Electroluminescent devices having a light-emitting layer

A method of making an electroluminescent device comprising the steps of:

• providing on a substrate a first electrode for injecting charge carriers of a first type;
• depositing on top of said first electrode a first emitter comprising an organic layer selected so that radiation of a first wavelength is emitted when charge carriers of opposite types are injected into the first emitter;
• providing on top of said first emitter a second electrode for injecting charge carriers of a second type;
• depositing on top of said second electrode a second emitter comprising an organic layer selected so that radiation of a second wavelength is emitted when charge carriers of opposite types are injected into the second emitter;
• providing on top of said second emitter a third electrode for injecting charge carriers of said first type;
• connecting the first and second electrodes to a first source for applying an electric field across the first emitter so that charge carriers of the first and second types are injected into the first emitter;
• and
• connecting the second and third electrodes to a second source for applying an electric field across the second emitter so that charge carriers of the first and second types are injected into the second emitter, the first and second source for applying electric fields being operable independently one from another whereby either radiation of said first wavelength or radiation of said second wavelength or both can be caused to be emitted and viewed.

An electroluminescent device comprising:

• a first emitter comprising an organic layer selected so that radiation of a first wavelength is emitted when charge carriers of opposite types are injected into the first emitter;
• first electrode means arranged to inject charge carriers into said first emitter, said first electrode means including first and second contact layers of different types arranged on either side of the first emitter and connected to a source for applying an electric field between the first and second contact layers so that charge carriers of respective first and second types are injected into the first emitter, the second contract layer comprising a semi-transparent metallic layer having a thickness of the order of a few hundred angstroms;
• a second emitter comprising an organic layer arranged in viewing overlap with said first emitter and selected so that radiation of a second wavelength is emitted when charge carriers of opposite types are injected into the second emitter;
• and
• second electrode means arranged to inject charge carriers into said second emitter, said second electrode means being operable independently of said first electrode means whereby either radiation of said first wavelength or radiation of said second wavelength or both can be caused to be emitted and viewed.