The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N′-dicarbazolyl-1,1′-biphenyl

Abstract

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. Using absorption, steady state, and time-resolved spectroscopy, we explore the singlet and triplet states in solid and solution samples of CBP. In solutions we observe two distinct short-lived states with well-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 saturated solutions and solid samples. Both of those species have different lifetimes. After UV exposure of very concentrated degassed solution the intensities of the LE bands starts to decrease. The longer the solution is exposed to UV, the less emission is seen at 404 and 424 nm, until it is totally gone. The spectrum 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 peaks correlates with an increase in UV exposure time. Similar behavior is observed in evaporated CBP film. We propose that this behavior is due to chemical instability of the weak N–C bonding of carbazolyl moiety—this creates new degradational species over time which dissociate after exposure to UV. We believe this to be the reason for variation in CBP fluorescence and delayed fluorescence spectra recorded by various research groups. Further, we detected two types of very long-lived states. One of these states (higher energy) is ascribed to molecular phosphorescence emission, the other to emission from low energy triplet trap states which we relate to degradational species. We propose that triplets are more easily caught by these latter sites when their hopping rate increases, and they emit inefficiently from these lower energy sites.

Is Poly(vinylcarbazole) a Good Host for Blue Phosphorescent Dopants in PLEDs? Dimer Formation and Their Effects on the Triplet Energy Level of Poly(N‐vinylcarbazole) and Poly(N‐Ethyl‐2‐Vinylcarbazole)

“…When we record the emission spectrum of spincoated film of P2VK, we observed emission only at 525 nm, irrespective of which wavelength we excited, indicating increased energy transfer via migration. This is very similar to previously observed behaviour of polyfluorene40 or 4,4'‐ N , N ′‐dicarbazolyl‐1,1′‐biphenyl38 containing low energy trap states.…”

“…Furthermore, by using sensitized phosphorescence quenching Klopffer et al22 have shown that these traps are populated via the triplet channel, probably by Dexter transfer, which is very reasonable in films where emissive sites can be within angstroms of each other, thus making this transfer much more efficient than in solutions, even if highly concentrated. The concentration of trap species might be extremely low, in the range of 5 × 10 −3 % as conjectured by Klopffer et al22 Hopping is very efficient in these types of films and triplets have long lifetimes to enable them to find a trap easily 36–38. However Chakrabarty et al23 estimate that nearly all carbazoles are paired at 10 K. Benten et al39 have performed calculation on bridged carbazole pairs, pointing out that the large red shift of the monomer to dimer phosphorescent cannot be simply attributed to excitonic delocalisation; substantial charge resonance effects must contribute, with large orbital overlap between the pairs.…”

Transparent InP Quantum Dot Light‐Emitting Diodes with ZrO2 Electron Transport Layer and Indium Zinc Oxide Top Electrode

“…But, because the conduction band minimum and the valence band maximum of InP QDs are higher than those of CdSe QDs, there are large hole injection barrier and small electron blocking barrier between InP QDs and CBP layers ( Figure a) . In Figure a, there is an additional emission peak at about 400 nm for the device with CBP HTL due to the electron–hole recombination in CBP layer caused by the unbalance of charge injection . To eliminate this additional emission peak, we used another HTL material, TAPC, which shows small hole injection barrier between TAPC and MoO 3 and large electron blocking barrier between TAPC and InP QDs (Figure b).…”

Engineering the singlet–triplet energy splitting in a TADF molecule

“…The PH spectra identified at long TD is clearly the PH of the host, CBP, having three well defined peaks (at 491 nm, 518 nm and 557 nm) with increasing emission intensity on cooling down the system. 27,28 In this thin film, the excited states were not confined on the DPO-TXO2 molecules, and a quenching of fluorescence was observed by the host. Therefore, the 1 CT was identified to be at 2.82 eV and the 1 LED level was not identified.…”