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(2phenylpyridyl)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. Figure 1. Chemical structures of CBP (left) and Ir(ppy) 3 (right).

Molecular Orientation Effects in Organic Light‐Emitting Diodes

“…It is therefore required to first re-parameterize the pre-existing force field sets, such as the Optimized Potential for Liquid Simulations (OPLS), against first-principle calculations. [108][109][110] For example, regarding the bonded interactions, the dihedral terms are usually fitted to the results of DFT calculations vacuum Figure 5. Schematic workflow of multiscale modeling of OLEDs.…”

“…Schematic illustration of different length scales involved in the molecular simulation methodology. [110] (Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). calculations using the hybrid B3LYP (Becke 3-parameters, Lee-Yang-Parr) [111,112] exchange-correlation functional and the popular 6-31G [113] split-valence doubleζ Gaussian basis set.…”

“…Since the molecular morphology highly depends on the thin-film deposition process, MD schemes have been developed to mimic the PVD process. [110,122,125,126] An example is described as follows. A substrate (the xy plane) is placed in a rectangular computational box, with a longer dimension along the z coordinate (out-of-plane direction) to accommodate molecular deposition.…”

“…Next, each molecule in the ensemble is inserted individually in the box once at a time, with the initial coordinates generated randomly, and the initial velocity drawn from an appropriate distribution function, such as the Maxwell-Boltzmann or Gaussian function, by taking into account of deposition temperature. An initial velocity in the negative z direction [110,122] or a weak attractive potential is often required in order to make sure the inserted molecule can travel towards the substrate. [124] The above procedure is repeated until the last molecule being inserted into the system, with the computation box coupled to a Berendsen thermostat to ensure sampling from the canonical (NVT) ensemble.…”

“…Both schemes have been tested in order to reveal the molecular orientation effects in OLEDs. [110,[117][118][119] More research efforts are required to develop faster and more universal methodologies so that molecular simulations may become an important tool for the experimentalists, in silico screening the OLED emitter-host pairs for optimizing the light outcoupling efficiencies. As pointed out by Bagchi et al, [129] the demonstrated correlation between the molecular orientations in vacuum-deposited films and at the surface of a supercooled liquid may open up a possible route towards a far less computationally expensive algorithm.…”