Synthesis and optical properties of ordered-vacancy perovskite cesium bismuth halide nanocrystals

Perovskite-phase cesium bismuth halide (Cs3Bi2X9; X = Cl, Br, I) nanocrystals were synthesized using a hot-injection approach. These nanocrystals adopted ordered-vacancy perovskite crystal structures and demonstrated composition-tunable optical properties. Growth occurred by initial formation of Bi0 seeds, and morphology was controlled by precursor and seed concentration. The Cs3Bi2I9 nanocrystals demonstrated excellent stability under ambient conditions for several months. Contrary to previous reports, we find that photoluminescence originates from the precursor material as opposed to the Cs3Bi2X9 nanocrystals

Surface Chemistry of Ternary Nanocrystals: Engineering the Deposition of Conductive NaBiS2 Films

The ability to engineer the surface chemistry of complex ternary nanocrystals is critical to their successful application in photovoltaic, thermoelectric, and other energy conversion devices. For many years, several studies have shed light into the surface chemistry of unary and binary semiconductor nanocrystals, as well as their surface modification with monodentate and multidentate ligands in a variety of applications. In contrast, our understanding of the surface chemistry and ligand modification of ternary and other complex multinary nanocrystals remains relatively limited. Recently, our group reported the synthesis of colloidal NaBiS2 semiconductor nanocrystals with sizes tunable between 2–60 nm, and a light absorption edge of ca. 1.4 eV. Here, we use a combination of infrared and nuclear magnetic resonance spectroscopies to show that the as-made NaBiS2 nanocrystals are capped by oleylamine and neodecanoate ligands. We investigate biphasic liquid-liquid exchange as a means to replace these native ligands with either carboxylate-terminated lipoic acid or with small iodide ligands, leading in both cases to solubility in polar solvents—such as methanol, water, and dimethylformamide. We also investigate a layer-by-layer, biphasic solid-liquid exchange approach to prepare films of NaBiS2 nanocrystals capped with halide ligands—iodide, bromide, chloride. Upon exchange and removal of the native ligands, we show that the resistance of NaBiS2 nanocrystal films greatly decreases, with their measured conductivity being comparable to that of films made of isostructural PbS nanocrystals, which have been used in solar cells. Lastly, we report the first solar cell device made of NaBiS2 nanocrystal films with a limited power conversion efficiency (PCE) of 0.07. Further nanostructuring and ligand optimization may enable the preparation of much more efficient energy conversion devices based on NaBiS2 as well as other non-toxic and Earth-abundant, biocompatible multinary semiconductors

Graphene-supported high-resolution TEM and STEM imaging of silicon nanocrystals and their capping ligands

Using graphene as an ultrathin support, highresolution transmission and scanning transmission electron microscopy (TEM and STEM) images of organic ligandstabilized silicon (Si) nanocrystals with unprecedented clarity were obtained. TEM images of Si nanocrystals are usually obscured on conventional amorphous carbon TEM supports because of low atomic number (Z) contrast. The atomically thin graphene supports enabled clear images of the crystalline Si cores and, for the first time, organic capping ligands. Various twin defects were observed, often accompanied by very significant lattice distortion and anisotropic strain

Composition-matched molecular ???solders??? for semiconductors

We propose a general strategy to synthesize largely unexplored soluble chalcogenidometallates of cadmium, lead, and bismuth. These compounds can be used as ???solders??? for semiconductors widely used in photovoltaics and thermoelectrics. The addition of solder helped to bond crystal surfaces and link nano- or mesoscale particles together. For example, CdSe nanocrystals with Na2Cd2Se3 solder was used as a soluble precursor for CdSe films with electron mobilities exceeding 300 square centimeters per volt-second. CdTe, PbTe, and Bi2Te3 powders were molded into various shapes in the presence of a small additive of composition-matched chalcogenidometallate or chalcogel, thus opening new design spaces for semiconductor technologies.close10

High Efficiency Solution Processed Sintered CdTe Nanocrystal Solar Cells: The Role of Interfaces

Solution processing of photovoltaic semiconducting layers offers the potential for drastic cost reduction through improved materials utilization and high device throughput. One compelling solution-based processing strategy utilizes semiconductor layers produced by sintering nanocrystals into large-grain semiconductors at relatively low temperatures. Using n-ZnO/p-CdTe as a model system, we fabricate sintered CdTe nanocrystal solar cells processed at 350 °C with power conversion efficiencies (PCE) as high as 12.3%. <i>J</i>_SC of over 25 mA cm^–2 are achieved, which are comparable or higher than those achieved using traditional, close-space sublimated CdTe. We find that the <i>V</i>_OC can be substantially increased by applying forward bias for short periods of time. Capacitance measurements as well as intensity- and temperature-dependent analysis indicate that the increased <i>V</i>_OC is likely due to relaxation of an energetic barrier at the ITO/CdTe interface

Graphene-Supported High-Resolution TEM and STEM Imaging of Silicon Nanocrystals and their Capping Ligands

Using graphene as an ultrathin support, high-resolution transmission and scanning transmission electron microscopy (TEM and STEM) images of organic ligand-stabilized silicon (Si) nanocrystals with unprecedented clarity were obtained. TEM images of Si nanocrystals are usually obscured on conventional amorphous carbon TEM supports because of low atomic number (<i>Z</i>) contrast. The atomically thin graphene supports enabled clear images of the crystalline Si cores and, for the first time, organic capping ligands. Various twin defects were observed, often accompanied by very significant lattice distortion and anisotropic strain