APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 9
26 AUGUST 2002
Fuzzy-junction organic light-emitting devices C.-W. Chen, T.-Y. Cho, and C.-C. Wua) Department of Electrical Engineering, Graduate Institute of Electro-Optical Engineering, and Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China
H.-L. Yu and T.-Y. Luh Department of Chemistry, National Taiwan University, Taipei, Taiwan 10617, Republic of China
共Received 20 May 2002; accepted for publication 1 July 2002兲 A ‘‘fuzzy-junction’’ organic light-emitting device 共OLED兲 containing a graded organic–organic interface is reported. Such graded junction is effectively produced utilizing interdiffusion through an ultrathin interfacial fusing layer sandwiched between two functional layers. With a glass transition temperature (T g ) lower than remaining layers, this fusing layer permits smooth interdiffusion and mixing of neighboring layers by annealing above its T g . With appropriate material combinations, fuzzy-junction OLEDs thus prepared exhibit both reduced voltage and enhanced emission efficiency in comparison with conventional abrupt-junction devices. As an instance, a green fluorescent OLED with such fuzzy junction shows a high peak power efficiency of ⬃20 lm/W, substantially higher than ⬃14 lm/W of a corresponding abrupt-junction device. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1502912兴
Organic light-emitting devices 共OLEDs兲 have been the subjects of intense investigation in recent years due to their applications in efficient, large-area, and full-color displays.1– 4 Heterojunctions in multilayer OLEDs have been effective in enhancing device performance due to their confinement effects on carriers and excitons.1,4 However, barriers at these heterojunctions may also lead to difficulty for carrier injection/transport and accumulation of space charges at these interfaces, limiting device performance in some cases.5,6 In view of these, recently there have been a few reports on the benefits of incorporating mixed material layers in device structures to eliminate abrupt junctions.5–9 In these previous reports, the mixed layer has been prepared by vacuum codeposition, which in principle complicates OLED fabrication particularly when additional doping and graded compositions may be necessary for device optimization.7,9 In this letter, we report a ‘‘fuzzy-junction’’ OLED, in which the graded organic–organic interface is effectively produced by interdiffusion induced with an interfacial fusing layer at elevated temperatures, instead of the more complicated codeposition process. The fuzzy-junction OLEDs exhibit both reduced operation voltage and enhanced emission efficiency in comparison with conventional abrupt-junction devices. The device structures and materials studied are shown in Fig. 1. The as-deposited structure on the glass substrate 关Fig. 1共a兲兴 has the typical structure of multiple organic layers sandwiched between the bottom indium–tin–oxide 共ITO兲 anode and the top Al cathode. The stack of organic layers consists of a hole-injection layer 共300 Å兲 of conducting polymer polyethylene dioxythiophene/polystyrene sulphonate 共PEDT/ PSS, Bayer Corp.兲,10 a hole-transport layer 共HTL, 400 Å兲 using either ␣-naphthylphenylbiphenyl diamine 共␣-NPD兲 or
N,N ⬘ -diphenyl-N,N ⬘ -bis兵4 ⬘ -关N,N-bis共naphth-1-yl兲-amino兴biphenyl-4-y其-benzidine 共a triarylamine tetramer, TATE兲,2,11 an electron-transport/emitting layer 共ETL/EL, 600 Å兲 containing pure or partially doped tris-共8-hydroxyquinoline兲 aluminum 共Alq兲,1 and a thin interfacial fusing layer consisting of either bis-4,4⬘ -关共diphenylmethylsilyl兲vinyl兴biphenyl 共DPSVB兲 or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline 共BCP兲 inserted between HTL and ETL.12,13 5 Å LiF deposited prior to Al cathode is used as the electron-injection layer,3 and green fluorescent dye C545T is used as emissive dopant for the first 300 Å of Alq in some structures.9 For comparison, control devices 共i.e., ‘‘conventional’’ devices兲 without the interfacial fusing layers were also fabricated. The PEDT/PSS layer was prepared by spin coating and other layers were deposited in a multiple-source vacuum chamber. A few different devices can be fabricated without breaking the vacuum, permitting reliable side-by-side comparison of various devices. In our previous work on the structure of ␣-NPD/DPSVB/Alq,12 DPSVB has been shown to be an effective hole-blocking material. Due to its T g 共⬃30 °C兲 being rather lower than ␣-NPD 共⬃100 °C兲 and Alq 共⬃170 °C兲, we have found that raising the device temperature substantially
a兲
FIG. 1. Structures of 共a兲 as-deposited device, 共b兲 fuzzy-junction device, and 共c兲 compounds used.
