Probing spin-dependent electronic transitions in a conjugated polymer using electrically detected magnetic resonance (EDMR) under hole injection with MoO₃ electrodes

Spin-dependent recombination in organic light emitting diodes (OLEDs) occurs when an active polymer layer is sandwiched between an electron and hole charge carrier injection layers. As spin-dependent electronic transitions rates change under magnetic resonant excitation, spin-dependent recombination can be studied using a method called electrically detected magnetic resonance spectroscopy (EDMR) [1-2]. Most past EDMR studies used the copolymer blend poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) as hole injector layer. Here, we report EDMR spectroscopy of OLEDs based on various π-conjugated poly-phenylenevinylenes as active materials and the Ca and MoO₃ layers as electron and hole injectors, respectively, in order to test whether Mo, (atomic number Z=42) is able to affect the charge carrier spin-orbit coupling (SOC) in the active layer, e.g. by suppression of spin-dependent processes. Our results indicate that no SOC effects can be observed and, thus, that SOC in hole injection layers does not affect pEDMR signals in OLEDs. [1] D. McCamey et al., Nature Materials, 7 (9), 723 (2008); [2] H. Malissa et al., Science 345 (6203), 1487 (2014).