Fabrication of rubrene OLEDs

The device structure of rubrene-based OLEDs was ITO (185 nm)/PEDOT:PSS (30 nm)/rubrene (35 nm)/C₆₀ (25 nm)/BCP (6 nm)/Ag (120 nm). Organic materials were used as purchased without further purification. PEDOT:PSS was spin-coated on the substrate at 4000 rpm for 60 s, followed by annealing at 150 °C for 20 min. The thickness of the PEDOTS:PSS layer was around 40 nm. The PEDOT:PSS-coated ITO substrates were then transferred into the thermal evaporation system. A 35-nm thin layer of rubrene and a 25-nm thin layer of C₆₀ were deposited at a constant deposition rate of 0.5 Å s⁻¹. The substrate temperature was maintained at 80 °C during deposition. Further deposition was done at room temperature. A 6-nm thin layer of BCP was deposited prior to the deposition of the top electrode. Devices were completed by evaporation of a 120-nm thin layer of Ag. Metal deposition was achieved through a shadow mask. The device area was 5.25 mm⁻² as defined by the overlapping area of the ITO films and top electrodes. All depositions were performed under a base pressure lower than 4 × 10⁻⁴Pa. The devices were encapsulated with UV epoxy (NOA81, Thorlabs)/cover glass before subsequent measurements.