EP1085496A2 - Driving method and drive for organic electroluminescence element and display employing the same - Google Patents
Driving method and drive for organic electroluminescence element and display employing the same Download PDFInfo
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- EP1085496A2 EP1085496A2 EP00307755A EP00307755A EP1085496A2 EP 1085496 A2 EP1085496 A2 EP 1085496A2 EP 00307755 A EP00307755 A EP 00307755A EP 00307755 A EP00307755 A EP 00307755A EP 1085496 A2 EP1085496 A2 EP 1085496A2
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
- G09G3/30—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
- G09G3/32—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
- G09G3/3208—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
- G09G3/3216—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using a passive matrix
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
- G09G3/30—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
- G09G3/32—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
- G09G3/3208—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
- G09G3/3225—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
- G09G3/3233—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2310/00—Command of the display device
- G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
- G09G2310/0243—Details of the generation of driving signals
- G09G2310/0254—Control of polarity reversal in general, other than for liquid crystal displays
- G09G2310/0256—Control of polarity reversal in general, other than for liquid crystal displays with the purpose of reversing the voltage across a light emitting or modulating element within a pixel
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2310/00—Command of the display device
- G09G2310/06—Details of flat display driving waveforms
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2320/00—Control of display operating conditions
- G09G2320/02—Improving the quality of display appearance
- G09G2320/0233—Improving the luminance or brightness uniformity across the screen
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2320/00—Control of display operating conditions
- G09G2320/04—Maintaining the quality of display appearance
- G09G2320/043—Preventing or counteracting the effects of ageing
Definitions
- the present invention relates to a driving method for an organic electroluminescence element, a drive for an organic electroluminescence element and a display employing the same.
- a display employing an organic eletroluminescence element (hereinafter referred to as an organic EL element) is actively subjected to research and development as one of such flat display elements.
- the display employing an organic EL element has average or high efficiency, is thin and lightweight, and has no viewing angle dependency.
- the organic EL element injects electrons and holes into a luminescent part from an electron injection electrode and a hole injection electrode respectively for recombining the electrons and the holes at the luminescence center and exciting organic molecules for fluorescing when the organic molecules return from the excited state to a ground state.
- the organic EL element is formed by a plurality of luminous elements arranged on a substrate in the form of a matrix.
- Such an organic EL element can advantageously be driven with a low voltage of about 5 V to 20 V. Further, the organic EL element, capable of obtaining luminous elements luminescing in proper colors by selecting fluorescent materials serving as luminescent materials, is expected for application to a multi- or full-color display. In addition, the organic EL element capable of surface luminescence with a low voltage can also be employed as a backlight for a display such as a liquid crystal display.
- Reliability is an important factor for putting the organic EL element into practice. While a number of organic luminescent materials and doping materials have heretofore been studied for improving the luminescence life of the organic EL element, a method of increasing the life of the organic EL element by controlling a current supplied to the organic EL element is recently studied.
- each of Japanese Patent Laying-Open Nos. 11-3060 (1999) and 11-8064 (1999) discloses a technique of successively applying a forward-bias voltage and a reverse-bias voltage to an organic EL element.
- An object of the present invention is to provide a driving method for an organic electroluminescence element capable of attaining a long luminescence life while ensuring high luminous efficiency over a long period.
- Another object of the present invention is to provide a drive for an organic electroluminescence element capable of attaining a long luminescence life while ensuring high luminous efficiency over a long period.
- Still another object of the present invention is to provide a display employing an organic electroluminescence element capable of attaining a long luminescence life while ensuring high luminous efficiency over a long period.
- a driving method for an organic electroluminescence element having a pair of electrodes comprises a step of applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than the luminescence starting voltage across the electrodes of the organic electroluminescence element.
- the luminous intensity is increased to the constant level in a short time upon application of the first voltage and the luminescence continues in the initial stage of application of the second voltage, whereby the total luminescent time is increased. Further, it is not necessary to apply a high voltage for obtaining prescribed average luminance. Consequently, high luminous efficiency is attained.
- the first and second voltages are alternately applied to the organic electroluminescence element so that the organic electroluminescence element intermittently luminesces, whereby reduction of the luminance remains small over a long period and a long life is attained.
- the second voltage is preferably substantially equal to the luminescence starting voltage. In this case, higher luminous efficiency can be attained.
- the duty ratio of the drive pulse is preferably greater than zero and not more than 50 %.
- the duty ratio of the drive pulse is more preferably not more than 10 %.
- the duty ratio of the drive pulse is further preferably at least 0.2 %. Further, the duty ratio or the drive pulse may be at least 1 %. Thus, the luminescence life of the organic electroluminescence element is further improved.
- a drive for driving an organic electroluminescence element having a pair of electrodes comprises a drive pulse application circuit applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than the luminescence starting voltage across the electrodes of the organic electroluminescence element.
- the first and second voltages are alternately applied to the organic electroluminescence element so that the organic electroluminescence element intermittently luminesces, whereby reduction of luminance remains small over a long period and a long life is attained.
- the second voltage is preferably substantially equal to the luminescence starting voltage. In this case, higher luminous efficiency can be attained.
- the duty ratio of the drive pulse is preferably greater than zero and not more than 50 %.
- the duty ratio of the drive pulse is more preferably not more than 10 %.
- the duty ratio of the drive pulse is further preferably at least 0.2 %. Further, the duty ratio or the drive pulse may be at least 1 %. In this case, the luminescence life of the organic electroluminescence element is further improved.
- a display comprises one or a plurality of organic electroluminescence elements each having a pair of electrodes and a drive applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than the luminescence starting voltage across the electrodes of each of one or a plurality of organic electroluminescence elements.
- luminous intensity is increased to a constant level in a short time upon application of the first voltage and the luminescence continues in an initial stage of application of the second voltage, whereby the total luminescent time is increased. Further, it is not necessary to apply a high voltage for obtaining prescribed average luminance. Consequently, high luminous efficiency is attained.
- the first and second voltages are alternately applied to the organic electroluminescence element so that the organic electroluminescence element intermittently luminesces, whereby reduction of luminance remains small over a long period and a long life is attained.
