Method and drive means for color correction in an organic electroluminescent device

1. A method for color correction in an organic electroluminescent device ( 1 ) having at least one pixel ( 6 ) comprising an electroluminescent material layer ( 5 ), which is sandwiched between a first and a second electrode ( 2 , 3 ), the pixel constituting a first and a second light-emitting element ( 6 R, 6 G), wherein said method comprises the steps of:

inputting a data signal (S) comprising information to be displayed by said light-emitting elements ( 6 R, 6 G),

generating a correction factor for each light-emitting element ( 6 R, 6 G), said correction factors being based on:

(i) a measured shift in a voltage (V) across a light-emitting element ( 6 R, 6 G) at a predetermined current (I s ) through said light-emitting element, and a relation between the measured shift in the voltage and a color point wavelength shift Δλ of said light-emitting element, or

(ii) a measured shift in a current (I) through a light-emitting element ( 6 R, 6 G) at a predetermined voltage (V s ) across said light-emitting element, and a relation between the measured shift in the current and a color point wavelength shift Δλ of said light-emitting element,

applying said correction factor to said data signal (S); and

supplying the corrected data signal (S) to the light-emitting elements ( 6 R, 6 G).

2. A method as claimed in claim 1 , wherein said correction means ( 10 ) comprise a look-up table containing pre-measured information regarding the relation between the voltage applied across a light-emitting element ( 6 R or 6 G), or current applied through said light-emitting element ( 6 R or 6 G), and the wavelength shift Δλ of said light-emitting element.

3. A method as claimed in claim 1 , further comprising the steps of feeding, with predetermined time intervals, one of said light-emitting elements ( 6 R; 6 G) with the predetermined current (I s ),

measuring the voltage (V) over the light-emitting element ( 6 R; 6 G) as the predetermined current (I s ) is fed through the light-emitting element ( 6 R; 6 G),

calculating a voltage shift ΔV between said measured voltage (V) and a previous voltage (V 0 ) for the predetermined current (I s ), and

outputting a correction factor corresponding to a wavelength shift Δλ of said light-emitting element ( 6 R; 6 G), based on said voltage shift ΔV.

4. A method as claimed in claim 3 , wherein the wavelength shift Δλ for a light-emitting element ( 6 R; 6 G) is calculated by:

Δλ= k·ΔV,

where k is a correction coefficient and wherein k is pre-stored for each light-emitting element ( 6 R; 6 G) or for each type of light-emitting element.

5. A method as claimed in claim 3 , wherein said previous voltage V 0 is an initial voltage across said light-emitting element ( 6 R; 6 G), measured during manufacture of the device ( 1 ).

6. A method as claimed in claim 3 , wherein said previous voltage V 0 is a voltage across said light-emitting element ( 6 R; 6 G), measured previously during the drive of the device.

7. A method as claimed in claim 1 , comprising the steps of feeding, with predetermined time intervals, one of said light-emitting elements ( 6 R; 6 G) with a predetermined voltage (V s ),

measuring the current (I) through said light-emitting element ( 6 R; 6 G) as the predetermined voltage (V s ) is applied across the light-emitting element ( 6 R; 6 G),

calculating a current shift ΔI between said measured current (I) and a previous current I O ,

outputting a correction factor corresponding to a wavelength shift Δλ of said light-emitting element ( 6 R; 6 G), based on said current shift ΔI.

8. A method as claimed in claim 7 , wherein the wavelength shift Δλ for said light-emitting element ( 6 R; 6 G)is calculated by:

Δλ=k·ΔI

where k is a correction factor and wherein k is pre-stored in said correction means ( 10 ) for each light-emitting element ( 6 R; 6 G) or for each type of light-emitting element.

9. A method as claimed in claim 1 , wherein said electroluminescent material layer ( 5 ) comprises a polymer light-emitting material, an organic light-emitting material, or a mixture of a polymer and an organic light-emitting material.

10. A method as claimed in claim 1 , wherein said correction factor is arranged to provide a substantially constant total color point for the pixel, based on the light output from each of said light-emitting elements ( 6 R, 6 G).

11. A drive means ( 7 ) for an organic electroluminescent device ( 1 ), comprising a layer ( 5 ) of electroluminescent material which is sandwiched between a first and a second electrode pattern ( 2 , 3 ), wherein said patterns define at least one pixel ( 6 ), comprising at least a first and a second light-emitting element ( 6 R, 6 G), said drive means ( 7 ) being connected to said electrodes ( 2 , 3 ) and arranged to apply a current (I) through said electroluminescent material in order to achieve light emission from said material, said drive means ( 7 ) comprising:

an input connection ( 8 ) for inputting a data signal (S), comprising information to be displayed by each of said light-emitting elements ( 6 R, 6 G),

a correction means ( 10 ) for applying a correction factor to said data signal (S), said correction factor being based on a relationship between a color point wavelength shift and a measured shift in one of a voltage (V) across at least one of said light-emitting elements ( 6 R, 6 G) and a current (I) through this light-emitting elements ( 6 R, 6 G), and

an output means ( 9 ) for outputting said color-corrected data signal to said light-emitting elements ( 6 R, 6 G).