On the Role of Molecular Structure on the Lifetime of Organic Light-Emitting Devices

Abstract

Organic light-emitting devices (OLEDs) are an established technology in mobile and large format display technologies due to their improved dark contrast and efficiency. The ability to create long-lived devices for lighting or high-brightness applications relies on understanding the degradation mechanisms inherent to excitonic organic semiconductor systems. This work discusses procedures and methods to characterize and understand the degradation in OLEDs, with emphasis on devices containing a phosphorescent light-emitter. Two structurally similar hosts, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and 4,4'-bis(carbazole-9-yl)-2,2'-dimethylbiphenyl (CDBP) are investigated as hosts in phosphorescent OLEDs, as a stark 10x difference in operational lifetime is observed despite their similarities. Using accelerated aging tests, molecular fragment analysis, and low-temperature phosphorescence spectroscopy, the reduced lifetimes of devices containing CDBP are correlated with the formation of triplet excimer states. These findings suggest that candidate host molecules should be screened for excimer formation, as host excimers may aggravate device degradation and lower device stability.