http://jjap.ipap.jp/link?JJAP/46/642/ | http://jjap.ipap.jp/link?JJAP/46/642/We investigated directional characteristics of the electroluminescence (EL) from microcavity organic light-emitting diodes (MOLEDs) to examine applicability of these diodes as a light source for optical interconnects. By measuring the angular dependence of a EL emission, we estimated the coupling efficiency between a MOLED and an optical fiber. The coupling efficiency was enhanced by 1.4-fold compared with that of a non cavity organic light-emitting diode (OLED), and reached 85% for an optical fiber with a numerical aperture (NA) of 0.5. The current efficiency of the MOLED also increased optimizing the device structure, and its maximum was 18.4 cd/A.textapplication/pdfjournal articl

16699

フクダ, タケシ

福田, 武司

English

Saitama University Cyber Repository of Academic Resources

Typeset using jjap.cls <ver.1.0.1>High Coupling Efficiency of Microcavity Organic Light-EmittingDiode with Optical Fiber for as Light Source for OpticalInterconnectsTakeshi Fukuda∗, Bin Wei1, Masakazu Ohashi, Musubu Ichikawa1 and Yoshio Taniguchi1Applied Electronics Technology Department, Optics and Electronics Laboratory, FujikuraLtd., 1440, Mutsuzaki, Sakura, Chiba, 285-8550, Japan1Department of Functional Polymer Science, Faculty of Textile Science and Technology,Shinshu University, 3-15-1, Tokida, Ueda, Nagano, 386-8567, Japan(Received )We investigated the directional characteristics of the electroluminescence (EL) frommicrocavity organic light-emitting diodes (MOLEDs) to examine applicability of thesediodes as a light source for optical interconnects. By measuring the angular dependenceof a EL emission, we estimated the coupling efficiency between a MOLED and an opticalfiber. The coupling efficiency was enhanced by 1.4-fold compared with that of a noncavity organic light-emitting diode (OLED) , and reached 85 % for an optical fiber witha numerical aperture (NA) of 0.5. The current efficiency of the MOLED also increasedoptimizing the device structure, and its maximum was 18.4 cd/A.KEYWORDS: OLED, microcavity structure, tris (8-hydroxyquinolato) aluminum, opticalinterconnect, light source∗E-mail address: fukuda@lab.fujikura.co.jp11. IntroductionIn recent years, several breakthroughs have led to significant enhancements in the per-formance of organic light-emitting diodes (OLEDs) , such as the improvement in charge-carrier balance,1,2) the development of low-work function electrode materials,3) and the re-alization of high carrier mobility and high EL quantum efficiency in organic light-emittinglayers,4) high fluorescence efficiency emitting materials,5–7) and phosphorescence emittingmaterials.8) Therefore, the internal quantum efficiency of OLEDs has been optimizedgradually close to the theoretical limit.6)On the other hand, only a small fraction of total photons generated inside the emittinglayer are generally usable, because of the internal reflection and the confinement effects ofhigh-refractive-index layers.9) For an OLED with a sandwiched structure between a metalreflector and a transparent anode, the fraction of light generated in the organic layer andescaping from the substrate into the air can be estimated to be 1/(2×n2org) . Where norgis the refractive index of the organic layer. Therefore, we can determine that the lightcoupling efficiency of OLEDs is typically only 20 % under the assumption of norg = 1.6.The rest of the light either is absorbed by the metal cathode and the polaron producedby applying voltage or escapes through waveguide mode of the substrate.10–12)The use of microcavity structures in OLEDs is an effective approach to enhancingextraction efficiency. Because a microcavity structure concentrates spontaneous emissionphotons to be perpendicular to a cavity structure, the emission intensity of a cavity deviceincreases as compared with the non