Molecular Orientations of Delayed Fluorescent Emitters in a Series of Carbazole-Based Host Materials

Abstract

Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (η ext) of 26% using an emission layer with horizontal orientation ratio () of 79%, which is higher than that of an almost randomly oriented emission layer with of 62% (η ext = 22%).

What Controls the Orientation of TADF Emitters?

“…Finally, we also want to note that chemical interactions between host and emitter molecules may cause specific combinations of them to orient better than predicted by the general T g trend. It has been reported recently, that specific molecular units may allow π-π stacking or weak hydrogen bonds between host and emitter and, thus, promote horizontal orientation (Watanabe et al, 2019 ; Sasabe et al, 2020 ). Probably, this could contribute to the observation that DMAC-TRZ is relatively more horizontally aligned in mCBP-CN as compared to other hosts with similar physical properties.…”

Identification of the Key Parameters for Horizontal Transition Dipole Orientation in Fluorescent and TADF Organic Light‐Emitting Diodes

“…[ 68 ] and Sasabe et al. [ 63 ] have started to exploit them toward the horizontal orientation of TADF emitters. Unfortunately, we were not able to incorporate any parameter related to these interactions into our analysis due to the difficulty of quantifying the variables involved (packing structure, number of bonding sites, bonding strength, etc.)…”

“…The figure was made using data from 53 separate reports that include EQE max and emitter orientation data on 124 materials systems. [ 20,22–24,37–85 ] While there is no correlation between EQE max and anisotropy across all data, it is clear that the highest efficiency in TADF‐ and non‐TADF‐based systems, respectively, is reached at low anisotropy factors.…”

What controls the orientation of TADF emitters?

“…Finally, we also want to note that chemical interactions between host and emitter molecules may cause specific combinations of them to orient better than predicted by the general T g trend. It has been reported recently, that specific molecular units may allow π-π stacking or weak hydrogen bonds between host and emitter and, thus, promote horizontal orientation (Watanabe et al, 2019;Sasabe et al, 2020). Probably, this could contribute to the observation that DMAC-TRZ is relatively more horizontally aligned in mCBP-CN as compared to other hosts with similar physical properties.…”