WO2001075853A1 - Organic electroluminescent device compensated pixel driver circuit - Google Patents

Organic electroluminescent device compensated pixel driver circuit Download PDF

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Publication number
WO2001075853A1
WO2001075853A1 PCT/GB2001/001460 GB0101460W WO0175853A1 WO 2001075853 A1 WO2001075853 A1 WO 2001075853A1 GB 0101460 W GB0101460 W GB 0101460W WO 0175853 A1 WO0175853 A1 WO 0175853A1
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WO
WIPO (PCT)
Prior art keywords
channel
transistors
driver circuit
current
pixel driver
Prior art date
Application number
PCT/GB2001/001460
Other languages
French (fr)
Inventor
Simon Tam
Original Assignee
Seiko Epson Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Seiko Epson Corporation filed Critical Seiko Epson Corporation
Publication of WO2001075853A1 publication Critical patent/WO2001075853A1/en

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Classifications

    • GPHYSICS
    • G09EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09GARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20Control 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
    • G09G3/22Control 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
    • G09G3/30Control 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
    • GPHYSICS
    • G09EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09GARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20Control 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
    • G09G3/22Control 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
    • G09G3/30Control 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
    • G09G3/32Control 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]
    • G09G3/3208Control 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]
    • G09G3/3225Control 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
    • G09G3/3233Control 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
    • G09G3/3241Control 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 the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror
    • G09G3/325Control 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 the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror the data current flowing through the driving transistor during a setting phase, e.g. by using a switch for connecting the driving transistor to the data driver
    • GPHYSICS
    • G09EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09GARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00Aspects of the constitution of display devices
    • G09G2300/08Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809Several active elements per pixel in active matrix panels
    • G09G2300/0842Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • GPHYSICS
    • G09EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09GARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00Aspects of the constitution of display devices
    • G09G2300/08Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809Several active elements per pixel in active matrix panels
    • G09G2300/0842Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G09G2300/0861Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor with additional control of the display period without amending the charge stored in a pixel memory, e.g. by means of additional select electrodes

