Organic electroluminescent diodes
`
`Cite as: Appl. Phys. Lett. 51, 913 (1987); https://doi.org/10.1063/1.98799
`Submitted: 12 May 1987 . Accepted: 20 July 1987 . Published Online: 04 June 1998
`
`C. W. Tang, and S. A. VanSlyke
`
`COLLECTIONS
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`Paper published as part of the special topic on APL Classic Papers
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`Appl. Phys. Lett. 51, 913 (1987); https://doi.org/10.1063/1.98799
`
`51, 913
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`© 1987 American Institute of Physics.
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`SAMSUNG EX. 1018 - 1/4
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`Organic electroluminescent diodes
`c. W. Tang and S. A. VanSlyke
`Research Laboratories. Corporate Research Group. Eastman Kodak Company. Rochester. New York 14650
`
`(Received 12 May 1987; accepted for publication 20 July 1987)
`
`A novel electroluminescent device is constructed using organic materials as the emitting
`elements. The diode has a double-layer structure of organic thin films, prepared by vapor
`deposition. Efficient injection of holes and electrons is provided from an indium-tin-oxide
`anode and an alloyed Mg:Ag cathode. Electron-hole recombination and green
`electroluminescent emission are confined near the organic interface region. High external
`quantum efficiency (1 % photon/electron), luminous efficiency (1.5 lm/W), and brightness
`( > 1000 cd/m2) are achievable at a driving voltage below IO V.
`
`Organic materials have previously been considered for
`the fabrication of practical electroluminescent (EL) de(cid:173)
`vices. I The primary reason is that a large number of organic
`materials are known to have extremely high fluorescence
`quantum efficiencies in the visible spectrum,2.3 including the
`blue region. some approaching 100%. In this regard, they
`are ideally suited for multicolor display applications.
`However, the development of organic EL devices has
`not been successful so far, one reason being that high voltage
`is generally required to inject charges into organic crystals
`(e.g., anthracene). In early attempts by Helfrich and
`Schneider,4 Dresner, I and Williams and Schadt,5 the drive
`voltage was on the order of 100 V or above in order to
`achieve a significant light output. Therefore, the EL device
`power-conversion efficiency is quite low, typically less than
`0.1 % W /W, despite the reported high external quantum ef(cid:173)
`ficiency of ~ 5% photon/electron. In an attempt to reduce
`the drive voltage, Vincett et al. n used thin organic films of
`similar materials in their EL devices. They reported EL op(cid:173)
`eration below 30 V. However, the quantum efficiency of
`their EL diodes was only about 0.05%, presumably owing to
`the inefficiency of electron injection and the inferior quality
`of the evaporated anthracene films. Other organic thin-film
`EL work 7.X reported similar performance. Another factor
`for the lack of development is perhaps the question of long(cid:173)
`term stability of organic EL diodes. There are very few re(cid:173)
`ported data on the organic EL stability in the literature.o
`In this letter, we report a novel thin-film organic device
`with superior EL characteristics. It is efficient and can be
`driven to high brightness by a low dc voltage. In contrast to
`most organic EL cells, which use a single layer of organic
`material sandwiched between two injecting electrodes, our
`EL diode consists of a double layer of organic thin films, with
`one layer capable of only monopolar transport. The organic
`materials were chosen such that the morphological, trans(cid:173)
`port, recombination, and luminescent properties were com(cid:173)
`patible with the construction and operation of the thin-film
`EL diodes. In addition, we used a low-work-function alloy
`prepared by vapor codeposition as the cathode for efficient
`electron injection.
