The photophysics of singlet, triplet, and degradation trap statesin 4,4-N, N'-dicarbazolyl-1,1'-biphenyl

摘要

In this paper we report the results of optical characterization of 4,4-N,N' -dicarbazolyl-1,1 '-biphenyl (CBP), known as a host material for phosphorescent light emitting devices. Usingabsorption, steady state, and time-resolved spectroscopy, we explore the singlet and triplet states insolid and solution samples of CBP. In solutions we observe two distinct short-lived states withwell-resolved emission originating from individual molecule singlet states (at 365 and 380 nm) and"quenching" low energy (LE) states (at 404 and 424 nm). The latter are seen only in saturatedsolutions and solid samples. Both of those species have different lifetimes. After UV exposure ofvery concentrated degassed solution the intensities of the LE bands starts to decrease. The longer thesolution is exposed to UV, the less emission is seen at 404 and 424 nm, until it is totally gone. Thespectrum of the highly concentrated solution is then the same as the spectrum of dilute solution, i.e.,only emission at 365 and 380 nm is present. An increase in intensities of the singlet emission peakscorrelates with an increase in UV exposure time. Similar behavior is observed in evaporated CBPfilm. We propose that this behavior is due to chemical instability of the weak N-C bonding ofcarbazolyl moiety—this creates new degradational species over time which dissociate afterexposure to UV. We believe this to be the reason for variation in CBP fluorescence and delayedfluorescence spectra recorded by various research groups. Further, we detected two types of verylong-lived states. One of these states (higher energy) is ascribed to molecular phosphorescenceemission, the other to emission from low energy triplet trap states which we relate to degradationalspecies. We propose that triplets are more easily caught by these latter sites when their hopping rateincreases, and they emit inefficiently from these lower energy sites.