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Electroluminescence (EL) and photoluminescence (PL) spectra are not identical: there's a small red-shift of the PL spectrum relative to the EL spectrum.

Why is this happening if both EL and PL are caused by radiative relaxation of the same excited states?

Photoluminescence (PL) spectra of the TPD∶2∶PC (left) and TPD∶poly(2)∶PC (right) films excited at 420 nm; electroluminescence (EL) spectra of the ITO/TPD∶2∶PC/Ca (solid) and ITO/TPD∶poly(2)∶PC/Ca OLEDs. All spectra are normalised.

Photoluminescence (PL) spectra of the TPD∶2∶PC (left) and TPD∶poly(2)∶PC (right) films excited at 420 nm; electroluminescence (EL) spectra of the ITO/TPD∶2∶PC/Ca (solid) and ITO/TPD∶poly(2)∶PC/Ca OLEDs. All spectra are normalised. (source)

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For an electroluminescence measurement you need to apply some voltage, which causes a Stark shift (splitting of otherwise degenerate energy levels). Photoluminescence measurements are usually done unbiased.

But why are the two spectra different from one another?

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  • $\begingroup$ Stark shift causes energy splitting, which I would expect to result in a red shift, not a blue shift. No..? $\endgroup$ – Sparkler Feb 18 '16 at 15:05
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The difference between the two spectra originates in light interference effects during electroluminescence.

Originally, electroluminescence and photoluminescence give the same spectrum. The reason for the shift comes from measurement conditions: while in photoluminescence all the molecules of the bulk participate in excitation-relaxation, in electroluminescence the relaxation occurs at recombination zones, typically at the center of the material. After light is produced at the center of the material, interference due reflection modifies the original spectrum.

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