Color-Stable and Low-Roll-Off Fluorescent White Organic Light Emitting Diodes Based on Nondoped Ultrathin Emitters

Copyright 5 by American Scientific Publishers All rights reserved. Printed in the United States of America Science of Advanced Materials Vol. 7, pp., 5 www.aspbs.com/sam Color-Stable and Low-Roll-Off Fluorescent White Organic Light Emitting Diodes Based on Nondoped Ultrathin Emitters Meijun Yang, Jing Wang, Xindong Shi, Jun Liu, Xinkai Wu, Yang Wang, Zhiyuan Min, and Gufeng He National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, People s Republic of China ABSTRACT We present highly efficient, color-stable, and low-efficiency-roll-off fluorescent white organic light emitting diodes containing two nondoped ultrathin emitters with complementary colors and an interlayer between them. The fabrication process was simplified by using nondoped emitters in the emissive layers. The influence of interlayer thickness on luminance efficiency, efficiency roll-off, and color stability was thoroughly studied. The recombination zone was considerably broadened in the optimized device, contributing to stable energy transfer to both emitters and suppressing concentration quenching. The peak current efficiency of the resulting white organic light emitting diodes reached 9. cd/a and the efficiency roll-off was as low as 8% at luminances ranging from 5 to 5 cd/m. The color coordinates were (, 5), with variation of only (8, 5) from cd/m to 88 cd/m in the Commission Internationale de L Eclairage 93 (x, y) chromaticity diagram. The fabricated white organic light emitting diode structures demonstrated various advantages such as easy fabrication, high stability, and high efficiency. KEYWORDS: WOLEDs, Ultrathin Emitter, Interlayer, Color Stability, Efficiency Roll-Off.. INTRODUCTION In recent years, white organic light emitting diodes (WOLEDs) have attracted extensive attention becaue of their potential applications in solid-state lighting and fullcolor displays. To ensure their effective application as lighting sources or backlights, WOLEDs must exhibit high efficiency, stable color, and a long lifetime. In general, white emission comprises three primary colors (red, green, and blue) or two complementary colors. To obtain high efficiency and low efficiency roll-off, emitters with different colors are typically doped into a single emissive layer (EML) or multiple emissive layers. Because phosphorescent emitters harvest both singlet and triplet excitons, they have been employed in OLEDs to achieve high efficiency. Recently, numerous highefficiency phosphorescent WOLEDs have been reported. 5 7 However, the efficiency of such devices drops rapidly as the current density increases because of the strong triplet triplet annihilation and triplet-polaron quenching. In addition, because of the poor stability of blue phosphorescent emissive materials, WOLEDs exhibit color Author to whom correspondence should be addressed. Email: gufenghe@sjtu.edu.cn Received: December Accepted: 9 March 5 shifts during operation and short lifetimes. All-phosphor WOLEDs still cannot meet real application demands. To eliminate the disadvantages of strong roll-off and unstable emission in phosphorescent WOLEDs, several research groups have adopted fluorescent materials for generating blue emission. 8 Yook reported a WOLED with suppressed efficiency roll-off achieved by managing the interlayer and host properties. Song reported a color-stable WOLED containing two white emissive layers (EMLs). The two EMLs contain the same dopants, blue and red fluorescent emitters, but different host materials in order to obtain highly balanced charge carriers. However, multipleemissive-layer and doped emitters were used in the aformentioned WOLEDs, complicating fabrication processes because of the difficulty of precisely controlling the doping concentration. In particular, fabrication processes have poor reproducibility, which leads to difficulty in production. Thus, the demand for easy fabrication processes for developing WOLEDs exhibiting high efficiency and reduced roll-off is increasing. This paper presents fluorescent WOLEDs containing two nondoped ultrathin emission layers, namely blue and yellow emission layers. In these devices, p-bis(p-n,n - di-phenyl-aminostyryl)benzene (DSA-ph), which is a promising fluorescent material and has been extensively studied, serves as the blue emitter. 3 Furthermore, Sci. Adv. Mater. 5, Vol. 7, No. xx 97-935/5/7// doi:./sam.5.99

