US 3564260 A
Description (OCR text may contain errors)
I United States Patent 1111 ,5
 Inventors Kazunobu Tanaka 2,988,646 6/1961 Wolfe et a1. 250/213 Kawasaki-shi, 3,217,168 11/1965 Kohashi 250/213 Tadao Kohaghi, Yokohamadapan 3,300,645 1/1967 Winslow... 250/213 [211 App]. No. 707,713 3,358,185 12/1967 Lally 313/108BX [221 Fi ed F b- 3. 9 OTHER REFERENCES  Patented l9 1 Thornton ac-dc Electroluminesce Ph ysrcal Rev1ew, 13 Assignee gsztlsgsljfi lnd'mrlal vol 1 13, Number 5, March 1, 1959, pp. 1 187- 90 250/213 8 a g Japan Primary Examiner-James W. Lawrence  Priority Feb. 24, 1967 Assistant Examiner-C. M. l eedom  Japan AttorneyStevens,Dav1s, Miller & Mosher  42/1200 5 soups- ENERGY RESPONSIVE ABTRACTz solid-state energy-responsive luminescent LUMINESCENT DEVICE device comprlsmg an electro-lummescent element which 15 7 Claims, 3 Drawing Figs excited by AC voltage and has been endowed with resistivity and a photoconductlve element whose photoconductive sen-  (I Y 250/2133 sitivity is controlled by superimposing a DC voltage on the AC 313/ 108; 31 operating voltage; an AC voltage and a DC voltage superim- [51) Cl H013 31/50 posed on said AC voltage being applied across said two ele-  Field ofSearch 250/213, mems; said device being constituted so that DC voltage i 833mg 313/94 108A 1083 tributed to the hotoconductive element decreases cord'td 'th 't fth ht d respon mg 0 ecrease 1n e resrs ance o e p o ocon uc- 1561 g gzxsgg s tive element relating to excitation by incident energy and that 2,905,830 9/1959 Kazan 2,272,692 2/l 9 6 l Thornton the AC photoconductive sensitivity of the photoconductive 7 element is controlled through the DC voltage in response to the intensity of the incident energy.
SEW/CONDUCT IVE EL LAYER SEMICOND LIGHT PERV/OUS ELECTRODE UCT/VE REFLECT/NG LA YER IMPERV/OUS SEMICUNOUCfll/E L A YER SOLID-STATE ENERGY-RESPONSIVE LUMINESCENT DEVICE This invention relates to a solid-state energy-responsive luminescent device in which input energy signal is converted or amplified and displayed through a solid-state image plate which consists of a combination of photoconductive element and electroluminescent element and is connected to a power source. This invention is intended to provide a wide range of controllability of the contrast, that is, gamma value, an improved characteristics in the low input energy range, and an extended input energy latitude (that is, an effective operating range of the input energy signal), by giving appropriate resistive impedances to respective appropriate elements out of constituents of said solid-state image plate, supplying an AC voltage superimposed with a DC voltage as the operating voltage and controlling the magnitude of this DC voltage. In the conventional solid-state image plates consisting of a combination of photoconductive element and electroluminescent element, in which the luminescence of the electroluminescent element is electrically controlled by variation in the AC impedance of the photoconductive element in response to the input energy signal, the AC impedance of the photoconductive element depends on capacitive impedance of the photoconductive element determined by its geometrical structure in low input energy range. From this reason and because the photoconductivity is essentially less sensitive when an AC voltage is being imposed on it, the rate of variation in the AC impedance is extremely low in the conventional image plates. Thus, the conventional image plates, partly because of the photoelectrical nonlinearity of the electroluminescent element, has a low sensitivity and a high gamma value, and accordingly, a very limited input energy latitude. According to this invention, the electrolumine element of the solid-state image plate and appropriate intermediate elements interposed between said electroluminescent element and the photoconductive element are endowed with resistive impedances, and the photoconductive element is of such property that its photoconductive sensitivity under an AC operating voltage is controllably increased by superimposing a DC voltage onto the AC voltage. In the operation, the DC voltage is superimposed on the AC voltage applied across the electroluminescent element and the photoconductive element, and by controlling the value of said DC voltage are attained an improved characteristics in low input energy range, a wide controllable range of gamma value and an extended input energy latitude.
