Harnessing charge and exciton distribution towards extremely high performance: the critical role of guests in single-emitting-layer white OLEDs

Baiquan Liu a, Lei Wang a, Dongyu Gao b, Miao Xu a, Xuhui Zhu a, Jianhua Zou *a, Linfeng Lan a, Honglong Ning a, Junbiao Peng *a and Yong Cao a
aInstitute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China. E-mail: zou1007@gmail.com; psjbpeng@scut.edu.cn
bNew Vision Opto-Electronic Technology Co., Ltd, Guangzhou 510530, China

Received 3rd April 2015 , Accepted 22nd July 2015

First published on 22nd July 2015


Currently, high efficiency, low efficiency roll-off and stable color white organic light-emitting diodes (WOLEDs) are still challenging to realize via simplified structures. Herein, the simplicity/extremely high efficiency/low efficiency roll-off/stable color trade-off has been accomplished in a single-emitting-layer (single-EML) phosphorescent WOLED. The WOLED exhibits total efficiencies of 111.7 cd A−1 and 75.5 lm W−1 at a practical luminance of 1000 cd m−2, these efficiencies are the highest among single-EML WOLEDs. Even at 5000 cd m−2, the efficiencies remain as high as 101.3 cd A−1 and 58.8 lm W−1. Besides, the color variation in the whole range of luminances is (0.00, 0.00), which is the first single-EML WOLED with an extremely stable color. The origin of the high performance is unveiled. Particularly, the role of guests, which is often ignored in single-EML WOLEDs, on the origin of the color-stability is systematically revealed and harnessing charge and exciton distribution by using multifunctional guests is demonstrated to be critical for stabilizing the color. Such marvelous results not only represent a significant step towards the realization of simplified WOLEDs, but also provide a new opportunity to achieve extremely stable colors [ΔCIE (Commission International de I'Eclairage) = (0.00, 0.00)].



Conceptual insights

White organic light-emitting diodes (WOLEDs) are the object of intense research since they are promising for future display and lighting applications. This work demonstrates that it is possible to accomplish simplicity/extremely high efficiency/low efficiency roll-off/stable color trade-off in a single-emitting-layer WOLED, although it is still a big challenge. Unprecedented efficiencies of 111.7 cd A−1 and 75.5 lm W−1 at 1000 cd m−2 have been achieved, which have already overtaken fluorescent lamps and most state-of-the-art multi-emitting-layer WOLEDs. Furthermore, a rather low efficiency roll-off is realized, since the efficiencies remain as high as 101.3 cd A−1 and 58.8 lm W−1 even at 5000 cd m−2. Moreover, this is the first single-EML WOLED with an extremely stable color, since the color variation across a whole range of luminances is (0.00, 0.00). Then, the origin of the high performance is unveiled. Particularly, we have systematically revealed the role of guests, which is often ignored, on the origin of the color-stability and demonstrated that harnessing charge and exciton distribution by using multifunctional guests is crucial to stabilize the color. The presented results here will be beneficial to the design of both material and device structure for extremely high-performance WOLEDs.

Introduction

White organic light-emitting diodes (WOLEDs) have been the object of intense research because of their striking advantages, like high efficiency, long lifetime and flexibility.1–5 In order to boost the efficiency, phosphorescent emitters are usually imperative to achieving an internal quantum efficiency of 100% by consuming both singlet and triplet excitons (generated at a ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]3 owing to their multiplicity).3 As a consequence, the thoughtful design and understanding of the emitting layer (EML) may be the area for further enhancing the performance of WOLEDs.

Generally, mixing two complementary colors (i.e., blue/orange and blue/red) or three primary colors (blue/green/red) is required to generate white emission. To date, numerous methods have been reported to realize WOLEDs, including managing singlet and triplet excitons in hybrid structures,6 harvesting triplet excitons from fluorescent blue emitters with high triplet energies,7 having carrier- and exciton-confining structures for reduced efficiency roll-off,8 combining blue/green/red phosphors with light-outcoupling enhancement techniques,9 adopting color down-conversion media,10 comprising ultrathin EMLs,11 exploiting tandem architectures,12 employing excimer/exciplex systems,13 and so forth. However, multiple EMLs are widely required to produce white emissions, resulting in complex structures. In addition, the recombination zone usually changes with increasing voltages, leading to unstable colors.14 Besides, the charge injection/transfer, exciton diffusion/confinement and energy transfer between or within the different EMLs should be considered, further complicating the structures. Moreover, more heterojunctions would emerge from the various EMLs, which is detrimental to the device lifetime.15

