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DOE OSTI2019

Electronic Structure and Performance Bottlenecks of CuFeO 2 Photocathodes

The path to realizing low-cost, stable, and earth-abundant photoelectrodes can be enabled through a detailed understanding of the optoelectronic properties of these materials by combining theory and experimental techniques. Of the limited set of oxide photocathode materials currently available, CuFeO 2 has emerged as a promising candidate warranting detailed attention. Here, highly compact thin films of rhombohedral (3R) CuFeO 2 were prepared via reactive co-sputtering. Despite its 1.43 eV indirect band gap, a cathodic photocurrent of 0.85 mA/cm 2 was obtained at 0.4 V versus reversible hydrogen electrode in the presence of a sacrificial electron acceptor. This unexpected performance was related to inefficient bulk charge separation because of the ultrafast (<1 ps) self-trapping of photogenerated free carriers. The electronic structure of 3R-CuFeO 2 was elucidated through a combination of optical and X-ray spectroscopic techniques and further complemented by first-principles computational methods including a many-body approach for computing the O K-edge X-ray absorption spectrum. Through resonant inelastic X-ray scattering spectroscopy, the visible absorption edges of CuFeO 2 were found to correspond to Cu → Fe metal-to-metal charge transfer, which exhibits a high propensity toward self-trapping. Findings of the present work enable us to understand the performance bottlenecks of CuFeO 2 photocathodes and suggest feasible strategies for improving material limitations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanoparticle-Templated Thickness Controlled Growth, Thermal Stability, and Decomposition of Ultrathin Tin Sulfide Plates

Tin monosulfide (SnS) is a layered two-dimensional semiconductor attractive for instance for photovoltaics, optoelectronics, and valleytronics. However, these applications require synthesis of SnS flakes with controlled thickness and shape. Here we demonstrate the possibility of using Au nanoparticles to seed the nucleation, determine the position and control the thickness of the growing SnS plates. Further, we use in-situ transmission electron microscopy on individual ultrathin SnS flakes during annealing at high temperatures to establish their thermal stability and decomposition pathways, important due to the large surface areas of ultrathin flakes. We find that ultrathin SnS plates decompose rapidly at temperatures above 400 ºC. The decomposition invariably starts at the edges of the flakes and is highly anisotropic. Real-time observations show that while the {010} and the {110} facets are relatively stable and retract slowly, rapid removal of material along the < 100 > directions leads to the fast decay of {100} facets, which are thus replaced by extended {110} facets. The comparison of shapes developed during growth and sublimation establishes that the typical crystal habits observed for SnS and related group IV monochalcogenides are indeed kinetic growth shapes rather than equilibrium (Wulff) shapes determined by different facet surface free energies. Furthermore, these findings can further guide the design of growth experiments for achieving particular shapes of flakes of SnS and related layered crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Hybrid Organic–Inorganic Halides (C 5 H 7 N 2 ) 2 MBr 4 (M = Hg, Zn) with High Color Rendering Index and High-Efficiency White-Light Emission

Low-dimensional hybrid organic–inorganic materials (HOIMs) are being widely investigated for their unique optoelectronic properties. Some of them exhibit broadband white-light (WL) luminescence upon UV excitation, providing a potential for the fabrication of single-component white-light-emitting diodes. We report new examples of low-dimensional HOIMs, based on 4-aminopyridinium (4AMP) and group 12 metals (Hg and Zn), for single-component WL emission. The 4AMP cation containing structures feature HgBr 4 and ZnBr 4 isolated tetrahedra in (C 5 H 7 N 2 ) 2 HgBr 4 ·H 2 O (1) and (C 5 H 7 N 2 ) 2 ZnBr 4 (2), respectively. The presence of isolated molecular units in the zero-dimensional structures results in strongly localized charges and bright WL luminescence with corresponding Commission Internationale de l’Eclairage color coordinates of (0.34, 0.38) and (0.25, 0.26), correlated color temperatures of 5206 K (1) and 11 630 K (2), and very high color rendering indexes (CRI) of 87 (1) and 96 (2). The visibly bright WL emission at room temperature is corroborated with high measured photoluminescence quantum yield values of 14.87 and 19.18% for 1 and 2, respectively. Notably, the high CRI values for these new HOIMs exceed the commercial requirements and produce both “warm” and “cold” WL depending on the metal used (Hg or Zn). Based on temperature- and powder-dependent photoluminescence (PL), PL lifetimes measurements and density functional theory calculations, the broadband WL emission is assigned to the 4AMP organic molecules emission and self-trapped states.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Small Cyclic Diammonium Cation Templated (110)-Oriented 2D Halide (X = I, Br, Cl) Perovskites with White-Light Emission