Now, this invention will be described in connection with an embodiment, referring to the attached drawings, in which:
FIG. 1 shows an equivalent circuit for explaining the principle of this invention;
FIG. 2 is a schematic diagram showing a portion of the solid-state image plate embodying this invention; and
FIG. 3 shows characteristic. curves concerning an embodiment of this invention.
Referring to FIG. 1 which is an equivalent circuit of a solidstate image place of DC controlled type for displaying a positive output image, the electroluminescent element and the photoconductive element used being of the form of layer, marking Cp indicates the capacitive component of the photoconductive layer (hereafter, referred to as a PC layer), Rp the resistive component of the PC layer, Ce the capacitive component of the electroluminescent layer (hereafter, referred to as an EL layer), and Re the resistive component of the EL layer. Value of Re is selected to be appropriately lower than the dark resistance of Rp. Mark Va indicates the operating AC voltage, Vb the variable DC voltage, L the incident energy signal and L: the output light. If Vb is zero volt, the circuit is equivalent to a conventional image plate of positive image type. Since the AC impedance due to the capacitance Cp determined by the geometrical structure is more dominant in the low input energy range than the AC photoconductive sensitivity of the PC layer, the characteristics of the output light intensity vs the input energy signal is not satisfactory, and
accordingly, the input energy latitude is not sufficiently broad and further, the gamma value is considerably high. However, a DC voltage Vb is applied; in a closed DC circuit composed by the resistive component Rp of the PC layer and the component Re of the EL layer, DC voltage Vbp determined by said resistive components Rp and Re is applied to the PC layer, being superimposed on AC voltage Vap. In this state, assuming that the input energy signal is very weak, the resistive component Rp of the PC layer is very high in comparison with the component Re, rendering the DC voltage Vbp nearly equal to the voltage Vb, thus a very high DC voltage being superimposed on the AC voltage Vap. A satisfactory PC layer of which the photoconductive sensitivity under AC voltage can be controllably increased by the superimposition of a DC voltage is the one containing powder of photoconductive material.
Generally, AC photoconductive sensitivity of photoconductive powder increases nonlinearly with the increase of the superimposed DC voltage. Therefore, the characteristics is remarkably improved in the low input energy range where the proportion of the superimposed DC voltage is high. While, with the increase of the input energy signal intensity, the resistive component Rp of the PC layer decreases, accompanied by the decrease of the component DC voltage Vbp, thus diminishing the improving effect of the DC voltage to the AC photoconductive sensitivity of the PC layer. Upon the intent energy reaching a high intensity range, the resistive component Rp of the PC layer decreases to a value negligible in comparison with the resistive component Re of the EL layer and the component DC voltage Vbp of the PC layer becomes nearly zero. Therefore, the intensity of the output light L in this input range becomes equivalent in the value to that of the conventional solid-state image plate in which the voltage Vb is zero. That is, whereas the characteristics representing the intensity of output light vs the intensity of input energy signal in a high input range is almost equivalent to that of the conventional solid-state image plate of positive image type when the DC voltage is zero, the similar characteristics in a low input range is remarkably improved by the effect of the component DC voltage Vbp imposed upon the PC layer by applying the DC voltage Vb to the device, the effective range being extended greatly to the low input energy range depending on the value of the DC voltage Vb, thereby enabling a wide range control of gamma value and providing an extended input energy latitude. Moreover, the contrast in the obtained output image is almost as excellent as that in Vb 0, as the luminescence of the EL layer is little effected by DC voltage.
Further, control of the resistive element Re of the EL layer concurrently varies the impedance of the EL layer. The contrast hitherto limited by the ratio of capacitances of the EL layer and PC layer, thus becomes freely controllable by adjusting the resistive component Re, without depending on the geometrical structure of the PC layer and EL layer.