- the drive may include a first driver supplying a drive signal to one of the electrodes of each of one or a plurality of organic electroluminescence elements and a second driver supplying a selection signal to the other one of the electrodes of each of one or a plurality of organic electroluminescence elements, and the drive signal and the selection signal may be so set as to apply the drive pulse across the electrodes of a selected organic electroluminescence element.
- the drive pulse alternately changing between the first voltage higher than the luminescence starting voltage and the second voltage higher than 0 V and not more than the luminescence starting voltage is applied across the electrodes of the selected organic electroluminescence element by the first and second drivers, so that the selected organic electroluminescence element luminesces.
- Each organic electroluminescence element includes a transparent substrate, a hole injection electrode, an organic luminescent layer and an electron injection electrode in this order, and the pair of electrodes are the hole injection electrode and the electron injection electrode.
- the second voltage is preferably substantially equal to the luminescence starting voltage. In this case, higher luminous efficiency can be attained.
- the duty ratio of the drive pulse is preferably greater than zero and not more than 50 %.
- the duty ratio of the drive pulse is more preferably not more than 10 %.
- the duty ratio of the drive pulse is further preferably at least 0.2 %. Further, the duty ratio or the drive pulse may be at least 1 %. Thus, the luminescence life of the organic electroluminescence element is further improved.
- Fig. 1 is a model diagram showing an exemplary structure of an organic electroluminescence element (hereinafter referred to as an organic EL element) 20 and a drive pulse application circuit 10.
- an organic electroluminescence element hereinafter referred to as an organic EL element
- a hole injection electrode 2 of ITO (indium-tin oxide) serving as a transparent conductive film is formed on a glass substrate 1.
- a hole transport layer 3 of 200 ⁇ in thickness made of MTDATA (4,4',4''-tris(3-methylphenylphenylamino)triphenyl-amine) having a molecular structure expressed in the following chemical formula (1) is formed on the hole injection electrode 2.
- An electron transport layer 5 of 500 ⁇ in thickness made of Alq 3 (tris(8-hydroxy-quinolinate)aluminum) having a molecular structure expressed in the following chemical formula (4) is formed on the organic luminescent layer 4.
- An electron injection electrode 6 of 2000 ⁇ in thickness made of MgIn is formed on the electron transport layer 5.
- the drive pulse application circuit 10 is connected between the hole injection electrode 2 and the electron injection electrode 6 of the organic EL element 20.
- the drive pulse application circuit 10 applies a drive pulse to the organic EL element 20 as described later.
- the organic luminescent layer 4 of the organic EL element 20 luminesces so that light 100 outgoes from the rear surface of the glass substrate 1.
- Fig. 2 is a voltage waveform diagram of the drive pulse applied by the drive pulse application circuit 10 shown in Fig. 1 to the organic EL element 20.
- the drive pulse alternately changes between a first voltage higher than a luminescence starting voltage (luminescence threshold voltage) V 0 and a second voltage higher than 0 V and lower than the luminescence starting voltage V 0 .
- the frequency of the drive pulse is in the range of 30 Hz to 10 kHz, for example.
- the period when the drive pulse is at the first voltage is referred to as a luminous period T1
- the period when the drive pulse is at the second voltage is referred to as a non-luminous period T2.
- the organic EL element 20 is substantially luminous in the luminous period T1 and substantially non-luminous in the non-luminous period T2. More correctly, the luminous period T1 includes a non-luminous time of the organic EL element 20, and the non-luminous period T2 includes a luminous time of the organic EL element 20, as described later.
- a continuous luminescence experiment was made by applying drive voltages to the organic EL element 20 by driving methods according to Inventive Example and comparative examples 1 and 2 for analyzing voltage waveforms, current waveforms and luminous intensity values and examining time changes of luminance and luminous efficiency.
- the drive pulse shown in Fig. 2 was applied to the organic EL element 20 by the drive pulse application circuit 10 shown in Fig. 1 in Inventive Example, while a constant voltage higher than the luminescence starting threshold voltage V 0 was applied to the organic EL element 20 in comparative example 1 and a forward bias voltage and a reverse bias voltage were alternately applied to the organic EL element 20 in comparative example 2.
- Fig. 3 shows the drive voltage applied to the organic EL element 20, a current flowing in the organic EL element 20 and luminous intensity of the organic EL element 20 in Inventive Example.
- Fig. 4 shows the drive voltage applied to the organic EL element 20, a current flowing in the organic EL element 20 and luminous intensity of the organic EL element 20 in comparative example 1.
- Fig. 5 shows the drive voltage applied to the organic EL element 20, a current flowing in the organic EL element 20 and luminous intensity of the organic EL element 20 in comparative example 2. Referring to Fig. 5, the period when the drive voltage is set to the forward bias voltage is referred to as a luminous period T1, and the period when the drive voltage is set to the reverse bias voltage is referred to as a non-luminous period T2.
- each drive voltage was so set that the average value of the current flowing in the organic EL element 20 was regularly constant from starting of luminescence to the end of the experiment, for driving the organic EL element 20 with a constant current.
- the organic EL element 20 is deteriorated by continuous luminescence and internal resistance is increased, therefore, it follows that the drive voltage gradually increases.
- Table 1 shows experimental conditions for Inventive Example and comparative examples 1 and 2.
- FREQUENCY:1kHz DUTY RATIO OF DRIVE VOLTAGE:50% FORWARD BIAS VOLTAGE (INITIAL):V 2 5.8V
- REVERSE BIAS VOLTAGE (CONSTANT):-V 1 -10V
- the drive voltage is set to a first voltage V 3 higher than the luminescence starting voltage V 0 in the luminous period T1 in Inventive Example. Further, the drive voltage is set to a second voltage identical to the luminescence starting voltage V 0 in the non-luminous period T2.
- the drive voltage is set to the luminescence threshold voltage V 0 in the non-luminous period T2
- the current flowing in the organic EL element 20 reaches zero.
- charges stored in the organic EL element 20 in the luminous period T1 flow in the organic EL element 20 for some time without flowing out.