Two-dimensional (2D) halide perovskites exhibit excellent potential for optoelectronics because of their outstanding physical properties and structural diversity. White-light emission is one property of 2D perovskites that originates from self-trapped excitons (STE) in the highly distorted structures. The so-called (110)-oriented 2D perovskites are generally distorted and believed to be good candidates for white-light emitting devices. Here, we report (110)-oriented 2D perovskites, C 4 N 2 H 12 PbX 4 (X = I, Br, Cl), templated by the small cyclic diammonium cation, 3-aminopyrrolidinium (3APr). Structural characterization by single-crystal X-ray diffraction reveals that the distortion of the inorganic part of the structures is influenced by the stereochemical conformation of the cation between the perovskite layers. The experimental bandgaps follow the trend I < Br < Cl (2.56 eV, 3.29 eV, 3.85 eV, respectively). Density functional theory calculations reveal a weak but significant electronic band dispersion along the stacking axis, suggesting a non-negligible interlayer electronic coupling caused by the short proximity of adjacent inorganic layers. The high level of distortion results in the emergence of white-light emission, rarely seen in iodide perovskites, as well as the bromide and chloride isostructural analogues, which provides perfect platform to compare the broad emission mechanism for all three halides. The bromide and chloride perovskites show longer lifetimes and higher color rendering index (CRI) (83 and 85), relevant to solid-state lighting. Lastly, temperature-dependent PL measurements confirm that the broad emission comes from different STE mechanism for different halides, with the peak broadening persisting even at low temperature for the chloride compound.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Machine Learning Optimization of p-Type Transparent Conducting Films

p-Type transparent conducting materials (p-TCMs) are important components of optoelectronic devices including solar cells, photodetectors, displays, and flexible sensors. Cu–Zn–S thin films prepared by chemical bath deposition (CBD) can have both high transparency in the visible range (>80%) as well as excellent hole conductivity (>1000 S cm –1 ). However, the interplay between the deposition parameters in the CBD process (metal and sulfur precursor concentrations, temperature, pH, complexing agents, etc.) creates a multidimensional parameter space such that optimization for a specific application is challenging and time-consuming. Here we show that strategic design of experiment combined with machine learning (ML) allows for the efficient optimization of p-TCM performance. The approach is guided by a figure of merit (FOM) calculated from the film conductivity and optical transmission in the desired spectral range. A specific example is shown using two steps of optimization using a selected subset of four experimental CBD factors. The ML model is based on support vector regression employing a radial basis function as the kernel function. 10-fold cross-validation was performed to mitigate overfitting. After the first round of optimization, predicted areas in the parameter space with maximal FOMs were selected for a second round of optimization. Films with optimal FOMs were incorporated into heterojunction solar cells and transparent photodiodes. Here, the optimization approach shown here will be generally applicable to any materials synthesis process with multiple parameters.