Definitions

  • the present invention relates to an organic electroluminescent device and particularly to a compensated pixel driver circuit thereof.
  • An organic electroluminescent device consists of a light emitting polymer
  • TFT thin film transistor
  • Transistor Tj is provided to address the pixel and transistor T is provided to convert a data
  • a data signal is stored by a storage capacitor C s t ora g e when the pixel is not addressed.
  • the LEP material does, however, have relatively uniform characteristics.
  • TFT analog circuit the variation of threshold voltage, ⁇ N T , from device to device.
  • ⁇ N T threshold voltage
  • the current driven circuit also known as the current programmed threshold
  • Transistor T operates as an analog current control to
  • Transistor T connects between the drain and
  • transistor T 2 toggles transistor T 2 to act either as a diode or in a saturation
  • Transistor T acts as a switch in response to an applied waveform N GP .
  • Transistor T or transistor T can be ON at any one time. Initially, at time ⁇ shown in the
  • transistors T and T are OFF, and transistor T is ON.
  • transistor T is OFF, transistors T ⁇ and T are ON, and a current I DAT of known value is
  • transistor T operates as a diode
  • the detected threshold voltage of transistor T is stored by a capacitor C
  • Transistor T 4 is then turned ON by driving waveform N GP and the
  • transistor T used solely to represent the threshold voltage of transistor T .
  • a constant current is provided, in theory, during a subsequent active programming
  • the reproduction stage starts at time t •
  • the present invention seeks to provide, therefore, an improved compensated pixel
  • pixel driver circuit for an electroluminescent device, the circuit comprising an n-channel
  • the current supplied to the electroluminescent device in combination, the current supplied to the electroluminescent device.
  • the compensated pixel driver circuit also comprises respective storage
  • the compensated pixel driver circuit may also comprise respective
  • the first switching means and the source of current data are identical to each other.
  • the first switching means the source of current data
  • control in combination, the supply current to the electroluminescent device.
  • the method further comprises providing respective storage capacitors for
  • n-channel and p-channel transistors and respective switching means connected so as to
  • the method may comprise providing a programming stage during
  • organic electroluminescent display device comprising a compensated pixel driver circuit as .
  • Fig. 1 shows a conventional OELD pixel driver circuit using two transistors
  • Fig. 2 shows a known current programmed OELD driver circuit with threshold
  • Fig. 3 illustrates the concept of a compensated pixel driver circuit including a
  • Fig. 4 shows plots of characteristics for the complementary driver transistors
  • Fig. 5 shows a compensated pixel driver circuit arranged to operate as a voltage
  • Fig. 6 shows a compensated pixel driver circuit arranged to operate as a current
  • Fig. 7 shows a compensated current programmed driver circuit in accordance with a
  • Figs 8 to 11 show SPICE simulation results for the circuit illustrated in Fig. 6.
  • An OELD device is coupled between two transistors T u and T 12
  • Transistor T u is a p-channel transistor and transistor T 15 is an n-channel
  • circuit design is the threshold voltage V ⁇ . Any variation, ⁇ N T within a circuit has a
  • TFT's can be achieved by employing a pair of TFT's, one p-channel TFT and one n-channel TFT,
  • the driving current flowing to the OELD is a driving current flowing to the OELD.
  • Figure 4 illustrates the variation in drain current, that is the current flowing through
  • Figure 5 shows a compensated pixel driver circuit configured as a voltage driver
  • the circuit comprises p-channel transistor T 12 and n-channel transistor T 15 acting as
  • a complementary pair to provide, in combination, an analog current control for the OELD.
  • the circuit includes respective storage capacitors C 12 and C 15 and respective switching
  • transistors T A and T B coupled to the gates of transistors T 12 and T 15 .
  • transistors T A and T B are switched ON data voltage signals Vj and V 2 are stored respectively in storage
  • transistors T A and T B are identical to transistors T A and T B .
  • Figure 6 shows a compensated driver circuit according to the present invention
  • p-channel transistor T 12 and n-channel transistor T 15 are coupled so as to function as
  • Transistors Tj and T 6 connect respectively
  • Transistor T 3 is also connected to receive waveform V SEL .
  • Transistors T x are also connected to receive waveform V SEL .
  • T 6 are both p-channel transistors to ensure that the signals fed through these transistors
  • circuit shown in figure 6 operates in a similar manner to known current
  • OELD is controlled by the complementary opposite channel transistors T 12 and T 15 .
  • transistor T 4 is ON and transistors T 1? T 3 and T 6
  • Transistor T 4 is turned OFF at time t x by the waveform V GP and transistors T l5 T 3 and T 6 are turned ON at time t 3 by the waveform V SEL . With transistors T x and T 6 turned
  • the p-channel transistor T 12 and the complementary n-charmel transistor T 15 act in a
  • the driving waveform for the frame period concerned is available
  • Transistors T x , T 3 and T 6 are then switched OFF at time t 4 and transistor T 4 is
  • VDD under the control of the p-channel and n-channel transistors T 12 and T 15 operating in a
  • transistors T 12 and T 15 any variation in threshold voltage in one of the transistors will be
  • T 3 is coupled to the p-channel transistor T 12 , with the source of the driving waveform I DAT
  • the switching transistor T 3 may as an alternative
  • I DAT operates as a
  • Figures 8 to 11 show a SPICE simulation of an improved compensated pixel driver
  • this shows the driving waveforms I DAT , V GP , V SEL and three
  • threshold voltage namely -Ivolt, Ovolts and + Ivolt used for the purposes of
  • figure 10 shows a magnified version of the response plots shown in figure 9.
  • Figure 11 shows that for levels of I DAT ranging from 0.2 ⁇ A to l.O ⁇ A, the improved
  • control of the OELD drive current is maintained by the use of the p-channel and opposite n-
  • transistor and an opposite n-channel transistor to provide, in combination, analog control of the drive current through an electroluminescent device provides improved compensation for
  • the TFT n-channel and p-channel transistors are fabricated as
  • the p-channel and n-channel transistors may be
  • the improved compensated pixel driver circuit of the present invention may be used
  • portable displays such as desktop computers, CCTV or photo albums; or industrial displays such as control room equipment displays.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • General Physics & Mathematics (AREA)
  • Theoretical Computer Science (AREA)
  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)
  • Control Of El Displays (AREA)

Abstract

A compensated pixel driver circuit comprises a p-channel transistor and an n-channel transistor connected as a complementary pair of transistors to provide analog control of the drive current for an organic electroluminescent device (OELD). The transistors, being of opposite channel, compensate for any variation in threshold voltage ΔVT and therefore provide a drive current to the OELD which is relatively independent of ΔVT. The complementary pair of transistors can be applied to either voltage driving or current driving pixel driver circuits.

