Bjorn Marsen

Growth of silicon nanostructures on graphite

Abstract Silicon nanostructures such as small clusters, superclusters, and elongated chains, with an average diameter of a few nanometers, have been synthesized by magnetron sputtering on cleaved highly oriented pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM) reveals that flat, defect-poor areas of the HOPG surface are covered with almost uniformly sized silicon clusters of 0.6±0.2xa0nm, 5.1±1.2xa0nm, and 15.4±3xa0nm diameter. Surface regions with defects such as pits and craters, descending a few layers into the graphite surface, are sparsely covered with silicon. Most of the deposited material, with an average diameter of 2xa0nm, is found to be attached to the monatomic step edges forming the crater rims. A simulation of the growth process, i.e. deposition of silicon atoms onto a surface with built-in defects, and subsequent surface diffusion and aggregation of the adatoms, convincingly reproduces most of the Si nanostructures observed in the STM topographs.

Use of amorphous silicon tandem junction solar cells for hydrogen production in a photoelectrochemical cell

We report the use of amorphous silicon (a-Si) tandem junctions as part of an integral hybrid photoelectrochemical (PEC) cell to produce hydrogen directly from water using sunlight. The device configuration consists of stainless steel (SS)/ni2pni1p/ZnO/WO3. When the device is immersed in an electrolyte and illuminated, O2 is evolved at the WO3/electrolyte interface and H2 is produced at the counter electrode. A voltage >1.23V is required to split water; typically 1.6-1.8V are needed, taking account of losses in a practical water-splitting system. We use a-Si tandem cells, deposited by plasma-enhanced chemical vapor deposition, to supply this voltage. Current matching in the two a-Si subcells is achieved by altering the thicknesses of the two layers (i1 and i2) while keeping their band gaps at ~1.75eV, which results in a device with an open circuit voltage >1.6V, short circuit current density (Jsc) >6mA/cm2 (on SS substrates), and a fill factor >0.6. Deposition on a textured SnO2 coated glass has resulted in Jsc >9mA/cm2. Photoactive WO3 films, deposited using the RF sputtering technique, have achieved photocurrents >3mA/cm2 at 1.6V vs. saturated calomel electrode (SCE). The PEC device operates at the point at which the WO3 photocurrent IV curve and the a-Si (filtered by WO3) light IV curve cross, leading to operating currents of 2.5mA/cm2 and solar-to-hydrogen (STH) conversion efficiency of >3%.

a-SiC:H films used as photoelectrodes in a hybrid, thin-film silicon photoelectrochemical (PEC) cell for progress toward 10% solar-to hydrogen efficiency

In this paper we describe the fabrication of amorphous SiC:H materials and using them as photoelectrodes in photoelectrochemical cells (PEC). With the increase of CH4 flow (in SiH4 gas mixture) during growth, the bandgap, Eg, increases from ~ 1.8eV to ~2.0eV, while the photoconductivity decreases from ~10-5 S/cm to ~10-8 S/cm. These high-quality a-SiC:H materials with Eg of 2.0eV included into a solar cell configuration led to a conversion efficiency,η~7% on textured Asahi U type SnO2 coated substrates, with the i-layer thickness of ~300nm. For a reduced i-layer thickness of ~100 nm, a current density, Jsc ~8.45mA/cm2 has been achieved, Immersing the a-SiC:H(p)/a-SiC:H(i) structure in 0.33M H3PO4 electrolytes, produced a photocurrent of ~7mA/cm2. With a further optimization we expect that the photocurrent could exceed 9mA/cm2. With the use of this configuration substrate/silicon tandem device (a-Si/a-Si or a- Si/nc-Si)/a-SiC:H(p)/a-SiC:H(i), it may therefore be possible to increase the solar-to-hydrogen (STH) efficiencies to beyond 10%.

