Versatile Electroluminescence Color-Tuning Strategy of an Efficient Light-Emitting Electrochemical Cell (LEC) by an Ionic Additive

Abstract

The color-tuning strategies of solid-state light-emitting devices (ss-LEDs) are mainly focused on engineering molecular structures. In this paper, for the first time, we developed a facile strategy for tuning the electroluminescence (EL) color from orange to green through the addition of the ionic additive TBAP (tetrabutylammonium perchlorate). To achieve the active ionic emissive compound for use in a light-emitting electrochemical cell (LEC), the neutral biscyclometalated bromo tetrazole iridium(III) [Ir(ppy) 2 (BrTz)] was exchanged to its cationic complex, [Ir(ppy) 2 (BrTz-Me)]ClO 4 (ppy = 2-phenyl pyridine, BrTz = 4-bromo-2-pyridine tetrazole, BrTz-Me = 4-bromo-2pyridine methyl tetrazole) with a new synthetic strategy. This method allows employing neutral Ir-cyclometalated complexes, which are ruled out for use in LECs because of their non-ionic behaviors. In the following, an LEC based on the new cationic [Ir(ppy) 2 (BrTz-Me)]ClO 4 as the emissive layer was fabricated between the FTO (fluorine-doped tin oxide) anode and Ga:In alloy cathode without using any additive or polymers, which makes this configuration the simplest ss-LED so far. By adding the ionic additives, the electroluminescence characteristics of [Ir(ppy) 2 (BrTz-Me)]ClO 4 were dramatically increased, including luminance (L) from 162.8 cd/m 2 for the device with an additive to 212.9 and 355.9 cd/m 2 for devices containing LiTFSI (bis(trifluoromethane)sulfonamide lithium salt) and TBAP, respectively. In particular, when TBAP was added to the [Ir(ppy) 2 (BrTz-Me)]ClO 4 complex, the irradiance was significantly increased from 166.4 to 220.8 μW/cm 2 with an efficacy of 1.78 cd/A and external quantum efficiency (EQE) value of 2.14%. The obtained EL results clearly showed that adding TBAP and LiTFSI significantly improved the electroluminescence characteristics and tuned the electroluminescence color.

Phenanthroimidazole as molecularly engineered switch for efficient and highly long-lived light-emitting electrochemical cell

“…However, further modification such as the selection of bulky counter anion (for example tetraphenylborate, ph 4 B − , Ref. 85 ) and use of various ionic additives into emissive layer 36,86 will contribute to creating the LECs based on extra ionic complexes with acceptable stability and performance.…”