Downconversion, also known as quantum cutting , has attracted much attention due to the potential application in photovoltaic cells. The downconversion materials placed on the front surface of solar cell are able to harvest the far above-bandgap light and split the high energy photons into two photons that can still be absorbed by the solar cells. This is an avenue to realize current (and thus efficiency) doubling for the high energy (blue, UV) part of the solar spectrum.
Even though downconversion has been demonstrated for a variety of materials, downconversion materials are still far from practical application. Downconversion materials rely on lanthanide ions. Their low absorption cross sections result in low excitation efficiency and only a small fraction of the solar spectrum can be harvested . Therefore, enhancing the efficiency by means of broadband absorption by so-called sensitizers is crucial. An important class of sensitizers, organic dyes, has been extensively investigated in upconversion materials because of the 3-4 order of magnitude higher absorption cross section [3-4], but so far not for downconversion. In this work, we demonstrate dye sensitized downconversion with a proof-of-concept experiment. Luminescence spectra and decay lifetime measurements shows the occurrence of Förster energy transfer from dye molecules absorbed on the surface on NaYF4:Pr,Yb nanocrystals. Energy transfer from the dye to Pr3+ is followed by energy transfer to two neighboring Yb3+ ions and emission of two infrared photons. The NaYF4:Pr3+Yb3+ nanocrystals have ~30 times stronger infrared emission intensity after dye sensitization. The decrease in lifetime of the dye emission after absorption at the nanocrystal surface serves as evidence for energy transfer and can be used to quantitatively determine the efficiency of Förster energy transfer. The present study demonstrates the feasibility of dye sensitized downconversion and the strategy can be used to enhance the downconversion efficiency for a wide range of lanthanide doped materials.
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