RSS 1.0Applied Physics
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Recent Submissions
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When JV Curves Conceal Material Improvements: The Relevance of Photoluminescence Measurements in the Optimization of Perovskite Solar Cells(Advanced Optical Materials, Wiley, 2023-09-13) [Article]Lead halide perovskites have prompted great interest, offering impressive photovoltaic performances. Most fundamental investigations and cell optimizations focus on solution-based solar cells, which are not easily extended to larger scales. Commonly in these cells, losses in the open-circuit voltage are attributed to arise primarily from interface recombination, and therefore the most studies have focused on optimization of the surface to eliminate defects states. In contrast, thermal evaporation is an alternative, solvent-free, and scalable method to deposit lead halide perovskites that is gaining attention. However, the number of reports showing high-efficiency solar cells (> 20%) prepared using thermal evaporation is still small. Here, the origins of non-radiative charge carrier recombination are investigated in perovskite cells that are deposited via thermal co-evaporation. This is done through a combination of photoluminescence spectroscopy, current-voltage characterization, and simulations. It is found that the non-radiative recombination in these cells is caused equally by bulk and interface defects. In general, it is advocated to perform a dual analysis of the photoluminescence spectroscopy of both the film and the photovoltaic device, in conjunction with current-voltage measurements. It is emphasized that such a dual analysis is needed to enable the identification of improvements and to unlock further advancements in this technology.
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Potential of AlP and GaN as barriers in magnetic tunnel junctions.(Nanoscale, 2023-09-13) [Article]AlP and GaN are wide band gap semiconductors used industrially in light emitting diodes. We investigate their potential as tunnel barriers in magnetic tunnel junctions, employing density functional theory and the non-equilibrium Green's function method for ground state and quantum transport calculations, respectively. We show that the valence band edges are dominated by pz orbitals and the conduction band edges are dominated by s orbitals. Both materials filter Bloch states of Δ1 symmetry at the Γ-point of the Brillouin zone. In the zero bias limit, we find for the Co/AlP/Co junction a high tunnel magnetoresistance of ∼200% at the Fermi energy and for the Co/GaN/Co junction a tunnel magnetoresistance of even ∼300% about 1.4 eV below the Fermi energy.
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Over 19% Efficiency in Ternary Organic Solar Cells Enabled by n-Type Dopants(ACS Energy Letters, American Chemical Society (ACS), 2023-09-11) [Article]Molecular doping has become a valuable technique for enhancing the efficiency of high-performance organic photovoltaic systems (OPVs). However, the number of known dopant molecules, especially n-type ones, that enhance the PCE of OPVs remains limited. In this study, two n-type dopants, ethyl viologen (EV) and methyl viologen (MV), are synthesized and incorporated into ternary PM6:BTP-eC9:PC71BM bulk heterojunction (BHJ) OPVs. Both dopants are found to enhance the OPV performance, yielding maximum PCE values of 19.03% and 18.61%, respectively. We show that EV and MV function as n-type dopants and microstructure modifiers, enhancing π–π stacking while increasing the absorption coefficient of the BHJs. Moreover, the n-doping balances the carrier mobility while increasing the carrier lifetime and reducing bimolecular recombination. Our results demonstrate the potential of EV and MV to improve the performance of highly efficient OPVs to levels beyond those achievable by the pristine BHJ.
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Stable Organic Solar Cells Enabled by Interlayer Engineering(Authorea, Inc., 2023-09-11) [Preprint]The development of high-performance organic solar cells (OSCs) with high operational stability is essential to accelerate their commercialization. Unfortunately, there is currently a lack of detailed understanding of the origin of instabilities in state-of-the-art OSCs based on bulk heterojunction (BHJ) featuring non-fullerene acceptors (NFAs). Herein, we developed NFA-based OSCs using different charge extraction interlayer materials and studied their storage, thermal, and operational stabilities. Despite the high power conversion efficiency (PCE) of the OSCs (17.54%), we found that cells featuring self-assembled monolayers (SAMs) as hole-extraction interlayers exhibited poor stability. The time required for these OSCs to reach 80% of their initial performance (T80) was only 6 h under continuous thermal stress at 85 °C in a nitrogen atmosphere and 1 h under maximum power point tracking (MPPT) in a vacuum. Inserting MoOx between ITO and SAM enhanced the T80 to 50 h and ~15 h after the thermal and operational stability tests, respectively, while maintaining a PCE of 16.9%. Replacing the organic PDINN electron transport layer with ZnO NPs further enhances the cells’ thermal and operational stability, boosting the T80 to 1000 and 170 h, respectively. Our work reveals the synergistic role of charge interlayers and device architecture in developing efficient and stable OSCs.
