ELECTROLUMINESCENT LAMPS

The comparatively recently discovered and still almost unstudied phenomenon of electroluminescence has already found practical application. At present, production has been set up of electroluminescent lamps which, although they have very low power, possess a whole series of interesting features that may make them useful. The reviewed article*) is devoted to a description of these features, as well as to an exposition of the scant information available about the phenomenon of electroluminescence itself.

The phenomenon of electroluminescence, characteristic of certain phosphors—primarily zinc sulfides activated with copper, lead, chlorine, and manganese—consists in the appearance of luminescence when the phosphor is placed in an alternating electric field. The most intense luminescence is observed in phosphors activated with relatively large quantities of copper with a small admixture of lead. The luminescence of phosphors of this type has a green tint and consists of two fairly broad and overlapping spectral regions that are practically visible to the eye: a blue region \((\lambda_{\max}=460\ \mathrm{m}\mu)\) and a green region \((\lambda_{\max}=520\ \mathrm{m}\mu)\). By changing the activator, phosphors with luminescence of the most varied colors can be obtained—from blue to orange. In particular, activation with manganese gives rise to the appearance of a new band with its center at \(\lambda_{\max}=590\ \mathrm{m}\mu\). The relative intensity of the bands depends, on the one hand, on the concentration of the activators and, on the other hand, on the frequency of the applied electric field (see below). The intensity of the blue component increases linearly with frequency, at least up to frequencies of the order of 2.5 thousand cycles. The intensity of the green component also increases with freque-

*) K. H. Butler, C. W. Jerome, J. C. Waymonth, Electrical Engineering 73, No. 6, 524 (1954).

the same, but already at low frequencies a gradual decrease in the rate of increase is observed, and at frequencies of about 1000 cps saturation occurs (Fig. 1). This indicates the presence of two different kinds of centers of luminescence with substantially different times required for their activation. Whereas for the blue luminescence this time is less than \(5 \cdot 10^{-4}\) sec, for the green luminescence it is close to \(10^{-3}\) sec.

Fig. 1. Dependence of the brightness of the blue and green luminescence on frequency for a lamp with a green phosphor.

In contrast to photoluminescence excited by ultraviolet light, electroluminescent luminescence does not encompass the entire volume of the luminescent crystallites, but occurs only in the form of small bright “spots” (1–2 \(\mu\) in diameter) on their surface (especially at points of reduced stress intensity in the electric field). In this connection, “spots” of two types are found. One of them corresponds to contacts between two apparently identical crystallites, whereas the other is observed at the pointed tips of single needle-shaped crystallites. The luminescence of the “contact” type is more intense when the electric field is directed perpendicular to the plane of contact; the luminescence of needle-shaped crystallites is more intense when the electric field is directed along their axes. As the direction of the field is changed, the intensity of the luminescence rapidly decreases. This makes it possible to conclude that electroluminescent “spots” arise as a result of concentration of the electric field in the region of luminescence. It is important to note that (at any rate for luminescence of the “contact” type) luminescence is observed over the course of only one half-period, with the brightness reaching a maximum when the phase of the applied voltage approaches \(\sim 50^\circ\). In this case an individual “spot” begins to glow at some minimum amplitude of the electric field, and thereafter the light intensity of the individual “spot” increases linearly with the amplitude.

Comparison of the spectra of electroluminescence with the spectra of other kinds of phosphorescence of the same substances shows that the principal difference is reduced to a difference in the mechanism of excitation, i.e., in the method of transfer of energy from the electric field to the centers of luminescence. At present the mechanism of excitation of electroluminescence has not yet been established. Two hypotheses appear the most probable. According to one of them, excitation occurs as a result of electron bombardment. An electron, freed from a donor level (either as a result of thermal motion or under the action of the electric field), is accelerated in the conduction band by the electric field. Having acquired sufficient kinetic energy, it excites or ionizes the luminescence center. According to the second hypothesis, excitation of the luminescence center occurs directly by ionization of this center under the action of the electric field, which transfers (as a result of the tunnel effect) an electron from a local level (in the luminescence center) into the conduction band of the crystal. In both cases it is essential that the electric field be alternating, i.e., that the conduction band “rock,” changing inclination.

The most serious objection to the hypotheses named is the fact that luminescence begins at comparatively small field strengths—500–1500 V/cm. However, if one assumes that the entire applied field,

...to the crystal, is concentrated in a luminous “spot,” say, of the “contact” type, then, taking into account the dimensions of the crystals (10–20 µ), we find that the field strength in the region of the “spot” may reach rather large values—up to \(10^6\) V/cm and more. One possible explanation for the concentration of the field in the region of the “spot” consists in the fact that the contact acts like a \(P\)—\(N\) contact, functioning as a rectifier. With the appropriate direction of the field, the potential drop in the contact region should then approach 100%.

Let us now turn to electroluminescent lamps. They are capacitors with transparent electrodes, filled with a phosphor. The shape and dimensions of the lamp are limited only by the technological conditions of their manufacture and may vary within very wide limits. Usually they are made as follows. A thin layer of powdered phosphor is applied to a sheet of electrically conducting glass; over it a thin aluminum film is deposited. This film, in turn, is covered with a layer of lacquer and ordinary glass or plastic.

