An UC device is designed to absorb subbandgap photons in an UC layer behind a bifacial solar cell. This layer radiatively absorbs two or more long-wavelength photons and emits a photon of higher energy above the bandgap of the bifacial solar cell. Thus, the current in the device is boosted by photons that would not normally be absorbed. As the UC does not interrupt the incidence of photons on the front surface, even a very low efficiency of UC gives a small current boost and hence an efficiency increase.
For application to photovoltaics there are two broad possibilities. Either the current in a silicon solar cell can be boosted through the application of a simply applied thick-film upconverting layer on the back surface of a bifacial silicon cell. This approach would be reasonably simple to apply to cells in a production line and hence offers a near-term boost to efficiency. Although it would boost the current of the device the energy levels of around 0.8 eV for the intermediate level and > 1.1 eV for the emission level (i. e. above the bandgap of silicon) are not optimum for the three-transition upconversion effect. Alternatively, a device in which both the intermediate and bandgap energies are optimised to about 1 and 2 eV, respectively, would give a higher limiting efficiency of just over 40%, for a symmetric device, [Ekins-Daukes and Schmidt, 2008]. The materials for this would both need to be tailor-made so this is not a near-term device, but does offer higher efficiencies.
The key to an efficient upconversion mechanism is for the transitional state – resulting from the absorption of one subbandgap photon – to be long-lived such that it lasts long enough to absorb a second low-energy photon and allow time for the reaction, boosting the energy of the state to a higher level to occur. If this second higher-energy state is above the bandgap, radiative emission from the state will boost the flux of above bandgap photons incident on the cell.
