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. Lanthanide-doped nanoparticles Lanthanide-doped nanoparticles emerged in the late 1990s owing to the increasing focus on nanotechnology. Although their optical transitions essentially resemble those in bulk materials, the nanostructure amenable to surface modifications results in improved or new characteristics. Besides, the small size of the particles allow their use as alternatives to molecular fluorophores for biological applications. Their unique optical properties, such as large Stokes shift and the lack of blinking, have enabled them to rival conventional luminescent probes