An international team including researchers from Osaka University, Japan, reports the development of a novel hybrid emitter that could revolutionise the design of OLEDs.
Organic light-emitting diode (OLED) displays are commonly used in smartphones and televisions. OLEDs have the advantages of being low cost, light, flexible, and easy to modify, making them ideal display materials. Current OLEDs that achieve commercially viable quantum efficiencies contain rare metal atoms such as iridium and platinum that increase costs and reduce sustainability. Now, an international team, including researchers from Osaka University, has reported the creation of the best performing heavy-atom-free OLED, which relies on a novel hybrid emitter.
The heavy-atom-free emitters that are currently available have limitations. Materials known as thermally activated delayed fluorescence (TADF) emitters are efficient; however, they typically have broad emission spectra that make them more suitable for use as light sources than as the precise emitters required for display applications.
Another type of heavy-atom-free emitter is room-temperature phosphorescence (RTP) emitters; however, the OLEDs using them show very low efficiencies of <1%, due to the quenching of long-lived triplet excitons in the device.
Osaka’s novel hybrid emitter
The researchers merged the mechanisms of TADF and RTP phenomena to produce a hybrid emitter that combines features of both systems. Their TADF/RTP material, named SiAz, contains only carbon, hydrogen, nitrogen, and silicon atoms, which are naturally abundant elements, making SiAz viable for widespread use.
Study corresponding author Youhei Takeda explains: “The energy level gaps in the excited states of an emitting material determine how the materials can behave upon the excitation and the emission they produce. Combining the two mechanisms meant that we could alter the way an excitonic molecule undergoes transition between the spin- and energetically-different states to produce the overall characteristics that we wanted. Specifically, by tuning energy levels, our material can utilise thermal upconversion system to produce RTP.”
The researchers achieved a high degree of control over the energy levels through careful selection of the host material that the emitter molecule was impregnated into — which allowed thermally activated transition of the energetically-lowest triplet excited state to the higher triplet state of the emitter molecules to irradiate pure RTP in an efficient way. The SiAz material was successfully used in a device that achieved an external quantum efficiency of 4%, which is the best reported to date for a heavy-atom-free OLED based on RTP.