In FIG. 2 which is showing schematically structure of a solid-state image plate embodying this invention and the manner in which the electric power is supplied to the device, numerals 101 to 107 indicate the constituting elements of the solid-state image plate, 101 being light-pervious support plate made of glass or the like, and 102 being light-pervious electrode, for example, made of metal oxide such as tin oxide. Numeral 103 indicates semiconductive electroluminescent layer of approximately 30 to 60 micron in thickness which comprises powder of electroluminescent material such as ZnSzCuAl and powder of semiconductive metal oxide such as Sn0 or Ti0 which has good reflexibility against the luminescent spectrum of said EL material, said powders being binded by a vitreous material and formed in a layer. Thus, by the endowment of resistivity to an EL layer by mixing powder of a concurrently reflective, resistive and semiconductive metal oxide, the luminescent output is effectively taken out from the EL layer without being absorbed by resistive powder. Further, the resistivity of the EL layer can be easily controlled over a wide range by varying the amount of the resistive powder to be mixed. Accordingly, the matching of the PC layer to the load circuit including the EL layer can be easily attained in the series connected resistive circuit, and a very effective control of the AC photoconductive sensitivity of the PC layer is achieved by the control of DC voltage, with the aid of usage of a PC layer containing powdered photoconductive material. Thus, gamma value becomes widely controllable and the effective operating range of the intensity of the input energy is extended.
Out of the resistive intermediate layers 104 and 105, the former is semiconductive reflecting layer of about 10 micron in thickness which comprises powder of a light-reflective and ferroelectric material such as BaTiO and powder of semiconductive metal oxide such as Snt) or Ti said powders being bonded with a vitreous material or a plastic material. in this case, a vitreous bonding material is preferable for making ohmic layer, while a plastic material is advantageous for nonohmic layer. Numeral 105 indicates impervious semiconductive layer of about micron in thickness which comprises, for example, black paint mixed with powder of nonlinear resistivity such as CdSzCl or powder of linear resistivity such as carbon black and which is formed in a layer. By providing such resistive intermediate layers, dielectric breakdown of EL layer by DC voltage or even by AC voltage is prevented. Moreover, resistivity of the series connected resistive load circuit including the EL layer can be adjusted by varying the resistivity of the intermediate layers, so that the resistivity of said load circuit is set at an appropriate value of the same order as the dark resistivity of the PC layer or lower than that. Therefore, limitation for the resistivity of the EL layer is much relieved. For example, if the EL layer has been produced with extremely low resistivity, the intermediate layer 104 is made so as to have an ohmic resistance asdescribed above, the resistivity being set at an appropriate value higher than that of the EL layer, thereby to attain the matching of the load to the PC layer in the resistances. As described above, the intermediate layers allow easy fabrication of the resistive EL layer and eliminate the effect of limitation of the resistivity to luminescent characteristics, and further facilitate easy matching of the DC resistances between the series connected load circuit including the EL layer and the PC layer, thereby permitting very effective control of the AC photoconductive sensitivity of the PC layer by the DC voltage.
As mentioned previously, by selecting the resistivity of each relevant element so that the transversal resistance of the DC load circuit including the EL layer is similar to or lower than the dark resistance of the PC layer, the DC voltage across the PC layer is made high when the intensity of input energy signal is zero or very low. Thus, gamma value and input-to-output characteristics in low input energy range are improved, and the effective operating range of intensity of input energy signal is remarkably extended.
Further, as the intermediate layers are endowed with resistivity by mixing of powder of resistive material, the resistivity can be freely controlled by varying the amount of the powder over a wide range. This makes very easy the adjustment of the resistance of the series-connected load circuit including the EL layer or the matching of resistances between said load circuit and the PC layer, and accordingly facilitates fabrication of the complete device, and further makes easy improvement of gamma value and the effective operating range of the intensity of input energy.
It will be noted that as the resistive intermediate layers contain powder of ferroelectric material such as BaTi0 the overall dielectric constant of the layers is raised. This raised dielectric constant lowers AC voltage loss in the intermediate layers. This fact is another advantage of the resistive intermediate layers, beside the above-mentioned advantages.
Further, the ferroelectric material such as BaTill has a high specific resistivity. Therefore, when resistive binder material is used for the intermediate layer, the use of the ferroelectric material also makes possible controlling of resistivity of the intermediate layer by varying the amount of the material to be mixed, beside it serves to raise the overall dielectric constant of the layer. On the other hand, in the intermediate layer the resistivity of which is presented by adding of resistive powder, particles of highly resistive ferroelectric material are intermixed with particles of resistive material, as the ferroelectric material is also used. As the result, two dimensional uniformity in the resistivity of the intermediate layer is improved, as the condensation and maldistribution of the resistive powder are thus prevented.