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above T g of DPSVB 共but still ⬍T g ’s of others兲 could cause the interdiffusion of neighboring layers through it and consequently the fusing of the initially separated ␣-NPD and Alq layers 关Fig. 1共b兲兴. This has been confirmed by effective resonant energy transfer between ␣-NPD and Alq in photoluminescence 共PL兲 studies of fused devices.12 As a result, the hole-blocking property is bypassed and device emission is altered from blue of ␣-NPD to green of Alq, making the device a ‘‘programmable’’ one. In this work, we find that by making DPSVB as thin as tens of angstroms, the fusing caused by this interfacial fusing layer can actually enhance device performance beyond that of a conventional ␣-NPD/ Alq device. Since the interdiffusion of ␣-NPD, DPSVB, and Alq is expected to cause blurring of the abrupt junction and to give a region of graded compositions of these three compounds between pure ␣-NPD and pure Alq regions, we have therefore named this type of device a ‘‘fuzzy-junction’’ OLED. Owing to the additional ‘‘fusing’’ step, after the device deposition, the completed structures were subject to annealing at elevated temperatures under dry nitrogen atmosphere to complete the fabrication of the fuzzy-junction device. Figures 2共a兲 and 2共b兲 compare the I – V – L and the efficiency characteristics of a conventional ␣-NPD/Alq device and a corresponding fuzzy-junction device 共with DPSVB 10 Å, annealing at 80 °C, 3 min兲. Both devices show the same Alq electroluminescence 共EL兲 spectra. However, the fuzzyjunction device exhibits substantially reduced voltage and enhanced efficiency 共1.6%, 5 cd/A vs 1.3%, 4 cd/A for the conventional device兲, resulting in higher brightness at a given voltage and improved power efficiency 关4.1 lm/W vs 3.3 lm/W at the peaks, Fig. 2共d兲兴. Although not shown, it has to be mentioned that characteristics of conventional devices with or without similar annealing were also examined and were found nearly identical, ensuring that the device enhancement was due to fusing rather than some other annealing effect. Furthermore, as shown in Fig. 2共b兲, the fuzzyjunction device exhibits highly rectified I – V characteristics 共rectification ratio ⬃108 兲 as in the conventional device, indicating that the fuzzy junction brings no detrimental effects on these characteristics. Both devices show identical onset voltage of ⬃2 V, yet the fuzzy junction gives steeper I – V and L – V characteristics 关Fig. 2共b兲兴, leading to reduced operation voltage. The device enhancement due to the fuzzy junction applies to devices with emissive dopants as well. Figure 2共c兲 shows the I – V – L and efficiency characteristics of the C545 T-doped 共1 wt % in the first 300 Å of Alq兲 conventional device and fuzzy-junction device 共DPSVB 10 Å, annealing at 80 °C, 3 min兲. Both devices exhibit pure green emission of C545T 关inset of Fig. 2共d兲兴. The fuzzy-junction device shows a substantially higher maximum efficiency than the conventional one 共⬃4.5%, 16.3 cd/A vs ⬃3.4%, 12.3 cd/A兲. It is also noteworthy that the efficiency of the fuzzy-junction device rises sharply to its efficiency maximum at ⬃2.5 V and stays rather constant beyond. In contrast, the efficiency of the conventional device still rises gradually beyond 3 V until a maximum is finally reached at ⬃8.5 V. Together with reduced operation voltage, enhanced efficiency gives the fuzzy junction a remarkably high power efficiency of 19.5 lm/W at
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FIG. 2. 共a兲 Linear scale, 共b兲 semilog scale I – V 共䊉兲, 共䉱兲 and L – V 共䊊兲, 共䉭兲 characteristics for a conventional ␣-NPD/Alq device 共upward triangles兲 and an ␣-NPD/DPSVB/Alq fuzzy-junction device 共circles兲. Inset of 共a兲 : efficiency for devices in 共a兲. 共c兲 I – V 共䊏兲, 共䉲兲 and L – V 共䊏兲, 共䉮兲 characteristics for a doped conventional device 共downward triangles兲 and a doped fuzzy-junction device 共squares兲. Inset of 共c兲 efficiency for devices in 共c兲. 共d兲 Power efficiency for devices in 共a兲 and 共c兲. Inset of 共d兲: EL spectra of nondoped and C545T doped devices.
