Towards Highly Efficient TADF Yellow-Red OLEDs Fabricated by Solution Deposition Methods: Critical Influence of the Active Layer Morphology

Abstract

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence emitters (TADF) in simple device structures fabricated by solution processing are strongly dependent on a suitable host molecular conformation and morphology. Herein, we report the fabrication of highly efficient yellow-red TADF-based OLEDs via solution processing in a simple, two-organic-layer device structure. The devices were fabricated at different weight concentrations of 5%, 8%, and 10% emitter in an n-/p-type mixed host matrix, and their characteristics were studied. The device performance was compared with different thickness parameters for both the emitting layer (EML) and the electron transport layer (ETL) in various solvents, including chlorobenzene, dichlorobenzene, and chloroform. By optimizing the mixed ratio of EML, yellow-red OLEDs of 2-[4 (diphenylamino)phenyl]-10,10-dioxide-9H-thioxanthen-9-one (TXO-TPA) emitter in an n-/p-type host matrix of poly(N-vinylcarbazole):1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (PVK:OXD-7) as a blend for the active layer were fabricated. In the best results, the device exhibited a lower turn-on voltage at around 6 V, with an external quantum efficiency (EQE) of 18.44%, current efficiency of 36.71 cd/A, and power efficiency of 14.74 Lm/W for the 8% emitter concentration. The importance of solvent for improving the electrical properties, together with organic layer thickness and host effect for the charge carrier’s transport and device characteristics are also discussed.

Suppressing dimer formation by increasing conformational freedom in multi-carbazole thermally activated delayed fluorescence emitters

“…1), are prone to dimerization, which may also be responsible for their mechanochromism [24][25][26][27][28] and thermochromism. 23 The investigation by Etherington et al has been followed by further detailed investigation into the stimuliresponsive behaviour of TADF emitters 13,[29][30][31][32][33][34] and a recent minireview. 35 This previous work established that dimerization causes significant shifts in the photoluminescence and electroluminescence colour ( 1 CT emission energy) of the TADF materials, beyond that expected by packing and polarizability effects alone.…”

Solution-Processed Dendrimer-Based TADF Materials for Deep-Red OLEDs

“…There have been a small number of recent reports of orange/red-emitting TADF-based solution-processed OLEDs. For example, OLEDs containing an orange-emitting small molecule blended with poly­(vinylcarbazole) have been reported with EQEs of up to 18.4%, with the results strongly dependent on the processing solvent . A solution-processed TADF main chain polymer-based OLED has been reported to have an EQE of 19.4% for orange emission, but there was insufficient device data to determine whether this was at a usable luminance .…”

“…For example, OLEDs containing an orange-emitting small molecule blended with poly(vinylcarbazole) have been reported with EQEs of up to 18.4%, with the results strongly dependent on the processing solvent. 14 A solution-processed TADF main chain polymer-based OLED has been reported to have an EQE of 19.4% for orange emission, but there was insufficient device data to determine whether this was at a usable luminance. 15 Other examples of solution-processed orange-red/red TADF 17 and a truxene-substituted pyrazinophenanthrene that has been reported to give an EQE of 15.4% at an undefined luminance and 12.3% at 100 cd m −2 for red emission.…”

Insights into Charge Transport in High-Efficiency Green Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with a Single Emitting Layer

“…15,47−49 The weight ratio of PVK and OXD-7 was chosen as the most efficient reported previously. 48,50,51 The substrates were sequentially cleaned in an ultrasonic bath with Hellmanex solution, acetone, and 2-propanol, and before use, with a UV ozone treatment for 5 min. Before deposition, the PEDOT:PSS was filtered with a 0.45 μm polyvinylidene difluoride filter.…”