US3187184A - Electroluminescent-photoconductive device with improved linearity response - Google Patents

Description

E C I m Dm? W05 IP9 Tl NMR ODYS SNT O l NCT..1 lomp LMEA MHN OPM@ TL. 1 NDi BEEF .mmm TMPn IM MIg U i LHr OTO RI Tw C E L E June l, 1965 FIG. 2

FIG. l

vw L luminescent apparatus constructed in accordance withY the present invention, and i FIG. 4 illustrates a modified form of the apparatus of FIG. 3. Elemental apparatus f FIG. I

Referring to FIG. 1 of the drawing, there is shown an elemental form of elecrtoluminescent apparatus constructed in accordance with the present invention. This apparatus of FIG. 1 is referred to as being elemental in Ynature because, for the .case of a complete image, it is capable of reproducing only a single image element, This is not, however, -intended to imply anyrlimitation as to the physical size of the apparatus.V Nor is it in 7 tended to imply that the apparatus is only useful for reproducing intelligible images because, for example, the apparatus might bek used as a radiationidetector or amplier where no image is involved. Also, for the sake of a concrete illustration of its use, the apparatus of FIG. 1, as well as that of the other figures, shall in general be described for Vthe case where the radiation of interest is visible light radiation.Y It is to be understood that the apparatus is equally useful with other types of electromagnetic radiation Vsuch as X-rays, ultraviolet radiation, or infrared radiation. g

Considering now the details of the electroluminescent apparatus of FIG. 1, such apparatus comprises an electroluminescent cell 10 of the form comprising a-quantity of electroluminescent material 11 ypositioned between-a pair of conductive electrodes 12 and 13. The electroluminescent material 11 may be, for example, copperactivated zinc s ulde. Also, one of the conductive electrodes, in this case the electrode 12, is preferably made to be radiation transparentfor enabling the radiant glow of the electroluminescent material 11 to escape from the cell 19. As is known in the art, such radiation transparent conductive electrode may consist of a very thin metallic lm of, for example, aluminum or a film of transparent conductive material such as tin oxide. Instead, such electrode might take the form of a grid mesh structure wherein the sizes of thevapertures are arranged so that the electrode is essentially transparent. In orderv to indicate that such electrode is radiation transparent, it is represented diagrammatically by means of a dashed line. Such conductive electrode may be formed Yon a sheet of dielectricrnaterial 14 which,for the case of visible light', may take the form of glass or a thin layer of mica. In the case of the FIG. 1 apparatus, the other conductive electrode 13 may be composed of `a Y relatively thick layer of metal and need notbe radiation to vary the impedance of the photoconductive material 16. This electrode 18 may take any of the forms previously mentioned in connection with the transparent electrode 12. of the cell 1G. Also, the other electrode 17 of cell 15 mayagain be a relatively thick layer of metal.

The apparatus also includes an electrical impedance represented, for example, by a resistor 2) and means for supplying an energizing voltage. The energizing voltage should, at least on the basisrof present knowledge, be of Ya uctuating nature and, hence, `may take the form of an alternating-current voltage of suitable frequency. Means for supplying such an energizing voltage is represented diagrarnmatically by an alternating-current voltage generator21 having a'pairof output terminals 22 and 23.

The apparatus of the present invention further includes means connecting the two cells in parallel with one another and in series with thev impedance and the voltage-supply means 21.11 Such connecting means includes a connecting wire 24 for connecting the two electrodes 13 and 17 to the terminal 23 of the voltagesupply means and includes connecting wires 25, l26, and 27 for connecting the other two electrodes, namely the conductive electrodes 12 and 18, to the other terminal 22 of the voltagesupply means. It is essential that the impedance represented in this case by the resistor `20 be connected in series so that the current flow to both the photoconductive material 16 and the electroluminescent material 11 must pass therethrough. In the case shown, this is accomplished by connecting the two wires and 26 to the side of the resistor 20 farthest removed from the voltage source 21. Y

Considering now the operation -of the electroluminescent apparatus just described, it will be noticed that the electroluminescent cell. 10 and the photoconductive cell 15 are connected in parallel with one `another and that this parallel combination is, in turn, connected to the voltage source 21 by way of the series impedance represented by resistor 20. As a result, the apparatus operates in the bright condition, that is to say, the condition in which it is highly sensitive to voltage changes, when the intensity of incident radiation I is small and the .photoconductive element 16 is most sensitive to changes inthe incident radiation I. By employing an applied voltage and seriesconnected impedance of suitable values, there can be made to occur a more linear change in the light output L in response to changes in the incident radiationI than can be obtained inrmost known devices of 4the type heretofore proposed where the photoconductive element and the electroluminescent element are connected in series with each other.

