Temperature-dependent nanomorphology-performance relations in binary iridium complex blend films for organic light emitting diodes

Effect of thermal annealing on an emissive layer containing a blend of a small molecule and polymer as host for application in OLEDs

In order to improve the performance of OLEDs, a host-guest mixture was used as an emissive layer. To have better host properties, a mixture of different materials with suitable properties can also be used as a host. In this study, we used a mixture of a polymer and a small molecule as the host and studied the effect of thermal annealing on the emissive layer properties by using Ir(ppy)3 as the emitter. UV-visible absorption, steady-state and time-resolved photoluminescence, scanning electron microscopy, atomic force microscopy, and optical microscopic studies were performed to study the film properties. Devices were fabricated and their current-voltage and luminance-voltage characteristics were studied. Charge-carrier mobility in the devices was studied by dark CELIV and transient electroluminescence methods. We show that, below the glass transition temperature of the polymer, the small molecules formed aggregates due to thermal annealing, which was beneficial for the device performance in the lower-temperature range, mainly due to the improved electron mobility. However, this aggregate formation was detrimental in the higher-temperature range, as it led to inefficient energy transfer due to the increased pure phase formation. At temperatures above the glass transition temperature of the polymer, the small molecules were seen to be distributed more uniformly into the polymer matrix. However, as a result of the degradation of the polymer property due to degradation of the primary chain of the phenyl ring of the polymer, this uniform distribution was not of any use and the device performance deteriorated.

Process Engineered Spontaneous Orientation Polarization in Organic Light-Emitting Devices

Polar molecules with appreciable permanent dipole moments (PDMs) are widely used as the electron transport layer (ETL) in organic light-emitting devices (OLEDs). When the PDMs spontaneously align, a macroscopic polarization field can be observed, a phenomenon known as spontaneous orientation polarization (SOP). The presence of SOP in the ETL induces considerable surface potential and charge accumulation that is capable of quenching excitons and reducing device efficiency. While prior work has shown that the degree of SOP is sensitive to film processing conditions, this work considers SOP formation by quantitatively treating the vapor-deposited film as a supercooled glass, in analogy to prior work on birefringence in organic thin films. Importantly, the impact of varying thin-film deposition rate and relative temperature is unified into a single framework, providing a useful tool to predict the SOP formation efficiency for a polar material, as well as in blends of polar materials. Finally, in situ photoluminescence characterization and efficiency measurements reveal that SOP-induced exciton-polaron quenching can be reduced through an appropriate choice of processing conditions, leading to enhanced OLED efficiency.

Effects of steric encumbrance of iridium(iii) complex core on performance of solution-processed organic light emitting diodes

Iridium(iii) complexes are the most frequently applied commercialized green and red emitters for organic light emitting diode (OLED) displays. Throughout years a significant research effort has been devoted to modify these compounds, in order to make them suitable for cost-effective solution-processing techniques, such as inkjet printing. To achieve this, the inherent tendency of the complex molecules to form poorly emissive aggregates needs to be suppressed. In many cases this has been achieved by an encapsulation of the iridium(iii) complex core with dendritic structures, composed of either passive or charge-transporting fragments. In order to validate this approach, we acquired three structural analogues of the conventional green emitter Ir(ppy)3, which possess gradually increasing sterical encumberment at the complex surface. Corresponding OLEDs were examined, with three distinctively different active emissive layer compositions in terms of charge transportation characteristics. The results show that in the all scenarios the unmodified Ir(ppy)3 outperforms the compounds with attached bulky groups. The in-device performance of the emitter is directly related to its charge trapping ability, which is being compromised in the presence of dendritic auxiliary substituents.

Correlation of Controllable Aggregation with Light-Emitting Property in Polymer Blend Optoelectronic Devices

The control of solution-processed emitting layers in organic-based optoelectronic devices enables cost-effective processing and highly efficient properties. However, a solution-based protocol for emitter fabrication is highly complex, and the link between the device performance and internal nanoscale features as well as three associated fabricating parameters (e.g., the employed solvents, annealing temperatures, and molecular concentration) needs to be understood. Here, this study investigates the influence of the solution-processing parameters on the nanostructure-property relationship in light emitters that consist of iridium complexes doped in polymer. The boiling points and evaporation rates of the selected solvents govern the nanomorphology of molecular aggregation in the as-processed state, and the aggregation is either needle-like, spherical, or even a mixture of needles and spheres. Furthermore, a direct observation via in situ heating microscopy indicates that annealing of emitters containing a needle-type aggregation promotes the associated molecular transport, leading to a substantial reduction in the surface roughness. Consequently, a nearly threefold increase in the current efficiency of the device is induced. These findings have important implications for the tuning of the aggregation of iridium complexes for emitters used in the new evolution of high-performance organic-based optoelectronic devices.