Open Access Publications - Materials Science and Engineering
Permanent URI for this collection
Open access publications by faculty, postdocs, and graduate students in the Department of Materials Science and Engineering
Browse
Recent Submissions
Item Conversion of Compositionally Diverse Plastic Waste over Earth-Abundant Sulfides(Journal of the American Chemical Society, 2025-04-02) Selvam, Esun; Schyns, Zoé O. G.; Sun, Jessie A.; Kots, Pavel A.; Kwak, Yeonsu; Korley, LaShanda T. J.; Lobo, Raul F.; Vlachos, Dionisios G.Chemical deconstruction of polyolefin plastic wastes via hydroconversion is promising for mitigating plastic accumulation in landfills and the environment. However, hydroconversion catalysts cannot handle complex feedstocks containing multiple polymers, additives, and heteroatom impurities. Here, we report a single-step strategy using earth-abundant metal sulfide catalysts to deconstruct these wastes. We show that NiMoSx/HY catalysts deconstruct polyolefin feedstocks, achieving ∼81–94% selectivity to liquid products. Postsynthetic zeolite modification enhances the catalyst’s activity by >2.5 times, achieving over 95% selectivity to liquid fuels with controllable product distribution in the naphtha, jet fuel, and diesel range. The catalyst is resilient to increasingly complex feedstocks, such as additive-containing polymers and mixed plastics composed of polyolefins and heteroatom-containing polymers, including poly(vinyl chloride). We extend the strategy to single-use polyolefin wastes that can generate toxic byproducts, such as HCl and NH3, and eliminate their emissions by integrating reaction and sorption in a one-step process.Item The Influence of Irradiation Wavelength on the Growth of Polymer Brushes by SI-PET-RAFT Polymerization(Journal of Polymer Science, 2025-02-22) Mérai, László; Rymsha, Khrystyna; Yadav, Jyoti; Pester, Christian W.; Fery, Andreas; Besford, Quinn A.An effective method to produce well-defined polymer brushes with high spatial, temporal, and sequence control is to employ a photoredox catalyst in photo-mediated polymerization. Generally, the excitation wavelength is chosen as the absorption maximum of the photocatalyst, however, it is not clear if that corresponds to the best photochemical activity for producing polymer brushes. Herein, we systematically examine wavelength-by-wavelength resolved polymer brush growth using surface-initiated photoinduced electron transfer-reversible addition-fragmentation chain transfer polymerization (SI-PET-RAFT), of four monomer types. The absorption spectra of the water-soluble photocatalyst (ZnTPPS44−) and the brush growth at each irradiation wavelength were examined as photochemical activity plots. Our results show a striking disparity between the absorbance of the photoredox catalyst and the maximum brush height. Moreover, the photochemical activity with wavelength was highly dependent on the nature of the monomer used. In addition to displaying a strong wavelength selectivity, a characteristic red-shift in the brush height relative to the lowest possible energy transition of the photocatalyst's absorbance spectrum was observed. We anticipate this work will better inform on wavelength choice for SI-PET-RAFT polymerization of polymer brushes. Graphical Abstract available at: https://doi.org/10.1002/pol.20241148 The use of SI-PET-RAFT has been gaining momentum for producing thick, homogenous, functional polymer brushes, in an oxygen-tolerant manner. However, the connection between irradiation wavelength and the brush height has not been clear. Herein, we systematically explore the effect of irradiation wavelength on the resulting brush properties, finding a disconnection between photocatalyst absorption and brush thicknesses.Item Toward Sustainable Materials: From Lignocellulosic Biomass to High-Performance Polymers(Accounts of Materials Research, 2025-02-21) Mahajan, Jignesh S.; Gottlieb, Eric R.; Kim, Jung Min; Epps, Thomas H., IIIConspectus Lignocellulosic biomass is an ideal feedstock for the next generation of sustainable, high-performance, polymeric materials. Although lignin is a highly available and low-cost source of natural aromatics, it is commonly burned for heat or disposed of as waste. The use of lignin for new materials introduces both challenges and opportunities with respect to incumbent petrochemical-based compounds. These considerations are derived from two fundamental aspects of lignin: its recalcitrant/heterogeneous nature and aromatic methoxy substituents. This