- holes and electrons recombine to contribute to the luminescence. Consequently, the luminescence continues in an initial time t3 of the non-luminous period T2. Thereafter the luminescence of the organic EL element 20 stops.
- the drive voltage is set to a constant voltage higher than the luminescence starting voltage V 0 in comparative example 1.
- a constant current continuously flows in the organic EL element 20, which in turn continuously luminesces with constant luminous intensity.
- the drive voltage is set to a forward bias voltage V 2 higher than the luminescence starting voltage V 0 in the luminous period T1 in comparative example 2. Further, the drive voltage is set to a reverse bias voltage -V 1 in the non-luminous period T2. In this case, the drive voltage in the luminous period T1 is set higher than the first voltage V 3 in Inventive Example and the drive voltage in comparative example 1, in order to equalize average luminance of comparative example 2 with those of Inventive Example and comparative example 1.
- the organic EL element 20 When the drive voltage is set to the forward bias voltage V 2 in the luminous period T1, a current flows in the organic EL element 20, which in turn luminesces. Charging is caused in an initial period of the luminous period T1 due to influence by the capacitance of the organic EL element 20 itself so that a large current flows in the organic EL element 20 and the current is thereafter stabilized to a constant value. Therefore, the organic EL element 20 remains non-luminous in the initial stage of the luminous period T1 while luminous intensity starts to increase in a delay by a prescribed time from application of the forward bias voltage V 2 and is thereafter stabilized to a constant level.
- a charging time t4 is lengthened since the drive voltage has been set to the reverse bias voltage -V 1 in the preceding non-luminous period T2. Consequently, a time t6 required for the luminous intensity to rise to the constant level is lengthened.
- Fig. 6 illustrates the relation between the luminous efficiency of the organic EL element 20 and the drive voltage.
- the luminous efficiency of the organic EL element 20 is maximized when the drive voltage is set to the luminescence starting voltage V 0 , and reduced as the drive voltage is increased. Therefore, the luminous efficiency is reduced when driving the organic EL element 20 with a high voltage.
- the luminous intensity is increased to the prescribed level in a short time in the luminous period T1 while the luminescence of the organic EL element 20 continues in the initial stage of the non-luminous period T2 as shown in Fig. 3, whereby the total luminescent time is increased.
- the drive voltage can be set lower than that in comparative example 2 in order to attain prescribed average luminance. Consequently, the luminous intensity is increased.
- Fig. 7 illustrates time changes of luminance in the continuous luminescence experiment on Inventive Example and comparative examples 1 and 2.
- the half-life of the luminance is regarded as the life.
- the life of comparative example 1 was about 3000 hours, while that of comparative example 2 was about 10000 hours.
- the life of Inventive Example was about 7000 hours, i.e., at least twice that of comparative example 1.
- Fig. 8 illustrates time changes of luminous efficiency in the continuous luminescence experiment on Inventive Example and comparative examples 1 and 2.
- comparative example 2 exhibited low initial luminous efficiency of 3.8 (lm/W), which was gradually reduced following continuous luminescence.
- Comparative example 1 exhibited the highest initial luminous efficiency of 5.0 (lm/W), which was abruptly reduced following continuous luminescence to reach a level similar to the luminous efficiency of comparative example 2 after a lapse of 3000 hours.
- Inventive Example exhibited initial luminous efficiency of 4.6 (lm/W) slightly lower than that of comparative example 1, reduction of the luminous efficiency following continuous luminescence was so small that the luminous efficiency of Inventive Example exceeded that of comparative example 1 after a lapse of several 100 hours. Further, Inventive Example constantly exhibited higher luminous efficiency than comparative example 2.
- a long luminescence life can be attained while ensuring high luminous efficiency, and reduction of the luminous efficiency remains small in continuous luminescence over a long period.
- the second voltage of the drive pulse is preferably set in the range of at least 70 % of the luminescence starting voltage V 0 and not more than the luminescence starting voltage V 0 for attaining higher luminous efficiency, more preferably set in the range of at least 80 % of the luminescence starting voltage V 0 and not more than the luminescence starting voltage V 0 , further preferably set in the range of at least 90 % of the luminescence starting voltage V 0 and not more than the luminescence starting voltage V 0 , and most preferably set to the luminescence starting voltage V 0 .
- the relation between the luminescence life and the duty ratio of the drive pulse was measured with reference to the organic EL element 20 shown in Fig. 1.
- the organic EL element 20 was driven with a constant current, and the drive pulse shown in Fig. 2 was applied to the organic EL element 20.
- the frequency of the drive pulse was set to 100 Hz
- the duty ratio of the drive pulse was set to 50 %, 10 % and 1%.
- the second voltage was set to the luminescence starting voltage V 0 .
- the organic EL element 20 was driven with a direct current (duty ratio: 100 %).
- Fig. 9 shows the results of measurement of drive pulse duty ratio dependency of the luminescence life.
- the horizontal axis shows the duty ratio of the drive pulse
- the vertical axis shows the ratio of improvement of the luminescence life upon pulse driving with respect to the luminescence life upon dc driving.
- the ratio of improvement of the luminescence life was about twice to three times that upon dc driving when the duty ratio of the drive pulse was 50 %, while a remarkable ratio of improvement of the luminescence life of at least 20 times was attained when the duty ratio of the drive pulse was not more than 10 %.
- the duty ratio of the drive pulse was not more than 1 %, the ratio of improvement of the luminescence life was slightly lowered as compared with that with the drive pulse having the duty ratio of 10 %.
- the duty ratio of the drive pulse was 0.2 %, a ratio of improvement of the luminescence life similar to that with the drive pulse having the duty ratio of 1 % was attained although this result is now shown in Fig. 9.
- the duty ratio of the drive pulse is preferably greater than zero and not more than 50 %.
- the duty ratio of the drive pulse is more preferably not more than 10 %.
- the duty ratio of the drive pulse is further preferably at least 0.2 %.
- Fig. 10 is a block diagram showing an exemplary structure of a display employing a drive according to the present invention.
- a plurality of organic EL elements 20 each having the structure shown in Fig. 1 are arranged on a common substrate in the form of a matrix of n rows and m columns, where n and m represent arbitrary integers of at least two.
