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A team of researchers at the Physics Department of Durham University have found that certain molecules long considered poor emitters are actually ideal for boosting efficiency and stability in next-generation blue OLED displays. The study reveals an overlooked molecular ‘blind spot’ that could enable major advances in energy-saving display technologies.
Outshining expectations
Organic light-emitting diodes (OLEDs), used in most smartphones and TVs today, rely on specialised organic molecules to emit light. However, obtaining efficient and stable blue OLED emission has remained a difficult challenge for researchers. Now, the research team has revealed that molecules previously dismissed as subpar light emitters can be used to triple the efficiency of blue OLEDs. A molecule called ACRSA was found to increase device efficiency from 10% to over 28% when used as a sensitiser in ‘hyperfluorescent’ OLEDs, where energy is transferred from the sensitiser to a separate terminal emitter molecule.
An illuminating discovery
Even more remarkably, using the greenish sensitiser ACRSA, blue emission can be achieved by transferring ACRSA’s energy to a blue terminal emitter. This green-to-blue approach reduces exciton energy compared to direct blue emitters, enabling more stable, longer-lasting blue OLEDs. The team's strategy provides a new blueprint for designing stable and highly efficient hyperfluorescent OLED displays. This new understanding opens up exciting possibilities for both fundamental research and practical applications.
More about OLEDs
Organic Light Emitting Diodes, “OLEDs” are becoming more commonplace in mobile phones and televisions. Picture quality is superb, but such perfection comes at a high cost in terms of battery life and energy consumption. To address this, they have studied the fundamental light generation processes in an OLED to produce extremely pure saturated colour at up to four times more efficiently than current OLEDs. To achieve pure red, green and blue colours, currently, colour filters are used that effectively throw away 70% of the light created, wasting large amounts of energy.
In their work, they demonstrate how the process of hyperfluorescence can be optimised in a non-intuitive fashion to produce intrinsically highly saturated colours at very high efficiency (close to 100%) without the hugely wasteful optical filters. Hyperfluorescence was first demonstrated in OLEDs nearly 10 years ago, however, it has been very difficult to control, especially for blue light. Here, two different molecules are used to generate the light in the OLED. The first converts electrical current into light at nearly 100% efficiency, but instead of simply emitting that light which requires optical filtering to achieve usable colours for the display, the light energy is passed on to a second terminal emitter molecule, in a sensitisation mechanism, this emits with an intrinsically narrow emission band yielding saturated colour without filters. This second molecule however cannot directly convert current to light at high efficiency.
Until their new work, the sensitiser molecule was always selected to have the best possible light generation efficiency, assuming that then it would transfer the most light to the terminal emitter molecule. In fact, achieving the most efficient sensitisation and obtaining the purest colours requires photophysical properties that make the sensitiser molecule a rather mediocre emitter in its own right, one that would never be used in a standard OLED design and previously would have been overlooked for hyperfluorescent applications. However, such sensitisers ensure 100% of their light energy is transferred to the terminal emitter, whereas very good emitting sensitizers emit light in competition with the terminal emitter giving non-saturated emission which requires optical filtering. In their study, they determine the optimal photophysical parameters for the sensitiser to give 100% sensitisation with no parasitic sensitiser emission. In the best examples, they take a sensitiser that on its own only produces 10% efficient OLEDs, but when combined with a hyperfluorescent emitter they achieved 28% efficient OLEDs giving saturated blue light, 4 times better than current state-of-the-art blue OLEDs, saving nearly 40% energy consumption. If they were to apply such energy savings across all displays globally, some 400-500 TWhrs of electricity could be saved annually.
Click here to read the paper titled, "Key requirements for ultraefficient sensitization in hyperfluorescence organic light-emitting diodes."
