Many of today’s screens, such as those found in your TV, laptop, and smartphone, rely on a material called an organic light-emitting diode, or OLED for short. OLED-based screens are known for being thin and lightweight, and for displaying deeper and darker black. However, these screens are stiff and brittle, like a piece of glass. What if these screens could be built with a skin-like softness, and the ability to wrap around your wrist or fold completely in half?
Previously, scientists have engineered new screen designs to impart flexibility in OLED displays. However, these attempts led to a reduction in resolution and image quality, and limited bendability. Other scientists have successfully designed stretchable screens using alternative light-emitting materials, such as fluorescents. However, these materials have inferior characteristics, like lower brightness and less power efficiency than OLED materials.
Recently, scientists from China and the USA modified the molecular design of an existing OLED material to make it more flexible while maintaining its ability to emit light. They focused on a type of OLED that uses energy absorbed from heat to excite an electron into a different energy state and emit light. This type of screen is called a thermally activated delayed fluorescence, or TADF, emitter. Unlike other OLED technology, TADF emitters don’t rely on heavy metals and are therefore safe for human-integrated applications, like wearables.
Most TADF emitters are made of small, inflexible molecules. In recent studies, scientists developed TADF emitters from long chains of molecules, called polymers, but these weren’t stretchable either. To add stretchability to their materials, these scientists added soft molecules consisting of carbon and hydrogen atoms, called alkyl chains, between the TADF polymer units. Their goal was to determine the longest alkyl chains they could add to impart flexibility without sacrificing the light-emitting properties. They synthesized four TADF polymers with alkyl chain lengths of 1, 3, 6, and 10 carbon atoms. They also synthesized a typical small molecule TADF emitter for comparison.
First the scientists tested the light emission properties of each emitter to learn if adding alkyl chains affected their performance. They observed that all five devices successfully emitted green light, with only minimal changes to its intensity. Then they measured the difference in electron energy states, a value that corresponds to how much heat energy is needed to excite an electron in the TADF process, and found it was nearly identical for all the emitters. They interpreted their results to indicate that adding soft alkyl chains to the TADF devices did not affect their ability to emit light.
Next the scientists stretched each emitter until it doubled in length, and observed any crack formation and changes in light emission. They observed the devices with longer alkyl chains had fewer, shorter cracks, meaning they had less damage due to stretching. They noted the TADF polymer with a 10 carbon alkyl chain, the longest alkyl chain they tested, remained fully intact with no cracking even when the emitter was stretched to twice its original size. They also measured more light emitted from the stretched version of that TADF polymer than from the samples made with shorter alkyl chains. They explained this was likely due to the cracks that formed in these emitters, which deteriorated electrical contacts and potentially prevented electrons from changing energy states.
The scientists then incorporated the TADF polymer with a 10 carbon alkyl chain into a stretchable OLED device. In a typical OLED device, the organic light-emitting material is sandwiched between two conducting layers, called electrodes, that allow electricity to flow between them. The scientists designed new stretchable transparent electrodes by adding silver nanowires to a flexible polymer similar to silicone. Then they sandwiched the TADF polymer between these two flexible electrodes. They found the resulting OLED device needed a low voltage to turn on and could be powered with a commercial battery.
The scientists found their new unstretched OLED device had a higher efficiency than what was previously reported for stretchable OLEDs. Then they were able to stretch the device to over 1.5 times its original size before it failed. They found the brightness of the display decreased by 40% when stretched this far. The scientists explained the decrease in brightness was likely due to increasing resistance in the silver nanowire electrodes and damage to the contact between the TADF and the electrode. They observed the device failure was mainly due to short circuiting between the electrodes.
The scientists suggested future work focus on improving stretchable electrodes. They also suggested developing a stretchable, transparent film that could encapsulate and protect the OLED display. They concluded their approach to stretchable TADF polymers could lead to technological advances in stretchable optoelectronic devices with skin-like softness.