Matthew G. Panthani

Synthesis of CuInS2, CuInSe2, and Cu(InxGa1-x)Se2 (CIGS) Nanocrystal “Inks” for Printable Photovoltaics

Chalcopyrite copper indium sulfide (CuInS2) and copper indium gallium selenide (Cu(InxGa(1-x))-Se2; CIGS) nanocrystals ranging from approximately 5 to approximately 25 nm in diameter were synthesized by arrested precipitation in solution. The In/Ga ratio in the CIGS nanocrystals could be controlled by varying the In/Ga reactant ratio in the reaction, and the optical properties of the CulnS2 and CIGS nanocrystals correspond to those of the respective bulk materials. Using methods developed to produce uniform, crack-free micrometer-thick films, CulnSe2 nanocrystals were tested in prototype photovoltaic devices. As a proof-of-concept, the nanocrystal-based devices exhibited a reproducible photovoltaic response.

Copper Selenide Nanocrystals for Photothermal Therapy

Ligand-stabilized copper selenide (Cu(2-x)Se) nanocrystals, approximately 16 nm in diameter, were synthesized by a colloidal hot injection method and coated with amphiphilic polymer. The nanocrystals readily disperse in water and exhibit strong near-infrared (NIR) optical absorption with a high molar extinction coefficient of 7.7 × 10(7) cm(-1) M(-1) at 980 nm. When excited with 800 nm light, the Cu(2-x)Se nanocrystals produce significant photothermal heating with a photothermal transduction efficiency of 22%, comparable to nanorods and nanoshells of gold (Au). In vitro photothermal heating of Cu(2-x)Se nanocrystals in the presence of human colorectal cancer cell (HCT-116) led to cell destruction after 5 min of laser irradiation at 33 W/cm(2), demonstrating the viabilitiy of Cu(2-x)Se nanocrystals for photothermal therapy applications.

Synthesis of Cu2ZnSnS4 Nanocrystals for Use in Low-Cost Photovoltaics

Cu(2)ZnSnS(4) (CZTS) is a promising new material for thin-film solar cells. Nanocrystal dispersions, or solar paints, present an opportunity to significantly reduce the production cost of photovoltaic devices. This communication demonstrates the colloidal synthesis of CZTS nanocrystals and their use in fabricating prototype solar cells with a power conversion efficiency of 0.23% under AM 1.5 illumination.

Spray-deposited CuInSe2 nanocrystal photovoltaics

Photovoltaic devices (PVs) were fabricated by spray-coating an ink of copper indium diselenide (CIS) nanocrystals as the light-absorbing layer. Without high-temperature post-deposition annealing, PVs were made on glass and plastic substrates with power conversion efficiencies of up to 1.9% and 1.1%, respectively, under AM1.5 illumination. The mild processing conditions also enabled fabrication of alternative device structures that are not compatible with conventional high-temperature PV processing, including substrate and superstrate designs, and devices with transparent back contacts of conducting indium tin oxide (ITO) and plastic substrates. Device performance is observed to be limited by poor charge extraction from the nanocrystal films, with the highest efficiencies being obtained from PVs with relatively thin absorber layers. To improve light absorption without sacrificing internal quantum efficiency, stacked PVs were fabricated, which exhibited improved short-circuit current and power conversion efficiency compared to stand-alone single junction devices.

CuInSe2 Quantum Dot Solar Cells with High Open-Circuit Voltage.

CuInSe2 (CISe) quantum dots (QDs) were synthesized with tunable size from less than 2 to 7 nm diameter. Nanocrystals were made using a secondary phosphine selenide as the Se source, which, compared to tertiary phosphine selenide precursors, was found to provide higher product yields and smaller nanocrystals that elicit quantum confinement with a size-dependent optical gap. Photovoltaic devices fabricated from spray-cast CISe QD films exhibited large, size-dependent, open-circuit voltages, up to 849 mV for absorber films with a 1.46 eV optical gap, suggesting that midgap trapping does not dominate the performance of these CISe QD solar cells.

