Effect of TADF Assistance on Performance Enhancement in Solution Processed Green Phosphorescent OLEDs

Abstract

Many methods have been proposed to increase the efficiency of organic electroluminescent materials applied as an emissive layer in organic light emitting diodes (OLEDs). Herein, we demonstrate enhancement of electroluminescence efficiency and operational stability solution processed OLEDs by employing thermally activated delayed fluorescence (TADF) molecules as assistant dopants in host-guest systems. The TADF assistant dopant (SpiroAC–TRZ) is used to facilitate efficient energy transfer from host material poly(N–vinylcarbazole) (PVK) to a phosphorescent Ir(III) emitter. We present the analysis of energy transfer and charge trapping—two main processes playing a crucial role in light generation in host–guest structure OLEDs. The investigation of photo-, electro- and thermoluminescence for the double-dopant layer revealed that assistant dopant does not only harvest and transfer the electrically generated excitons to phosphorescent emitter molecules but also creates exciplexes. The triplet states of formed PVK:SpiroAC–TRZ exciplexes are involved in the transport process of charge carriers and promote long–range exciton energy transfer to the emitter, improving the efficiency of electroluminescence in a single emissive layer OLED, resulting in devices with luminance exceeding 18 000 cd/m2 with a luminous efficiency of 23 cd/A and external quantum efficiency (EQE) of 7.4%.

Electric-field modulated energy transfer in phosphorescent material- and fluorescent material-codoped polymer light-emitting diodes

“…Fig. 1(b) illustrates the molecular structures and energy level diagrams of PVK, 31,32 Fir6, 33 and rubrene. 34 The commonly used blue phosphorescent Fir6 as the energy donor and highly efficient uorescent rubrene as the energy acceptor were co-doped in the large bandgap polymer host PVK, forming a single-layer organic light-emitting system where energy transfer was involved.…”

Efficient OLEDs Based on Slot-Die-Coated Multicomponent Emissive Layer

“…The doping ratio of Ir complex in TADF-enriched matrix should be optimized to acquire high energy transfer efficiency [67,70]. The optimal EML composition of PVK + 25 wt.% SpiroAC-TRZ + 2 wt.% Ir resulted from our previous studies [71].…”

“…This process in particular can have a drastic effect on the performance of OLEDs with EML based on PVK + SpiroAC-TRZ + Ir due to the high concentration of TADF material. Furthermore, the formation of exciplex triplet states between TADF and PVK [71] and their subsequent energy transfer to the Ir complex can be disturbed. When such scenarios operate, the quenching of iridium complex emission by oxygen molecules seems to be obvious.…”

Complementary Study Based on DFT of Optical and Electronic Properties of New Copolymer PVK-F8T2

“…Indeed, polymers have many applications, such as OLED [ 1 , 2 , 3 ] and organic photovoltaic (OPV) [ 4 , 5 , 6 , 7 ], because of their ease of shaping as well as their low production cost. For this reason, the current scientific and industrial challenge is to design new ideally soluble [ 8 ] conjugated polymers with the aim of obtaining a new material having modular optoelectronic properties endowed with the desired performance, thus improving their solubility [ 9 , 10 ] in order to act as active layers in optoelectronic devices [ 11 , 12 , 13 ].…”