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Elecrtroluminescence and Band Gap in Anthracene

Elecrtroluminescence and Band Gap in Anthracene - 2920...

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Unformatted text preview: 2920 Electroluminescence and Band Gap in Anthracene MIZUKA SANG, MARTIN POPE, AND HARTMUT KALLMANN Department of Physics, Radiation and Solid State Laboratory New York University, New York, New York (Received 14 December 1964) N this paper, we describe the occurrence of delayed electroluminescence (EL) and give evidence that By, the energy of the lowest state that is conducting in anthracene, is greater than 3.1 eV.1 The electrodes consisted of a silver—epoxy-paste electrode (called the point electrode), about 2 mm in diameter, on one side of a crystal (typically 20 M thick) and a transparent vacuum-evaporated silver electrode on the opposite side. There was a voltage range in which light was emitted from the point electrode regardless of its polarity.” At a lower voltage range, light was emitted only when the point was negative. We worked only in the lower voltage range. Upon application of a (lscps) ac voltage to the crystal, EL appeared as small bright spots at the cathode. Upon application of a negative voltage pulse 10‘7 sec in duration a delayed EL was observed, in which an appreciable fraction of the total light output appeared after the applied voltage disappeared, some- times in the form of a scoond peak. The spectrum of the delayed EL was checked only at 421 and 444 my, and there were EL maxima at both wavelengths. We assume from this that the energy origin of the delayed EL is at least as high as the excited singlet state. The appearance of the second peak led us to the conclusion that the delayed EL was caused by the recombination of carriers. In order to show that charge-carrier recombination produced delayed EL also in those crystals that were without the second peak, the following experiment was performed.3 A second voltage pulse (V2) of variable duration was applied (within 1 psec) after the first voltage pulse (V1). In one case, V2 had the same polarity as V1 (+V2), and in the other case, V2 was of opposite polarity to V1 ( -— V2). The voltage V2 produced no EL in the absence of V1. The results are shown in Fig. 1. The electric field of V2 strongly modified the delayed EL; the delayed EL decreased during the duration of (+V2), and a new EL peak appeared upon the disaps pearance of (+Vz). At the onset of the application of (-Vz), the delayed EL was enhanced and dropped eventually to a level below that produced by V1 alone. We take these results as evidence that an appreciable part of the delayed BL is caused by carrier recombina« tion; singlet formation by triplet—triplet annihilation can occur in addition. The first voltage pulse (with point electrode negative) is responsible for the creation of pairs of free carriers, not all of which are discharged LETTERS TO THE EDITOR during the duration of V1. The up an opposing polarization polarization voltage V,, and, on V1, carrier recombination takes 111 (+V2) prevents this recoma r» transposing the delayed EL peak l the pulse (—Vg) together with combination. We conclude that the. normal conduction level because ' present during the delayed EL I , V2 alone produces a current equal current, and V2> VP, V, cannot Thus, recombination occurs bet A trons that are either in the lowest were released from traps. Since the the delayed EL were those of the Ecr>3.1 eV. Thus, photoconducfii energy less than 3.1 eV1 is not monomolecular process. - This conclusion in regard to Ec: is mechanism of carrier generation;‘ are not produced by injection beat with point negative, and close to the 20 Eteclroluminescence Intensity (arbitrary unil) Pulse Vollage (volts) ITlme (microseconds) FIG. 1. Elect of electric field on dela ed - in anthracene crystal. Top: 21, (V1) = 30 (V1) =780 V, (+V2)=320 V; c, (V1)=780.Y§ Crystal thickness: 20 u; temperature: —67°C_- Hg. Bottom: Pulse (V1) width: 0.1 psec; p psec. equilibrium density of holes is practically zero temperatures; furthermore, holes cannot gain '5 the field due to the narrowness of the hole ‘hus, the only possibility of producing holes is elerated electrons. Since the normal electron arrow“ (~01 eV), no effective acceleration place in this band. We conclude that the ‘ hon takes