Oxadiazole based bipolar host materials employing planarized triarylamine donors for RGB PHOLEDs with low efficiency roll-off

Abstract

A series of 6 novel triarylamine-containing oxadiazole compounds (o-PCzPOXD, o-ICzPOXD, o-TPATOXD, o-PCzTOXD, o-ICzTOXD, o-CzTOXD) have been designed, synthesized and characterized concerning applications as host materials in PHOLED devices. To further improve the ortho-linkage concept, the impact of incorporating planarized electron-donating triarylamine (TAA) structures on intramolecular charge transfer was examined. The effect was evaluated for two series of electron-accepting oxadiazole scaffolds, realizing ortho-linkage on the benzene (POXD) and the thiophene (TOXD) core. Thermal analysis shows increased glass-transition temperatures for planarized structures indicating an improved morphological stability. A higher degree of planarization also results in significantly increased singlet and triplet energy values, revealing the impact on the intramolecular charge transfer. Employing the developed materials, red (o-TPATOXD: CEmax: 28.8 cd A-1, EQEmax: 16.9%), green (o-PCzPOXD: CEmax: 62.9 cd A-1, EQEmax: 17.1%) and blue (o-PCzPOXD: CEmax: 29.8 cd A-1, EQEmax: 13.4%) devices were achieved showing remarkably low efficiency roll-off for planarized donors. Hence, this is the first report of efficient blue devices for this specific class of host materials. It is proposed that the results correlate with an increasing ortho-linkage effect and decreasing donor strength of the TAA moiety by planarization and, thus, tackling one of the major challenges in PHOLED research: improving both triplet energy and compound stability

Azaindolo[3,2,1‐jk]carbazoles: New Building Blocks for Functional Organic Materials

“…Recently, we introduced indolo[3,2,1‐ jk ]carbazole (ICz) as new building block for optoelectronic materials . ICz can be considered a fully planarized derivative of triphenylamine (TPA).…”

“…Although arylamines are widely employed as electron‐donating moieties, reports on the application of ICz have been scarce for a long time, owing to the elaborate preparation of the planarized scaffold (e.g., vacuum flash pyrolysis) . However, a new synthetic strategy based on modern C−H activation renders the wider application of this building block possible . Compared with TPA and phenylcarbazole (PCz), ICz is a weaker electron donor (Figure ).…”

The pseudo-inversion symmetry of 9,90′ (1,3,4-oxadiazole-2,5-diyldi(1,1′-biphenyl)-2′,4- diyl)bis[9H-carbazole] in the light of OD theory

“…(I) was synthesized as described by [11]. Large single crystals were grown by recrystallization from boiling toluene.…”

“…During routine structural characterization of organic molecules designed for use in phosphorescent organic light emitting diodes [11], we determined the crystal structure of 9,9 0 -(1,3,4-oxadiazole-2,5-diyldi(1,1 0 -biphenyl)-2 0 ,4-diyl)bis[9H-carbazole] (I). (Fig.…”

Thieno[3,4-c]pyrrole-4,6-dione as novel building block for host materials for red PhOLEDs

“…[17][18][19][20][21] In particular, the application of bipolar host materials consisting of donor and acceptor subunits proved successful, owing to their balanced charge transport properties and thus simplified device structures and broad recombination zones. [22][23][24] The primary source of electron donating groups for optoelectronic materials are aromatic amines, such as triphenylamines, 3,9,[25][26][27][28][29] carbazoles, 10,17,28,[30][31][32][33] indolocarbazoles, [34][35][36][37] phenoxazines 15,[38][39][40] and dihydroacridines. 15,41 In contrast, the nature of the electron accepting moieties exhibits more variations.…”