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Nonlinear Optics and Quantum Optics, Vol. 37, pp. 9–19 Reprints available directly from the publisher Photocopying permitted by license only

Bright White Organic Light-Emitting Diode With Dual Doped Blue and Yellow-Orange Emitting Layers BERNARD GEFFROY, NOE¨ LLA LEMAˆITRE, JE´ RE´ MIE LAVIGNE, CHRISTINE DENIS, PASCAL MAISSE, AND PAUL RAIMOND CEA/DRT/LITEN/DSEN/GENEC, Laboratoire Cellules et Composants, CEA/Saclay, 91191 Gif sur Yvette Cedex, France E-mail: [email protected]

Organic Light Emitting Diodes (OLED) are actively considered as potential next generation of solid state lighting sources due to the tremendous progress in device luminous efficiency and the emerging viability as a commercial display technology. A white OLED has been fabricated employing a highly efficient doped blue material and a yellow-orange doped layer as emitting species. The device structure was ITO/CuPc/NPB:rubrene/DPVBi:PR3491/Alq3/LiF/Al. The blue electroluminescent layer is based on a DPVBi (4, 4’-bis(2, 2’-diphenylvinyl)biphenyl) host doped with a derivative of distyryl biphenyl molecule synthesized in the laboratory. White diodes can be obtained by further doping the hole transport layer (NPB) with rubrene, a highly efficient yellow fluorescent molecule. By properly tuning the doping rate in order to balance the blue and yellow-orange contribution of the diode emission, a fairly pure white light has been obtained with Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of (0.31; 0.34) and external efficiencies of 3.4% and 8.7 cd/A at a current density of 10 mA/cm2 . Moreover, the CIE coordinates of the emitted light are quite stable for luminance values ranging from 500 cd/m2 to 5000 cd/m2 . Key words: White OLED, electroluminescence, DPVBi, rubrene, doping layers, Recombination zone, charge trapping, solid state lighting

1 INTRODUCTION Organic light emitting diodes (OLED) have attracted great attention for flat panel displays since the demonstration of efficient electroluminescent

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devices by Tang et al. [1]. Moreover, the tremendous progress in device luminous efficiency [2] and the emerging viability as a commercial display open new markets for white OLED (WOLED) like backlight for liquid crystal displays or lighting sources. WOLED can be considered as potential large area next generation of Solid State Lighting sources to replace traditional incandescent white light sources, thanks to their potential of energy saving, their high efficiencies and their possibility to fabricate thin and flexible devices. For high quality white light illumination, sources with chromaticity CIE (Commission Internationale de l’Eclairage) coordinates similar to that of the black body radiator with a CCT between 2500K and 6500K (ref D65) and a CRI above 80 are required [3]. Due to the limited spectral bandwidth of organic molecules to about one third of the visible spectrum, a white emission cannot be provided from a single molecule. Generally white light is obtained by mixing the three primary colors (red (R), green (G) and blue (B)) [4] or two complementary colors [5] (cyan and red for example) in the OLED. Several WOLED architectures can be considered in order to combine the emission from multiple emitters [3] like (i) multilayer devices with R, G and B emission layers, (ii) WOLED with a single multiply doped (with R, G and B fluorescent dyes) emissive layer, (iii) diode with a microcavity effect of one emissive layer [6], (iv) OLED with emission based upon exciplex formation [7]. A device made of a single emitting layer is preferable because its CIE coordinates are almost independent of the driving current density but its EL efficiency is generally quite low. Multi-emitting layer devices are much efficient but their CIE coordinates are more sensitive to the driven current due to the shift of exciton recombination zone with the internal electrical field. In this paper, we have focused on the approach of the two complementary colors in order to minimize the number of emissive materials and layers and to simplify the device architecture. Deep blue OLED have been realized by evaporation of small molecules with the following structure ITO/CuPc/α-NPB/DPVBi/Alq3/LiF/Al. The results have been improved by doping the host DPVBi with a home made blue dopant named PR3491. White diodes have then been fabricated by adding a highly fluorescent yellow-orange material in the previous blue device. The rubrene molecule has been chosen as a complementary color efficient emitter of the blue device [8]. Rubrene is probably one of the most interesting and extensively studied emitting materials due to its attractive properties like high photoluminescent quantum efficiency (near unity), bipolar character [9] and ability to enhance device efficiency and stability [10, 11]. The concentration effects and the location of the yellow-orange dopant in the multilayer have been investigated.

