Method of driving an organic electroluminescent device comprising a phosphorescent light emitter

1. A method of driving an organic electroluminescent device comprising a phosphorescent light emitter having an excited state emission decay time τ, which method comprises applying to the organic electroluminescent device a series of electrical pulses, each having a duration t d , such that the ratio t d /τ is less than or equal to 0.1 at a pulse frequency which is less than 1/τ.

2. A method according to claim 1 , wherein T d /τ is less than or equal to 0.05.

3. A method according to claim 1 , wherein τ is from 0.05 to 1 ms.

4. A method according to claim 1 , wherein td is less than or equal to 5 μs.

5. A method according to claim 1 , wherein the electrical pulses are applied at a current density of from 0.1 to 100 mA/cm 2 .

6. A method according to claim 1 , wherein the electrical pulses are applied at a frequency of from 10 Hz to 1 kHz.

7. A method according to claim 1 , wherein the phosphorescent light emitter is an organolanthanide phosphor compound.

8. A method according to claim 7 , wherein the organolanthanide phosphor compound is a compound of formula (I):

M 3+ (L n− ) x A y   (I)

in which M 3+ is a trivalent lanthanide metal ion,

L n− is an anionic ligand such that n.x is 3,

A is an electrically neutral co-ligand which may be monodentate or bidentate, and

y is 0, 1 or 2.

9. A method according to claim 8 , wherein the organolanthanide phosphor compound is europium tris (2-thenoyltrifluoroacetone) 1,10-phenanthroline (Eu(TTA) 3 phen).

10. A method according to claim 8 , wherein the organolanthanide phosphor compound is terbium tris(1-phenyl-3-methyl-4-(2-methylbutan-1-oyl)pyrazolin-5-one (Tb2B).

11. A method according to claim 1 , wherein the phosphorescent light emitter is 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (II) (PtOEP).

12. A method according to claim 1 , wherein the organic electroluminescent device is passively addressed.