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 There are three basic mechanisms 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 for a single-junction cell.
Sensitized triplet-triplet annihilation Sensitized triplet-triplet annihilation (sTTA) based photon upconversion is a bimolecular process that through a number of energy transfer steps, efficiently combines two low frequency photons into one photon of higher frequency. TTA systems consist of one absorbing species, the sensitizer, and one emitting species, the emitter (or annihilator). Emitters are typically polyaromatic chromophores with large singlet-triplet energy splitting, such as anthracene and its derivatives. The first step in sensitized triplet-triplet annihilation is absorption of a low energy photon by the sensitizer. The sensitizer then populates its first triplet excited state (3Sen*) after intersystem crossing (ISC). The excitation energy on the sensitizer then transfers through a Dexter type triplet energy transfer (TET) to a ground state emitter, generating a triplet excited emitter (3Em*). Two triplet excited emitters then interact in a second energy transfer process, known as triplet-triplet annihilation (TTA). Upon TTA the triplet energies are fused leaving one emitter in its excited singlet state (1Em*) and the other emitter in its ground state. From the singlet excited state the emitter returns to the ground state through the emission of a photon. In this way two low energy photons are converted into one photon of higher energy. The principle relies on long lived triplet states to temporarily store the photon energy. Since molecular oxygen effectively quenches triplet states it is important that samples are thoroughly degassed or encapsulated to function efficiently. Photon upconversion through sensitized triplet-triplet annihilation has the advantage of being efficient even at low excitation intensities making it potentially useful for converting sun light to enhance solar cell efficiencies.
Upconverting nanoparticles
Although photon upconversion was first studied in bulk crystals and optical fibers, it became better known with the development of nanomaterials. This happened due to the many ways in which nanostructures with photon upconversion properties can be applied. This new class of materials may broadly be referred to as upconverting nanoparticles or UCNPs.
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