Toward Efficient Carbon Nanotube/P3HT Solar Cells: Active Layer Morphology, Electrical, and Optical Properties

We demonstrate single-walled carbon nanotube (SWCNT)/P3HT polymer bulk heterojunction solar cells with an AM1.5 efficiency of 0.72%, significantly higher than previously reported (0.05%). A key step in achieving high efficiency is the utilization of semiconducting SWCNTs coated with an ordered P3HT layer to enhance the charge separation and transport in the device active layer. Electrical characteristics of devices with SWCNT concentrations up to 40 wt % were measured and are shown to be strongly dependent on the SWCNT loading. A maximum open circuit voltage was measured for SWCNT concentration of 3 wt % with a value of 1.04 V, higher than expected based on the interface band alignment. Modeling of the open-circuit voltage suggests that despite the large carrier mobility in SWCNTs device power conversion efficiency is governed by carrier recombination. Optical characterization shows that only SWCNT with diameter of 1.3–1.4 nm can contribute to the photocurrent with internal quantum efficiency up to 26%. Our results advance the fundamental understanding and improve the design of efficient polymer/SWCNTs solar cells

Structure and exfoliation mechanism of two-dimensional boron nanosheets.

Exfoliation of two-dimensional (2D) nanosheets from three-dimensional (3D) non-layered, non-van der Waals crystals represents an emerging strategy for materials engineering that could significantly increase the library of 2D materials. Yet, the exfoliation mechanism in which nanosheets are derived from crystals that are not intrinsically layered remains unclear. Here, we show that planar defects in the starting 3D boron material promote the exfoliation of 2D boron sheets-by combining liquid-phase exfoliation, aberration-corrected scanning transmission electron microscopy, Raman spectroscopy, and density functional theory calculations. We demonstrate that 2D boron nanosheets consist of a planar arrangement of icosahedral sub-units cleaved along the {001} planes of β-rhombohedral boron. Correspondingly, intrinsic stacking faults in 3D boron form parallel layers of faulted planes in the same orientation as the exfoliated nanosheets, reducing the {001} cleavage energy. Planar defects represent a potential engineerable pathway for exfoliating 2D sheets from 3D boron and, more broadly, the other covalently bonded materials

Morphological stability during solidification of silicon incorporating metallic impurities

We study the stability of a planar solidification front during pulsed laser melting-induced rapid solidification of silicon containing high concentrations of ion-implated metallic impurities. We calculate the critical impurity concentration for destabilizing plane-front solidification, and introduce the "amplification coefficient", which is an empirical parameter describing the degree of amplification that must accord between the time the planar liquid-solid interface first becomes unstable, and the time of formation of morphological features of interface breakdown that are later observed in the microstructure. By connecting our calculations to experimental observations from the literature we determine this parameter for Au, Co, Cr, Fe, Ga, In, and Zn in (100) Si and Ti in (111) Si, and find that it increases with impurity diffusive speed vd approximately as vd^.56. We present an approximate but simple method of estimating the maximum impurity concentration that may be incorporated in a surface layer of a given thickness without the appearance of cellular breakdown.Engineering and Applied Science

Direct correlation between structural and optical properties of III-V nitride nanowire heterostructures with nanoscale resolution,” Nano Lett

ABSTRACT Direct correlation of structural and optical properties on the nanoscale is essential for rational synthesis of nanomaterials with predefined structure and functionality. We study optical properties of single III-V nitride nanowire radial heterostructures with measured spatial resolution of <20 nm using cathodoluminescence (CL) technique coupled with scanning transmission electron microscopy (STEM). Enhanced carrier recombination in nanowire quantum wells and reduced light emission from regions containing structural defects were directly observed. Using newly developed parallel-detection-mode CL-STEM, we show that optical properties can vary within a single nanowire heterostructure as a function of nanowire morphology. Nanostructured materials exhibit interesting size-and morphology-dependent properties and offer unique opportunities for fundamental studies and applications of light-matter interactions. 1-3 In particular, semiconductor nanowires and nanowire heterostructures have emerged as an important class of nanomaterials for applications in nanophotonics and optoelectronics. The optical excitation of single nanowire cavities have produced wavelength-tunable stimulated emission 4 and lasing with low lasing thresholds, 5 electrically injected light-emitting diodes have been demonstrated using radial 6 and axial 7 nanowire heterostructures, and axial nanowire heterostructures can act as single photon emitters. 8 Relevant electron energy levels in nanowire heterostructures are position dependent; consequently, nanowire device functionalities, such as emission wavelength and extraction efficiency, are sensitive to miniscule changes in interface quality and interdiffusion between nanowire segments or surface passivation layers. Only methods that combine a multitude of complementary techniques on the nanometer scale can provide direct insight into the complex structureproperties interplay within semiconductor nanowires, as well as in other nanostructured materials. Here, we show that scanning transmission electron microscopy (STEM) coupled with cathodoluminescence (CL) provides simultaneous structural, compositional, and optical information with high spatial resolution. CL is a spectroscopic technique based on the light emission as a result of electronic excitation. 9 A highly focused electron beam generates electron-hole pairs within a sample, which diffuse and recombine through radiative or nonradiative processes. Far-field CL inside a scanning electron microscope (CL-SEM) is a common technique; but CL-STEM, although scarcely available, provides improved spatial resolutio

