In photoluminescence (PL), electrons and holes are photo excited and the luminescence emitted from the radiative recombination of these carriers. Electroluminescence (EL) is similar to PL, except that in EL, the holes and electrons are produced by current injection, and EL is measured on OLEDs. PL and EL are determined from the different characteristics of the material. What may be the closest absolute method to optimize OLEDs by a preliminary work of PL measurements? What factors may affect the agreement of EL and PL?
One thing to consider is the temporal difference between PL and EL and the states that precede emission. In PL measurement the precursor state is usually the singlet exciton state. Once a singlet exciton is created by the excitation photons, it can undergo energy transfer to nearby molecules and undergo what is typically called exciton diffusion. Energy transfer will favor jumping to lower energy sites and over time the exciton will relax into the tail of the density of exciton states. Depending on the exciton lifetime and the width of the DOS, steady state emission may not come from completely relaxed excitons.
In an EL measurement, the exciton is not formed until charges transport through the material and meet somewhere in the film. The charges also are moving through a density of states and will relax over time as they move. Depending on the time it takes for them to meet and the width of the DOS, the exciton that forms may or may not also be completely relaxed.
Moving now to blends, things can become even more complicated as there are morphological issues to consider as well. Depending on the tranport pathways allowed in the morphology, charges injected from the electrodes may not have full access to all interfacial sites, and as a result, the emission spectra will be biased to a subset of the overall population of interfacial sites, whereas the PL spectrum of the same blend may give an emission spectra coming from "all" of the interfacial sites.
In reality, these are complicated issues and something that still needs more investigation, especially in blends. I am attaching a paper explaining the morphology issue in polymer:fullerene blend that may help get you started with your literature search. Good luck!
Article On the Dissociation Efficiency of Charge Transfer Excitons a...
Thank you Michael for providing a thorough answer. I am still doing research and it is really hard to justify explanations for blends and what is going on with them during PL and EL measurements. I would be glad if anything else comes to your mind that might help me.
There are quite a few papers discussing EL and PL of blends, so the best thing for your right now is to probably spend some time reading up on previous work to get a good overall understanding of what's been done. This should help you understand some of the more common explanations for different behaviors and help you identify where to focus your new research. Good luck!
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