Phosphorescent organic light-emitting devices (OLEDs) can suffer a significant reduction in device efficiency under high current density excitation. This steady-state efficiency roll-off is frequently modeled by including losses from exciton-exciton and exciton-polaron quenching. Despite success in modeling the steady-state efficiency roll-off, the corresponding transient electroluminescence behavior has not been modeled as effectively using the same quenching processes. In this work, both the steady-state and transient electroluminescence behavior of phosphorescent OLEDs based on tris[2-phenylpyridinato-C2,N]Iridium(III) (Ir(ppy)3) are successfully reproduced by considering a dynamic polaron population. Within this model, polarons are able to either form excitons or leak through the device emissive layer, reducing the overall efficiency. This formalism permits a natural and rigorous connection between exciton and polaron dynamics and device charge balance, with the charge balance cast as the efficiency of exciton formation. The full dynamics model reproduces both the rise and decay of transient electroluminescence, as well as the full dependence of the external quantum efficiency on current density. Fit parameters are independently verified using separate studies of transient and steady-state photoluminescence. The model provides a complete picture for the dynamics present during the electrical operation of phosphorescent OLEDs, while also offering a direct route to elucidate exciton formation.

Degradation in organic light-emitting devices (OLEDs) is generally driven by reactions involving excitons and polarons. Accordingly, a common design strategy to improve OLED lifetime is to reduce the density of these species by engineering an emissive layer architecture to achieve a broad exciton recombination zone. Here, the effect of exciton density on device degradation is analyzed in a mixed host emissive layer (M-EML) architecture which exhibits a broad recombination zone. To gain further insight into the dominant degradation mechanism, losses in the exciton formation efficiency and photoluminescence (PL) efficiency are decoupled by tracking the emissive layer PL during device degradation. By varying the starting luminance and M-EML thickness, the rate of PL degradation is found to depend strongly on recombination zone width and hence exciton density. In contrast, losses in the exciton formation depend only weakly on the recombination zone, and thus may originate outside of the emissive layer. These results suggest that the lifetime enhancement observed in the M-EML architectures reflects a reduction in the rate of PL degradation. Moreover, the varying roles of excitons and polarons in degrading the PL and exciton formation efficiencies suggest that kinetically distinct pathways drive OLED degradation and that a single degradation mechanism cannot be assumed when attempting to model the device lifetime. This work highlights the potential to extract fundamental insight into OLED degradation by tracking the emissive layer PL during lifetime testing, while also enabling diagnostic tests on the root causes of device instability.

The analysis of organic light-emitting device degradation is typically restricted to fitting the overall luminance loss as a function of time or the characterization of fully degraded devices. To develop a more complete understanding of degradation, additional specific data are needed as a function of luminance loss. The overall degradation in luminance during testing can be decoupled into a loss in emitter photoluminescence efficiency and a reduction in the exciton formation efficiency. Here, we demonstrate a method that permits separation of these component efficiencies, yielding the time evolution of two additional specific device parameters that can be used in interpreting and modeling degradation without modification to the device architecture or introduction of any additional post-degradation characterization steps. Here, devices based on the phosphor tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3) are characterized as a function of initial luminance and emissive layer thickness. The overall loss in device luminance is found to originate primarily from a reduction in the exciton formation efficiency which is exacerbated in devices with thinner emissive layers. Interestingly, the contribution to overall degradation from a reduction in the efficiency of exciton recombination (i.e., photoluminescence) is unaffected by thickness, suggesting a fixed exciton recombination zone width and degradation at an interface.

We describe here three alkynyl substituted naphthalenes that display promising luminescence characteristics. Each compound is easily and efficiently synthesized in three steps by capitalizing on the hexadehydro-Diels–Alder (HDDA) cycloisomerization reaction in which an intermediate benzyne is captured by tetraphenylcyclopentadienone, a classical trap for benzyne itself. These compounds luminesce in the deep blue when stimulated either optically (i.e., photoluminescence in both solution and solid films) or electrically [in a light-emitting diode (LED)]. The photophysical properties are relatively insensitive to the electronic nature of the substituents (H, OMe, CO2Me) that define these otherwise identical compounds. Overall, our observations suggest that the twisted nature of the five adjacent aryl groups serves to minimize the intermolecular interaction between core naphthalene units in different sample morphologies. These compounds represent promising leads for the identification of others of value as the emissive component of organic LEDs (OLEDs).

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.