To alleviate the bottleneck, elaborate structures have to be developed, aside from the synthesis of elegant materials. Since single-EML WOLEDs, which only need one EML comprised of a versatile host doped with different color guests to furnish white emission, can offer many unique merits (i.e., simpler structures, shorter fabrication processes, easier control, lower voltages and lower cost, and so forth), compared with their multi-EML counterparts, they have been studied for both polymer and small-molecule WOLEDs.16 For example, Zhang et al. recently developed a single-EML WOLED based on the solution-processed dendritic host and new orange-emitting Ir-complex, achieving a maximum forward-viewing power efficiency (PE) of 47.6 lm[thin space (1/6-em)]W−1 (23.3 lm[thin space (1/6-em)]W−1 at 1000 cd m−2), which is the most efficient polymer WOLED.17 In the case of small-molecule WOLEDs, plenty of endeavors have been made using the single-EML method as well. Ma et al. used the p-type material mCP as the host to set up a single-EML WOLED with a PE of 42.5 lm W−1 at ∼7 cd m−2 (19 lm[thin space (1/6-em)]W−1 at 500 cd m−2).18 Huang et al. also fabricated a single-EML WOLED by using a unipolar host, obtaining a maximum efficiency of 14.2 lm[thin space (1/6-em)]W−1 (6.8 lm[thin space (1/6-em)]W−1 at 1000 cd m−2).19 However, relatively low efficiency, serious efficiency roll-off and poor color-stability are commonplace for the single-EML WOLEDs comprising unipolar hosts. The unsatisfactory performance can be attributed to the intrinsic quality of unipolar host materials, since they usually only transport holes or electrons, resulting in the charge accumulation/recombination close to the EML/electron transporting layer (ETL) interface or the hole transporting layer (HTL)/EML interface. As a consequence, narrow recombination zones and then triplet–triplet annihilation (TTA), triplet-polaron quenching (TPQ) and exciton diffusion can occur, which is detrimental to device performance.20

To solve these issues, the introduction of single-EML WOLEDs employing bipolar hosts is one of the most ideal solutions. As bipolar materials facilitate both hole and electron transport, more balanced charges as well as broader recombination regions within the EMLs can be guaranteed, resulting in devices with the potential of higher efficiency, lower efficiency roll-off and more stable color.21 Gong et al. utilized the new silicon-bridged molecule as the bipolar material of a single-EML WOLED, achieving a maximum PE of 42.7 lm[thin space (1/6-em)]W−1 at ∼30 cd m−2 (25.2 lm[thin space (1/6-em)]W−1 at 1000 cd m−2) and ΔCIE (Commission International de I'Eclairage) = (0.01, 0.00) between 8–12 V.22 They also developed a single-EML WOLED by utilizing the bipolar tetraarylsilane compound as the host, achieving a maximum PE of 51.9 lm[thin space (1/6-em)]W−1 (22.2 lm[thin space (1/6-em)]W−1 at 1000 cd m−2) and ΔCIE = (0.01, 0.00) during 8–12 V.23 Wu and Wong's group constructed a single-EML WOLED by incorporating a CN group into mCP to design a bipolar host, attaining a maximum PE of 49.7 lm[thin space (1/6-em)]W−1 at ∼1 cd m−2 (25 lm[thin space (1/6-em)]W−1 at 1000 cd m−2) but with no color-stability information.24 More recently, Pan et al. prepared a single-EML WOLED with bipolar hosts, yielding a maximum efficiency of 60.4 lm[thin space (1/6-em)]W−1 at ∼10 cd m−2 (27.4 lm[thin space (1/6-em)]W−1 at 1000 cd m−2, ΔCIE = (0.02, 0.01) between 5–10 V).25 Previously, we also developed a series of high-performance polymer/small-molecule WOLEDs with single-EML structures due to the above mentioned merits.26–28 Thus, the use of the single-EML architecture is a very promising, simplified, as well as effective, method to realize WOLEDs. However, despite the performances of single-EML WOLEDs steadily increasing, they still face at least three challenges: (i) the efficiency is not high enough, particularly, single-EML WOLEDs with a forward-viewing PE exceeding 30 lm[thin space (1/6-em)]W−1 at the practical luminance of 1000 cd m−2 are rarely reported; (ii) the efficiency roll-off is afflicting the development of single-EML WOLEDs, although numerous bipolar hosts are adopted; (iii) the color-stability is not good enough, the reported colors are only relatively stable during a very limited luminance/voltage. In fact, no single-EML WOLED with extremely stable color [ΔCIE = (0.00, 0.00)] has been documented.