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A single n-type semiconducting polymer-based photo-electrochemical transistor(Nature communications, Springer Science and Business Media LLC, 2023-09-07) [Article]Conjugated polymer films, which can conduct both ionic and electronic charges, are central to building soft electronic sensors and actuators. Despite the possible interplay between light absorption and the mixed conductivity of these materials in aqueous biological media, no single polymer film has been utilized to create a solar-switchable organic bioelectronic circuit that relies on a fully reversible and redox reaction-free potentiometric photodetection and current modulation. Here we demonstrate that the absorption of light by an electron and cation-transporting polymer film reversibly modulates its electrochemical potential and conductivity in an aqueous electrolyte, which is harnessed to design an n-type photo-electrochemical transistor (n-OPECT). By controlling the intensity of light incident on the n-type polymeric gate electrode, we generate transistor output characteristics that mimic the modulation of the polymeric channel current achieved through gate voltage control. The micron-scale n-OPECT exhibits a high signal-to-noise ratio and an excellent sensitivity to low light intensities. We demonstrate three direct applications of the n-OPECT, i.e., a photoplethysmogram recorder, a light-controlled inverter circuit, and a light-gated artificial synapse, underscoring the suitability of this platform for a myriad of biomedical applications that involve light intensity changes.
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Ferromagnetism and ferroelectricity in a superlattice of antiferromagnetic perovskite oxides without ferroelectric polarization(npj Computational Materials, Springer Science and Business Media LLC, 2023-09-07) [Article]We study the structural, electronic, and magnetic properties of the SrCrO3/YCrO3 superlattice and their dependence on epitaxial strain. We discover that the superlattice adopts A-type antiferromagnetic (A-AFM) ordering in contrast to its constituents (SrCrO3: C-AFM; YCrO3: G-AFM) and retains it under compressive strain while becoming ferromagnetic (5 μB per formula unit) at +1% strain. The obtained ferroelectric polarization is significantly higher than that of the R2NiMnO6/La2NiMnO6 (R = Ce to Er) series of superlattices [Nat. Commun. 5, 4021 (2014)] due to a large difference between the antipolar displacements of the Sr and Y cations. The superlattice is a hybrid-improper multiferroic material with a spontaneous ferroelectric polarization (13.5 μC/cm2) approaching that of bulk BaTiO3 (19 μC/cm2). The combination of ferromagnetism with ferroelectricity enables multistate memory applications. In addition, the charge-order-driven p-type semiconducting state of the ferromagnetic phase (despite the metallic nature of SrCrO3) is a rare property and interesting for spintronics. Monte Carlo simulations demonstrate a magnetic critical temperature of 90 K for the A-AFM phase without strain and of 115 K for the ferromagnetic phase at +5% strain, for example.
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Strong magnon-magnon coupling in an ultralow damping all-magnetic-insulator heterostructure(arXiv, 2023-09-06) [Preprint]Magnetic insulators such as yttrium iron garnets (YIGs) are of paramount importance for spin-wave or magnonic devices as their ultralow damping enables ultralow power dissipation that is free of Joule heating, exotic magnon quantum state, and coherent coupling to other wave excitations. Magnetic insulator heterostructures bestow superior structural and magnetic properties and house immense design space thanks to the strong and engineerable exchange interaction between individual layers. To fully unleash their potential, realizing low damping and strong exchange coupling simultaneously is critical, which often requires high quality interface. Here, we show that such a demand is realized in an all-insulator thulium iron garnet (TmIG)/YIG bilayer system. The ultralow dissipation rates in both YIG and TmIG, along with their significant spin-spin interaction at the interface, enable strong and coherent magnon-magnon coupling with a benchmarking cooperativity value larger than the conventional ferromagnetic metal-based heterostructures. The coupling strength can be tuned by varying the magnetic insulator layer thickness and magnon modes, which is consistent with analytical calculations and micromagnetic simulations. Our results demonstrate TmIG/YIG as a novel platform for investigating hybrid magnonic phenomena and open opportunities in magnon devices comprising all-insulator heterostructures.
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Functionalized Carbon Honeycomb Membranes for Reverse Osmosis Water Desalination(Advanced Materials Interfaces, Wiley, 2023-08-30) [Article]Reverse osmosis desalination is a common technique to obtain fresh water from saltwater. Conventional membranes suffer from a trade-off between salt rejection and water permeability, raising a need for developing new classes of membranes. C-based membranes with porous graphene and carbon nanotubes offer high salt rejection, water permeability, and fouling resistance. However, controlling the pore size of these membranes is challenging. Therefore, a carbon honeycomb membrane is studied using classical molecular dynamics simulations. It is reported that functionalization with −COO– groups provides 100% salt rejection with around 1000 times higher water permeability than conventional polyamide membranes. Atomic-level understanding of the effect of the functional groups' location on salt rejection and water permeability is developed.
