Dependence of Organic Interlayer Diffusion on Glass-Transition Temperature in OLEDs

Abstract

Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (T) of the material with the lowest T. Diffusion occurred when the material with the lowest T formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative T's. If the supercooled liquid formed at a temperature significantly lower than the T of the higher-T material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two T's were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the "burn in" often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes.

Record High External Quantum Efficiency of 19.2% Achieved in Light‐Emitting Diodes of Colloidal Quantum Wells Enabled by Hot‐Injection Shell Growth

“…Nevertheless, a low EQE of 7.2% is obtained (Figure S16, Supporting Information). Furthermore, a stepwise HTL NPB (30 nm)/4,4′,4″‐tris(carbazol‐9‐yl)‐triphenylamine (TCTA, 30 nm) has been used to develop CQW‐LEDs, since TCTA has a HOMO (5.7 eV) locating between the HOMO of NPB (5.3 eV) and the valence band maximum of CdSe/Cd 0.25 Zn 0.75 S. Additionally, TCTA exhibits a high morphological stability ( T g = 152 °C) . As shown in Figure S17 (Supporting Information), T 50 for this device at 100 cd m −2 is 57.0 h, which is 4.5‐fold longer than that of CBP‐based CQW‐LEDs.…”

“…Furthermore, a stepwise HTL NPB (30 nm)/4,4′,4″-tris(carbazol-9-yl)-triphenylamine (TCTA, 30 nm) has been used to develop CQW-LEDs, since TCTA has a HOMO (5.7 eV) [63] locating between the HOMO of NPB (5.3 eV) [62] and the valence band maximum of CdSe/Cd 0.25 Zn 0.75 S. Additionally, TCTA exhibits a high morphological stability (T g = 152 °C). [64] As shown in Figure S17 Adv. Mater.…”

Sputter Depth‐Profile Study of Accelerated Interface Mixing by Thermal Annealing in Solution‐Processed Organic Light‐Emitting Diodes

“…However, the thermal annealing fundamentally modifies the properties of each organic layer and the interface and alters the performance of the devices significantly. In conventional vapor‐deposited OLEDs, it is known that the heat treatment of the device at a temperature higher than the glass transition temperature ( T g ) of the organic material induces interfacial mixing . The organic materials at such temperature turn into liquid‐like phase and mix with each other relatively easily.…”

Development of a Library of Novel Compounds for Use in Organic Solar Cells

“…Tools such as X-ray diffraction (XRD), neutron scattering, atomic force microscopy (AFM), transmission electron microscopy (TEM), and ellipsometry are all viable methods to probe the 3D characteristics of a film. [189][190][191][192][193][194][195] However, these imaging techniques are better regarded as complementary tools in expanding the understanding of previously analysed device output characteristics. As these techniques are often time and cost intensive (or not widely available), they are not practical for everyday predictive optimisation processes.…”

“…Particularly effective on systems containing materials with a glass transition temperature (Tg), thermally induced reorganisation processes enable significant changes to the BHJ morphology. 194,[197][198][199][200] A strong relationship between the annealing procedure and I-V characteristics is demonstrated across a number of systems due to crystallisation and diffusion of the active layer materials. [201][202][203][204][205][206][207][208][209] High boiling solvent additives (e.g., diiodooctane (DIO)) are added to adjust the rate at which one or more components are deposited from solution.…”