Showing papers by "Nathan S. Lewis published in 2009"

Flexible Polymer‐Embedded Si Wire Arrays

Abstract: Arrays of Si rods are embedded in PDMS and removed from the rigid growth substrate, resulting in a composite material that merges the benefits of single-crystalline silicon with the flexibility of a polymer. With this technique, solar cell absorber materials with the potential to achieve high efficiency can be prepared by high-temperature processing and transformed into a flexible, processable form.

10 μm minority-carrier diffusion lengths in Si wires synthesized by Cu-catalyzed vapor-liquid-solid growth

Abstract: The effective electron minority-carrier diffusion length, L_(n,eff), for 2.0 µm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, L_(n,eff) was limited by surface recombination to a value of ≤ 0.7 µm. However, a value of L_(n,eff) = 10.5±1 µm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large L_(n,eff) values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes.

Combinatorial synthesis and high-throughput photopotential and photocurrent screening of mixed-metal oxides for photoelectrochemical water splitting

Abstract: A high-throughput method has been developed using a commercial piezoelectric inkjet printer for synthesis and characterization of mixed-metal oxide photoelectrode materials for water splitting. The printer was used to deposit metal nitrate solutions onto a conductive glass substrate. The deposited metal nitrate solutions were then pyrolyzed to yield mixed-metal oxides that contained up to eight distinct metals. The stoichiometry of the metal oxides was controlled quantitatively, allowing for the creation of vast libraries of novel materials. Automated methods were developed to measure the open-circuit potentials (Eoc), short-circuit photocurrent densities (Jsc), and current density vs. applied potential (J–E) behavior under visible light irradiation. The high-throughput measurement of Eoc is particularly significant because open-circuit potential measurements allow the interfacial energetics to be probed regardless of whether the band edges of the materials of concern are above, close to, or below the values needed to sustain water electrolysis under standard conditions. The Eoc measurements allow high-throughput compilation of a suite of data that can be associated with the composition of the various materials in the library, to thereby aid in the development of additional screens and to form a basis for development of theoretical guidance in the prediction of additional potentially promising photoelectrode compositions.

Photoluminescence-based measurements of the energy gap and diffusion length of Zn3P2

Abstract: The steady-state photoluminescence spectra of zinc phosphide (Zn_3P_2) wafers have revealed a fundamental indirect band gap at 1.38 eV, in close proximity to the direct band gap at 1.50 eV. These values are consistent with the values for the indirect and direct band gaps obtained from analysis of the complex dielectric function deduced from spectroscopic ellipsometric measurements. Bulk minority carrier lifetimes of 20 ns were observed by time-resolved photoluminescence decay measurements, implying minority-carrier diffusion lengths of ≥ 7 µm.

Predicted efficiency of Si wire array solar cells

Abstract: Solar cells based on arrays of CVD-grown Si nano- or micro-wires have attracted interest as potentially low-cost alternatives to conventional wafer-based Si photovoltaics [1–6], and single-wire solar cells have been reported with efficiencies of up to 3.4% [7]. We recently presented device physics simulations which predicted efficiencies exceeding 17%, based on experimentally observed diffusion lengths within our wires [8]. However, this model did not take into account the optical properties of a wire array device - in particular the inherently low packing fraction of wires within CVD-grown wire arrays, which might limit their ability to fully absorb incident sunlight. For this reason, we have combined a device physics model of Si wire solar cells with FDTD simulations of light absorption within wire arrays to investigate the potential photovoltaic efficiency of this cell geometry. We have found that even a sparsely packed array (14%) is expected to absorb moderate (66%) amounts of above-bandgap solar energy, yielding a simulated photovoltaic efficiency of 14.5%. Because the wire array comprises such a small volume of Si, the observed absorption represents an effective optical concentration, which enables greater operating voltages than previously predicted for Si wire array solar cells.

Growth, Characterization, and Electrochemical Properties of Doped n-Type KTaO3 Photoanodes

Abstract: The effects of compositionally induced changes on the semiconducting properties, optical response, chemical stability, and overall performance of KTaO_3 photoanodes in photoelectrochemical (PEC) cells have been investigated. Single crystals of n-type Ca- and Ba-doped KTaO_3 with carrier concentrations ranging from 0.45 to 11.5×10^(19) cm^(−3) were grown and characterized as photoanodes in basic aqueous electrolyte PEC cells. The PEC properties of the crystals, including the photocurrent, photovoltage, and flatband potential in contact with 8.5 M NaOH(aq) were relatively independent of whether Ca or Ba was used to produce the semiconducting form of KTaO_3. All of the Ca- or Ba-doped KTaO_3 single-crystal photoanodes were chemically stable in the electrolyte and, based on the open-circuit potential and the band-edge positions, were capable of unassisted photochemical H_2 and O_2 evolution from H_2O. The minority-carrier diffusion lengths values were small and comparable to the depletion region width. Photoanodic currents were only observed for photoanode illumination with light above the bandgap (i.e., λ<340 nm). The maximum external quantum yield occurred at λ=255 nm (4.85 eV), and the depletion width plus the minority-carrier diffusion length ranged from 20 to 65 nm for the various KTaO_3-based photoanode materials.