Color-Stable and Low-Roll-Off Fluorescent WOLEDs Based on Nondoped Ultrathin Emitters Yang et al.,8-di(t-butyl)-5,-di[(t-butyl) phenyl]-,-diphenylnaphthacene (tetra(t-butyl)rubrene) TBRb serves as the yellow fluorescent emissive material. The electroluminescence (EL) spectrum of DSA-ph overlaps the TBRb absorption spectrum, ensuring efficient energy transfer from DSAph to TBRb. A -methyl-9,-di(-naphthyl)anthracene (MADN) interlayer was inserted between the two emitters to tune the color. The optimized WOLEDs exhibit high efficiency, stable color, and reduced efficiency roll-off. The fabrication approach can be applied to other high efficiency emitters to enhance performance.. EXPERIMENTAL DETAILS Glass substrates with patterned indium tin oxide (ITO) were cleaned with detergent, deionized water, acetone, and isopropyl alcohol separately (5 min for each cleaning process) in an ultrasonication process and then dried using a nitrogen gun. All devices were fabricated using a vacuum thermal evaporation technique at a base pressure of Torr. The evaporation rates of MoO 3, organic layers, LiF, and Al were approximately.,,., and 5 Å/s, respectively. The basic structure of the devices is ITO/MoO 3 ( nm)/n,n -bis- (-naphthyl)-n,n -diphenyl-, -biphenyl-, -diamine (NPB) ( nm)/eml/,7-diphenyl-,-phenanthroline (Bphen) (35 nm)/lif ( nm)/al ( nm). MoO 3 and NPB serve as the hole injection layer and hole transporting layer (HTL), respectively. LiF and Bphen serve as the electron injection layer and electron transporting layer (ETL), respectively. Devices A and B are blue fluorescent OLEDs containing a typical MADN:DSA-ph (5%, 5 nm) EML and ultrathin DSA-ph EML ( nm), respectively. Devices C and D are WOLEDs containing yellow TBRb emitters inserted in various locations. MADN interlayers with different thicknesses (5,, and 5 nm) were inserted between TBRb and DSA-ph in devices E G. Table I lists detailed structures of devices A G. Figure shows the molecular structures of TBRb, DSA-ph, and MADN. The Current density voltage luminance (I V L) characteristics and color coordinates were measured using a Keithley source meter and Topcon BM-7A Luminance Colorimeter. The EL spectra were recorded using a Labsphere CDS-, and the absorption spectra were measured using a Mapada UV-3PC Spectrophotometer. All Table I. Device Device A Device B Device C Device D Device E Device F Device G Structures of devices A G. Structures of EML MADN:DSA-ph (5%,5 nm) DSA-ph( nm) DSA-ph( nm)/tbrb(. nm) TBRb(. nm)/dsa-ph( nm) TBRb(. nm)/madn(5 nm)/dsa-ph( nm) TBRb(. nm)/madn( nm)/dsa-ph( nm) TBRb(. nm)/madn(5 nm)/dsa-ph( nm) Fig.. N TBRb DSA -ph MADN Molecular structures of TBRb, MADN, and DSA-ph. the devices were measured immediately after fabrication in an ambient atmosphere without encapsulation. 3. RESULTS AND DISCUSSION Figure illustrates the I V L and current efficiency current density characteristics of devices A and B. The EML in Device A is a 5-nm DSA-ph layer doped in MADN(5%)andtheEMLindeviceBisa-nmDSAph layer. Device B exhibited a slightly higher current density and luminance than those of device A at the same voltage because of the low thickness of the organic layer. The turn-on voltage of device B was only.7 V and the maximum current efficiency was 5.9 cd/a, slightly lower than that of device A; this difference is attributable to the smaller amount of emitter molecules in device B. However, when the thickness of the nondoped EML was increased, the efficiency decreased (Fig. (c)), indicating the variation of the current efficiency relative to the EML thickness. This variation is mainly because of concentration quenching on thick nondoped EMLs and reduced the current efficiency. To obtain white color, a complementary yellow emission should be added to a blue device. The yellow emitter, TBRb, exhibited strong absorption at wavelengths between 5 and 55 nm, whereas DSA-ph exhibited peak emission at 7 nm and a shoulder at 5 nm (Fig. 3). Optimal overlap between DSA-ph emission and TBRb absorption leads to efficient Förster energy transfer from DSA-ph to TBRb. 5 We inserted a.-nm TBRb layer after or before DSA-ph nondoped EMLs were implemented in devices C and D. Figure 3 shows that the yellow emission intensities in both devices are considerably higher than the blue emission intensities, indicating efficient energy transfer from DSA-ph to TBRb. However, the blue emission intensity in device D containing TBRb on the anode side was higher than that in device C; moreover, the current efficiency in device D was higher than that in device C. Figure illustrates energy band diagrams, indicating that the lowest unoccupied molecular orbital N Sci. Adv. Mater., 7,, 5

