Green and low-cost approach to modify the indium tin oxide anodes in organic light-emitting diodes by electrochemical treatment in NaCl aqueous solution

We demonstrate an environment-friendly, simple, and low energy cost approach as an alternative to conventional O2 plasma treatment to modify the surface of indium tin oxide (ITO) anodes for use in organic light-emitting diodes (OLEDs). ITO is electrochemically treated in NaCl aqueous solution. A chlorinated ITO (Cl-ITO) electrode with a work function of 5.41 eV was obtained, which is 0.66 eV higher than that of pre-cleaned ITO. The increase of work function is due to the anodic oxidation reactions occurred on the surface of ITO. The power dissipation is only ∼3 mW in our approach, which is five orders of magnitude lower than that of O2 plasma treatment (∼100 W). We fabricated the OLEDs with the configuration of Cl-ITO/NPB(35 nm)/CBP:Ir(ppy)3 (15 nm, 8 wt%)/TPBi:Ir(ppy)3 (10 nm, 8 wt%)/TPBi (10 nm)/Bphen (50 nm)/Cs2CO3 (2 nm)/Al (100 nm), where NPB is N, Ń-di-1-naphthyl-N, Ń-diphenylbenzidine, CBP is 4′-bis(carbazol-9-yl)biphenyl, TPBi is 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole), Ir(ppy)3 is bis(3-phenylpyridine) iridium(III) and Bphen is 4,7-diphenyl-1,10-phenanthroline. A maximum power efficiency of 95.0 lm W−1 and external quantum efficiency (EQE) of 24.2% were achieved, respectively, which was slightly higher than that of the OLED fabricated on O2-plasma-treated ITO (91.2 lm W−1, EQE = 23.1%).