We describe the structural, optical, and electrical properties of high-quality films of PbSe nanocrystals fabricated by a layer-by-layer (LbL) dip-coating method that utilizes 1,2-ethanedithiol (EDT) as an insolubilizing agent. Comparative characterization of nanocrystal films made by spin-coating and by the LbL process shows that EDT quantitatively displaces oleic acid on the PbSe surface, causing a large volume loss that electronically couples the nanocrystals while severely degrading their positional and crystallographic order of the films. Field-effect transistors based on EDT-treated films are moderately conductive and ambipolar in the dark, becoming p-type and 30-60 times more conductive under 300 mW cm(-2) broadband illumination. The nanocrystal films oxidize rapidly in air to yield, after short air exposures, highly conductive p-type solids. The LbL process described here is a general strategy for producing uniform, conductive nanocrystal films for applications in optoelectronics and solar energy conversion.

We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that produces an exceptionally large short-circuit photocurrent (>21 mA cm -2 ) by way of a Schottky junction at the negative electrode. The PV cell consists of a PbSe NC film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55-65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This NC device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport. Figure 1 shows the structure, current-voltage performance, EQE spectrum, and proposed band diagram of our device. Device fabrication consists of depositing a 60-300 nm-thick film of monodisperse, spheroidal PbSe NCs onto patterned indium tin oxide (ITO) coated glass using a layer-by-layer dip coating method, followed by evaporation of a top metal contact. In this LbL method, 1 a layer of NCs is deposited onto the ITO surface by dip coating from a hexane solution and then washed in 0.01 M 1,2-ethanedithiol (EDT) in acetonitrile to remove the electrically insulating oleate ligands that originally solubilize the NCs (see Supporting Information). Large-area, crack-free and mildly conductive (σ ) 5 × 10 -5 S cm -1 ) NC films result. The NCs pack randomly in the films, are partially coated in adsorbed ethanedithiolate, and show p-type conductivity under illumination. 1 X-ray diffraction and optical absorption spectroscopy established that the NCs neither ripen nor sinter in response to EDT exposure. We have found that using methylamine instead of EDT yields similar device performance (Supporting Information, Figure 1). 2 We have also fabricated working devices from PbS and CdSe NCs (Supporting Information, Figures 2 and 3), which indicates that the approach adopted here is not restricted to EDT-treated PbSe NCs and that it should be possible to improve cell efficiency by engineering the surface of the NCs to attain longer carrier diffusion lengths and higher photovoltages through surface state passivation and prevention of Fermi level pinning.When tested in nitrogen ambient under simulated 1-sun test conditions (100 ( 5 mW cm -2 ELH white light illumination), EDT-treated PbSe devices exhibit large shortcircuit photocurrent densities (J SC ) and modest open-circuit voltages (V OC ) and fill factors (FF), with one of the most efficient devices yielding J SC ) 24.5 mA cm -2 , V OC ) 239 mV, FF ) 0.41 and a mismatch-corrected 3 AM1.5G efficiency of 2.1% (Figure 1a; see Supporting Information regarding spectral mismatch). The mismatch-corrected J SC values of these devices are reproducibly larger than those of other nanostructured solar cells, including the best organic 4 and dye-sensitized devices, 5 which is remarkable considering the unsintered, glassy microstructure of our NC films and the fact that the NCs retain quantum confinement...

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.

We describe the structural, optical, and electrical properties of films of spin-cast, oleate-capped PbSe nanocrystals that are treated thermally or chemically in solutions of hydrazine, methylamine, or pyridine to produce electronically coupled nanocrystal solids. Postdeposition heat treatments trigger nanocrystal sintering at approximately 200 degrees C, before a substantial fraction of the oleate capping group evaporates or pyrolyzes. The sintered nanocrystal films have a large hole density and are highly conductive. Most of the amine treatments preserve the size of the nanocrystals and remove much of the oleate, decreasing the separation between nanocrystals and yielding conductive films. X-ray scattering, X-ray photoelectron and optical spectroscopy, electron microscopy, and field-effect transistor electrical measurements are used to compare the impact of these chemical treatments. We find that the concentration of amines adsorbed to the NC films is very low in all cases. Treatments in hydrazine in acetonitrile remove only 2-7% of the oleate yet result in high-mobility n-type transistors. In contrast, ethanol-based hydrazine treatments remove 85-90% of the original oleate load. Treatments in pure ethanol strip 20% of the oleate and create conductive p-type transistors. Methylamine- and pyridine-treated films are also p-type. These chemically treated films oxidize rapidly in air to yield, after short air exposures, highly conductive p-type nanocrystal solids. Our results aid in the rational development of solar cells based on colloidal nanocrystal films.