14 SOLAR ENERGY↗
DOE OSTI2019

Thin Film Synthesis of Semiconductors in the Mg–Sb–N Materials System

Nitrides feature many interesting properties, such as a wide range of bandgaps suitable for optoelectronic devices including light-emitting diodes (LEDs), and piezoelectric response used in microelectromechanical systems (MEMS). Nitrides are also significantly underexplored compared to oxides and other chemistries, with many being thermochemically metastable, sparking interest from a basic science point of view. This paper reports on experimental and computational exploration of the Mg-Sb-N material system, featuring both metastable materials and semiconducting properties. Using sputter deposition, we discovered a new Mg 2 SbN 3 nitride with a wurtzite-derived crystal structure, and synthesized the antimonide-nitride Mg 3 SbN with an antiperovskite crystal structure for the first time in thin film form. Theoretical calculations indicate that Mg 2 SbN 3 is metastable and has properties relevant to LEDs and MEMS, whereas Mg 3 SbN has a large dielectric constant (28ε o ) and low hole effective masses (0.9m o ), of interest for photovoltaic solar cell absorbers. The experimental solar-matched 1.3 eV optical absorption onset of the Mg 3 SbN antiperovskite agrees with the theoretical prediction (1.3 eV direct, 1.1 eV indirect), and with the measurements of room-temperature near-bandgap photoluminescence. Furthermore, these results make an important contribution towards understanding semiconductor properties and chemical trends in the Mg-Sb-N materials system, paving the way to future practical applications of these novel materials.

14 SOLAR ENERGY↗
DOE OSTI2020

Efficient Control of Atom Arrangement in Ternary Metal Chalcogenide Nanoparticles Using Precursor Oxidation State

Controlling both the concentration and the distribution of elements in a given material is often crucial to extracting and optimizing synergistic properties of the various constituents. An interesting class of such multielement materials is metal chalcogenide nanoparticles, which exhibit a wide range of composition-dependent optoelectronic properties including both bandgap-mediated processes and localized surface plasmon resonance properties, each of which is useful in applications ranging from energy conversion to sensing. Because metal chalcogenide nanoparticles can support several different metal elements in a variety of chalcogen lattices, this material class has particularly benefited from the ability to control both atom concentration and atom arrangement to tailor final particle properties. The primary method to access complex, multimetallic chalcogenide particles is via a postsynthetic cation exchange strategy. One-pot syntheses have been less explored to access these complex particles, although this route is desirable for economy and scalability. In this work, we compare the composition and morphology outcomes from cation exchange and one-pot preparation approaches using a Cu/Ag/Se system, which is already known to exhibit both binary and ternary metal chalcogenide phases. We show that at similar concentrations of the two metal cations, initial reaction conditions for the one-pot method yield multicomponent nanoparticles, whereas cation exchange yields homogeneous ternary metal chalcogenide structures. We then show that by tuning the precursor oxidation state for the one-pot method, this approach can be used to access homogeneous ternary metal chalcogenide particles that are similar in atom arrangement to the particles obtained using cation exchange. Taken together, our results demonstrate reliable synthetic methods that yield a variety of controlled compositions and composition morphologies in the Cu/Ag/Se system. Importantly, we demonstrate that this entire collection of architectures can all be accessed via a one-pot method simply by modifying metal precursor chemistry. The mechanistic insights gained and the resulting streamlined syntheses outlined indicate pathways to easily scaled, highly tailorable syntheses for rapid translation into downstream technologies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Organic–Inorganic Perovskites: Structural Versatility for Functional Materials Design

Although known since the late 19th century, organic–inorganic perovskites have recently received extraordinary research community attention because of their unique physical properties, which make them promising candidates for application in photovoltaic (PV) and related optoelectronic devices. In this review, we will explore beyond the current focus on three-dimensional (3-D) lead(II) halide perovskites, to highlight the great chemical flexibility and outstanding potential of the broader class of 3-D and lower dimensional organic-based perovskite family for electronic, optical, and energy-based applications as well as fundamental research. The concept of a multifunctional organic–inorganic hybrid, in which the organic and inorganic structural components provide intentional, unique, and hopefully synergistic features to the compound, represents an important contemporary target.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Conductivity Spectroscopy for Investigation and Discovery of Photovoltaic Materials