the peak 共with a few cd/m2兲 and 14 lm/W at 100 cd/m2 共vs 13.5 lm/W at the maximum and 8 lm/W at 100 cd/m2 for the conventional device兲, as shown in Fig. 2共d兲. Such improvements in turn-on characteristics and power efficiency are of particular significance to the active-matrix OLED displays, in which OLEDs are driven near device onset. Since the present scheme makes use of interdiffusion capability of organic glasses, one may be concerned about thermal stability of fuzzy-junction devices. However, after extended annealing 共80 °C, ⬎60 min兲 of DPSVB fuzzyjunction devices, they exhibited almost identical characteristics as those shown previously, showing no indication of degradation. Such thermal stability may suggest that the interdiffusion induced by an interfacial layer at an elevated temperature be self-limiting. That is, initially in pure DPSVB, T g is low and interdiffusion is feasible. With interdiffusion and mixing, the concentration of DPSVB declines, leading to a rise of local T g and eventually suppression of further diffusion. In recent years, there is also a trend of employing high-T g materials in OLEDs to extend opera-
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FIG. 3. Comparison of I – V 共䊉兲, 共䉱兲 and L – V 共䊊兲, 共䉭兲 characteristics of a conventional TATE/Alq device 共triangles兲 and a TATE/BCP/Alq fuzzyjunction device 共circles兲. Inset: efficiency vs voltage for the above two devices.
tional temperature range.11 To test whether interfacial fusing can be applied to such high-T g systems, we replaced ␣-NPD and DPSVB with higher-T g hole-transport TATE (T g ⬃150 °C) and hole-blocking BCP (T g ⬃80 °C), respectively. Since T g of the fusing layer is raised, the fusing temperature required correspondingly increases to 120 °C 共but still ⬍T g ’s of other materials兲. Figure 3 compares the I – V – L and efficiency characteristics of the conventional TATE/Alq device and the corresponding BCP fuzzy-junction device 共with 50 Å BCP, annealing at 120 °C, 3 min兲. As in previous cases, both devices exhibit EL of Alq, yet the thermally induced fuzzy junction leads to reduced voltage and enhanced efficiency. This result suggests that the fuzzy junction may be a general approach applicable to various material combinations and that careful choice of material combinations may be carried out to obtain desired or optimized device characteristics. To acquire further insight of present fuzzy-junction devices and its differentiation from conventional ones, we have also fabricated the following devices with codeposited mixed layers to mimic the situation of fuzzy junction: 共I兲 ITO/ PEDT/␣-NPD 375 Å/␣-NPD:DPSVB 共5:1 wt, 25 Å兲/Alq: DPSVB 共5:1 wt, 25 Å兲/Alq 575 Å/LiF/Al, 共II兲 ITO/PEDT/␣NPD 375 Å/␣-NPD:BCP 共5:1 wt, 25 Å兲/Alq:BCP 共5:1 wt, 25 Å兲/Alq 575 Å/LiF/Al, 共III兲 ITO/PEDT/␣-NPD 375 Å/␣NPD:Alq:DPSVB 共5:5:2 wt, 50 Å兲/Alq 575 Å/LiF/Al, 共IV兲 ITO/PEDT/␣-NPD 375 Å/␣-NPD:Alq 共1:1 wt, 50 Å兲/Alq 575 Å/LiF/Al. All these devices have a 50 Å codeposited mixed region between pure HTL 共␣-NPD兲 and pure ETL 共Alq兲 and all show pure EL of Alq. Figure 4 compares the
FIG. 4. Comparison of I – V and efficiency characteristics of devices I 共䊏兲, II共〫兲, III共䉲兲, and IV 共*兲 with those of the conventional device 共䉭兲 and the fuzzy-junction device 共䊉兲 in Fig. 2共a兲.
