Biochemical networks use reaction cascades to selectively reduce CO2 using energy from sunlight, but can similar selectivity be achieved by applying a cascade approach to an engineered system? Here, we report the design and implementation of a two-step photoelectrochemical (PEC) cascade to a liquid solar fuel: reduction of CO2 to CO and subsequent reduction of CO to methanol. The potentials required to perform the reductions were generated using custom-made III-V-based three-terminal tandem (3TT) solar cells. Cobalt phthalocyanine immobilized on multi-walled carbon nanotubes (CoPc/MWCNT) catalyzed both reactions. Multiphysics simulations of electrolyte flow and non-illuminated electrochemical measurements were used to narrow the operating parameters for the CoPc/MWCNT 3TT photocathodes. The champion integrated photocathode produced methanol with 3.8 ± 0.4% Faradaic efficiency (FE), with tested photocathodes having 0.7-3.8% methanol FE. Products were quantified by nuclear magnetic resonance spectroscopy and gas chromatography. The current output of the tested photocathodes was highly stable, and methanol production continued over multiple experiments. The low methanol yield is attributed to insufficient CO flux to, and CO2 depletion at, the methanol-producing subcell when both contacts are active, which is supported by the observation that a control photoelectrode slightly outperformed the methanol production of the 3TT device. Methanol production ceased when the 3TT subcell driving CO reduction was deactivated, supporting the assignment of a cascade mechanism. The major factors resulting in low methanol FE by the CoPc/MWCNT 3TT photocathodes are insufficient CO2 depletion at the methanol-producing contact and uncertainty in operating potential selection using the 3TT design. Although the CoPc/MWCNT 3TT photocathode is not yet highly selective, this work develops the basic science principles underlying the PEC cascade, demonstrates the co-design of a 3TT-based photoelectrode to produce carbon-based fuels, and finally discusses routes for improving product yields with this concept, including CO2 supply optimization and alternative photoelectrode and catalyst materials.

Combining two or more junctions into a tandem solar cell promises to deliver a leap in power conversion efficiency that will help to sustain continued growth in installed photovoltaic (PV) capacity. Although tandems are now on the roadmaps of many PV manufacturers, much work remains before they are ready for mass deployment. Accelerating their development requires advances on many fronts. In this article, we outline the fundamentals and status of tandem PV, considering multiple PV technology pairings and architectures. We then present the challenges that must be overcome and a general timeline of activities that are required to translate tandems to commercial products. Our intent is to spur researchers and manufacturers to work together to address important aspects of tandem design, reliability, and scaling to enable more rapid progress toward mass production.

The Front Cover illustrates a TCE sheet, where the rainbow is the light reaching the photoelectrode, the spheres are the conductive pathway through the polymer matrix to the electrochemical interface, and the methyl viologen redox couple is reduced in the solution. Cover design by Talysa Klein (www.tk2.design). More information can be found in the Research Article by G. A. Rome et al.

Carbon-neutral electricity is rapidly becoming available worldwide as solar and wind technologies advance, but storing energy in chemical bonds will remain a critical need for the transportation sector, as planes and other energy intensive processes will still require liquid fuels. Solar fuels, utilizing sunlight to directly convert CO 2 into useful chemicals, are a renewable and net carbon-neutral way to produce needed liquid fuels. However, a common problem with photoelectrochemical solar fuel production is semiconductor degradation from submersion in aqueous environments. An ideal protective layer should 1) prevent solution from reaching the semiconductor, 2) maintain charge transfer to and from the solution, and 3) be transparent to light above the semiconductor band gap. While there presently are protective layer options that meet all three requirements, such as leaky TiO 2 and MoS 2 , they are not easily adaptable to new semiconductor surfaces and/or to new electrochemical reactions. This can make protection difficult for newly developed photoabsorbers and catalytic reaction pairings. In this work, we demonstrate the use of transparent conductive encapsulants (TCEs) to meet these three requirements while also allowing for photoelectrode- and catalysis-agnostic adaptability. TCEs are composed of an ethyl vinyl acetate (EVA) matrix with embedded conductive metal-coated poly(methyl methacrylate) (PMMA) microspheres that can be attached to substrates through a lamination process. First, we characterize the electrochemical behavior of TCE-coated electrodes using the reduction of methyl viologen, demonstrating electrical conduction through the TCE layer. Results from a pinhole detection apparatus suggest the TCE is initially defect free and thus able to prevent solution from reaching the substrate. Then, we perform photoelectrochemical measurements of TCE-covered semiconductors to demonstrate the flexibility of this protection scheme for multiple materials. We also show the results of long-term photoelectrochemical measurements designed to probe the efficacy of TCEs as protective layers. These findings demonstrate that TCEs are an effective protective layer for a variety of photoelectrochemical applications.