Conductivity spectroscopy is an extremely powerful set of methods for probing the properties of optoelectronic materials, especially photovoltaics, where photoconductivity is one of the best spectroscopic proxies for performance. Despite this power, they are substantially less commonly used than time-resolved photoluminescence (for instance) because they tend to be more expensive to implement (THz) and/or require specialized knowledge (GHz) to construct instruments, which are not widely available. The goal of this review is to illustrate the utility of these experiments in the discovery and study of photovoltaic absorber materials and simultaneously make them more accessible to the community by providing a central tutorial resource. We provide a comprehensive review of how conductivity spectroscopy has developed over the past decade and been applied in the discovery and development of photovoltaic materials, with a primary focus on emerging solution-processable technologies. Along the way we aim to demystify conductivity spectroscopy with focused tutorial sections that explain the physical models used to fit the data and illustrate how to think about “high-frequency conductivity”.

14 SOLAR ENERGY↗
DOE OSTI2016

Wood-Derived Materials for Green Electronics, Biological Devices, and Energy Applications

With the arising of global climate change and resource shortage, in recent years, increased attention has been paid to environmentally friendly materials. Trees are sustainable and renewable materials, which give us shelter and oxygen and remove carbon dioxide from the atmosphere. Trees are a primary resource that human society depends upon every day, for example, homes, heating, furniture, and aircraft. Wood from trees gives us paper, cardboard, and medical supplies, thus impacting our homes, school, work, and play. All of the above-mentioned applications have been well developed over the past thousands of years. However, trees and wood have much more to offer us as advanced materials, impacting emerging high-tech fields, such as bioengineering, flexible electronics, and clean energy. Wood naturally has a hierarchical structure, composed of well-oriented microfibers and tracheids for water, ion, and oxygen transportation during metabolism. At higher magnification, the walls of fiber cells have an interesting morphology--a distinctly mesoporous structure. Moreover, the walls of fiber cells are composed of thousands of fibers (or macrofibrils) oriented in a similar angle. Nanofibrils and nanocrystals can be further liberated from macrofibrils by mechanical, chemical, and enzymatic methods. The obtained nanocellulose has unique optical, mechanical, and barrier properties and is an excellent candidate for chemical modification and reconfiguration. Wood is naturally a composite material, comprised of cellulose, hemicellulose, and lignin. Wood is sustainable, earth abundant, strong, biodegradable, biocompatible, and chemically accessible for modification; more importantly, multiscale natural fibers from wood have unique optical properties applicable to different kinds of optoelectronics and photonic devices. Today, the materials derived from wood are ready to be explored for applications in new technology areas, such as electronics, biomedical devices, and energy. The goal of this study is to review the fundamental structures and chemistries of wood and wood-derived materials, which are essential for a wide range of existing and new enabling technologies. The scope of the review covers multiscale materials and assemblies of cellulose, hemicellulose, and lignin as well as other biomaterials derived from wood, in regard to their major emerging applications. Structure-properties-application relationships will be investigated in detail. Understanding the fundamental properties of these structures is crucial for designing and manufacturing products for emerging applications. Today, a more holistic understanding of the interplay between the structure, chemistry, and performance of wood and wood-derived materials is advancing historical applications of these materials. This new level of understanding also enables a myriad of new and exciting applications, which motivate this review. There are excellent reviews already on the classical topic of woody materials, and some recent reviews also cover new understanding of these materials as well as potential applications. This review will focus on the uniqueness of woody materials for three critical applications: green electronics, biological devices, and energy storage and bioenergy.