In operation, with no incident light I falling on the photoconductive layer 16 through the transparent electrode 18, there is a high impedance between the electrodes 17 and 18 so that only a small current lilows in the circuit and a relatively high voltage is applied across the electroluminescent layerll located between the electrodes 12 and 13. The value of the voltage actually appearing across the electroluminescent layer 11,-of course, depends on the relative impedances of the series resistor 20 and the electroluminescent layer 11 Vat the supply frequency. To this end, the construction of the layers and the impedance andsupply voltage values are chosen so that the electroluminescentlayer 11 is operating at the maximum brightness required for this condition of no incidentA illumination.

Then, as gradually increasing amounts of incident light, as represented by the arrows I, are allowed to fall on the photoconductive layer 16, `the resistance of the photoconductive materialV decreases. 'The current through the resistor 20 thereupon increases and the voltage across the electroluminescent layer `11 is'reduce'd,l thereby resulting in a decreased light output L therefrom.

Although the changes in the lightY output L will be in the reverse direction to changes in the incident radiation I, Iby employing two units of apparatus, each constructed in accordance vwith FIG. 1 and arranged so that the light L emitted by the electroluminescent element of the first is directed onto the photocoriductive element of the second, an overfall operation is obtained inwhich the light output varies in :the same sense as theincident radiation.

energies Witha single unit of apparatus, as shown 1n FIG. 1, a more uniform change in light output L with steady changes in the incident light I on the photoconductive layer 16 is obtained. This can be more readily appreciated by reference to the graph of FIG. 2 which shows a curve relating the brightness of the light output L from electroluminescent cell l@ to the RMS. value of the alternating-current voltage applied across such an electroluminescent cell 10 at a constant frequency. It caribe seen that as the voltage is increased from zero there is an apparent threshold Voltage V1 at which a significant amount of light is lirst emitted by the cell 10. The rate of change of brightness with respect to voltage, that is, the incremental slope of the curve of FlG. 2, then increases as the voltage is increased but subsequently decreases after a voltage V2 is reached. This continues until the cell 10 is operating near the maximum brightness obtainable at the frequency employed.

For the case of prior art devices of the type previously proposed where the photoconductive layer and the electroluminescent layer are connected in series, the relative impedances are such that without employing a supply voltage of an undesirably high value onli,7 a relatively small voltage is applied across the electroluminescent layer when the incident illumination on the photoconductive layer is small. This situation continues until the voltage across the electroluminescent layer becomes greater than the effective threshold voltage V1 and, until this threshold voltage is exceeded, no significant amount of light will be emitted by the electroluminescent layer. Furthermore, in such prior art devices further increases in the amount of incident lightresult in higher voltages being applied across the electroluminescent layer but it can be seen that changes in the amount of light emitted for uniform changes in the incident light become gradually greater and a substantially linear variation in the light output with uniform changes in the amount of incident light is not obtained.

With the shunt-connected or parallel-connected apparatus of the present invention, on the other hand, the applied Voltage is chosen so that the electroluminescent cell 10 operates in the region of V2 when there is little or no incident illumination on the photoconductive cell. An increase in the incident illumination then causes the operating Voltage across the electroluminescent cell l0 to decrease towards V1. As a result of this mode of operation, the threshold effect is substantially eliminated for incident radiation of weak intensity. Also, both the electroluminescent cell lil and the photoconductive cell 15 are in their most sensitive condition when the incident radiation is small. Furthermore, by using elements of suitable materials and dimensions together with a series impedance of an appropriate value, changes in the incident radiation produce a substantially linear change in the brightness of the electroluminescent cell 1G over anY appreciable range of values. Also, as mentioned, by employing two units of apparatus with the light emitted by the electroluminescent cell of the first unit arranged to be incident on the photoconductive cell of the second unit, an output which is substantially directly proportional to the incident light can be obtained. By a suitable choice of circuit components, a light-amplification edect may also be obtained.

Another feature of the present invention is that it makes practical the use of radiation feedback from the electroluminescent cell to the photoconductive cell of the same -unit of apparatus without impairing the operation ofthe apparatus. As a result, an even more linear change in the light output in response to changes in the incident illumination may be obtained. This, of course, is only applicable where the photoconductive material is sensitive to the same type of radiation as emitted .by the electroluminescent material. The amount of feedback may, of course, be controlled by limiting the amount of light from the electroluminescent element whichV is allowed to fall on the photoconductive element. The light feedback will, of course, vary in the reverse direction tovariations in the incident radiation, that is to say, it will constitute negative feedback and, by controlling the amount of feed-V back, for example, by the use of a screen of a suitable transparency, a variable light output giving a substantial linear response to changes in the input radiation can be obtained. This form of operation may most readily be obtained by using a modified form of construction as will now be explained in connection with EEG. 3.

