Role of host excimer formation in the degradation of organic light-emitting devices

Abstract

Host-guest structures are used in most state-of-the-art organic light-emitting devices, with the host transporting charge and confining excitons on the guest. While the host often plays a critical role in achieving high efficiency and stability, predicting and understanding these effects is a persistent design challenge which slows the discovery of new active materials. Closely related host molecules, which differ only by several functional groups, often show drastically different degradation behavior. Here, we explore this observation for the archetypical carbazole hosts 4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′-bis(carbazole-9-yl)-2,2′-dimethylbiphenyl (CDBP). While devices based on these hosts show similar efficiencies, CDBP-based devices show a tenfold lower lifetime than CBP devices when paired with phosphorescent or fluorescent emitters. Using optically and electrically pumped degradation tests, mass spectrometry, compositional analysis, and low-temperature phosphorescence spectroscopy, the lifetimes of devices containing CDBP are shown to correlate with the formation of intermolecular 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.

Photoluminescence and electrochemiluminescence of thermally activated delayed fluorescence (TADF) emitters containing diphenylphosphine chalcogenide-substituted carbazole donors

“…Newcomb et al has identified the necessity to screen potential TADF OLED emitters for exciplex formation, as the presence of exciplexes was shown to lower device lifetime. 119 Masui et al also identified large EQE roll-off in OLEDs caused by charge imbalance in the emissive layer that are exacerbated at higher current densities. 109,120 The exciplex formation in these compounds is characteristic of compounds that also exhibit OLPL, thereby supporting OLECL emission as the dominant mechanism.…”

Discrimination of Degradation Mechanisms for Organic Light-Emitting Diodes by In Situ, Layer-Specific Spectroscopic Analysis

“…As the device was under operation, the EL intensity showed a gradual decrease with no change in the spectral shape (Figure 2a), suggesting no new emissive materials forming in the EML and no significant shift of the recombination zone, which implies that the out-coupling efficiency of the device remains unchanged during operation. 25,26 The T50 (time that the EL intensity drops to 50% of its initial intensity) of the device was 2715 h, as determined by fitting the EL intensity drop to a stretched exponential decay function (Figure S1c). No dark spots were formed during operation, which means that external degradation factors such as particles introduced in the device fabrication process or electrode oxidation can be excluded.…”

Photoluminescence and Electrochemiluminescence of Thermally Activated Delayed Fluorescence (TADF) Emitters Containing Diphenylphosphine Chalcogenide-Substituted Carbazole Donors

“…Newcomb et al has identified the necessity to screen potential TADF OLED emitters for exciplex formation, as the presence of exciplexes was shown to lower device lifetime. 112 Masui et al also identified large EQE roll-off in OLEDs caused by charge imbalance in the emissive layer that are exacerbated at higher current densities. 101,113 The exciplex formation in these compounds is characteristic of compounds that also exhibit OLPL, thereby supporting OLECL emission as the dominant mechanism.…”