Yang et al. Color-Stable and Low-Roll-Off Fluorescent WOLEDs Based on Nondoped Ultrathin Emitters. E-3 E- E-5 MADN:DSA-ph (5%) DSA-ph ( nm) 5 3 Luminance (cd/m ) Absorption Intensity (a.u.) TBRb DSA-ph Ralative Intensity (a.u.) E- Current Efficiency (cd/a) (c) Current Efficiency (cd/a) 7 5 3 Voltage (V) MADN:DSA-ph (5%) DSA-ph ( nm) 8 7 5 3 8. nm nm.3 nm nm Fig.. I V L characteristics of devices A and B. Current efficiency-current density characteristics of devices A and B. (c) Current efficiency variation in nondoped EML with various thicknesses. (LUMO) and highest occupied molecular orbital (HOMO) of TBRb were 3. and 5.9 ev, respectively, and that those of DSA-ph were.7 and 5. ev, respectively. Because the LUMO level of Bphen was 3. ev, electrons were easily transported from Bphen to TBRb in device C and trapped in TBRb because of the.5-ev energy gap between TBRb and DSA-ph, leading to low blue emission. Furthermore, because the LUMO energy gap between DSA-ph and NPB was only.3 ev, some electrons on the 3 5 7 DSA-ph/TBRb TBRb/DSA-ph Current Efficiency (cd/a) 8 8 Fig. 3. Absorption spectrum of TBRb and emission spectrum of DSA-ph. EL spectra at ma/cm and current efficiency of devices CandD. LUMO level of DSA-ph were injected into the NPB layer, resulting in current leakage. Nevertheless, in device D, electrons in DSA-ph were easily transported to TBRb and trapped within the EML because of the substantial difference in the LUMO level between TBRb and NPB ( ev), leading to the electrons being blocked in the HTL. Moreover, the HOMO energy barrier between DSA-ph and Bphen was as high as. ev, blocking the holes in the ETL. Thus, the recombination of holes and electrons is confined in TBRb and DSA-ph emitters, which leads to an improvement of the current efficiency in OLEDs. 7 9 To obtain stable and proper white emission, the blue emission must be enhanced. Therefore, on the basis of device D, an MADN interlayer with high exciton energy was inserted between DSA-ph and TBRb. MADN is commonly used as the host of DSA-ph because of the efficient energy transfer from MADN to DSA-ph. Because the interlayer thickness plays a crucial role in the efficiency and color coordinates of WOLEDs, we fabricated devices E G with different MADN thicknesses (ranging from 5 to 5 nm). Figure 5 illustrates the I V L and current efficiency-luminance characteristics of devices D G. At the same voltage, the current density and luminance Sci. Adv. Mater., 7,, 5 3

Color-Stable and Low-Roll-Off Fluorescent WOLEDs Based on Nondoped Ultrathin Emitters Yang et al. Fig... ev NPB 3. ev TBRb 5.5 ev 5.9 ev.5 ev MADN 5.5 ev.7 ev DSAph 5. ev 3. ev Bphen.5 ev LUMO HOMO Energy level diagrams of each material used in the devices. decreased successively from device D to device G because of the increased thickness of the interlayer. The turn-on voltages of devices D G were approximately.5,.7,.8, and.9 V, respectively. Device D without an interlayer exhibited the lowest turn-on voltage and highest current density among the devices because carriers were directly transported between TBRb and DSA-ph. Figure 5 illustrates that devices E G containing an MADN interlayer exhibited considerably higher current. E 3 E Device D MADN Device E MADN 5 nm Device F MADN nm Device G MADN 5 nm E 5 3 5 7 8 Voltage (V) Current Efficiency (cd/a) 8 8 8 Luminance (cd/m ) Device D MADN Device E MADN 5 nm Device F MADN nm Device G MADN 5 nm Fig. 5. I V L characteristics of devices D G. Current efficiency-current density decay characteristics of devices D G. Luminance (cd/m ) Table II. Summary of device characteristics. @5 cd/m Roll-off CE PE EQE CIE (5 5, Device (cd/a) (lm/w) (%) (x, y) cd/m )(%) Device B 5.9.7 3.7 (.7,.33) 3% Device D.9..7 (55, 89) % Device E 7. 5.77 3.9 (.387, ) % Device F 8.9.9 3.3 (.393, 5) 8% Device G 8.8 5.89 3.7 (.378, ) % efficiencies than that of device D and that the device current efficiency increased with the interlayer thickness. Nevertheless, the efficiency of device G showed no obvious improvement compared with that of device F. However, a thicker interlayer leads to a lower current density and lower luminance at the same driving voltage. Regarding efficiency and operating voltage, device F exhibited higher performance than that of the other devices. Table II shows a summary of the performance of devices B, D, E, F, and G. The peak current efficiency of the optimized WOLED reached 9. cd/a. The efficiency rolloff of device F was only approximately 8%, ranging from 8.9 cd/a at 5 cd/m to 8. cd/a at 5 cd/m, whereas that of device D was nearly %, ranging from.9 cd/a at 5 cd/m to 3.8 cd/a at the highest luminance of cd/m. In device D, holes and electrons recombine on the blue and yellow emitters. However, the molecules in the ultrathin emitters are limited, leading to high concentrations in the recombination zone. The severity of concentration quenching increases at high luminance, resulting in strong efficiency roll-off. In devices with interlayers, most excitons are formed on the interlayers and transfer energy to the blue and yellow emitters. The recombination zone is greatly broadened, and the exciton concentration decreases. Thus, improved luminance efficiency is obtained and efficiency roll-off is suppressed. Figure illustrates the EL spectra of devices D, F, and G at 8 V. In device