After being an active and intense area of research for well over a decade, organic light-emitting diode (OLED) displays have now achieved commercial success for displays used in products ranging from cellphones to large-format televisions. Advances in the technology built upon both OLED device fundamentals and new-materials development are leading to improvements in device efficiency and lifetime. Despite these successes, an effective way to stretch device lifetime is needed if OLEDs are to achieve wider adoption—particularly in products with longer replacement cycles.
Understanding What Causes OLED Degradation
Achieving this goal while developing the next generation of OLED materials requires a thorough understanding of all the potential degradation modes that cause OLED device failure. Although external influences on OLED device failure (e.g., processing and handling) can be substantial, careful control greatly minimizes their impact on today’s OLED devices. Better control of intrinsic failure modes will further improve device performance—making this the primary focus of ongoing research and new materials design efforts.
Each of the diﬀerent organic layers in an OLED device stack can be susceptible to unique decomposition pathways, but stability toward excitons is critical for emissive layer (EML) materials, as well as any layer near the recombination zone. The work discussed in this post focuses on degradation modes within an OLED’s hole transport layer (HTL). The goal is to identify the general decomposition paths occurring in an operating device and use this information to design new derivatives that can block these pathways.
Material selection, device fabrication and performance
The pursuit of novel HTL materials generates some challenges necessitating computational predictions to ensure optimal material selection and prioritization. Figure 1 compares the molecular structures of several different HTL material options. Compound 1 has the highest occupied molecular orbital (HOMO), similar to that of typical HTL materials TPD and HPD, with the lowest unoccupied molecular orbital (LUMO).
Figure 1. Structures and properties of novel HTL 1 and typical HTL materials NPD and TPD
The next step involved testing compound 1 for hole-transporting ability. To do this, OLED devices were fabricated using the novel compound, as well as NPD and TPD as reference HTL materials, to serve as a performance benchmark. The results indicated that compound 1 achieved efficiencies and CIE performance that were fairly comparable to the standard materials, while requiring slightly higher voltage to drive. However, the use of the new compound led to significantly lower device lifetime.
Post-mortem analysis of OLED devices
What happened? An initial assessment of the stark disparity in device lifetimes suggested a problem with compound 1’s weak aliphatic C-N bond, so further analysis was performed to confirm this instability and to improve upon the fundamental understanding of intrinsic failure modes. Careful examination revealed a new material formed as a byproduct of aging on the device with compound 1 HTLs. However, the new material had a mass corresponding to loss of hydrogen molecules, disproving the belief that the lower device lifetime could be blamed on compound 1’s aliphatic C-N bond.
Further examination of aged and unaged pairs of devices containing the various HTLs indicated that the new product forms on compound 1 HTLs as a result of device aging. Finally, ultraviolet photoelectron spectroscopy (UPS) measurements were carried out on compound 1 and NPD to determine the HOMO levels in a thin film, and this value for both NPD and compound 1 changed as a function of irradiation time.
To better understand the degradation observed under device operation, a solution-based technique was developed to model the decomposition chemistry. Given the localization of the decomposition—most prominent near the hole transport layer (HTL)/emissive layer (EML) interface—an exciton-based degradation pathway was initially suspected, so photoexcitation was used to initiate degradation in these model studies.
Two signiﬁcant new peaks were observed in compound 1 after just 30 seconds’ exposure to a 450W mercury vapor UV lamp. One peak matched the retention time and mass of the impurity found in the aged devices described previously. Intrinsic degradation of compound 1 generated a product with mass corresponding to two H atoms less than the parent compound; the additional smaller peak with a retention time of 9.6 minutes was apparently due to subsequent degradation, showing a mass of four H atoms less than the parent.
Figure 1a. MS/MS spectra for compound 1 (a) and its major photolysis products, 1-H2 (b) and 1−2H2 (c)
The resulting fragmentation patterns for compound 1 indicate a loss of diphenylamine as a major fragmentation pathway for the material before irradiation, and a second loss of diphenylamine as expected (Figure 1a). This led the 1−H2 and 1−2H2 products being structured such that cyclization would form a carbazole that would not easily fragment away from the main core structure.
Modeling cyclization to form carbazoles
Photolysis experiments pointed toward an interesting new exciton-based intrinsic degradation pathway for HTL materials, and its observation in aged devices supported this having an impact on OLED device stability. However, the observation of a difference in stability between NPD and compound 1 provided no insight into the cause of this difference. To design materials that would provide improved device lifetime, it is necessary to predict their ability to cyclize.
To that end, the last step in the experimentation process was to undertake computational efforts to understand the differences in reactivity between compound 1 and NPD or TPD, focusing initially on the transition state between for C-C bond formation.
Post-mortem analyses of several aged OLED devices revealed a new intrinsic HTL degradation pathway. With the assistance of SIMS, UPS, CV, photolysis and MS/MS analytical approaches, this pathway was determined to be active in a typical HTL molecule, NPD, as well as a novel HTL material, compound 1. This degradation pathway is parallel to and faster than the previously described fragmentation for materials such as NPD.
Photolysis studies of NPD and compound 1 showed diﬀerent photolytic stabilities, suggesting materials that have a lower propensity to undergo cyclization may possess longer device lifetime. The work also showed that this novel HTL degradation is exciton-induced and could be facilitated by a low-transition-state barrier for the developing C−C bond from a triplet intermediate.
The bottom line: This discovery has great potential for further impacting molecular design of HTL molecules and leveraging this approach can enable developers to design longer-lived OLED devices.