A method of making an electroluminescent device having a substrate, and at least one blue light emitting layer and at least one non-emissive layer containing an emissive material that emits light longer in wavelength than blue light, these two layers being directly separated by and in direct contact with a non-emissive buffer layer; and heating the electroluminescent device after fabrication to cause the long wavelength emissive material to diffuse from the non-emissive layer into at least the buffer layer such that the long wavelength emissive material comes into contact with the blue light-emitting layer such that the recombination energy in the emitting layer is preferentially transferred to the diffused emissive material compared to the blue emissive material and the light emitted is longer in wavelength than blue light.