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Strain-Induced Sulfur Vacancies in Monolayer MoS2(ACS Materials Letters, American Chemical Society (ACS), 2023-08-25) [Article]The tuning of two-dimensional (2D) materials offers significant potential to overcome nanoelectronic limitations. As strain engineering is a nondestructive approach, we examine in this study the influence of biaxial strain on the chalcogen vacancy formation energy in transition metal dichalcogenides, employing a combination of calculations and experiments, specifically density functional theory, spherical-corrected scanning transmission electron microscopy, X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopy, Kelvin probe force microscopy, and I–V characterization. We demonstrate that compressive/tensile biaxial strain decreases/increases the chalcogen vacancy formation energy, increasing/decreasing the probability of creating chalcogen vacancies during the growth. Thus, differently strained areas within a sample can have different chalcogen vacancy densities, opening up a way to customize the work function and a route for defect engineering.
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Ionically driven synthesis and exchange bias in Mn4N/MnNx heterostructures(Applied Physics Letters, AIP Publishing, 2023-08-22) [Article]Ferrimagnets have received renewed attention as a promising platform for spintronic applications. Of particular interest is the Mn4N from the ϵ-phase of the manganese nitride as an emergent rare-earth-free spintronic material due to its perpendicular magnetic anisotropy, small saturation magnetization, high thermal stability, and large domain wall velocity. We have achieved high-quality (001)-ordered Mn4N thin film by sputtering Mn onto η-phase Mn3N2 seed layers on Si substrates. As the deposited Mn thickness varies, nitrogen ion migration across the Mn3N2/Mn layers leads to a continuous evolution of the layers to Mn3N2/Mn2N/Mn4N, Mn2N/Mn4N, and eventually Mn4N alone. The ferrimagnetic Mn4N, indeed, exhibits perpendicular magnetic anisotropy and forms via a nucleation-and-growth mechanism. The nitrogen ion migration is also manifested in a significant exchange bias, up to 0.3 T at 5 K, due to the interactions between ferrimagnetic Mn4N and antiferromagnetic Mn3N2 and Mn2N. These results demonstrate a promising all-nitride magneto-ionic platform with remarkable tunability for device applications.
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Fluctuation mediated spin-orbit torque enhancement in the noncollinear antiferromagnet Mn3Ni0.35Cu0.65N(Research Square Platform LLC, 2023-08-10) [Preprint]The role of spin fluctuations near magnetic phase transitions is crucial for generating various exotic phenomena, including anomalies in the extraordinary Hall effect, excess spin-current generation through the spin-Hall effect (SHE), and enhanced spin-pumping, amongst others. In this study, we experimentally investigate the temperature dependence of spin-orbit torques (SOTs) generated by Mn3Ni0.35Cu0.65N (MNCN), a member of the noncollinear antiferromagnetic family that exhibits unconventional magnetotransport properties. Our work uncovers a strong and nontrivial temperature dependence of SOTs, peaking near the Néel temperature of MNCN, which cannot be explained by conventional intrinsic and extrinsic scattering mechanisms of the SHE. Notably, we measure a maximum SOT efficiency of 30%, which is substantially larger than that of commonly studied nonmagnetic materials such as Pt. Theoretical calculations confirm a negligible SHE and a strong orbital Hall effect which can explain the observed SOTs. We propose a previously unidentified mechanism wherein fluctuating antiferromagneticmoments trigger the generation of substantial orbital currents near the Néel temperature. Our findings present an approach for enhancing SOTs, which holds promise for magnetic memory applications by leveraging antiferromagnetic spin fluctuations to amplify both orbital and spin currents.
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Bandgap Engineering of Melon Using Highly Reduced Graphene Oxide for Enhanced Photoelectrochemical Hydrogen Evolution(Wiley, 2023-08-07) [Article]The uncondensed form of polymeric carbon nitrides (PCN), generally known as melon, is a stacked two-dimensional structure of poly(aminoimino)heptazine. Melon is used as a photocatalyst in solar energy conversion applications, but suffers from a poor photoconversion efficiency due to weak optical absorption in the visible spectrum, high activation energy, and inefficient separation of photoexcited charge carriers. We report experimental and theoretical studies to engineer the bandgap of melon with highly reduced graphene oxide (HRG). Three HRG@melon nanocomposites with different HRG:melon ratios (0.5%, 1%, and 2%) were prepared. The 1% HRG@melon nanocomposite showed a higher photocurrent density (71 μA cm−2) than melon (24 μA cm−2) in alkaline conditions. The addition of a hole scavenger further increased the photocurrent density to 630 μA cm−2 relative to the reversible hydrogen electrode (RHE). These experimental results were validated by calculations using density functional theory (DFT), which revealed that HRG results in a significant charge redistribution and an improved photocatalytic hydrogen evolution reaction (HER).
