Stability of the Halide Double Perovskite Cs₂AgInBr₆

Cs2AgInBr6 is among the lead-free halide perovskites of interest, predicted by first-principles calculations to be stable with a direct band gap, but there has been only one report of its synthesis. Herein we report the formation of Cs2AgInBr6 thin films through thermal evaporation of CsBr, AgBr, and InBr3 and subsequent annealing between 130 °C and 250 °C. Cs2AgInBr6 appears stable in this temperature range. However, Cs2AgInBr6 thin films are thermodynamically unstable at room temperature, remaining cubic only long enough to be characterized but not long enough to be useful for practical devices. Cs2AgInBr6 decomposed into Cs2AgBr3, Cs3In2Br9, AgBr, and InBr3 upon cooling from 130 °C to 250 °C to room temperature. This conclusion did not depend on illumination, film thickness, annealing environment, or details of the film formation, pointing to an intrinsic thermodynamic instability of the material. Optical absorption measurements may be interpreted as Cs2AgInBr6 having a direct band gap of 1.57 ± 0.1 eV

Crossover From Nanoscopic Intergranular Hopping to Conventional Charge Transport in Pyrite Thin Films

Pyrite FeS₂ is receiving a resurgence of interest as a uniquely attractive thin film solar absorber based on abundant, low-cost, nontoxic elements. Here we address, <i>via ex situ</i> sulfidation synthesis, the long-standing problem of understanding conduction and doping in FeS₂ films, an elusive prerequisite to successful solar cells. We find that an abrupt improvement in crystallinity at intermediate sulfidation temperatures is accompanied by unanticipated crossovers from intergranular hopping to conventional transport, and, remarkably, from hole-like to electron-like Hall coefficients. The hopping is found to occur between a small volume fraction of conductive nanoscopic sulfur-deficient grain cores (beneath our X-ray diffraction detection limits), embedded in nominally stoichiometric FeS₂. In addition to placing constraints on the conditions under which useful properties can be obtained from FeS₂ synthesized in diffusion-limited situations, these results also emphasize that FeS₂ films are <i>not</i> universally p-type. Indeed, with no knowledge of the active transport mechanism we demonstrate that the Hall coefficient alone is insufficient to determine the sign of the carriers. These results elucidate the possible transport mechanisms in thin film FeS₂ in addition to their influence on the deduced carrier type, an enabling advancement with respect to understanding and controlling doping in pyrite films

Effect of Nanocrystal Size and Carbon on Grain Growth during Annealing of Copper Zinc Tin Sulfide Nanocrystal Coatings

Polycrystalline films were prepared by annealing coatings cast from colloidal dispersions of Cu₂ZnSnS₄ (CZTS) nanocrystals in sulfur vapor. This nanocrystal dispersion-based route is a promising potential low-cost approach for production of low-cost thin-film solar cells. We studied the effects of nanocrystal size, sulfur pressure, and carbon concentration on the microstructure development and grain growth during annealing. Coatings prepared from dispersions of CZTS nanocrystals with an average diameter of either 5 or 35 nm were annealed for 10–60 min at 600 °C in 50 or 500 Torr of sulfur. The CZTS nanocrystal size influenced both the rate and mechanism of grain growth. When coatings composed of 5 nm nanocrystals are annealed, abnormal grain growth forms micrometer-scale CZTS grains on the surface of the coating. In contrast, when CZTS coatings composed of 35 nm nanocrystals are annealed, grains grow uniformly via normal grain growth. Grain growth rates increased with sulfur pressure regardless of the nanocrystal size. The presence of carbon, originating from ligands used to stabilize nanocrystal dispersions, enhances abnormal grain growth, but too much carbon eventually inhibits all grain growth. On the basis of these observations, we propose a mechanism for microstructure development during annealing of CZTS nanocrystal coatings in sulfur. While much research effort has been expended on the reduction of carbon from nanocrystal coatings prior to sulfidation or selenization by means of ligand exchange or preannealing treatments in the belief that reduced carbon concentration aids CZTS microstructure development and solar cell efficiencies, this work indicates that carbon plays a more complex and significant role in CZTS grain growth than previously assumed: carbon may be beneficial or even required for rapid grain growth during sulfidation

Microstructure Evolution and Crystal Growth in Cu₂ZnSnS₄ Thin Films Formed By Annealing Colloidal Nanocrystal Coatings

