Molecular Stacking Effect on Small-Molecular Organic Light-Emitting Diodes Prepared with Solution Process

Abstract

The light-emitting layer (EML) is generally prepared by mixing the host and dopant to realize an organic light-emitting diode (OLED). However, phase separation is often observed during the fabrication process to prepare OLEDs, depending on the structure of the host materials. In particular, phase separation because of π−π stacking is frequently observed during thermal annealing for the solution process. The annealing process is required for solvent removal and complete relaxation of the molecule. Hence, the materials with a high glass transition temperature (T g ) are ideal because phase separation occurs because of π−π stacking during the annealing process, if T g is too low. To understand this phenomenon, we compared two host materials with similar molecular weights but different threedimensional connectivity, which causes different rotational freedom. Then, we investigated the effect on the device properties, depending on the annealing conditions. In both materials, when the annealing temperature rises above 120 °C, the dopant completely escaped from the EML. However, the material that does not disturb the molecular stacking order by annealing because of its limited free rotation through the internal bond shows much better device characteristics even after annealing at a higher temperature than T g . The results show that interdiffusion at the interface and unstable internal density distribution with annealing temperature are responsible for the device degradation behavior.

An indenocarbazole-based host material for solution processable green phosphorescent organic light emitting diodes

“…However, according to a recent report by Lee et al, the phosphorescent dopant can be quasi-dewet from EML and escape during annealing procedure during solution processes if the thermal stability of the host materials is very low. 3 To conrm such a possibility, we proceeded with differential scanning calorimetry (DSC) analysis of the PCIC material and, in the end, conrmed that the glass transition temperature (T g ) is very low, about 98 C, and the T m is about 234 C (Table 1). So, we performed molecular dynamics simulation to predict how the relative distance between molecules can be changed on series of annealing processes.…”

“…In this process, the phosphorescent dopant is separated from the host material, resulting in poor thermal stability. In fact, in a previous paper we reported that the red dopant escaped from the host at 150 C. 3 Phosphorescent devices are inherently affected by many triplettriplet annihilation (TTA) or triplet-polaron quenching (TPQ) processes under high current density driving conditions, and these dopant agglomeration can be a major cause of reduced efficiency and lifetime. 4,5 Apart from this problem, the mixing of the hole transport layer (HTL) and emitting layer (EML) interface suppresses the device lifetime.…”

Identification of SARS‐CoV‐2 Main Protease Inhibitors Using Structure Based Virtual Screening and Molecular Dynamics Simulation of DrugBank Database

“…The root mean square deviation (RMSD) and root mean square fluctuation (RMSF) of ligand‐protein complex backbone were plotted and the stability of the ligand‐protein complex was determined. The analysis of RMSD, RMSF, and hydrogen bonding interactions were performed using the trajectory analysis incorporated in the Desmond [59] …”

“…The analysis of RMSD, RMSF, and hydrogen bonding interactions were performed using the trajectory analysis incorporated in the Desmond. [59] …”

10.1: Invited Paper: Importance of Interface Control in Solution Processed Organic Light Emitting Diodes

“…4 In particular, such a phase separation is frequently observed during thermal annealing processes. 5,6 Thus, we need to anneal in the range not more than 20 o C than glass transition temperature (Tg) because there could be a deterioration of the device performances. 7 However, it is not clear why annealing should be performed under those conditions.…”