Triplet-triplet annihilation upconversion (TTA-UC)

Shockley-Queisser efficiency limit (SQ limit) of the maximum theoretical efficiency of a semiconductor solar cell places maximum energy conversion efficiency around 33.7% (single p-n junction with a band gap of 1.34 eV, AM 1.5 solar spectrum). This limit arises from the fact that red and near-infrared (NIR) part of solar spectrum possess too low energy to cause electrons and holes formation in a semiconductor material. Triplet-triplet annihilation upconversion is the only known physical phenomenon that allows to overcome SQ limit. It can be achieved if upconversion material, transforming light of lower energy into light of higher energy (e.g. red into blue), is placed underneath the cell along with a reflector as shown in the figure below.