cavity device.13,14) We can also realize spectrallynarrowed13,15) and highly directional EL emissions.15–17) Therefore, strongly directedpure RGB ELs are emitted from a microcavity organic-light-emitting diode (MOLED).15) These effects are useful as not only OLED displays but also light sources for opticalinterconnects.To date, OLEDs that intrinsically possess a rapid time response have been investigatedas light sources for optical interconnects.18–20) OLED light sources have already demon-strated a transmission speed of 100 MHz.18) However, the optical coupling between OLEDlight sources and optical fibers is crucial in achieving high efficiency signal transmission,2because optical signals from OLEDs should enter into an optical fiber. Therefore, it isessential to optimize the directional characteristics of EL from an OLED. Furthermore,there is as yet no report on the coupling efficiency between OLEDs and fibers, to thebest of our knowledge. Because the EL distribution of conventional OLEDs is usuallyclose to the Lambertian distribution, the coupling efficiency of OLEDs with optical fibersdecreases in contrast to that in the case of semiconductor lasers. In this study, we in-vestigated the angular dependence of EL from several MOLEDs with different structures,and also evaluated the coupling efficiency between a MOLED and an optical fiber on thebasis of its dependence on angle.2. MOLED DesignIn MOLEDs, a microcavity structure is defined by a metal cathode and a distributedBragg reflector (DBR) formed on a glass substrate. We used MgAg layer as the metalcathode and several SiO2 and Ta2O5 pairs of SiO2 and Ta2O5 quarter wavelength layersas the DBR. The cavity length is expressed by13,15)L(λ) =λ2(neff∆n)+∑jnjLj +∣∣∣∣ϕm4π λ), (2.1)where neff and ∆n are the effective refractive index of the DBR and the refractive indexdifference between SiO2 and Ta2O5, and the nj and Lj are the refractive index and thethickness of the organic and indium tin oxide (ITO) layers, respectively. ϕm is the phaseshift at the metal cathode expressed by21)ϕm = arctan(2nskmn2s − n2m − k2m), (2.2)where ns is the refractive index of the organic layer in contact with the metal cathode, andnm and km are the real and imaginary parts of the refractive index of the metal cathode,respectively. Cavity modes are given by the relationship mλ = 2L(λ), where m is themode index. The mode index can be adjusted by varying the cavity length L (λ) .3. ExperimentWe fabricated three kinds of MOLED devices (Fig. 1) . The mode indexes are 4 fordevices A and C, and 5 for device B. First, we prepared a DBR consisting of several pairs3of SiO2 and Ta2O5 and an indium tin oxide (ITO) anode on a glass substrate. The DBRand ITO layers are deposited by electron beam deposition and RF sputtering, respectively.The thicknesses of the SiO2 and Ta2O5 layers were set to be equal to the quarter opticallengths of a center wavelength (510 nm) ; they are 86.32 and 56.85 nm, respectively. Thenumber of the SiO2/Ta2O5 pair varied from 1 to 7 with a chang in the reflectivity of theDBR. The thicknesses of the ITO anode are 100 nm for devices A and C, and 150 nmfor device B. The prepared glass substrate was cleaned in deionized water, detergent, andisopropyl alcohol sequentially under ultrasonic waves, and then treated with 50 W oxygenplasma for 5 min just before use.Next, we deposited organic layers and a metal cathode using a conventional vacuumdeposition system at a base pressure of below 5.0 × 10−6 Torr. Each organic layers con-sisted of 4,4’-bis [N- (1-napthyl) -N-phenyl-amino] -biphenyl (α-NPD) as a hole transportlayer (HTL) , tris (8-hydroxyquinoline) aluminum (Alq3) doped with 0.4- wt% coumarin6 (for devices A and B) or rubrene (for device C) as a light-emitting layer (EML) , Alq3 asan electron transport layer (ETL) , LiF as an electron injection layer (EIL) , and MgAg(9:1 mol ratio) as a