In this paper, we have accomplished simplicity/extremely high efficiency/low efficiency roll-off/excellent color-stability trade-off in a single-EML WOLED. The optimized device exhibits total efficiencies of 111.7 cd A−1 and 75.5 lm W−1 at the practical luminance of 1000 cd m−2, which are, to the best of our knowledge, the highest efficiencies among single-EML WOLEDs so far. Besides, a maximum current efficiency (CE) and PE of 113.6 cd A−1 and 92.5 lm W−1 are obtained, respectively, which only slightly decrease to 101.3 cd A−1 and 58.8 lm W−1 even at an ultrahigh luminance of 5000 cd m−2, indicating a rather low efficiency roll-off. Additionally, the color variation across a whole range of luminances is (0.00, 0.00), which is the first single-EML WOLED with an extremely stable color. The origin of the high performance has been comprehensively unveiled. In particular, we have systematically revealed the role of guests, which is often ignored in single-EML WOLEDs, on the origin of the color-stability and demonstrated that harnessing charge and exciton distribution by using multifunctional guests is crucial to stabilize the color. Such marvelous results not only represent a significant step towards the realization of simplified WOLEDs, but also provide a new opportunity to achieve extremely stable colors [ΔCIE = (0.00, 0.00)].

Materials and methods

As shown in Fig. 1, the WOLED (W1) has the configuration of ITO (indium tin oxide)/MeO-TPD (N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine)[thin space (1/6-em)]:[thin space (1/6-em)]F4-TCNQ (tetrafluorotetracyanoquinodimethane) (100 nm, 4%)/TAPC (1-bis[4-[N,N-di(4-tolyl)amino]phenyl]cyclohexane, 20 nm)/26DCzPPy[thin space (1/6-em)]:[thin space (1/6-em)]FIrpic[thin space (1/6-em)]:[thin space (1/6-em)]Ir(dmppy)2(dpp) (11 nm, 1[thin space (1/6-em)]:[thin space (1/6-em)]22.5%[thin space (1/6-em)]:[thin space (1/6-em)]1.1%)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (200 nm), in which iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2] (FIrpic) and bis(2-phenyl-4,5-dimethylpyridinato)[2-(biphenyl-3-yl)pyridinato] iridium(III) [Ir(dmppy)2(dpp)] are used as the blue and orange emitters, respectively, to co-dope into the host, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy). ITO, MeO-TPD[thin space (1/6-em)]:[thin space (1/6-em)]F4-TCNQ, TAPC, 26DCzPPy[thin space (1/6-em)]:[thin space (1/6-em)]FIrpic[thin space (1/6-em)]:[thin space (1/6-em)][Ir(dmppy)2(dpp)], TmPyPB, LiF and Al are the anode, hole injection layer (HIL), HTL, EML, ETL, electron injection layer (EIL) and cathode, respectively. These material layers have been evaporated in sequence at a base pressure of 2 × 10−7 Torr without breaking the vacuum, the ITO surface was cleaned by utilizing ultrasonic water for 3 minutes and then dried at 80 °C for 20 minutes. All of these materials were commercially purchased with no further purification. After preparation under a nitrogen atmosphere using epoxy glue and glass lids, the devices were immediately encapsulated. The CIE color coordinates and electroluminescent (EL) spectra were obtained by a Konica Minolta CS2000 spectra system. The emission area of the devices is 3 × 3 mm2 as defined by the overlapping area of the anode and cathode. The luminance–current density (J)–voltage (V) characteristics were recorded simultaneously, using a computer-controlled source meter (Keithley 2400) and multimeter (Keithley 2000) with a calibrated silicon photodiode. All the measurements were carried out at room temperature under ambient conditions.
image file: c5mh00051c-f1.tif
Fig. 1 Top: The structure of W1 and the chemical structure of guests. Bottom: The proposed highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the device.