Account highlights four key efforts from the Epps group and collaborators that established innovative methods/processes to synthesize polymers from lignin deconstruction products to unlock application potential, with a particular focus on the polymerization of biobased monomer mixtures, development of structure–property–processing relationships for diverse feedstocks, functional benefits of methoxy substituents, and scalability of lignin deconstruction. First, lignin-derivable polymethacrylate systems were probed to investigate the polymerization behavior of methacrylate monomers and predict thermomechanical properties of polymers from monomer mixtures. Notably, the glass transition temperatures (Tgs) of lignin-derivable polymethacrylates (∼100–200 °C) were comparable to, or significantly above, those of petroleum-based analogues, such as polystyrene (∼100 °C), and the Tgs of the complex, biobased copolymers could be predicted by the Fox equation prior to biomass deconstruction. Second, an understanding of structure–property relationships in polymethacrylates was applied to create performance-advantaged pressure-sensitive adhesives (PSAs) using phenolic-rich bio-oil obtained from the reductive catalytic fractionation of poplar wood. The use of actual lignin-derived monomers as the starting material was an important step because it underscored that nanostructure-forming, multiblock polymers could be readily made despite the complexity of real lignin deconstruction products. This work also highlighted that lignin-based phenolics could be used to make colorless/odorless PSAs, without complex separations/purifications, and still perform as well as commercial adhesives. Third, an intensified reductive catalytic deconstruction (RCD) process was developed to deconstruct lignin at ambient conditions, and the deconstructed products were successfully employed in 3D printing. The reactive distillation-RCD process operated at ambient pressure using a low-volatility and biobased solvent (glycerin) as a hydrogen donor, which reduced capital/operating costs, energy use, and safety hazards associated with conventional RCD. Technoeconomic analysis showed that such optimization could lead to a 60% reduction in cost to make the PSAs described above. Fourth, lignin-derivable bisguaiacols/bissyringols were explored as potential alternatives to petroleum-derived bisphenol A (BPA) in diamine-cured epoxy resins. A distinguishing feature of the lignin monomers (vs. BPA/bisphenol F [BPF]) was the presence of methoxy groups on the aromatic rings, and these methoxy moieties enabled tuning of application-specific properties, such as Tg, degradation temperature (Td), and glassy storage modulus (E′), to achieve improved processing and performance. The lignin-derivable thermosets exhibited Tgs above 100 °C, Tds above 300 °C, and E′s above 2 GPa, all values that were comparable to those of BPA-/BPF-based analogues. Moreover, the methoxy groups on these lignin-derivable compounds sterically hindered hormone receptor binding and could mitigate many of the toxicity concerns associated with BPA/BPF. This Account concludes with suggestions on future research needed to advance lignin-derived materials as sustainable and performance-advantaged alternatives by leveraging recycling/upcycling strategies and scaling-up/commercializing biomass waste.Item Coarse-grained molecular dynamics simulations of mixtures of polysulfamides(RSC Applied Polymers, 2025-02-20) Shah, Jay; Jayaraman, ArthiPolysulfamides are a new class of polymers that exhibit favorable chemical and physical properties, making them a sustainable alternative to commodity polymers like polyurea. To advance the fundamental understanding of this new class of polymers, Wu et al. [Z. Wu, J. W. Wu, Q. Michaudel and A. Jayaraman, Macromolecules, 2023, 56, 5033–5049]conducted experiments and coarse-grained (CG) molecular dynamics (MD) simulations to connect the polysulfamide backbone design to the assembled structure of polysulfamides due to hydrogen bonding between sulfamides. Their CG MD simulations qualitatively reproduced experimentally observed trends in crystallinity for analogous variations in polysulfamide backbone designs. To bring chemical specificity to this generic CG model of Wu et al. and to facilitate quantitative agreement with experiments in the future, in this work, we modify this older CG model of Wu et al. using structural information from atomistic simulations. Atomistic angle and dihedral distributions involving the sulfamide