Dramatic efficiency improvement in single-layer orange phosphorescent organic light-emitting devices with suppressed efficiency roll-off

Abstract

We have successfully demonstrated efficient single-layer organic light-emitting devices (OLEDs) with a current efficiency of 31.38 cd A⁻¹. The efficiencies still remain as high as 31.36 cd A⁻¹, 30.76 cd A⁻¹ and 29.98 cd A⁻¹ at the luminance of 1000 cd m⁻², 5000 cd m⁻² and 10 000 cd m⁻². The key feature of the device concept is uniformly doping iridium-bis-(4,6-difluorophenyl-pyridinato-N,C2)-picolinate (FIrPic) into a single organic layer to balance the transport of charge carriers. To better understand the mechanism of single-layer OLEDs, the position of the recombination zone and the influence of FIrPic on the transport properties are also studied in detail. Our work clearly reveals that the performance of the single-layer OLEDs can be dramatically improved by intentionally doping with a phosphorescent dye to balance the transport of charge carriers. This novel and versatile device concept provides a promising simple method to achieve high performance single-layer OLEDs.

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1. Introduction

Organic light-emitting devices (OLEDs) have potential applications in solid-state lighting and flat panel display due to their numerous virtues, such as flexibility, broad operating temperature, self-luminescence, light weight characteristics and low power consumption. During the past two decades, the efficiency and lifetime of OLEDs have been considerably improved.^1–5 In particular, phosphorescent OLEDs have been rapidly developed for their intrinsic property of harvesting both singlet and triplet excitons which enables them to obtain 100% internal quantum efficiency in principle.^6–12 The representative phosphorescent OLEDs have multiple organic layers which contain a hole injection layer (HIL), hole transport layer (HTL), hole blocking layer, electron transport layer (ETL), and electron injection layer (EIL). However, these organic functional layers which are considered vital parts in promoting device performance complicate the manufacturing process and hamper phosphorescent OLEDs from entering the practical market. Thus, to simplify the device structure, a variety of device concepts have been proposed.^13–16

Recently, single-layer OLEDs have attracted great interests because of their low manufacturing cost.^13,14 To achieve high-efficiency single-layer OLEDs, a key factor is realizing the balance of charge carriers injection and transport. Therefore, the single organic material should possess balanced and high electron and hole mobilities. Unfortunately, such ambipolar organic materials are very rare, which restrains their application in high performance single-layer OLEDs. Chang et al. demonstrated a new tricarbazole phosphine oxide bipolar material for efficient single-layer blue phosphorescent OLEDs with maximum current efficiency of 21.3 cd A⁻¹.¹³ Joo et al. reported a developed single-layer device by using a novel phosphine oxide type host material, which obtained efficiency of 11.4 cd A⁻¹ at 1000 cd m⁻².¹⁴ However, these single-layer OLEDs show low efficiency and noteworthy efficiency roll-off, especially at high luminance. And most previous report mainly focused on the synthesis of new ambipolar organic materials. Thus, it is essential to come up with novel device concept for single-layer OLEDs with high efficiency and low efficiency roll-off.

In this work, we demonstrated efficient simplified single-layer orange phosphorescent OLEDs with maximum efficiency of 31.38 cd A⁻¹. The key feature of the device concept is uniformly doping iridium-bis-(4,6-difluorophenyl-pyridinato-N,C2)-picolinate (FIrPic) into single organic layer to balance the transport of charge carriers. To better understand the mechanism of the designed single-layer OLEDs, the position of recombination zone and the influence of FIrPic on transport properties were also studied respectively. It was found that the blue phosphorescent dye plays important roles in achieving balanced charge carriers, suggesting promising simple method to achieve highly efficient single-layer OLEDs with extremely low efficiency roll-off.