BRIGHT WHITE ORGANIC LIGHT-EMITTING DIODE

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FIGURE 1 Structure of the blue and white (I/II/III) multilayer OLEDs.

2 EXPERIMENTAL 2.1 Device fabrication Blue and white EL devices, based on a multilayer structure have been fabricated onto patterned ITO coated glass substrates from Asahi (sheet resistance 10 /). The different structures of the blue and white diodes are depicted in Figure 1. The common structure of all the devices is the following: a thin layer (10 nm thick) of CuPc is used as hole injection layer (HIL) and 50nm of α-NPB as hole transporting layer (HTL). The blue emitting layer consists of DPVBi, a commercially available molecule as host material (from Syntec). In order to increase the EL efficiency and to improve the blue color, the emitting layer is doped with a derivative of distyryl biphenyl, designated as PR3491. This product has been synthesized and purified in the laboratory. The doping rate is controlled by co-evaporation of host and dopant in the range of 1 to 4 wt.% A thin layer of Alq3 is used as electron transporting layer (ETL). Finally, a cathode consisting of 1.2 nm of LiF capped with 100 nm of Al is deposited onto the organic stack. The entire device is fabricated in the same run without breaking the vacuum. In this study, the thicknesses of the different organic layers were kept constant for all the devices. Rubrene, a well known highly fluorescent molecule is used as the yellow-orange emitter. The chemical structures of the blue and yellow-orange emitting materials are shown in Figure 2. For the white diodes, three types of devices were fabricated. In type I device,

FIGURE 2 Chemical structures of the blue and yellow-orange emitters.

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the emitting blue layer is doped with the yellow-orange emitter while in type II and III devices, the yellow-orange molecule is co-evaporated with the HTL layer. In type III device, the DPVBi blue host is further doped with PR3491 molecule in order to improve the efficiency of the white device comparatively to the type II structure. All the organic materials (from Aldrich and Syntec) are deposited onto the ITO anode by sublimation under high vacuum (< 5.10−6 Torr) at a rate of 0.2 – 0.3 nm/s. The active area of the devices defined by the overlap of the ITO anode and the metallic cathode was 0.3 cm2 . 2.2 Measurements The current-voltage-luminance (I-V-L) characteristics of the devices were measured with a regulated power supply (ACT100 Fontaine) combined with a multimeter and a 1 cm2 area silicon calibrated photodiode (Hamamatsu). The spectral emission was recorded with a SpectraScan PR650 spectrophotometer. All the measurements were performed at room temperature and at ambient atmosphere with no further encapsulation of devices. 3 RESULTS AND DISCUSIONS 3.1 Blue EL devices The current density (J) and luminance (L) versus applied voltage (V) characteristics and the EL spectrum of the blue devices are presented in Figure 3. The electroluminescent (EL) performances of the blue devices are reported in Table 1. For the non-doped device, the EL spectrum (Figure 3b) is characterized by a broad peak at 452 nm with a Full Width at Half Maximum (FWHM) of 170 nm. The threshold voltage of such a device is 5.3 V and its luminance is proportional to the current density (insert Figure 3a). Its external quantum efficiency and current efficiency were measured to be 3.1% and 3.3 cd/A respectively (Table 1) with deep blue CIE chromaticity coordinates of x = 0.157 and y = 0.125. The DPVBi

FIGURE 3 J (open symbols)-V-L (full symbols) characteristics (a) and emission spectra (b) of non-doped and PR3491 doped DPVBi. Insert in (3a) shows the L-J characteristics of the devices.

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BRIGHT WHITE ORGANIC LIGHT-EMITTING DIODE TABLE 1 EL characteristics of the blue devices. Efficiencies Device (doping rate %) Non sublimed DPVBi Sublimed DPVBi Sublimed DPVBi PR3491 (1.8%)