I – V and efficiency characteristics of these devices with those of the conventional and fuzzy-junction devices in Fig. 2共a兲. It is noticed that codepositing HTL and ETL only as in device 共IV兲 does not provide for efficiency enhancement as compared to the conventional abrupt-junction device, consistent with previous reports.7,9 On the other hand, from the efficiency enhancement observed for devices I–III, it appears that mixing hole-blocking materials 共DPSVB or BCP兲 near the junction is responsible for efficiency enhancement in present fuzzy-junction devices and for the difference in performance from previous mixed-layer devices.7,9 Meanwhile, the fuzzy junction induced with a hole-blocking fusing layer has the further advantage of substantial voltage reduction, compared to mixing hole-blocking materials simply by codeposition. Although detailed mechanisms require further investigation, it is currently speculated that the graded composition and the hole-blocking compounds mixed near the fuzzy junction subtly affect the scenario of carrier injection/ transport, and distributions of carriers and electric fields near the junction, together contributing to overall device enhancement. In summary, we report a ‘‘fuzzy-junction’’ OLED, in which the graded organic–organic interface is effectively produced by a functional interfacial fusing layer. The ultrathin interfacial fusing layer has lower T g than remaining layers such that annealing the structure with a temperature above its T g causes the fusing 共mixing兲 of neighboring layers, forming a fuzzy junction. The fuzzy-junction OLEDs exhibit both reduced operation voltage and enhanced emission efficiency in comparison with conventional abruptjunction devices. This approach may be generalized to appropriate material combinations and to various organic– organic interfaces in a heterostructure OLED to obtain desired or optimized device characteristics. The authors would like to acknowledge financial support from National Science Council 共Grant No. NSC 90-2215-E002-025兲 and Ministry of Education 共Grant No. 89-N-FA012-4-2兲 of the Republic of China. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913 共1987兲. J. Shi and C. W. Tang, Appl. Phys. Lett. 70, 1665 共1997兲. 3 L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett. 70, 152 共1997兲. 4 C. W. Tang, S. A. VanSlyke, and C. H. Chen, J. Appl. Phys. 65, 3610 共1989兲. 5 H. Riel, W. Bru¨tting, T. Beierlein, E. Haskal, P. Mu¨ller, and W. Riess, Synth. Met. 111–112, 303 共2000兲. 6 J. Shen and J. Yang, J. Appl. Phys. 87, 3891 共2000兲. 7 V.-E. Choong, S. Shi, J. Curless, C.-L. Shieh, H.-C. Lee, F. So, J. Shen, and J. Yang, Appl. Phys. Lett. 75, 172 共1999兲. 8 H. Aziz, Z. D. Popovic, N.-X. Hu, A.-M. Hor, and G. Xu, Science 283, 1900 共1999兲. 9 A. B. Chwang, R. C. Kwong, and J. J. Brown, Appl. Phys. Lett. 80, 725 共2002兲. 10 A. Elschner, F. Bruder, H. W. Heuer, F. Jonas, A. Karbach, S. Kirchmeyer, S. Thurm, and R. Wehrmann, Synth. Met. 111, 139 共2000兲. 11 S. Tokito, H. Tanaka, K. Noda, A. Okada, and Y. Taga, Appl. Phys. Lett. 70, 1929 共1997兲. 12 C.-C. Wu, C.-W. Chen, Y.-T. Lin, H.-L. Yu, J.-H. Hsu, and T.-Y. Luh, Appl. Phys. Lett. 79, 3023 共2001兲. 13 Y. Kijima, N. Asai, and S. Tamura, Jpn. J. Appl. Phys., Part 1 38, 5274 共1999兲. 1 2