Fig. 2. Dependence of the brightness of a phosphor powder’s glow on the applied voltage at a frequency of 60 cps.

As a result, a two-dimensional light source of rather large area and small thickness (less than 6 mm) is obtained. The lamps produced are designed for operation at a voltage of 120 or 600 V and a frequency of 60 cps; however, both the amplitude and the frequency of the applied voltage may be substantially increased. Since a lamp is formed by a multitude of phosphor particles differing in shape, size, and orientation relative to the field, the electro-optical characteristics of the lamp turn out to be averaged and differ from the characteristics of individual particles. We have already indicated that, for an individual particle, luminescence begins at a certain threshold value of the amplitude of the applied voltage, and the brightness of the glow increases linearly with amplitude. The dependence of the brightness of the glow on the applied voltage for a lamp is shown in Fig. 2. The nonlinearity is due to the fact that, as the voltage increases, not only does the brightness of the glow of the particles increase, but the number of luminous particles also increases. At voltages below 100 V the glow does not disappear, but becomes too weak for its brightness to be represented on the scale of Fig. 2.

As follows from Fig. 2, increasing the applied voltage entails a substantial increase in brightness, practically limited by safety requirements and the possibility of breakdown. (Let us recall that the brightness of the glow is determined, properly speaking, not by the magnitude of the applied voltage, but by the field strength.) An additional and very significant enhancement of brightness can be obtained by increasing the frequency. This is illustrated by Fig. 3. On passing from a frequency of 60 cps to a frequency of 4000 cps, the brightness

Fig. 3. Dependence of the brightness of phosphor luminescence on the frequency of the applied voltage at different amplitudes.

increases by more than 20 times. The authors note that an obstacle to increasing the frequency is the high resistance of the conducting glass used as the electrode of the transparent electrode (normally about \(500\ \Omega/\text{cm}^2\)). As the frequency increases, the current increases and, consequently, so does heat generation in the glass, reducing both the viability of the phosphor and the brightness of its glow. Evidently, further investigations are needed here.

The dependence of brightness on frequency finds a simple explanation. Fig. 4 gives simultaneous oscillograms of the glow, current, and applied voltage for a typical lamp at a frequency of 60 cps. First of all, it is noteworthy that the glow appears in the form of “flashes,” occurring during the increase of the voltage, i.e. at the beginning of each of the half-periods. This is explained by the fact that the phosphor grains contain particles oriented in all possible directions and, consequently, “flashing” either at the beginning of the first or at the beginning of the second half-periods. With increasing frequency the number of “flashes” per second increases and, consequently, so does the integral light output.

Further, from Fig. 4 it is seen that the glow, appearing only when the direction of the applied electric field changes, is accompanied by an increase in the current flowing through the capacitor. At the same time the current and voltage prove to be in phase, i.e. the capacitor consumes energy. The dependence of light output on consumed power is shown in Fig. 5, relating to a lamp operating at a frequency of 60 cps with voltage amplitudes up to \(600\ \text{V}\) and having an area of \(220\ \text{cm}^2\). The efficiency increases with increasing

Fig. 4. Simultaneous oscillograms of luminescence, current, and applied voltage. a — for nearly rectangular pulses; b — for sinusoidal voltage.

Labels in the figure: “Light,” “Current,” “Voltage.”

voltage up to 500 V, and then decreases again. The maximum efficiency is about 4.24 lumens per watt.

An important feature of electroluminescent lamps is their long life. Fig. 6 shows the dependence of the lamp brightness on its operating time (in hours). At first the brightness increases rapidly, and then, after several tens of hours, begins to decrease slowly according to an approximately exponential law. The main reason for the failure of lamps is electrical breakdown. Since breakdown is caused by short-term increases of voltage in the network and then “heals,” so that only a black speck remains at the site of the breakdown, the consequence of breakdowns is a gradual decrease in the brightness of the lamp. But since the brightness decreases very slowly, the lamps can serve for a very long time—up to 10,000 hours or more. At present, lamps are manufactured in three types:

1) 120 V, 60 Hz, with a brightness of about 0.1 foot-lambert,

2) 600 V, 60 Hz, with a brightness of about 10 foot-lamberts, and

3) 600 V, designed for high frequency and having a brightness on the order of 100 foot-lamberts.

Fig. 5. Dependence of the luminous output of a typical lamp with a green phosphor on the power consumed.

The authors recommend lamps of the first type for illuminating the dials of clocks and the scales of measuring instruments, as night-lights, etc. In particular, the authors note the possibility of changing the intensity of the glow without changing its color.

Fig. 6. Dependence of the brightness of the glow on the lifetime of the lamp (60 Hz, 600 V).

More powerful lamps may be used for the same purposes, and also as street signals, house numbers, advertisements, etc.

It should be assumed that the distinctive properties of this new light source will find numerous and important applications, extending far beyond the limits indicated by the authors of the article.

T. R.