Us (V) 5.3 5.3 5.0

% 3.1 3.6 5.7

cd/A 3.3 4 7

CIE lm/W 1.0 1.2 2.3

x 0.157 0.155 0.154

y 0.125 0.130 0.138

is known to be a highly efficient blue emitter in multilayer OLED [12]. Performances of this blue device have then been studied as a function of the chemical purity of the matrix and of the percentage of blue PR3491 dopant incorporated in the emitting layer. The use of a sublimed grade product leads to an improvement of the efficiencies by 20% (Table 1). Moreover, doping the DPVBi matrix with the fluorescent distyryl biphenyl derivative leads to the modification of the EL spectrum (Figure 3b). By comparison of the non-doped device, two new peaks appear at 445 and 472 nm respectively and the FWHM decreases to around 130 nm. This leads to deep blue light with CIE coordinates of (0.154; 0.138). The improvement of external efficiencies of the doped DPBVi device is function of the doping concentration. The optimum doping rate is found to be 1.8 wt.% of rubrene into the DPVBi host material. In this case, the quantum efficiency can lead to values as high as 5.7% and current efficiency of 7 cd/A (Table 1). The improvement of EL efficiency upon doping has been reported previously [13] and can be explained by an efficient energy transfer mechanism from the matrix to the dopant. Furthermore, the threshold voltage and the electrical characteristics of the device (Table 1 and Figure 3a) are lightly improved after doping the DPVBi layer. 3.2 White electroluminescent devices The EL spectra of the three type devices are shown in Figure 4 as a function of the rubrene doping concentration. The evolution of the blue TABLE 2 Blue (1st line) and yellow (2nd line) peak intensity (W/sr/m2 × 100) for the three white devices as a function of the rubrene concentration. Rubrene concentration (% wt.) Devices Type I Type II Type III

0 11.6 0 11.6 0 17.1 0

0.1 4.5 5.7 9.1 3.3

0.2 4.0 6.7

0.6

0.7

3.7 6.5 5.7 5.8

3.8 6.5

0.8 1.2 6.4

1.0

1.4

2.3 7.4 3.0 7.5

1.9 8.3

1.8 0.3 5.6

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FIGURE 4 EL spectra of the three type devices as a function of the rubrene doping concentration recorded at 30 mA/cm2 .

and yellow main peaks is reported in Table 2 and the EL characteristics are collected in Tables 3, 4 and 5 for type I, II and III devices respectively. As shown in Figure 4, the wavelength of the blue peak (452 nm for type I & type II devices and 444 nm & 472 nm for type III devices) is quite constant upon doping with rubrene, while it is shifted for the yellow one from 550 nm for low concentration of dopant to 560 nm for higher concentration. The main emission peak of pure rubrene is generally observed at 560 nm. By doping the DPVBi matrix with rubrene (Figure 4a), we can observe an incomplete energy transfer or a partial carrier trapping at the dopant site so that the light emission comes from both the host material and dopant [14]. Varying the percentage of rubrene from 0.1 to 1.8 %, we TABLE 3 EL characteristics of type I devices as a function of the doping rate. Efficiencies Rubrene in DPVBi (wt. %) 0 0.1 0.2 0.8 1.8

Us (V) 5.3 4.2 4.4 4.2 4.3

% 3.6 3.5 3.4 2.9 2.5

cd/A 4 9.4 9.3 8.9 8.2

CIE lm/W 1.2 3 2.9 2.8 2.5

x 0.155 0.316 0.346 0.409 0.462

y 0.130 0.339 0.383 0.454 0.495

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BRIGHT WHITE ORGANIC LIGHT-EMITTING DIODE TABLE 4 EL characteristics of type II devices as a function of the doping rate. Efficiencies Rubrene in α-NPB (wt. %) 0 0.1 0.7 1.0

