As many colours as the human eye can see, these represent only a small range in the full spectrum of light. Just beyond it lies near-infrared (NIR) light—an invisible yet vital part of the spectrum used in night vision, environmental monitoring and medical imaging. However, to make NIR signals accessible for everyday applications, they must be transformed into visible light through a process known as photon upconversion.
“Conventional photon upconversion systems often suffer from significant energy losses, particularly during the diffusion and transfer of energy states called triplet excitons,” said Le Yang, Group Leader at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE). This inefficiency stems from using a single-component sensitiser to both absorb the invisible photons and transfer energy to emitter molecules.
Together with collaborators from the Institute of Chemistry Chinese Academy of Sciences and Shandong University in China, as well as Nanyang Technological University, Singapore, Yang and A*STAR IMRE Research Scientist Pengqing Bi led a team in designing a bulk-heterojunction (BHJ) donor-acceptor (D-A) sensitiser. This D–A type sensitiser efficiently generates a large number of free charge carriers, which subsequently recombine at the sensitiser–emitter interface to form triplet excitons. In contrast to conventional upconversion systems that rely on triplet exciton diffusion and energy transfer, this approach significantly suppresses related energy losses and enhances upconversion efficiency.
“We aimed to reduce the threshold amount of NIR light needed to trigger photon upconversion, while also improving the conversion efficiency,” explained Yang. “Think of it as trying to light a bulb with as little electricity as possible and making sure it glows brightly.”
Both sensitiser components worked in tandem to accelerate the formation of triplet excitons on the emitter, enhancing the overall upconversion performance.
Despite the promise of the BHJ strategy, the researchers observed that the photon upconversion efficiency still decreased significantly under low-light scenarios where NIR signals are much too weak. They then integrated electroluminescence-based light compensation to achieve more reliable NIR detection. “Under weak NIR illumination, the device can be switched to a mode where it actively emits light using electrical input, which helps visualise the NIR photons even when upconversion alone is insufficient,” Yang said.
By combining NIR sensing, photon upconversion and electroluminescence into a single platform, the team’s new multifunctional optoelectronic device effectively adapts to the intensity of NIR light present.
“We essentially built a smart ‘skin’ around our light-converting material, so the device can do more than just see NIR—it can interact with its environment,” said Yang. This could support the development of various practical applications, such as night-time security systems that can see in complete darkness, non-invasive blood flow monitors and other advanced healthcare sensors.
The researchers are now working to enhance their multifunctional optoelectronic device by improving the upconversion efficiency, potentially enabling it to function under even weaker NIR light conditions. In addition, they aim to develop flexible array-type devices with higher spatial resolution to enable real-time NIR detection and imaging capabilities.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