functional groups are used to modify the donor and acceptor bead positions in the new CG model. Using MD simulations with this new atomistically informed CG model, we confirm that we obtained the structural trends with varying polysulfamide backbone length, bulkiness, and non-uniformity of the segments in repeat units as seen in the previous work by Wu et al. These key structural trends are as follows: (a) shorter contour lengths of segments between sulfamide groups enhance H-bonding between sulfamides, (b) increased bulkiness in the segment hinders sulfamide–sulfamide H-bonding and reduces orientational order among chains in the assembled structure, and (c) non-uniformity in the segments along the backbone does not affect orientational order in the assembled structure. While the trends qualitatively matched between the two models, we observe quantitatively higher positional order and lower orientational order among the assembled chains in the new CG model as compared to the older CG model. This difference in local chain packing arises from a change in the donor–acceptor H-bonding pattern between the two models. In this work, we also use the new CG model to study mixing and demixing in two types of mixtures of polysulfamides: one mixture has chains with varying segment lengths between sulfamide groups and another mixture has chains with different degrees of bulkiness in the backbone. We find that increasing dissimilarity (bulkiness or length) between the two types of chains promotes demixing despite the presence of sulfamide–sulfamide H-bonding interactions.Item Molecular-Scale Simulation of Wetting of Actin Filaments by Protein Droplets(The Journal of Physical Chemistry B, 2025-01-12) Andrews, James; Weirich, Kimberly; Schiller, Ulf D.Liquid phase-separating proteins can form condensates that play an important role in spatial and temporal organization of biological cells. The understanding of the mechanisms that lead to the formation of protein condensates and their interactions with other biomolecules may lead to processing routes for soft materials with tailored geometry and function. Fused in sarcoma (FUS) is an example of a nuclear protein that forms stable complexes, and recent studies have highlighted its ability to wet actin filaments and bundle them into networks. We perform coarse-grained molecular dynamics simulations to investigate the wetting and spreading of FUS droplets on actin filaments. We employ the Martini model and rescale the protein–protein and protein–actin interactions to tune the interfacial and wetting properties of FUS droplets. By measuring the molecular displacements in the three-phase region, we are able to relate contact angle, contact line velocity, and contact line friction in terms of a linear approximation of molecular kinetic theory. The results show that the rescaled Martini model can be used to study the molecular mechanisms of dynamic wetting at the nanoscale and to obtain quantitative predictions of the contact line friction and contact angles during dynamic wetting.Item Further exploration of the physicochemical nature of μ2-bridge-relevant deprotonations via the elucidation of four kinds of alditol complexes(Physical Chemistry Chemical Physics, 2024-12-12) Wu, Yi; Xie, Linchen; Jiang, Ye; He, Anqi; Li, Da; Yang, Limin; Xu, Yizhuang; Liu, Kexin; Ozaki, Yukihiro; Noda, IsaoSingle-crystal structures of four alditol complexes are presented. In LuCl3/galactitol and ScCl3/myo-inositol complexes, μ2-bridge-relevant deprotonations were observed. The polarization from two rare earth ions in the μ2-bridge activates the chemically inert OH and promotes deprotonation. Additionally, mass spectrometry, pH experiments, and quantum chemistry calculations were conducted to enhance our understanding of the μ2-bridge-relevant deprotonations. A common structural feature of the complexes where μ2-bridge-relevant deprotonation takes place is that two metal ions and two oxygen atoms in two μ2-bridges form an M2O2 cluster. The four atoms in the M2O2 cluster make up a parallelogram. Such a structure is useful to balance the strong coulombic repulsions between two M3+ and between two O−. In the ScCl3/myo-inositol complex, the deprotonation exhibits a characteristic of regional/chiral selectivity. Galactitol is a third alditol ligand where μ2-bridge-relevant deprotonation is observed. The flexible backbone of the galactitol allows the formation of more five-membered chelating rings and six-membered chelating rings, which are used to stabilize