- Hole injection electrodes 2 (see Fig. 1) of a plurality of organic EL elements 20 forming each row are connected with a drive signal line 31.
- a plurality of such drive signal lines 31 corresponding to the plurality of rows are connected with a row driver 30.
- the row driver 30 applies a drive signal to the plurality of drive signal lines 31 respectively.
- Electron injection electrodes 6 (see Fig. 1) of a plurality of organic EL elements 20 forming each column are connected with a column selection signal line 41.
- a plurality of such column selection signal lines 41 corresponding to the plurality of columns are connected with a column driver 40.
- the column driver 40 sequentially applies a column selection signal to the plurality of column selection lines 41.
- the drive signal applied to the drive signal lines 31 by the row driver 30 and the column selection signal applied to the column signal selection lines 41 by the column driver 40 are so set as to apply the first voltage V 3 higher than the luminescence starting voltage V 0 to luminous organic EL elements 20 while applying the second voltage, equal to the luminescence starting voltage V 0 , to non-luminous organic EL elements 20.
- Fig. 11 is a waveform diagram showing exemplary drive pulses applied to organic EL elements 20 of first to n-th rows of the display shown in Fig. 10.
- the first voltage higher than the luminescence starting voltage V 0 is applied to the organic EL elements 20 in a luminous state while the second voltage equal to the luminescence starting voltage V 0 is applied to the organic EL elements 20 in a non-luminous state.
- the luminance varies with the level of the first voltage applied to the organic EL elements 20 in the luminous state.
- the organic EL element 20 shown in Fig. 1 has the organic luminescent layer 4 formed by doping TPD with rubrene, the drive and the driving method according to the present invention are also readily applicable to various organic EL elements employing other organic materials.
Abstract
Description
- The present invention relates to a driving method for an organic electroluminescence element, a drive for an organic electroluminescence element and a display employing the same.
- Following recent diversification of information apparatuses, requirement for flat display elements having smaller power consumption as compared with generally used CRTs (cathode ray tubes) is increased. A display employing an organic eletroluminescence element (hereinafter referred to as an organic EL element) is actively subjected to research and development as one of such flat display elements. The display employing an organic EL element has average or high efficiency, is thin and lightweight, and has no viewing angle dependency.
- The organic EL element injects electrons and holes into a luminescent part from an electron injection electrode and a hole injection electrode respectively for recombining the electrons and the holes at the luminescence center and exciting organic molecules for fluorescing when the organic molecules return from the excited state to a ground state. The organic EL element is formed by a plurality of luminous elements arranged on a substrate in the form of a matrix.
- Such an organic EL element can advantageously be driven with a low voltage of about 5 V to 20 V. Further, the organic EL element, capable of obtaining luminous elements luminescing in proper colors by selecting fluorescent materials serving as luminescent materials, is expected for application to a multi- or full-color display. In addition, the organic EL element capable of surface luminescence with a low voltage can also be employed as a backlight for a display such as a liquid crystal display.
- Reliability is an important factor for putting the organic EL element into practice. While a number of organic luminescent materials and doping materials have heretofore been studied for improving the luminescence life of the organic EL element, a method of increasing the life of the organic EL element by controlling a current supplied to the organic EL element is recently studied.
- For example, each of Japanese Patent Laying-Open Nos. 11-3060 (1999) and 11-8064 (1999) discloses a technique of successively applying a forward-bias voltage and a reverse-bias voltage to an organic EL element.
- In order to apply the organic EL element to a display, however, it is necessary to improve luminous efficiency (the ratio of luminance to applied power (unit: lm/W)) while attaining a long life. Therefore, it is desired to attain a long luminescence life while ensuring high luminous efficiency.
- An object of the present invention is to provide a driving method for an organic electroluminescence element capable of attaining a long luminescence life while ensuring high luminous efficiency over a long period.
- Another object of the present invention is to provide a drive for an organic electroluminescence element capable of attaining a long luminescence life while ensuring high luminous efficiency over a long period.
- Still another object of the present invention is to provide a display employing an organic electroluminescence element capable of attaining a long luminescence life while ensuring high luminous efficiency over a long period.
- According to an aspect of the present invention, a driving method for an organic electroluminescence element having a pair of electrodes comprises a step of applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than the luminescence starting voltage across the electrodes of the organic electroluminescence element.
- When the first voltage exceeding the luminescence starting voltage is applied across the electrodes of the organic electroluminescence element in the driving method, a current flows to the organic electroluminescence element so that the organic electroluminescence element luminesces.
- At this time, charging is caused due to influence by the capacitance of the organic electroluminescence element itself when rising to the first voltage so that the current is instantaneously increased and thereafter stabilized to a constant value. In this case, the second voltage higher than 0 V and not more than the luminescence starting voltage has been applied across the electrodes of the organic electroluminescence element before rising to the first voltage, whereby the charging time is reduced. Therefore, luminous intensity is increased to a constant level in a shorter time.
- When the second voltage is applied across the electrodes of the organic electroluminescence element, charges stored in the organic electroluminescence element upon application of the first voltage flow in the organic electroluminescence element for some time without flowing out. Thus, holes and electrons recombine to contribute to the luminescence. Consequently, the luminescence continues for a constant time also after falling to the second voltage, and thereafter stops.
- Thus, the luminous intensity is increased to the constant level in a short time upon application of the first voltage and the luminescence continues in the initial stage of application of the second voltage, whereby the total luminescent time is increased. Further, it is not necessary to apply a high voltage for obtaining prescribed average luminance. Consequently, high luminous efficiency is attained.
- The first and second voltages are alternately applied to the organic electroluminescence element so that the organic electroluminescence element intermittently luminesces, whereby reduction of the luminance remains small over a long period and a long life is attained.
- The second voltage is preferably substantially equal to the luminescence starting voltage. In this case, higher luminous efficiency can be attained.
