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Glass is the most widely used transparent material for photovoltaic (PV) panels. Unfortunately, the glass introduces an additional interface and therefore optical losses, which can be mitigated using antireflection (AR) coatings. This article reports a scalable method to prepare AR coatings from ZnO nanowires (NWs) sorted by their size. The measured AR performance depends on the coating coverage and ZnO NW size, which is explained by scattering and absorption cross section simulations. Randomly oriented and size fractioned ZnO NWs with an average diameter of 19–30 nm and lengths of 121–273 nm demonstrate increased transmittance and decreased reflectance compared to a glass substrate in case of low coverage. The experimental PV modules with ZnO NW coatings show an 1.2% increase of the short circuit current. A strategy to decouple ZnO NW synthesis from AR coating preparation allows to tune NW reflectance and other optical properties by varying the length, diameter, and orientation of the constituent NWs. The proposed AR coating made from size‐sorted nanomaterials is not limited to ZnO and the method could be adapted to other materials and nanostructures. [ABSTRACT FROM AUTHOR]

The reflectance, transmittance, and absorptance of GaAs nanowire (NW) arrays are calculated by solving Maxwell's equations using the finite element method. The model is compared with measurement results from well-ordered periodic GaAs NW arrays fabricated by dry etching. The model results are also compared with the reflectance measured from NWs grown by the Au-assisted vapor-liquid-solid method. The optimum NW diameter, periodicity (spacing between NWs), and length are determined to maximize absorptance of the AM1.5G solar spectrum and short circuit current density in a NW array solar cell. The optimum NW diameter, period, and length were 180 nm, 350 nm, and 5 μm, respectively, giving a photocurrent density from the NW of 27.3 mA/cm2 and corresponding to 91.3% absorption of the AM1.5 G solar spectrum. The photocurrent density saturated for NW lengths greater than 5 μm. A gold nanoparticle at the top of the NWs (used in the vapor-liquid-solid NW growth process) substantially reduced the optimum photocurrent density, while a polymer filling the space between NWs and a planar indium tin oxide contact had a relatively minor influence. [ABSTRACT FROM AUTHOR]

The continuity and Poisson equations are solved numerically to obtain J-V characteristics and photoconversion efficiency of a two-junction solar cell. The cell consists of a top junction comprised of nanowires with bandgap of 1.7 eV grown on a bottom junction comprised of a Si substrate. The lattice relaxation possible in nanowires permits lattice-mismatched III-V material growth on Si, thereby achieving the optimum bandgaps in a two-junction cell. The model indicates a limiting efficiency of 42.3% under a concentration of 500 Suns (AM1.5 D spectrum). This limiting efficiency is similar to that calculated for the planar lattice-matched triple-junction Ge/InGaAs/InGaP cell. Methods of fabricating the nanowire/Si cell are discussed including requirements for nanowire sidewall surface passivation. The model indicated that passivation of the nanowire sidewall surfaces that produces a surface recombination velocity of 3000 cm·s-1 and surface trap density of 1012 cm-2 should be sufficient to yield high efficiency solar cells. [ABSTRACT FROM AUTHOR]

Numerical simulation of current-voltage (J-V) characteristics of III-V nanowire core-shell p-n junction diodes under illuminated conditions is presented with an emphasis on optimizing the nanowire design for photoconversion efficiency. Surface recombination and depletion effects are found to play a dominant role in the J-V characteristics. The impact of surface charge density, surface recombination velocity, doping concentration, and nanowire geometry are investigated. Investigation of contacting methodology indicated that solar cell efficiency is degraded with electrical contacts on the sidewalls of the nanowire due to Fermi level pinning at the metal/semiconductor interface. On the other hand, contacts on the top of nanowires with sidewall passivation provide solar cell performance close to the detailed balance efficiency limit of ∼30%. Elimination of the thin film between nanowires produces a smaller dark current and improved cell performance. [ABSTRACT FROM AUTHOR]

The mechanical and electronic properties of GaP nanowires are investigated by computing the Young's modulus, Poisson's ratio, energy band gap and effective carrier masses using first-principles calculations based on density functional theory. The wurtzite structural nanowires with diameters upper limited to Å are strained by uniaxial strains in the range of -. The Young's moduli of nanowires are found to be decreased with increase of the size in the direction of the Young's modulus of the bulk GaP. The Poisson's effect is determined to be stronger in GaP nanowires than in the bulk. The energy band gaps of the unstrained and strained nanowires are obtained to be enlarged with decrease of the size due to the quantum size effect. The confinement effect is found larger in the compressed nanowires than in the stretched ones. All the unstrained nanowires except the largest one are indirect band gap materials. Indirect to direct band gap transition is determined to be size and strain dependent. The effective carrier masses in all unstrained nanowires are found small compared to the ones in the bulk GaP. The effective electron and hole masses are obtained to be modulated in nanowires of this work by the compressive and both compressive/tensile strains, respectively. [ABSTRACT FROM AUTHOR]

The results of studies of photoelectric properties of an array of axial n-type GaAs/AlGaAs ( x ≈ 0.3) nanowires grown using molecular beam epitaxy on a p-type silicon substrate are presented. The ability to separate charges efficiently in a wide spectral range (from 450 to 1100 nm) is demonstrated. Such properties are important for designing active elements of photodetectors and solar cells. [ABSTRACT FROM AUTHOR]

