Sky-blue-emitting cationic iridium complexes with carbazole-type counter-anions and their use for efficient solution-processed organic light-emitting diodes

Abstract

The control of counter-anion is a facile approach to tune the overall properties of cationic iridium complexes for optoelectronic applications. Here, we report for the first time the use of electron-rich carbazole-type counter-anions in cationic iridium complexes and the use of such complexes as dopants in solution-processed organic light-emitting diodes (OLEDs). PF₆⁻, 4-(9H-carbazol-9-yl)benzenesulfonate (CAZ-SO₃⁻), and 4-(9′H-[9,3′:6′,9′′-tercarbazol]-9′-yl)benzenesulfonate (TCAZ-SO₃⁻) have been employed as the counter-anions of sky-blue-emitting complexes R, 1 and 2, respectively. The carbazole-type counter-anions do not largely disturb the phosphorescence of the cations and the complexes show similar emission properties in solution and films. The close proximity of the carbazole-type anions to the phosphorescent cations allows efficient energy-transfer from the former to the latter in films. When used as dopants in solution-processed OLEDs, complexes 1 and 2 show higher performances than complex R because the hole-trapping effects of the carbazole-type counter-anions largely improve the carrier-recombination balance in the emissive layers. In particular, the double-layer sky-blue device based on complex 2 with strong hole-trapping by TCAZ-SO₃⁻ affords a high peak current efficiency of 27.1 cd A⁻¹. The work reveals that electron-rich carbazole-type anions are promising counter-anions for cationic iridium complexes toward optoelectronic applications.