Excitation of luminescing biolabels via two-photon absorption processes allows the use of near-infrared (NIR) light, which is only weakly absorbed by biological tissue.[1] Excitation in the NIR induces only a very weak autofluorescence background and avoids photodegradation in biotagging applications, thus simplifying the detection of the labeled target molecules and increasing the sensitivity of the method. Organic dyes as well as semiconductor nanoparticles can be employed as emitters. [1±3] Due to the nature of the two-photon absorption (TPA) process which involves a non-stationary (ªvirtualº) quantum mechanical state, however, its efficiency is very low, requiring high excitation densities. To avoid thermal decomposition processes in the sample, expensive pulsed lasers are frequently employed as light sources with pulse durations in the pico-or even femtosecond range. Photon upconversion is an alternative process for the generation of visible radiation by NIR excitation. It is based on sequential absorption and energy transfer steps involving real metastable excited states of the chromophore.[4] Therefore its efficiency can be much higher than for TPA processes, and continuous wave (CW) laser or lamp excitation is possible. Typical excitation densities fall in the range of 1±10 3 W cm ±2for upconversion, compared to 10 6 ±10 9 W cm ±2 for twophoton absorption. [2,4,5] Applications range from display devices [6] and lasers [7] to commercially used reporters for nucleic acid microarrays. [8,9] [10] Crystalline materials doped with these ion couples, however, normally consist of sub-micrometer [8,9] to micrometer-sized grains [10] which do not form transparent colloids and are much too large to substitute for molecular dyes in biological tagging applications. Therefore it was a challenging task to synthesize nanocrystals of these materials which can be transparently dispersed in solution. Recently we have been able to dope lanthanide phosphate nanocrystals with these ion couples and have demonstrated upconversion emission in transparent colloidal solution for the first time.[11] Due to competing radiationless processes, however, the efficiency of the upconversion luminescence was still rather poor. In the present paper we report on the successful synthesis and very intense multicolor upcon- nanocrystals transparently dispersed in solution. The upconversion efficiency of such solutions is about eight orders of magnitude higher than for the previously reported colloids of lanthanide-doped phosphate nanocrystals. We believe that this enormous improvement of the upconversion efficiency of these materials opens the door for interesting future applications in the field of biolabeling. The characterization of NaYF 4 :20 %Yb,2 %Er is summarized in Figure 1. The transmission electron microscopy (TEM) images (Fig. 1A) show crystalline particles of roughly spherical shape. A histogram of the particle size distribution, deduced from several overview TEM images, is given in Figure 1B and shows a relatively broad distributi...

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Erbium-doped sodium yttrium fluoride ͑NaYF 4 :Er 3+ ͒ up-conversion phosphors were attached to the rear of a bifacial silicon solar cell to enhance its reponsivity in the near-infrared. The incident wavelength and light intensity were varied and the resulting short circuit current of the solar cell was measured. A close match between the spectral features of the external quantum efficiency and the phosphor absorption is consistent with the energy transfer up-conversion process. The peak external quantum efficiency of the silicon solar cell was measured to be ͑2.5± 0.2͒% under 5.1 mW laser excitation at 1523 nm, corresponding to an internal quantum efficiency of 3.8%.