Us (V) 5.3 4.6 4.8 5

% 3.6 4 3.4 3.5

cd/A 4 6.9 9.6 10.5

CIE lm/W 1.2 2.3 3.3 3.5

x 0.155 0.222 0.352 0.389

y 0.130 0.219 0.390 0.430

can see an evolution of EL spectra with a diminution of the blue emission (peak intensity at 452 nm from 11.6 to 0.3 for 0% to 1.8% of rubrene, respectively). For the yellow emission, a saturation effect is seen with quite a constant value of the rubrene peak intensity (Table 2) independently of the rubrene concentration. As a result of the decrease of the blue emission and of the saturation of the yellow one, the external quantum efficiency decreases from 3.5 to 2.5% with the rubrene concentration (Table 3). For type I devices, the energy transfer from DPVBi to rubrene or the charge trapping by rubrene is preponderant even at really low concentration of rubrene since the dopant material is located within the emitting layer. It has been reported that rubrene dispersed in a host layer acts as trap charge carrier and as efficient recombination center [15]. Moreover, a saturation effect of the rubrene emission is observed at really low concentration of dopant (0.1%). The decrease of the blue emission with the concentration of rubrene added to the saturation effect leads to a drop of the current efficiencies from concentrations of rubrene of 0.1% (Figure 5). The white emission can be obtained but for low concentration of rubrene (below 0.2%) as shown in Table 3 (see CIE coordinates). Nevertheless, the control of such low values is quite difficult within the co-evaporation process. Contrary to type I devices, a balanced emission has been observed from the blue and the yellow emitters by changing the rubrene concentration for type II and type III devices. An effective decrease of the blue peak and a concomitant increase of the yellow one are observed for these two devices (see Figure 3b & 3c and Table 2). The different EL mechanisms of type II and III versus type I come from the fact that rubrene is no longer in the TABLE 5 EL characteristics of type III devices as a function of the doping rate. Rubrene in α-NPB (wt. %) 0 0.2 0.6 1 1.4

PR3491 in DPVBi (wt. %) 1.8 1.6 1.6 1.8 1.8

Efficiencies Us (V) 5.0 4.3 4.2 4.1 4.2

% 5.7 3.4 3.2 3.6 3.7

cd/A 7 8.7 9.1 11.0 11.5

CIE lm/W 2.3 2.9 3.1 3.9 4.0

x 0.154 0.308 0.343 0.382 0.423

y 0.138 0.338 0.390 0.422 0.449

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FIGURE 5 Evolution of the current efficiencies of WOLED of type I, II and III as a function of the rubrene concentration.

DPVBi layer but in the HTL layer. The mechanism of emission probably depends on interlayer energy transfers from blue emitter to rubrene. It may also be due to a shift in the recombination zone due to the doped HTL layer by rubrene. Zhang et al. shown that doping the HTL with rubrene facilitates the injection of both holes and electrons [10] and Murata et al. reported that rubrene acts as both a charge trap and a recombination center [15]. The rubrene trapping-dopant molecules tend to reduce the holes mobility in the NPB layer and to improve the balance of electrons and holes in the device. This is the reason why a better balance between the blue and yellow light is observed for the rubrene doped HTL layer. The external quantum efficiencies are quite similar (∼ 3.5%) for type II and type III devices after doping the HTL layer with rubrene (see Tables 4 & 5). This result is rather surprising and effectively a drop of the external quantum efficiency (from 5.7% to 3.5%) is obtained in type III device upon doping with rubrene. It means that the EL mechanism is mainly dominated by the rubrene specie which is an efficient charge trap as previously mentioned. Both type II and III systems lead to higher current efficiencies at high concentration of rubrene (Figure 5), especially for type III. Moreover, efficient white emission (∼9 cd/A) can be obtained for percentages of rubrene from 0.2 to ∼0.7% (versus < 0.2% for type I), which is more convenient for the WOLED elaboration process, as the co-evaporation of rubrene and its host is easier to control at higher percentage. Type III devices should probably lead to more efficient WOLED by optimizing the blue doping concentration. Moreover, doped emissive layers should be more suitable when device stability is of concern. As shown in Tables 3, 4 and 5, type I, II and III structures emit a white light under specific doping ratio. Their white color quality is close to the


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FIGURE 6 Evolution of the CIE coordinates as a function of the current density for WOLED of type I, II and III.

D65 reference and their CIE coordinates do not change much with driving current as depicted in Figure 6. For all devices, one can say that the rubrene emission is not governed by photoluminescence of this yellow-orange emitter since the layer optical density is rather low (small concentration of dopant and thin film thickness). However, the emission mechanisms of the two complementary colors involved in type I and type II/III are not the same. For the rubrene doped-DPVBi device, the excitons recombine mainly within the rubrene HOMO-LUMO levels while for type II and III devices, they recombine both in the HTL and the blue emitting layers. 4 CONCLUSIONS Blue and white devices have been fabricated from a common structure ITO/CuPc/α-NPB/DPVBi/Alq3/LiF/Al. Highly efficient blue diodes with external quantum and current efficiencies of up to 5.7 % and 7 cd/A have been obtained for doped DPVBi emitting layer. WOLED have been also realized doping the blue emitter or the HTL with an efficient complementary color emitter, the rubrene. By properly tuning the doping rate in order to balance the blue and yellow-orange contribution of the diode emission, fairly pure white lights have been obtained with CIE chromaticity coordinates close to the D65 reference and external current efficiencies of about 9 cd/A at a current density of 10 mA/cm2 . Moreover, the CIE coordinates of the emitted light are quite stable as a function of the current density. We have shown that different mechanisms are involved within the blue/yellow emission