- The duty ratio of the drive pulse is preferably greater than zero and not more than 50 %. Thus, the luminescence life of the organic electroluminescence element is improved. In particular, the duty ratio of the drive pulse is more preferably not more than 10 %. Thus, the luminescence life of the organic electroluminescence element is further improved. The duty ratio of the drive pulse is further preferably at least 0.2 %. Further, the duty ratio or the drive pulse may be at least 1 %. Thus, the luminescence life of the organic electroluminescence element is further improved.
- According to another aspect of the present invention, a drive for driving an organic electroluminescence element having a pair of electrodes comprises a drive pulse application circuit applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than the luminescence starting voltage across the electrodes of the organic electroluminescence element.
- In this drive, luminous intensity is increased to a constant level in a short time upon application of the first voltage and the luminescence continues in an initial stage of application of the second voltage, whereby the total luminescent time is increased. Further, it is not necessary to apply a high voltage for obtaining prescribed average luminance. Consequently, high luminous efficiency is attained.
- The first and second voltages are alternately applied to the organic electroluminescence element so that the organic electroluminescence element intermittently luminesces, whereby reduction of luminance remains small over a long period and a long life is attained.
- The second voltage is preferably substantially equal to the luminescence starting voltage. In this case, higher luminous efficiency can be attained.
- The duty ratio of the drive pulse is preferably greater than zero and not more than 50 %. Thus, the luminescence life of the organic electroluminescence element is improved. In particular, the duty ratio of the drive pulse is more preferably not more than 10 %. Thus, the luminescence life of the organic electroluminescence element is further improved. The duty ratio of the drive pulse is further preferably at least 0.2 %. Further, the duty ratio or the drive pulse may be at least 1 %. In this case, the luminescence life of the organic electroluminescence element is further improved.
- According to still another aspect of the present invention, a display comprises one or a plurality of organic electroluminescence elements each having a pair of electrodes and a drive applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than the luminescence starting voltage across the electrodes of each of one or a plurality of organic electroluminescence elements.
- In the display, luminous intensity is increased to a constant level in a short time upon application of the first voltage and the luminescence continues in an initial stage of application of the second voltage, whereby the total luminescent time is increased. Further, it is not necessary to apply a high voltage for obtaining prescribed average luminance. Consequently, high luminous efficiency is attained.
- The first and second voltages are alternately applied to the organic electroluminescence element so that the organic electroluminescence element intermittently luminesces, whereby reduction of luminance remains small over a long period and a long life is attained.
- The drive may include a first driver supplying a drive signal to one of the electrodes of each of one or a plurality of organic electroluminescence elements and a second driver supplying a selection signal to the other one of the electrodes of each of one or a plurality of organic electroluminescence elements, and the drive signal and the selection signal may be so set as to apply the drive pulse across the electrodes of a selected organic electroluminescence element.
- In this case, the drive pulse alternately changing between the first voltage higher than the luminescence starting voltage and the second voltage higher than 0 V and not more than the luminescence starting voltage is applied across the electrodes of the selected organic electroluminescence element by the first and second drivers, so that the selected organic electroluminescence element luminesces.
- Each organic electroluminescence element includes a transparent substrate, a hole injection electrode, an organic luminescent layer and an electron injection electrode in this order, and the pair of electrodes are the hole injection electrode and the electron injection electrode.
- The second voltage is preferably substantially equal to the luminescence starting voltage. In this case, higher luminous efficiency can be attained.
- The duty ratio of the drive pulse is preferably greater than zero and not more than 50 %. Thus, the luminescence life of the organic electroluminescence element is improved. In particular, the duty ratio of the drive pulse is more preferably not more than 10 %. Thus, the luminescence life of the organic electroluminescence element is further improved. The duty ratio of the drive pulse is further preferably at least 0.2 %. Further, the duty ratio or the drive pulse may be at least 1 %. Thus, the luminescence life of the organic electroluminescence element is further improved.
- The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
- Fig. 1 is a model diagram showing an exemplary structure of an organic EL element and a drive pulse application circuit;
- Fig. 2 is a voltage waveform diagram of a drive pulse applied by the drive pulse application circuit shown in Fig. 1 to the organic EL element;
- Fig. 3 illustrates a drive voltage applied to the organic EL element, a current flowing in the organic EL element and luminous intensity of the organic EL element in Inventive Example;
- Fig. 4 illustrates a drive voltage applied to the organic EL element, a current flowing in the organic EL element and luminous intensity of the organic EL element in comparative example 1;
- Fig. 5 illustrates a drive voltage applied to the organic EL element, a current flowing in the organic EL element and luminous intensity of the organic EL element in comparative example 2;
- Fig. 6 illustrates the relation between luminous efficiency of the organic EL element and the drive voltage;
- Fig. 7 illustrates time changes of luminance in a continuous luminescence experiment on Inventive Example and comparative examples 1 and 2;
- Fig. 8 illustrates time changes of luminous efficiency in the continuous luminescence experiment on Inventive Example and comparative examples 1 and 2;
- Fig. 9 illustrates results of measurement of drive pulse duty ratio dependency of a luminescence life;
- Fig. 10 is a block diagram showing an exemplary structure of a display employing a drive according to the present invention; and
- Fig. 11 is a waveform diagram showing exemplary drive pulses applied to organic EL elements of respective rows of the display shown in Fig. 10.