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The Optical Properties of Organic Photovoltaic PM6:Y6 Thin Films for Solar Cell Applications(2023-08-05) [Thesis]
Advisor: Laquai, Frédéric
Committee members: Fatayer, Shadi P.; Baran, DeryaAs Organic solar cells (OSCs) become a promising complementary to traditional inorganic solar cells, studying the optical properties of OSCs plays a critical role to understand and improve the performance of organic solar cells. Studying optical properties is essential because it can help to understand how light interacts with the materials used in organic solar cells, which can help to improve the efficiency of organic solar cells. In this work, the optical properties of the organic photovoltaic system PM6:Y6 prepared from two different solvents, namely, chloroform and o-xylene, were investigated. The optical constants, specifically the refractive index and absorption coefficient of thin films of these materials, and the effects of thermal annealing on the optical properties were studied. The optical properties of isotropic and anisotropic organic materials were also compared, and the obtained optical constants were used to simulate the optical properties of the devices using the transfer matrix approach. The results suggest that more accurate measurements and analysis of the optical constants help to achieve more accurate simulations. This, in turn, provides more information about how the molecular orientation affects the optical properties of OSCs. However, it is important to note that the optical properties of PM6:Y6 blends that were studied are limited to those obtained under the conditions used to prepare the films. In fact, changes in the thickness or concentrations of solutions will need to be considered as well. Lastly, the glass transition temperature was determined using the change in the ellipsometric data (Ψ). This helps to select and test different thermal annealing temperatures for the material system, which could improve the efficiency of the respective solar cells. -
Hexanary blends: a strategy towards thermally stable organic photovoltaics(Nature Communications, Springer Science and Business Media LLC, 2023-08-01) [Article]Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 °C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells.
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Semiconductor Emitters in Entropy Sources for Quantum Random Number Generation(Annalen der Physik, Wiley, 2023-08-01) [Article]Random number generation (RNG) is needed for a myriad of applications ranging from secure communication encryption to numerical simulations to sports and games. However, generating truly random numbers can be elusive. Pseudorandom bit generation using computer algorithms provides a high random bit generation rate. Nevertheless, the reliance on predefined algorithms makes it deterministic and predictable once initial conditions are known. Relying on physical phenomena (such as measuring electrical noise or even rolling dice) can achieve a less predictable sequence of bits. Furthermore, if the physical phenomena originate from quantum effects, they can be truly random and completely unpredictable due to quantum indeterminacy. Traditionally, physical RNG is significantly slower than pseudorandom techniques. To meet the demand for high-speed RNG with perfect unpredictability, semiconductor light sources are adopted as parts of the sources of randomness, i.e., entropy sources, in quantum RNG (QRNG) systems. The high speed of their noise, the high efficiency, and the small scale of these devices make them ideal for chip-scale QRNG. Here, the applications and recent advances of QRNG are reviewed using semiconductor emitters. Finally, the performance of these emitters is compared and discuss their potential in future technologies.
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Controllable Skyrmionic Phase Transition between Néel Skyrmions and Bloch Skyrmionic Bubbles in van der Waals Ferromagnet Fe3-δGeTe2(Advanced Science, Wiley, 2023-07-28) [Article]The van der Waals (vdW) ferromagnet Fe3-δGeTe2 has garnered significant research interest as a platform for skyrmionic spin configurations, that is, skyrmions and skyrmionic bubbles. However, despite extensive efforts, the origin of the Dzyaloshinskii–Moriya interaction (DMI) in Fe3-δGeTe2 remains elusive, making it challenging to acquire these skyrmionic phases in a controlled manner. In this study, it is demonstrated that the Fe content in Fe3-δGeTe2 has a profound effect on the crystal structure, DMI, and skyrmionic phase. For the first time, a marked increase in Fe atom displacement with decreasing Fe content is observed, transforming the original centrosymmetric crystal structure into a non-centrosymmetric symmetry, leading to a considerable DMI. Additionally, by varying the Fe content and sample thickness, a controllable transition between Néel-type skyrmions and Bloch-type skyrmionic bubbles is achieved, governed by a delicate interplay between dipole–dipole interaction and the DMI. The findings offer novel insights into the variable skyrmionic phases in Fe3-δGeTe2 and provide the impetus for developing vdW ferromagnet-based spintronic devices.
