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Gallium phosphide exhibits a short diffusion length relative to its optical absorption length, and is thus a candidate for use in wire array geometries that allow light absorption to be decoupled from minority carrier collection. Herein is reported the photoanodic performance of heteroepitaxially grown gallium phosphide on planar and microwire-array Si substrates. The n-GaP|n-Si heterojunction results in a favorable conduction band alignment for electron collection in the silicon. A conformal electrochemical contact to the outer GaP layer is produced using the ferrocenium/ferrocene (Fc^+/Fc) redox couple in acetonitrile. Photovoltages of ∼750 mV under 1 sun illumination are observed and are attributed to the barrier formed at the (Fc^+/Fc)|n-GaP junction. The short-circuit current densities of the composite microwire-arrays are similar to those observed using single-crystal n-GaP photoelectrodes. Spectral response measurements along with a finite-difference-time-domain optical model indicate that the minority carrier diffusion length in the GaP is ∼80 nm. Solid-state current–voltage measurements show that shunting occurs through thin GaP layers that are present near the base of the microwire-arrays. The results provide guidance for further studies of 3D multi-junction photoelectrochemical cells.

Arrays of B-doped p-Si microwires, diffusion-doped with P to form a radial n+ emitter and subsequently coated with a 1.5-nm-thick discontinuous film of evaporated Pt, were used as photocathodes for H_2 evolution from water. These electrodes yielded thermodynamically based energy-conversion efficiencies >5% under 1 sun solar simulation, despite absorbing less than 50% of the above-band-gap incident photons. Analogous p-Si wire-array electrodes yielded efficiencies

In this paper a statistically significant study of 1096 individual GaN nanowire (NW) devices is presented. We have correlated the effects of changing growth parameters for hot-wall chemically-vapour-deposited (HW-CVD) NW sf abricated via the vapour–liquid–solid mechanism. We first describe an optical lithographic method for creating Ohmic contacts to NW field effect transistors with both top and bottom electrostatic gates to characterize carrier density and mobility. Multiprobe measurements show that carrier modulation occurs in the channel and is not a contact effect. We then show that NW fabrication runs with nominally identical growth parameters yield similar electrical results across sample populations of >50 devices. By systematically altering th eg rowth parameters we were able to decrease the average carrier concentration for these as-grown GaN NWs ∼10-fold, from 2.29 × 10 20 to 2.45 × 10 19 cm −3 ,a nd successfully elucidate the parameters that exert the strongest influence on wire quality. Furthermore, this study shows that nitrogen vacancies, and not oxygen impurities, are the dominant intrinsic dopant in HW-CVD GaN NWs.

Tandem Ga1-xInxP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Coupled full-field optical and device physics simulations of a Ga0.51In0.49P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped “buffer” layer between the bottom cell and tunnel junction is assumed to harbor a high density of lattice mismatch and heteroepitaxial defects. Under simulated AM1.5G illumination, the device structure explored in this work has a simulated efficiency of 23.84% with realistic top cell SRH lifetimes and surface recombination velocities. The relative insensitivity to surface recombination is likely due to optical generation further away from the free surfaces and interfaces of the device structure. To move towards realizing these device structures, GaP and Ga1-xInxP layers were grown heteroepitaxially with metalorganic chemical vapor deposition on Si microwire array substrates. The layer morphology and crystalline quality have been studied with scanning electron microscopy and transmission electron microscopy, and they provide a baseline for the growth and characterization of a full device stack.

Many proposed next-generation photovoltaic devices have complicated nano- and micro-structured architectures that are designed to simultaneously optimize carrier collection and light absorption. Characterization of the electrical properties of these highly structured materials can be challenging due to the difficulty of creating electrical contacts, as well as the need to decouple the properties of the contact from that of the semiconductor. Regenerative photoelectrochemistry is a powerful technique to characterize the electrical properties of such systems, providing a conformal liquid contact that can be ohmic or rectifying, depending on the system used. We demonstrate the use of the methyl viologen regenerative electrochemical system to characterize different stages of the fabrication of radial junction Si microwire (SiMW) solar cells. Photoelectrochemical characterization, combined with other more traditional measurements allows evaluation of how the different processing steps affect the device performance, without having to construct a fully integrated device. We describe the operating principle of this technique, and demonstrate that it can be applied to semiconductor materials with complex architectures.