By combining nonhydrolytic reaction with seed-mediated growth, high-quality and monodisperse spinel cobalt ferrite, CoFe(2)O(4), nanocrystals can be synthesized with a highly controllable shape of nearly spherical or almost perfectly cubic. The shape of the nanocrystals can also be reversibly interchanged between spherical and cubic morphology through controlling nanocrystal growth rate. Furthermore, the magnetic studies show that the blocking temperature, saturation, and remanent magnetization of nanocrystals are solely determined by the size regardless the spherical or cubic shape. However, the shape of the nanocrystals is a dominating factor for the coercivity of nanocrystals due to the effect of surface anisotropy. Such magnetic nanocrystals with distinct shapes possess tremendous potentials in fundamental understanding of magnetism and in technological applications of magnetic nanocrystals for high-density information storage.

We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. We demonstrate that the MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which has been shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in organic solvents at approximately 4 and 4.5 times the effective band gap. A slightly modified analysis is applied to extract the MEG efficiency and the absorption cross section of each sample at the pump wavelength. We compare the absorption cross sections of our samples to that of bulk PbSe. We find that both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in solution.

Interfacial trap-assisted non-radiative recombination and residual stress impede the further increase of power conversion efficiency (PCE) and stability of the methylammonium-free (MA-free) perovskite solar cells (PSCs). Here, we report an interfacial defect passivation and stress release strategy through employing the multi-active-site Lewis base ligand (i.e., (5-mercapto-1,3,4-thiadiazol-2-ylthio)­acetic acid (MTDAA)) to modify the surface and grain boundaries (GBs) of MA-free perovskite films. Both experimental and theoretical results confirm strong chemical interactions between multiple active sites in the MTDAA molecule and undercoordinated Pb2+ at the surface or GBs of perovskite films. It is demonstrated theoretically that multi-active-site adsorption is more favorable thermodynamically as compared to single-active-site adsorption, regardless of PbI2 termination and formamidinium iodide (FAI) termination types. MTDAA modification results in much reduced defect density, increased carrier lifetime, and almost thoroughly released interfacial residual stress. Upon MTDAA passivation, the PCE is boosted from 20.26% to 21.92%. The unencapsulated device modified by MTDAA maintains 99% of its initial PCE after aging under the relative humidity range of 10–20% for 1776 h, and 91% after aging at 60 °C for 1032 h.

A combination of hard phase CoFe(2)O(4) and soft phase MnFe(2)O(4) as the bimagnetic nanocrystals in a core-shell architecture has been synthesized, and their magnetic properties have been systematically studied. Both HRTEM and EDS results confirmed the formation of bimagnetic core-shell structured nanocrystals. On the basis of the systematic and comparative studies of the magnetic properties of a mechanical mixture of pure CoFe(2)O(4) and MnFe(2)O(4) nanocrystals, chemically mixed Co(1-x)Mn(x)Fe(2)O(4) nanocrystals, and bimagnetic core-shell CoFe(2)O(4)@MnFe(2)O(4) and MnFe(2)O(4)@CoFe(2)O(4) nanocrystals, the bimagnetic core-shell nanocrystals show very unique magnetic properties, such as the blocking temperature and coercivity. Our results show that the coercivity correlates with the volume fraction of the soft phase as the theoretical hard-soft phase model has suggested. Furthermore, switching the hard phase CoFe(2)O(4) from the core to the shell shows great changes in the coercivity of the nanocrystals. The bimagnetic core-shell nanocrystals evidently demonstrate the rational design capability to separately control the blocking temperature and the coercivity in magnetic nanocrystals by varying the materials, their combination, and the volume ratio between the core and the shell and by switching hard or soft phase materials between the core and shell. Such controls via a bimagnetic core-shell architecture are highly desirable for magnetic nanocrystals in various applications.