Vahid A. Akhavan

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.

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.

Silicon Nanowire Fabric as a Lithium Ion Battery Electrode Material

A nonwoven fabric with paperlike qualities composed of silicon nanowires is reported. The nanowires, made by the supercritical-fluid-liquid-solid process, are crystalline, range in diameter from 10 to 50 nm with an average length of >100 μm, and are coated with a thin chemisorbed polyphenylsilane shell. About 90% of the nanowire fabric volume is void space. Thermal annealing of the nanowire fabric in a reducing environment converts the polyphenylsilane coating to a carbonaceous layer that significantly increases the electrical conductivity of the material. This makes the nanowire fabric useful as a self-supporting, mechanically flexible, high-energy-storage anode material in a lithium ion battery. Anode capacities of more than 800 mA h g(-1) were achieved without the addition of conductive carbon or binder.

Alkyl passivation and amphiphilic polymer coating of silicon nanocrystals for diagnostic imaging.

A method to produce biocompatible polymer-coated silicon nanocrystals for medical imaging is shown. Silica-embedded Si nanocrystals are formed by HSQ thermolysis. The nanocrystals are then liberated from the oxide and terminated with Si-H bonds by HF etching, followed by alkyl monolayer passivation by thermal hydrosilylation. The Si nanocrystals have an average diameter of 2.1 nm ± 0.6 nm and photoluminesce with a peak emission wavelength of 650 nm, which lies within the transmission window of 650-900 nm that is useful for biological imaging. The hydrophobic Si nanocrystals are then coated with an amphiphilic polymer for dispersion in aqueous media with the pH ranging between 7 and 10 and an ionic strength between 30 mM and 2 M, while maintaining a bright and stable photoluminescence and a hydrodynamic radius of only 20 nm. Fluorescence imaging of polymer-coated Si nanocrystals in biological tissue is demonstrated, showing the potential for in vivo imaging.

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.

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.

Multiexciton Solar Cells of CuInSe2 Nanocrystals

Peak external quantum efficiencies (EQEs) of just over 120% were observed in photovoltaic (PV) devices of CuInSe2 nanocrystals prepared with a photonic curing process. The extraction of more than one electron/hole pair as a result of the absorption of a single photon can occur if multiple excitons are generated and extracted. Multiexciton generation (MEG) in the nanocrystal films was substantiated by transient absorption spectroscopy. We propose that photonic curing leads to sufficient electronic coupling between nanocrystals to enable multiexciton extraction under typical solar illumination conditions. Under low light conditions, however, the EQE drops significantly, indicating that photonic curing-induced ligand desorption creates a significant amount of traps in the film that limit the overall power conversion efficiency of the device.

A Photovoltaic-Driven and Energy-Autonomous CMOS Implantable Sensor

An energy-autonomous and MRI-compatible CMOS implantable sensor is presented that operates by harvesting the energy of the light which penetrates into the tissue. On-chip P+/N-well diodes are used as on-chip photovoltaic cells and in-vivo physiological data is transmitted neuromorphically to the skin surface.

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.