Effect of electrical operation on the defect states in organic semiconductors

Abstract

We have investigated the role of the trapping process in degradation mechanisms of poly(9,9- dihexylfluorene- co-N,N-di(9,9- dihexyl- 2- fluorenyl)- N- phenylamine) (PF) based diodes, after aging (at half lifetime) by electrical stress. By using the Charge based Deep Level Transient Spectroscopy, we have determined the trap parameters in PF light emitting devices. The mean activation energies of the traps are in the range 0.13- 0.60 eV from the band edges with capture cross sections of the order of 10(-18) to 10(-20) cm(2). The trap densities are in the range of 10(-16) to 10(-17) cm(-3). Upon aging, no new trap levels have been found indicating that the electrical stress did not create additional defect level in the polymer in contrast to previous investigations on other organic materials, which reported that the degradation of devices in humid atmosphere lead to the onset of new traps acting as recombination centers. Furthermore, aging would not affect uniformly the defect levels in the polymer. Shallow trap states (below 0.3 eV) remain stable, whereas the enhancement in trap density of deeper trap levels (above 0.3 eV) have been observed, suggesting that degradation by electrical stress leads to an increase in density of deep levels.