Molecular Origin of Balanced Bipolar Transport in Neat Layers of the Emitter CzDBA

Recently, an efficient single-layer organic light-emitting diode has been reported, consisting of a neat layer of the diboron-based thermally activated delayed fluorescence emitter 5,10-bis(4-(9H-carbazol-9-yl)-2,6-dimethylphenyl)-5,10-dihydroboranthrene, exhibiting remarkably balanced bipolar electron and hole transport. Here, the donor–acceptor–donor architecture of the molecule is linked to the transport characteristics of its neat amorphous films. It is found that energetic disorder is larger for holes than for electrons, explaining the experimentally observed difference in temperature activation of the mobility. Although a difference in energetic disorder would suggest unbalanced charge transport, it is demonstrated that it is partly counteracted by larger coupling elements for holes.