EFFICIENT ULTRAVIOLET LIGHT FREQUENCY DOWN-SHIFTING BY A THIN FILM OF ZnO NANOPARTICLES

The maximal effciency of a single junction solar cell (SC) is defined as the Shockley-Queisser limit, which determines the maximal output power which can be furnished by a SC as a function of the bandgap of the semiconductor constituting the cell. The short wavelength spectral response of a SC can be improved if a luminescent down-converting layer is added to the SC structure. We propose the use of a layer containing ZnO nanoparticles (NPs) as a luminescent down-shifter. ZnO is able to absorb efficiently the ultraviolet light (lambda< 400 nm), where the SC spectral response is low and to re-emit lower energy photons (longer wavelength photons) for which the SC spectral response is enhanced, thus increasing the total photocurrent. The stoichiometry and crystallinity of ZnO NPs can be controlled and adjusted to obtain the highest visible photoluminescent emission, indicator of an efficient down-shifting. The ratio between the ZnO UV absorption and visible emission is also estimated and from these results we expect the increase of the SC efficiency using ZnO NPs as a down-shifting layer placed on the front side of the SC.