Electroluminescent materials, those that are able to emit light in response to the application of an electric current or strong electric field, have been central components to multiple modern technologies. These include displays, wireless data and power transmission and medical therapies.
One of the most advanced and well-known electroluminescent technologies are organic light-emitting diodes, or OLEDs, which can be found in most advanced displays, from watches and cell phones, to large television screens. After decades of research and development their ubiquity in modern society results from their high efficiency, high brightness, low-voltage operation, low cost, large-area scalability and mechanical bendability.
Recent advances have sought to develop electroluminescent materials that have properties close to those of human skin. This would open the door to developing devices that could be intimately integrated with the human body. Imagine an on-skin display, such as a smart watch, or sensors that can monitor blood oxygen saturation or blood sugar levels, or even devices that could implanted to optically stimulate medical therapies. These are just a few of the exciting possible applications that this technology could enable. There are significant challenges that have caused stretchable electroluminescent devices to lag in their development, combining a highly efficient and highly stretchable electroluminescent material. This report, that describes the collaborative work of the seven research groups, outlines a design approach that addresses this combination of properties.
The IMOD group of Dmitri Talapin (University of Chicago), worked in collaboration with the groups of Sihong Wang (University of Chicago), Juan J. de Pablo (University of Chicago and Argonne National Laboratory), Xiaohong Zhang (Soochow University, China), Jie Xu (Argonne National Laboratory), Lixiang Wang (Changchun Institute of Applied Chemistry, China), and Shiyang Shao (Hainan University), to develop electroluminescent materials that are both stretchable and efficient.
The stretchable electroluminescent devices developed to date have used two design approaches; embedding non-stretchable inorganic electroluminescent materials into a stretchable polymer (resulting in poor resolution when stretched – think about printing a picture on a balloon and then blowing it up), using stretchable electroluminescent organic materials. While this second approach is promising, the materials that have been developed thus far using inferior materials – essentially where light-emitting diodes were decades ago.
When a material is excited, by either light or an electrical charge, the electrons in the molecule are promoted to a higher energy level, that is they move slightly further away from the nucleus, but not far enough away to be completely removed. When these excited electrons return to their original state energy is released in the form of what we call an exciton. There are two forms, that differ in their quantum mechanical properties, singlet and triplet excitons. Work to date has employed materials that emit only using the singlet excitons. Only 25% of all excitons formed are singlet, the rest, ~75%, and the vast majority, are in the triplet state. This means that the best materials in this regime will remain in-efficient.
This team took a new approach and developed a design strategy that made use a type of materials called thermally activated delayed fluorescence (TADF) emitters. This class of materials is organic, so can be directly built into the polymer and can emit light using the triplet excitons – removing the limits imposed by only using singlet excitons.
Using a multipronged approach that brought together a range of skills the team were able to develop electroluminescent devices that require low voltages to operate, record efficiencies, even under 60% strain (though at higher strains it is not the polymer that fails, rather shorting between the electrodes).
This new approach toward bright, efficient, and stretchable OLEDs reveals new opportunities for devices in displays, sensors, and medical applications. The design approach was shown to be systematic and generalizable for future researchers.