OLED fabrication, characterization, and operation

The OLED devices used in this work were fabricated at the ASU Flexible Electronics and Display Center (FEDC) using patterned indium-tin oxide (ITO) on flexible substrates. The green OLED device structure has a peak emission intensity at 515 nm and consisted of a 10 nm layer of hexaazatriphenylene hexacarbonitrile (HAT-CN) hole injection layer (HIL), followed by a 40 nm 4-4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPD) hole transport layer (HTL). A 10–50 nm green emissive layer (EML) was deposited onto the HTL and comprised a co-host structure of [Host1:Host2:Green dopant]. Next, a 10–30 nm thick hole blocking layer (HBL) was deposited onto the EML, followed by a 30 nm electron transport layer (ETL) consisting of doped 8-hydroxyquinolinolato-lithium (Liq). A 2 nm Liq electron injection layer (EIL) was deposited onto the ETL, followed by a 100 nm layer of MgAg or Al cathode metal. Device area was 0.05 cm². Films were deposited by vacuum thermal evaporation in a Sunicel Plus 400 system made by Sunic Systems (Suwon, South Korea) at pressures below 5 × 10⁻⁷ Torr. Films were patterned using metal shadow masks with no break in vacuum between layers. The devices were encapsulated using a thin-film barrier material from 3 M Company. HAT-CN was purchased from Lumtec (Hsin-Chu, Taiwan), and the other organic materials were supplied by Universal Display Corporation (New Jersey, USA).

Encapsulated devices were tested in ambient conditions. Current density/voltage, radiance and electroluminescence measurements were made using a Keithley 2400 source meter, calibrated Si photodiode model 818-UV from Newport (Irvine, CA, USA), and Ocean Optics HR-4000 spectrometer, using Spectra Suite software (Ocean Optics, Dunedin, FL, USA). For all reported test results, the OLED was pulsed at 6 Hz, with an 8.8 volt forward bias, providing an instantaneous optical output power of 0.8 mW. Similar to a semiconductor diode, increasing the direct current (DC) bias voltage exponentially increases current, which results in the OLED light intensity to also increase exponentially. However, we observed that increasing the DC bias above 7 volts to obtain higher light intensity started to degrade the OLEDs organic emissive layers due to current induced, localized joule heating in the OLED organic layers^35,47. As a solution, we found that pulsing the OLED power supply at less than 10 Hz, combined with applying a thin conformal metal foil heat sink to the cathode, the OLED operating voltage could be increased above 7 volts to subsequently increase the instantaneous light intensity without degrading or damaging the OLED organic layers. Essentially, pulsing the OLED at a low frequency appears to give the organic layers in the OLED a chance to recover and cool down before applying a voltage bias in the next period.