Infrared Vibrational Spectroscopy of Isotopically Labeled Ethyl-Terminated Si(111) Surfaces Prepared Using a Two-Step Chlorination/Alkylation Procedure

Abstract: Fourier transform infrared (FTIR) spectroscopy was used to investigate C2H5−Si(111) surfaces prepared using a chlorination/alkylation method. After alkylation, in addition to ethyl groups, such surfaces showed the presence of hydrogen bonded to atop silicon surface atoms. Systematic isotopic substitution of protic solvents and reagents with their fully or partially deuterated counterparts revealed the origin of the surface-bound hydrogen on the C2H5−Si(111) surfaces. The presence or absence of the Si−H stretch at 2080 cm−1 and the Si−D stretch at ∼1510 cm−1, respectively, indicated that the hydrogen originated from the methyl group of the ethyl Grignard reagent.

A Perspective on Forward Research and Development Paths for Cost‐Effective Solar Energy Utilization

Abstract: Solar electricity has long been recognized as a potential energy source that holds great promise. Several approaches towards converting sunlight into energy are elaborated in this Viewpoint, and discussed with respect to their feasibility for large-scale application.

Behavior of Electrodeposited Cd and Pb Schottky Junctions on CH3-Terminated n-Si(111) Surfaces

Abstract: n-Si/Cd and n-Si/Pb Schottky junctions have been prepared by electrodeposition of Cd or Pb from acidic aqueous solutions onto H-terminated and CH3-terminated n-type Si(111) surfaces. For both nondegenerately (n-) and degenerately (n+-) doped H–Si(111) electrodes, Cd and Pb were readily electroplated and oxidatively stripped, consistent with a small barrier height (Phib) at the Si/solution and the Si/metal junctions. Electrodeposition of Cd or Pb onto degenerately doped CH3-terminated n+-Si(111) electrodes occurred at the same potentials as Cd or Pd electrodeposition onto H-terminated n+-Si(111). However, electrodeposition on nondegenerately doped CH3-terminated n-Si(111) surfaces was significantly shifted to more negative applied potentials (by −130 and −347 mV, respectively), and the anodic stripping of the electrodeposited metals was severely attenuated, indicating large values of Phib for contacts on nondegenerately doped n-type CH3–Si(111) surfaces. With either Cd or Pb, current–voltage measurements on the dry, electrodeposited Schottky junctions indicated that much larger values of Phib were obtained on CH3-terminated n-Si(111) surfaces than on H-terminated n-Si(111) surfaces. Chronoamperometric data indicated that CH3–Si(111) surfaces possessed an order-of-magnitude lower density of nucleation sites for metal electrodeposition than did H–Si(111) surfaces, attesting to the high degree of structural passivation afforded by the CH3–Si surface modification.

Silicon and tungsten oxide nanostructures for water splitting

Abstract: Inorganic semiconductors are promising materials for driving photoelectrochemical water-splitting reactions. However, there is not a single semiconductor material that can sustain the unassisted splitting of water into H 2 and O 2 . Instead, we are developing a three part cell design where individual catalysts for water reduction and oxidation will be attached to the ends of a membrane. The job of splitting water is therefore divided into separate reduction and oxidation reactions, and each catalyst can be optimized independently for a single reaction. Silicon might be suitable to drive the water reduction. Inexpensive highly ordered Si wire arrays were grown on a single crystal wafer and transferred into a transparent, flexible polymer matrix. In this array, light would be absorbed along the longer axial dimension while the resulting electrons or holes would be collected along the much shorter radial dimension in a massively parallel array resembling carpet fibers on a microscale, hence the term "solar carpet". Tungsten oxide is a good candidate to drive the water oxidation. Self-organized porous tungsten oxide was successfully synthesized on the tungsten foil by anodization. This sponge-like structure absorbs light efficiently due to its high surface area; hence we called it "solar sponge".

High-throughput screening and device for photocatalysts

Abstract: The disclosure relates to compositions, devices and methods for screening of photocatalysts for water-splitting.