cathode. The thicknesses of all the layers are shown in Fig. 1. Figure1 also shows the chemical structures of the dopants. The deposition rates ranged from0.08 to 0.12 nm/s for the ETL and HTL, 0.01 to 0.02 nm/s for the EIL, and 5.0 nm/s forthe EML and cathode. The active area was 2 × 2 mm2.We measured luminance-current density-voltage characteristics with a source measureunit (HP4140B, Hewlett-Packard) and a luminance color meter (BM-7, TOPCON) . Thecolor meter was placed normal to the test device to measure the front luminance at asolid angle of 1 deg.4. Results and DiscussionFigure 2 shows the reflection spectra of the DBRs. As clearly shown in Fig. 2, thecenter wavelengths all the DBRs are about 550 nm and the reflectivity at the centerwavelength increases with increasing number of SiO2 and Ta2O5 pairs. The DBRs havea sufficiently broad stop band to confine the entire EL emission except for the cavityresonance wavelength.4Figures 3(a)-3(c) show the current efficiencies defined here as the ratio of the frontluminance and the current density, and the full width at half maximum (FWHM) of theEL spectra of all the devices as functions of the number of SiO2 and Ta2O5 pairs. Currentefficiency was measured at a current density of 60 mA/cm2. Although the non cavityOLEDs (number of pairs, 0) exhibited a spectrally broad emission with FWHMs of 64.8nm for device A, 88.0 nm for device B, and 67.0 nm for device C, the FWHMs decreasedwith increasing number of SiO2 and Ta2O5 pairs and then to minimum values of 8.7 nmfor device A, 6.0 nm for device B, and 9.3 nm for device C. As shown in Figs. 3(a) and3(b), current efficiency decreased with increasing number of SiO2 and Ta2O5 Macro 1s fordevices A and B which had the same EML (coumarin-6-doped Alq3).On the other hand, the current efficiency of device C (rubrene-doped Alq3 EML device)initially increased up to 18.4 cd/A when the number of SiO2 and Ta2O5 pairs was 2,and then decreased with an increase in that number. A microcavity causes photonsof spontaneous emission to be concentrated perpendicular to itself. Therefore, currentefficiency can increase with increasing number of reflections of the DBR. The decrease incurrent efficiency can be explained as follows: The number of such reflections at the metalcathode increases using more reflective DBRs.16) Metal mirrors have some absorption lossdue to their electric resistivity; such absorption loss increases with the boosting reflectivityof a DBR. Thus, the combination of concentration and absorption loss is led to the resultshown in Fig. 3(c).On the other hand, current efficiency monotonically decreased for devices A and B,as shown in Figs. 3(a) and 3(b), respectively. Coumarin-6-doped Alq3, which is usedas EML of devices A and B, absorbed the EL emission itself, because it has a higherabsorption coefficient than rubrene-doped Alq3, as clearly shown in Fig. 4. In addition,the total propagation length of the EL emission in the organic layer for MOLEDs increasedcompared with that for non cavity conventional OLEDs. Therefore, current efficiencymonotonically decreased with increasing reflectivity of the DBR for devices A and B.We also measured the angular dependence of EL spectra from the MOLEDs. Figure 5shows the EL spectra of device A with a 4-SiO2/Ta2O5-pair DBR together with those fromanother device A without a DBR, as shown in Fig. 5(b), where the mode index of device5A is 4. For the device with a DBR, the resonance wavelengths as the angle against normalto the device increases, and peak intensity decreases gradually. Meanwhile, another peakin the red region appears, as shown in the inset of Fig. 5(a).Figure 6 shows the angular dependence of the EL spectra of device B whose mode indexis 5. Resonance wavelength is also shifted toward shorter wavelengths as the detectionangle is tilted, and another peak at approximately 630 nm also