Fabrication of OLEDs and films of emission layers of OLEDs

The OLEDs were fabricated as follows: metals and organic materials were thermally evaporated through a shadow mask in a vacuum chamber at a base pressure of 10⁻⁷ mbar (Angstrom Engineering Inc., EvoVac). The OLED layer stack consisted of 40 nm 2,2′,7,7′-tetrakis(N,N′-di-p-methylphenylamino)-9,9′-spirobifluorene (Spiro-TTB) p-doped with 2,2′- (perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) (4 wt%) as a hole transport layer (HTL), 10 nm N,N′-di(naphtalene-1-yl)-N,N′-diphenylbenzidine (NPB) as an electron blocking layer (EBL), 20 nm emission layer (EML, detailed below), 10 nm bis-(2-methyl-8-quinolinolato)-(4-phenyl-phenolato)-aluminum (III) (BAlq) as a hole blocking layer (HBL), and 40 nm cesium-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) as an n-doped electron transport layer (ETL). Calibrated quartz crystal microbalances were used to control deposition rates and the final thicknesses of each layer. All materials were purchased from commercial suppliers and used without further purification. All OLEDs were encapsulated under nitrogen atmosphere using 1.1 mm thick glass lids (Shanghai Amerina Optoelectronic Co., Ltd.), epoxy resin (Norland Products Inc., Norland Optical Adhesive 68), and an additional moisture getter (Dynic Corporation, HD-071210T-50S). A similar procedure was used to make films of the emission layers of the OLEDs although films for PL quantum yield were not encapsulated.

The G1-OLED was fabricated onto 1.1 mm thick glass substrates coated with a 90 nm thick pre-patterned indium tin oxide (ITO) anode (Thin Film Devices Inc.). The device stack was capped with a 100 nm thick aluminum cathode. The EML consisted of the fluorescent emitter 2,5,8,11-tetra-tert-butylperylene (TBPe), which was doped at 1.5 wt% in the host 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN). The active area of these G1-OLEDs was 16.1 mm².

The G2- and G3-OLEDs were fabricated on silicon substrates with a thickness of 675 µm coated with a 300 nm silicon dioxide (SiO₂) layer on both sides to prevent electrical leakage through the substrate. Aluminum anodes were evaporated onto the SiO₂ layer through a shadow mask with different aperture sizes. The organic material stack was subsequently evaporated as detailed above. The EML of the G2- and G3-OLEDs consisted of TBPe, which was doped at 1.5 wt% in MADN. The EML of G3-FEM-OLEDs consisted of 4,4-bis[4-(diphenylamino)styryl]biphenyl (BDAVBi) doped at 3 wt% in MADN. A 30 nm silver layer was evaporated on top as a semi-transparent cathode and the stack was finished with a 50 nm thick NPB capping layer. Shadow masks were changed without breaking vacuum. A single layer of aluminum was used as a bottom electrode instead of the commonly used multi-layer of aluminum and silver^58,59 as this was found to give higher device fabrication yield and more stable operation. The active areas were measured from electroluminescence (EL) images of the operating OLEDs under a microscope (ZEISS, Axio Lab A1, (see Fig. 1d)).