Using magneto-electroluminescence as a fingerprint to identify the carrier-to-photon conversion process in dye-doped OLEDs

Abstract

Long-range Förster energy transfer (FET) and short-range charge trapping (CT) are two competing basic mechanisms in the carrier-to-photon process of dye-doped organic diode, but little is known about which of FET and CT governs the electroluminescence process for a given dye. Here we report that magneto-electroluminescence (MEL) response can serve to identify the fundamental issue. (1) In relative high magnetic field (>20 mT), dramatic decrease in MEL response implies that CT dominates carrier-to-photon process for a common fluorescent dye of 4-(dicyano-methylene)-2-methyl-6-(p-dimethylaminostyryl)-4H- pyran (DCM1), whereas saturating of MEL response indicates that FET is dominant for another common fluorescent dye of Coumarin 540 (C540). (2) In low field (<15 mT), overlapping between MEL responses of DCM1-doped and control OLED shows that prior to being trapped by DCM1 molecule, electron and hole have formed electrostatically bound e-h polaron pair in adjacent Alq 3 molecules. Both of the observations are finally confirmed by intermolecular correlated quantum calculation. © 2011 American Chemical Society.