y) at 10 mA cm^−2.
1.8 wt%	3.3	63 152	30.46 (10.6%)	21.32	30.45 (10.6%, 4.9 V)	(0.546, 0.439)
2.0 wt%	3.4	62 190	35.20 (12.3%)	24.39	33.35 (11.6%, 5.2 V)	(0.553, 0.436)
2.2 wt%	3.0	60 395	32.07 (11.1%)	21.42	29.97 (10.4%, 5.2 V)	(0.552, 0.438)
2.4 wt%	3.3	55 555	30.09 (10.7%)	21.35	27.04 (9.4%, 5.1 V)	(0.555, 0.435)
3.0 wt%	3.3	46 171	29.69 (10.3%)	21.81	26.68 (9.3%, 5.3 V)	(0.559, 0.431)

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Meanwhile, five double-EMLs devices with the structure of ITO/MoO₃ (3 nm)/TAPC (40 nm)/Ir(MDQ)₂(acac) (x wt%):TcTa (10 nm)/Ir(MDQ)₂(acac) (x wt%):26DCzPPy (10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) were also designed and fabricated by adjusting x to be 1.8, 2.0, 2.2, 2.4 and 3.0, respectively. Fig. 3 depicted the doping concentration dependence of EL efficiency. Brightness–current density–voltage characteristics of these double-EMLs devices were depicted in the inset of Fig. 3. With increasing doping concentration, EL efficiency increased to a maximum value and then decreased gradually. As listed in Table 2, the 2.0 wt% doped double-EMLs device gave the maximum current efficiency of 44.76 cd A⁻¹ (EQE = 15.5%) and the power efficiency of 40.19 lm W⁻¹. Even at the certain brightness of 1000 cd m⁻², EL efficiency as high as 40.59 cd A⁻¹ (EQE = 14.1%) can be retained by this device. On the other hand, the 2.2 wt% doped double-EMLs device displayed the maximum brightness of 80 263 cd m⁻². Compared with single-EML devices, double-EMLs devices displayed relatively higher brightness, higher efficiencies and slower roll-off of EL efficiencies, which demonstrates the efficacy of designing double-EMLs devices with stepwise energy levels. Amongst both single-EML and double-EMLs devices, the 2.0 wt% doped devices achieved the highest EL efficiencies, so the optimal concentration of Ir(MDQ)₂(acac) can be determined to be 2.0 wt%.

Fig. 3 EL efficiency–current density (η–J) characteristics of the double-EMLs devices with Ir(MDQ)₂(acac) at different doping concentrations. Inset: brightness–current density–voltage (B–J–V) characteristics of the double-EMLs devices with Ir(MDQ)₂(acac) at different doping concentrations.

Table 2 Key performance parameters of the double-EMLs devices with Ir(MDQ)₂(acac) at different doping concentrations

Device	Vturn-on (V)	B^a (cd m^−2)	ηc^b (EQE^c) (cd A^−1)	ηp^d (lm W^−1)	ηc^e (cd A^−1) (EQE^f) (1000 cd m^−2)	CIEx,y^g
a The data for maximum brightness (B).b Maximum current efficiency (ηc).c Maximum external quantum efficiency (EQE).d Maximum power efficiency (ηp).e Current efficiency (ηc) at the certain brightness of 1000 cd m^−2.f External quantum efficiency (EQE) at the certain brightness of 1000 cd m^−2.g Commission Internationale de l'Eclairage coordinates (CIEx,y) at 10 mA cm^−2.
1.8 wt%	3.4	76 721	42.32 (14.4%)	31.30	40.82 (13.9%, 4.9 V)	(0.553, 0.437)
2.0 wt%	3.4	73 675	44.76 (15.5%)	40.19	40.59 (14.1%, 5.2 V)	(0.556, 0.435)
2.2 wt%	3.3	80 263	40.06 (13.9%)	27.33	39.41 (13.7%, 5.3 V)	(0.557, 0.436)
2.4 wt%	3.4	71 441	39.90 (13.8%)	29.95	38.84 (13.5%, 5.2 V)	(0.559, 0.433)
3.0 wt%	3.4	54 525	38.06 (13.2%)	30.20	34.41 (11.9%, 5.3 V)	(0.564, 0.427)