Microwire solar cells have demonstrated promising optical and photovoltaic performance in arrays of single junction Si wires. Seeking higher efficiencies, we have numerically investigated III-V on Si 1−x Ge x architectures as candidates for tandem microwire photovoltaics via optical and electronic transport modeling. Optical modeling indicates that light trapping is an important design criterion. Absorption is more than doubled by the presence of Al 2 O 3 scattering particles around the wires, leading to high overall light collection despite low wire packing fraction. Texturing of the microwire outer surface, which was found to occur experimentally for GaP/Si microwires, is also shown to enhance absorption by over 50% relative to wires with smooth surfaces, allowing for the use of thinner layers. Finally, full optoelectronic simulations of GaAs on Ge structures revealed that current matching is attainable in these structures and that wire device efficiencies can approach those of planar cells.

Crystalline Si wires, grown by the vapor–liquid–solid (VLS) process, have emerged as promising candidate materials for lowcost, thin-film photovoltaics. Here, we demonstrate VLS-grown Si microwires that have suitable electrical properties for high-performance photovoltaic applications, including long minority-carrier diffusion lengths (L_n » 30 µm) and low surface recombination velocities (S « 70 cm·s^(-1)). Single-wire radial p–n junction solar cells were fabricated with amorphous silicon and silicon nitride surface coatings, achieving up to 9.0% apparent photovoltaic efficiency, and exhibiting up to ~600 mV open-circuit voltage with over 80% fill factor. Projective single-wire measurements and optoelectronic simulations suggest that large-area Si wire-array solar cells have the potential to exceed 17% energy-conversion efficiency, offering a promising route toward cost-effective crystalline Si photovoltaics.

Si microwire-array solar cells with Air Mass 1.5 Global conversion efficiencies of up to 7.9% have been fabricated using an active volume of Si equivalent to a 4 μm thick Si wafer. These solar cells exhibited open-circuit voltages of 500 mV, short-circuit current densities (J_(sc)) of up to 24 mA cm^(-2), and fill factors >65% and employed Al_2O_3 dielectric particles that scattered light incident in the space between the wires, a Ag back reflector that prevented the escape of incident illumination from the back surface of the solar cell, and an a-SiN_x:H passivation/anti-reflection layer. Wire-array solar cells without some or all of these design features were also fabricated to demonstrate the importance of the light-trapping elements in achieving a high J_(sc). Scanning photocurrent microscopy images of the microwire-array solar cells revealed that the higher J_(sc) of the most advanced cell design resulted from an increased absorption of light incident in the space between the wires. Spectral response measurements further revealed that solar cells with light-trapping elements exhibited improved red and infrared response, as compared to solar cells without light-trapping elements.

Si wire arrays have recently demonstrated their potential as photovoltaic devices [1–3]. Using these arrays as a base, we consider a next generation, multijunction wire array architecture consisting of Si wire arrays with a conformal GaN x P 1−x−y As y coating. Optical absorption and device physics simulations provide insight into the design of such devices. In particular, the simulations show that much of the solar spectrum can be absorbed as the angle of illumination is varied and that an appropriate choice of coating thickness and composition will lead to current matching conditions and hence provide a realistic path to high efficiencies. We have previously demonstrated high fidelity, high aspect ratio Si wire arrays grown by vapor-liquid-solid techniques, and we have now successfully grown conformal GaP coatings on these wires as a precursor to considering quaternary compound growth. Structural, optical, and electrical characterization of these GaP/Si wire array heterostructures, including x-ray diffraction, Hall measurements, and optical absorption of polymer-embedded wire arrays using an integrating sphere were performed. The GaP epilayers have high structural and electrical quality and the ability to absorb a significant amount of the solar spectrum, making them promising for future multijunction wire array solar cells.

Silicon wire arrays, though attractive materials for use in photovoltaics and as photocathodes for hydrogen generation, have to date exhibited poor performance. Using a copper-catalyzed, vapor-liquid-solid–growth process, SiCl_4 and BCl_3 were used to grow ordered arrays of crystalline p-type silicon (p-Si) microwires on p^+-Si(111) substrates. When these wire arrays were used as photocathodes in contact with an aqueous methyl viologen^(2+/+) electrolyte, energy-conversion efficiencies of up to 3% were observed for monochromatic 808-nanometer light at fluxes comparable to solar illumination, despite an external quantum yield at short circuit of only 0.2. Internal quantum yields were at least 0.7, demonstrating that the measured photocurrents were limited by light absorption in the wire arrays, which filled only 4% of the incident optical plane in our test devices. The inherent performance of these wires thus conceptually allows the development of efficient photovoltaic and photoelectrochemical energy-conversion devices based on a radial junction platform.