Returning to FIG. 2, numeral 106 indicates the photoconductive layer or PC layer of about 200 to 500 micron in thickness, which is formed of photoconductive powder bound by plastics or a similar binding material, said photoconductive powder being a material which is sensitive not only to the visible light but to a radiation such as X-ray, infrared ray and ultra-violet ray, such as, for example, cadmium sulfide activated with an element of IB group such as Cu or Ag and an element of VII B group such as C1, the latter element of Vll B group being able to be substituted by an element of III B group such as All or Ga. Numeral 107 indicates electrode of vapourdeposited metal, for example, aluminum. This electrode is pervious not only to a radiation such as X-ray, but to the visible light, and can be formed in a gappy pattern such as equispaced parallel lines, lattice or mesh. Numeral 108 represents input energy signal, which is not limited to the visible light, but can be other radiation such as ultra-violet ray, infrared ray or X-ray. Numeral 109 indicates output visible image. Numerals 110 and 111 indicate voltage sources for the solid-state image plate applied across the electrodes 102 and 107, 110 being the AC operating voltage source and 111 being variable DC voltage source. Among the above-described elements 101 to 107, the most important one for realizing the above-mentioned control by DC voltage is the semiconductive EL layer 103.
The resistivity of the EL layer is theoretically required to be appropriately lower than the dark value of resistive component of the PC layer; that is, to be in the semiconductive range of the order of 10 to 10 ohm-cm, where the characteristics is fairly linear. However, this requirement for the resistivity presents several technical problems including difficulties relating to construction and manufacturing method. The conventional techniques for imparting electroconductivity to a solid layer made of a highly resistive material such as plastic or glass, include a process of dispersing resistive material into the plastic or glass body. However, the resistivity obtained by such process is limited to one very near to that of a conductor or to one having directivity, and a solid layer which has a resistivity of 10 to 10 ohm-cm. belonging to the semiconductor range and which maintains ohmic characteristics up to a considerably high electric field, has not been obtained. Though carbon black is comparatively satisfactory as a resistive material, it is not suitable for the material for imparting semiconductivity to the EL layer, as it absorbs the luminescent light from the EL powder. Use of metal powder such as Cu or Sn also presents a difficult problem, since such material is apt to be oxidized or deteriorated at a high temperature expected in the manufacturing process and further, since pulverizin g of such material has a limitation because of its high malleability. Moreover, resistivity of such metallic material is too low to allow easy control of the resistivity of the layer. Especially, if a plastic binding material is used, the ohmic resistivity will be maintained by no means up to a high electric field, partly because of the inferior thermal property of the plastic.
In order to overcome such difficulties, the semiconductive EL layer 103 as shown in FIG. 2 has been introduced by this invention. According to this invention, the resistive material is selected from the semiconductive metal oxides including Sn0,, W0 Sb O and Tit! which are stable at a considerably high temperature in the atmospheric environment and readily available in the form of pulverized product, and which have high reflexibility to the light in the visible spectrum emitted from the EL powder. As the binder of the EL layer, is used a vitreous material which is thermally stable up to a considerably high temperature and into which metal oxide such as Sn0 is preferably fusible in some extent, the ohmic characteristics of the resistivity and thermal stability of the electrical properties being taken into consideration. It is required to select a vitreous binder the softening point of which is lower than that of the support plate 101 and the heat expansion coefficient of which is substantially the same as that of the support plate, so as to ensure satisfactory application of the EL layer to the support plate. Of course, the binder must be light-pervious, as it is used in the EL layer. The control of the resistivity of the semiconductive EL layer according to the above-mentioned construction is made by varying the volumetric ratio of the metal oxide powder in relation to the total volume. In this process, it is important to properly select the relative gradings of the resistive powder, EL powder and binding vitreous powder, in order to ensure sufficient adhesion among the resistive powder, EL powder and support plate 101 and to obtain smooth layer. The following table 1 shows the gradings and volumic percentages of the ingredients of the above-described mixture: that is, Sn0 powder used as the resistive material, ZnS:CuAl powder as the electroluminescent material and the vitreous material as the binder. Table 2 shows an example of the composition of the vitreous binder. Table 3 shows volume expansion coefficients and softening points of the vitreous binder and the light-pervious support plate (a glass plate).
coefiicient point, C.