place by injection of the electrons 9' upper and broad electron conduction band, 3 ey can gain energy. to acknowledge the valuable contribution i- rt Laupheimer in designing our electronic nt and for stimulating discussions. Work was supported by the U.S. Oflice of Naval : m, using materials prepared with US. Army Re— Oflice support. For a review of band gap studied, see L. E. Lyons, in i and Chemistry of the Organic Solid State, edited by D. M. Labes, and A. Weissberger (John Wiley & Sons, York, 1963). (b) R. G. Kepler, Phys. Rev. 119, 1226 . “so , H. Kallmann, and P. Magnante, J. Chem. Phys. ‘ 1963). experiment was performed after submission of this paper. Katz, S. A. Rice, S. Choi, and J. Jortner, J. Chem. Phys. 5‘ (1963). in on Spin Resonance Transitions In- Simultaneous Changes in Spin tates of Two Neighboring Protons" WALLACE SNIPES AND WILLIAM BERNHARD ‘ I1» lmenl of Biophysics, The Pennsylvania State University University Park, Pennsylvania (Received 6 July 1965) 'i! ELLITE lines arising from simultaneous changes 1,: the spin state of a single proton and the spin ,f the unpaired electron are occasionally seen in ‘. n spin resonance spectra. These transitions, first {:1 d in irradiated frozen acids,1 are induced by a dipole—dipole coupling between the electron 'tic moment and the magnetic moment of the 'noring proton. During the investigation of the :of an irradiated single crystal of barbituric acid i ate, we have observed an additional set of its lines which arise from changes in the spin of two neighboring protons concurrent with the . ,.,.e in spin state of the unpaired electron. These . 'tions have been predicted by Trammell et al.2 H our knowledge have not been observed in ESR 1' a of free radicals. fu ua irradiation of barbituric acid dihydrate ces a free radical having an unpaired electron -u mainly in a 21m orbital on a carbon atom.3 ue interaction with a single proton splits the . tron into a doublet. This doublet is shown in a, (a) for an orientation of the crystal in the mag- .' field for which all four molecules per unit cell4 are .esetically equivalent. At higher spectrometer sensi- LETTERS TO THE EDITOR 2921 (a) sens. = 5. POWER = P. (b) SENS. = 3 3. POWER = P9 (Cl SENS.= IOO So POWER = JOO Po 0“— BO GAUSS— FIG. 1. Second-derivative ESR tracing of an irradiated single crystal of barbituric acid dihydrate with the external magnetic field in the crystallographic be plane. The magnetic-field intensity increases from left to right, with the arrow indicating the field position of the resonance of DPPH for which g=2.0036. Relative spectrometer sensitivities and microwave power levels are in- dicated for each tracing. Observations were made at a microwave frequency of 9510 Mc/sec. tivity [Fig 1(b)], satellite lines appear corresponding to a change in Spin state of a single proton along with the change in spin state of the electron. The separation of these satellite lines from the main hyperfine lines, in frequency units, is close to the proton resonance fre- quency for the particular magnetic-field strength at which the observation is made.2 Calculated values of this separation for magnetic-field strengths of 3030 and 3390 G are 4.6 and 5.2 G; our observed values for these splittings are 5.3 and 5.8 G. The separation of .the satellite lines from the main hyperfine lines is inde- pendent of the orientation of the crystal in the external magnetic field. Figure 1(c) shows a spectrum taken at an even higher spectrometer sensitivity so that the set of satellite lines shown in Fig. 1(b) is now off scale. Within experimental error, the additional satellite lines which now appear are separated from the original satellite lines by exactly the same amount that the original satellite lines are separated from the main hyperfine lines. This was observed at magnetic-field strengths of both 3030 and 3390 G. Although there should be eight of these satellite lines corresponding to changes in spin states of two protons, four of them fall identically under the main hyperfine lines. While the most clearly resolved of these lines are those which lie outside the remainder of the spectrum, the line at the center of the spectrum is actually a superposition of two of these satellite lines. If it were possible to make observations at crystal orientations for which ...
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