2. Experiment

All single-layer OLEDs were manufactured on pre-cleaned glass substrates covered by semitransparent indium tin oxide (ITO) with a sheet resistance of 20 Ω sq.⁻¹. We cleaned ITO substrates with de-ionized water, acetone and methanol in sequence. Considering that 4,4′-bis(carbazol-9-yl)biphenyl (CBP) is a common bipolar host material with good hole and electron transport mobilities, here we used CBP as the single organic material. Bis(4-phenylthieno[3,2-c]pyridinato-N,C2′) acetylacetonate iridium(III) (PO-01) served as orange emission dye for the high efficiency. Lithium quinolate (Liq) was selected to assist electron injection from Al cathode. Devices were fabricated in thermal vacuum evaporation chamber at a base pressure ∼10⁻⁶ Torr. CBP was deposited at the rate of 1–2 Å s⁻¹. PO-01, FIrPic were deposited at the rate of 0.1 Å s⁻¹, 0.2 Å s⁻¹, respectively. Liq and MoO₃ were deposited at the rate 0.02 Å s⁻¹, and Al was deposited at the rate of 5 Å s⁻¹. The thickness of the red ultra-thin emission layer (0.1 nm) can be accurately controlled with the deposition rate of 0.01 Å s⁻¹. The layer thickness and the deposition rate of materials were monitored in situ by an oscillating quartz thickness monitor. The luminance–voltage–current characteristics were measured simultaneously with the measurement of the electroluminescent (EL) spectra by combining a PR655 spectroscan spectrometer with a programmable Keithley 2400 voltage–current source.

3. Results and discussions

The reported works have confirmed that CBP could be used as HIL and HTL. We also demonstrated efficient orange and white OLEDs adopting CBP as HIL and HTL, as well as the host for emission dyes.^7,17 But there is also some doubt whether CBP could be used as EIL and ETL. Liq is often used to enhance electron injection from cathode to EIL by lowering interfacial energy barrier.^18,19 What is uncertain is whether Liq could work well with CBP. The two uncertain factors can be answered by observing performance of the following devices with the structures of ITO/MoO₃ (1.5 nm)/CBP: 8% PO-01 (100 nm)/Liq (X nm)/Al. Here, X was varied among 0, 1, 3, 5 corresponding to devices A–D.

The EL performance characteristics of devices A–D are shown and summarized in Fig. 1 and Table 1. It is clearly to see that device A exhibits the lowest efficiency among the four devices. As expected, the introduction of Liq evidently improves device performance. Device B shows the highest efficiency of 16.34 cd A⁻¹. However, when the Liq layer gets thicker, it causes a decrease in the efficiency. For instance, device C has efficiency of 16.21 cd A⁻¹ which is higher than device D. From Fig. 1(b) we can see, device B shows the highest current density, indicating that electrons could be injected effectively from cathode to CBP when the Liq layer is relatively thin. The observed current density decrease in devices C and D reveals that further increase in the thickness of Liq will obstacle the injection and transport of electrons. The drastic improvement of performance with a introduction of a thin insulating layer was always explained by the dipole model.²⁰ According to the model, a generated dipole layer decreases the surface potential of the contact materials, thus lowering the electronic barrier height for electron injection. In our future research work, we will carry out more experiments to make in-depth analysis on the underlying mechanism. Based on above results, we set the thickness of Liq as 1 nm for the subsequent experiments. It is regrettable that the optimum device showed low efficiency especially at high luminance, which prevents single-layer OLEDs from realizing commercialization. The low efficiency and high efficiency roll-off might be due to the unbalanced charge injection and transport which could be estimated from the position of the recombination zone.

Fig. 1 The current efficiency–luminance (a) and the current density–voltage (b) characteristics of devices A–D.