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depending on the layer where the rubrene is located. The rubrene doping within the α-NPB layer allows a more convenient control of the percentage of rubrene to get WOLED. Finally, an efficient double doped system has been demonstrated. The lifetime measurements have to be investigated. ACKNOWLEDGMENTS We thank the European Commission (Photoledd project IST-2001-37, 181) and the national PEGS program for financial support. REFERENCES [1] Tang, C. W., and Vanslyke, S. A. (1987). Organic electroluminescent diodes. Appl. Phys. Lett., 51, 913–915. [2] Ikai, M., Tokito, S., Sukamoto, Y., Suzuki T., and Taga, Y. (2001). Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer. Appl. Phys. Lett., 79, 156–159. [3] D’Andrale, B. W., and Forrest, S. R. (2004). White organic light-emitting devices for solid-state lighting. Adv. Mater., 16(18), 1585–1595. [4] Chuen, C. H., and Tao, Y. T. (2002). Highly-bright white organic light-emitting diodes based on a single emission layer. Appl. Phys. Lett., 81(24), 4499–4501. [5] Zhang, Z. L., Jiang, X. Y., Zhu, W. Q., Zheng, X. Y., Wu, Y. Z., and Xu, S. H. (2003). Blue and white emitting organic diodes based on anthracene derivative. Synthetic Metals, 137, 1141–1142. [6] Shiga, T., Fujikawa, H., and Taga, Y. (2003). Design of multiwavelength resonant cavities for white organic light-emitting diodes. J. Appl. Phys., 93(1), 19–22. [7] Kim, J.-S., Seo, B.-W., and Gu, H.-B. (2003). Exciplex emission and energy transfer in white light-emitting organic electroluminescent device. Synthetic Metals, 132, 285–288. [8] Tsuji, T., Naka, S., Okada, H., and Onnagawa, H. (2004). Improved white organic electroluminescent devices using fine mesh as an evaporation mask. Current Applied Physics, 1, 1–4. [9] Hamada, Y., Sano, T., Shibata, K., and Kuroki, K. (1995). Influence of the emission site on the running durability of organic electroluminescent devices. Jpn. J. Appl. Phys., 34, L824–L826. [10] Zhi-Lin, Z., Xue-Yin, J., Shao-Hong, X., Nagatomo, T., and Omoto, O. (1998). The effect of rubrene as a dopant on the efficiency and stability of organic thin film electroluminescent devices. J. Phys. D: Appl. Phys., 31, 32–35. [11] Aziz, H., and Popovic, Z. D. (2002). Study of organic light emitting devices with a 5,6,11,12-tetraphenylnaphthacene (rubrene)-doped hole transport layer, Appl. Phys. Lett., 80(12), 2180–2182. [12] Shaheen, S. E., Jabbour, G. E., Morrell, M. M., Kawabe, Y., Kippelen, B., Peyghambarian, N., Nabor, M.-F., Schlaf, R., Mash, E. A., and Armstrong, N. R. (1998). Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode. J. Appl. Phys., 84(4), 2324–2327. [13] Hosakawa, C., Higashi, H., Nakamura, H., and Kusumoto, T. (1995). Highly efficient blue electroluminescence from a distyrylarylene emitting layer with a new dopant. Appl. Phys. Lett., 67(26), 3853–3855.


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[14] Deshpande, R. S., Bulovi´c, V., and Forrest, S. R. (1999). White-light-emitting organic electroluminescent devices based on interlayer sequential energy transfer. Appl. Phys. Lett., 75(7), 888–890. [15] Murata, H., Meritt, C. D., and Kafafi, Z. H. (1998). Emission mechanism in rubrene-doped molecular organic light-emitting diodes: direct carrier recombination at luminescent centers. IEEE J. Sel. Top. Quantum Electron., 4, 119–124.

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