the rare earth ions of the μ2-bridge. The coordination makes the backbone of galactitol deviate from the zigzag conformation. The above results are helpful in the rational design of high-performance catalysts.Item A bio-inspired approach to engineering water-responsive, mechanically-adaptive materials(Molecular Systems Design & Engineering, 2025-02-20) Jang, Daseul; Wong, Yu-Tai; Korley, LaShanda T. J.Inspired by a diverse array of hierarchical structures and mechanical function in spider silk, we leverage building blocks that can form non-covalent interactions to develop mechanically-tunable and water-responsive composite materials via hydrogen bonding modulation. Specifically, self-assembling peptide blocks consisting of poly(β-benzyl-l-aspartate) (PBLA) are introduced into a hydrophilic polyurea system. Using these peptide–polyurea hybrids (PPUs) as a hierarchical matrix, cellulose nanocrystals (CNCs) are incorporated to diversify the self-assembled nanostructures of PPUs through matrix–filler interactions. Our findings reveal that higher PBLA content in the PPUs reduces the magnitude of the stiffness differential due to the physical crosslinking induced by the peptide blocks. Additionally, the inclusion of CNCs in the PPU matrix increases the storage modulus in the dry state but also diminishes the wet-state modulus due to the shift of physical associations from peptidic arrangements to PBLA–CNC interactions, resulting in variations in the morphology of the PPU/CNC nanocomposites. This molecular design strategy allows for the development of adaptable materials with a broad range of water-responsive storage modulus switching , spanning from ∼70 MPa to ∼400 MPa. This investigation highlights the potential of harnessing peptide assembly and peptide–cellulose interactions to achieve mechanical enhancement and water-responsiveness, providing insights for engineering next-generation responsive materials.Item Standard purification methods are not sufficient to remove micellular lipophilic dye from polymer nanoparticle solution(RSC Pharmaceutics, 2025-03-06) Sterin, Eric H.; Weinstein, Laura A.; Chowdhury, Chitran Roy; Guzzetti, Emma C.; Day, Emily S.Tracking nanoparticles’ location is imperative for understanding cellular interactions, pharmacokinetics, and biodistribution. DiD is a lipophilic dye commonly used to label nanoparticles for trafficking studies. Herein, we show that DiD micelles form in polymer NP solutions during synthesis and can lead to false positive results in downstream assays. Potential methods to remove these micelles are also described.Item Non-Isothermal Melt Crystallization of a Biodegradable Polymer Studied by Two-Dimensional Infrared Correlation Spectroscopy(Molecules, 2025-03-01) Noda, IsaoThe non-isothermal melt crystallization process of poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoateate] (PHBHx) was monitored using attenuated total reflection infrared (ATR IR) measurement. The resulting time- and temperature-dependent spectra were subjected to the two-dimensional correlation spectroscopy (2D-COS) analysis. The C=O stretching region of the PHBHx sample consisted of several distinct IR contributions attributable to the population of amorphous component, well-ordered type I lamellar crystal, and less ordered inter-lamellar type II crystal. The spectral intensity change in type I crystal occurs in the earlier stage of the crystallization at a higher temperature range compared to the overall intensity decrease in the amorphous component occurring throughout the crystallization process. The growth of the type II crystal started in a later stage at a lower temperature than the creation of the type I crystal. An early decrease in a small but distinct portion of the amorphous component may be related to a crystallization precursor species with some level of molecular order. Hetero-mode correlation analyses revealed that the crystalline band intensity changes in the C-H stretching and fingerprint regions all occur later than the population changes in crystalline species reflected by the carbonyl stretching bands. This observation suggests that the spectral intensity changes in the C-H stretching and fingerprint regions do not directly represent the population dynamics of the crystalline and amorphous species but probe instead the molecular state of the crystalline entities still undergoing the evolutionary changes.Item Ordered assemblies of peptide nanoparticles with only positive charge(Nature Communications, 