-
- Fig. 1 is a model diagram showing an exemplary structure of an organic electroluminescence element (hereinafter referred to as an organic EL element) 20 and a drive
pulse application circuit 10. - In the
organic EL element 20 shown in Fig. 1, ahole injection electrode 2 of ITO (indium-tin oxide) serving as a transparent conductive film is formed on aglass substrate 1. Ahole transport layer 3 of 200 Å in thickness made of MTDATA (4,4',4''-tris(3-methylphenylphenylamino)triphenyl-amine) having a molecular structure expressed in the following chemical formula (1) is formed on thehole injection electrode 2. An organicluminescent layer 4 of 300 Å in thickness prepared by doping TPD (N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) having a molecular structure expressed in the following chemical formula (2) with 5 % of rubrene having a molecular structure expressed in the following chemical formula (3) is formed on thehole transport layer 3. Anelectron transport layer 5 of 500 Å in thickness made of Alq3 (tris(8-hydroxy-quinolinate)aluminum) having a molecular structure expressed in the following chemical formula (4) is formed on the organicluminescent layer 4. Anelectron injection electrode 6 of 2000 Å in thickness made of MgIn is formed on theelectron transport layer 5. - The drive
pulse application circuit 10 is connected between thehole injection electrode 2 and theelectron injection electrode 6 of theorganic EL element 20. The drivepulse application circuit 10 applies a drive pulse to theorganic EL element 20 as described later. Thus, the organicluminescent layer 4 of theorganic EL element 20 luminesces so that light 100 outgoes from the rear surface of theglass substrate 1. - Fig. 2 is a voltage waveform diagram of the drive pulse applied by the drive
pulse application circuit 10 shown in Fig. 1 to theorganic EL element 20. - As shown in Fig. 2, the drive pulse alternately changes between a first voltage higher than a luminescence starting voltage (luminescence threshold voltage) V0 and a second voltage higher than 0 V and lower than the luminescence starting voltage V0. The frequency of the drive pulse is in the range of 30 Hz to 10 kHz, for example.
- The period when the drive pulse is at the first voltage is referred to as a luminous period T1, and the period when the drive pulse is at the second voltage is referred to as a non-luminous period T2.
- The
organic EL element 20 is substantially luminous in the luminous period T1 and substantially non-luminous in the non-luminous period T2. More correctly, the luminous period T1 includes a non-luminous time of theorganic EL element 20, and the non-luminous period T2 includes a luminous time of theorganic EL element 20, as described later. - A continuous luminescence experiment was made by applying drive voltages to the
organic EL element 20 by driving methods according to Inventive Example and comparative examples 1 and 2 for analyzing voltage waveforms, current waveforms and luminous intensity values and examining time changes of luminance and luminous efficiency. - The drive pulse shown in Fig. 2 was applied to the
organic EL element 20 by the drivepulse application circuit 10 shown in Fig. 1 in Inventive Example, while a constant voltage higher than the luminescence starting threshold voltage V0 was applied to theorganic EL element 20 in comparative example 1 and a forward bias voltage and a reverse bias voltage were alternately applied to theorganic EL element 20 in comparative example 2. - Fig. 3 shows the drive voltage applied to the
organic EL element 20, a current flowing in theorganic EL element 20 and luminous intensity of theorganic EL element 20 in Inventive Example. Fig. 4 shows the drive voltage applied to theorganic EL element 20, a current flowing in theorganic EL element 20 and luminous intensity of theorganic EL element 20 in comparative example 1. Fig. 5 shows the drive voltage applied to theorganic EL element 20, a current flowing in theorganic EL element 20 and luminous intensity of theorganic EL element 20 in comparative example 2. Referring to Fig. 5, the period when the drive voltage is set to the forward bias voltage is referred to as a luminous period T1, and the period when the drive voltage is set to the reverse bias voltage is referred to as a non-luminous period T2. - In this continuous luminescence experiment, each drive voltage was so set that the average value of the current flowing in the
organic EL element 20 was regularly constant from starting of luminescence to the end of the experiment, for driving theorganic EL element 20 with a constant current. When theorganic EL element 20 is deteriorated by continuous luminescence and internal resistance is increased, therefore, it follows that the drive voltage gradually increases. - Table 1 shows experimental conditions for Inventive Example and comparative examples 1 and 2.
INVENTIVE EXAMPLE FREQUENCY:1kHz DUTY RATIO OF DRIVE VOLTAGE:50% FIRST VOLTAGE (INITIAL):V3=5.4V SECOND VOLTAGE:V0=3.8V COMPARATIVE EXAMPLE 1 DRIVE VOLTAGE: DC 5 V COMPARATIVE EXAMPLE2 FREQUENCY:1kHz DUTY RATIO OF DRIVE VOLTAGE:50% FORWARD BIAS VOLTAGE (INITIAL):V2=5.8V REVERSE BIAS VOLTAGE (CONSTANT):-V1=-10V - In this continuous luminescence experiment, the drive voltages were so adjusted as to attain the same average luminance in Inventive Example and comparative examples 1 and 2. Initial average luminance levels of Inventive Example and comparative examples 1 and 2 were set to the same level of 300 cd/m2.
- As shown in Fig. 3, the drive voltage is set to a first voltage V3 higher than the luminescence starting voltage V0 in the luminous period T1 in Inventive Example. Further, the drive voltage is set to a second voltage identical to the luminescence starting voltage V0 in the non-luminous period T2.
- When the drive voltage is set to the first voltage V3 in the luminous period T1, a current flows in the
organic EL element 20, which in turn luminesces. When the drive voltage rises, charging is caused due to influence by the capacitance of theorganic EL element 20 itself so that the current is instantaneously increased and thereafter stabilized to a constant value. In this case, a charging time t1 is reduced since the drive voltage has been set to the luminescence threshold voltage V0 in the preceding non-luminous period T2. Therefore, luminous intensity is increased to a constant level in a shorter time t2. - When the drive voltage is set to the luminescence threshold voltage V0 in the non-luminous period T2, the current flowing in the
organic EL element 20 reaches zero. At this time, charges stored in theorganic EL element 20 in the luminous period T1 flow in theorganic EL element 20 for some time without flowing out. Thus, holes and electrons recombine to contribute to the luminescence. Consequently, the luminescence continues in an initial time t3 of the non-luminous period T2. Thereafter the luminescence of theorganic EL element 20 stops. - As shown in Fig. 4, the drive voltage is set to a constant voltage higher than the luminescence starting voltage V0 in comparative example 1. Thus, a constant current continuously flows in the
organic EL element 20, which in turn continuously luminesces with constant luminous intensity. - As shown in Fig. 5, the drive voltage is set to a forward bias voltage V2 higher than the luminescence starting voltage V0 in the luminous period T1 in comparative example 2. Further, the drive voltage is set to a reverse bias voltage -V1 in the non-luminous period T2. In this case, the drive voltage in the luminous period T1 is set higher than the first voltage V3 in Inventive Example and the drive voltage in comparative example 1, in order to equalize average luminance of comparative example 2 with those of Inventive Example and comparative example 1.
