Our research group is currently focusing on exploring the unconventional photochemical phenomenon, called upconversion. Upconversion (UC) refers to a process that achieves photon-frequency amplification or anti-Stokes shift via sequential absorption of two or more photons with lower energy and subsequent emission of one higher energy photon. By making use of UC materials, one can produce higher energy photon from the photochemical fusion lower energy photons (e.g. VIS-to-UV, NIR-to-VIS, Red-to-Blue, etc). UC has been gathering more attention in academia and industry, due to its unique photoluminescence characteristic.
The well-known primary application of UC is the photovoltaics. Most of the current photovoltaics work by absorbing only some portion of the solar spectrum. The rest part of the solar spectrum that has lower energy than the photovoltaics' threshold energy cannot be utilized for activating the system. This has been considered as a serious bottleneck for enhancement the photovoltaics efficiency. UC can be employed as a breakthrough for solving this problem by providing additional available photons from the fusion of lower energy photons which is currently being wasted. Though the primary application of UC is the photovoltaics, UC is now broadening its application spectrum to various energy, environmental, biomedical, and material engineering where anti-Stokes emission can be utilized.
Among a few processes that achieve UC, our major interest is in triplet−triplet annihilation (TTA)-based UC. TTA-UC is achieved through synchronous energy transfer between organic chromophores – sensitizer and acceptor –that fulfill specific energetic criteria, i.e., triplet−triplet energy transfer (TTET) between the sensitizer and acceptor as well as TTA between two excited acceptors. Below are the advantages of TTA-UC.
· High upconversion quantum yield (up to 40%)
· Capability to upconvert a non-coherent light source at low irradiation power (10~100 mW/cm2)
· No need for a high-temperature sintering step
More specifically, our research area includes:
· Fundamental study of TTA-UC mechanism
· Synthesis of new sensitizers and acceptors for TTA-UC
· Fabrication of efficient TTA-UC materials with various configuration
· Application of TTA-UC materials to the practical devices
Fabrication of efficient TTA-UC materials
Our research group is specialized in making highly efficient and photo-chemically stable TTA-UC materials with various configuration, that includes nano- or microcapsule, thin film, micro-patterned film, or freely-designed shapes. To achieve this, varieties of techniques are employed independently or integrated together. These techniques include micro-emulsion, glass capillary microfluidics, spin-coating, interfacial polymerization, photo-polymerization, photolithography, and soft lithography. We are also trying to graft new techniques onto making novel TTA-UC materials.
Application of TTA-UC materials to the practical devices
TTA-UC materials can be primarily integrated with photonic devices that work by absorbing solar spectrum such as photovoltaics or solar water splitting cell to provide enhanced energy conversion efficiency. By making use of the TTA-UC materials, we continue to develop new TTA-UC-hybridized photonic devices. Besides of photovoltaics, we are also largely interested in the application of TTA-UC materials to the bio-medical and the material engineering where Anti-Stokes emission can be utilized (e.g. ultra-high S/N bio-imaging and novel anti-counterfeiting technique).