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We demonstrated a novel-structure white organic light-emitting devices (WOLEDs) composed of a greenish blue fluorescent emitting layer and a red fluorescent dye-doped hole injection layer. The spectra of WOLEDs show no change at a wide range current density operation and long-term continuous operation. We concluded that carriers recombined inside the emitting layer in the WOLEDs to produce a greenish blue electroluminescence (EL), and a part of the EL was absorbed by the red fluorescent guest doped hole injection layer and converted into red photoluminescence (PL); the whole white emission from the device was a mixture of the greenish blue EL and red PL.
©2007 Optical Society of America
1. Introduction
In recent years, organic light-emitting devices (OLEDs) for flat panel displays (FPDs) have been attracting many researchers due to their advantages such as excellent image quality, light weight, easy manufacturability, low cost, and large-area extendibility.
White organic light-emitting devices (WOLEDs) are the potential light source to make full color FPDs with micro-patterned color filters [1]. Furthermore, WOLEDs were also applied to lightening as another application. To date, there are some efforts to realizing WOLEDs, including a single emissive layer structure doped with some different fluorescent materials [2–3], and a stacked multi emissive layer structure in which each layer emits a different-color light to generate white-light [4–7] or electrophosphorescent WOLED [8], as well as a tandem cell structure [9–10]. In the single emissive layer WOLEDs, in order to control an exciton distribution balance among each emissive material, a doping concentration of a red material shall be very low. On the other hand, in the multi-emissive layer WOLEDs, carrier recombination balances in each emissive layer are essential to obtain better white lights at higher efficiency. In order to control the recombination balance, a hole-blocking layer is usually inserted between the two emissive layers [4–5]. In all former researches, it is a crucial problem that the luminescence color of WOLEDs is easy to shift with increasing the drive current and/or operating times. This probably results from the change of energy distribution in different emissive materials or of the recombination site due to different electric field dependences of charge carriers in WOLEDs emissive layer.
The other approaches to producing white emission is the use of blue OLED in combination with outside color conversion materials in both sides of glass substrate[11–12]. This technology utilized the down-conversion mechanism to avoid the color instability with the increase in current density, moreover, the light output was found to increase by absorption/reemission of light scattering by down-conversion layers. Due to relatively complicated fabricated process, we propose one simple structure WOLEDs incorporating color conversion layers into OLED configuration with blue emission.
In this paper, we report a new structure WOLEDs whose white luminescence has no color change even in a wide range current density operation and for a long-term continuous operation. The WOLED structure consists of a red luminescent color conversion hole injection layer (HIL); a hole transport layer (HTL); a greenish blue emitting layer (EML); an electron transport layer (ETL), and an electron injection layer.
2. Experimental
The WOLEDs were fabricated by conventional vacuum deposition of the organic layers and cathode on an indium-zinc-oxide (IZO) coated glass substrate under a base pressure lower than 5.0×10-6 Torr. We fabricated two different structures of OLEDs: the device structure | is IZO/red luminescent color conversion HIL/ /HTL/EML/ETL//LiF/Al, and the device structure II is IZO/HIL/HTL/EML/ETL/LiF/Al. The device structure II with no doping the red fluorescent to the HIL that is prepared for comparison emits greenish-blue electroluminescence (EL). In these device structures, IZO and Al were used as an anode and a cathode, respectively. The IZO-coated glass substrates were cleaned by a UV ozone treatment at 150°C.A fused aromatic ring red fluorescence material, P1TM [13] and an oligo-amine hole injection material, HI-406TM [14] from Idemitsu Kosan Co., Ltd. were used as a red luminescent dopant and HIL, respectively. According to the previous publications, even if the red emitter P1 was doped in an EML of OLED at a very high concentration, luminescence quenching was not remarkable [13]. Moreover, it was also reported that P1 has a good hole transport property [13]. The following chemicals were used, respectively, 4,4’-bis[N-(1-naphthyl)-N-phenylamino] biphenyl (NPB) as a HTL; 2-t-butyl-9,10-di-2-naphthylanthracene (TBADN) as the host material of EML, p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-ph) as the guest material of EML; and tris(8-hydroxyquinoline)aluminum (Alq3) as an ETL. The EML was 5.0 wt% DSA-ph in TBADN. The thickness of NPB, 5.0 wt% DSA-ph in TBADN, Alq3, LiF and Al cathode were 20, 25, 20, 0.5 and 130 nm, respectively. In addition, the thickness of non-doped HIL or doped HIL with red dye P1 was 200 nm. The absorption peak and maximum absorbance of 25 wt% P1 doped HIL film was 530 nm and 0.20 while those of 50 wt% P1 doped HIL was 530 nm and 0.43. The entire organic layer stack and the Al cathode were deposited without exposure to the atmosphere. The deposition rates for the organic materials excepting for the guest materials (P1 and DSA-ph), LiF and Al were typically 0.1 nm/s, 0.025 nm/s and 0.5 nm/s, respectively.