Results and discussion

The forward-viewing efficiencies of W1 as a function of the luminance have been displayed in Fig. 2. The CE and PE peaks are 66.8 cd A−1 (201 cd m−2) and 54.4 lm W−1(33 cd m−2), respectively. These remained at 65.7 cd A−1, 44.4 lm W−1 at the illumination-relevant level of 1000 cd m−2. Since illumination sources have been generally represented by the total emitted power,1–7 the peak total CE and PE of W1 are 113.6 cd A−1 and 92.5 lm W−1, respectively, which only slightly decreased to 111.7 cd A−1 and 75.5 lm W−1 at the practical luminance of 1000 cd m−2. In fact, the efficiency has already overtaken fluorescent lamps (with a typical PE of 40–70 lm W−1, at 1000 cd m−2) demonstrating a significant step in simplified WOLEDs.17 Besides, it is noted that the efficiencies have exceeded most state-of-the-art WOLEDs with multi-EML structures, as shown in Table 1. Table 1 also shows that the efficiency/efficiency roll-off/color-stability trade-off is rarely accomplished by these multi-EML WOLEDs, which further demonstrates the advantages of our single-EML WOLED.
image file: c5mh00051c-f2.tif
Fig. 2 Forward-viewing CE and PE in dependence of luminance for device W1. Inset: Normalized EL spectra in the whole range of luminances.
Table 1 Summary of most advanced, representative performances of multi-EML WOLEDs in the last four years (without outcoupling technology)
Device PEmax/100/1000[thin space (1/6-em)]a (lm W−1) CIE1000[thin space (1/6-em)]b Color shift
a PE in the order of maximum, at 100 cd m−2 and at 1000 cd m−2. b The CIE coordinates of devices at 1000 cd m−2. c Color shift in the whole range of luminances.
Ref. 29 (2 EMLs) 34/—/— (0.35, 0.44) (0.006, 0.009)@6–10 V
Ref. 30 (2 EMLs) 49.5/—/30.0 (0.44, 0.47) (0.00, 0.00)@3–7 V
Ref. 31 (2 EMLs) 68.8/—/60.0 (0.347, 0.461)
Ref. 32 (2 EMLs) 47.2/32.9/15.6 (0.42, 0.42) (0.01, 0.01)@5–8 V
Ref. 33 (2 EMLs) 63/—/52 (0.359, 0.498) Good@1–10 mA cm−2
Ref. 34 (3 EMLs) 61/37/12 (0.34, 0.44)
Ref. 35 (4 EMLs) —/42.6/33.8 (0.44, 0.46)
Ref. 36 (4 EMLs) 48.2/45.3/26.8 (0.38, 0.45) (0.08, 0.12)@10–10[thin space (1/6-em)]000 cd m−2
Ref. 37 (4 EMLs) ∼40/—/35.8 (0.507, 0.449)
This work (single-EML) 92.5/88.9/75.5 (0.288, 0.485) (0.00, 0.00)c


Remarkably, even at an ultrahigh luminance of 5000 cd m−2, the CE of 101.3 cd A−1 and PE of 58.8 lm W−1 have still been recorded, demonstrating that W1 has a very low efficiency roll-off.1–37 Notably, W1 exhibits an excellent color-stability, as illustrated in Fig. 2 inset. As the luminance ranges from 1, 1000 to 50[thin space (1/6-em)]000 cd m−2, the color experiences almost no shift from (0.2858, 0.4811), (0.2876, 0.4853) to (0.2864, 0.4856). The total variation in the whole range of luminance is ΔCIE ≤ (0.0022, 0.0045). To appropriately compare it with the color-stability of previous WOLEDs, the total variation is (0.00, 0.00) if we compute to two decimal places. Unlike the previous concept that phosphorescent WOLEDs should exhibit unstable colors since they comprise blue phosphors (i.e., FIrpic) which are usually detrimental to the color-stability,6 we have demonstrated that the excellent color-stability can be realized in single-EML phosphorescent WOLEDs, indicating a significant breakthrough in WOLEDs.

Galvanized by the superior properties obtained from this simplified device, we then make a comprehensive investigation in order to reveal the origin of the high performance. In particular, we will unveil the role of guests in the color-stability. Before we explore the impact of guests, several other important factors for the high performance will be discussed.

First, two-complementary color (blue/orange) WOLEDs can not only lower the manufacturing cost but also show higher efficiency, compared with three-color WOLEDs.3 Besides, the correlated color temperature (i.e., cool or warm white) are readily modified by regulating the concentration of blue and orange guests.34 Therefore, high-performance WOLEDs can be expected by taking advantage of this strategy.