09 BIOMASS FUELS↗
DOE OSTI2021

Structural, Optical, and Electronic Properties of Two Quaternary Chalcogenide Semiconductors: Ag 2 SrSiS 4 and Ag 2 SrGeS 4

Quaternary chalcogenide materials have long been a source of semiconductors for optoelectronic applications. Recent studies on the I2-II-IV-X4 (I = Ag, Cu, Li; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) materials have shown particular versatility and promise among these compounds. These semiconductors take advantage of a diverse bonding scheme and chemical differences among cations to target a degree of anti-site defect resistance. Within this set of compounds, the materials containing both Ag and Sr have not been experimentally studied and leave a gap in the full understanding of the family. Here, we have synthesized powders and single crystals of two Ag- and Sr-containing compounds, Ag 2 SrSiS 4 and Ag 2 SrGeS 4 , each found to form in the tetragonal I4¯2m structure of Ag 2 BaGeS 4 . During the synthesis targeting the title compounds, two additional materials, Ag 2 Sr 3 Si 2 S 8 and Ag 2 Sr 3 Ge 2 S 8 , have also been identified. Furthermore, these cubic compounds represent impurity phases during the synthesis of Ag 2 SrSiS 4 and Ag 2 SrGeS 4 . We show through hybrid density functional theory calculations that Ag 2 SrSiS 4 and Ag 2 SrGeS 4 have highly dispersive band edge states and indirect band gaps, experimentally measured as 2.08(1) and 1.73(2) eV, respectively. Second-harmonic generation measurements on Ag 2 SrSiS 4 and Ag 2 SrGeS 4 powders show frequency doubling capabilities in the near-infrared range.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Formation Pathway of Wurtzite-like Cu 2 ZnSnSe 4 Nanocrystals

Cu 2 ZnSnSe 4 is a direct band gap semiconductor composed of Earth-abundant elements, making it an attractive material for thin film photovoltaic technologies. Cu 2 ZnSnSe 4 crystallizes in the kesterite structure type as a bulk material, but it can also crystallize in a metastable wurtzite-like crystal structure when synthesized on the nanoscale. The wurtzite-like polymorph introduces unique and useful properties to Cu 2 ZnSnSe 4 materials, including widely tunable band gaps and superior composi-tional flexibility as compared to kesterite Cu 2 ZnSnSe 4 . Here, we investigate the formation pathway of colloidally prepared wurtzite-like Cu 2 ZnSnSe 4 nanocrystals. We show that this quaternary material forms through a chain of reactions, starting with binary Cu 3 Se 2 nanocrystals that, due to both kinetic and thermodynamic reasons, preferentially react with tin to yield hexagonal copper tin selenide intermediates. Furthermore, these ternary intermediates then react with zinc to form the resulting wurtzite-like Cu 2 ZnSnSe 4 nanocrystals. Based on this formation pathway, we suggest synthetic methods that may prevent the for-mation of unwanted impurity phases that are known to hamper the efficiency of Cu 2 ZnSnSe 4 -based optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Cubic Crystal Structure Formation and Optical Properties within the Ag–B II –M IV –X (B II = Sr, Pb; M IV = Si, Ge, Sn; X = S, Se) Family of Semiconductors

