Method and circuit arrangement for the ageing compensation of an organic light-emitting diode and circuit arrangement

• US 20060214888A1
• Filed: 09/20/2005
• Published: 09/28/2006
• Est. Priority Date: 09/20/2004
• Status: Active Grant

• Current Assignee: Novaled AG
• Original Assignee: Novaled AG
• Inventors: Schneider, Oliver, Birnstock, Jan
• Granted Patent: US 7,656,370 B2
• Time in Patent Office: Days
• US Class Current: 345/76
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2320/029 by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element

Abstract

The invention relates to a method and a circuit arrangement for the ageing compensation of an organic light-emitting diode (OLED) which is fed from a supply voltage and is switched by means of a driver transistor operated in saturation operation, by means of a driving of the light-emitting diode. The method comprises the following steps of: storing at least one desired current-voltage value pair of a desired current-voltage characteristic curve of the light-emitting diode; transferring the driver transistor from saturation operation to linear operation during a measurement cycle; measuring a current value for the current through the light-emitting diode by means of a current measuring circuit in the measurement cycle; determining at least one present current-voltage value pair of a present current-voltage characteristic curve of the light-emitting diode by means of the measured current value; comparing the at least one present current-voltage value pair of the light-emitting diode with the desired current-voltage value pair of the light-emitting diode; and generating driving parameters for driving the light-emitting diode in a manner dependent on the result of the comparison.

Claims

1. Method for the ageing compensation of an organic light-emitting diode (OLED) which is fed from a supply voltage and is switched by means of a driver transistor operated in saturation operation, by means of a driving of the light-emitting diode, the method comprising the following steps of:

storing at least one desired current-voltage value pair of a desired current-voltage characteristic curve of the light-emitting diode,

transferring the driver transistor from saturation operation to linear operation during a measurement cycle,

measuring a current value for the current through the light-emitting diode by means of a current measuring circuit in the measurement cycle,

determining at least one present current-voltage value pair of a present current-voltage characteristic curve of the light-emitting diode by means of the measured current value,

comparing the at least one present current-voltage value pair of the light-emitting diode with the desired current-voltage value pair of the light-emitting diode, and

generating driving parameters for driving the light-emitting diode in a manner dependent on the result of the comparison.

2. Method according to claim 1, characterized in that the driving parameters are stored in a memory until the ageing is determined anew.

3. Method according to claim 1, characterized in that the ageing compensation is implemented in a display having a matrix of a plurality of light-emitting diodes (OLEDs), in which case

all of the plurality of light-emitting diodes of the matrix are switched off during the measurement cycle,

a total current I_Doff of the matrix through a display supply line is measured,

afterwards, one of the plurality of light-emitting diodes which is to be measured and an associated driver transistor are switched on,

for the light-emitting diode which is to be measured and the associated driver transistor, two measurements of the current in saturation operation of the driver transistor, I_Don1 and I_Don2, at two different gate voltages U_GS1, U_GS2, are carried out, and

a threshold voltage of the associated driver transistor for the light-emitting diode which is to be measured is calculated from the total current I_Doff, the currents I_Don1, I_Don2 and the gate voltages U_GS1, U_GS2.

4. Method according to claim 1, characterized in that the ageing compensation is implemented in a display having a matrix of a plurality of light-emitting diodes (OLEDs), in which case

a supply voltage V_DD is reduced to an extent such that the driver transistors of the plurality of light-emitting diodes are transferred from saturation operation to linear operation,

firstly, all of the plurality of light-emitting diodes are switched off,

afterwards, one of the plurality of light-emitting diodes which is to be measured is switched on and for an associated driver transistor of the light-emitting diode which is to be measured the source-drain current I_Donlinear through the display supply line is measured,

the source-drain voltage of the associated driver transistor for the light-emitting diode which is to be measured is determined by means of the characteristic curve of the associated driver transistor, the gate voltage and the measured source-drain current,

a forward voltage value for the light-emitting diode which is to be measured is calculated from the difference between the supply voltage and the calculated source-drain voltage, and

a characteristic curve alteration is determined from the comparison of the desired current-voltage value pair with the present current-voltage value pair for the light-emitting diode which is to be measured.

5. Method according to claim 4, characterized in that after the plurality of light-emitting diodes have been switched off a dark current I_Dofflinear through the display supply line is measured.

6. Method according to claim 4, characterized in that, by multiple application of the method with alteration of the gate voltage of the associated driver transistor, a characteristic curve segment of the OLED characteristic curve is recorded and this characteristic curve segment is subsequently used for more precise compensation of the ageing.

7. Method according to claim 4, characterized in that, by multiple application of the method with alteration of the supply voltage V_DD, a characteristic curve segment of the OLED characteristic curve is recorded and this characteristic curve segment is subsequently used for more precise compensation of the ageing.

8. Method according to claim 1, characterized in that the ageing compensation is carried out upon every turn-on.

9. Method according to claim 1, characterized in that a brightness compensation of the light-emitting diode is carried out with the aid of the driving.

10. Method according to claim 1, characterized in that a gamma correction adaptation of the plurality of light-emitting diodes of the matrix is carried out with the aid of the driving.

11. Circuit arrangement for carrying out the method according to claim 1 having a circuit, having a light-emitting diode (OLED) connected together with a drain-source path of a driver transistor, the gate of which is connected to a driving transistor, in a current path between a supply voltage V_DD and earth, and having a driving circuit connected to the circuit in a manner driving the latter, characterized in that a current measuring circuit (7) is connected into the current path.

12. Circuit arrangement according to claim 11, characterized in that the current measuring circuit is arranged between a terminal of the supply voltage V_DD and the light-emitting diode.

13. Circuit arrangement according to claim 11, characterized in that the circuit is arranged multiply in rows and columns of a display matrix, all circuits having a common connection to the supply voltage V_DD, and in that the current measuring circuit is located in the common connection of the circuits to the supply voltage V_DD.

14. Circuit arrangement according to claim 13, characterized in that the current measuring circuit is arranged spatially in or at the driving circuit.

15. Circuit arrangement according to claim 13, characterized in that the current measuring circuit is arranged spatially on a substrate of the display matrix.