Theoretical studies on geometrical and electronic structure of electroplex at the NPB/PBD interface in organic light-emitting diodes

The studies on geometrical and electronic structure of electroplex (NPB+PBD−), which is formed by NPB+ and PBD−, were carried out by simulation calculation. The calculation results indicate that the (NPB+PBD−) is formed efficiently when the positions of PBD− and NPB+ are appropriate. The (NPB+PBD−) is energetically favored compared to isolated ions NPB+ and PBD−. So the ionic state NPB+ and PBD− at the interface NPB/PBD inside organic light-emitting diode (OLED) tend to form electroplex. The analysis of geometrical structure data of the (NPB+PBD−) suggests that the electron transfer occurs from the PBD− side to the NPB+ side. The lowest unoccupied molecular orbital (LUMO) of (NPB+PBD−) is localized on the PBD− side and the highest occupied molecular orbital (HOMO) of (NPB+PBD−) on the NPB+ side. The energy gap of the (NPB+PBD−) is 1.61 eV, which approximately equals to the energy difference of 1.63 eV between the LUMO of PBD and the HOMO of NPB. The emission of electroplex is theoretically intermolecular optical transition from the LUMO of PBD to the HOMO of NPB. Because of little overlap probability between LUMOs of PBD and NPB at the NPB/PBD interface inside OLED, the exciplex of PBD and NPB is not efficiently produced.