Elecrtroluminescence in Organic Crystals

Elecrtroluminescence in Organic Crystals - 2042 A a I 9 In...

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Unformatted text preview: 2042 A a I 9 In 5 7 9 "'9 _, 6 \ '0- 5 _ r. 4 ,. IO 8 U > X 3 a m. N — 2 | O 0 0.2 0.4 0.6 0.8 |.O X/L 1 5 = A, 7; L=10 A, 1—6; 11:3.7X1017 ions/cmz-sec, 3, 4; . = 1, 2, 5, 6, 7; qa/e=106 V/Cm, 1—4; qo/e= —106, 5‘7; DC(0)/ 7 28X10‘7ions/cm2-sec, 1—7. .6><1018 ions/cm“, 2, 4, e, 7; C(0)=1.6><10‘9, 1, 3, for nonequilibrium systems in which transport occurs. The surface charge density 0' used for these compu— tations is equivalent to an E(0) =qcr/e; hence the re— sults are pertinent to recent experimental work3 for growth of Si02 on Si in a field. An evaluation of Eq. (3) of I at x=L shows that in a homogeneous field the growth rate becomes zero for a limiting film thickness L0,, given by (/eT/qE) ln C(L)/C(0). The product qELm=q¢,,, where ¢>e is the electrical potential needed to halt growth at. Leo, is thus equal to the Gibbs’ free energy for transport through the film. This can be smaller in magnitude than the free energy AF of the oxidation process since the bulk concentration at the reaction interface may not be directly proportional to AF and at the opposite interface the bulk concentra— tion should be more characteristic of the processes at that interface. This may explain why a (be less than the potential equivalent to AF was sufficient3 to halt oxidation of Si. In Conclusion, the author wishes to thank P. B. Bailey and G. W. Petznick for the computer program used for the computations. * Work done under the auspices of the United States Atomic Energy Commission. 1 A. T. Fromhold, Jr., J. Chem. Phys. 38, 282 (1963). 2T. B. Grimley, Oxidation of Metals, Chemistry of the Solid State (Butterworths Scientific Publications Ltd., London, 1955), 2nd ed., p. 340. 3 P. I. Iorgensen, J. Chem. Phys. 37, 874 (1962). LETTERS TO THE EDITOR Electroluminescence in Organic Crystals" M. Poms, H. P. KALLMANN, AND P. MAGNANTE Physics Departmenl, New York University, New Y or]? (Received 10 December 1962) LECTROLUMINESCENCE has been observed in single crystal anthracene, and in single crystal anthracene with about 10—1 mole% tetracene as an impurity.1 The luminescence in the case of anthracene was the usual anthracene fluorescence; in the case of anthracene with tetracene impurity, it was that of tetracene. The crystals were 10 to 20 [.1. thick and were prepared by sublimation2 and from solution.3 Two electrode con- figurations were used, the results with each differing markedly. In one, silver paste (epoxy base) electrodes were used, the area of one electrode being made smaller than the opposite electrode. In a typical case, the small electrode was 1 mm in diam. compared with an 8 mm diam. opposing electrode. In the second configuration, the electrodes were 0.1M solutions of NaCl, 0.1 cm‘3 in area, and were symmetrically disposed on both sides of the crystals.4 The field was applied along the c’ direc- tion of the crystal. Experiments were carried out either with no current limiting resistor or with one of 105 9, when current measurements were made. Light was de- tected with a photomultiplier using a DuMont 6292 tube. \Vith the small electrodes, dc electroluminescence (E.L.) was first observed above about 400 V. When a slowly varying ac field was applied to the crystal, luminescence appeared at the same voltage and both light output and dark current were in phase with the voltage. The E.L. appeared first when the small elec- trode was positive, but at slightly higher voltages EL. was obtained during both halves of the ac cycle. The integrated light output per unit time was approximately constant from 0.1 cps, independent of ac frequency, dur- ing both halves of the ac cycle. There was good sym- , metry in the EL. and current response to the increasing and decreasing voltages. The current through the crystal was more than 1 MA; this represents a current density near the small electrode of about 100 uA/cmg. Experiments carried out with biased voltages showed that the EL. depended only on the instantaneous volt- age and not on any previous voltage history. The E.L. brightness data could be very well represented by a linear plot of 1118 against V‘i. The current voltage data could be slightly better represented as ioc V" where n was greater than 10. The EL. was also observed when the crystal was placed in a vacuum of 0.5 a and also when the crystal was submerged in silicone oil. The light could be seen in a dimly lighted room. The physical and chemical integrity of the crystal was checked after an hour of do E.L. operation at 400 V and 1 [1A. There was no visual evidence of irreversible effects d the current—voltage relationship was the same be- ”u. e and after the EL. In the case of anthracene doped with 0.1 mole% vi acene, the EL. appeared either at a voltage in- ,endth of the polarity of the small electrode, or ap— . red first with the small electrode negative. The I'ltage required to produce E.L. was always much Higher for these doped crystals. The voltage dependence ‘the brightness and the current were the same as that the pure anthracene. The behavior of the electrolyte electrodes was quite different and light was observed