TADF, a process demonstrated at Kyushu University’s Center for Organic Photonic and Electronics Research (OPERA) in 2012, allows fluorescent molecules (which can be metal-free) to approach the theoretical 100% charge conversion of phosphorescent materials which need to include expensive metal ions such as platinum or iridium. Previous fluorescent molecules only converted ~25% of charge.
“While our initial TADF devices lost 5% of their brightness after only 85 hours,” said Dr Daniel Tsang or OPERA, “we have now extended that more than eight times just by making a simple modification to the device structure.”
The modification to the devices, which are made using vacuum deposition, is to put two 1-3nm layers of the lithium-containing molecule ‘Liq’ in the OLED stack, on each side of the hole blocking layer, which brings electrons to the TADF material, while preventing holes from exiting the device before contributing to emission. The TADF material in this case was 4CzIPN which emits green.
“Devices will last even longer in practical applications because the tests are performed at extreme brightnesses [1,000cd/m2] to accelerate the degradation,” said the University.
Applying extra, previously reported, optimisations delayed the 5% droop time to >1,300 hours, over 16x that of the initial devices.
“What we are finding is that the TADF materials themselves can be very stable, making them really promising for future displays and lighting,” said OPERA director Professor Chihaya Adachi.
Liq layers also improved a similar device structure with a phosphorescent emitter.
How does it work?
One contribution to the longer life is that devices with Liq layers contain a much lower number of trap defects which prevent charge from moving freely.
One of the next challenges for TADF is to find a stable efficient blue emitter – something that is also difficult with phosphorescent emitters, said the University. “We think that TADF has the potential to solve the challenge of efficient and stable blue emission,” said Adachi.
Paper ‘Operational stability in organic light-emitting diodes with ultrathin Liq interlayers‘ on-line in Scientific Reports (01 March 16) covers the work.
This paper, which is available in full, also explains TADF emission: “The key characteristic of efficient TADF emitters is a small energy gap between the lowest singlet-excited and triplet-excited states, allowing efficient up-conversion of triplet excitons into emissive singlet excitons. Through the careful tuning of molecular structures, TADF emitters and OLEDs employing them can obtain nearly 100% upconversion efficiency and top-tier quantum efficiencies. Since the molecular design strategy for TADF emitters provides broad freedom by simply using conventional aromatic moieties, TADF materials can open a wide variety of possibilities in future OLEDs.”
Published at the same time, in Advanced Materials, are solution-processed TADF emitters from Pohang University of Science & Technology (POSTECH).
This team tackled efficiency issues with solution-processed TADF OLEDs with a multi-functional buffer hole injection layer (Buf-HIL) that can increase the hole injection into the emitting layer because of its high work function, and improve the luminescence efficiency by preventing exciton quenching at the interface between hole injection layer and emissive layer.
“Furthermore, a new polar aprotic solvent improved the device efficiency by improving the solubility of pure-organic [non-metal] TADF emitters, reducing the surface roughness and the aggregation of dopants, and managing the exciton quenching in the emitting layer,” said POSTECH.
“This technology is a big leap toward the development of inexpensive OLED displays and solid-state lightings because this method uses only low-cost pure-organic molecules and simple solution process to realise high-efficiency OLEDs,” said Professor Tae-Woo Lee of POSTECH.
Black line – early TADF emitter
Red line – modified with 3nm Liq on the emitter side and 2nm on the electron transport side of the hole blocking layer.
Green line – also doping the emitter and host layer with an electron transport material, and doping the electron transport layer with Liq.