Utilizing sunlight to directly perform photoelectrochemical reactions is a promising route to renewable, net carbon-neutral fuels. However, a common problem with solar fuel production is semiconductor degradation in aqueous environments. An ideal protection layer should (1) prevent solution from reaching the semiconductor, (2) maintain charge transfer to and from solution, and (3) be transparent to light above the semiconductor band gap. While there have been substantial advances toward layers that meet these requirements, they are not easily adapted to new surfaces or new reactions, which can make protection difficult for newly developed photoabsorbers and (photo)electrochemical reaction pairings. In this work, we demonstrate the use of transparent conductive encapsulants (TCEs) to meet these requirements while also allowing for photoelectrode- and reaction-agnostic adaptability. TCEs are composed of an ethyl-vinyl acetate matrix with embedded conductive metal-coated microspheres that can be laminated to semiconductors. First, the electrochemical behavior of TCE-coated electrodes for the reduction of methyl viologen is characterized, demonstrating through-TCE electrical conduction. Then, photoelectrochemical measurements on TCE-protected semiconductors demonstrate the flexibility of this protection scheme. Finally, long-term photoelectrochemical measurements probe the efficacy of TCEs as protection layers. These findings demonstrate the potential of TCEs as adaptable protection layers in various photoelectrochemical applications.

Magnetic semiconductors may soon improve the energy efficiency of computers, but materials exhibiting these dual properties remain underexplored. Here, we report the computational prediction and realization of a new magnetic and semiconducting material, MnSnN$_2$, via combinatorial sputtering of thin films. Grazing incidence wide angle X-ray scattering and laboratory X-ray diffraction studies show a wide composition tolerance for this wurtzite-like MnSnN$_2$, ranging from $20\% <$ Mn/(Mn+Sn) $< 65$\% with cation disorder across this composition space. Magnetic susceptibility measurements reveal a low-temperature transition ($T^{\mathrm{*}} \approx 10$ K) for MnSnN$_2$ and strong antiferromagnetic correlations, although the ordering below this transition may be complex. This finding contrasts with bulk MnSiN$_2$ and MnGeN$_2$, which exhibited antiferromagnetic ordering above 400 K in previous studies. Spectroscopic ellipsometry identifies an optical absorption onset of 1 eV for the experimentally-synthesized phase exhibiting cation disorder, consistent with the computationally-predicted 1.2 eV bandgap for the cation-ordered structure. Electronic conductivity measurements confirm the semiconducting nature of this new phase by showing increasing conductivity with increasing temperature. This work adds to the set of known semiconductors that are paramagnetic at room temperature and will help guide future work targeted at controlling the structure and properties of semiconducting materials that exhibit magnetic behavior.

GaN/ZnGeN 2 /GaN double heterojunctions were grown by molecular beam epitaxy; we demonstrate coherent interfaces between ZnGeN 2 and GaN and highlight defects and associated properties of interest with respect to optoelectronic applications.

Using the reciprocal space, two types of structures are simple to identify: simple periodic (photonic crystals), which have high diffractive efficiencies but sparse resonances (narrow-band), and random structures, with a continuous reciprocal space (broadband) but suffering from low diffraction efficiencies. A third type, quasirandom structures, lies in between; these provide high diffractive efficiency over a target wavelength range, which is broader than simple photonic crystals but narrower than a random structure. These structures are promising for ultrathin solar cells due to their broader nature. We present our numerical work towards evolving simple photonic crystals in quasirandom structures, and our fabrication approach based on polymer-blend lithography, with initial results on solar cells.

Photoelectrochemical fuel generation is a promising route to sustainable liquid fuels produced from water and captured carbon dioxide with sunlight as the energy input. Development of such technologies requires photoelectrode materials that are both photocatalytically active and operationally stable in harsh oxidative and/or reductive electrochemical environments. Such photocatalysts can be discovered based on co-design principles, wherein design for stability is based on the propensity for the photocatalyst to self-passivate under operating conditions and design for photoactivity is based on the ability to integrate the photocatalyst with established semiconductor substrates. Here we report on synthesis and characterization of zinc titanium nitride (ZnTiN2) that follows these design rules by having a wurtzite-derived crystal structure and showing self-passivating surface oxides created by electrochemical polarization. The sputtered ZnTiN2 thin films have optical absorption onsets below 2 eV and n-type electrical conduction of 0.1 S/cm. The band gap of this material is reduced from the 3.5 eV theoretical value by cation site disorder, and the impact of cation antisites on the band structure of ZnTiN2 is explored using density functional theory. Under electrochemical polarization, the ZnTiN2 surfaces have TiO2- or ZnO-like character, consistent with Materials Project Pourbaix calculations predicting the formation of stable solid phases under near-neutral pH. These results show that ZnTiN2 is a promising candidate for photoelectrochemical liquid fuel generation and demonstrate a new materials design approach to other photoelectrodes with self-passivating native operational surface chemistry.

New optoelectronic materials are needed for improving the efficiency and reliability of devices such as solar cells. Cation ordering presents one means of controlling optoelectronic properties while introducing potential to also diversify the mineral constituents of electronic devices; however, the mechanisms of ordering are not yet well understood. To better understand cation ordering in a system integratable with current devices, we assess short- and long-range order parameters of $$\hbox {ZnGeP}_{2}$$ , a material closely lattice matched to Si. Structures are simulated using cluster-based Monte Carlo and first-principles calculations to compare structural distortions, periodicity, and local coordination environments in $$\hbox {ZnGeP}_{2}$$ to experimental data both from the literature and presented here. Comparing order parameters, we relate the transition in order parameters of $$\hbox {ZnGeP}_{2}$$ to that of $$\hbox {ZnGeP}_{2}$$ , discuss the reduction of band gaps with disorder, and show that traditional structural characterization alone is insufficient for understanding order in $$\hbox {ZnGeP}_{2}$$ .

ZnSnP 2 , an emerging inorganic material for solar cells, was characterized by deep level transient spectroscopy (DLTS) and photoluminescence (PL). Acceptor- and donor-like traps with shallow energy levels were detected by DLTS analysis. The previous study based on first-principle calculation also suggested such traps were due to antisite defects of Zn and Sn. PL measurements also revealed sub-gap transitions related to these trap levels. Additionally, DLTS found a trap with a deep level in ZnSnP 2 . A short lifetime of minority carrier in previous work might be due to such trap, coming from phosphorus vacancies and/or zinc interstitials suggested by the first-principle study.

Conventional solar cells, including multijunction solar cells, are based on pn junctions as building blocks. In contrast, the three‐terminal heterojunction bipolar transistor solar cell (3T‐HBTSC) explores the use of a bipolar transistor structure to build a solar cell. The limiting efficiency of this transistor structure equals that of a double‐junction solar cell. However, since the 3T‐HBTSC does not require tunnel junctions, its minimal structure has only three semiconductor layers, while the minimal structure of a double junction solar cell has six. This work reviews the operation principles of this solar cell and the steps carried out towards its practical implementation. Experimental results on a GaInP/GaAs HBTSC prototype with bottom interdigitated contacts are presented.

The pursuit of ever-higher solar cell efficiencies has focused heavily on multijunction technologies. In tandem cells, subcells are typically either contacted via two terminals (2T) or four terminals (4T). Simulations show that the less-common three-terminal (3T) design may be comparable to 4T tandem cells in its compatibility with a range of materials, operating conditions, and methods for subcell integration, yet the 3T design circumvents shading losses of the 4T intermediate conductive layers. This study analyzes the performance of two superstrate 3T III-V-on-Si (III-V//Si) tandem cells: One has slightly greater current contribution from the Si bottom cell (GaInP//Si) and the other has substantially greater current contribution from the GaAs top cell (GaAs//Si). Our results show that both tandem cells exhibit the same efficiency (21.3%), thereby demonstrating that the third terminal allows for flexibility in the selection of the top cell material, similar to the 4T design.

Ultrathin solar cells reduce material usage and allow the use of lower-quality materials thanks to their one order of magnitude smaller thickness than their conventional counterparts. However, efficient photonic light-trapping is required to harvest the incident light efficiently for an otherwise insufficient absorber thickness. Quasi-random photonic crystals are predicted to have high efficient light-trapping while being more robust under angle and thickness variations than simple photonic crystals. Here we experimentally demonstrate a light-trapping solution based on quasi-random photonic crystals fabricated by polymer blend lithography. We control the average lattice parameter by modifying the spin-coating speed. We demonstrate an ultrathin GaAs cell of 260 nm with a rear quasi-random pattern with submicron features, and a Jsc = 26.4 mA/cm2 and an efficiency of 22.35% under the global solar spectrum.

Growing energy storage demands on lithium-ion batteries necessitate exploration of new electrochemical materials as next-generation battery electrode materials. In this work, we investigate the previously unexplored electrochemical properties of earth-abundant and tunable Zn1−xSn1+xN2 (x = −0.4 to x = 0.4) thin films, which show high electrical conductivity and high gravimetric capacity for Li insertion. Enhanced cycling performance is achieved compared to previously published end-members Zn3N2 and Sn3N4, showing decreased irreversible loss and increased total capacity and cycle stability. The average reversible capacity observed is >1050 mAh/g for all compositions and 1220 mAh/g for Zn-poor (x = 0.2) films. Extremely Zn-rich films (x = −0.4) show improved adhesion; however, Zn-rich films undergo a phase transformation on the first cycle. Zn-poor and stoichiometric films do not exhibit significant phase transformations which often plague nitride materials and show no required overpotential at the 0.5 V plateau. Cation composition x is explored as a mechanism for tuning relevant mechanical and electrochemical properties, such as capacity, overpotential, phase transformation, electrical conductivity, and adhesion. The lithiation/delithiation experiments confirm the reversible electrochemical reactions. Without any binding additives, the as-deposited electrodes delaminate resulting in fast capacity degradation. We demonstrate the mechanical nature of this degradation through decreased electrode thinning, resulting in cells with improved cycling stability due to increased mechanical stability. Combining composition and electrochemical analysis, this work demonstrates for the first time composition dependent electrochemical properties for the ternary Zn1−xSn1+xN2 and proposes earth-abundant ternary nitride anodes for increased reversible capacity and cycling stability.

Journal Article Exploration of Chemical Ordering in Ternary Pnictides Using Electron-Channeling-Based Methods Get access Allison Mis, Allison Mis Metallurgy and Materials Department, Colorado School of Mines, Golden, CO, USA Corresponding Author: amis@mines.edu Search for other works by this author on: Oxford Academic Google Scholar Geoff L Brennecka, Geoff L Brennecka Metallurgy and Materials Department, Colorado School of Mines, Golden, CO, USA Search for other works by this author on: Oxford Academic Google Scholar Adele Tamboli, Adele Tamboli National Renewable Energy Laboratory, Golden, CO, USA Search for other works by this author on: Oxford Academic Google Scholar Nestor J Zaluzec Nestor J Zaluzec Argonne National Laboratory, Lemont, IL, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2560–2562, https://doi.org/10.1017/S143192762200976X Published: 01 August 2022

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Modern optoelectronic devices are constrained to a fixed collection of band gap and lattice parameter combinations by the limited number of semiconductors that can be epitaxially integrated with high crystal quality. II-IV-V2 compounds are promising materials to break this paradigm as changes to the cation lattice site disorder can modify the band gap without a substantial change to the lattice parameter. ZnGeP2 is a particularly interesting member of this group as it is lattice-matched to Si and GaP, but substantial work is needed to understand and improve the epitaxial growth of ZnGeP2. In this paper, we report on the growth of epitaxial ZnGeP2 on Si and GaP substrates via reactive combinatorial sputtering in phosphine gas. Reciprocal space maps revealed that films on both GaP and Si have high crystalline quality, matching that of the substrate. The out-of-plane lattice parameter was found to increase with increasing Ge content, displaying an alloy-like behavior. Films deposited on Si displayed a much larger range for the (004) peak full width at half maximum (FWHM) than those deposited on GaP. Due to the growth of a lower-symmetry material, ZnGeP2, on a higher-symmetry substrate, Si, it is likely that the films grown on Si have antiphase domains and larger threading dislocation densities than those on GaP. Electron channeling contrast imaging revealed the films on GaP to be largely dislocation-free. In the films deposited on Si, the optical absorption onset energies trended toward lower energies with larger (004) FWHM values. These results suggest that the defects in the films on Si that result in a broadened (004) FWHM cause sub-band gap absorption. This work provides the first combinatorial study of epitaxial ZnGeP2 on Si and GaP and demonstrates the strong potential for the growth of high-quality epitaxial ZnGeP2 with future work optimizing synthesis conditions and substrate preparation.