Description

Organic ElectroLuminescent Device Compensated Pixel Driver Circuit
The present invention relates to an organic electroluminescent device and particularly to a compensated pixel driver circuit thereof.
An organic electroluminescent device (OELD) consists of a light emitting polymer
(LEP) layer sandwiched between an anode layer and a cathode layer. Electrically, this
device operates like a diode. Optically, it emits light when forward biased and the intensity
of the emission increases with the forward bias current. It is possible to construct a display
panel with a matrix of OELDs fabricated on a transparent substrate and with one of the
electrode layers being transparent. It is also possible to integrate the driving circuit on the
same panel by using low temperature poly silicon thin film transistor (TFT) technology.
In a basic analog driving scheme for an active matrix OELD display, a mmimum of
two transistors are required per pixel. Such a driving scheme is illustrated in Figure 1.
Transistor Tj is provided to address the pixel and transistor T is provided to convert a data
voltage signal NData into current which drives the OELD at a designated brightness. The
data signal is stored by a storage capacitor Cstorage when the pixel is not addressed.
Although p-channel TFTs are shown in the figure, the same principle can also be applied
for a circuit utilising n-channel TFTs.
There are problems associated with TFT analog circuits and OELDs do not act like
perfect diodes. The LEP material does, however, have relatively uniform characteristics.
Due to the nature of the TFT fabrication technique, spatial variation of the TFT
characteristics exists over the extent of the display panel. One of the most important
considerations in a TFT analog circuit is the variation of threshold voltage, ΔNT, from device to device. The effect of such variation in an OELD display, exacerbated by the non
perfect diode behaviour, is the non-uniform pixel brightness over the display area of the
panel, which seriously affects the image quality. Therefore, a built-in compensation circuit
is required.
A simple threshold voltage variation compensation, current driven, circuit has been
proposed. The current driven circuit, also known as the current programmed threshold
voltage compensation circuit, is illustrated in figure 2. In this circuit, transistor T is
provided for addressing the pixel. Transistor T operates as an analog current control to
provide the driving current to the OELD. Transistor T connects between the drain and
gate of transistor T2 and toggles transistor T2 to act either as a diode or in a saturation
mode. Transistor T acts as a switch in response to an applied waveform NGP. Either
Transistor T or transistor T can be ON at any one time. Initially, at time ^ shown in the
timing diagram of Figure 2, transistors T and T are OFF, and transistor T is ON. When
transistor T is OFF, transistors Tι and T are ON, and a current IDAT of known value is
allowed to flow into the OELD, through transistor T_. This is the programming stage
because the threshold voltage of transistor T2 is measured with transistor T3 turned ON
which shorts the drain and gate of transistor T Hence transistor T operates as a diode
while the programming current is allowed to flow through transistors T and T and into the
OELD. The detected threshold voltage of transistor T is stored by a capacitor C
connected between the g °ate and source terminals of transistor T 2 when transistors T 3 and T 1 are switched OFF. Transistor T4 is then turned ON by driving waveform NGP and the
current through the OELD is now provided by supply NDD. If the slope of the output characteristics for transistor T were flat, the reproduced current would be the same as the
programmed current for any threshold voltage of T detected and stored in capacitor C
However, by turning ON transistor T , the drain-source voltage of transistor T is pulled up,
so a flat output characteristic will maintain the reproduced current at the same level as the
programmed current. Note that ΔNT2 shown in figure 2 is imaginary, not real. It has been
used solely to represent the threshold voltage of transistor T .
A constant current is provided, in theory, during a subsequent active programming
stage, which is signified by the time interval to t in the timing diagram shown in figure
2. The reproduction stage starts at time t •
The circuit of Figure 2 does provide an improvement over the circuit shown in
Figure. 1 but variations in the threshold value of the control transistor are not fully
compensated and variations in image brightness over the display area of the panel remain.
The present invention seeks to provide, therefore, an improved compensated pixel
driver circuit in which variations in the threshold voltages of the pixel driver transistor can
be further compensated, thereby providing a more uniform pixel brightness over the display