appears. The intensityof the other peak is significantly higher than that of the peak of device A [see the insetof Fig. 5(a)], and is independent of angle. On the other hand, the peak intensity atthe resonance wavelength decreased with increasing angle. In other words, the relativeintensity of the peak at approximately 630 nm increases with tilting angle. Therefore, thedirectionality of device B with a mode index of 5 is lower than that of device A. Note thatthe other peak at approximately 630 nm means a higher mode of resonance.13) Figure7 shows the angular dependence of EL spectra for devices A, B, and C. The EL spectrawere measured with a luminance color meter by tilting the samples from -80 deg to 80deg in steps of 5 deg.Finally, we estimated the coupling efficiency between an OLED and an optical fiberto examine applicability of OLEDs as a light source for optical interconnects. Figure 8shows the coupling efficiency as a function of the number of SiO2/Ta2O5 pairs for eachdevice. The efficiency was calculated from the angular dependence of EL intensity shownin Fig. 7, where the numerical aperture (NA) of a coupled optical fiber was assumed tobe 0.5. In devices A and C with a mode index of 4, coupling efficiency initially increasesand then decreases as the number of SiO2 and Ta2O5 pairs increases. In particular, thecoupling efficiency of device A reaches 85 % when the number of SiO2/Ta2O5 pair is 3.On the other hand, the coupling efficiency of the device B monotonically decreased withincreasing number of such SiO2 and Ta2O5 pairs. This is because, the relative intensity ofthe mode index peak higher than the resonance wavelength increases as mentioned before.These results indicate that a mode index of 4 is necessary to realize the high couplingefficiency of MOLEDs with an optical fiber. By optimizing the reflectivity of the DBR,the coupling efficiency of what was enhanced by 1.4-fold with that of a non cavity OLED.65. ConclusionsWe investigated high directionality MOLEDs to evaluate their applicability as lightsources for optical interconnects. The directionality of the MOLEDs is improved byoptimizing the reflectivity of the DBR, developing organic materials of the EMLs, andincreasing the mode index of the cavity. The coupling efficiency between a MOLED andan optical fiber was calculated from the angular dependence of the EL spectra. Thedependence of current efficiency on the reflectivity of the DBR has been investigated fora coumarin-6 or rubrene-doped Alq3 EML device. The current efficiency increased byoptimizing the reflectivity of the DBR.7References1) J. Kido and T. Matsumoto: Appl. Phys. Lett. 73 (1998) 2866.2) L. S. Hung, C. W. Tang, and M. G. Mason: Appl. Phys. Lett. 70 (1997) 152.3) I. D. Parker: J. Appl. Phys. 75, (1994) 1656.4) L. S. Hung and C. H. Chen: Mater. Sci. Eng. R 39 (2002) 143.5) C. W. Tang, S. A VanSlyke, and C. H. Chen: J. Appl. Phys. 65 (1989) 3610.6) C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest: J. Appl. Phys. 90 (2001)5048.7) Y. Cao. I. Parker, G. Yu, Z. Gang, and A. Heeger: Nature (London) 397 (1999) 414.8) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest: Appl.Phys. Lett. 75 (1999) 4.9) A. Chutinan, K. Ishikawa, T. Asano, M. Fujita, and S. Noda: Org. Electron. 6 (2005)3.10) T. Tsutsui, E. Aminaka, C. P. Lin, and D.-U. Kim: Philos. Trans. R. Soc. London,Ser. A 355 (1997) 801.11) N. Patel, K. S. J. Cina, and H. Burroughes: IEEE J. Sel. Top. Quantum Electron. 8(2002) 346.12) K. Meerholz and D. C. Muller: Adv. Funct. Mater. 