Solution-Deposited F:SnO₂/TiO₂ as a Base-Stable Protective Layer and Antireflective Coating for Microtextured Buried-Junction H₂‑evolving Si Photocathodes

Protecting Si photocathodes from corrosion is important for developing tandem water-splitting devices operating in basic media. We show that textured commercial Si-pn⁺ photovoltaics protected by solution-processed semiconducting/conducting oxides (plausibly suitable for scalable manufacturing) and coupled to thin layers of Ir yield high-performance H₂-evolving photocathodes in base. They also serve as excellent test structures to understand corrosion mechanisms and optimize interfacial electrical contacts between various functional layers. Solution-deposited TiO₂ protects Si-pn⁺ junctions from corrosion for ∼24 h in base, whereas junctions protected by F:SnO₂ fail after only 1 h of electrochemical cycling. Interface layers consisting of Ti metal and/or the highly doped F:SnO₂ between the Si and TiO₂ reduce Si-emitter/oxide/catalyst contact resistance and thus increase fill factor and efficiency. Controlling the oxide thickness led to record photocurrents near 35 mA cm^–2 at 0 V vs RHE and photocathode efficiencies up to 10.9% in the best cells. Degradation, however, was not completely suppressed. We demonstrate that performance degrades by two mechanisms, (1) deposition of impurities onto the thin catalyst layers, even from high-purity base, and (2) catastrophic failure via pinholes in the oxide layers after several days of operation. These results provide insight into the design of hydrogen-evolving photoelectrodes in basic conditions, and highlight challenges

Cobalt–Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism

Cobalt oxides and (oxy)­hydroxides have been widely studied as electrocatalysts for the oxygen evolution reaction (OER). For related Ni-based materials, the addition of Fe dramatically enhances OER activity. The role of Fe in Co-based materials is not well-documented. We show that the intrinsic OER activity of Co_1–<i>x</i>Fe_<i>x</i>(OOH) is ∼100-fold higher for <i>x</i> ≈ 0.6–0.7 than for <i>x</i> = 0 on a per-metal turnover frequency basis. Fe-free CoOOH absorbs Fe from electrolyte impurities if the electrolyte is not rigorously purified. Fe incorporation and increased activity correlate with an anodic shift in the nominally Co^2+/3+ redox wave, indicating strong electronic interactions between the two elements and likely substitutional doping of Fe for Co. <i>In situ</i> electrical measurements show that Co_1–<i>x</i>Fe_<i>x</i>(OOH) is conductive under OER conditions (∼0.7–4 mS cm^–1 at ∼300 mV overpotential), but that FeOOH is an insulator with measurable conductivity (2.2 × 10^–2 mS cm^–1) only at high overpotentials >400 mV. The apparent OER activity of FeOOH is thus limited by low conductivity. Microbalance measurements show that films with <i>x</i> ≥ 0.54 (i.e., Fe-rich) dissolve in 1 M KOH electrolyte under OER conditions. For <i>x</i> < 0.54, the films appear chemically stable, but the OER activity decreases by 16–62% over 2 h, likely due to conversion into denser, oxide-like phases. We thus hypothesize that Fe is the most-active site in the catalyst, while CoOOH primarily provides a conductive, high-surface area, chemically stabilizing host. These results are important as Fe-containing Co- and Ni-(oxy)­hydroxides are the fastest OER catalysts known

Selective Area Epitaxy of GaAs Microstructures by Close-Spaced Vapor Transport for Solar Energy Conversion Applications

Close-spaced vapor transport is a plausibly low-cost, high-rate method to grow III–V materials for photovoltaic and photoelectrochemical device applications. We report the first homoepitaxial growth of GaAs microstructures on (100)- and (111)­B-oriented GaAs substrates using patterned SiO_<i>x</i> and Al₂O₃ masks and show that the resulting microstructured GaAs is an efficient semiconductor absorber for photovoltaic and photoelectrochemical applications. Cross-sectional transmission electron microscopy reveals an unusually low density of twin-plane defects in the (111)-oriented microstructures and the occurrence of stacked twin-plane defects in the (100)-oriented microstructures. Nonaqueous photoelectrochemical measurements show similar short-circuit currents of 9.7 and 9.1 mA cm^–2 for (100)- and (111)-oriented microstructures, respectively, with promising external quantum efficiencies. Together, the low twin density and good electronic properties indicate that micro- or nanostructures grown by selective area epitaxy in close-spaced vapor transport are promising for device applications that take advantage of their three-dimensional structure