Table 1 The performance characteristics of devices A–D. η_max, η₁₀₀₀, η₃₀₀₀, η₅₀₀₀, η₁₀₀₀₀: maximum current efficiency (cd A⁻¹), efficiencies at 1000 cd m⁻², 3000 cd m⁻², 5000 cd m⁻², 10 000 cd m⁻²

Devices	ηmax	η1000	η3000	η5000	η10000
Device A	0.43	—	—	—	—
Device B	16.34	16.22	15.62	14.85	12.16
Device C	16.21	15.56	14.71	13.81	9.86
Device D	15.62	15.51	14.38	13.65	10.75

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Keeping the main recombination zone away from the electrodes is very necessary because there are no additional carriers or excitons blocking layers in single-layer OLEDs. In order to investigate the precise location of exciton recombination zone, we fabricated another series of devices E–H shown in Fig. 2(a). To sense the intensity of excitons, 0.1 nm ultrathin red emission layer bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)-iridium(III) [Ir(MDQ)₂(acac)] was inserted into CBP: 8% PO-01 layer at a distance of Y nm away from the interface of CBP: 8% PO-01/MoO₃. Y was varied being 20, 45, 60, 70 for devices E–H respectively. The normalized EL spectra of the four devices E–H at 9 V are shown in Fig. 2(b). For the purpose of observing the relative intensity of red emission from Ir(MDQ)₂(acac), the spectra was normalized to the orange emission peak. In Fig. 2(b), it is obvious that the strongest red emission can be achieved as the ultrathin sensing layer was sited at 45 nm away from the interface of CBP: 8% PO-01/MoO₃, indicating where the density of excitons reaches highest due to the accumulation of the largest densities of holes and electrons. With Y varying from 45 to 70, the relative intensity of red emission decreases, as well as the efficiency. These phenomena mean that the main excitons recombination zone is predominantly close to anode side. Considering that the hole mobility of CBP is about one order of magnitude larger than that of electron,²¹ the above phenomena are attributed to the more efficient electron injection. Balanced charge carriers injection and transport are the determinate factors for the performance of single-layer OLEDs. Thus, to further improve performance of the single-layer OLEDs, special attention should be paid to balancing the charge carriers by facilitating the injection and transport of holes.

Fig. 2 The structure (a) and the normalized EL spectra (b) of devices E–H.

Synthesising new materials and proposing novel device concept with existing materials are the two effective ways to promote devices performance.^22–24 Ouyang et al. have demonstrated the first time with improved dopant concentration by designing and synthesising an ideal saturated deep-blue fluorophore.²⁴ Some typical phosphorescent dyes have been doped into emission layers in traditional multiple-layer OLEDs to enhance the device performance.^22,23 But there has been no relevant report about the use in single-layer OLEDs. Here we uniformly doped a typical blue phosphorescent dye of FIrPic at the concentration of 15% into single organic layer to further improve the performance of orange single-layer OLEDs. And the overall thickness of the single-layer OLEDs was also optimized to create an efficient recombination zone. The doping concentration of PO-01 was fixed at 8% in the devices of ITO/MoO₃ (1.5 nm)/CBP: 15% FIrPic: 8% PO-01 (Z nm)/Liq (1 nm)/Al, where Z was varied among 100, 110, 120, 130 corresponding to devices O₁–O₄.

Fig. 3(a) shows the normalized spectra of single-layer device O₃ doped with 15% FIrPic. We can see that the device doped with FIrPic has pure emission from PO-01. The absence of emission from CBP or FIrPic indicates that the generated excitons are finally transferred to PO-01. As can be seen in Fig. 3(b) and Table 2 that device O₃ with a 120 thickness achieves the highest current efficiency of 31.38 cd A⁻¹ which is a 1.9 times of that of device B without FIrPic. Another phenomenon we should pay close attention is that device O₃ shows much lower efficiency roll-off. The efficiencies still remain as high as 31.36 cd A⁻¹, 31.27 cd A⁻¹, 30.76 cd A⁻¹ and 29.98 cd A⁻¹ at the luminance of 1000 cd m⁻², 3000 cd m⁻², 5000 cd m⁻², 10 000 cd m⁻², corresponding to extremely low efficiency roll-off of 0.06%, 0.35%, 1.97% and 4.46%. Comparatively, device B represents much higher efficiency roll-off of 0.73%, 4.40%, 9.11% and 25.58% at the luminance of 1000 cd m⁻², 3000 cd m⁻², 5000 cd m⁻² and 10 000 cd m⁻².

