Photon Upconversion to
Light is an electromagnetic wave, as shown by the fact that monochromatic light obtained using a glass prism has a unique wavelength. Light also behaves as a collection of elementary particles called photons, whose existence is manifested by the photoelectric effect. Light energy (photoenergy) is especially important among many possible forms of energy. Sunlight is the only energy supplied to the Earth from space. In addition, light has many important practical functions to convert energy and matter to more useful forms. For example, using photovoltaics, photoenergy can be converted to more useful electrical energy. Using photosynthesis materials or photocatalysts, photoenergy can be converted to various storable forms of chemical energy like sugars, proteins, hydrocarbon fuels, and hydrogen gas.
However, the efficiencies of these photoenergy conversion systems suffer from a fundamental limitation. This originates from the fact that only photons possessing higher energies than the bandgap energy of semiconductors or the gap energy between the highest occupied and lowest unoccupied molecular orbitals of molecular materials can be used. Photons with lower energy than these intrinsic gap energies of materials are wasted at present, regardless of the power of the incident light. Because the energy of a photon is inversely proportional to the wavelength of light, there is a threshold wavelength, which represents a boundary separating the usable and unusable spectral regions of sunlight. That is, incident light that has a longer wavelength than this threshold wavelength is useless for photoenergy conversion, resulting in the aforementioned limitation of conversion efficiency. For example, the water-splitting reaction to generate hydrogen gas by a photocatalyst can usually be achieved only by light that has a wavelength shorter than blue (∼450 nm). Photosynthesis in plants can only be driven by light whose wavelength is red or shorter. Light that can generate electric power in amorphous silicon photovoltaics is limited to wavelengths shorter than dark red (∼720 nm). Therefore, given the solar spectrum on Earth, numerous photoenergy conversion systems display fundamental losses of energy.
Photon upconversion (UC) is a technology that circumvents these fundamental losses and converts presently unused photons to usable ones. Previously, UC was only possible for high-intensity laser light possessing linear polarization. Recently, a method of UC that uses properly designed intermolecular energy transfer between organic molecules emerged and has been found to be applicable to low-intensity and randomly polarized light, including normal sunlight. Thus, this UC method has great versatility with broader application prospects. Realization of such UC with high efficiency would not only increase the efficiencies of the aforementioned photoenergy conversions but also broaden the possible applications of light.
We started studies on this UC technology soon after its emergence and have reported several unique achievements. This research field is currently rapidly growing, through which exciting frontiers and questions are emerging. Our present aims are to elucidate such questions and develop novel UC materials suitable for application. To attain these goals, we are conducting highly interdisciplinary research encompassing physical chemistry, mechanical engineering, photochemistry, and materials science.
