Study of trap states in polyspirobifluorene based devices: Influence of aging by electrical stress
Résumé
The influence of aging by electrical stress on the formation of defects has been studied in two types of polymers: blue emitting spiro copolymer and white emitting spiro blend polymer. The white emitting spiro polymer was obtained by adding green and red chromophores into the host blue copolymer. The defect states were investigated by using two complementary techniques: charge based deep level transient spectroscopy and thermally stimulated current (TSC). Six trap levels were retrieved in blue emitting diodes with a depth of 0.17-0.85 eV, and the onset of an additional trap with a depth of 0.49 eV was observed in white emitting ones. The density of these traps is of the order of 10(16)-10(17) cm(-3). The fractional TSC experiments allowed us to determine the distribution of five trap types in white emitting diodes, which could be described by Gaussian functions. The aging of devices was performed by electrical stress and not by exposure of samples to air or oxygen, in contrast to previous investigations on other organic materials. Upon aging, no new traps have been created or suppressed in both doped and undoped polymers. Furthermore, an enhancement in trap density has been observed. The analysis of the trap characteristics has highlighted that the aging would not uniformly affect the defect levels in the polymer. The density of deeper traps (above 0.3 eV) is increased, and their distribution is also modified, while the shallow traps (below 0.3 eV) remain stable. Furthermore, the investigation of the doping influence on the degradation process has shown that the chromophore addition seems to stabilize the polymer, whose degradation rate is slower than that of the undoped material.