Next, the bipolar host 26DCzPPy plays a vital role, because: (i) the triplet energy (T1) is (2.71 eV),38 satisfying the blue FIrpic guest (2.65 eV)38 as well as the orange Ir(dmppy)2(dpp) emitter (<2.25 eV),39 which can not only prevent the reverse energy transfer from FIrpic/Ir(dmppy)2(dpp) to 26DCzPPy but also confine triplets in the whole EML, effectively enhancing the efficiency.3 (ii) The similar hole and electron mobility [∼10−5 cm2 (V s)−1]38 can broaden the exciton recombination zone and achieve balanced charge transport for the recombination procedure. Therefore, TTA and TPQ can be effectively eliminated via 26DCzPPy, leading to high efficiency, low efficiency roll-off as well as good color-stability.22 (iii) The LUMO and HOMO of 26DCzPPy are 2.65 eV and 6.05 eV, respectively, matching well with those of HTL and ETL to diminish the charge transport barrier, which can lower the driving voltages and further boost the efficiency. To illustrate the importance of 26DCzPPy, other devices using 4,4-N,N-dicarbazolebiphenyl (CBP) and TAPC hosts have been fabricated, which exhibit poor performances, as shown in Fig. S1 and S2 (ESI), respectively.

Additionally, the utilization of a carrier/exciton confining structure has a substantial influence on ensuring the performance. As shown in Fig. 3, TAPC is selected as the HTL due to (i) the high hole mobility (10−2 cm2 (V s)−1),40 giving an effective hole injection, (ii) a higher T1 (2.87 eV) than the host and guests,40 effectively confining excitons in the EML and (iii) a high LUMO (1.8 eV),40 a great energy offset of LUMO (0.85 eV) between TAPC and 26DCzPPy, decreasing the electron leakage.3 On the other hand, TmPyPB is chosen as the ETL because of: (i) its high electron mobility [10−3 cm2 (V s)−1],41 resulting in an enhanced charge balance due to the effective electron injection, (ii) a higher T1 (2.75 eV) than the host and guests,41 confining triplets in the whole EML and (iii) a deep HOMO of 6.7 eV,41 the big difference in HOMO levels (0.55 eV) between TmPyPB and the guest FIrpic (or 0.65 eV between TmPyPB and the host 26DCzPPy), reducing the hole leakage. Therefore, both charges (holes and electrons) and excitons (singlets and triplets) can be well confined within the EML via this excellent confining structure, enhancing the efficiency, efficiency roll-off and color-stability, as illustrated in Fig. 3. To illustrate the importance of TAPC and TmPyPB, other devices using N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) as the HTL or bis(10-hydroxybenzo[h]quinolinato)beryllium complex (Bebq2) as the ETL have been fabricated, which exhibit poor performances, as shown in Fig. S3 and S4 (ESI), respectively.


image file: c5mh00051c-f3.tif
Fig. 3 The charge/exciton confining structure for device W1 and the schematic diagram of EL procedures.

Furthermore, the appropriate thickness of the EML is one of the main reasons for the high performance. Although no work has systematically investigated the effect of EML thickness in single-EML phosphorescent WOLED, a further understanding of its impact is still needed. As shown in Fig. 4 and Table 2, when a thin EML thickness is used, a poor color-stability is observed. For example, devices comprising 4 nm, 6 nm and 8 nm EML exhibit the variation of (0.0150, 0.0169), (0.0093, 0.0086) and (0.0024, 0.0045) during 1–10[thin space (1/6-em)]000 cd m−2, respectively. The color shift of these devices in dependence of the thickness of EML is clearly shown in Fig. 4d. On the other hand, devices comprising a 4 nm, 6 nm and 8 nm EML exhibit forward-viewing efficiencies of 54 cd A−1 (42.6 lm W−1), 60.3 cd A−1 (44.1 lm W−1) and 62.8 cd A−1 (44.3 lm W−1) at 1000 cd m−2, respectively, which are lower than those of W1, as shown in Fig. 4e. This is because when the thickness of the EML is too thin, the emission zone is very narrow, which easily causes TTA due to a high triplet exciton density, leading to evident deterioration of the color-stability.42 As a result, the device comprising 4 nm or 6 nm EML shows a serious color shift of (0.02, 0.02) and (0.01, 0.01) if we compute to two decimal places, respectively. However, the device comprising 8 nm EML shows a stable color with the color shift of (0.00, 0.00), almost similar to that of W1, suggesting that the 8 nm EML may be enough to widen the main recombination zone. Besides, an EML that is too thin cannot effectively trap or harvest excitons, leading to a low efficiency.3 However, there is an offset between CE and PE, as shown in Fig. 4e. A higher CE can be expected when a thick EML thickness is used, however, the PE will be weakened since the thick EML thickness usually results in higher voltages. This is the reason why W1 shows a higher CE but almost the same PE as the device comprising 8 nm EML. This may also be the reason why previous WOLEDs comprising relatively thick EMLs can be used to pursue very high CE but poor PE.29 However, the thickness of W1 (11 nm) is still much thinner than most previous WOLEDs, lowering voltages and thus improving the efficiency.