Depth-resolved cathodoluminescence spectroscopy of silicon supersaturated with sulfur

We investigate the luminescence of Si supersaturated with S (Si:S) using depth-resolved cathodoluminescence spectroscopy and secondary ion mass spectroscopy as the S concentration is varied over 2 orders of magnitude $(10^{18}–10^{20} cm^{−3})$. In single-crystalline supersaturated Si:S, we identify strong luminescence from intra-gap states related to Si self-interstitials and a S-related luminescence at 0.85 eV, both of which show a strong dependence on S concentration in the supersaturated regime. Sufficiently high S concentrations in Si $(>10^{20} cm^{−3})$ result in complete luminescence quenching, which we propose is a consequence of the overlapping of the defect band and conduction band.Engineering and Applied Science

Supersaturating silicon with transition metals by ion implantation and pulsed laser melting

We investigate the possibility of creating an intermediate band semiconductor by supersaturating Si with a range of transition metals (Au, Co, Cr, Cu, Fe, Pd, Pt, W, and Zn) using ion implantation followed by pulsed laser melting (PLM). Structural characterization shows evidence of either surface segregation or cellular breakdown in all transition metals investigated, preventing the formation of high supersaturations. However, concentration-depth profiling reveals that regions of Si supersaturated with Au and Zn are formed below the regions of cellular breakdown. Fits to the concentration-depth profile are used to estimate the diffusive speeds, v [subscript D], of Au and Zn, and put lower bounds on v [subscript D] of the other metals ranging from 10[superscript 2] to 10[superscript 4] m/s. Knowledge of v [subscript D] is used to tailor the irradiation conditions and synthesize single-crystal Si supersaturated with 10[superscript 19] Au/cm[superscript 3] without cellular breakdown. Values of v [subscript D] are compared to those for other elements in Si. Two independent thermophysical properties, the solute diffusivity at the melting temperature, D [subscript s](T [subscript m]), and the equilibrium partition coefficient, k [subscript e], are shown to simultaneously affect v [subscript D]. We demonstrate a correlation between v [subscript D] and the ratio D [subscript s](T [subscript m])/k [subscript e] [superscript 0.67], which is exhibited for Group III, IV, and V solutes but not for the transition metals investigated. Nevertheless, comparison with experimental results suggests that D [subscript s](T [subscript m])/k [subscript e] [superscript 0.67] might serve as a metric for evaluating the potential to supersaturate Si with transition metals by PLM.National Science Foundation (U.S.) (Faculty Early Career Development Program ECCS-1150878)Chesonis Family FoundationUnited States. Army Research Laboratory (United States. Army Research Office Grant W911NF-10-1-0442)National Science Foundation (U.S.) (United States. Dept. of Energy NSF CA EEC-1041895

Silicon-in-silica spheres via axial thermal gradient in-fibre capillary instabilities

The ability to produce small scale, crystalline silicon spheres is of significant technological and scientific importance, yet scalable methods for doing so have remained elusive. Here we demonstrate a silicon nanosphere fabrication process based on an optical fibre drawing technique. A silica-cladded silicon-core fibre with diameters down to 340 nm is continuously fed into a flame defining an axial thermal gradient and the continuous formation of spheres whose size is controlled by the feed speed is demonstrated. In particular, spheres of diameter \u3c 500 nm smaller than those produced under isothermal heating conditions are shown and analysed. A fibre with dual cores, p-type and n-type silicon, is drawn and processed into spheres. Spatially coherent break-up leads to the joining of the spheres into a bispherical silicon \u27p-n molecule\u27. The resulting device is measured to reveal a rectifying I-V curve consistent with the formation of a p-n junction