area of the panel and, therefore, improved image quality.
According to a first aspect of the present invention there is provided a compensated
pixel driver circuit for an electroluminescent device, the circuit comprising an n-channel
transistor and a complementary p-channel transistor connected so as to operatively control,
in combination, the current supplied to the electroluminescent device.
Preferably, the compensated pixel driver circuit also comprises respective storage
capacitors for the n-channel and p-channel transistors and respective switching means connected so as to establish when operative respective paths to the n-charmel and p-channel
transistors for respective data voltage pulses.
Advantageously, the compensated pixel driver circuit may also comprise respective
storage capacitors for storing a respective operating voltage of the n-channel and the p-
channel transistors during a programming stage, a first switching means connected so as to
establish when operative a first current path from a source of current data signals through
the n-channel and p-channel transistors and the electroluminescent device during the
programming stage, and a second switching means connected to establish when operative a
second current path through the n-channel and p-channel transistors and the
electroluminescent device during a reproduction stage.
In a further embodiment, the first switching means and the source of current data
signals , are connected so as to provide when operative a current source for the
electroluminescent device
In an alternative embodiment, the first switching means the source of current data
signals are connected so as to provide when operative a current sink for the
electroluminescent device.
According to a second aspect of the present invention there is also provided a
method of compensating the supply current to an electroluminescent device comprising
providing an n-channel transistor and a p-channel transistor connected so as to operatively
control, in combination, the supply current to the electroluminescent device.
Preferably, the method further comprises providing respective storage capacitors for
the n-channel and p-channel transistors and respective switching means connected so as to
establish when operative respective paths to the n-channel and p-channel transistors for respective data voltage pulses thereby to establish, when operative, a voltage driver circuit
for the electroluminescent device.
Advantageously, the method may comprise providing a programming stage during
which the n-channel and p-channel transistors are operated in a first mode and wherein a
current path from a source of current data signals is established through the n-channel and
the p-channel transistors and the electroluminescent device and wherein a respective
operating voltage of the n-channel transistor and the p-channel transistor is stored in
respective storage capacitors, and a reproduction stage wherein a second mode and a second
current path is established through the n-channel transistor and the p-channel transistor and
the electroluminescent device.
According to a third aspect of the present invention, there is also provided an
organic electroluminescent display device comprising a compensated pixel driver circuit as .
claimed in any one of claims 1 to 11.
The present invention will now be described by way of further example only, with
reference to the accompanying drawings in which: -
Fig. 1 shows a conventional OELD pixel driver circuit using two transistors;
Fig. 2 shows a known current programmed OELD driver circuit with threshold
voltage compensation;
Fig. 3 illustrates the concept of a compensated pixel driver circuit including a
complementary pair of driver transistors for providing threshold voltage compensation in
accordance with the present invention;
Fig. 4 shows plots of characteristics for the complementary driver transistors
illustrated in Fig. 3 for various levels of threshold voltages; Fig. 5 shows a compensated pixel driver circuit arranged to operate as a voltage
driver circuit in accordance with a first embodiment of the present invention.
Fig. 6 shows a compensated pixel driver circuit arranged to operate as a current
programmed driver circuit in accordance with a second embodiment of the present
invention;
Fig. 7 shows a compensated current programmed driver circuit in accordance with a
third embodiment of the present invention, and
Figs 8 to 11 show SPICE simulation results for the circuit illustrated in Fig. 6.
The concept of a compensated pixel driver circuit according to the present invention
is illustrated in Fig. 3. An OELD device is coupled between two transistors Tu and T12
which operate, in combination, as an analog current control for the current flowing through
the . OELD. Transistor Tu is a p-channel transistor and transistor T15 is an n-channel