`Figure I shows the structure of the present EL cell. The
`substrate is an indium-tin-oxide (ITO) coated glass with a
`sheet resitance of about 10-20 %
`(Nesatron ™ from PPG
`Industries). It was cleaned by ultrasonication in a mixture of
`
`isopropyl alcohol and water ( I: 1) and degreased in toluene
`vapor. The first organic layer (about 750 A) on top of the
`substrate is an aromatic diamine9 of molecular structure
`shown in Fig. 1. The second organic layer is the luminescent
`film, about 600 A. It belongs to a class of fluorescent metal
`chelate complexes. 10 The specific example shown in Fig. I is
`8-hydroxyquinoline aluminum (Alq3)' The top electrode is
`an alloy or mixture of magnesium and silver with an atomic
`ratio of 10: I. The organic layers, as well as the Mg:Ag elec(cid:173)
`trode, were all deposited by vacuum deposition (_10- 5
`Torr). The substrate was nominally at room temperature
`and the deposition rates for the organic layers were about 2-
`5 A/s. The Mg:Ag electrode was deposited by simultaneous
`evaporation from two separate sources at a total rate of
`about 10 A/s.
`The organic diode shown in Fig. I can be operated in a
`continuous dc or pulsed mode. It behaves like a rectifier, the
`forward bias corresponding to a positive voltage on the ITO
`electrode. Light emission, seen only in forward bias, was
`measurable from as low as about 2.5 V. Figure 2 shows the
`continuous dc current vs voltage (1- V) and the radiance exi(cid:173)
`tance vs voltage (B- V) curves. The shape of the J- V curves
`for most diodes is relatively independent of the thickness of
`the diamine layer but strongly dependent on that of the Alq3
`layer, indicating that most of the bias voltage is across the
`Alq3layer. The J- V curve can be fitted to an injection-limited
`model where the electron current is limited by electron emis(cid:173)
`sion from the cathode into the Alq3 layer. The radiance exi(cid:173)
`tance in mW /cm 2 was measured from a diode with an emit-
`
`FIG. \. Configuration ofEL cell and molecular structures.
`
`913
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`Appl. Phys. Lett. 51 (12). 21 September 1987 0003-6951/87/380913-03$01.00
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`© 1987 American Institute of Physics
`
`913
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`SAMSUNG EX. 1018 - 2/4
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`FIG. 2. Brightness-current-voltage characteristics of an ITO/diaminei
`Alq,/Mg:Ag EL cell.
`
`ting area of 0.1 cm:' by a radiometer (EGG model 550-1).
`The emitting surface is Lambertian for viewing angles within
`60° from the normal to the surface. The brightness in units of
`cd/m~ was separately measured by a spot photometer (Min(cid:173)
`olta Luminance Meter, III - 1/3°). A radiance exitance of
`0.1 m W /cm:' is equivalent to a brightness of 100 cd/m:' for
`the EL diode with Alq, as the emitter. As shown in Fig. 2,
`the EL diode can be driven to produce high brightness
`( > 1000 cd/m:') with a dc voltage of less than 10 V. In
`pulsed operation, the response of the diode has a rise and
`decay time on the order of a few microseconds.
`The light output from the EL diode is linearly propor(cid:173)
`tional to the input current in the current range from 10 - I to
`1O~ mA/cm~. The external quantum efficiency of the EL
`diode is about 1%. At the power output of 0.1 mW/cm:',
`which is visible in ambient lighting, the required drive vol(cid:173)
`tage is 5.5 V and the corresponding power conversion effi(cid:173)
`ciency is 0.46%. The equivalent luminous efficiency is 1.5
`Im/W, which compares favorably with the commercially
`available light-emitting diodes or ZnS-based EL devices. II
`The emission spectrum of the EL diode is shown in Fig.
`3. The peak intensity is at 550 nm, the FWHM is about 100
`nm, and the color is green. The EL emission spectrum is
`independent of the drive voltage or current but is sensitive to
`the thickness of the organic layers. The latter effect is due to
`the interference phenomenon of emission in front of a re(cid:173)
`flecting mirror. I:' For thin organic films (as in Fig. I) the EL
`
`400
`
`700
`600
`500
`Wavelength (nm)
`FIG. J. Ekctroluminescence spectrum of ITO/diaminel Alq ,IMg:Ag.