Vitreous binder X- Support plate (glass) As to the grading of the powder, it is important that the diameter of the particles of vitreous binder is always smaller than those of the other two materials. If this relation is reversed, the mixture will not unite. The percentage of Sn0 powder can be varied in a range of 10 to percent, causing corresponding variation in the resistivity. The heating temperature was set at 640 C. in this embodiment. The resistivity of thus obtained EL layer showed a fairly good linearity in the semiconductor range of 10 to 10 ohm-cm. Moreover, thus obtained EL layer is highly resistive against heat and environmental conditions. The resistivity of the intermediate layers, that is, the semiconductive reflection layer 104 and the semiconductive nonpervious layer 105, is selected so that the total of the resistances of the two intermediate layers and the EL layer is at most not higher than the dark resistance (that is, resistance under no light input) of the PC layer 106. Generally, the resistance of the intermediate layers may be approximately the same or lower in comparison with that of said EL layer, and may have a nonlinear current-voltage characteristics. Thus, AC voltage loss in the intermediate layers is decreased by the above-mentioned nonlinearity of resistance, thereby the AC voltage being effectively impressed on the EL layer. Further, drop of the distinction in the output image due to dispersion of AC current in the intermediate layers is prevented. Therefore, the nonlinearity of the characteristics is rather preferable, when satisfactory matching between the resistance of the EL layer and the dark resistance of the PC layer is obtainable without adjustment of the resistance of the intermediate layers.
It is important that softening point of the vitreous binder used in the intermediate layers is lower than that of the vitreous binder used in the EL layer, and that volume expansion coefficients of the two binders are approximately the same.
In the above-described constitution of the device of this invention, at least either one of the semiconductive reflection layer 104 and the semiconductive nonpervious layer 105 can be omitted. If the nonpervious layer 105 and further the reflection layer 104 have been removed, the luminescent output from the EL layer 103 can be fed back to the PC layer 106. Accordingly, if the spectral characteristics of the EL material and the PC material are selected so that the PC layer effectively responds to the light from the EL layer, sensitivity of the device to the input energy will be improved and at the same time, gamma value can be raised appropriately under such improved sensitivity. In such case, the EL layer 103 can be made in the form of a compound layer consisting of a layer for displaying output image and a layer for feedback of the light, with a semiconductive nonpervious layer therebetween.
FIG. 3 shows input versus output characteristics of the embodiment shown in FIG. 2, as plotted on a logarithmic chart, where the AC operating voltage is fixed at 450v., its frequency being 1 kc. and the DC voltage is varied, as the parameter, from zero to 400v. In the diagram, the intensity of input energy signal is represented by dose rate of continuous X-ray from a 113 kVP X-ray tube. As is seen from the diagram, gamma value is variable widely and continuously in a range of l to 3, and input energy latitude is extended by nearly one hundred times of the conventional value.
As described above, by giving resistivity to the EL element and other appropriate elements, superimposing a DC voltage on the AC voltage applied to the series connected EL element and PC element, and providing means for varying said DC voltage, the AC photoconductive sensitivity of the PC layer in relation to the intensity of input energy can be controlled, and characteristics in low input energy range, gamma value and effective operating range for the input energy intensity can be adjusted widely and continuously.
Further, in the above-described example, if the DC voltage is superimposed in such a manner that the electrode in the EL element side is of positive polarity and the electrode in the PC element side is negative, the above-mentioned improvements are realized with much higher sensitivity or effectiveness in comparison with the case where the DC voltage is applied in opposite direction.
Accordingly, in the above embodiment, if means for changing the polarity of the DC voltage Vb, for example, a changeover switch is provided to the voltage source, the operating characteristics can be varied only by changing the polarity of the DC voltage without varying magnitude of the voltage. Therefore, a wide variability of the operating characteristics is obtained by providing means for controlling at least either one of magnitude or polarity of the DC voltage to be superimposed on the AC voltage.
