Exciton-Induced Degradation of Carbazole-Based Host Materials and Its Role in the Electroluminescence Spectral Changes in Phosphorescent Organic Light Emitting Devices with Electrical Aging

We investigate the origins of the long-wavelength bands that appear in the emission spectra of carbazole-based host materials and play a role in the electroluminescence (EL) spectral changes of phosphorescent organic light emitting devices (PhOLEDs) with electrical aging. 4,4'-Bis(carbazol-9-yl)biphenyl (CBP) is used as a model carbazole host material and is studied using photoluminescence, EL, and atomic force microscopy measurements under various stress scenarios in both single and bilayer devices and in combination with various electron transport layer (ETL) materials. Results show that exciton-induced morphological aggregation of CBP is behind the appearance of those long-wavelength bands and that complexation between the aggregated CBP molecules and ETL molecules plays a role in this phenomenon. Comparisons between the effects of exciton and thermal stress suggest that exciton-induced aggregation may be limited to short-range molecular ordering or pairing (e.g., dimer or trimer species formation) versus longer-range ordering (crystallization) in the case of thermal stress. The findings provide new insights into exciton-induced degradation in wide band gap host materials and its role in limiting the stability of PhOLEDs.