Fig. 3 The normalized spectra of device O₃ at different voltages (a). The current efficiency–luminance (b), the current density–voltage (c) and the luminance–voltage (d) characteristics of devices O₁–O₄.

Table 2 The performance characteristics of devices O₁–O₄

Devices	ηmax	η1000	η3000	η5000	η10000
Device O1	30.26	27.56	29.99	30.06	29.04
Device O2	28.24	26.66	28.19	28.01	27.36
Device O3	31.38	31.36	31.27	30.76	29.98
Device O4	30.29	30.25	29.87	29.33	28.40

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To further investigate the influence of FIrPic on the transport properties of single-layer OLEDs, we fabricated hole-only and electron-only devices with the structures of ITO/MoO₃ (1.5 nm)/NPB (35 nm)/CBP: FIrPic (30 nm, 0% or 15%)/NPB (35 nm)/MoO₃ (1.5 nm)/Al and ITO/Liq (1 nm)/TPBi (35 nm)/CBP: FIrPic (30 nm, 0% or 15%)/TPBi (35 nm)/Liq (1 nm)/Al, respectively.

Fig. 4 describes the current density–voltage characteristics of the hole-only and electron-only devices respectively. It can be seen from Fig. 4(a) that the hole-only current density increases when doping 15% FIrPic into CBP. The obvious enhancement in current indicates FIrPic molecules act as hole transport channels in the doping system of CBP: 15% FIrPic. Therefore, uniformly doping FIrPic into CBP facilitates the transport of holes, which is beneficial to the balance of the holes and electrons in the single-layer OLEDs. In contrast, it is noted that electron-only current density greatly decreases when FIrPic was doped into CBP. From the detailed energy level diagram of the structures shown in Fig. 5, it is clearly to see that FIrPic has the lowest unoccupied molecular orbital level of 3.1 eV which is much deeper than CBP (2.9 eV) and PO-01 (2.7 eV), indicating that FIrPic acts as trapping sites for electrons. Overall, the above observation proves that doping FIrPic into CBP helps the injection and transport of holes and acts as electrons trapping sites simultaneously. The two procedures lead to more balanced charge carriers in single-layer OLEDs, causing a slight shift of the recombination zone from the anode side to the center of the device. Therefore, the improved efficiency and the extremely low efficiency roll-off in single-layer device O₃ doped with FIrPic are attributed to more balanced charge carriers. The state-of-art performance makes our single-layer OLEDs among the best reported results based on only one organic material.

Fig. 4 The current density–voltage characteristics of hole-only devices (a) and electron-only devices (b).

Fig. 5 The energy diagram of devices O₁–O₄ and chemical structures of the organic materials.

4. Conclusion

In summary, we have succeed in demonstrating efficient orange single-layer OLEDs with suppressed efficiency roll-off by uniformly doping 15% FIrPic into CBP. The resulting device achieved maximum current efficiency of 31.38 cd A⁻¹ and extremely low efficiency roll-off over a large range of luminance. The efficiencies are 31.36 cd A⁻¹, 30.76 cd A⁻¹ and 29.98 cd A⁻¹ at the luminance of 1000 cd m⁻², 5000 cd m⁻² and 10 000 cd m⁻². The excellent performance can be attributed to the balanced and efficient carrier injection and transport. We firmly believe that there would be further investigations for improving single-layer phosphorescent OLEDs with novel proposes, for example, by bringing in more advanced ambipolar host materials and phosphorescent dopants.

Acknowledgements

The work was funded by the National Natural Science Foundation of China (61377026, 61275024, 61275033, 61274002), Program of international science and technology cooperation (2014DFG12390), the Ministry of Science and Technology of China (2013CB834802), the National High Technology Research and Development Program of China (2011AA03A110) and the Project of Science and Technology Development Plan of Jilin Province (20140101204JC, 20130206020GX, 20150101045JC, 20140520071JH).

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