transistor which act therefore, in combination, as a complementary pair for analog control
of the current through the OELD.
As mentioned previously, one of the most important parameters in a TFT analog
circuit design is the threshold voltage Vτ. Any variation, ΔNT within a circuit has a
significant effect on the overall circuit performance. Variations in the threshold voltage can
be viewed as a rigid horizontal shift of the source. to drain current versus the gate to source
voltage characteristic for the transistor concerned and are caused by the interface charge at
the gate of the transistor.
It has been realised with the present invention that in an array of TFT devices, in
view of the fabrication techniques employed, neighbouring or relatively close TFT's have a
high probability of exhibiting the same or an almost similar value of threshold voltage ΔNT.
Furthermore, it has been realised that as the effects of the same ΔNT on p-channel and n- channel TFT's are complementary, compensation for variations in threshold voltage ΔNT
can be achieved by employing a pair of TFT's, one p-channel TFT and one n-channel TFT,
to provide analog control of the driving current flowing to the OELD. The driving current
can, therefore, be provided independently of any variation of the threshold voltage. Such a
concept is illustrated in figure 3.
Figure 4 illustrates the variation in drain current, that is the current flowing through
the OELD shown in figure 3, for various levels of threshold voltage ΔNT, ΔVT1, ΔNΩ for
the transistors Tu and T12. Voltages Vl5 N2 and VD are respectively the voltages appearing
across transistor Tπ, T12 and the OELD from a voltage source VDD. Assuming that the
transistors Tπ and T12 have the same threshold voltage and assuming that ΔVT = O, then
the current flowing through the OELD is given by cross-over point A for the characteristics
for the p-channel transistor Tn and the n-channel transistor T12 shown in figure.4. This is
shown by value IQ.
Assuming now that the threshold voltage of the p-channel and n-channel transistors
changes to ΔVT1, the OELD current lj is then determined by crossover point B. Likewise,
for a variation in threshold voltage to ΔV2, the OELD current I2 is given by crossover point
C. It can be seen from figure 4 that even with the variations in the threshold voltage there
is minimal variation in the current flowing through the OELD.
Figure 5 shows a compensated pixel driver circuit configured as a voltage driver
circuit. The circuit comprises p-channel transistor T12 and n-channel transistor T15 acting as
a complementary pair to provide, in combination, an analog current control for the OELD.
The circuit includes respective storage capacitors C12 and C15 and respective switching
transistors TA and TB coupled to the gates of transistors T12 and T15. When transistors TA and TB are switched ON data voltage signals Vj and V2 are stored respectively in storage
capacitors C12 and Cι5 when the pixel is not addressed. The transistors TA and TB function
as pass gates under the selective control of addressing signals φj and φ2 applied to the gates
of transistors TA and TB.
Figure 6 shows a compensated driver circuit according to the present invention
configured as a current programmed OELD driver circuit. As with the voltage driver
circuit, p-channel transistor T12 and n-channel transistor T15 are coupled so as to function as
an analog current control for the OELD. Respective storage capacitors Cl5 C2 and
respective switching transistors Tj and T6 are provided for transistors T12 and T15. The
driving waveforms for the circuit are also shown in figure 6. Either transistors Tl5 T3 and
T6, or transistor T4 can be ON at any one time. Transistors Tj and T6 connect respectively
between the drain and gate of transistors T12 and Tι5 and switch in response to applied
waveform VSEL to toggle transistors T12 and T15 to act either as diodes or as transistors in
saturation mode. Transistor T3 is also connected to receive waveform VSEL. Transistors Tx
and T6 are both p-channel transistors to ensure that the signals fed through these transistors
are at the same magnitude. This is to ensure that any spike currents through the OELD
during transitions of the waveform VSEL are kept to a minimum.
The circuit shown in figure 6 operates in a similar manner to known current
programmed pixel driver circuits in that a programming stage and a display stage are
provided in each display period but with the added benefit that the drive current to the
OELD is controlled by the complementary opposite channel transistors T12 and T15.
Referring to the driving waveforms shown in figure 6, a display period for the driver circuit
extends from time ^ to time t6. Initially, transistor T4 is ON and transistors T1? T3 and T6
are OFF. Transistor T4 is turned OFF at time tx by the waveform VGP and transistors Tl5 T3 and T6 are turned ON at time t3 by the waveform VSEL. With transistors Tx and T6 turned