11 (2001) 346.13) A. Dodablapur, L. J. Rothberg, R. H. Jordan, T. M. Miller, R. E. Slusher, and J. M.Philips: J. Appl. Phys. 80 (1996) 6954.14) K. Neyts: Appl. Surf. Sci. 244 (2005) 517.15) S. Tokito, T. Tsutsui, and Y. Taga: J. Appl. Phys. 86 (1999) 2407.16) J. Grüuner, F. Cacialli, and R. H. Friend: J. Appl. Phys. 80 (1996) 207.17) H. Riel, S. Karg, T. Beierlein, W. Rieß, and K. Neyts: J. Appl. Phys. 94 (2003) 5290.18) H. Kajii, T. Tsukagawa, T. Taneda, K. Yoshino, M. Ozaki, A. Fujii, M. Hikita, S.Tomaru, S. Imamura, H. Takenaka, J. Kobayashi, F. Yamamoto, and Y. Omori: Jpn.J. Appl. Phys. 41 (2002) 2746.19) B. Wei, K. Furukawa, J. Amagai, M. Ichikawa, T. Koyama, and Y. Taniguchi: Semi-cond. Sci. Technol. 19 (2004) L56.820) B. Wei, K. Furukawa, M. Ichikawa, T. Koyama, and Y. Taniguchi: Jpn. J. Appl. Phys.44 (2005) 4248.21) A. Chin and T. Y. Chang: J. Lightwave Technol. 9 (1991) 321.910Figure captionsFig. 1. (a) Configuration of MOLEDs and (b) chemical structures of coumarin 6 andrubrene used as dopant light-emitters.Fig. 2. Reflectivity of DBR consisting of several pairs of SiO2/Ta2O5. The number ofSiO2 and Ta2O5 pairs range from 1 to 7.Fig. 3. Relationship between current efficiency at 60 mA/cm2 and number of SiO2/Ta2O5pairs of DBR.Fig. 4. Absorption coefficients of coumarin-6-doped Alq3 and rubrene-doped Alq3 solidthin films.Fig. 5. EL emission spectra measured at different angles for device A with a mode indexof 4 and coumarin-6-doped Alq3 EML device. (a) MOLED with 4 SiO2 and Ta2O5pairs of and (b) non cavity OLED.Fig. 6. EL emission spectra measured at different angles for device B with a mode indexof 5 and coumarin-6-doped Alq3 EML device. (a) MOLED with 4 SiO2 and Ta2O5pairs of and (b) non cavity OLED.Fig. 7. Angular dependence of EL spectra for (a) device A, (b) device B, and (c) deviceC. The number of the DBR is varied between 1 and 7.Fig. 8. Coupling efficiency between MOLED and optical fiber with NA of 0.5.11DBR(SiO2/Ta2O5 1-6 pairs) α-NPD (28nm)Coumarin6:Alq3 (20nm)Alq3 (22nm)ITO (100nm)Device A Device BLiF (0.4nm)MgAg (150nm)DBR(SiO2/Ta2O5 1-7 pairs) α-NPD (75nm)Coumarin6:Alq3 (20nm)Alq3 (50nm)ITO (150nm)Device CLiF (0.4nm)MgAg (150nm)DBR(SiO2/Ta2O5 1-6 pairs) α-NPD (40nm)Rubrene:Alq3 (20nm)Alq3 (40nm)ITO (100nm)LiF (0.4nm)MgAg (150nm)Glass substrate Glass substrate Glass substrate Rubrene Coumarin6 (a)(b)ONSEL2N O12020406080100400 450 500 550 600 650 700Wavelength （nm)Reflectivity (%)1 pair2 pairs3 pairs4 pairs5 pairs6 pairs7 pairs130510152025300 1 2 3 4 5 6 7Number of SiO2/Ta2O5 pairsCurrent efficiency (cd/A)020406080100FWHM (nm)Current efficiencyFWHM0510152025300 1 2 3 4 5 6 7Number of SiO2/Ta2O5 pairsCurrent efficiency (cd/A)020406080100FWHM (nm)Current efficiencyFWHM0510152025300 1 2 3 4 5 6 7Number of SiO2/Ta2O5 pairsCurrent efficiency (cd/A)020406080100FWHM (nm)Current efficiencyFWHM(a)(b)(c)140123456400 450 500 550 600Wavelength [nm]Absorption coefficient ×106  (%/m)coumarin-6-doped Alq3rubrene-doped Alq33315450 500 550 600 650 700Wavelength (nm)EL intensity (arb. units)450 500 550 600 650 700Wavelength (nm)EL intensity (arb. units)(a)(b)0°10°20°30°40°0°10°20°30°40°450 500 550 600 650 700Wavelength (nm)EL intensity (arb. units)16450 500 550 600 650 700Wavelength (nm)EL intensity (arb. units)450 500 550 600 650 700Wavelength (nm)EL intensity (arb. units)450 500 550 600 650 700Wavelength (nm)EL intensity (arb. units)(a)(b)0°10°20°40°40°30°20°10° 0°0°10°20°30°40°30°17-80 -60 -40 -20 0 20 40 60 80Angle (deg)EL Intensity (arb. units)-80 -60 -40 -20 0 20 40 60 80Angle (deg)EL Intensity (arb. units)-80 -60 -40 -20 0 20 40 60 80Angle (deg)EL Intensity (arb. units)(a) (b) (c)0 pair1 pair2 pair3 pair4 pair5 pair6 pair0 pair1 pair2 pair3 pair4 pair5 pair6 pair0 pair1 pair2 pair3 pair4 pair7 pair180204060801000 1 2 3 4 5 6 7SiO2/Ta2O5 pairCoupling efficiency (%)device Adevice Bdevice C19

High coupling efficiency of microcavity organic light-emitting diode with optical fiber for as light source for optical interconnects

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High coupling efficiency of microcavity organic light-emitting diode with optical fiber for as light source for optical interconnects

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