The amount of solar energy that a solar cell can convert into electrical energy is presently limited by its bandgap, such that only the photons with higher energies can be converted, leaving photons with lower energies useless. This limit is also called the Shockley-Queisser limit and in order to address such hindrance in achieving optimum solar efficiency, photon upconversion was created.
Photon upconversion is the process of combining low-energy photons to transform them into high-energy photons. It can increase the efficiency of solar cells with single bandgaps surpassing their Shockley Queisser limit. It may also be useful in light emitting devices and biological imaging.
One possible way of executing photo upconversion is through the method called triplet-triplet annihilation (TTA). TTA involves two types of chromophores, which are made up of organic molecules. One type is a sensitizer, which absorbs a photon and the other is an emitter. When a sensitizer absorbs a photon, it is rapidly transformed into an annihilator or emitter of excited triplet state.
The moment that two annihilators meet, TTA occurs, in which one annihilator becomes excited and simultaneously, the other annihilator becomes relaxed. The excited annihilator finally emits a photon that has a higher energy or twice that of the initial photon absorbed. Overall, the energies of the two initial photons add up to the energy of the photon emitted during a TTA.
As promising as this method can be, the organic solvents involved are hazardous and have high vapor pressures and lack thermal stability. Ionic liquids were also selected to replace these organic solvents, however, they were expensive and have poor biodegradability. Researchers at Tokyo Tech have recently developed a new type of solvent that surpasses these limitations. It is called the deep eutectic solvents (DESs).
DESs are relatively cheaper, safer, and more biodegradable alternative to those previously used solvents. Experimentally, DESs resulted in a photon upconversion quantum yield of 0.21, which is equivalent to an upconversion quantum efficiency of 42 percent, a relatively high efficiency. This result signifies that DESs have opened an opportunity for practical application of the upconversion technology.