Elucidating the Spatial Arrangement of Emitter Molecules in Organic Light‐Emitting Diode Films

Abstract

The effect of varying the emitter concentration on the structural properties of an archetypal phosphorescent blend consisting of 4,4'-bis(N-carbazolyl)biphenyl and tris(2-phenylpyridyl)iridium(III) has been investigated using nonequilibrium molecular dynamics (MD) simulations that mimic the process of vacuum deposition. By comparison with reflectometry measurements,w es howt hat the simulations provideanaccurate model of the average density of such films. The emitter molecules were found not to be evenly distributed throughout film, but rather they can form networks that providec harge and/or energy migration pathways, even at emitter concentrations as lowas% 5weight percent. At slightly higher concentrations,percolated networks form that span the entire system. While such networks would give improved charge transport, they could also lead to more non-radiative pathwaysf or the emissive state and ar esultant loss of efficiency.

Revealing the Interplay between Charge Transport, Luminescence Efficiency, and Morphology in Organic Light‐Emitting Diode Blends

“…For this analysis, a molecule or cluster was considered isolated if the nearest complex had an Ir–Ir distance of greater than 1.4 nm. This distance means that there is insufficient space for a TCTA molecule to be placed between neighboring Ir(ppy) 3 molecules based on the diameter of Ir(ppy) 3 being ≈1 nm . In addition, a distance of 0.4 nm is compatible with hopping transport of charges and Dexter energy transfer between neighboring complexes.…”

“…All simulations were performed using the GROMACS simulation package version 2018.3 with GPU acceleration . Molecules were randomly deposited onto a 34.1 nm by 33.4 nm graphene sheet using a deposition protocol similar to that previously reported by Tonnelé et al During the deposition process, two new molecules were inserted 2 nm from the top of the film every 10 ps with a random orientation and an initial velocity toward the surface to ensure they reached the growing film. The initial positions and orientations of the molecules above the surface were selected randomly such that the centers‐of‐mass were separated in the plane of the substrate by at least 10 nm.…”

“…Charges, van der Waals interactions, and bonded interactions for TCTA were assigned using the Automated Topology Builder (ATB) version 3.0 . The charges and interactions for Ir(ppy) 3 were identical to those used by Tonnelé et al The topology and optimized coordinate files are provided in the Supporting Information. Bonds involving hydrogen atoms were constrained using the LINCS algorithm.…”

“…In this manuscript, we use charge extraction experiments with a linearly increasing voltage based on a metal–insulator–semiconductor diode structure (MIS‐CELIV) to measure the hole mobility in blend films comprising Ir(ppy) 3 and TCTA across a range of Ir(ppy) 3 concentrations. The results of these experimental measurements are then related to atomic level morphologies generated using molecular dynamics simulations that mimic the evaporative formation of these blend films. The aim was to determine whether these transport measurements together with a range of other photophysical measurements were consistent with the simulated film morphologies.…”

Influence of Dopant Concentration and Steric Bulk on Interlayer Diffusion in OLEDs

“…High efficiency phosphorescent OLEDs generally have the emissive guest at low doping concentrations on the order of 6–10 wt% and it is generally assumed that the emissive guest is evenly distributed throughout the EML. However, recent modeling studies of the deposition process of such emissive layers have shown that this is not necessarily the case and from an experimental perspective, there is clear evidence that increasing the emissive dopant concentration above a certain level leads to a decrease in efficiency …”