Solar cells based on arrays of CVD-grown Si nano- or micro-wires have attracted interest as potentially low-cost alternatives to conventional wafer-based Si photovoltaics [1–6], and single-wire solar cells have been reported with efficiencies of up to 3.4% [7]. We recently presented device physics simulations which predicted efficiencies exceeding 17%, based on experimentally observed diffusion lengths within our wires [8]. However, this model did not take into account the optical properties of a wire array device - in particular the inherently low packing fraction of wires within CVD-grown wire arrays, which might limit their ability to fully absorb incident sunlight. For this reason, we have combined a device physics model of Si wire solar cells with FDTD simulations of light absorption within wire arrays to investigate the potential photovoltaic efficiency of this cell geometry. We have found that even a sparsely packed array (14%) is expected to absorb moderate (66%) amounts of above-bandgap solar energy, yielding a simulated photovoltaic efficiency of 14.5%. Because the wire array comprises such a small volume of Si, the observed absorption represents an effective optical concentration, which enables greater operating voltages than previously predicted for Si wire array solar cells.

The use of silicon nanostructures in solar cells offers a number of benefits, such as the fact they can be used on flexible substrates. A silicon wire-array structure, containing reflecting nanoparticles for enhanced absorption, is now shown to achieve 96% peak absorption efficiency, capturing 85% of light with only 1% of the silicon used in comparable commercial cells. Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.

Solar cells based on arrays of CVD-grown Si nano- or micro-wires are being considered as a potentially low-cost route to implementing a vertical multijunction cell design via radial p-n junctions. This geometry has been predicted to enable efficiencies competitive with planar multicrystalline Si designs, while reducing the materials and processing costs of solar cell fabrication [1]. To further assess the potential efficiency of cells based on this design, we present here experimental measurements of minority carrier diffusion lengths and surface recombination rates within nanowires via fabrication and characterization of single-wire solar cell devices. Furthermore, we consider a potential Si wire array-based solar cell design, and present device physics modeling of single-wire photovoltaic efficiency. Based on experimentally observed diffusion lengths within our wires, we model a radial junction wire solar cell capable of 17% photovoltaic energy conversion efficiency.

Single-nanowire solar cells were created by forming rectifying junctions in electrically contacted vapor-liquid-solid-grown Si nanowires. The nanowires had diameters in the range of 200 nm to 1.5 microm. Dark and light current-voltage measurements were made under simulated Air Mass 1.5 global illumination. Photovoltaic spectral response measurements were also performed. Scanning photocurrent microscopy indicated that the Si nanowire devices had minority carrier diffusion lengths of approximately 2 microm. Assuming bulk-dominated recombination, this value corresponds to a minimum carrier lifetime of approximately 15 ns, or assuming surface-dominated recombination, to a maximum surface recombination velocity of approximately 1350 cm s(-1). The methods described herein comprise a valuable platform for measuring the properties of semiconductor nanowires, and are expected to be instrumental when designing an efficient macroscopic solar cell based on arrays of such nanostructures.

We report conformal, epitaxial growth of GaP layers on arrays of Si microwires. Silicon wires grown using chlorosilane chemical vapor deposition were coated with GaP grown by metal-organic chemical vapor deposition. The crystalline quality of conformal, epitaxial GaP/Si wire arrays was assessed by transmission electron microscopy and x-ray diffraction. Hall measurements and photoluminescence show p- and n-type doping with high electron mobility and bright optical emission. GaP pn homojunction diodes on planar reference samples show photovoltaic response with an open circuit voltage of 660 mV.

The effective electron minority-carrier diffusion length, L_(n,eff), for 2.0 µm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, L_(n,eff) was limited by surface recombination to a value of ≤ 0.7 µm. However, a value of L_(n,eff) = 10.5±1 µm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large L_(n,eff) values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes.

Indium nitride nanowires (NWs) grown by a catalyst-free, vapor-solid method are shown to be high-purity, single-crystal hexagonal wurtzite and intrinsic n type with uniform diameters that range from 70to150nm and lengths that vary between 3 and 30μm. Single NWs were fabricated into field-effect transistors and the electronic material parameters of the wires were extracted and are found to be identical to comparable bulk InN.