High‑κ Lanthanum Zirconium Oxide Thin Film Dielectrics from Aqueous Solution Precursors

Metal oxide thin films are critical components in modern electronic applications. In particular, high-κ dielectrics are of interest for reducing power consumption in metal–insulator–semiconductor (MIS) field-effect transistors. Although thin-film materials are typically produced via vacuum-based methods, solution deposition offers a scalable and cost-efficient alternative. We report an all-inorganic aqueous solution route to amorphous lanthanum zirconium oxide (La₂Zr₂O₇, LZO) dielectric thin films. LZO films were spin-cast from aqueous solutions of metal nitrates and annealed at temperatures between 300 and 600 °C to produce dense, defect-free, and smooth films with subnanometer roughness. Dielectric constants of 12.2–16.4 and loss tangents <0.6% were obtained for MIS devices utilizing LZO as the dielectric layer (1 kHz). Leakage currents <10^–7 A cm^–2 at 4 MV cm^–1 were measured for samples annealed at 600 °C. The excellent surface morphology, high dielectric constants, and low leakage current densities makes these LZO dielectrics promising candidates for thin-film transistor devices

Amorphous In–Ga–Zn Oxide Semiconducting Thin Films with High Mobility from Electrochemically Generated Aqueous Nanocluster Inks

Solution processing is a scalable means of depositing large-area electronics for applications in displays, sensors, smart windows, and photovoltaics. However, solution routes typically yield films with electronic quality inferior to traditional vacuum deposition, as the solution precursors contain excess organic ligands, counterions, and/or solvent that leads to porosity in the final film. We show that electrolysis of aq. mixed metal nitrate salt solutions drives the formation of indium gallium zinc oxide (IGZO) precursor solutions, without purification, that consist of ∼1 nm radii metal–hydroxo clusters, minimal nitrate counterions, and no organic ligands. Films deposited from cluster precursors over a wide range of composition are smooth (roughness of 0.24 nm), homogeneous, dense (80% of crystalline phase), and crack-free. The transistor performance of IGZO films deposited from electrochemically synthesized clusters is compared to those from the starting nitrate salt solution, sol–gel precursors, and, as a control, vacuum-sputter-deposited films. The average channel mobility (μ_<i>AVE</i>) of air-annealed cluster films (In:Ga:Zn = 69:12:19) at 400 °C was ∼9 cm² V^–1 s^–1, whereas those of control nitrate salt and sol–gel precursor films were ∼5 and ∼2 cm² V^–1 s^–1, respectively. By incorporating an ultrathin indium–tin–zinc oxide interface layer prior to IGZO film deposition and air-annealing at 550 °C, a μ_<i>AVE</i> of ∼30 cm² V^–1 s^–1 was achieved, exceeding that of sputtered IGZO control films. These data show that electrochemically derived cluster precursors yield films that are structurally and electrically superior to those deposited from metal nitrate salt and related organic sol–gel precursor solutions and approach the quality of sputtered films

Role of Combustion Chemistry in Low-Temperature Deposition of Metal Oxide Thin Films from Solution

Metal-oxide thin films find many uses in (opto)­electronic and renewable energy technologies. Their deposition by solution methods aims to reduce manufacturing costs relative to vacuum deposition while achieving comparable electronic properties. Solution deposition on temperature-sensitive substrates (e.g., plastics), however, remains difficult due to the need to produce dense films with minimal thermal input. Here, we investigate combustion thin-film deposition, which has been proposed to produce high-quality metal-oxide films with little externally applied heat, thereby enabling low-temperature fabrication. We compare chemical composition, chemical structure, and evolved species from reactions of several metal nitrate [In­(NO₃)₃, Y­(NO₃)₃, and Mg­(NO₃)₂] and fuel additive (acetylacetone and glycine) mixtures in bulk and thin-film forms. We observe combustion in bulk materials but not in films. It appears acetylacetone is removed from the films before the nitrates, whereas glycine persists in the film beyond the annealing temperatures required for ignition in the bulk system. From analysis of X-ray photoelectron spectra, the oxide and nitrate content as a function of temperature are also inconsistent with combustion reactions occurring in the films. In­(NO₃)₃ decomposes alone at low temperature (∼200–250 °C) without fuel, and Y­(NO₃)₃ and Mg­(NO₃)₂ do not decompose fully until high temperature even in the presence of fuel when used to make thin films. This study therefore distinguishes bulk and thin-film reactivity for several model oxidizer-fuel systems, and we propose ways in which fuel additives may alter the film formation reaction pathway