`
`800
`
`emISSIOn spectrum is identical to the photoluminescence
`spectrum of the Alq.l thin film. This result indicates that the
`radiative recombination of injected electrons and holes takes
`place in the Alq, layer. Detailed analysis 1\ shows that this
`recombination is confined to the Alq \ layer adjacent to the
`diamine layer to a distance of about 300 A. The diamine
`layer, which is known to transport holes only, 'I blocks the
`electrons injected from the Mg:Ag electrode. Therefore, the
`interface between the diamine and Alq, layer effectively con(cid:173)
`trols the recombination processes.
`The morphological properties of the organic layers are
`critical in the construction of thin-film devices without pin(cid:173)
`holes. It is necessary that both layers in the EL device be
`smooth and continuous. The transmission electron micro(cid:173)
`graphs show that the evaporated diamine layer appears to be
`amorphous, whereas the Alq, film is microcrystalline with
`an average grain size of about 500 A. The ability to form
`smooth films in both layers in the present EL diode is related
`to the low order of symmetry as well as large molecular ge(cid:173)
`ometry of the constituent molecules. In addition, the two(cid:173)
`layer structure partially alleviates the shorting problem by
`minimizing the probability of having overlapping pinholes.
`The Mg:Ag alloy used as the cathode is important in the
`reduction of the drive voltage. Mg is a low-work-function
`metal ideally suited for electron injection into organic mate(cid:173)
`rials. However, it is susceptible to atmospheric oxidation
`and corrosion. The incorporation of Ag in the Mg:Ag film is
`found to retard these degradation processes. In addition, Ag
`improves the sticking coefficient of Mg on the organic film
`during vapor deposition. Other common cathode materials
`such as In, Ag, and Al generally result in much higher vol(cid:173)
`tage drive and inferior stability.
`The EL diode has been tested for stability in a contin(cid:173)
`uous dc operation. Under constant current drive of 5 mAl
`cm=' and in an argon ambient, the EL emission (with initial
`output of about 0.05 m W /cm=' or 50 cd/m~) shows a rela(cid:173)
`tively fast degradation in the initial hours (about 30% loss in
`10 h) and then decays at a much slower rate to about half of
`the initial value at the end of a 100-h test The steady degra(cid:173)
`dation is accompanied by a concomitant increase in the drive
`voltage from the initial 6 or 7 V to about 14 V during this test
`period. The nature of degradation is not clearly understood.
`Some of the failure is attributed to the degradation of both
`hole and electron injecting contacts, the latter resulting in
`the formation of dark nonemissive spots.
`In conclusion, we have shown a novel organic electrolu(cid:173)
`minescent diode with a double-organic-Iayer structure. The
`diode has unique characteristics of high electroluminescent
`emission efficiency, fast response, low voltage drive, and
`simplicity offabrication. It demonstrates that organic mate(cid:173)
`rials can indeed be viable alternatives for optoelectronic ap(cid:173)
`plications such as displays.
`
`'1. Dresner, RCA Rev. 30,322 (1969).
`'K. H. Drexhage, in Topics in Applied Physics: Dye Lasers, edited by F. P.
`Schafer (Springer, New York, 1977), Vol. I, p. 144.
`'H. Gold. in The Chemistry a/Synthetic Dyes, edited by K. Venkataraman
`
`914
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`AppL Phys. Lett., Vol. 51, No. 12,21 September 1987
`
`C. W. Tang and S. A. VanSlyke
`
`914
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`SAMSUNG EX. 1018 - 3/4
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`ilL. E. Tannas. Flat Panel Displays and CRTs (Van Nostrand. New York.
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`Amsterdam. 1974). Vol. 12. p. 165.
`"c. W. Tang. C. H. Chen. and S. A. VanSlyke (unpublished).
`
`915
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`Appl. Phys. Lett .• Vol. 51. No. 12.21 September 1987
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`c. W. Tang and S. A. VanSlyke
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`915
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`SAMSUNG EX. 1018 - 4/4
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