Variable sensitivity of organic light-emitting diodes to operation-induced chemical degradation: Nature of the antagonistic relationship between lifetime and efficiency

Abstract

The efficiency of any organic light-emitting diode (OLED) decreases with prolonged operation. In fluorescent OLEDs containing the standard hole-transporting material NPB [N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine], prolonged operation also results in chemical degradation of the NPB. Qualitatively and quantitatively, the extent and location of chemical changes are consistent with the hypothesis that chemical degradation of NPB is largely responsible for the loss of device efficiency and that the degradation is initiated by bond-breaking in excited-state NPB. Blue fluorescent OLEDs with low operating voltages and high luminance efficiencies tend to lose efficiency much faster than OLEDs with higher operating voltages and lower efficiencies. Even so, the two types exhibit approximately the same kind and degree of chemical degradation after operation for equal times at equal current densities. In the low-voltage OLEDs, the electric field in the light-emitting layer is weaker, and the concentration of NPB radical-cations near the emission zone is smaller than in the higher-voltage devices. Apparently for this reason, degradation products impair the luminescent efficiency more severely in the low-voltage OLEDs. Such differing sensitivity of low-voltage and high-voltage OLEDs to the same amount of chemical degradation is further demonstrated by experiments with OLEDs that have been doped intentionally with a contaminant modeling a degradation product at realistic concentrations. It is also supported by a substantial recovery of efficiency after a conversion of a degraded low-voltage device into a high-voltage device by replacement of the cathode and electron-injecting interface.