Cs 200

The substrates were cleaned with ultrapurified water and organic solvents (acetone, then isopropanol), and then dry-cleaned for 30 min through exposure to UV–ozone. Organic layers were deposited onto ITO substrates under vacuum (=10⁻⁵ Pa), successively. LiF and Al were patterned using a shadow mask with an array of 2 mm × 2 mm openings without breaking the vacuum (=10⁻⁵ Pa). The electroluminescent (EL) were taken using an optical multichannel analyzer Hamamatsu Photonics PMA-11. The current density–voltage and luminance–voltage characteristics were measured using a Keithley 2400 source measure unit and a Minolta CS200 luminance meter, respectively.

Glass substrates pre-coated with a 95 nm thin layer of indium tin oxide (ITO) with a sheet resistance of 20 Ω per square were thoroughly cleaned for 10 min in ultrasonic bath of acetone, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol and then treated with O₂ plasma for 5 min in sequence. Organic layers were deposited onto the ITO-coated substrates by high-vacuum (< 5 × 10⁻⁴ Pa) thermal evaporation. Deposition rates were controlled by independent quartz crystal oscillators, which were 1 ~ 2 Å s⁻¹ for organic materials, 0.1 Å s⁻¹ for LiF, and 6 Å s⁻¹ for Al, respectively. The emission area of the devices was 3 × 3 mm⁻² as shaped by the overlapping area of the anode and cathode. All the device characterization steps were carried out at room temperature under ambient laboratory conditions without encapsulation except spectrum collection process. EL spectra were taken by an optical analyzer, Photo Research PR705. Current density and luminance vs. driving voltage characteristics were measured by Keithley 2,420 and Konica Minolta chromameter CS-200, respectively. External quantum efficiencies were calculated by assuming that the devices were Lambertian light sources.

Stimuli were presented on a liquid crystal monitor (UP2516D, Dell; 2560 × 1440 pixels, 59 Hz frame rate, Adobe RGB). The monitor was connected via a HDMI cable to the onboard graphics card (Intel HD Graphics 620, 8 bits) of a PC running Microsoft Windows 10 Home Edition (64 bits). Color and luminance were calibrated with the chroma meter CS-200 (Konica Minolta, Japan) under the same illumination condition as the experiment. Pixel intensity was measured in sub-pixel resolution using a microscope (Dinolite basic, Dinolite), and the confirmed gray lines were the same across stimuli. The experiment was controlled using custom software developed using MATLAB 2018a (Mathworks) and Psychtoolbox 3.0.15^{35 (link)}.