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

Abstract

Understanding the mechanism responsible for the temperature-dependent performances of emitting layers is essential for developing advanced phosphorescent organic light emitting diodes. We described the morphological evolution occurring in PVK:Ir(ppy)3 binary blend films, with respect to thermal annealing up to 300 °C, by coupling atomic force microscopy and transmission electron microscopy. In particular, in situ temperature-dependent experimental characterization was performed to directly determine the overall sequence of morphological evolution occurring in the films. The device thermally annealed at 200 °C exhibits a noticeable enhancement in the performances, compared to the devices in the as-processed state and to the devices annealed at 300 °C. Our approaches reveal that the Ir(ppy)3 molecules, with a needle-like structure in the as-processed state, were aggregated, and thus diffused into PVK without a morphological change at the temperature regime between 150 °C and 200 °C. Moreover, both network-like and droplet patterns existed in the devices annealed at 300 °C, which was beyond the glass temperature of PVK, leading to a profound increase in the surface roughness. The observed pattern formation is discussed in terms of viscoelastic phase separation. Based on our experimental findings, we propose that the performances of the devices are significantly controlled by the diffusion of dopant molecules and the morphological evolution of the host materials in binary blend systems.

Process Engineered Spontaneous Orientation Polarization in Organic Light-Emitting Devices

“…The same “shifting” procedure can be applied to the temperature-dependent SOP data for TPBi reported by Bangsund et al, where the average orientation was calculated from the GSP slope obtained from a two-point estimation based on DCM data. Although extra care should be taken in the analysis of the samples fabricated at elevated temperatures due to potential changes in the film morphology − and impurity incorporation, , the temperature-dependent data for TPBi appear to follow the same general trend as shown in Figure a. The extracted shift factor value of 0.075 decade/K means that for the SOP effect under investigation, raising the substrate temperature by 13.3 K is equivalent to depositing 10 times more slowly (1/S.F.…”

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

“…Moreover, based on in situ heating microscopy approach ranging from room temperature to 200 °C, the temperature‐dependent morphological evolution that occurred within the emitters produced from the different solvents was associated with molecular dynamics. The details for the CB‐processed emitters have been published elsewhere . Regardless of the employed solvents, the dark contrast of the needle‐like Ir(ppy) 3 aggregation observed for the CB‐, THF‐, CF‐, and DCM‐processed films became gradually bright from the corner regions of the needle‐like and spherical aggregations from a temperature of 150 °C (see the red arrows), while the contrast of spherical Ir(ppy) 3 aggregates with a radius less than 100 nm changed (Figure b–d).…”

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

“…NMR spectra and the elemental composition of the synthesized materials are consistent with the proposed structures. 13 C spectra of 3TPY indicate a chemical equivalence of 2-phenylpyridine (ppy) ligands and ancillary groups, underlining C 3 symmetry of the complex core. The symmetry is being broken in the case of 1TPY and 2TPY, resulting in an observation of three chemically inequivalent sets of ppy signals.…”

“…10,12 In the solution-processed lms the aggregation of Ir(ppy) 3 is even more pronounced, causing large-scale phase separation between emitter and host materials. 13,14 Again, this is governed by the attracting force, as the aggregation process between individual Ir(ppy) 3 molecules is proven to take place even in solutions. 15 As a consequence, wet-processed OLEDs based on Ir(ppy) 3 show notably reduced performance characteristics in comparison to vacuum deposited analogues.…”