A potentially low-cost approach for making copper zinc tin sulfide (CZTS) solar cells relies on annealing colloidal nanocrystal films cast on suitable substrates in a sulfur-containing environment to form thin films with micrometer-sized grains. The film microstructure and grain size affects the solar cell performance. We conducted a systematic study of the factors controlling crystal growth and microstructure development during annealing of films cast from colloidal dispersions of CZTS nanocrystals. The film microstructure is controlled by concurrent normal and abnormal grain growth. At 600 to 800 °C and low sulfur pressures (50 Torr), abnormal CZTS grains up to 10 μm in size grow on the surface of the CZTS nanocrystal film via transport of material from the nanocrystals to the abnormal grains. Meanwhile, the nanocrystals coarsen, sinter, and undergo normal grain growth. The driving force for abnormal grain growth is the reduction in total energy associated with the high surface area nanocrystals. The eventual coarsening of the CZTS nanocrystals reduces the driving force for abnormal crystal growth. Increasing the sulfur pressure by an order of magnitude to 500 Torr accelerates both normal and abnormal crystal growth though sufficient acceleration of the former eventually reduces the latter by reducing the driving force for abnormal grain growth. For example, at high temperatures (700 °C) and sulfur pressures (500 Torr) normal grains quickly grow to ∼500 nm which significantly reduces abnormal grain growth. The use of soda lime glass as the substrate, instead of quartz, accelerates normal grain growth. Normal grains grow to ∼500 nm at lower temperatures and sulfur pressures (i.e., 600 °C and 50 Torr) than those required to grow the same size grains on quartz (700 °C and 500 Torr)

Self-Regulation of Cu/Sn Ratio in the Synthesis of Cu₂ZnSnS₄ Films

There has been a tremendous recent surge of interest in copper zinc tin sulfide (Cu₂ZnSnS₄, CZTS) as a photovoltaic material, because its optical and electronic properties are well-suited for solar cells, and its elemental constituents are abundant in the earth’s crust. Here we have studied the formation mechanisms of CZTS films, and the factors that control the cation stoichiometry during ex situ sulfidation of precursor Cu–Zn–Sn alloy films in a closed isothermal system. We find that the Cu/Sn ratio in CZTS is self-regulating and approaches 2, regardless of the initial composition of the precursor films, provided that adequate Sn is available in the sulfidation system. If precursor films are initially tin rich, excess Sn evaporates in the form of SnS. If precursor films are initially Sn-deficient, the inclusion of solid Sn in the sulfidation ampule readily generates SnS vapor, which mitigates the films’ Sn deficiency to return the Cu/Sn ratio to 2. When sulfidized for sufficiently long times at sufficiently high temperatures (<i>e.g.,</i> 600 °C, 8 h), films with similar Cu/Zn ratios exhibit similar phase compositions, such that if Cu/Zn >2, a Cu₂SnS₃ impurity phase is present in addition to CZTS, and if Cu/Zn < 2, a ZnS impurity phase occurs. To achieve phase-pure, void-free films, Sn-deficient precursor films with Cu/Zn in the desired range (typically close to, but slightly less than 2) can be sulfidized with excess Sn in a closed system, or a system that maintains a SnS vapor pressure over the film. Time-dependent sulfidation experiments were performed to elucidate the mechanism of this Sn self-regulation. During the formation of CZTS, almost all of the Sn is found to leave the film as SnS, later reincorporation of the Sn occurring through reactions between SnS vapor and CuS to form Cu₂SnS₃. The ZnS and Cu₂SnS₃ phases within the films then interdiffuse to form CZTS. Because Cu/Sn is 2 in both Cu₂SnS₃ and CZTS, the Cu/Sn ratio tends to 2 when sufficient Sn is included in the system to consume all Cu. This strategy is useful for avoiding Cu–S minority phases, provided the films are sulfidized to the point of equilibrium phase composition

Formation of Copper Zinc Tin Sulfide Thin Films from Colloidal Nanocrystal Dispersions via Aerosol-Jet Printing and Compaction

A three-step method to create dense polycrystalline semiconductor thin films from nanocrystal liquid dispersions is described. First, suitable substrates are coated with nanocrystals using aerosol-jet printing. Second, the porous nanocrystal coatings are compacted using a weighted roller or a hydraulic press to increase the coating density. Finally, the resulting coating is annealed for grain growth. The approach is demonstrated for making polycrystalline films of copper zinc tin sulfide (CZTS), a new solar absorber composed of earth-abundant elements. The range of coating morphologies accessible through aerosol-jet printing is examined and their formation mechanisms are revealed. Crack-free albeit porous films are obtained if most of the solvent in the aerosolized dispersion droplets containing the nanocrystals evaporates before they impinge on the substrate. In this case, nanocrystals agglomerate in flight and arrive at the substrate as solid spherical agglomerates. These porous coatings are mechanically compacted, and the density of the coating increases with compaction pressure. Dense coatings annealed in sulfur produce large-grain (>1 μm) polycrystalline CZTS films with microstructure suitable for thin-film solar cells

