Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-05-31
2004-10-26
Clinger, James (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169400, C345S077000, C349S069000
Reexamination Certificate
active
06809482
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an OLED (organic light emitting device) panel in which a light emitting element such as an OLED formed on a substrate is sealed between the substrate and a cover member, and to a method of driving the OLED panel. The invention also relates to an OLED module obtained by mounting an IC that includes a controller to the OLED panel. In this specification, a light emitting device is used as the generic term for the OLED panel and the OLED module. Also included in the present invention is electronic equipment using the light emitting device.
2. Description of the Related Art
In recent years, a technique of forming a TFT on a substrate has made great advancement to promote application of TFTs to active matrix display devices. In particular, TFTs using polysilicon have higher field effect mobility (also called mobility) than conventional TFTs that use amorphous silicon and therefore can operate at high speed. This makes it possible to control pixels, which has conventionally been controlled by a driving circuit external to the substrate, by a driving circuit formed on the same substrate on which the pixels are formed.
With various circuits and elements formed on the same substrate, active matrix display devices can have many advantages including lowering of manufacture cost, reduction in display device size, an increase in yield, and improvement in throughput.
An active matrix light emitting device having an OLED as a self-luminous element (hereinafter simply referred to as light emitting device) is being researched actively. A light emitting device is also called as an organic EL display (OELD) or an organic light emitting diode (OLED).
Being self-luminous, an OLED does not need back light which is necessary in liquid crystal display devices (LCDs) and is therefore easy to make a thinner device. In addition, a self-luminous OLED has high visibility and no limitation in terms of viewing angle. These are the reasons why light emitting devices using OLEDs are attracting attention as display devices to replace CRTs and LCDs.
An OLED has a layer containing an organic compound (organic light emitting material) that provides luminescence (electroluminescence) when an electric field is applied (the layer is hereinafter referred to as organic light emitting layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). A light emitting device according to the present invention can use one or both types of light emission.
In this specification, all the layers that are provided between an anode and a cathode of an OLED together make an organic light emitting layer. Specifically, an organic light emitting layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc.
A basic structure of an OLED is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order, or the like.
A pixel portion of a light emitting device generally has the structure shown in
FIG. 21. A
pixel portion
1701
is provided with a plurality of gate signal lines
1705
, a plurality of source signal lines
1706
, and a plurality of power supply lines
1707
.
A region that has one of the gate signal lines
1705
, one of the source signal lines
1706
, and one of the power supply lines
1707
corresponds to a pixel
1702
. The pixel
1702
and similarly structured pixels form a matrix in the pixel portion
1701
. Each pixel has an OLED
1703
. The OLED
1703
has an anode and a cathode. In this specification, the cathode is called an opposite electrode (second electrode) when the anode is used as a pixel electrode (first electrode) whereas the anode is called the opposite electrode when the cathode serves as the pixel electrode.
The opposite electrode in every OLED
1703
receives a given voltage from a power supply
1704
that is external to the OLED panel. The voltage between the opposite electrode and the pixel electrode is called an OLED drive voltage in this specification.
An enlarged view of the pixel
1702
is shown in FIG.
22
. The pixel
1702
has the OLED
1703
, a first TFT
1708
that functions as a switching element, a second TFT
1709
that controls a current flowing between the pixel electrode and opposite electrode of the OLED
1703
, and a capacitor (storage capacitor)
1710
.
A gate electrode of the first TFT
1708
is connected to one of the gate signal lines
1705
. The first TFT
1708
has a source region and a drain region one of which is connected to one of the source signal lines
1706
for receiving digital signals and the other of which is connected to a gate electrode of the second TFT
1709
.
The second TFT
1709
has a source region and a drain region one of which is connected to one of the power supply lines
1707
and the other of which is connected to a pixel electrode of the OLED
1703
. Of two electrodes the capacitor
1710
has, one is electrically connected to one of the power supply lines
1707
and the other is electrically connected to the gate electrode of the second TFT
1709
.
Next, a method of driving the light emitting device shown in
FIGS. 21 and 22
is described. The description here takes as an example gray scale display using n-bit digital signals.
When an image is displayed using n-bit digital signals, one frame period is divided into at least n sub-frame periods. Each sub-frame period consists of a period for inputting digital signals to pixels (writing period) and a period for display by the pixels in accordance with the bits of the digital signals written.
In a writing period, the voltage of the opposite electrode of every OLED
1703
is kept at the same level as the voltage of the power supply lines
1707
by the power supply
1704
. The plural gate signal lines
1705
are selected one by one and the first TFT
1708
is turned ON in order when a gate signal line to which its gate electrode is connected is selected. In this specification, a signal line being selected means turning ON every TFT whose gate electrode is connected to the selected signal line.
When digital signals are inputted to the plural source signal lines
1706
separately, the digital signals are inputted to the gate electrode of the second TFT
1709
through the first TFT
1708
that has been turned ON. The voltage of the digital signals is held in the capacitor
1710
.
A digital signal has ‘0’ information or ‘1’ information. A ‘0’ signal is a Lo voltage signal and a ‘1’ signal is a Hi voltage signal, or it may be the other way around.
The gate signal lines
1705
are selected one by one until all of them are selected once and digital signals are inputted to every pixel. Inputting a digital signal to a pixel means inputting a digital signal to the gate electrode of the second TFT
1709
. A period required to complete inputting digital signals to all pixels in the pixel portion
1701
is called a writing period.
When completing inputting digital signals to all pixels, a writing period is ended to start a display period. As a display period is started, the power supply
1704
changes the voltage of the opposite electrode in each OLED
1703
to generate a voltage between the opposite electrode and the power supply lines
1707
.
If a digital signal inputted to a pixel during a writing period has information of ‘0’, the second TFT
1709
is turned OFF and the OLED
1703
does not emit light. On the other hand, if the digital signal has information of ‘1’, the second TFT
1709
is turned ON to g
Clinger James
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Semiconductor Energy Laboratory Co,. Ltd.
Tran Chuc
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