C9920 12

The devices were prepared using precleaned indium tin oxide (ITO) substrates. The different organic layers were sequentially thermally evaporated under vacuum lower than 3 × 10⁻⁴ Pa. Finally, the LiF/Al electrodes were deposited through a shadow mask on top of the multilayer structure. The device active area was 4 mm². To avoid any degradation and emission quenching due to oxygen and moisture, the OLEDs were encapsulated in a glovebox filled with nitrogen. The J–V–L characteristics were collected using a source meter (Keithley 2400, Keithley Instruments Inc.) and an absolute external quantum efficiency measurement system (C9920-12, Hamamatsu Photonics). An optical fiber connected to a spectrometer (PMA-12, Hamamatsu Photonics) was used to record the electroluminescence spectra.

All compounds were subjected to temperature-gradient sublimation under high vacuum before use. OLEDs were fabricated on the ITO-coated glass substrates with multiple organic layers sandwiched between the transparent-bottom ITO anode and the top metal cathode. Before device fabrication, the ITO glass substrates were precleaned carefully. All material layers were deposited by vacuum evaporation in a vacuum chamber with a base pressure of 10⁻⁶ torr. The deposition system permits the fabrication of the complete device structure in a single vacuum pump-down without breaking vacuum. The deposition rate of organic layers was kept at 1 to 2 Å/s. The doping was conducted by coevaporation from separate evaporation sources with different evaporation rates. The current density, voltage, luminance, external quantum efficiency, EL spectra, and other characteristics were measured with a Keithley 2400 source meter and an absolute EQE measurement system in an integrating sphere at the same time. The EQE measurement system is Hamamatsu C9920-12, which equipped with Hamamatsu PMA-12 Photonic multichannel analyzer C10027-02 whose longest detection wavelength is 1100 nm. Device encapsulation was carried out in glove box for following performance characterizations and lifetime measurements. Device lifetimes were measured using an OLED aging lifetime tester (ZJZCL-1, Shanghai University).

The OLEDs were fabricated by vacuum deposition process without exposure to ambient air. After fabrication, the devices were immediately encapsulated with glass lids using epoxy glue in a nitrogen-filled glove box (O₂ ~ 0.1 ppm, H₂O ~ 0.1 ppm). The indium–tin oxide surface was cleaned ultrasonically and sequentially with acetone, isopropanol and deionized water, then dried in an oven, and finally exposed to ultraviolet light and ozone for about 10 min. Organic layers were deposited at a rate of 1 Å/s. Subsequently, Liq and Al were deposited at 0.3 and 1 Å/s, respectively. The device area is ~ 0.04 cm². The EQE and J-V-L measurements were performed using a Keithley 2400 source meter and an absolute external quantum efficiency (EQE) measurement system (C9920-12, Hamamatsu Photonics, Japan). For the device lifetime tests, the luminance and EL spectra of the driving devices in the normal direction were measured using a luminance meter (SR-3AR, TOPCON, Japan) under constant current density driving conditions with an initial luminance of 10³ cd m⁻².

OLEDs were fabricated by vacuum depositing the materials at ca. 3.0 × 10⁻⁴ Pa onto ITO-coated glass substrates (sheet resistance of ca. 15 Ω/□). Before device fabrication, the ITO-coated glass substrates were cleaned in sequential ultrasonic baths of acetone, ethanol, and deionized water, dried in an oven, and then exposed to UV/ozone for about 30 min. After depositing the organic layers (deposition rates of 2–3 Å s⁻¹), the devices were unloaded into a nitrogen-filled glovebox and affixed to metal masks that defined the cathode area. The devices were loaded back into the same evaporation chamber for deposition of a cathode of LiF (0.2 Å s⁻¹) and Al (ca. 4 Å s⁻¹). The emitting area of all the OLEDs was determined by the overlap of the two electrodes (0.04 cm²). The J–V–L characteristics were measured using a Keithley 2400 source meter in conjunction with an absolute EQE measurement system (C9920–12, Hamamatsu Photonics, Japan).

The current density–voltage (J–V), external quantum efficiency–current density curves, and EL spectra at DC operation were measured using an external quantum efficiency measurement system (C9920-12, Hamamatsu Photonics). Pulsed voltage operation was measured using rectangular pulses with a pulse width of 400 ns, repetition frequency of 1 kHz, and varying peak voltages were applied to the devices at ambient temperature using a pulse generator (NF, WF1945) while applied voltage (V_CH1) was monitored on a multichannel oscilloscope (MSO6104A, Agilent Technologies). To measure current flow through the device a 51.4 Ω resistor was connected in serial connection to the OLED and voltage across the resistor (V_CH2) was monitored on the oscilloscope (Supplementary Fig. 8). Therefore, in situ voltage across the OLED device can be reported as V_CH1–V_CH2 based on voltage division rule (Supplementary Fig. 8).

All compounds used in this manuscript were twice purified by a vacuum sublimation approach. The ITO glass substrates were carefully precleaned before fabricating the corresponding devices. All devices were constructed under high vacuum with a pressure of 1 × 10⁻⁵ Pa. The thermally evaporated rates for all organic materials were in the range of 1.0–1.5 Å s⁻¹. An absolute EQE measurement system (C9920‐12, Hamamatsu Photonics) was adopted to measure the forward‐viewing electrical characteristics of those OLEDs at room temperature under ambient laboratory conditions. The active area of the OLEDs is 3 × 3 mm². And four repetitions were carried out for each device and errors in EQE_maxs of those devices were within 5%.