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Encouraged by the outstanding improvement of EL performances, EL spectra of these single-EML and double-EMLs devices operating at the current density of 10 mA cm⁻² were measured and shown in Fig. 4 and 5, respectively. Apart from the characteristic emission of Ir(MDQ)₂(acac) (peaked at 600 nm), weak 26DCzPPy emission (peaked at 390 nm) was observed in these devices.^10,14 In this case, the emergence of weak 26DCzPPy emission in EL spectra can be attributed to the accumulation of holes and electrons within EML(s), which result in the transfer of few carriers from emitter onto host molecules. Consequently, few holes and electrons recombine on 26DCzPPy molecules. Among these devices, the 2.0 wt% doped single-EML and double-EMLs devices displayed the weakest 26DCzPPy emission, which provides further evidence for the conclusion that 2.0 wt% is the optimal doping concentration. At the current density of 10 mA cm⁻², the 2.0 wt% doped single-EML device displayed the Commission Internationale de l'Eclairage (CIE) coordinate of (0.553, 0.436), while the 2.0 wt% doped double-EMLs device displayed the CIE coordinate of (0.556, 0.435). With increasing current density, no distinguishable discrepancy of EL spectra was found in these devices, which demonstrates the superior color stability of these devices.

Fig. 4 Normalized EL spectra of the single-EML devices operating at the current density of 10 mA cm⁻².

Fig. 5 Normalized EL spectra of the double-EMLs devices operating at the current density of 10 mA cm⁻².

On the other hand, as shown in Fig. 6, EL spectra of 2.0 wt% doped single-EML and double-EMLs devices operating at the current density of 100 mA cm⁻² were also measured. In addition, photo-luminescent (PL) spectra of TcTa and 26DCzPPy were also given in Fig. 6. Compared with single-EML device, double-EML device exhibited a slightly red shift of host emission from 390 toward 400 nm, which means the contribution of TcTa emission. This result indicates the recombination of holes and electrons on TcTa molecules, thus the broadening recombination zone in double-EML device. Interestingly, as shown in the inset of Fig. 6, current density–voltage characteristics of 2.0 wt% doped single-EML and double-EMLs devices are nearly identical when the operation voltage is higher than the turn-on voltage. This phenomenon enlighten us that the insertion of TcTa EML facilitate the transfer of holes, which compensates the negative effect of voltage drop on TcTa EML.²⁴

Fig. 6 Normalized EL spectra of the 2.0 wt% doped single-EML and double-EMLs devices operating at the current density of 100 mA cm⁻². Inset: brightness–current density–voltage (B–J–V) characteristics of the 2.0 wt% doped single-EML and double-EMLs devices.

To better understand the EL mechanisms of these single-EML and double-EMLs devices, we have further analyzed the distribution of holes and electrons within EML(s). For single-EML devices, as shown in Fig. 7(a), holes and electrons accumulate in HTL and EML, respectively, near the interface of HTL/EML. Compared with single-EML devices, double-EMLs devices possess better hole injection ability from HTL into EMLs because the HOMO level of TcTa situate between those of TAPC and 26DCzPPy, which causes the low hole injection barrier.²⁴ So for double-EMLs devices, as shown in Fig. 7(b), holes and electrons accumulate in EML1 and EML2, respectively, near the interface of EML1/EML2. Consequently, double-EMLs devices obtained better balance of carriers within EMLs and thus higher EL efficiencies compared with single-EML devices. On the other hand, double-EMLs devices possess wider recombination zone and thus the lower density of triplet excitons, which provides the reasonable interpretation for why double-EMLs devices displayed slower roll-off of efficiencies and higher brightness. In addition, the broadening recombination zone helps to reduce the densities of holes and electrons within EMLs, thus facilitating the trapping of holes and electrons on emitter molecules, which is helpful in improving both efficiency and color purity.

Fig. 7 Schematic representation of carriers' distribution within the EML(s) of single-EML device (a) and double-EMLs device (b). Symbols − and + represent electrons and holes, respectively.

4. Conclusion

In summary, we have designed and fabricated the high performance red EL devices by doping Ir(MDQ)₂(acac) into host materials with stepwise energy levels. Compared with single-EML devices, double-EMLs devices displayed higher EL efficiency, slower roll-off of efficiency, and higher brightness attributed to better balance of holes and electrons, broadening recombination zone and improved trapping of holes and electrons. Finally, pure red EL device with maximum current efficiency of 44.76 cd A⁻¹, power efficiency of 40.19 lm W⁻¹, and external quantum efficiency of 15.5% was obtained. Even at the high brightness of 1000 cd m⁻², EL efficiency as high as 40.59 cd A⁻¹ (EQE = 14.4%) can be retained by the same device.

Acknowledgements

The authors are grateful to the financial aid from Research Equipment Development Project of Chinese Academy of Sciences (YZ201562), Youth Innovation Promotion Association of Chinese Academy of Sciences (2013150), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20030300), National Natural Science Foundation of China (21521092, 21590794 and 21210001), and National Key Basic Research Program of China (No. 2014CB643802).

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