Light-emitting devices

Details Light-emitting devices Light-emitting devices

2003-06-18

2004-10-05

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Plural power supplies

Plural cathode and/or anode load device

C257S103000

Reexamination Certificate

active

06801000

ABSTRACT:

DESCRIPTION
BACKGROUND OF THE INVENTION
The present invention relates to a light-emitting device, particularly an electroluminescent device and displays incorporating such devices. The electroluminescence for the electroluminescent device may be provided by means of an organic light-emissive material (see for example International Publication WO90/13148 which describes electroluminescent semi-conductive conjugated polymers, such as PPV).
By way of background,
FIG. 1
shows the typical cross-sectional structure of an organic light-emissive device. The device is fabricated on a substrate (
1
) coated with a transparent first electrode (
2
) such as indium-tin-oxide. The coated substrate is overcoated with at least one layer of a thin film of an electroluminescent organic material (
3
) and a final layer forming a second electrode (
4
) which is typically of metal. By using a transparent substrate (e.g. of glass or plastics material), light generated in the film (
3
) is able to leave the device by passing through the first electrode (
2
).
The performance of electroluminescent devices has advanced rapidly over the past few years. Due to their high efficiencies, the devices show potential for a wide range of display applications, from simple backlights to graphic displays, such as television screens, computer monitors and palm-top devices which may consist of several million pixels. However, there is considerable variation in the active lifetimes of red, green and blue organic electroluminescent systems, including polymer systems. For the purposes of the present specification, the active lifetime of an electroluminescent element is defined as the maximum time for which the element is able to produce at least a display-monitor level of brightness (for example, set at 100 cd/m
2
) when operating under a given drive scheme. For example, an electroluminescent device with a red light emitting polymer may have an active life of 30,000 hours at 5 volts, whereas a device with a blue light-emitting polymer may have an active life of only 1500 hours at the same voltage (see table 1).
The disparity in active lifetimes of organic light-emissive materials is significant because one factor in determining the useful life or service life of a graphic display incorporating such materials is governed by the shortest of the active lifetimes of the different polymers employed. (Another factor concerns decay rates causing colour shift, which can reduce overall colour purity, i.e. ‘white’ becomes ‘off-white’, and possibly also produce non-uniformity in the display). Accordingly, attempts have been made to improve the service life of graphic displays. For example, research has been conducted into upgrading the active lifetime of the ‘weak link’ in such displays; namely, the relatively short-lived blue light-emitting polymers. Also, systems have been devised to compensate the device driving current—either by using a sensing mechanism or by predicting the rate of performance decay in complex drive compensation electronics—to maintain optimal performance with time. However, compensation mechanisms require complex and expensive circuitry which may also impose restrictions on the available aperture ratio.
SUMMARY OF THE INVENTION
An object of the present invention is to improve the service life of graphic displays incorporating organic light-emissive materials.
In accordance with a first aspect of the present invention, there is provided a light-emitting device comprising: a first electroluminescent element for emitting light of a first colour when energised; and a second electroluminescent element for emitting light of a second colour when energised, the first electroluminescent element having an active lifetime which is greater than that of the second electroluminescent element, characterised in that the second element is configured to operate at a lower brightness than the first element.
The brightness or luminescence of light emitted by an object may be measured in candela per square meter, and is a measure of the amount of light (number of photons) emitted per second per unit solid angle per unit area, as corrected for the sensitivity of the eye. The instantaneous brightness may vary—intentionally or otherwise—from one moment to another. When considering light-emitting devices for use in graphic displays, variations in instantaneous brightness may occur too rapidly to be detected by the human eye. Accordingly, the “brightness” which is of interest to the present invention is time-averaged to the extent necessary to smooth out localised or high speed variations in instantaneous brightness.
The first and second electroluminescent elements may comprise organic light emissive materials, and may be polymeric materials such as those discussed in WO90/13148 or WO92/03490.
The present applicant has appreciated the implication of the correlation between brightness and service life of devices employing organic light-emissive materials. The correlation is illustrated schematically in
FIG. 2
for two electroluminescent elements employing different organic light-emissive materials, for example a red light emitter (R) and a blue light emitter (B) which have different active lifetimes. The correlation for each may be summarised as a relatively high brightness being indicative of a relatively short service life and vice versa. If both organic light-emissive materials are operated continuously at the same level of brightness, the material with the shortest active lifetime will fail (i.e. reach the end of its active life) first (in the example, the blue will fail before the red) and the device will be judged to have failed prematurely at t
1
. However if the material with the shortest active lifetime is operated continuously at a lower level of brightness than the other material, the service life of the device will be extended to t
2
.
The ratio between the brightness (B
1
) of the first electroluminescent element and the brightness (B
2
) of the second electroluminescent element may be substantially equal to the ratio between the active lifetime (&tgr;
1
) of the first element and the active lifetime (&tgr;
2
) of the second element (i.e. B
1
/B
2
=&tgr;
1
/&tgr;
2
) Suppose, for example, that there is an order of magnitude difference in the active lifetime of the two elements (e.g. active lifetime of the first element is 30,000 hrs and the active lifetime of the second element is 3,000 hrs). If the two elements are to fail at substantially the same time, it may be necessary to operate the second element at one tenth of the brightness of the first element.
There may be another advantage to operating the second element (shorter active lifetime) at a lower brightness than the first element. When operated continuously, the amount of light emitted per unit time by the first and, second elements may decrease or decay with time, with the rate of decay perhaps being greater for the second element. Thus, the perceived colour of the light-emitting device with both elements energised will drift with time because the contribution by the second element to the overall light output slowly decreases. However, by operating the second element at a lower brightness than the second element may have the effect of retarding the rate of decay in the amount of light emitted per unit time. In other words, the rates of decay in the amount of light emitted per unit time by the first and second elements may become more even. Hence, the problem of perceived colour drift with time may be alleviated.
The first and second electroluminescent elements may be energised by a common potential difference, for example by using a common cathode. The correlation between brightness (or luminance in cd/m
2
) and voltage is illustrated schematically in
FIG. 3
for two electroluminescent elements employing different materials, for example a red light emitter (RED) and a blue light emitter (BLUE) which have different driving voltage characteristics. In fact, the red light emitter (RED) has a lower driving voltage characteristic than the blue light emit

Affiliated with

Also associated with

Rating

Light-emitting devices does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Light-emitting devices, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Light-emitting devices will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3287946