ON, the p-channel transistor T12 and the complementary n-charmel transistor T15 act in a
first mode as diodes. The driving waveform for the frame period concerned is available
from the current source IDAT at time t^ and this is passed by the transistor T3 when it
switches on at time tg. The detected threshold voltages of transistors T12 and T15 are stored
in capacitors Cx and C2. These are shown as imaginary voltage sources ΔVT12 and ΔVT15 in
figure 6.
Transistors Tx, T3 and T6 are then switched OFF at time t4 and transistor T4 is
switched ON at time t5 and the current through the OELD is then provided from the source
VDD under the control of the p-channel and n-channel transistors T12 and T15 operating in a
second mode, i.e. as transistors in saturation mode. It will be appreciated that as the
current through the OELD is controlled by the complementary p-channel and n-channel
transistors T12 and T15, any variation in threshold voltage in one of the transistors will be
compensated by the other opposite channel transistor, as described previously with respect
to figure 4.
In the current programmed driver circuit shown in figure 6, the switching transistor
T3 is coupled to the p-channel transistor T12, with the source of the driving waveform IDAT
operating as a current source. However, the switching transistor T3 may as an alternative
be coupled to the n-channel transistor T15 as shown in figure 7, whereby IDAT operates as a
current sink. In all other respects the operation of the circuit shown in figure 7 is the same
as for the circuit shown in figure 6.
Figures 8 to 11 show a SPICE simulation of an improved compensated pixel driver
circuit according to the present invention. Referring to figure 8, this shows the driving waveforms IDAT, VGP, VSEL and three
values of threshold voltage, namely -Ivolt, Ovolts and + Ivolt used for the purposes of
simulation to show the compensating effect provided by the combination of the p-channel
and n-channel transistors for controlling the current through the OELD. From figure 8, it
can be seen that, initially the threshold voltage ΔVT was set at -Ivolt, increasing to Ovolts at
0.3 x 10"4 seconds and increasing again to + Ivolt at 0.6 x 10"4 seconds. However, it can be
seen from figure 9 that even with such variations in the threshold voltage the driving current
through the OELD remains relatively unchanged.
The relative stability in the driving current through the OELD can be more clearly
seen in figure 10, which shows a magnified version of the response plots shown in figure 9.
It can be seen from figure 10 that, using a value of 0 volts as a base for the threshold
voltage ΔVT, that if the threshold voltage ΔVT changes to -Ivolts there is a change of
approximately 1.2% in the drive current through the OELD and if the threshold voltage ΔVT
is changed to + Ivolt, there is a reduction in drive current of approximately 1.7% compared
to the drive current when the threshold voltage ΔVT is 0 volts. The variation of drive
current of 8.7% is shown for reference purposes only as such a variation can be
compensated by gamma correction, which is well known in this art and will not therefore be
described in relation to the present invention.
Figure 11 shows that for levels of IDAT ranging from 0.2μA to l.OμA, the improved
control of the OELD drive current is maintained by the use of the p-channel and opposite n-
channel transistors in accordance with the present invention.
It will be appreciated from the above description that the use of a p-channel
transistor and an opposite n-channel transistor to provide, in combination, analog control of the drive current through an electroluminescent device provides improved compensation for
the effects which would otherwise occur with variations in the threshold voltage of a single
p-channel or n-channel transistor.
Preferably, the TFT n-channel and p-channel transistors are fabricated as
neighbouring or adjacent transistors during the fabrication of an OELD display so as to
maximise the probability of the complementary p-channel and n-channel transistors having
the same value of threshold voltage ΔVT. The p-channel and n-channel transistors may be
further matched by comparison of their output characteristics.
The improved compensated pixel driver circuit of the present invention may be used
in display devices incorporated in many types of equipment such as mobile displays e.g.
mobile phones, laptop personal computers, DVD players, cameras, field equipment;
portable displays such as desktop computers, CCTV or photo albums; or industrial displays such as control room equipment displays.
The aforegoing description has been given by way of example only and it will be
appreciated by a person skilled in the art that modifications can be made without departing
from the scope of the present invention.