In Vivo Whole Animal Fluorescence Imaging of a Microparticle-Based Oral Vaccine Containing (CuInSexS2–x)/ZnS Core/Shell Quantum Dots

Zinc sulfide-coated copper indium sulfur selenide (CuInSexS2-x/ZnS core/shell) nanocrystals were synthesized with size-tunable red to near-infrared (NIR) fluorescence with high quantum yield (40%) in water. These nanocrystals were tested as an imaging agent to track a microparticle-based oral vaccine administered to mice. Poly(lactic-co-glycolic acid) (PLGA) microparticle-encapsulated CuInSexSe2-x/ZnS quantum dots were orally administered to mice and were found to provide a distinct visible fluorescent marker in the gastrointestinal tract of living mice.

Thickness-limited performance of CuInSe 2 nanocrystal photovoltaic devices

This paper reports our latest results using colloidal CuInSe₂ nanocrystal inks to prepare photovoltaic (PV) devices. Thus far, devices with nanocrystal layers processed under ambient conditions with no post-deposition treatment have achieved power conversion efficiencies of up to 3.1%. Device efficiency is largely limited by charge carrier trapping in the nanocrystal layer, and the highest device efficiencies are obtained with very thin layers-less than 150 nm-absorbing only a fraction of the incident light. Devices with thicker nanocrystal layers had lower power conversion efficiency, despite the increased photon absorption, because the internal quantum efficiency of the devices decreased significantly. The thin, most efficient devices exhibited internal quantum efficiencies as high as 40%, across a wide spectrum. Mott-Schottky measurements revealed that the active region thickness in the devices is approximately 50 nm.

Phase transitions, melting dynamics, and solid-state diffusion in a nano test tube.

Confined Germanium Thermodynamics When a material is confined to nanoscale volumes, the very high proportion of surface to bulk can alter its thermodynamic properties. This has been studied using in situ electron microscopy, but in most cases the volume of the material is not constrained. Holmberg et al. (p. 405) studied the thermodynamics of a germanium nanowire attached to a gold seed and coated with a carbon shell to restrict its volume, measuring the reaction temperature, as well as the liquid composition without changes in volume throughout the heating cycle. This enabled monitoring of phase behavior while the germanium was being heated, and tracking solid-state diffusion across the confined interface. A carbon coating allows the thermodynamic behavior of a germanium nanowire to be probed under constant-volume conditions. Confined nanoscale geometry greatly influences physical transformations in materials. The electron microscope enables direct visualization of these changes. We examined the evolution of a germanium (Ge) nanowire attached to a gold (Au) nanocrystal as it was heated to 900°C. The application of a carbon shell prevented changes in volume and interfacial area during the heating cycle. Au/Ge eutectic formation was visualized, occurring 15°C below the bulk eutectic temperature. Capillary pressure pushed the melt into the cylindrical neck of the nanowire, and Ge crystallized in the spherical tip of the carbon shell. Solid-state diffusion down the length of the confined Ge nanowire was observed at temperatures above 700°C; Au diffusion was several orders of magnitude slower than in a bulk Ge crystal.

Nanocrystals for Electronics

Semiconductor nanocrystals are promising materials for low-cost large-area electronic device fabrication. They can be synthesized with a wide variety of chemical compositions and size-tunable optical and electronic properties as well as dispersed in solvents for room-temperature deposition using various types of printing processes. This review addresses research progress in large-area electronic device applications using nanocrystal-based electrically active thin films, including thin-film transistors, light-emitting diodes, photovoltaics, and thermoelectrics.

Comparison of the photovoltaic response of oleylamine and inorganic ligand-capped CuInSe2 nanocrystals.

Thin film photovoltaic devices (PVs) were fabricated with CuInSe(2) (CIS) nanocrystals capped with either oleylamine, inorganic metal chalcogenide-hydrazinium complexes (MCC), or S(2-), HS(-), and OH(-). A CIS nanocrystal layer deposited from solvent-based inks without high temperature processing served as the active light-absorbing material in the devices. The MCC ligand-capped CIS nanocrystal PVs exhibited power conversion efficiency under AM1.5 illumination (1.7%) comparable to the oleylamine-capped CIS nanocrystals (1.6%), but with significantly thinner absorber layers. S(2-)-capped CIS nanocrystals could be deposited from aqueous dispersions, but exhibited lower photovoltaic performance.