Plasmonic Interactions through Chemical Bonds of Surface Ligands on PbSe Nanocrystals

When functional films are cast from colloidal dispersions of semiconductor nanocrystals, the length and structure of the ligands capping their surfaces determine the electronic coupling between the nanocrystals. Long chain oleic acid ligands on the surface of IV–VI semiconductor nanocrystals such as PbSe are typically considered to be insulating. Consequently, these ligands are either removed or replaced with short ones to bring the nanocrystals closer to each other for increased electronic coupling. Herein, using high-angle annular dark-field scanning transmission electron microscopy imaging combined with electron energy loss spectroscopy, we show that partial oxidation of PbSe nanocrystals forms conjugated double bonds within the oleic ligands, which then facilitates enhanced plasmonic interaction among the nanocrystals. The changes in the geometric configurations of the ligands are imaged directly and correlated with the changes in the surface plasmon intensities as they oxidize and undergo structural modifications

Microstructure Evolution During Selenization of Cu₂ZnSnS₄ Colloidal Nanocrystal Coatings

Annealing colloidal nanocrystal coatings in a selenium-containing environment to form polycrystalline thin films of the earth-abundant solar absorber copper zinc tin sulfoselenide (CZTSSe) is an attractive approach for making solar cells. We used a closed selenization system to investigate how coatings comprising copper zinc tin sulfide (CZTS) nanocrystals evolve into polycrystalline CZTSSe thin films and studied the effects of selenium vapor pressure, annealing temperature, and heating rate. These studies revealed two different types of microstructures and two different grain growth mechanisms depending on whether the CZTS nanocrystals are exposed to selenium vapor only or to both selenium vapor and liquid selenium. Coatings annealed in the presence of selenium vapor form a microstructure comprising micron-size CZTSSe grains on top of a nanocrystalline, carbon-rich, CZTSSe layer. The film microstructure is controlled by concurrent normal and abnormal grain growth, and the grain size distribution is bimodal, similar to that observed when CZTS nanocrystal coatings are annealed in sulfur vapor. The size of the abnormal crystals increases with selenium pressure and temperature to as large as 4 μm after annealing at 700 °C in 450 Torr of selenium. Carbon, initially present on nanocrystals as dispersion stabilizing ligands, segregates to the region between the CZTSSe grains and the substrate instead of desorbing from the coating as volatile reaction products such as CSe₂. Experiments suggest that carbon segregation occurs due to the tendency for CSe₂ to polymerize and form (CSe_2‑<i>x</i>)_<i>n</i>. Coatings annealed in the presence of liquid selenium exhibit neither the bimodal grain size distribution nor the carbon-rich layer between CZTSSe grains and the substrate. In the presence of liquid selenium, the CZTS nanocrystals selenize, grow, and coarsen to ∼1 μm in size, forming compact CZTSSe films through liquid phase sintering, a mechanism wherein both grain size coarsening and film densification are mediated by the presence of a liquid phase

Computational Study of Structural and Electronic Properties of Lead-Free CsMI₃ Perovskites (M = Ge, Sn, Pb, Mg, Ca, Sr, and Ba)

Electronic structure calculations of five crystallography-imitated structures of CsMI₃ perovskites with M = Ge, Sn, Pb, Mg, Ca, Sr, and Ba were performed. The formation energy of different perovskite phases, their relative stability, and structural and electronic properties were explored. The sensitivity of the calculations to the choice of the density functional was investigated, and the predictions were compared with experimental results. The outcome of this study is that Mg and Ba perovskites are unlikely to form in the cubic, tetragonal, or orthorhombic phases because they have positive formation energies. Although Ca and Sr perovskites have negative formation energies with respect to the metal-iodide precursors, they exhibit wide band gaps and high hygroscopicity, making these unlikely candidates for applications in photovoltaic devices. Our results suggest that the performance of a local density functional with a nonseparable gradient approximation (NGA) is similar to that of hybrid functionals in terms of band gap predictions, when M in CsMI₃ is a p-block element (Pb, Sn, and Ge). However, local density functionals with NGA predictions for the band gap are similar to other local functionals with a generalized gradient approximation (PBE, PBEsol, and PBE-D3) and are worse than those of HSE06, when M is an s-block element (Mg, Ca, Sr, and Ba)