OLED fabrication and characterization

ITO substrates were cleaned in an ultrasonic bath using acetone and isopropanol, dried under a N₂ stream and treated in an O₂ plasma chamber for 10 min. A 40 nm layer of PEDOT:PSS (purchased from Sigma Aldrich) was spin-coated at 4000 rpm from a 2.8 wt% dispersion in water. The active layer was spin-coated from the same solutions used for the PL characterization, and a Ca/Al (30/200 nm) cathode was thermally evaporated on top. For the multilayered OLEDs, a film of TFB was spin-cast onto PEDOT:PSS from a 10 mg/ml solution in toluene at 3000 rpm and then annealed at ~185 °C for 1 h in a N₂ environment. The TFB film was then spin-rinsed with toluene to obtain a final thickness of <10 nm⁵³, measured with a Dimension Icon atomic force microscope. The samples were then measured under an ~10⁻² mbar vacuum using a Keithley 2400 source-meter for both the current measurement and the voltage supply. The optical output of the OLEDs was measured with a calibrated silicon photodiode, and the EL spectra were collected with the same spectrometer employed for the PL experiments.