Towards Printed Organic Light‐Emitting Devices: A Solution‐Stable, Highly Soluble Cu^I–NHetPHOS

Abstract

The development of iridium-free, yet efficient emitters with thermally activated delayed fluorescence (TADF) was an important step towards mass production of organic light-emitting diodes (OLEDs). Progress is currently impeded by the low solubility and low chemical stability of the materials. Herein, we present a Cu -based TADF emitter that is sufficiently chemically stable under ambient conditions and can be processed by printing techniques. The solubility is drastically enhanced (to 100 g L ) in relevant printing solvents. The integrity of the complex is preserved in solution, as was demonstrated by X-ray absorption spectroscopy and other techniques. In addition, it was found that the optoelectronic properties are not affected even when partly processing under ambient conditions. As a highlight, we present a TADF-based OLED device that reached an efficiency of 11±2 % external quantum efficiency (EQE).

Solid‐State Step‐Scan FTIR Spectroscopy of Binuclear Copper(I) Complexes

“…Two different ligands are arranged around the Cu 2 I 2 core: a bidentate 2‐diphenylphosphino‐4‐methylpyridine (MePyrPHOS) ligand and a monodentate ligand, which is either triisopropoxyphosphine P( i PrO) 3 ( 1 , Figure , Figure ) or tris( p ‐methoxyphenyl)phosphine P( p ‐MeOPh) 3 ( 2 , Figure , Figure ). The MePyrPHOS ligand in combination with the Cu 2 I 2 core is mainly responsible for the photophysical properties of these complexes, whereas the phosphine ligands affect mainly the solubility …”

“…In the case of phosphorescence (up to 100 % internal efficiency is possible) it is difficult to achieve deep blue emission. In the TADF process, collection of both singlet and triplet excitons results in a maximum possible internal efficiency of up to 100 % with a blueshift relative to phosphorescence . It was the aim of this work to further establish the method of solid‐state step‐scan FTIR spectroscopy in a KBr matrix and to characterize the structure and structural changes between the S 0 and T 1 states of the investigated Cu I complexes by IR spectroscopy in combination with DFT calculations.…”

“…The MePyrPHOS ligand in combination with the Cu 2 I 2 core is mainly responsible for the photophysical properties of these complexes,w hereas the phosphine ligandsaffect mainly the solubility. [17] In 1987 VanSlyke and Ta ng already presenteda nO LED prototype that used the fluorescent metal chelate complex tris(8hydroxyquinolinato)aluminum as emitter. [18] Afterwards, expensive iridium and platinum emitter materials were in the focus of researchers because of their high quantumy ield in solution.…”

“…[23] AT ADF mechanism has the benefits of ah igh quantum yield (nearly unity for some emitters), al ong excited-state lifetime (about af ew microseconds in general)c ompared to usual fluorescencel ifetimes, and as hort excited-state lifetimec ompared to phosphorescence. [24] The high quantum yield results from the fact that both the excitons from the singlet and triplet state are "harvested", whereas in fluorescente mitterso nly 25 %i nternal efficiency is possible, [17,24] because the triplet excitons are not convertedt of luorescence. In the case of phosphorescence (up to 100 %i nternal efficiency is possible) it is difficult to achieved eepb lue emission.I nt he TADF process, collection of both singlet and triplet excitons resultsi namaximum possible internal efficiency of up to 100 %w ith ab lueshift relative to phosphorescence.…”

Taking a snapshot of the triplet excited state of an OLED organometallic luminophore using X-rays

“…In this regard, one of the main challenges is to develop efficient electroluminescent materials, which do not require rare and expensive transition metal ions, as this has a significant impact on the production cost of OLED devices and especially on possible applications for large scale area lighting 8,9 . During the last few years, a class of luminophores based on cost-efficient Cu coordination complexes has appeared, triggered by the discovery of the temperature-activated delayed fluorescence (TADF) effect [10][11][12][13] . A remarkable photoluminescence quantum yield >99% was recently achieved for such materials 14,15 .…”