OLED fabrication and characterization

OLED devices were fabricated by high-vacuum (10⁻⁷ Torr) thermal evaporation on ITO-coated glass substrates with a sheet resistance of 15 Ω/⎕. Substrates were cleaned by sonication in non-ionic detergent, deionised water, acetone and isopropyl alcohol and subjected to an oxygen plasma treatment for 10 min. Layers were deposited at rates of 0.1–2 Ås⁻¹. o-CBP was synthesized according to the literature procedure³⁴. TAPC, TCP and UGH2 were purchased from Luminescence Technology Corp. TPBi, DPEPO and TSPO1 were purchased from Shine Materials. All purchased materials were used as received. OLED current density–voltage measurements were made using a Keithley 2400 source-meter unit. The luminance was measured on-axis using a 1-cm² calibrated silicon photodiode at a distance of 15 cm from the front face of the OLED. Electroluminescence spectra were measured using a calibrated OceanOptics Flame spectrometer. Lifetime measurements were measured with a Keithley 2400 source-meter unit and a 0.75-cm² silicon photodiode. The devices were held under rough vacuum (~10⁻³ Torr).