Development of a corrosion-resistant amorphous silicon carbide photoelectrode for solar-to-hydrogen photovoltaic/photoelectrochemical devices

Photoelectrochemical (PEC) water splitting at a semiconductor-electrolyte interface using sunlight is of considerable interest as it offers a clean approach to hydrogen production. PEC cells require semiconductor photoelectrode materials fulfilling a number of important requirements, such as band-edge alignment, corrosion resistance to electrolyte, and adequate current generation. We report the development of RF-PECVD-deposited hydrogenated amorphous silicon carbide (a-SiC:H) photoelectrodes with improved durability, which, when combined with a-Si:H tandem photovoltaic devices, should produce hydrogen directly from water under sunlight. The a-SiC:H is commonly grown with a bandgap in excess of 2.0 eV and completes the PEC device by providing contact with the electrolyte, proper band-edge alignment, and acts as a buffer for the a-Si:H tandem structure. Effects of the pH of electrolyte, type of substrates, and a platinum nanoparticle coating on the durability of a-SiC photoelectrodes will be presented. From these studies we surmise that corrosion or damage mechanism occurring on a-SiC:H layer could be divided into different aspects of physical and chemical. From the physical point of view, defects associated with spikes in textured TCO substrates, roughness of stainless steel, or other sources of pinholes may initiate delamination as confirmed by SEM (Scanning Electron Microscopy) and EDS (Energy-Dispersive X-ray Spectroscopy) studies. Chemically, the production of hydrogen involves reactions that may etch the electrode, especially when physical defects are involved. We observe that reducing the acidity of the electrolyte (increasing the pH from 0 to 2) significantly reduces corrosion while the useful photocurrent output of the a-SiC:H p/i structure is unaffected.

Copper gallium diselenide photocathodes for solar photoelectrolysis

Copper chalcopyrite films exhibit properties suitable for solar energy conversion processes such as direct bandgap, and excellent carrier transport. To explore the possibilities of solar-powered hydrogen production by photoelectrolysis using these materials, we have synthesized p-type polycrystalline CuGaSe2 films by vacuum co-evaporation of the elemental constituents, and performed physical and electrochemical characterizations of the resulting films and electrodes. Based on CuGaSe2 material with 1.65 eV bandgap, a 2.2 micron thick electrode exhibited an outdoor 1-sun photocurrent of 16 mA/cm2, while a 0.9 micron thin device still produced 12.6 mA/cm2 in conjunction with vigorous gas evolution. Flatband potential measurements and bias voltage requirements for saturation photocurrents indicate a valence band position to high for practical device implementation. Future photoelectrolysis devices may be based on copper chalcopyrites with lower valence band maximum in conjunction with a suitable auxiliary junction.

Coulomb blockade effects in charged Si7 clusters on a graphite substrate

Abstract The present work deals with the interpretation of recent high-resolution scanning tunneling microscopy (STM) measurements in which Si7 clusters were assembled through quasi-free growth on a clean highly oriented pyrolytic graphite (HOPG) surface. It was found that at low bias, some clusters exhibited the Coulomb blockade phenomenon, acquiring a negative charge that blocks the tunneling current from the microscope tip to the cluster. However, upon a switch of polarity, conceivably neutralizing the charged system, the clusters proved to be detectable within a wide range of positive and negative values of the STM bias. We attempt to understand this effect in terms of an electronic structure change of the Si7 unit associated with a transition from a singly charged Si7 anion in a spin quartet state to a neutral Si7 cluster in a spin triplet state, performing density functional computations for a Si7C54H18 cluster which simulates the combined system of the Si7 unit and the graphite layer.

Silicon Nanostructures Grown by Vapor Deposition on HOPG

Silicon nanostructures such as small clusters, superclusters, elongated chains, and tube- or rod-like structures with an average diameter of a few nanometers, have been synthesized by magnetron sputtering on cleaved highly oriented pyrolytic graphite (HOPG). Scanning tunneling microscopy (STM) exhibits that flat, defect-poor areas of the HOPG surface are covered with almost uniformly sized spherical structures of 0.6 ± 0.2 nm, 5.1 ± 1.2 nm, and 15.4 ± 3 nm diameter. Surface regions with defects such as foldings, pits and craters descending a few layers into the graphite surface, are sparsely covered with silicon. In such defect rich regions most of the deposited material is found to be attached to the monatomic step edges forming the crater rims. The average diameter of the silicon nanoparticles that are attached to these steps is 2nm ± 0.5 nm. A simulation of the growth process, i.e., deposition of silicon atoms onto a surface with built-in defects, and subsequent surface diffusion and aggregation of the adatoms, reproduces convincingly most of the Si nanostructures observed in the STM topographs.