Identification of the nature of trapping centers in polyspirobifluorene based diodes by using electrical characterization

International audienceThe trap parameters in hole-only diodes using a blue emitting polyspirobifluorene copolymer were explored by using two complementary techniques such as: charge based deep level transient spectroscopy and thermally stimulated current. The trap analyses have been further completed by the measurements of the current-voltage characteristics as a function of the temperature. The use of unipolar structures allowed us to determine hole traps in the devices and further to identify electron traps in bipolar structures. Two hole traps which are located at 0.19 and 0.37 eV from the highest occupied molecular orbital, have been identified in blue polyspirobifluorene based devices. In bipolar diodes, four electron traps are determined and are located at 0.32, 0.58, 0.74, and 0.83 eV from the lowest unoccupied molecular orbital. We also showed that the use of a palladium electrode introduced new trap states of mean activation energy of 0.4 eV. As the nature of the cathode changes from Ba/Al to Pd, these traps are inferred to be related to the interface of polymer/Pd contact