Elemental apparatus of FIG. 3

Referring now to FG. 3 of the drawing, there is shown a further form of elemental.electroluminescent apparatus constructed in accordance with the present invention wherein the electroluminescent and photo conductive cells are, so to speak, arranged back-to-back to one another with a single conductive electrode common to each cell. The electroluminescent cell includes a quantity of electroluminescent material 39 positioned between a pair of conductive electrodes 31 and 32. Similarly, the photoconductive cell includes a quantity of photoconductive material 33 positioned between a pair of conductive electrodes 32 and 34. As is apparent, the center conductive electrode 32 is thus an electrode which is common to the two cells. The outer two conductive electrodes 3l and 34 are radiation transparent and may be superimposed on corresponding layers 35 and 36 of a dielectric material such as glass. The series impedance is, in this case, repf resented by a condenser 37" and is connected in series with the voltage-supply means 21 and its terminals 22 and 23.

The operation of the apparatus of FlG. 3 is generally the same as that of FIG. 1 apparatus because of the fact that the electroluminescent and photoconductive cells of FIG. 3 are effectively coupled in parallel with one another. There is one important exception, however, in that radiation f edback from the electroluminescent layer 30 to the photoconductive layer 3.3 automatically occurs unless some means is included for blocking such feedback. As mentioned, such feedback is desirable in that it serves to improve the linearity of the device. Where no such radiation feedback is desired, however, such feedback may be prevented by including a thin layer of opaque material between either the electroluminescent layer 3i? and the conductive electrode 32 or between the conductive electrode 32 and the photoconductive layer 33 or both. Another way of achieving the same result would be to make the common conductive electrode 32 of suti'icient thicknessso as to be substantially opaque to the radiation emitted by the electroluniinescent layer 3?. 0f course, in most cases the radiation feedback,

which is a form of negative'feedback, will be desired.

The amount of such feedback may be controlled by either suitably selecting the transparency factor of the common `conductive electrode 32 or by utilizing semitransparent Elemental apparatus of FIG.

Referring now to FIG. 4 of the drawing, there is shown a modified form of electroluminescent apparatus which is generally similar to that of FIG. 3 except that Vprovision has been made whereby the desired series impedance is built into the device itself thus eliminating the need for an external circuit component. To this end,

each of the dielectric layers 35 and S may include additional conductive electrodes 4t? and il positioned adjacent the outer surfaces thereof. These additional elecenergie/i trodes 40 and 41, which shouldbe radiation transparent, may be formed, for example, by coating suitable films of transparent conductive material on the outer surfaces ofthe dielectric layers V35 and 36. These additional conductive electrodes 40 and 41 are Vthen coupled Vto the terminal 23 of the voltage-supply means. In addition, what was formerly the outer conductiveV electrodes, namely the electrodes 31 and 34, are connected together, for example, by the external connecting wire 42.

In operation, the capacitance across each oi` the dielectric layers 35 and 36 serves to form the desired series impedance which is connected in series between the voltage-supply means 21 and the parallel-connected 'electroluminescent and photoconductive cells. It is essential that the conductive electrodes 31 and 34 be connected together as indicated by the external connecting wire 42 in order that this built-in capacitance of the dielectric layers 35 and 36 `may be common to both the electroluminescent and photoconductive cells. In other words, the capacitances of the dielectric layers 35 and 36 are effectively in parallel with one another and this parallel combination is in series with the parallel combination formed by the electroluminescentiand photoconductive cells. if desired, either the conductive electrode 40 or the conductive electrode 41 may be omitted, in which case the series capacitance is correspondingly reduced.

Y It will be appreciated, of course, that electroluminescent apparatus constructed in accordance with the present invention are not restricted to casesin which the incident radiation lies within the visible range but may also be used to convert nonvisible radiation such as, for example, X-rays, ultraviolet radiation, yor infrared radiation into visible radiation `by use of an appropriate photoconductive material. Y

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and rnodiiicationsmay be made therein without departing from the invention, and it is, therefore, aimed yto cover all such changes and modifications as tall within the Ytrue spirit and scope of the invention.