We propose a novel nanostructure semiconductor which integrated the nanowire (NW) template and the zero dimensional quantum dot (QD) active region. We successfully fabricated InGaN quantum dots on the nonpolar facet of n-GaN NWs a new self-catalyst method using In droplets. The resultant InGaN QDs via were of a quadrangle shape. Their average width and height were 30 and 7 nm, respectively. HR-TEM analysis verified that the InGaN QDs were epitaxially grown on the -plane facet of n-GaN NWs and had m high crystalline quality without any defects. The low-temperature PL spectra of InGaN QDs indicated a prominent emission in the range of 387-425 nm which consisted of several overlapping sharp peaks owing to the different sizes of InGaN QDs. As a result, we expect that this unique InGaN QD growth method can be widely applied to other nanostructure semiconductor systems and that this newly integrated nanostructure semiconductor will be a very promising material for high-quality optoelectronic device applications. [ABSTRACT FROM AUTHOR]

This article reviews recent developments in nanowire‐based photovoltaics (PV) with an emphasis on III–V semiconductors including growth mechanisms, device fabrication and performance results. We first review the available nanowire growth methods followed by control of nanowire growth direction and crystal structure. Important device issues are reviewed, including optical absorption, carrier collection, strain accommodation, design for high efficiency, tunnel junctions, Ohmic contact formation, passivation and doping. Performance data of III–V nanowire cells and the primary challenges in nanowire PV are summarized. Many of the issues discussed here are also applicable to other nanowire devices such as photodetectors. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) Part of Focus Issue on “Semiconductor Nanowires” (Eds.: Chennupati Jagadish, Lutz Geelhaar, Silvija Gradecak) To overcome the limitations of current III–V multi‐junction solar photovoltaic (PV) cells, III–V nanowire‐based PV is being aggressively pursued by numerous groups. This article provides a comprehensive review of nanowire‐based PV covering all aspects of design for high power conversion efficiency including nanowire growth methods, growth direction, crystal structure, optical absorption, carrier collection, strain accommodation, tunnel junctions, Ohmic contact formation, passivation and doping. [ABSTRACT FROM AUTHOR]

In the fabrication of III-V semiconductor nanostructures for electronic and optoelectronic devices, techniques that are capable of removing material with monolayer precision are as important as material growth to achieve best device performances. A robust chemical treatment is demonstrated using sulfur (S)-oleylamine (OA) solution, which etches layer by layer in an inverse epitaxial fashion and simultaneously passivates the surface. The application of this process to push the limits of top-down nanofabrication is demonstrated by the realization of InP-based high optical quality nanowire arrays, with aspect ratios more than 50, and nanostructures with new topologies. The findings are relevant for other III-V semiconductors and have potential applications in III-V device technologies. [ABSTRACT FROM AUTHOR]

A modified form of detailed balance theory is applied to a tandem semiconductor solar cell formed from a vertical nanowire array on a planar substrate. The concept of an effective packing fraction is introduced to account for the light trapping properties of the nanowire array. Our model provides important design criteria for the nanowire-planar system, including the requirement for optimum performance of a lower energy nanowire bandgap than in an ideal all-planar tandem cell. Using realistic design parameters for a III-V nanowire-silicon bottom cell system we predict an efficiency enhancement of 22-27% relative to a planar silicon cell. [ABSTRACT FROM AUTHOR]

In this work, we present a systematic study of the optical reflectivity of GaAs nanowire arrays as a function of nanowire size, morphology, and arrangement on the substrate. To analyze the results, we extend a recently proposed model, which describes the nanowire mat as an ensemble of diffuse optical reflectors, by including the effective medium description and the low or zero-absorption regime. The modified model is very general and well accounts for the behavior of all the samples analyzed in this work, which covers a wide range of common situations: from the array of disordered parallel oriented and small nanowires (1.5-2 μm long and 30-50 nm large) to the randomly oriented wire array with non-uniform distribution of the sizes. The results that we report here for GaAs nanowires can be applied to any other type of semiconductor nanowires and provide useful insights to design more efficient solar cells. [ABSTRACT FROM AUTHOR]

Colloidal PbS quantum dots (QDs) have provoked a revolution in the field of optoelectronic devices owing to their low‐cost fabrication processing and excellent physical properties. Recently, the fabrication of nanostructured PbS QD photovoltaic (PV) devices based on zinc oxide (ZnO) nanowire array appears as an effective strategy for improving the overall device performance. Despite its potentially strong impact on the device performance, the role of nanowire areal density on photon absorption and exciton dynamics has not yet been studied and still remains unexplored. Here, for the first time, the areal density of ZnO nanowires is tuned through controlling the precursor concentration and its impact on PbS QD PV performance is studied. It is found that the device with optimized ZnO nanowire areal density yields significantly increased power conversion efficiency (PCE) (10.1% vs 8.5% of control nanowire‐based device) due to improved antireflection effect and reduced surface recombination states. To further improve the photovoltaic performance, the ZnO nanowire surface is treated with hydrogen plasma. Transient photovoltage (TPV) measurement reveals that this passivation process noticeably reduces the nonradiative charge recombination yielding a champion device with a remarkable PCE of 10.8%. [ABSTRACT FROM AUTHOR]