Quaternary chalcogenide semiconductors are promising materials for energy conversion and nonlinear optical applications, with properties tunable primarily by varying the elemental composition and crystal structure. Here, we first analyze the connections among several cubic crystal structure types, as well as the orthorhombic Ag 2 PbGeS 4 -type structure, reported for select members within the Ag–B II –M IV –X (B II = Sr, Pb; M IV = Si, Ge, Sn; X = S, Se) compositional space. Focusing on the Ag–Pb–Si–S and Ag–Sr–Sn–S systems, we show that one structure type, with the formulas Ag 2 Pb 3 Si 2 S 8 and Ag 2 Sr 3 Sn 2 S 8 , is favored. We have prepared powder and single-crystal samples of Ag 2 Pb 3 Si 2 S 8 and Ag 2 Sr 3 Sn 2 S 8 , showing that each takes on the noncentrosymmetric cubic space group I$\bar{43}$d and is isostructural to the previously reported compound Ag 2 Sr 3 Ge 2 Se 8 . Through hybrid density functional theory calculations, these cubic compounds are demonstrated to be (quasi-)direct band gap semiconductors with high densities of states at the band maxima. Furthermore, the band-gap energies are measured by reflectance spectroscopy as 1.95(3) and 2.66(4) eV for Ag 2 Pb 3 Si 2 S 8 and Ag 2 Sr 3 Sn 2 S 8 , respectively. We further measure the optical properties and show the electronic band structures of three other isostructural A I –B II –M IV –X-type materials, i.e., Ag 2 Sr 3 Si 2 S 8 , Ag 2 Sr 3 Ge 2 S 8 , and Ag 2 Sr 3 Ge 2 Se 8 , showing that the band gaps can be predictably tuned by element substitution. Detailed visual analyses of the different structures and of their relationships with other members of the Ag–B II –M IV –X compositional family provide a basis for a broader understanding of the structure formation and optoelectronic properties within the quaternary chalcogenide semiconductor family.

14 SOLAR ENERGY↗
DOE OSTI2025

Interplay between Cation “Coloring” and Stereochemically Active Lone Pairs in AgBiS 2 Thin Films

Solution-processed AgBiS 2 thin films were fabricated using novel thiol− amine precursor inks to investigate the stereochemical activity of Bi 3+ 6s 2 lone pairs and their impact on the structure. A dual-space analysis combining Bragg diffraction and hard X-ray photoelectron spectroscopy (HAXPES) revealed a rock salt-like average structure with local distortions linked to cation coloring. Density functional theory (DFT) and crystal orbital Hamilton population (COHP) analyses confirmed that local Bi-rich and Ag-rich nanodomains amplify stereochemical activity, whereas more mixed and cation-order nanodomains are less stereochemically active. This local, nanoscopic mixing of segregated and ordered domains would indeed explain an average Fm3̅m structure that is rock salt-like and that does not manifest the full anharmonicity and noncentrosymmetry evidenced in canonical structures with stereochemical expression. These findings provide insights into the local structural and electronic complexities governing the optoelectronic properties of AgBiS 2 thin films.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Solid-State Syntheses, Crystallographic Spatial Disorders, Thermal Behaviors, and Bandgaps of Hybrid Organic-Inorganic Manganese Halides: A2Mn(Cl/Br)4 (A = NH4+, C(NH2)3+, & C3H4N2+)

By systematically optimizing solid-state synthesis via the concerted use of differential scanning calorimetry and in-situ variable-temperature X-ray diffraction, we report the discovery of four new hybrid organic-inorganic manganese halides A2Mn(Cl/Br)4 (A = NH4+, C(NH2)3+, C3H4N2+), with emphasis on how the organic fragment geometry, polarity, and electron-donating properties influence crystallographic spatial disorders, thermal behaviors, and optical band gaps. With a nonpolar tetrahedral cation, as in (NH4)2MnCl4 (P21/c, mP30) or (NH4)2MnBr4 (C2/c, mC180), the direct optical bandgaps (4.36–4.28 eV) are wider than those with an organic nonpolar planar geometry, as in (C(NH2)3)2MnBr4 (P21/c, mP300, 4.07 eV), or an organic polar planar geometry, as in (C3H4N2+)2MnBr4 (I41/a, tI384, 4.05 eV). Furthermore, the organic components affect the spatial arrangement and dimensionality of the resulting inorganic frameworks comprised of Mn(Cl/Br)6 octahedra, with varying distortions, or spatially disordered MnBr4 tetrahedra. These resulting trends, coupled with the systematic investigation into synthesis, and the motivation behind elucidating the nuanced role of the organic cation, altogether expand upon the phase-space of hybrid materials beyond perovskites and demonstrate the potential for a rational design of hybrid materials with tailored optoelectronic functionalities for advanced energy applications.

Lee, Shannon J.↗

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