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New Haven, MI, United States

Foley J.M.,Program in Applied Physics | Price M.J.,Program in Applied Physics | Maldonado S.,Program in Applied Physics | Maldonado S.,930 iversity
Energy and Environmental Science | Year: 2012

The solar energy conversion properties of thin Si and GaP nanowire photoelectrodes in photoelectrochemical cells have been examined through sets of finite-element simulations. A discussion describing the motivation behind nanostructured, high aspect ratio semiconductor photoelectrode designs and a brief survey of current experimental results reported for nanostructured semiconductor photoelectrodes in photoelectrochemical cells are presented first. An analysis is then shown that outlines the primary recombination pathways governing the steady-state current-potential behaviors of thin, cylindrical nanowire photoelectrodes, with explicit expressions detailing the differences between planar and cylindrical photoelectrodes arising from the solution of carrier fluxes in planar and cylindrical geometries. Results from finite-element simulations used to model the key features of thin nanowire photoelectrodes under low-level injection conditions are shown that illustrate which recombination pathway(s) is operative under various experimental conditions. Specifically, the respective effects of non-uniform doping, tapering along the length, variation in charge carrier mobilities and lifetimes, changes in nanowire radius, and changes in the density of surface defects on the observable photocurrent-potential responses are reported. These cumulative results serve as guides for future experimental work aimed at improving the attainable solar energy conversion efficiencies of doped semiconductor nanowire photoelectrodes. Lastly, separate simulations that model lightly doped nanowire photoelectrodes under high-level injection conditions are discussed. These results suggest discrete, ohmic-selective contacts may afford a way to circumvent the stringent doping requirements discussed herein for thin nanowire photoelectrodes. © 2011 The Royal Society of Chemistry. Source

Brown E.S.,930 iversity | Hlynchuk S.,930 iversity | Maldonado S.,930 iversity
Surface Science | Year: 2016

Chemically modified Si(111) surfaces have been prepared through a series of wet chemical surface treatments that simultaneously show resistance towards surface oxidation, selective reactivity towards chemical reagents, and areal defect densities comparable to unannealed thermal oxides. Specifically, grazing angle attenuated total reflectance infrared and X-ray photoelectron (XP) spectroscopies were used to characterize allyl-, 3,4-methylenedioxybenzene-, or 4-[bis(trimethylsilyl)amino]phenyl-terminated surfaces and the subsequently hydroxylated surfaces. Hydroxylated surfaces were confirmed through reaction with 4-(trifluoromethyl)benzyl bromide and quantified by XP spectroscopy. Contact angle measurements indicated all surfaces remained hydrophilic, even after secondary backfilling with CH3- groups. Surface recombination velocity measurements by way of microwave photoconductivity transients showed the relative defect-character of as-prepared and aged surfaces. The relative merits for each investigated surface type are discussed. © 2015 Elsevier B.V. All rights reserved. Source

Hagedorn K.,930 iversity | Forgacs C.,930 iversity | Collins S.,930 iversity | Maldonado S.,930 iversity
Journal of Physical Chemistry C | Year: 2010

The steady-state photoelectrochemical responses of semiconductor nanowire arrays in a nonaqueous regenerative photoelectrochemical cell were analyzed. Experimental and numerical simulation data were collected to determine the extent that dopant density levels, ND, have on the efficiency of semiconductor nanowire photoelectrodes with radii (r) comparable to the width of the depletion region (W). Films of Si nanowires (r < 40 nm) were prepared by metal-assisted chemical etching of single-crystalline Si(111) substrates with known bulk optoelectronic properties and utilized as photoelectrodes in a methanolic electrolyte containing dimethylferrocene (dmFc) and dimethylferrocenium (dmFc+). This photoelectrochemical system featured definable values for the rate of heterogeneous charge-transfer, the interfacial equilibrium barrier height (φb), and the rate of surface recombination. Under white light illumination, the photocurrent- potential responses of Si nanowire arrays were strongly influenced by the ratio between the nanowire radius and the depletion region width (r/W). Lightly doped Si nanowire arrays consistently showed lower light-saturated photocurrents than heavily doped Si nanowire arrays despite having hole diffusion lengths, L p, that were larger by a factor of 2. Measurement of the wavelength-dependent external quantum yields for the Si nanowire arrays separated out the effects from the underlying Si substrate and confirmed that carrier-collection was either significantly enhanced or suppressed by the Si nanowires depending on the value of r/W established by the φb and ND. Digital simulations of nanowire heterojunctions using a two-dimensional semiconductor analysis software package (TeSCA) and known system parameters are presented that further explore the quantitative interplay between r/W and collection efficiency for nanowire photoelectrodes. The implications for designing low-cost semiconductor photoelectrodes using nanowire-based heterojunction architectures are examined, and tolerances for control over doping levels in semiconductor nanowire photoelectrodes are discussed. © 2010 American Chemical Society. Source

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