image file: c5mh00051c-f4.tif
Fig. 4 Normalized EL spectra of the devices comprising various EML thicknesses during 1–10[thin space (1/6-em)]000 cd m−2: (a) 4 nm, (b) 6 nm, (c) 8 nm, (d) the color variation as a function of EML thickness and (e) Forward-viewing efficiencies at 1000 cd m−2 as a function of EML thickness.
Table 2 Summary of WOLEDs with EMLs of various thicknesses
Device CE1000[thin space (1/6-em)]a (cd A−1) PE1000[thin space (1/6-em)]b (lm W−1) CIE1000[thin space (1/6-em)]c Color shift
a Forward-viewing CE at 1000 cd m−2. b Forward-viewing PE at 1000 cd m−2. c The CIE coordinates of devices at 1000 cd m−2.
W1 (11 nm EML) 65.7 44.4 (0.288, 0.485) (0.00, 0.00)
4 nm EML 54 42.6 (0.251, 0.479) (0.02, 0.02)
6 nm EML 60.3 44.1 (0.260, 0.487) (0.01, 0.01)
8 nm EML 62.8 44.3 (0.268, 0.490) (0.00, 0.00)


Apart from the above-mentioned reasons (i.e., adopting the rational design strategy, introducing the bipolar host, using carrier/exciton confining structures, and exploiting the appropriate EML thickness), other factors may also be attributed to the high performance, particularly for the color-stability. Since the influence of each organic layer has been investigated, the effect of guests should also be considered despite being usually overlooked. Therefore, we will systematically reveal the role of guests on the origin of the color-stability.

For excellent color-stability, the recombination ratio should be constant, or else the color variation is easily observed.3,42–44 Since charge transport can be slowed down by the host–guest energy level discrepancy in the EML, the recombination ratio can be constant, thus greatly stabilizing the color. As shown in Fig. 3, since the HOMOs of 26DCzPPy and FIrpic are 6.05 eV and 6.15 eV, respectively,38 there is a positive barrier (0.1 eV) between the host and guest. As a result, iit is possible for njected holes tocollision with this barrier and scatter, which reduces the drift velocity because of the added transit path.43 Therefore, hole transport is slowed down via the blue guest FIrpic. Besides, the HOMO of the orange guest Ir(dmppy)2(dpp) is 5.05 eV,39 which is much higher than that of 26DCzPPy (6.05 eV) and lower than that of TAPC (5.3 eV),40 leading to holes being trapped by Ir(dmppy)2(dpp).3 As a result, the hole transport is greatly decreased via the orange guest because of additional trap residing times.43 Therefore, both the blue and orange guest can lower the hole mobility due to the host–guest energy level discrepancy. On the other hand, since the LUMOs of FIrpic and Ir(dmppy)2(dpp) are 3.47 eV38 and 2.8 eV,39 respectively, which are lower than those of TmPyPB (2.7 eV) and 26DCzPPy (2.65 eV), electrons can be easily trapped guests.3 As a consequence, the electron transport is vastly reduced. In a word, both holes and electron transport have been greatly reduced due to these two multifunctional guests, resulting in the recombination ratio being unchanged with the rising luminance/current, which is beneficial to obtaining stable colors.

To demonstrate the above analyses, hole-only and electron-only devices have been fabricated comprising configurations of anode/HIL/HTL/EML/TAPC(50 nm)/cathode and anode/LiF (1 nm)/TmPyPB (20 nm)/EML/ETL/EIL/cathode, respectively, in which the anode, HIL, HTL, ETL, EIL and cathode are the same as those of W1, while the 11 nm EML constitutes the host 26DCzPPy and guests are not incorporated into device H1 and E1, 1.1% Ir(dmppy)2(dpp) is included in device H2 and E2, 22.5% FIrpic in device H3 and E3, 22.5% FIrpic and 1.1% Ir(dmppy)2(dpp) codoped in device H4 as well as E4. For these single-charge transport devices, electrons cannot be injected from the cathode due to the high LUMO of TAPC (1.8 eV) and holes are impossible to be injected from the anode due to the LiF as well as the deep HOMO of TmPyPB (6.7 eV).3 As shown in Fig. 5a, both the J of H2 and H3 are lower than that of H1 due to the effect of the guests. In particular, although the concentration of Ir(dmppy)2(dpp) is low (1.1%) and the concentration of FIrpic is high (22.5%), H2 shows a lower J than H3, suggesting that Ir(dmppy)2(dpp) exhibits a strong hole trapping ability. Hence, it is no wonder that H4 shows the smallest J among these four devices. On the other hand, as shown in Fig. 5b, both the J of E2 and E3 are lower than that of E1 owing to the electron trapping of guests. In particular, although FIrpic is an electron-type material,7 FIrpic exhibits a great electron trapping ability, vastly reducing the electron transport in E3 in comparison to the J of E1. As expected, it is reasonable that E4 shows the smallest J in these four electron-only devices. Therefore, both the hole and electron mobility are greatly decreased via these two guests, experimentally demonstrating the above analysis.