3. Results and discussion
Figure 1 shows the energy level diagram of the WOLEDs and structures of chemicals used in this study excluding P1 and HI-406 whose structures are not disclosed. As shown in the diagram, the highest occupied molecular orbital (HOMO) level of P1 is same to that of HI-406 (5.2 eV). An energy barrier between LUMO (lowest unoccupied molecular orbital) level of NPB (2.3 eV) and that of TBADN can make sure of the carrier recombination in EML only.
Fig. 1. Molecular structures of chemicals used in this study without P1 and HI-406 (from Idemitsu Kosan) and energy diagram of the device.
Figure 2(a) shows photoemission spectra of devices with the structure |and ∥at a current density of 37 mA/cm2, revealing the greenish-blue emission of device structure ∥ and the white emissions of the devices with the structure |. The CIE coordinates of the WOLEDs with different P1 dopant concentrations were (0.242; 0.353) for 25 wt% and (0.262; 0.329) for 50 wt%, respectively. Figure 2(b) shows the film absorption and PL spectra of the HI-406 doped with P1. The luminescence spectra of the WOLEDs additionally contain a red component. We supposed that the red emission ingredient was generated from P1 in HIL layer, which absorbs a part of the greenish-blue EL light, and convert this to red PL lights; namely, the P1 doped HIL worked as a color conversion layer. In order to verify the supposition, we have changed the EML of the devices; a red fluorescent emission material, 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) was doped into the greenish blue EMLs of both device (structure I and II) at a concentration of 10.0 wt%. Then, it was found that the emissive spectra from the two DCJTB doped devices were the emission of DCJTB only, and the red EL spectra of the DCJTB doped devices overlapped well each other. Because it is widely believed without any doubt that charge carrier recombination and excitons formations occurred at the EML in the case of the DCJTB device (structure II) concerning with its conventional device structure, the recombination probably occurred at the EML even in the other DCJTB device with the P1 doped HIL (structure I).
Fig. 2. (a) Normalized EL spectra of the WOLEDs (structure I) with different P1 dope concentrations together with the greenish blue OLED (structure II). (b) Absorption and PL spectra of the P1 doped HI-406 thin-films with different P1 concentrations, which were utilized as a color conversion HIL in the WOLEDs. (c) Operation current dependence of CIE color coordinates of the WOLEDs.
In addition, it was reported that the diffusion length of excitons derived by the recombination were not more than 17nm in HTL [15]. Because HIL functions as color version layer, it should suitably absorb the blue emission and effectively convert into low-energy emission. In this study, the 31% and 51% of forward-direction blue emission can be absorbed by 25% and 50% PI doped HILs, respectively. So, we can safely conclude that the red emission ingredient of the WOLEDs with the P1 doped HIL is caused by color conversion of EL from the EML in the HIL, not excited by the carrier recombination in HIL. As shown in Fig. 2(c), when drive current densities were changed at the range from 0.001 A/cm2 to 1.0 A/cm2, emission color of both WOLEDs exhibit little change (CIE coordinate migration less than (±0.002; ±0.003)). In addition, after a continuous long term operation for 1600 hour at a current density of 37 mA/cm2, the emission spectrum for each two WOLED still kept the same CIE coordinates: (0.242; 0.353) for the 25 wt% P1 device and (0.262; 0.329) for the 50 wt% P1 device, respectively. Thus, the demonstrated structure WOLED has excellent color stability against drive current density and a long operation time. These two color stabilities of the WOLED have a significant advantage over the former WOLEDs [2–10].
Fig. 3. Long-term durability of PL intensity of the P1 doped HI-406 thin-films under an exposure of an intense filtered Xe lamp (>420 nm). Inset: P1 concentration dependence of PL quantum efficiency of P1 doped HI-406 thin-films.
