Migration of small molecules during the degradation of organic light-emitting diodes

Degradation process of organic light-emitting diodes (OLEDs) is examined directly by X-ray photoelectron spectroscopy with in situ high-energy C60+ and low-energy Ar+ co-sputtering. It is proven that this analytical technique clearly indicates the elemental depth profile of as-prepared OLED device (Al/LiF/2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)/4,4′-bis(carbazol-9-yl)biphenyl:bis(3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) iridium (III) (CBP:FIrpic)/poly(ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS)/ITO). Devices operated for different durations are subjected to this profiling technique for studying the change of elemental distribution. In addition to some of the accepted degradation mechanisms, it is observed that small TPBi molecules migrate towards the ITO anode under a direct driving voltage while retaining its original structure. It is also observed that oxygen diffused into the device through the Al cathode and along the Al–organic interface. Molecules with high stereo-hindrance may have less degradation due to the electron migration.