image file: c5mh00051c-f5.tif
Fig. 5 The JV characteristics of single-charge transport devices with guests FIrpic, Ir(dmppy)2(dpp), (MPPZ)2Ir(acac) and Ir(piq)3-doped the host 26DCzPPy. (a) Hole-only devices. (b) Electron-only devices.

In addition, based on the above analysis, it can be concluded that excitons can be directly formed on the guests, improving the efficiency since the need to electrically excite the host is eliminated, while allowing for efficient carrier collection, exciton formation, and recombination at the guest molecular sites.3 Moreover, the charge trapping procedure is also responsible for the low efficiency roll-off, because the proportion of trapped charges are unnecessary to suffer the diffusive transport loss, together with the fact that the decreased charge and exciton accumulation at the EML broaders can reduce TTA and TCQ.45

To further verify the above analysis and illustrate the importance of the orange guest Ir(dmppy)2(dpp) in making a contribution to maintaining the color constancy resulting from the decreased hole mobility, we have fabricated another device (W21) by replacing Ir(dmppy)2(dpp) with iridium(III) diazine complexes [(MPPZ)2Ir(acac)], who has a HOMO of 5.31 eV and LUMO of 3.13 eV.46 The configuration of W21 is similar to that of W1 except the difference in orange guest (MPPZ)2Ir(acac). As shown in Fig. 6a, the CIE coordinate variation of W21 is (0.03, 0.00) during 1–10[thin space (1/6-em)]000 cd m−2, suggesting that W21 exhibits an unstable color compared with W1, which can be understood as follows. According to previous reports, low-concentration guests play a negligible role in the electrical property if the energy barrier between charge transport layers and guests is similar.39,47 Because of a low concentration of the guest (MPPZ)2Ir(acac) (1.1%) together with the fact that (MPPZ)2Ir(acac) has a similar HOMO with TAPC, it is inferred that (MPPZ)2Ir(acac) cannot lower the hole mobility. To demonstrate this phenomenon, we have fabricated a hole-only device (H5) with the same configuration as H2 except a difference in the orange guest (MPPZ)2Ir(acac). As shown in Fig. 5a, H5 shows a similar J with H1, indicating that (MPPZ)2Ir(acac) cannot lower the hole transport characteristics.


image file: c5mh00051c-f6.tif
Fig. 6 (a) Normalized EL spectra of W21. (b) Normalized EL spectra of W22. (c) Diagrams of EL processes in W21, W22 and W1.

On the other hand, to illustrate the importance of Ir(dmppy)2(dpp) in making a contribution to maintaining color consistency by decreasing the electron mobility, we have fabricated another device (W22) by replacing Ir(dmppy)2(dpp) with tris(1-phenylisoquinolinolato-C2,N) iridium(III) [Ir(piq)3], which has a HOMO of 5.0 eV and LUMO of 2.7 eV.48 The configuration of W22 is similar to that of W1 except a difference in the guest Ir(piq)3. As shown in Fig. 6b, the total CIE coordinate variation is (0.04, 0.02) during 1–10[thin space (1/6-em)]000 cd m−2, indicating that W22 exhibits an unstable color. Owing to the low concentration of Ir(piq)3 (1.1%) and Ir(piq)3 has a similar LUMO with TmPyPB, it is inferred that Ir(piq)3 cannot lower the electron mobility.39,47 To demonstrate this phenomenon, we have fabricated an electron-only device (E5) with the same configuration as H2 except a difference in the guest Ir(piq)3. As shown in Fig. 5a, E5 shows a similar J with E1, indicating that Ir(piq)3 cannot lower the electron transport characteristics.