- When the drive voltage is set to the forward bias voltage V2 in the luminous period T1, a current flows in the
organic EL element 20, which in turn luminesces. Charging is caused in an initial period of the luminous period T1 due to influence by the capacitance of theorganic EL element 20 itself so that a large current flows in theorganic EL element 20 and the current is thereafter stabilized to a constant value. Therefore, theorganic EL element 20 remains non-luminous in the initial stage of the luminous period T1 while luminous intensity starts to increase in a delay by a prescribed time from application of the forward bias voltage V2 and is thereafter stabilized to a constant level. In this case, a charging time t4 is lengthened since the drive voltage has been set to the reverse bias voltage -V1 in the preceding non-luminous period T2. Consequently, a time t6 required for the luminous intensity to rise to the constant level is lengthened. - When the drive voltage is set to the reverse bias voltage -V1 in the non-luminous period T2, a large reverse current first flows due to discharge of charges stored in the
organic EL element 20, and thereafter the current approaches zero due to rectification of theorganic EL element 20 and reaches zero at a time t5. In this case, the charges stored in theorganic EL element 20 are immediately reversely discharged and hence the luminescence stops simultaneously with application of the reverse bias voltage -V1. - Fig. 6 illustrates the relation between the luminous efficiency of the
organic EL element 20 and the drive voltage. As shown in Fig. 6, the luminous efficiency of theorganic EL element 20 is maximized when the drive voltage is set to the luminescence starting voltage V0, and reduced as the drive voltage is increased. Therefore, the luminous efficiency is reduced when driving theorganic EL element 20 with a high voltage. - In the driving method according to Inventive Example, the luminous intensity is increased to the prescribed level in a short time in the luminous period T1 while the luminescence of the
organic EL element 20 continues in the initial stage of the non-luminous period T2 as shown in Fig. 3, whereby the total luminescent time is increased. Further, the drive voltage can be set lower than that in comparative example 2 in order to attain prescribed average luminance. Consequently, the luminous intensity is increased. - In the driving method according to comparative example 2, a long time is required for increasing the luminous intensity to a prescribed level in the luminous period T1 while the
organic EL element 20 is absolutely non-luminous in the non-luminous period T2, and hence the total luminescent time is reduced. Further, the drive voltage must be set high for attaining prescribed average luminance. Thus, the luminous efficiency is reduced. - When setting the drive voltage not to the reverse bias voltage -V1 but to 0 V in the non-luminous period T2, the luminous efficiency exhibits a tendency similar to that shown in Fig. 5 although the same is slightly improved as compared with comparative example 2.
- Fig. 7 illustrates time changes of luminance in the continuous luminescence experiment on Inventive Example and comparative examples 1 and 2. The half-life of the luminance is regarded as the life.
- The life of comparative example 1 was about 3000 hours, while that of comparative example 2 was about 10000 hours. The life of Inventive Example was about 7000 hours, i.e., at least twice that of comparative example 1.
- Fig. 8 illustrates time changes of luminous efficiency in the continuous luminescence experiment on Inventive Example and comparative examples 1 and 2.
- As shown in Fig. 8, comparative example 2 exhibited low initial luminous efficiency of 3.8 (lm/W), which was gradually reduced following continuous luminescence. Comparative example 1 exhibited the highest initial luminous efficiency of 5.0 (lm/W), which was abruptly reduced following continuous luminescence to reach a level similar to the luminous efficiency of comparative example 2 after a lapse of 3000 hours. While Inventive Example exhibited initial luminous efficiency of 4.6 (lm/W) slightly lower than that of comparative example 1, reduction of the luminous efficiency following continuous luminescence was so small that the luminous efficiency of Inventive Example exceeded that of comparative example 1 after a lapse of several 100 hours. Further, Inventive Example constantly exhibited higher luminous efficiency than comparative example 2.
- According to a drive and a driving method of Inventive Example, as hereinabove described, a long luminescence life can be attained while ensuring high luminous efficiency, and reduction of the luminous efficiency remains small in continuous luminescence over a long period.
- In the driving method according to the present invention, the second voltage of the drive pulse is preferably set in the range of at least 70 % of the luminescence starting voltage V0 and not more than the luminescence starting voltage V0 for attaining higher luminous efficiency, more preferably set in the range of at least 80 % of the luminescence starting voltage V0 and not more than the luminescence starting voltage V0, further preferably set in the range of at least 90 % of the luminescence starting voltage V0 and not more than the luminescence starting voltage V0, and most preferably set to the luminescence starting voltage V0.
- The relation between the luminescence life and the duty ratio of the drive pulse was measured with reference to the
organic EL element 20 shown in Fig. 1. Theorganic EL element 20 was driven with a constant current, and the drive pulse shown in Fig. 2 was applied to theorganic EL element 20. In this case, the frequency of the drive pulse was set to 100 Hz, and the duty ratio of the drive pulse was set to 50 %, 10 % and 1%. The second voltage was set to the luminescence starting voltage V0. For the purpose of comparison, theorganic EL element 20 was driven with a direct current (duty ratio: 100 %). - Fig. 9 shows the results of measurement of drive pulse duty ratio dependency of the luminescence life. Referring to Fig. 9, the horizontal axis shows the duty ratio of the drive pulse, and the vertical axis shows the ratio of improvement of the luminescence life upon pulse driving with respect to the luminescence life upon dc driving. When the luminance is reduced to 70 % of the initial level, the
organic EL element 20 is regarded as getting to the end of its life. - As shown in Fig. 9, the ratio of improvement of the luminescence life was about twice to three times that upon dc driving when the duty ratio of the drive pulse was 50 %, while a remarkable ratio of improvement of the luminescence life of at least 20 times was attained when the duty ratio of the drive pulse was not more than 10 %. When the duty ratio of the drive pulse was not more than 1 %, the ratio of improvement of the luminescence life was slightly lowered as compared with that with the drive pulse having the duty ratio of 10 %. Also when the duty ratio of the drive pulse was 0.2 %, a ratio of improvement of the luminescence life similar to that with the drive pulse having the duty ratio of 1 % was attained although this result is now shown in Fig. 9.