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Dissecting the structure-stability relationship of Y-series electron acceptors for real-world solar cell applications(Joule, Elsevier BV, 2023-07-24) [Article]Despite striking progress toward improving the photovoltaic (PV) performance of organic solar cells (OSCs) with recent Y-series non-fullerene acceptors (Y-NFAs), knowledge about their outdoor performance under real-world conditions and photodegradation mechanisms remains elusive, which is urgently needed to close the lab-to-fab gap of OSCs. Herein, for the first time, we study the structure-outdoor-stability relationship of Y-NFAs. We show that Y-NFAs with long internal side-chains exhibit high energy barriers for photoisomerization, and fluorinated end-groups can enhance the structural confinement to inhibit the photodegradation pathway and thereby improve device stability. Furthermore, the performance loss of Y-NFA-based OSCs under illumination is mainly driven by increased trap-assisted recombination over time. The structure-stability correlation and demonstration of outdoor performance of these state-of-the-art Y-NFA cells provided in this study highlight molecular engineering of device stability control to minimize power output losses in real-world climates.
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Full Counting Statistics of Superconductor Hybrid Structures Involving a Time-Dependent Field(Elsevier BV, 2023-07-20) [Preprint]In this article, we discuss the charge transfer statistics of superconductor-quantum dot-superconductor contacts in non-equilibrium and involving a time-dependent field. In this case, the effects of multiple Andreev reflections as well as the Josephson effect show distinctive features. In addition to the effects observable in the conductance, we will also discuss the effects observable in the noise.
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Anomalous Ultralow Lattice Thermal Conductivity in Mixed-Anion Ba4Sb2Se and Ba4Sb2Te(ACS Applied Electronic Materials, American Chemical Society (ACS), 2023-07-17) [Article]Mixed-anion compounds currently attract great attention, and there are recent indications that they may also be of interest for thermoelectric applications. In this context, the lattice thermal conductivities of mixed-anion Ba4Sb2Se and Ba4Sb2Te are investigated using density functional theory and Boltzmann transport theory. We observe a 24% increase in the lattice thermal conductivity at room temperature when the atomic mass increases from Se to Te, which is counterintuitive given that lighter atoms typically result in higher phonon group velocities and lower phonon scattering rates. This anomalous behavior is attributed to a specific weak Ba–Se bond in Ba4Sb2Se as compared to the corresponding Ba–Te bond in Ba4Sb2Te, which generates numerous low-frequency optical phonons with low group velocities and enhances the phonon scattering. These findings provide avenues to customize the lattice thermal conductivity without the usual reliance on heavy atoms.
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Theoretical Investigation of Monolayer C6N3 as Anode Material for Li-, Na-, and K-Ion Batteries(2023-07-13) [Thesis]
Advisor: Schwingenschlögl, Udo
Committee members: Lanza, Mario; Laquai, FrédéricLithium-ion batteries (LIBs) are widely applied in a variety of applications such as mobile phones, laptop chargers, and electric vehicles. Thanks to a high energy density of about 120 to 220 Wh kg-1, LIBs are used for a long time, however, the present technology is unable to satisfy the increasing energy storage requirements. Therefore, increasing the energy density of LIBs to improve the performance is very important. Because of that the specific capacity and operation voltage of the anode and cathode materials determine the energy density, improving these two parameters is the key point. This can be achieved in two ways, one being the optimization of the electrode materials of existing LIBs, both cathode and anode, the other is the development of new battery systems to replace LIBs, potassium-ion batteries (KIBs) and sodium-ion batteries (NIBs) are examples of such new systems. In any case, the selection of the electrode materials is crucial. With a rapid development of two-dimensional (2D) materials, leading directly to an increase interest in exploring 2D materials in order to serve as possible electrode materials, based on their unique 2D structures, large conductivity, and most importantly, wide specific surface area. Among them lays graphene-like carbon-nitride materials with lightweight properties. These materials have collected spotlights in multiple fields that are concerned with energy harvesting and storage. The metallic monolayer C6N3 is a very recently discovered member in this family, which is chemically, mechanically, dynamically, and thermodynamically stable through the first-principal calculations. In this work, we investigate the monolayer C6N3 performance as a potential and promising foundation for the anode material of LIBs/NIBs/KIBs. According to our theoretical investigation, the metallic monolayer C6N3 should be an effective anode material for the LIBs/NIBs/KIBs, which combines high specific capacity and low average open-circuit voltage.