2024-11-20) Shi, Yi; Zhang, Tianren; Guo, Rui; Zhang, Zihan; McCahill, Amanda L.; Tang, Yao; Liskey, Sabrina E.; Yang, Dai-Bei; Kloxin, Christopher J.; Saven, Jeffery G.; Pochan, Darrin J.Surface charge patchiness of different charge types can influence the solution behaviours of colloidal particles and globular proteins. Herein, coiled-coil ‘bundlemer’ nanoparticles that display only a single type of surface charge (SC) are computationally designed to compare their solution behaviours to mixed charge-type (MC) counterparts with both positively and negatively charged side chains. Nematic and columnar liquid crystal phases are discovered in low concentrations of SC particles, indicative of particle end-to-end stacking into columns combined with lateral electrostatic repulsion between columns, while MC particles with the same net charge and particle shape produced only amorphous, soluble aggregates. Similarly, porous lattices are formed in mixtures of SC/MC particles of opposite charges while MC/MC mixtures of opposite charges produce only amorphous aggregates. The lattice structure is inferred with a machine learning optimization approach. The differences between SC and MC particle behaviours directly show the importance of surface electrostatic patchiness.Item Compressing slippery surface-assembled amphiphiles for tunable haptic energy harvesters(Science Advances, 2025-01-15) Jani, Pallav K.; Yadav, Kushal; Derkaloustian, Maryanne; Koerner, Hilmar; Dhong, Charles; Khan, Saad A.; Hsiao, Lilian C.A recurring challenge in extracting energy from ambient motion is that devices must maintain high harvesting efficiency and a positive user experience when the interface is undergoing dynamic compression. We show that small amphiphiles can be used to tune friction, haptics, and triboelectric properties by assembling into specific conformations on the surfaces of materials. Molecules that form multiple slip planes under pressure, especially through π-π stacking, produce 80 to 90% lower friction than those that form disordered mesostructures. We propose a scaling framework for their friction reduction properties that accounts for adhesion and contact mechanics. Amphiphile-coated surfaces tend to resist wear and generate distinct tactile perception, with humans preferring more slippery materials. Separately, triboelectric output is enhanced through the use of amphiphiles with high electron affinity. Because device adoption is tied to both friction reduction and electron-withdrawing potential, molecules that self-organize into slippery planes under pressure represent a facile way to advance the development of haptic power harvesters at scale.Item Phase stability and transition behaviors of (BixIn1−x)2Se3 alloy(Applied Physics Letters, 2025-01-03) Wang, Huachun; Cai, Xuefen; Li, Wei; Wang, Bin; Evangelista, Igor; Janotti, AndersonThe Bi2Se3–In2Se3 layered system has garnered significant attention and extensive research due to its versatile properties, yet its structural properties and phase stability remain elusive. Here, using first-principles calculations with van der Waals interactions, we systematically study the phase stability and transition behavior of (BixIn1−x)2Se3 alloys. Our results reveal a contrasting stability profile between Bi2Se3 and In2Se3, with the former exhibiting a distinct preference for the β phase over the α phase, while the latter shows similar stabilities in both phases, thus partially addressing previously reported ground-state inconsistencies. Exploring composition–structure relationships, we demonstrate that Bi incorporation in low concentrations stabilizes the β phase, consistent with early experimental observations. Further analysis based on the cation orbital properties indicates that the preference of Bi for octahedral sites over tetrahedral ones drives the small critical composition for the α→β phase transition. This work enhances our understanding of phase stability in (BixIn1−x)2Se3 alloys, providing insights for future design of monophasic materials and advanced applications.Item Defects in semiconductors(Journal of Applied Physics, 2024-11-20) Dreyer, Cyrus E.; Janotti, Anderson; Lyons, John L.; Wickramaratne, DarshanaDefects are crucial to understanding semiconductor materials and designing semiconductor-based devices. In using the term “defects,” we include not only native point defects (such as vacancies and interstitials), but also dopant impurities, unintentional contaminants, and complexes between these species. While some defects can lead to