- It is understood from the aforementioned results that the duty ratio of the drive pulse is preferably greater than zero and not more than 50 %. The duty ratio of the drive pulse is more preferably not more than 10 %. The duty ratio of the drive pulse is further preferably at least 0.2 %.
- Fig. 10 is a block diagram showing an exemplary structure of a display employing a drive according to the present invention.
- In the display shown in Fig. 10, a plurality of
organic EL elements 20 each having the structure shown in Fig. 1 are arranged on a common substrate in the form of a matrix of n rows and m columns, where n and m represent arbitrary integers of at least two. - Hole injection electrodes 2 (see Fig. 1) of a plurality of
organic EL elements 20 forming each row are connected with adrive signal line 31. A plurality of suchdrive signal lines 31 corresponding to the plurality of rows are connected with arow driver 30. Therow driver 30 applies a drive signal to the plurality ofdrive signal lines 31 respectively. - Electron injection electrodes 6 (see Fig. 1) of a plurality of
organic EL elements 20 forming each column are connected with a columnselection signal line 41. A plurality of such columnselection signal lines 41 corresponding to the plurality of columns are connected with acolumn driver 40. Thecolumn driver 40 sequentially applies a column selection signal to the plurality of column selection lines 41. - The drive signal applied to the
drive signal lines 31 by therow driver 30 and the column selection signal applied to the columnsignal selection lines 41 by thecolumn driver 40 are so set as to apply the first voltage V3 higher than the luminescence starting voltage V0 to luminousorganic EL elements 20 while applying the second voltage, equal to the luminescence starting voltage V0, to non-luminousorganic EL elements 20. - Fig. 11 is a waveform diagram showing exemplary drive pulses applied to
organic EL elements 20 of first to n-th rows of the display shown in Fig. 10. As shown in Fig. 11, the first voltage higher than the luminescence starting voltage V0 is applied to theorganic EL elements 20 in a luminous state while the second voltage equal to the luminescence starting voltage V0 is applied to theorganic EL elements 20 in a non-luminous state. The luminance varies with the level of the first voltage applied to theorganic EL elements 20 in the luminous state. - While the
organic EL element 20 shown in Fig. 1 has the organicluminescent layer 4 formed by doping TPD with rubrene, the drive and the driving method according to the present invention are also readily applicable to various organic EL elements employing other organic materials. - Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.
Claims (14)
- A driving method for an organic electroluminescence element having a pair of electrodes,comprising a step of applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than said luminescence starting voltage across said electrodes of said organic electroluminescence element.
- The driving method according to claim 1, whereinsaid second voltage is substantially equal to said luminescence starting voltage.
- The driving method according to claim 1, whereinthe duty ratio of said drive pulse is greater than zero and not more than 50 %.
- The driving method according to claim 3, whereinthe duty ratio of said drive pulse is not more than 10 %.
- A drive for driving an organic electroluminescence element having a pair of electrodes, comprising:a drive pulse application circuit applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than said luminescence starting voltage across said electrodes of said organic electroluminescence element.
- The drive according to claim 5, whereinsaid second voltage is substantially equal to said luminescence starting voltage.
- The drive according to claim 5, whereinthe duty ratio of said drive pulse is greater than zero and not more than 50 %.
- The drive according to claim 7, whereinthe duty ratio of said drive pulse is not more than 10 %.
- A display comprising:one or a plurality of organic electroluminescence elements each having a pair of electrodes; anda drive applying a drive pulse alternately changing between a first voltage exceeding a luminescence starting voltage and a second voltage higher than 0 V and not more than said luminescence starting voltage across said electrodes of each of said one or a plurality of organic electroluminescence elements.
- The display according to claim 9, whereinsaid drive includes:a first driver supplying a drive signal to one of said electrodes of each of said one or a plurality of organic electroluminescence elements, anda second driver supplying a selection signal to the other one of said electrodes of each of said one or a plurality of organic electroluminescence elements, andsaid drive signal and said selection signal are so set as to apply said drive pulse across said electrodes of selected said organic electroluminescence element.
- The display according to claim 10, whereineach of said organic electroluminescence elements include a transparent substrate, a hole injection electrode, an organic luminescent layer and an electron injection electrode in this order, and said pair of electrodes are said hole injection electrode and said electron injection electrode.
- The display according to claim 10, whereinsaid second voltage is substantially equal to said luminescence starting voltage.
- The display according to claim 10, whereinthe duty ratio of said drive pulse is greater than zero and not more than 50 %.
- The display according to claim 13, wherein the duty ratio of said drive pulse is not more than 10 %.
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EP1434469A4 (en) * | 2001-08-09 | 2006-08-09 | Idemitsu Kosan Co | Organic electroluminescence display and its driving method |
US9153168B2 (en) * | 2002-07-09 | 2015-10-06 | Semiconductor Energy Laboratory Co., Ltd. | Method for deciding duty factor in driving light-emitting device and driving method using the duty factor |
CN1525428B (en) * | 2003-06-02 | 2011-04-20 | 友达光电股份有限公司 | Apparatus, method, and control method for driving led with ac power |
US7733032B2 (en) | 2005-10-26 | 2010-06-08 | Panasonic Electric Works Co., Ltd. | OLED driver and lighting apparatus equipped with the same |
WO2010146523A1 (en) * | 2009-06-19 | 2010-12-23 | Koninklijke Philips Electronics N.V. | Method and device for driving an oled device |
US8749166B2 (en) | 2009-06-19 | 2014-06-10 | Koninklijke Philips N.V. | Method and device for driving an OLED device |
RU2546959C2 (en) * | 2009-06-19 | 2015-04-10 | Конинклейке Филипс Электроникс Н.В. | Method and device for excitation of oled device |
US10416466B2 (en) | 2012-05-09 | 2019-09-17 | Semiconductor Energy Laboratory Co., Ltd. | Display device and electronic device |
Also Published As
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EP1085496A3 (en) | 2002-08-07 |
JP2001085159A (en) | 2001-03-30 |
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