The efficiency of a single band-gap solar cell is specified by the Shockley-Queisser limit, which defines the maximal output power as a function of the solar cell's band-gap. One way to overcome this limit is by using a down-conversion process whereupon a high energy photon is split into two lower energy photons, thereby increasing the current of the cell. Here, we provide a full analysis of the possible efficiency increase when placing a down-converting material on top of a pre-existing solar cell. We show that a total 7% efficiency improvement is possible for a perfectly efficient downconverting material. Our analysis covers both lossless and lossy theoretical limits, as well as a thermodynamic evaluation. Finally, we describe the advantages of nanoparticles as a possible choice for a down-converting material. V

A mesoscale dissipative particle dynamics model of single wall carbon nanotubes is designed and demonstrated. The coarse-grained model is produced by grouping together carbon atoms and by bonding the new lumped particles through pair and triplet forces. The mechanical properties of the simulated tube are determined by the bonding forces which are derived by virtual experiments. Through the introduction of van der Waals interactions, tube-tube interactions were studied. Owing to the reduced number of particles, this model allows the simulation of relatively large systems. The applicability of the presented scheme to model carbon nanotube based mechanical devices is discussed.

Self-assembled peptide nanotubes are unique, newly developed nano-structures which exhibit many exciting properties that may establish them as preferred nano-technological building blocks, especially for nano-fluidics, biological sensing and self-assembly applications. Integrating peptide nanotube materials in standard micro-fabrication processes is inhibited by some of their specific characteristics, which make them susceptible to some of the chemicals used in standard lithography. Here, we present an adjusted photo-lithography compatible scheme that allows the integration of these novel new nano-materials in batch processing techniques. Specifically, a scheme for creating nano-fluidic channels using peptide nanotubes, as well as contacting nanotubes to electrodes, is demonstrated. In addition, some of the incompatible fabrication methods are delineated. The modified micro-fabrication processes described here can be extended to other types of sensitive nano-materials.

We describe and demonstrate a method of creating arrays of patterned, individual, single-walled carbon nanotubes, including the spectroscopic mapping of the array. The process consists of creating networks of nanotubes suspended between silicon pillars, which are then transferred onto other substrates by an innovative process of direct stamping. Raman spectroscopy is used to spatially map and assign the specific properties of the suspended network prior to transfer. This method provides a simple and inexpensive means for deriving nanoscale devices utilizing individually assigned carbon nanotubes in a robust and non-surface-specific technique.

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The thermodynamic efficiency of a single junction solar cell is bounded by the Shockley-Queisser detailed balance limit at ∼30% [W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510 (1961)]. This maximal efficiency is considered achievable using a semiconductor within a restricted bandgap range of 1.1-1.5 eV. This work upends this assumption by demonstrating that the optimal material bandgap can be shifted to lower energies by placing selective reflectors around the solar cell. This technique opens new possibilities for lower bandgap materials to achieve the thermodynamic limit and to be effective in high efficiency solar cells.

Solar cell efficiency is maximized through multijunction architectures that minimize carrier thermalization and increase absorption. Previous proposals suggest that the maximum efficiency for a finite number of subcells is achieved for designs that optimize for light trapping over radiative coupling. We instead show that structures with radiative coupling and back reflectors for light trapping, e.g. spectrum-splitting cells, can achieve higher conversion efficiencies. We model a compatible geometry, the polyhedral specular reflector. We analyze and experimentally verify the effects of spectral window and radiative coupling on voltage and power. Our results indicate that radiative coupling with back reflectors leads to higher efficiencies than previously studied architectures for practical multijunction architectures (i.e., #20 subcells).The photovoltaic community is closer than ever to achieving ultra-high multijunction solar cell efficiencies (>50%).1-8 Subcells from III-V compound semiconductors are approaching ideal Shockley-Queisser behavior and emit signicant radiation of photons with energies equal to or above the optical bandgap because nonradiative recombination has been minimized with advanced growth processes.6,9 The optical environment of a solar cell controls where the radiated photons from a subcell are directed and this greatly affects its efficiency.2,3 Thus the optical design of multijunction architectures is crucial for maximizing performance. To date, (1) light trapping and (2) radiative coupling have been investigated as promising optical design strategies. Light trapping inhibits the radiative emission of a subcell in order to reduce the dark current and increase voltage.For example, this can be achieved by including a back reector on a cell.9 By contrast, radiative coupling directs radiative emission between neighboring subcells for reconversion. 2,8Cells that have a high degree of radiative coupling have higher currents and are more tolerant of spectral mismatch because photons can be redistributed and boost carrier generation in the current-limited subcells.10-15 Thus including both strong light trapping and radiative coupling could yield very high efficiencies. However, only geometries that optimize for either strong light trapping or strong radiative coupling have been considered in the previous literature.2 Until now, a proposed structure that only optimizes for light trapping and completely blocks radiative coupling using frequency selective reectors matched to the band gap emission of each subcell has been assumed to be the most efficient structure for discrete numbers of junctions. This 'selective reector' design has been shown to give the highest efficiencies for time symmetric structures comprised of a realistic number Broader contextEven with the recent advances in photovoltaics research, 50% solar cell efficiencies have not yet been achieved. Previous designs have focused on a tandem stack structure where semiconductor layers are epitaxially grown or wafer bonded on top of e...

Interactions between atoms of bound single-walled carbon nanotubes (SWNTs) are known to cause measurable distortion to the tube's original circular cross-section frame. High-resolution transmission electron microscope (TEM) investigation was used here to directly image and verify these radial deformations. The data presented here provide, for the first time, direct measurements of the deformations due to the interactions between isolated pairs of nanotubes. The deformation data is compared to previously reported experimental and simulation results.