detrimental nonradiative recombination and carrier trapping, other defects can be used to provide free carriers that are necessary for the design of transistors, light-emitting devices, and solar cells. Since the advent of semiconductors, a significant amount of research has focused on how to deduce and control the behavior of these defects. While traditional materials (such as silicon, germanium, and gallium arsenide) continue to present challenges in terms of understanding defects, a surge of interest in power electronics has motivated the study of newer classes of materials such as two-dimensional semiconductors and wide-bandgap nitrides and oxides. Growing interest in this area has sustained the relevance of longstanding international conferences such as the International Conference on Defects in Semiconductors.1 As sources of electrical, optical, magnetic, and vibrational signals, defects in semiconductors provide an excellent testing ground for both theory and experiment. This Special Topic brings together contributions from researchers with wide-ranging expertise in the field of defects in semiconductors, documenting advances in our understanding of established materials like silicon carbide and gallium arsenide, but also includes progress in promising new materials, such as the II-IV-VI ternary compounds and ultrawide-bandgap oxides. While showcasing the latest breakthroughs in defects in semiconductors, we also wish to acknowledge the passing of our dear colleagues Audrius Alkauskas2 and Wladyslaw Walukiewicz,3 both of whom made fundamental contributions to this field.Item Electronic properties of corundum-like Ir2O3 and Ir2O3-Ga2O3 alloys(Applied Physics Letters, 2024-11-11) Khalid, Shoaib; Janotti, AndersonIn the hexagonal, corundum-like structure, -Ga2O3 has a bandgap of 5.1 eV, which, combined with its relatively small electron effective mass, high Baliga's figure of merit, and high breakdown field, makes it a promising candidate for power electronics. Ga2O3 is easy to dope n-type, but impossible to dope p-type, impeding the realization of some electronic device designs. Developing a lattice-matched p-type material that forms a high-quality heterojunction with n-type Ga2O3 would open new opportunities in electronics and perhaps optoelectronic devices. In this work, we studied Ir2O3 as a candidate for that purpose. Using hybrid density functional theory calculations we predict the electronic band structure of -Ir2O3 and compare that to -Ga2O3, and study the stability and electronic properties of -(IrxGa1−x)2O3 alloys. We discuss the band offset between the two materials and compare it with recently available experimental data. We find that the Ir d bands that compose the top of the valence band in -Ir2O3 are much higher in energy than O p bands in -Ga2O3, possibly enabling effective p-type doping. Our results provide an insight into using the Ir2O3 or Ir2O3-Ga2O3 alloys as p-type material lattice-matched to -Ga2O3 for the realization of p–n heterojunctions.Item Visualizing fiber end geometry effects on stress distribution in composites using mechanophores(Soft Matter, 2024-11-14) Haque, Nazmul; Chang, Hao Chun; Chang, Chia-Chih; Davis, Chelsea S.Localized stress concentrations at fiber ends in short fiber-reinforced polymer composites (SFRCs) significantly affect their mechanical properties. Our research targets these stress concentrations by embedding nitro-spiropyran (SPN) mechanophores into the polymer matrix. SPN mechanophores change color under mechanical stress, allowing us to visualize and quantify stress distributions at the fiber ends. We utilize glass fibers as the reinforcing material and employ confocal fluorescence microscopy to detect color changes in the SPN mechanophores, providing real-time insights into the stress distribution. By combining this mechanophore-based stress sensing with finite element analysis (FEA), we evaluate localized stresses that develop during a single fiber pull-out test near different fiber end geometries—flat, cone, round, and sharp. This method precisely quantifies stress distributions for each fiber end geometry. The mechanophore activation intensity varies with fiber end geometry and pull-out displacement. Our results indicate that round fiber ends exhibit more gradual stress transfer into the matrix, promoting effective stress distribution. Also, different fiber end geometries lead to distinct failure mechanisms. These findings demonstrate that fiber end geometry plays a crucial role in stress distribution