The Impacts of Solvents, Heat Treatments and Hole Injection Layers on the Electroluminescent Lifetime of Organic Light-Emitting Devices

Abstract

Since their invention over three decades ago, organic light-emitting devices (OLEDs) have attracted tremendous interest for display and solid-state lighting applications and have already been commercialized in smartphones, tablets and television screens. However, the most coveted potential of OLED technology is to enable ultra-low cost, roll-to-roll manufacturing of large-area panels on flexible substrates. To date, commercial OLED products rely on high-cost vacuum deposition techniques and thus fail to realize this potential. In particular, the lifetime, of solution-based (and thus printable) devices remains well below commercially acceptable standards.

The significant lifetime limitations of solution-based devices demand a more thorough understanding of the impact of the unique factors involved in the fabrication of these devices. Solution-processable hole injection layers (HILs), solvents, and heat or drying treatments are three such factors that play a crucial role in solution-processed devices. The principle aim of this work is to understand the influence of these factors on OLED lifetime in vacuum-deposited devices, independent of the multitude and variability of other parameters (e.g.: drying conditions, solubility, solution concertation) involved in most solution-processing methods; and to demonstrate proof-of-concept strategies to mitigate potentially adverse effects for application in solution-processed OLEDs.

Results show that solution-processed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) HILs are susceptible to electron-induced degradation, a mechanism that can lead to relatively short OLED electroluminescent (EL) lifetimes. This degradation can be minimized by selecting hole transporting materials and device structures that minimize electron leakage to the HIL, resulting in a lifetime improvement of up to 20x.

The effects of solvent and heat treatments on device efficiency and EL lifetime across a variety of hole injection and hole transport materials were found to vary considerably depending on the specific material combination. The extent of the morphological changes induced by the two treatments is highly material-dependent and does not necessarily correlate with device efficiency and EL lifetime. This suggests that additional, material-specific factors should be likely be considered in future correlations of device characteristics to the morphology of corresponding organic films for solution-processed devices.

Finally, solvent treatment of carbazole hole transport layers was found to induce substantial aggregation and lead to shorter EL lifetimes and lower device efficiency. The origin of this effect was found to be a decrease in photoluminescence quantum yield resulting from this aggregation. Material intermixing was shown to suppress this aggregation and resulted in improved device efficiency and a 2.5x increase in EL lifetime.