Active-oxidation of Si as the source of vapor-phase reactants in the growth of SiOₓ nanowires on Si

Gold-coated silicon wafers were annealed at temperatures in the range from 800–1100 °C in a N₂ ambient containing a low (3–10 ppm) residual O₂ concentration. A dense network of amorphous silicananowires 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

Secondary growth and photoluminescence from erbium implanted silica nanowires

Gold-catalyzed silicananowires were grown using vapor from the active oxidation of the silicon substrate and then implanted with erbium and annealed. During prolonged annealing at 1100 °C, where the concentration of vapor-phase reactants is sufficient to support nanowire growth, the erbium rich precipitates act as catalysts for the growth of a second generation of nanowires. These secondary nanowires increase in photoluminescence as they grow, suggesting that a fraction of the optically active erbium is incorporated into the growing wire. The resulting luminescent nanostructures have a very large surface-to-volume fraction and are well suited for optical-sensing applications.A.S. acknowledges funding from the Australian Research Council ARC

Imaging of the relative saturation current density and sheet resistance of laser doped regions via photoluminescence

We present an approach to characterize the relative saturation current density (J oe) and sheet resistance (RSH) of laser doped regions on silicon wafers based on rapid photoluminescence (PL) imaging. In the absence of surface passivation layers, the RSH of laser doped regions using a wide range of laser parameters is found to be inversely proportional to the PL intensity (I PL ). We explain the underlying mechanism for this correlation, which reveals that, in principle, I PL is inversely proportional to J oe at any injection level. The validity of this relationship under a wide range of typical experimental conditions is confirmed by numerical simulations. This method allows the optimal laser parameters for achieving low RSH and J oe to be determined from a simple PL image.The authors acknowledge financial support from the Australian Solar Institute (ASI)/Australian Renewable Energy Agency (ARENA) under the ANU PV Core project, Postdoctoral Fellowship and Australia-Germany Collaborative Solar Research and Development projects. The authors also acknowledge support from the Australian Government’s NCRIS/EIF funding programs for access to Heavy Ion Accelerator Facilities at the Australian National University

Implementation of a Monte Carlo method to model photon conversion for solar cells.

A physical model describing different photon conversion mechanisms is presented in the context of photovoltaic applications. To solve the resulting system of equations, a Monte Carlo ray-tracing model is implemented, which takes into account the coupling of the photon transport phenomena to the non-linear rate equations describing luminescence. It also separates the generation of rays from the two very different sources of photons involved (the sun and the luminescence centers). The Monte Carlo simulator presented in this paper is proposed as a tool to help in the evaluation of candidate materials for up- and downconversion. Some application examples are presented, exploring the range of values that the most relevant parameters describing the converter should have in order to give significant gain in photocurrent

Application of NaYF[sub 4]:Er[sup 3+] up-converting phosphors for enhanced near-infrared silicon solar cell response

Erbium-doped sodium yttrium fluoride (NaYF₄:Er³⁺) 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±0.2)% under 5.1 mW laser excitation at 1523 nm, corresponding to an internal quantum efficiency of 3.8%

An experimental study on the molecular organization and exciton diffusion in a bilayer of a porphyrin and poly(3-hexylthiophene)

The exciton root-mean-square displacement (?D) in regioregular poly(3-hexylthiophene) (P3HT) deposited onto meso-tetrakis (n-methyl-4-pyridyl) porphyrin tetrachloride (H2TMPyP) 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 H2TMPyP and P3HT can undergo interfacial charge separation by electron injection into the H2TMPyP and hole injection into the P3HT. The incident photon to current efficiency (IPCE) exceeds 20% over a broad wavelength regime. The theoretical analysis of the IPCE values gives a value for ?D in H2TMPyP 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 H2TMPyP. In contrast, in the case of P3HT deposited onto TiO2, 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 H2TMPyP 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.DelftChemTechApplied Science