The role of polymers in the luminescence conversion of sunlight for enhanced solar cell performance

The application of luminescent devices to existing silicon solar cells has the potential to bridge the gap between first and third generation photovoltaics (PV) and enhance the energy conversion efficiency. Two mechanisms that limit the efficiencies of conventional solar cells are, firstly, the transmission of sub-bandgap light and, secondly, the thermalisation of charge carriers generated by the absorption of photons with an energy greater than the silicon bandgap (E-g) of the semiconductor. In this paper we discuss ways of reducing these losses via the application of passive optical devices called up- and down-converters, respectively. Down-conversion (DC) results in the generation of more than one lower energy photon (>E-g) being generated per incident high-energy photon (>2E(g)), while up-conversion (UC) generates one photon with energy >E-g for every two or more sub-bandgap (<E-g) photons absorbed. A final process, down-shifting (DS), is similar to DC except that the external quantum efficiency of the DS process is less than unity. For a low cost and large area PV product, the luminescent centres are incorporated into large area polymer sheets. The desirable properties of such polymer hosts are compared for the three scenarios of UC, DC and DS.