Claims

1. A compensated pixel driver circuit for an electroluminescent device, the circuit
comprising an n-channel transistor and a complementary p-channel transistor connected so
as to operatively control, in combination, the current supplied to the electroluminescent
device.
2. A compensated pixel driver circuit as claimed in claim 1, wherein the
complementary n-channel and p-channel transistors comprise polysilicon thin film
transistors.
3. A compensated pixel driver circuit as claimed in claim 2, wherein the
complementary n-channel and p-channel transistors are spatially arranged in close proximity
to each other for providing a complementary pair of n-channel and p-channel transistors
having approximately equal threshold voltages.
4. A compensated pixel driver circuit as claimed in any one of claims 1 to 3 connected
so as to establish when operative a voltage driver circuit comprising respective storage
capacitors for the n-channel and p-channel transistors and respective switching means
connected so as to establish when operative respective paths to the n-channel and p-channel
transistors for respective data voltage pulses.
5. A compensated pixel driver circuit as claimed in any one of claims 1 to 3 comprising
respective storage capacitors for storing a respective operating voltage of the n-channel and
the p-channel transistors during a programming stage, a first switching means connected so
as to establish when operative a first current path from a source of current data signals
through the n-channel and p-channel transistors and the elecfroluminescent device during the
programming stage, and a second switching means connected to establish when operative a
second current path through the n-channel and p-channel transistors and the electroluminescent device during a reproduction stage.
6. A compensated pixel driver circuit as claimed in claim 5, wherein the first switching
means and the source of current data signals are connected so as to provide when operative
a current source for the elecfroluminescent device.
7. A compensated pixel driver circuit as claimed in claim 5, wherein the first switching
means and the source of current data signals are connected so as to provide when operative
a current sink for the electroluminescent device.
8. A compensated pixel driver circuit as claimed in any one of claims 5 to 7, further
comprising respective further switching means respectively connected to bias the n-channel
transistor and the p-channel transistor to act as diodes during the programming stage.
9. A compensated pixel driver circuit as claimed in claim 8, wherein the respective
further switching means comprise p-channel transistors.
10. A compensated pixel driver circuit as claimed in any one of claims 5 to 9, wherein
the circuit is implemented with poly silicon thin film transistors.
11. A compensated pixel driver circuit as claimed in claim 4, wherein the circuit is
implemented using poly silicon thin film transistors.
12. A method of compensating the supply current to an electroluminescent device
comprising providing an n-channel transistor and a p-channel transistor connected so as to
operatively control, in combination, the supply current to the electroluminescent device.
13. A method as claimed in claim 11, comprising the further step of providing the n-
channel transistor and the p-channel transistor as polysilicon thin film transistors.
14. A method as claimed in claim 12 comprising the further step of spatially arranging
the n-channel and p-channel polysilicon thin film transistors in close proximity to each
other.
15. A method as claimed in any one of claims 12 to 14 comprising providing respective
storage capacitors for the n-channel and p-channel transistors and respective switching means connected so as to establish when operative respective paths to the n-channel and p-
channel transistors for respective data voltage pulses thereby to establish, when operative, a
voltage driver circuit for the elecfroluminescent device.
16. A method as claimed in any one of claims 12 to 14 comprising providing a
programming stage during which the n-channel and p-channel transistors are operated in a
first mode and wherein a current path from a source of current data signals is established
through the n-channel and the p-channel transistors and the elecfroluminescent device and
wherein a respective operating voltage of the n-channel transistor and the p-channel
transistor is stored in respective storage capacitors, and a reproduction stage wherein a
second mode and a second current path is established through the n-channel transistor and
the p-channel transistor and the electroluminescent device.
17. A method as claimed in claim 15, wherein the first mode comprises operating the n-
channel and p-channel transistors as diodes.
18. An organic electroluminescent display device comprising a compensated pixel driver
circuit as claimed in any one of claims 1 to 11.
PCT/GB2001/001460 2000-03-31 2001-03-30 Organic electroluminescent device compensated pixel driver circuit WO2001075853A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0007879.0 2000-03-31
GB0007879A GB2360870A (en) 2000-03-31 2000-03-31 Driver circuit for organic electroluminescent device

Publications (1)

Publication Number Publication Date
WO2001075853A1 true WO2001075853A1 (en) 2001-10-11

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KR (1) KR100493555B1 (en)
GB (3) GB2360870A (en)
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Also Published As

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GB2364593A (en) 2002-01-30
GB2364592A (en) 2002-01-30
GB0016816D0 (en) 2000-08-30
KR100493555B1 (en) 2005-06-10
GB2360870A (en) 2001-10-03
KR20020032571A (en) 2002-05-03
GB0007879D0 (en) 2000-05-17
GB0016815D0 (en) 2000-08-30

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