As a means to effectively utilize limited energy resources, light energy conversion technology is an indispensable element in modern society. Among them, photon upconversion via triplet-triplet annihilation (TTA-UC) is a chemical reaction process that converts two photons into one higher-energy photon. This process operates even when incoherent low-intensity light is incident, making it possible to effectively utilize light of various wavelengths contained in sunlight. Therefore, TTA-UC attracts attention as a promising means to improve the light energy conversion efficiency in organic solar cells and photocatalysis which are expected to use sunlight as an energy source. Furthermore, because TTA-UC can realize the light-energy conversion using weak excitation lights compatible with the human body, it is expected to be useful in medical fields including bioimaging , photodynamic therapy , and optogenetics ,. A typical TTA-UC system often consists of a combination of a sensitizer and an TTA emitter molecule and goes through a complex process. The quantum yield of the overall TTA-UC process (𝛷 !" ) is represented by the following formula: