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The key to improve the conversion efficiency of perovskite solar cells lies in the identification and control of different limiting factors. Both intrinsic and extrinsic losses are shown here to be detrimental on conversion efficiency well below the thermodynamic limit. The effect of varying radiative and Auger recombination processes as inevitable intrinsic losses on device performance is shown in this work. The extrinsic losses are shown to impose severe bounds on efficiency limits. Such extrinsic losses include realistic material optical properties, finite diffusion length, ideality factor, parasitic resistance, and parasite absorption. Thus, this work presents the roadmap and the possible approaches in achieving performance beyond what is currently demonstrated in the highest efficiency perovskite solar cells. Additionally, the impact of light concentration, important in Auger limited devices is investigated. Finally, the impact of Auger recombination for perovskite with finite diffusion length in a two-terminal perovskite/silicon tandem device is investigated.
Silicon emerged as an important substrate material for photonics because of its transparency in the near infrared and its superior planar waveguide properties. Active optoelectronic devices in the infrared wave length regime need semiconductor heterostructures with smaller band gaps as silicon, preferably from the group IV material system. This paper describes fundamental properties of lattice mismatched group IV heterostructures on silicon and their synthesis with epitaxy methods. Special emphasis is given to the aspects of strain management in lattice mismatched device structures and to the realization of metastable non-equilibrium materials. Well-defined strain status is obtained by growth on virtual substrates which consist of silicon substrates with strain relaxed silicon germanium buffer layers. Epitaxy methods at low growth temperatures pushed the synthesis of germanium tin alloys with tin concentrations more than ten times the equilibrium value of about 1%. These achievements pave the way for silicon photonics to efficient light emission and mid infrared operation.
Recent advancement of inorganic dissolvable electronics nucleates around a realization that single-crystal silicon nanomembrane undergoes hydrolysis in biologically relevant conditions. The silicon-based high-performance dissolvable electronic devices are initially conceived for biomedical implants that function for a programmed timeframe followed by a complete dissolution to eliminate the need for recollection. The technology developed for biomedicine also presents unique opportunities in security devices that physically destruct and in environmentally benign electronics that dissolve without a trace to reduce electronic wastes. The new class of devices with this emerging technology complements the existing efforts in organic biodegradable devices. Compatible with state-of-the-art fabrication facilities for commercial microelectronics, the technology has a huge potential for future commercialization. This mini review will first discuss the relevant materials for the inorganic dissolvable electronics and then present the demonstrated applications in functional devices, followed by a perspective for the future developments.
A facile method that allows chemical functionalization of graphene sheets is described. These result in a solution processable graphene-based material, namely F-graphene, which can be integrated in organic photoelectronic devices, due to its unique structural and photophysical properties. The resultant poly(3-hexylthiophene)(P3HT):F-graphene are soluble in common organic solvents, facilitating the structure/property characterization and the device fabrication by solution processing. The synthesized F-graphene is blended with the conjugated polymer in optimized concentration. The high and sensitive photoresponse of P3HT:F-graphene was demonstrated by the photodetector. A heterojunction photovoltaic device based on the solution-cast P3HT:F-graphene (with a BHJ structure of ITO/ZnO/P3HT:F-graphene/MoO3/Ag) showed a power conversion efficiency of 1.9% under AM1.5 illumination (100 mW/cm2). It provides a new method for graphene application in organic photoelectronics. It can easily enhance the performance of devices by optimizing the structure and bulk heterojunction blend in the near future.
Mechanical reliability is a critical issue in all forms of energy conversion, storage, and harvesting. In Li-ion batteries, mechanical degradation caused by the repetitive swelling and shrinking of electrodes upon lithiation cycles is now well recognized; however, the impact of mechanical stresses on Li transport and hence the capacity of batteries is less obvious and underestimated. In particular, the stress field within the heterogeneous electrodes is complex, making the characterization of the chemomechanical behaviors of electrodes a challenging task. We develop a finite element program that computes the coupled Li diffusion and stresses in three-dimensional composite electrodes. We employ the reconstructed models of both cathode and anode materials to investigate the mechanical interactions of the constituents and their influence on the accessible capacity. The state of charge in the percolated particles is highly inhomogeneous regulated by the stress field. An ample space of design is open for the optimization of the capacity and mechanical performance of electrodes by tuning the size, shape, and pattern of active particles, as well as the properties of the inactive matrix.
JMR Early Career Scholars in Materials Science Annual Issue
We evaluate the potential of inserting metallic, metal-dielectric core-shell, and fully dielectric nanoparticles in ultrathin chalcopyrite solar cells to enhance absorption which experiences a significant drop for absorber thicknesses below 500 nm. For different integration positions at the front or at the rear of the solar cell structure theoretical expectations and potential benefits originating from light scattering, near-field enhancement and coupling into waveguide modes by the nanoparticles are presented. These benefits are always balanced against experimental challenges arising for particular geometries due to the very specific fabrication processes of chalcopyrite solar cells. In particular high absorber deposition temperatures as well as contact layers that are relatively thick compared to other devices need to be considered. Based on this, we will need to go beyond some geometries that have proven beneficial for other types of solar cells and identify the most promising configurations for chalcopyrite-based devices.
With the emergence of flexible/stretchable electronics, flexible solar cells (SCs) are able to attract much academic and industrial attention due to its advantages of lightweight, foldability, low cost, and extensive applications. Wearable technology has become a hot topic in the tech industry in this few years, shirts that read wearer's biological and physiological information are just beginning to make their way into society and will change the way that we interact with technology. The high strength and good electronic properties of graphene fiber make it a good candidate for some specific applications, such as wearable SCs, since it can be obtained at relatively low cost and it is amongst the strongest commercial yarns in existence. In this review, a summarized state of the art regarding wearable SCs is presented including several applications of graphene and its derivatives with their remarkable unconventional applications.
Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) absorbers are considered promising alternatives to commercial thin film technologies including CdTe and Cu(In,Ga)Se2 (CIGSe) owing to the earth abundance and non-toxicity of their constituents. However, to be competitive with the existing technologies, the photovoltaic performance of CZTSSe solar cells needs to be improved beyond the current record conversion efficiency of 12.6%. In this study, nanoscale elemental mapping using Auger nanoprobe microscopy (NanoAuger) and nano secondary ion mass spectrometry (NanoSIMS) are used to provide a clear picture of the compositional variations between the grains and grain boundaries in Cu2ZnSn(S,Se)4 kesterite thin films. NanoAuger measurements revealed that the top surfaces of the grains are coated with a Zn-rich (Zn,Sn)Ox layer. While thick oxide layers were observed at the grain boundaries, their chemical compositions were found to be closer to SnOx. NanoSIMS elemental maps confirmed the presence of excess oxygen deeper within the grain boundary grooves, as a result of air annealing of the CZTSSe films.
Historically, alloy development with better radiation performance has been focused on traditional alloys with one or two principal element(s) and minor alloying elements, where enhanced radiation resistance depends on microstructural or nanoscale features to mitigate displacement damage. In sharp contrast to traditional alloys, recent advances of single-phase concentrated solid solution alloys (SP-CSAs) have opened up new frontiers in materials research. In these alloys, a random arrangement of multiple elemental species on a crystalline lattice results in disordered local chemical environments and unique site-to-site lattice distortions. Based on closely integrated computational and experimental studies using a novel set of SP-CSAs in a face-centered cubic structure, we have explicitly demonstrated that increasing chemical disorder can lead to a substantial reduction in electron mean free paths, as well as electrical and thermal conductivity, which results in slower heat dissipation in SP-CSAs. The chemical disorder also has a significant impact on defect evolution under ion irradiation. Considerable improvement in radiation resistance is observed with increasing chemical disorder at electronic and atomic levels. The insights into defect dynamics may provide a basis for understanding elemental effects on evolution of radiation damage in irradiated materials and may inspire new design principles of radiation-tolerant structural alloys for advanced energy systems.
We apply n- and p-type polycrystalline silicon (poly-Si) films on tunneling SiOx to form passivated contacts to n-type Si wafers. The resulting induced emitter and n+/n back surface field junctions of high carrier selectivity and low contact resistivity enable high efficiency Si solar cells. This work addresses the materials science of their performance governed by the properties of the individual layers (poly-Si, tunneling oxide) and more importantly, by the process history of the cell as a whole. Tunneling SiOx layers (<2 nm) are grown thermally or chemically, followed by a plasma enhanced chemical vapor deposition growth of p+ or n+ doped a-Si:H. The latter is thermally crystallized into poly-Si, resulting in grain nucleation and growth as well as dopant diffusion within the poly-Si and penetration through the tunneling oxide into the Si base wafer. The cell process is designed to improve the passivation of both oxide interfaces and tunneling transport through the oxide. A novel passivation technique involves coating of the passivated contact and whole cell with atomic layer deposited Al2O3 and activating them at 400 °C. The resulting excellent passivation persists after subsequent chemical removal of the Al2O3. The preceding cell process steps must be carefully tailored to avoid structural and morphological defects, as well as to maintain or improve passivation, and carrier selective transport. Furthermore, passivated contact metallization presents significant challenges, often resulting in passivation loss. Suggested remedies include improved Si cell wafer surface morphology (without micropyramids) and postdeposited a-Si:H capping layers over the poly-Si.
Soft robots are being developed to mimic the movement of biological organisms and as wearable garments to assist human movement in rehabilitation, training, and tasks encountered in functional daily living. Stretchable artificial muscles are well suited as the active mechanical element in soft wearable robotics, and here the performance of highly stretchable and compliant polymer coil muscles are described and analyzed. The force and displacements generated by a given stimulus are shown to be determined by the external loading conditions and the main material properties of free stroke and stiffness. Spring mechanics and a model based on a single helix are used to evaluate both the coil stiffness and the mechanism of coil actuation. The latter is directly coupled to a torsional actuation in the twisted fiber that forms the coil. The single helix model illustrates how fiber volume changes generate a partial fiber untwist, and spring mechanics shows how this fiber untwist generates large tensile strokes and high gravimetric work outputs in the polymer coil muscles. These analyses highlight possible as yet unexplored means for further enhancing the performance of these systems.
Fibrillar collagen networks template and direct biocompatible silica mineralization to produce hybrid materials for biomedical applications. Silica mineralization kinetics is critical for precision-tuning material properties, including mechanical strength, microstructure, and interface thickness. We investigated the effect of varying collagen template fibril volume fraction (0.2–0.8) and elasticity (G′ 200–1500 Pa) on silica mineralization rates. Measurement of the depletion of mono- and disilicic acids by silicomolybdic acid titration showed that silica condensation on collagen fibrils follows third-order kinetics. Resulting third-order rate constants increased linearly with storage modulus and quadratically with fibril volume fraction. A unique rheological approach used to probe the collagen template surface elasticity in real-time during silicification suggested a two-phase mechanism with high levels of surface-localized gelation in Phase 1 and high levels of bulk solution-localized gelation in Phase 2. These results provide a tool for controlling hybrid collagen-silica material properties by controlling local silica condensation rates.
The microstructural evolution of a HfNbTaTiZr high-entropy alloy subjected to cold rolling and subsequent annealing was investigated. The dislocation activity dominates the deformation process. The microstuctural evolution of the alloy during cold rolling can be described as follows: (i) formation of dislocation tangles, (ii) formation of microbands, (iii) formation of thin laths and microshear bands containing thin laths, (iv) the transverse breakdown of the lath to elongated segment, and (v) formation of fine grains. During annealing at 800 and 1000 °C, decomposition of the metastable high-temperature body-centered cubic phase proceeded through a phase separation reaction. Annealing at 800 °C resulted in a nonrecrystallized microstructure with abundant second-phase particles distributed randomly. The second-phase particles with an average size of ∼145 nm were enriched in Ta and Nb, while the chemical composition of the matrix was close to the average composition of the alloy. Meanwhile, an unknown phase slightly enriched in Hf, Zr, and Ti was detected in the interfacial region between the second-phase particles.
Data mining has revolutionized sectors as diverse as pharmaceutical drug discovery, finance, medicine, and marketing, and has the potential to similarly advance materials science. In this paper, we describe advances in simulation-based materials databases, open-source software tools, and machine learning algorithms that are converging to create new opportunities for materials informatics. We discuss the data mining techniques of exploratory data analysis, clustering, linear models, kernel ridge regression, tree-based regression, and recommendation engines. We present these techniques in the context of several materials application areas, including compound prediction, Li-ion battery design, piezoelectric materials, photocatalysts, and thermoelectric materials. Finally, we demonstrate how new data and tools are making it easier and more accessible than ever to perform data mining through a new analysis that learns trends in the valence and conduction band character of compounds in the Materials Project database using data on over 2500 compounds.
Magnetic nanocomposites, annealed under stress, are investigated for application in inductive devices. Stress annealed Co-based metal/amorphous nanocomposites (MANCs) previously demonstrated induced magnetic anisotropies greater than an order of magnitude larger than field annealed Co-based MANCs and response to applied stress twice that of Fe-based MANCs. Transverse magnetic anisotropies and switching by rotational processes impact anomalous eddy current losses at high frequencies. Here we review induced anisotropies in soft magnetic materials and show new Co-based MANCs having seven times the response to stress annealing as compared to Fe-based MANC systems. This response correlates with the alloying of early transition metal elements (TE) that affect both induced anisotropies and resistivities. At optimal alloy compositions, these alloys exhibit a nearly linear B–H loop, with tunable permeabilities. The electrical resistivity is not a function of processing stress but trends in electrical resistivity and induced anisotropy with choice and concentration of TE content are clearly resolved. Previously reported and record-level induced anisotropies, Ku, ∼20 kJ/m3 (anisotropy fields, HK ∼ 500 Oe), in stress annealed Co-rich MANCs are increased to Ku ∼ 70 kJ/m3 (HK > 1800 Oe) in new systems.
Inorganic oxides exhibit numerous applications influenced by particle size and morphology. While industrial methods for forming oxides involve harsh conditions, nature has the ability to form intricate structures of silicon dioxide (silica) using small peptides and polyamines under environmentally friendly conditions. Recent research has demonstrated that these biomaterials will precipitate other inorganic oxides, such as titanium dioxide (titania). Using the diatom-derived R5 peptide, new peptides with systematic changes (e.g., truncation and substitution) in the R5 primary structure were surveyed for reactivities and the impact on the morphology of the titania. The results demonstrated that (i) basic residues are vital to initiating the reaction, and a minimum local concentration is necessary to sustain the precipitation, (ii) residues containing hydroxyl side chains are important to imparting morphological control on the precipitate, and (iii) buffer conditions can dramatically alter both precipitation and morphology.
Advances and Challenges in Carbon-based Tribomaterials
While materials design in the context of texture dependent properties is well developed, theoretical tools for microstructure design in the context of grain boundary sensitive properties have not yet been established. In the present work, we present an invertible relationship between texture and grain boundary network structure for the case of spatially uncorrelated two-dimensional textures. By exploiting this connection, we develop mathematical tools that permit the rigorous optimization of grain boundary network structure. Using a specific multi-objective materials design case study involving elastic, plastic and kinetic properties, we illustrate the utility of this texture mediated approach to grain boundary network design. We obtain a microstructure that minimizes grain boundary network diffusivity while simultaneously improving yield strength by an amount equal to half of the theoretically possible range. The theoretical tools developed here could complement experimental grain boundary engineering efforts to help accelerate the discovery of materials with improved performance.