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%.

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The exciton root-mean-square displacement ͑⌳ D ͒ in regioregular poly͑3-hexylthiophene͒ ͑P3HT͒ deposited onto meso-tetrakis ͑n-methyl-4-pyridyl͒ porphyrin tetrachloride ͑H 2 TMPyP͒ has been determined from the photovoltaic response of a device based on these materials in a bilayer configuration. Excitons formed on illumination that reach the interface between H 2 TMPyP and P3HT can undergo interfacial charge separation by electron injection into the H 2 TMPyP and hole injection into the P3HT. The incident photon to current efficiency ͑IPCE͒ exceeds 20% over a broad wavelength regime. The theoretical analysis of the IPCE values gives a value for ⌳ D in H 2 TMPyP that amounts to 14 nm, while for P3HT a value of 18 nm is obtained. The latter value exceeds literature values reported for P3HT by almost a factor of 3. X-ray diffraction analysis shows that in the studied bilayer the P3HT backbones are aligned parallel to the interface with H 2 TMPyP. In contrast, in the case of P3HT deposited onto TiO 2 , for which ⌳ D has been reported to equal only 7 nm, hardly any organization of the P3HT backbones is observed. The excitonic coupling between P3HT backbones deposited onto H 2 TMPyP is as high as 125 cm −1 , a factor of 3 larger than the excitonic coupling between the disordered P3HT backbones that amounts to 47 cm −1 . The difference illustrates the importance of controlling the molecular organization for the realization of efficient energy transfer in organic optoelectronics.

Gold-coated silicon wafers were annealed at temperatures in the range from 800-1100°C in a N 2 ambient containing a low ͑3-10 ppm͒ residual O 2 concentration. A dense network of amorphous silica nanowires was only observed on samples annealed at temperatures above 1000°C and was correlated with the development of faceted etch-pits in the Si surface. Comparison with known thermodynamic data for the oxidation of Si and vapor-pressures of reactants shows that nanowire growth is mediated by a vapor-liquid-solid mechanism in which the dominant vapor-phase source of reactants is SiO produced by the active oxidation of Si.