Design of a heteroleptic green Ir(iii) complex for eco-friendly solution-processed phosphorescent organic light-emitting diodes with external quantum efficiency exceeding 22

Abstract

The development of eco-friendly solution-processed PHOLEDs (s-PHOLEDs) is a significant issue due to the toxicity of halogenated solvents. However, high-performance s-PHOLEDs have predominantly relied on halogenated solvents, owing to their superior ability to dissolve organic materials and facilitate high-quality film formation in the EMLs. To advance the development of eco-friendly s-PHOLEDs, several criteria are required to solve; the development of emitters and host materials having high solubility in eco-friendly solvents and the establishment of optimal conditions for achieving high-quality films, including uniformity, thickness, and morphology. Herein, we developed eco-friendly s-PHOLEDs using a novel green phosphorescent emitter, MECF-pop, and a new thermally cross-linkable hole transport material, C2DPACzPVB, using 2-methylanisole (2-MA) as a food additive solvent for device fabrication. We utilized an exciplex forming mixed hosts system consisting of TCTA as a hole transport type material and TPBi as an electron transport type material. Interestingly, the variation of exciplex formation was observed in the EMLs due to the changed dispersion of TCTA and TPBi according to film-forming conditions. The optimized eco-friendly s-PHOLEDs using chlorobenzene and 2-MA showed maximum external quantum efficiencies of 26.4 and 22.7% and current efficiencies of 94.8 and 81.2 cd A^-1, respectively.

Scheme 1. Synthetic route to MECF-pop.

Scheme 2. Synthetic route to C2DPACzPVB.

Fig. 1. (a) UV-vis absorption, (b) PL spectra of MECF-pop and Ir(CF₃BNO)₂-pop. (c) UV-vis absorption, RTPL (FL) and LTPL (Ph) spectra of C2DPACzPVB.

Fig. 2. (a) Energy level diagram of solution-processed PHOLEDs. (b) Molecular structure of TCTA, TPBi, and MECF-pop, (c) J–V–L curve, (d) EL spectra, (e) EQE–L curve, and (f) CE–L–PE curve of Devices A–D.

Fig. 3. (a) PL properties of TCTA, TPBi, and mixed films (TCTA and TPBi). (b) Hole mobility of mixed films depending on film forming conditions using CB and 2-MA. ToF-SIMS 3D analysis of (c) TCTA (C₅₄H₃₇N⁺), (d) TPBi (C₄₅H₂₁N6⁺), and (e) mixed TCTA and TPBi distribution according to film forming conditions.

Fig. 4. (a) Energy level diagram of Device E and molecular structure of C2DPACzPVB. (b) J–V–L curves. (c) EL spectrum. (d) EQE–L curve. (e) CE–L–PE curves of Device E.