Regarding the origin of the delayed fluorescence of conjugated polymers

In the first part of this work we revisit and reevaluate the experimental data that lead to the assignment of the origin of the delayed fluorescence (DF) to triplet-triplet annihilation for polyfluorene and to geminate pair recombination in the case of the ladder-type polyparaphenylene (MeLPPP); the ambiguity of this classification is unveiled. Next, new data about the DF of MeLPPP under applied electric field are presented. Here, the DF intensity completely recovers once the field is turned off, which rules out geminate pairs as the origin of the DF and in turn provides clear evidence of the triplet-triplet annihilation picture. Finally, we show and discuss how recombination of space charge layers may also give rise to electric field induced delayed fluorescence, whereby the formation of these space charge layers strongly depends on device configuration and purity of the materials.