There are several reasons, mainly depending on the emitter type. For fluorescent emitters it can be dimmer formation, excimer formation (most likely) or you have oxidised centres which act as charge traps and preferentially emit (in the green). For phosphors it can again be dimmer formation or emission from states that have scrambled their ligands. You may see electroplex states talked about in the literature, here you need an excited state with a large dipole moment i.e. charge transfer states, which is perturbed by the bias E-field and the surrounding dielectric of highly dipolar excited states.

I'd imagine it has to do with singlet and triplet excitonic states.

In photoluminescence all the excited states are singlets due to quantum mechanical transition rules for combining an electron and photon. If there is only weak or negligible inter-system crossing (LS coupling) to an available triplet state you'll only observe fluorescence.

In electroluminescence there are four ways to combine the wavefunctions of electron and hole polarons, only one of which is a singlet state and the other three are triplets. Therefore if the material does have an emissive triplet state then it would be strongly populated by this mechanism and you would observe phosphorescence, which is always redder.

You could check this by measuring the excited state lifetimes of both the photoluminescence and electroluminescence emission, the latter should be much longer (ms or us). You should also be able to observe some singlet emission in the electroluminescence spectrum, though this depends on the relative quantum yields of the singlet and triplet states.

I remember working with Coumarin 102 (a.k.a 480) which shows some phosphorescence when excited by light, though this is swamped by the efficient fluorescence. I only noticed it in a dark room when removing my sample from the path of the excitation source and observed the sample glowing for almost a second afterwards.

There are several reasons, mainly depending on the emitter type. For fluorescent emitters it can be dimmer formation, excimer formation (most likely) or you have oxidised centres which act as charge traps and preferentially emit (in the green). For phosphors it can again be dimmer formation or emission from states that have scrambled their ligands. You may see electroplex states talked about in the literature, here you need an excited state with a large dipole moment i.e. charge transfer states, which is perturbed by the bias E-field and the surrounding dielectric of highly dipolar excited states.

Was photo- and electroluminescence measured at the same device, the photoluminescence in transmission? Otherwise, photoluminescence in reflection sees less self-absorption (meaning the emitted light from the singlet excition can be affected by absorption of the film). Therefore, electroluminescence generated in the center of the device sees "more absorption" than photoluminescence on the surface. This will depend largely on the thickness of the device and also how much spacing there is between the singlet absorbtion and the singlet emission.

In photoluminescence, once excitons form they have a short time to localise on lower energy sites before they emit. In electroluminescence, you should also consider that as the charges migrate through the device, they will naturally move to lower energy sites even before exciton formation. This extra process in electroluminescence typically gives a red-shift of the spectrum compared to photoluminescence.

The mechanisms for a fluorescent molecule and electroluminescent material ( in this case a organic light - emitting diodes - OLEDs) are generally different but depending on your molecules may have similarities . One possibility is the Förster resonance energy transfer ( FRET ) or Fluorescence resonance energy transfer ( FRET ), that is a mechanism describing energy transfer between two chromophores . If a fluorescent molecule has several aromatic rings, it can have energy transfer between them, and the shift to the red spectral region is inevitable. Generally the organic moieties chromophores in the molecule are not identical. But it is not a rule to have similar moieties. In the case of electroluminescent material, the excitation source is electricity that generates pairs of exciton with electron - hole recombination in the organic material, this leads to defects and non- radiative energy losses and if the organic material has several aromatic rings can also occur FRET with fluorescence and quenching. The electron transfer Dexter mechanism explains the electron transfer (it is the quenching mechanism in which an excited state electron transfers from one molecule ( the donor ) to a second molecule (an acceptor ) ) . If the degree of overlap between the wave functions of the organic layer and the semiconductor wave functions are varying between 1 and 100% , maybe will shift to the red spectral region . Because the maximum emission bands will move changing intensity.

In most cases,it is cavity effect! e.g.you can continously change te thickneess of ETL to modify the position of the emission zone, you see the shift of the EL spectrum. It could be red-shift, also blue-shift, really depends on your device structure.

@Sheldon Feng. Thanks for the suggestion. In my device I am not using any ETL as my active material acts also as ET material. My device structure is ITO/HIL/HTL/EL/EIL/Cathode. Will variation of HTL thickness effect EL spectrum ? I am using NPB as HTL.

@Edson Abreu. Thanks for the explanation. Can you please suggest any review papers/ Books on this. Is there any way using which we can suppress this phenomena. Please do suggest.

@Oliver Fenwick, Andy Monkman, Adam Green. Thanks for the suggestions and explanations

@Raoul Schroeder. Thanks for the suggestion.

Photo-luminescence was taken only for the material thermally coated on glass substrate and not for the device. It gave blue PL. But when used as active material in a device it gave green EL.

If you change the thickness of NPB, you can also see the shift of EL spectrum, however, the shift is not as much as changing the thickness of ETL. If you are using some material like Alq3 as both ETL and EM, you can still change the thickness of EM, which leads to the change of the position of emission zone, so you will see the shift of the spectrum.

@Sheldon Feng. Thanks for the suggestion. Since my material is a good ET material, I am not using AlQ3.

Stark effect - the red shift under the high electric field. This is, likely, the case when the material is used in the active matrix.

The only reason is die to the steric effect which is similar to what happens to the material used in active matrices

@Hatem Maraqah. Thanks for explanation.

Dear Dr Hidayath

Thank you for the letter please upvote my answer by pressing on the gree arrow below my answer

You did not mention the excitation wave length. It may be temperature dependent fluorrescence.

excitation is around 350 nm for fluorescence and voltage is less than 10 V for electroluminescence.

What material? Phosphor or Semiconductor?