Lifetime study for solution processed organic light emitting diodes

Abstract

In this paper, blue fluorescent organic light-emitting diodes (OLED) are fabricated by solution processes. UV Ozone exposure time on the ITO surface and the thickness of the hole injection layer (HIL), electron transport layer (ETL) and electron injection layer (EIL) are modulated to study the effects on device characteristics and lifetime. ITO/MADN:13% UBD-07/Al is used as the basic structure. First, HIL (PEDOT: PSS) is added. It is found that although the hole current is increased, the device does not emit light. Secondly, ETL (TPBi) is added to facilitate the injection of electrons from cathode to EML, and in such a way, hole-electron can be recombined in EML and emit light. With an increase in the time to expose the ITO into UV Ozone from 100s and 200s to 400s, the total current of the OLED is increased, and luminance is improved, but the current efficiency is not obviously improved. Its role facilitating the electron injection is observed by adjusting the LiF thickness of EIL. In addition to increasing the hole current, the addition of PEDOT as HIL may reduce the chance of a device shorten resulting from the spikes on ITO surface, and the lifetime of the OLED with this structure is 121 min. In order to prolong device lifetime and better balance the injection quantity of electrons and holes, the structure of ETL is adjusted to be TPBi+Alq3. When TPBi+Alq3 is 10nm/15nm thick, current efficiency is 1.29 cd/A at 8V, and the lifetime is extended to 252 min.