The degradation behavior of the P1 doped HI-406 thin-film was tested under an exposure for 190-W/m2-filtered Xe lamp (>420 nm) as shown in Fig. 3. It is found that the P1 doped HI-406 thin-film has excellent stability under the intense light irradiation. Because of this, WOLED emission spectrum was hard to shift under a long operating. The inset of Fig. 3 presents the fluorescence quantum efficiencies of P1 doped HI-406 thin-films as a function of P1 concentrations as revealed by an integrating sphere survey [16]. As presented in the inset, the fluorescence quantum yield of P1 doped HI-406 thin-films depended on the dope concentration. The fluorescence quantum yield was 58% and 42% for the thin-films with doping concentration at 25wt% and 50wt%, respectively.
Figure 4 shows the current density-voltage characteristics of the devices with the structures I and II, and the inset shows quantum efficiency–current density characteristics of the devices. From the Fig. 4, the driving voltage of the device with the structure I became higher than that of the device with the structure II. For example, the driving voltages at a current density of 0.8 A/cm2 were, respectively, 19 V, 22 V and 15 V for the 25 wt% doped device (structure I), 50 wt% doped device (structure I) and non-doped device (structure II). The higher drive voltages are probably contributed to slow hole mobility of P1 doped HI-406 layers which results from increasing energy disorders due to mixing different molecular species (P1 and HI-406) [17]. In order to reduce the drive voltage of the WOLEDs, it needs to develop new red emissive materials causing no voltage increase. As shown in the inset of Fig. 4, the quantum efficiencies of both WOLEDs (P1 25 wt% and 50 wt% doped) were lower than that of the greenish-blue device (structure II). This is due to a low PL quantum efficiency of the P1 doped HI-406 thin-film. In order to improve the external quantum efficiency of the WOLED, an effective way is to employing a color conversion HIL with higher PL quantum efficiency. On the other hand, due to the use of a thick P1 doped HIL layer in device, the carrier mobility may decrease, resulting in a relatively low charge carrier balance in EML. This low carrier-pair recombination in EML is assumed to be one reason of the low efficiency. It should be noted here that the demonstrated structure WOLED can give a higher external quantum efficiency than that of the original OLED without the color conversion HIL, as been similar to the case of a side-coupling color conversion technology [18]. Since the out-coupling efficiency is only 20% fraction of the generated light for a conventional OLEDs, as shown in Fig. 5(a), a remaining fraction was almost equally distributed to the waveguide-mode (confined in the thin-film stack) and the substrate-mode (propagate through a glass substrate) [19]. However, in our structure OLEDs, the color conversion HIL can effectively absorb the ITO/organic-guided modes and one part of substrate guided modes, and convert into low-energy PL emission. The 20% fraction of transferred PL in this layer reemit into external modes, resulting in an increase in the out-coupling efficiency, as shown in Fig. 5(b).
Fig. 4. Current density vs. voltage characteristics of the WOLEDs (structure I) and the reference greenish blue OLED (structure II). The inset shows the external quantum efficiency vs. current density characteristics of the devices.
Fig. 5 Schematic diagram showing light out from the devices (a) with non-doped HIL and (b) with P1 doped HIL
In order to improve WOLED color purity, we optimized the thickness of color conversion HIL (P1 25 wt%). When the HIL thickness increased to be 340 nm, 70% of forward-direction blue emission can be absorbed, and the device emission color became very pure white with a CIE coordinate of (0.31, 0.33). However, while increasing the HIL thickness to 400 nm, only 52% of forward-direction blue emission can be absorbed, and the emission color became poor white with a CIE coordinate of (0.27, 0.32). This is attributed to the reason that a greenish part of the EL emission resulting from carrier recombination at the EML becomes weak due to interference effects when thickness of the HIL is 400 nm [20]. In this case, P1 in the HIL will not able to absorb enough EL light and convert to red PL emission.
4. Conclusions
In conclusion, we have demonstrated a highly stable WOLED structure with a color conversion hole injection layer. As an example of the proposed structure we presented WOLEDs composed of the P1, the red emitter, doped HIL and the greenish-blue EL emissive layer. The WOLEDs worked as planed; a greenish blue EL was generated by charge carrier recombination at the EML, and then a part of the EL was converted into red luminescence. The WOLEDs exhibited high stability of luminescence color against both changing operation current densities and long-term continuous operations. In addition, the demonstrated structure potentially has a higher external quantum efficiency than the original OLED without a color conversion HIL. To suppress the increase of operation voltage due to introducing the color conversion HIL, which is the big issue of our WOLED structure, further studies are in progress.
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