Photon upconversion (UC) is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength. It is an anti-Stokes type emission. An example is the conversion of infrared light to visible light. Upconversion can take place in both organic and inorganic materials, through a number of different mechanisms. Organic molecules that can achieve photon upconversion through triplet-triplet annihilation are typically polycyclic aromatic hydrocarbons (PAHs). Inorganic materials capable of photon upconversion often contain ions of d-block or f-block elements. Examples of these ions are Ln3+, Ti2+, Ni2+, Mo3+, Re4+, Os4+, and so on. Physical mechanisms behind photon upconversion There are three basic mechanism for photon upconversion in inorganic materials and at least two distinct mechanisms in organic materials. In inorganic materials photon upconversion occurs through energy transfer upconversion (ETU), excited-state absorption (ESA) and photon avalanche (PA). Such processes can be observed in materials with very different sizes and structures, including optical fibers, bulk crystals or nanoparticles, as long as they contain any of the active ions mentioned above. Organic molecules can upconvert photons through sensitized triplet-triplet annihilation (sTTA) and energy pooling. Upconversion should be distinguished from two-photon absorption and second-harmonic generation. These two physical processes have a similar outcome to photon upconversion (emission of photons of shorter wavelength than the excitation) but the mechanism behind is different. An early proposal (a solid-state IR quantum counter) was made by Nicolaas Bloembergen in 1959 and the process was first observed by François Auzel in 1966. A thermal upconversion mechanism is also possible. This mechanism is based on the absorption of photons with low energies in the upconverter, which heats up and re-emits photons with higher energies. To improve this process, the density of optical states of the upconverter can be carefully engineered to provide frequency- and angularly-selective emission characteristics. For example, a planar thermal upconverting platform can have a front surface that absorbs low-energy photons incident within a narrow angular range, and a back surface that efficiently emits only high-energy photons. These surface properties can be realized through designs of photonic crystals, and theories and experiments have been demonstrated on thermophotovoltaics and passive radiative cooling. Under best criterion, energy conversion efficiency from solar radiation to electricity by introducing up-converter can go up to 73% using AM1.5D spectrum and 76% considering sun as a black body source at 6,000 K