Therefore, although (MPPZ)2Ir(acac) exhibits a lower LUMO (3.13 eV) than TmPyPB (2.7 eV) and 26DCzPPy (2.65 eV), electrons can be trapped by (MPPZ)2Ir(acac), decreasing the electron mobility, the hole mobility of W21 cannot be lowered by (MPPZ)2Ir(acac). On the other hand, Ir(piq)3 exhibits a higher HOMO (5.0 eV) than TAPC (5.3 eV) and 26DCzPPy (6.05 eV), holes can be trapped by Ir(piq)3, decreasing the hole mobility, the electron mobility of W22 cannot be lowered by Ir(piq)3. As a result, compared with W1 in which both the hole and electron mobility are reduced via Ir(dmppy)2(dpp), the recombination ratio is much more inconstant in W21 and W22, leading to poor color-stabilities.3,42–44 Besides, since (MPPZ)2Ir(acac) cannot lower the hole transport and Ir(piq)3 cannot lower the electron transport, more holes are accumulated at the 26DCzPPy/TmPyPB interface in W21 and more electrons are accumulated at the TAPC/26DCzPPy interface in W22, compared with W1, as shown in Fig. 6c. As a result, a narrower exciton generation zone is formed and an increasing TTA and TPQ can occur in W21 and W22, which is further detrimental to the color-stability.42 In the case of W1, the charges and excitons distribution are well harnessed, resulting in the recombination ratio being more constant. Therefore, it is reasonable that the color-stability of W21 or W22 is not as stable as W1. These results also show that excellent color-stability cannot be achieved by only using bipolar hosts, since the host of W1, W21 and W22 is similar, indicating that the role of guests should be paid more attention.

Since efficiencies of WOLEDs have been step-by-step enhanced in the last few years, recent research focuses have been shifted towards their drawbacks, like prolonging device lifetimes and achieving extremely stable colors [ΔCIE = (0.00, 0.00)], to meet the demand of future display and lighting applications. In terms of extremely stable colors, only a few approaches have been reported. Zhao et al. reported a double-EML hybrid WOLED with ΔCIE = (0.00, 0.00) during 100–10[thin space (1/6-em)]000 cd m−2 by using a bipolar spacer to control the exciton recombination.49 Sun et al. reported a triple-EML hybrid WOLED with ΔCIE = (0.00, 0.00) during 500–10[thin space (1/6-em)]000 cd m−2via a bipolar blue host to suppress the mutual quenching between the fluorescent and phosphorescent emitters.50 However, virtually no phosphorescent WOLED with ΔCIE = (0.00, 0.00) in the whole range of luminance has been documented. Herein, we, for the first time, have demonstrated that the single-EML WOLED can show extremely stable color, by using the multifunctional role of each guest to harness the charge and exciton distribution, providing a new opportunity to achieve extremely stable colors [ΔCIE = (0.00, 0.00)]. Finally, it is worth noting that W1 exhibits a cool white color with a color-correlated temperature of 6718 K at 1000 cd m−2, and the color rendering index is not high enough (i.e., 43 at 1000 cd m−2) due to the two-color system.

Conclusions

In summary, a single-EML WOLED has been designed, which can simultaneously achieve simplicity, extreme efficiency, reduced efficiency roll-off and excellent color-stability. The resulting WOLED shows a maximum CE and PE of 113.6 cd A−1 and 92.5 lm W−1, respectively, which only slightly decreases to 111.7 cd A−1 and 75.5 lm W−1 at 1000 cd m−2 as well as 101.3 cd A−1 and 58.8 lm W−1 at 5000 cd m−2. Besides, excellent color-stability with a color variation of (0.00, 0.00) is obtained. The origin of the high performance is comprehensively unveiled. In particular, we have systematically revealed the role of guests on the origin of the color-stability and demonstrated that harnessing charge and exciton distribution by using multifunctional guests is crucial to stabilize the color. Such marvelous results not only represent a significant step towards the realization of simplified WOLEDs, but also provides a new opportunity to achieve extremely stable colors, which will be beneficial to the design of both material and device structures for extremely high-performance WOLEDs.

Acknowledgements

This work was supported by the National “973” Project of China (Grant No. 2015CB655000), National Natural Science Foundation of China (Grant No. 61401156 and U1301243), Guangdong Innovative Research Team Program (Grant No. 201101C0105067115), Educational Commission of Guangdong Province (No. 2014KZDXM010 and 2014GKXM012), National Laboratory for Infrared Physics Open Project (Grant No. M201406) and Guangzhou Science and Technology Plan (Grant No. 2013Y2-00114).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c5mh00051c

This journal is © The Royal Society of Chemistry 2015