What is claimedisi' 21. `An electroluminescent device comprising conti guousV layers of different materials positioned one after the other in the lfollowing order: a first layer of conductive material; a layer of photo-impedance material; a second layer of conductive material; a layer of electroluminescent material; a third layer of conductive material; first andv sec-V ond layers of dielectric material contiguous with the outer side of said rst and third conductive layers, respectively; and fourth and iitth layers of conductive material and means connecting the second conductive `layer to another terminal ofthe voltage-supply means.

3. Electroluminescent apparatus comprising; an electroluminescent cell comprising a quantity of electroluminescent material positioned between a pair of conductive electrodes, a iirst of these electrodes being radiation transparent for enabling the radiant glow of the electroluminescent material to escape from the `cellga photo-irnpedance cellicomprising a quantity of photo-impedance material positioned between a pair of conductive electrodes, a iirst of these electrodes being radiationk transparent for enablingy incident radiationzto vary the impedance of the photo-impedance material; the cells being located adjacent one another so that a single common conductive electrode forms the second electrode for both cells, the radia-tion transparency yfactor of this common electrode being chosento tix the radiation feedback from the electroluminescent material to the photoimpedance material at a proper value for improving the response linearity of the apparatus;'an electrical impedance; means for supplying an energizing voltage; and means connecting the two cells in parallel with one another and in rseries with the impedance and Ithe voltage-supply means,

contiguous withvthe other side of said rst .and second Y layers of dielectric material, respectively.

2. Electrolulminescent apparatus comprising: contiguous layers of different materials positioned' one after the second layers of'dielectric material, respectively; means for supplying an energizing voltage; means/,connecting the iirst and third conductive layers -to one another'so as to be electrically common; means connecting the fourth conductive layer to one terminal of the voltage-supply means;

whereby variations in the intensity of the radiation incident on the photo-impedance material will cause the intensity of glow of the'electroluminescent material to vary in an inverse manner.

4. Electroluminescent apparatus comprising: an electrolurninescent cell comprising a quantity of electroluminescent material positioned ybetween a pair of conductive electrodes, a first of these electrodes being radiation trans-Y parent' for enabling the radiant glow of the electroluminescentl material to escape from the cell; a photo-irnpedance cell comprising a quantity of photoimpedance material positioned .between a pair of conductive electrodes, a liirst of these electrodes being radiation transparent for enabling incident radiation to vary the impedance of the photo-impedance material; the cells being located adjacent one another kso that a single common radiation transparent conductive electrode forms the second electrode for both cells; a quantity of semitransparent material located between at least one of said electroluminescent and photo-impedance materials and said common electrode for fixing the radiationfeedback from the electroluminescent material to the photo-impedance material at .a desired value [for improving the response linearity of the apparatus; an electrical impedance; means for supplying .an energizing voltage; and means connecting the two cells in parallel with one another and in series with the impedance and the voltage-'supply means, whereby variations in the intensity of the radiation incident on the photo-impedance material will cause the intensity of glow of the electroluminescent material to vary in an inverse manner.

References Citedbythe Examiner UNITED STATES PATENTS 2,882,419 4/59 Diemer et al Z50-213 2,886,556 4/59 Jenny et al. Z50-213 2,896,087 7/59 Kazan Z50-213 V2,896,088 7/59 Lempert 250-213 V2,906,884- 9/59 Gill 250-213 2,909,667 10/59 Orthuber et al. Z50-213 2,914,679 1l/59 Loebner 250- 213 2,987,624 6/61 Diemer 250--213 RALPH G. NusoN, Primm Examiner. ARCI-IIE R, BORCHELT, Examiner,

Claims (1)

1. AN ELECTROLUMINESCENT DEVICE COMPRISING CONTIGUOUS LAYERS OF DIFFERENT MATERIALS POSITIONED ONE AFTER THE OTHER IN THE FOLLOWING ORDER; A FIRST LAYER OF CONDUCTIVE MATERIAL; A LAYER OF PHOTO-IMPEDANCE MATERIAL; A SECOND LAYER OF CONDUCTIVE MATERIAL; A LAYER OF ELECTROLUMINESCENT MATERIAL; A THIRD LAYER OF CONDUCTIVE MATERIAL; FIRST AND SECOND LAYERS OF DIELECTRIC MATERIAL CONTIGUOUS WITH THE OUTER SIDE OF SAID FIRST AND THIRD CONDUCTIVE LAYERS, RESPECTIVELY; AND FOURTH AND FIFTH LAYERS OF CONDUCTIVE MATERIAL CONTIGUOUS WITH THE OTHER SIDE OF SAID FIRST AND SECOND LAYERS OF DIELECTRIC MATERIAL, RESPECTIVELY.

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