Amorphous TiO_2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation

Although semiconductors such as silicon (Si), gallium arsenide (GaAs), and gallium phosphide (GaP) have band gaps that make them efficient photoanodes for solar fuel production, these materials are unstable in aqueous media. We show that TiO_2 coatings (4 to 143 nanometers thick) grown by atomic layer deposition prevent corrosion, have electronic defects that promote hole conduction, and are sufficiently transparent to reach the light-limited performance of protected semiconductors. In conjunction with a thin layer or islands of Ni oxide electrocatalysts, Si photoanodes exhibited continuous oxidation of 1.0 molar aqueous KOH to O_2 for more than 100 hours at photocurrent densities of >30 milliamperes per square centimeter and ~100% Faradaic efficiency. TiO_2-coated GaAs and GaP photoelectrodes exhibited photovoltages of 0.81 and 0.59 V and light-limiting photocurrent densities of 14.3 and 3.4 milliamperes per square centimeter, respectively, for water oxidation

Chemical vapor deposition growth

The objective was to investigate and develop chemical vapor deposition (CVD) techniques for the growth of large areas of Si sheet on inexpensive substrate materials, with resulting sheet properties suitable for fabricating solar cells that would meet the technical goals of the Low Cost Silicon Solar Array Project. The program involved six main technical tasks: (1) modification and test of an existing vertical-chamber CVD reactor system; (2) identification and/or development of suitable inexpensive substrate materials; (3) experimental investigation of CVD process parameters using various candidate substrate materials; (4) preparation of Si sheet samples for various special studies, including solar cell fabrication; (5) evaluation of the properties of the Si sheet material produced by the CVD process; and (6) fabrication and evaluation of experimental solar cell structures, using impurity diffusion and other standard and near-standard processing techniques supplemented late in the program by the in situ CVD growth of n(+)/p/p(+) sheet structures subsequently processed into experimental cells

Chemical vapor deposition growth

A laboratory type CVD reactor system with a vertical deposition chamber and sample pedestal heated by an external RF coil has been extensively modified by installation of mass flow controllers, automatic process sequence timers, and special bellows-sealed air-operated valves for overall improved performance. Various film characterization procedures, including classical metallography, SEM analyses, X ray diffraction analyses, surface profilometry, and electrical measurements (resistivity, carrier concentration, mobility, spreading resistance profiles, and minority-carrier lifetime by the C-V-t method) area used to correlate Si sheet properties with CVD parameters and substrate properties. Evaluation procedures and measurements are given. Experimental solar cell structures were made both in epitaxial Si sheet (on sapphire substrates) and in polycrystalline material on alumina substrates, the former to provide an indication of what might be an upper limit on performance of the latter. Preliminary results are given, as obtained in cell structures not specially designed to allow for the unique properties of the sheet material, and fabricated in material known to be far from optimum for photovoltaic performance. Low power conversion efficiencies have been obtained in the epitaxial as well as the polycrystalline Si sheet

Materials Design toward High Performance Electrodes for Advanced Energy Storage Applications

Thesis advisor: Udayan MohantyRechargeable batteries, especially lithium ion batteries, have greatly transformed mobile electronic devices nowadays. Due to the ever-depletion of fossil fuel and the need to reduce CO2 emissions, the development of batteries needs to extend the success in small electronic devices to other fields such as electric vehicles and large-scale renewable energy storage. Li-ion batteries, however, even when fully developed, may not meet the requirements for future electric vehicles and grid-scale energy storage due to the inherent limitations related with intercalation chemistry. As such, alternative battery systems should be developed in order to meet these important future applications. This dissertation presents our successes in improving Li-O2 battery performance for electric vehicle application and integrating a redox flow battery into a photoelectrochemical cell for direct solar energy storage application. Li-O2 batteries have attracted much attention in recent years for electric vehicle application since it offers much higher gravimetric energy density than Li-ion ones. However, the development of this technology has been greatly hindered by the poor cycling performance. The key reason is the instability of carbon cathode under operation conditions. Our strategy is to protect the carbon cathode from reactive intermediates by a thin uniform layer grown by atomic layer depostion. The protected electrode significantly minimized parasitic reactions and enhanced cycling performance. Furthermore, the well-defined pore structures in our carbon electrode also enabled the fundamental studies of cathode reactions. Redox flow batteries (RFB), on the other hand, are well-suited for large-scale stationary energy storage in general, and for intermittent, renewable energy storage in particular. The efficient capture, storage and dispatch of renewable solar energy are major challenges to expand solar energy utilization. Solar rechargeable redox flow batteries (SRFBs) offer a highly promising solution by directly converting and storing solar energy in a RFB with the integration of a photoelectrochemical cell. One major challenge in this field is the low cell open-circuit potential, mainly due to the insufficient photovoltages of the photoelectrode systems. By combining two highly efficient photoelectrodes, Ta3N5 and Si (coated with GaN), we show that a high-voltage SRFB could be unassistedly photocharged and discharged with a high solar-to-chemical efficiency.Thesis (PhD) — Boston College, 2018.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Chemistry