Now, turning to the PC layer 106, it will be noted that the photoconductive material CdSzCuCl is essentially less sensitive to the radiations such as X-ray than to the visible light. In order to improve the sensitivity to such radiations, an appropriate amount of radiation luminescent fluorescent powder (for example, orange luminescent ZnCdSzAg) is added to the photoconductive powder of CdSzCuCl, and the mixture is bound by a plastic binder. By such composition, the sensitivity is increased by more than ten tifies of the original value. This is because the radiation luminescent fluorescent material is excited by the incident radiation (for example, X-ray) at the same time when the PC material is excited, and the PC material is further excited by the visible light converted by the radiation luminescent material. As the utility factor of the converted visible light corresponds to 41:- solid angle, increase of the sensitivity is remarkable. For example, an experiment with a PC layer containing CdSzCuCl added with 10 volumic percents of ZnCdSzAg, showed that the characteristic curves in FIG. 3 were shifted laterally to lower input range by one decimal scale by said addition of ZnCdSzAg. Especially, in this case, resistivity and dielectric strength of the fluorescent powder show much higher values in comparison with those of the PC powder under no input light. That is, it will be seen that the addition of the fluorescent powder improves dark resistivity and dielectric strength of the PC layer.
As described above, according to this invention, operating characteristics of the energy-responsive luminescent device is improved particularly in low input range, and gamma value of the solid-state image plate becomes widely variable, and further the input energy latitude is remarkably extended, by imparting resistive impedances to the EL layer and the required intermediate layers interposed between the EL layer and the PC layer with unique constitution and manufacturing method and by controlling the DC voltage superimposed on the AC operating voltage.
Further, the EL layer of this invention is formed from a mixture which contains powder of vitreous material, powder of BL fluorescent material and powder of at least one lightreflective and semiconductive metal oxide selected from group containing Sn Ti0 W0 and Sb 0 the mixture being heated to fuse the vitreous material. Thus, the EL layer of this invention presents an ohmic resistivity which is well stable even in high voltage range and a highly efficient EL luminescence, without possible decrease of the luminescent efficiency.
1. A solid-state energy-responsive luminescent device comprising an electroluminescent layer which is excited to luminescence by an AC voltage applied thereto, a photoconductive layer provided on said electroluminescent layer, the AC impedance of said photoconductive layer being variable depending on the intensity of an incident energy and said AC impedance in a dark state being higher than the AC impedance of said electroluminescent layer, a pair of electrodes sandwiching the combination of said two layers therebetween, at least that one of said electrodes which is disposed on the electroluminescent layer being light-pervious, and means for applying an AC voltage and a DC voltage superimposed on said AC voltage across said layers by means of said electrodes, whereby the AC voltage across said electroluminescent layer, and therefore the luminescent output, is controlled substantially according to the variation of the AC impedance of said photoconductive layer due to the variation in the intensity of the incident energy thereto, wherein said photoconductive layer comprises a powder of photoconductive material bound with a binding material so that the photoconductive sensitivity of said photoconductive layer under an AC voltage is substantially increased as the superimposed DC voltage is increased and so that the DC resistance of said electroluminescent layer is lower than the DC resistance of said photoconductive layer in a dark state so that the DC voltage across said photoconductive layer in the range of low incident energy is high enough to enhance the photoconductive sensitivity of said photoconductive layer.
2. A solid-state energy-responsive luminescent device as defined in claim 1, wherein said electroluminescent layer is composed of powder of electroluminescent fluorescent material and powder of resistive metal oxide which has reflective power against the light from said fluorescent material,
said two powders being mixed with a vitreous medium.
3. A solid state energy-responsive luminescent device as defined in claim 2, wherein the specific resistivity of said electroluminescent layer is in a range of 10'' to l0 ohm-cm.
4. A solid-state energy-responsive luminescent device as defined in claim 3, wherein said resistive metal oxide is at least one selected from a group including Sn0 Ti0 W0 and Sb 0 5. A solid-state energy-responsive luminescent device as defined in claim 1, wherein at least one resistive intermediate layer is interposed between said electroluminescent layer and said photoconductive layer, and the total-DC resistance of the combined layer comprising said electroluminescent layer and said intermediate layer is lower than the DC resistance of said photoconductive element in a dark state.
6. A solid-state energy-responsive luminescent device as defined in claim 5, wherein said resistive intermediate layer is a compound layer consisting of a light-reflective layer positioned on the electroluminescent side and an impervious layer positioned on the photoconductive side.
7. A solid-state energy-responsive luminescent device as defined in claim 2, wherein one of said pair of electrodes comprises light pervious conductive film of metal oxide deposited on a light pervious glass substrate.