Predicting the Operational Stability of Phosphorescent OLED Host Molecules from First Principles: A Case Study

Abstract

Low operational stability is the main limiting factor for commercialization of the blue phosphorescent organic light emitting diodes (PhOLEDs). The high energy and long lifetime of triplet excitons in blue PhOLEDs makes them more prone to degradation. Degradation of the host molecules in the emitting layer of PhOLEDs is one of the possible mechanisms leading to the luminosity loss in the course of device operation. Although possible degradation mechanisms are proposed in the literature, predicting the degradation kinetics is not straightforward because the evolution of excited states should be accurately described. We propose a computational scheme to assess the operational stability of PhOLED host materials. Our protocol relies on the usage of the multireference CASSCF/XMCQDPT2 method. In the present work we consider the degradation of four prototypical blue PhOLED host molecules in the charged and excited states as well as the degradation induced by exciton–polaron and exciton–exciton annihilation processes with the focus on breaking of exocyclic C–C or C–N bonds and triazine ring fission. By analyzing the calculated activation energies for different mechanisms we found the least stable states and the most probable dissociation pathways. On the basis of our computations, we derived a stability series for the studied molecules and determine the structural features that provide higher stability with respect to the unimolecular dissociation.

Mer‐Ir(ppy)3 to Fac‐Ir(ppy)3 Photoisomerization

“…Facial/meridional stereoisomerism in trisbidentate Ir(III) complexes strongly determines their photophysical properties and thereby their efficiency and stability within a phosphorescence-based organic light-emitting diode (PhOLED) device. [9] In recent years, numerous computational [10][11][12] and experimental [13][14][15][16] efforts have been made to decipher the underlying degradation mechanisms, and still many open questions remain. These investigations highlight the protagonistic role of a non-previously described 3 MC (triplet metal-centred) state bearing one stretched trans Ir-N position.…”

“…[1,2] To further improve their device operational lifetimes, and especially in the case of blue phosphors, it is crucial to acquire durable and efficient materials that do not suffer intrinsic chemical degradation upon PhOLED operation. [9] In recent years, numerous computational [10][11][12] and experimental [13][14][15][16] efforts have been made to decipher the underlying degradation mechanisms, and still many open questions remain. [9] In recent years, numerous computational [10][11][12] and experimental [13][14][15][16] efforts have been made to decipher the underlying degradation mechanisms, and still many open questions remain.…”

Predicting Operational Stability for Organic Light‐Emitting Diodes with Exciplex Cohosts

“…Recent studies have demonstrated that susceptible weak chemical bonds and their possible fragments could be theoretical simulated via identification of dissociated activation energy (E A ) under time dependent density functional theory (TDDFT) calculations. [17][18][19][20] However, the actual relationship between Organic light-emitting diodes (OLEDs) employing exciplex cohosts have gained attractive interest due to the promising high efficiency, low driving voltage, and potential low cost in future solid-state lighting sources and full-color displays. However, their device lifetime is still the most challenging weakness and rarely studied, which is regarded as a time consuming and complicated work.…”

“…As charged-excited state is extremely short-lived and difficult to measure clearly, however, their formative probabilities show positive dependence to the polaron and exciton density. [19] Fortunately, the amount of polarons could be properly controlled by optimizing HTM:ETM ratios. [34] Thus, we mainly discuss coordinated relationship between average exciton lifetime (<τ>) and ΔE EPA here.…”

“…Dissociated products would further react with neighboring molecules to form defects, such as carrier traps, nonradiative recombination centers and luminescence quenchers. [19,20,45] Thus, it is of interest to understand their inherent degrading behaviors to further confirm the dominant degradation mechanism by comparing the corresponding pristine and degraded devices. Figure 5 illustrates the TrEL decay profiles by inputting a voltage pulse in PQ2Ir emission.…”