only in some of the crystals tried. In those crystals where E.L. was ob- served, the applied voltages were high (normally about “10‘ V/cm). Light appeared only when square wave voltage forms were applied and only during the rising and falling portions of the wave, the intensity being the same in these intervals. There was no light during the times when the voltage was constant and the current was typical of normal dark currents. A current surge occurred as E.L. appeared. This current was not capaci- tive; it appeared as a sharp peak superimposed on the capacitive current. The intensity of light emitted per cycle was greater at lower frequencies. Using biased voltages, it was found that the brightness depended only on the peak-to-peak square wave voltage. Thus, the EL. depended on the voltages during the preceding period. Two of the prerequisites for maximum E.L. per cycle were a rapidly changing voltage and a rela- 1‘ tively long duration for the applied field in between the voltages changes. In a typical case, the voltage change was 2000 V in 1 msec, and EL. could not be detected when the duration of the applied field was less than 10 msec. ‘ In the case of electrolyte electrodes, the field was more uniform than in the case of the silver paste elec- trodes. Very high voltages, quite close to breakdown, were required to observe luminescence. We feel that the success of the square wave excitation may be attri— buted to the superposition of the internal polarization field and the externally applied field during the time the voltage is changing rapidly. During this period the field at the surface is being decreased so that it is mainly the polarization current that is producing the light. The carriers in this case would therefore be produced in the bulk of the crystal and would be accelerated by the field to energies sufficient to excite the optical modes of the crystal. In the case with metal paste electrodes, the bright- ness—voltage relation is similar to that found for ac- celeration—collision excitation mechanism for EL.5 As has been pointed out by Piper and Williams,5 accelera- tion mechanisms of excitation can avoid catastrophic consequences of dielectric breakdown if the field is in- homogeneous. Thus, a narrow high—field region in series with a broader low—field region can permit high carrier accelerations that terminate in the low-field region LETTERS TO THE EDITOR 2043 before destructive avalanches can develop. In the small electrode case, these conditions probably prevail at the edges of the electrodes. The question of the origin of the carriers is difficult to decide without doing temperature studies and studies with different electrodes. They may be injected from the electrode across an exhaustion barrier near the small electrode. Since the EL. is asso— ciated with a large current, we feel that these carriers are not thermally generated—but. are field generated at the electrode and accelerated to energies sufficient to excite optical modes. Once having been excited, these modes would display the normal fluorescence characteristics of anthracene; and in the case of anthracene doped with tetracene, the fluorescence of tetracene would be observed. We wish to express our appreciation to K. W. Boer for stimulating discussions, and to R. Laupheimer for designing the electronic equipment. * Grateful acknowledgment is made for the assistance rendered this work by the Office of Naval Research and the Air Force Cambridge Research Center. ‘ For E. L. in organic films, see A. Bernanose, J. Chem. Phys. 52, 396 (1955); and S. Namba, M. Yoshizawa, and H. Tamara, J. Appl. Phys. (Japan) 28, 439 (1959). 2 F. R. Lipsett, Can. J. Phys. 35, 284 (1957). 3 H. Kallmann and M. Pope, Rev. Sci. Instr. 29, 993 (1958). 4 H. Kallmann and M. Pope, Rev. Sci. Instr. 30, 44 (1959). 5 W. W. Piper and F. E. Williams, Brit. J. Appl. Phys. Suppl. 4, S 39 (1955). Silver—Catalyzed Surface Recombination in a Step—Function Flow of Atomic Oxygen* A. L. Mvnnsox Cornell Acrrmaulical Laboraiory, I no, Buflalo, New Y 07/6 (Received 13 November 1962) N a recent presentation,‘ the author described a new method of studying the surface-catalyzed recom- bination of atomic species. The method2 applies a de- fined step-function flow of atomic oxygen to a thin—film resistance thermometer. The thin film is capable of following recombination heat—transfer rates and hence catalytic efficiency with millisecond resolution. This new method difiers fundamentally from previous in- vestigations (e.g., references 3-6) which require estab— lishment of a steady state. This communication reports a unique phenomenon dealing with the first several milliseconds of exposure to atomic oxygen of a silver surface previously virgin to encounter with atomic oxygen. These observations throw considerable light on the mechanism of oxygen-atom recombination on silver. Briefly, the atom detector is a 4—mm o.d. Pyrex tube with two thin-film laminates (0.1 p. titanium dioxide on 0.1 ,u platinum) a few millimeters apart. On one of these laminates is silver foil (0.1 1.]. thick). Applica~ ...
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Elecrtroluminescence in Organic Crystals - 2042 A a I 9 In...

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