First row transition metal complexes for application to dye-sensitised solar cells

Ruthenium (II) complexes are used extensively in photoelectrochemical and photophysical devices, such as Dye-Sensitized Solar Cells (DSSCs). The use of Cu(I) as a possible replacement for Ru(II) has to date had limited exploration, but has obvious advantages in terms of low cost and high abundance. However, Cu(I) typically undergoes conformational change from tetrahedral towards square planar upon oxidation or MLCT excitation, often leading to reduced stability, reduced electron transfer rates and reduced excited state lifetime, thus impairing useful function. Typically, steric constraints are used to prevent this; however these can often be synthetically intensive, involving multi-step and low yielding synthetic pathways. In this work, we explore “blocking” functionality using two different ligands combined with a range of bipyridyl ligands with varying substituent groups. The study has looked into the synthesis of heteroleptic Cu(I) complexes of the general formula: [Cu(POP)(bipyridyl)][BF4], where POP = bis[2-(diphenylphosphanyl)phenyl] ether, and [Cu(pmppE)(bipyridyl)], where pmppE = hydrazono pyrazol-5-thiones(one). The work presented in this thesis focuses on the synthesis, and subsequent photoelectrochemical and photophysical characterisation of Cu(I) complexes, yielding results that open new avenues for design of functional Cu(I) systems. Solar cell testing also revealed photovoltages comparable to those of existing Cu(I) DSSC sensitisers. An extensive spectroscopic study of [Cu(POP)(dmbpy)]+ and [Cu(POP)(tmbpy)]+ has revealed the latter to have the significantly larger quantum yield: 65 % and 4% respectively in PMMA at 300 K. A complimentary computational investigation was carried out in order to gain a better understanding of how structural rigidity affects emission properties

High efficiency silicon solar cell review

An overview is presented of the current research and development efforts to improve the performance of the silicon solar cell. The 24 papers presented reviewed experimental and analytic modeling work which emphasizes the improvment of conversion efficiency and the reduction of manufacturing costs. A summary is given of the round-table discussion, in which the near- and far-term directions of future efficiency improvements were discussed

Bismuth ferrite sensitization of nanostructured titanium dioxide and/or zinc oxide-based for photovoltaic device applications

Bismuth ferrite (BFO) is a ‘mid-range’ band gap, multiferroic (ferroelectric, antiferromagnetic) material of interest in numerous applications39,96,100-106. Though its use in photovoltaic applications has been investigated39,96,100-102,104,105 with interesting result, such a material has not yet been used as a photovoltaic sensitizer/thin absorber in sensitized solar cell (SSC) or extremely thin absorber solar cell (eta-SC) devices. A band gap (Eg~2.2-2.8eV) 40,100,103,105,117,139,147 within the visible light range (albeit high) makes BFO a potential candidate for such application. Moreover, BFO is ferroelectric (Ec ~500-600kV/cm, Pr = 60μC/cm2)100,159, which provides the material with an internal electric field (which can be directed/’poled’ towards one electrode or another in a(n) SSC/eta-device), which may provide an additional mechanism for either or both charge separation and transport.CuSCN/ZnO, CuSCN/TiO2, CuSCN/Bi-Fe-Zn-O/ZnO, CuSCN/BFO/TiO2 thin-film ‘sandwich-like’ structures were fabricated on transparent-conducting-oxide-glass (TCO) substrates, via combinations of electrodeposition and suspension or sol-gel (requiring ‘high temperature’ for crystallization) dip-coating, and characterized at various stages of production to assess material/phases present, optical absorbance characteristics, and preliminary electronic device performance. ‘High-temperature’ heat treatments in air or N2 of Bi-Fe-Zn-O/ZnO samples result in films yielding crystalline non-BFO phases, while O2 annealing of similar samples appear promising. BFO has been successfully crystallized on nearly-pure anatase TiO2 synthesized/deposited two ways, as well as on F:SnO2-glass. Moreover, BFO is found to enhance absorbency in at least a portion the visible portion of the electromagnetic spectrum. Such are promising signs that thin-absorber-PV devices based on either TiO2 or ZnO may be viable for development in the near future.M.S., Materials Science and Engineering -- Drexel University, 201

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Handbook of bibliographic and subject information processing, storage, and retrieval at NASA SCIENTIFIC and Technical Information Facilit

New Advances in Semiconductors

New Advances in Semiconductors brings together contributions from important researchers around the world on semiconductor materials and their applications. It includes seven chapters in two sections: “Calculations and Simulations in Semiconductors” and “Semiconductor Materials.” The world will emerge different after the social and economic reorganizations caused by the COVID-19 pandemic and will be even more dependent on semiconductors than ever before. New Advances in Semiconductors is a book that brings together the contributions of important researchers around the world and is able to give an idea about the different characteristics of semiconductor materials and their applications. There is a section dedicated to theory, calculations and logic and another dedicated to the development and characterization of semiconductor materials of great future interest. I really hope that this book will help to spread knowledge about this research field to other researchers and students working in this area or even to those interested in starting their more advanced studies