management, critical for optimizing composite design and enhancing the reliability of SFRCs in practical applications. By integrating mechanophores for real-time stress visualization, we can accurately map quantified stress distributions that arise during loading and identify failure mechanisms in polymer composites, offering a comprehensive approach to enhancing their durability and performance.Item Mechanical Deformation Behavior of Polymer Blend Thin Films(Macromolecular Rapid Communications, 2024-12-31) Pokhrel, Geeta; Jo, Hyungyung; Christ, Nicholas M.; Son, Hyeyoung; Howarter, John A.; Davis, Chelsea S.Examining the mechanical properties of polymer thin films is crucial for high-performance applications such as displays, coatings, sensors, and thermal management. It is important to design thin film microstructures that excel in high-demand situations without compromising mechanical integrity. Here, a polymer blend of polystyrene (PS) and polyisoprene (PI) is used as a model to explore microscale deformation behavior under uniaxial mechanical testing. Six thin film compositions ranging from pure PS to a 4.5:5.5 ratio of PS to PI are fabricated. The thin films are deformed under compression, tension, and cyclic loadings, while monitoring the behavior utilizing a micromechanical stage and optical microscopy. To calculate the plane strain modulus, a strain-induced elastic buckling instability technique is employed. The results show that as the PI concentration increases, the plane strain modulus of the films decreases while the fracture strain increases. For the 4.5:5.5 ratio of PS to PI with a continuous rubbery PI phase, the thin films show major recoverable mechanical performance. This behavior is attributed to the mechanical strength of glassy PS combined with the strain energy absorption capability of rubbery PI, enabling elastic recovery. These fundamental observations provide valuable insights for designing mechanically resilient thin films for coatings and flexible devices.Item Two-Step Close-Space Vapor Transport of MAPbI3 Solar Cells: Effects of Electron Transport Layers and Residual PbI2(ACS Applied Energy Materials, 2022-09-11) Kuba, Austin G.; Harding, Alexander J.; Richardson, Raphael J.; McCandless, Brian E.; Das, Ujjwal K.; Dobson, Kevin D.; Shafarman, William N.The effect of the electron transport layer (ETL) on the growth of methylammonium lead iodide (MAPbI3) thin films by two-step close-space vapor transport (CSVT) is reported. Nanocrystalline CdS, as well as amorphous SnO2 and C60, were selected as ETLs on indium tin oxide-coated glass substrates prior to two-step CSVT. The ETL affected the PbI2 growth, leading to different morphological and crystallographic properties. These differences carried over through the methylammonium iodide reaction to the MAPbI3 phase, but compact films with a reasonable morphology could be made on each ETL. The ETL also affected the PbI2-to-MAPbI3 reaction rate. Solar cells processed on each ETL showed a low level of residual PbI2 was important for good photovoltaic conversion efficiency (PCE). The PCEs were similar on average, but trade-offs in J–V parameters depended on the ETL selection. When films on each ETL were reacted beyond an optimal PbI2 residual content, solar cells had lower performance driven by decreases in different J–V parameters. The ETL also had practical effects on J–V performance, namely, hysteresis and air stability. The hysteresis of solar cells on C60 was much less than on SnO2 and CdS. However, the solar cells with C60 ETLs were not stable in air, exhibiting FF and Jsc losses in as little as 15 min of air exposure, while those made on the other ETLs were stable for hours. Thus, the choice of ETL for two-step CSVT affects the growth of PbI2 and its reaction to MAPbI3, but interfacial chemistry considerations and effects on current and atmospheric stability appear to be more important for device performance and yield.Item Air-Induced Conductivity Loss in Fullerene ETLs Can Drive Charge Extraction Losses in Vapor-Deposited Perovskite Solar Cells(ACS Applied Energy Materials, 2024-12-04) Kuba, Austin G.; Santiwipharat, Chaiwarut; Richardson, Raphael J.; Das, Ujjwal K.; Dobson, Kevin D.; Shafarman, William N.The effect of air exposure on all-vapor processed perovskite solar cells using C60 fullerene electron transport layers (ETLs) was investigated. C60 is used in lead halide perovskite solar cells as an ETL to decrease hysteresis and improve stabilized power output. However, air exposure