D without the MADN interlayer, the blue emission intensity increased with the driving voltage. Although the LUMO difference between DSA-ph and Bphen was as high as.5 ev, electrons still tunneled through the blue EML and were injected into the yellow EML because the thickness of the blue EML was nm. The electrons and holes directly recombined on the yellow emitters. Furthermore, the DSA-ph excitons efficiently transferred energy to TBRb molecules as discussed, contributing to the yellow emission. Therefore, stronger yellow emission was observed in device D. Nonetheless, as the driving voltage increased, most of the yellow emitters were in excited states and the energy transfer from DSA-ph to TBRb was less efficient, resulting in stronger blue emission. Furthermore, a high TBRb exciton density within such a thin EML leads to concentration quenching, resulting in lower efficiency. Sci. Adv. Mater., 7,, 5

Yang et al. Color-Stable and Low-Roll-Off Fluorescent WOLEDs Based on Nondoped Ultrathin Emitters (c) V 5 V V 7 V 8 V V 5 V V 7 V 8 V V 5 V V 7 V 8 V Fig.. EL spectra of devices D, F, and (c) G at 8 V. In device G containing the 5-nm interlayer, the blue emission intensity decreased as the bias increased (Fig. (c)). As mentioned, electrons tunneled through the blue EML because of its low thickness (i.e., nm). However, the LUMO difference between MADN and Bphen was as high as.7 ev, and the interlayer thickness was 5 nm. At a low bias, only a few electrons were injected into the interlayer through tunneling, whereas most electrons were blocked in the blue EML by the interlayer. Holes and electrons recombined and formed excitons mainly at the interface of the interlayer and blue EML. Nevertheless, as the bias increased, the electrons overcame the LUMO difference between MADN and Bphen and were injected to the interlayer. The recombination zone of electrons and holes shifted toward the yellow EML, leading to greater yellow emission. For device F, the EL spectra remained almost constant (Fig. ) as the driving voltage increased. This indicates that the electrons overcame the LUMO difference between MADN and Bphen and were transferred to the yellow emitter at V. The recombination zone was mainly at the TBRb and MADN interface and remained almost constant at higher voltages because electrons transferred to TBRb were trapped within the layer because of the high LUMO difference between NPB and TBRb. Therefore, the spectrum of device F remained almost unchanged. In addition, we could hardly observe the blue emission from MADN, which was located at 3 nm, indicating that energy transfer from MADN to DSA-ph and TBRb was relatively efficient. Comparing the EL spectra of devices D, F, and G at V revealed that the blue emission of device G was the highest and that of device D was the lowest. As mentioned, electrons were blocked in the blue EML and excitons formed at the interface of the interlayer and the blue EML, leading to stronger blue emission in device G than that in other devices. In device D, the two emitters are thin to the extent that they can be considered blended. Furthermore, because the LUMO energy level of DSA-ph was higher than that of TBRb, electrons were easily injected into the yellow emitter. In addition, efficient energy transfer from DSA-ph to TBRb resulted in weak blue emission. However, comparing devices D, F, and G at 5 8 V indicated that device F exhibited the strongest blue emission. At 5 8 V, electrons already overcame the LUMO difference between MADN and Bphen and were injected into the MADN interlayer in devices F and G, and the recombination zone was close to the yellow EML because the MADN interlayer transferred electrons faster than holes did. Therefore, thicker interlayers increase the difficulty of transferring energy to the blue emitter, leading to weaker blue emission. Color stability over the luminance is also a highly crucial factor in lighting and display applications. All devices with ultrathin nondoped EMLs exhibited only slight color variation. For example, the color coordinates of device F were (.393, 5) at a luminance of 3 cd/m and (, 55) at 887 cd/m, and the variation in this wide luminance range was only (8, 5), evidencing high color stability.. CONCLUSION In summary, this study presents highly efficient, color stable, and low efficiency roll-off fluorescent WOLEDs containing nondoped ultrathin emitters. A peak current efficiency of 9. cd/a at 3. cd/m was obtained when a -nm MADN interlayer was inserted between nondoped ultrathin blue and yellow emitters. We attribute the high efficiency to efficient energy transfer and reduced concentration quenching, which were realized by optimizing the interlayer thickness. The optimized interlayer also ensures Sci. Adv. Mater., 7,, 5 5

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