to n-i-p solar cells using C60 ETLs without encapsulation or doping can result in performance degradation due to FF loss and the onset of s-shaped J–V curves. This is correlated to orders of magnitude increase in C60 resistivity upon air exposure. Drift-diffusion simulations provide evidence that a change in the C60 carrier concentration or mobility can lead to the FF loss and s-shaped J–V curve. The degradation does not occur when using inorganic ETLs but does occur in p-i-n architecture using C60 ETLs, further confirming that the C60 layer is the source of the degradation. This is an additional pathway for perovskite solar cell degradation upon air exposure beyond the instability of the perovskite itself. The loss of efficiency can be reduced in p-i-n solar cells using a LiF interlayer, and a better combination of hysteresis and air stability can be achieved in n-i-p solar cells using a C60/SnO2 bilayer ETL.Item Driving factors for the peculiar bond length dependence and tetragonal distortion of (Ag,Cu)(In,Ga)Se2 and other chalcopyrites(JPhys Energy, 2024-12-04) Falk, Hans H.; Eckner, Stefanie; Ritter, Konrad; Levcenko, Sergiu; Pfeiffelmann, Timo; Larsen, Jes; Shafarman, William N.; Schnohr, Claudia S.The chalcopyrite alloy (Ag,Cu)(In,Ga)Se2 is a highly efficient thin film solar cell absorber, reaching record efficiencies above 23%. Recently, a peculiar behavior in the bond length dependence of (Ag,Cu)GaSe2 was experimentally proven. The common cation bond length, namely Ga–Se, decreases with increasing Ag/(Ag + Cu) ratio even though the crystal lattice expands. This is opposite to the behavior observed for Cu(In,Ga)Se2, where all bond lengths increase with increasing lattice size. To better understand this peculiar bond length behavior, element-specific bond lengths of (Ag,Cu)InSe2 and Ag(In,Ga)Se2 alloys are determined using extended x-ray absorption fine structure spectroscopy. They show that the peculiar bond length dependence occurs only for (Ag,Cu) alloys, independent of the species of common cation (In or Ga). The bond lengths are used to determine the anion displacements and to estimate their contribution to the bandgap bowing. Again, both behaviors differ significantly depending on the type of alloyed cation. A valence force field approach, relaxing bond lengths and bond angles, is used to describe the structural distortion energy for a comprehensive set of I–III–VI2 and II–IV–V2 chalcopyrites. The model reveals bond angle distortions as main driving factor for the tetragonal distortion and reproduces the literature values with less than 10% deviation. In contrast, the peculiar bond length dependence is not reproduced, demonstrating that it originates from electronic effects beyond the scope of this structural model. Thus, a fundamental understanding of bond length behavior and tetragonal distortion is achieved for chalcopyrite materials, benefiting their technological applications such as high efficiency thin film photovoltaics.Item Formation of bijels stabilized by magnetic ellipsoidal particles in external magnetic fields(Soft Matter, 2024-10-08) Karthikeyan, Nikhil; Schiller, Ulf D.Bicontinuous interfacially-jammed emulsion gels (bijels) are increasingly used as emulsion templates for the fabrication of functional porous materials including membranes, electrodes, and biomaterials. Control over the domain size and structure is highly desirable in these applications. For bijels stabilized by spherical particles, particle size and volume fraction are the main parameters that determine the emulsion structure. Here, we investigate the use of ellipsoidal magnetic particles and study the effect of external magnetic fields on the formation of bijels. Using hybrid Lattice Boltzmann-molecular dynamics simulations, we analyze the effect of the magnetic field on emulsion dynamics and the structural properties of the resulting bijel. We find that the formation of bijels remains robust in the presence of magnetic fields, and that the domain size and tortuosity become anisotropic when ellipsoidal particles are used. We show that the magnetic fields lead to orientational ordering of the particles which in turn leads to alignment of the interfaces. The orientational order facilitates enhanced packing of particles in the interface which leads to different jamming times in the directions parallel and perpendicular to the field. Our results highlight the potential of magnetic particles for fabrication and processing of emulsion systems with tunable properties.