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We report ultrathin photonic metagratings where anisotropic scattering is designed to achieve self-stabilizing dynamics in a collimated beam of laser light. Stability necessitates a delicate balance between all scattered orders of light, which we demonstrate in a monolithic material platform suited for efficient propulsion in space. Our suspended structures are fabricated in silicon nitride membranes, which is a promising lightsail material candidate due to its wafer-level scalability and favorable mechanical and optical properties. Lightsail prototype designs are optically characterized by angle-resolved photocurrent measurements of the intensities and angles of the asymmetric ±1 diffraction orders. We infer the optically induced forces and torques from refracted and reflected light measurements and show that these are restoring along one axis by providing them as input functions to numerical simulations of lightsail dynamics. Our experimental results represent a first step toward full dynamical verification of realistic lightsail designs and pave the way for realization of stable beam-riding lightsails composed of ultrathin dielectric membranes.

Perovskite photovoltaics (PVs) are under intensive development for promise in terrestrial energy production. Soon, the community will find out how much of that promise may become reality. Perovskites also open new opportunities for lower cost space power. However, radiation tolerance of space environments requires appropriate analysis of relevant devices irradiated under representative radiation conditions. We present guidelines designed to rigorously test the radiation tolerance of perovskite PVs. We review radiation conditions in common orbits, calculate nonionizing and ionizing energy losses (NIEL and IEL) for perovskites, and prioritize proton radiation for effective nuclear interactions. Low-energy protons (0.05–0.15 MeV) create a representative uniform damage profile, whereas higher energy protons (commonly used in ground-based evaluation) require significantly higher fluence to accumulate the equivalent displacement damage dose due to lower scattering probability. Furthermore, high-energy protons may “heal” devices through increased electronic ionization. These procedural guidelines differ from those used to test conventional semiconductors.

We present a detailed design treatment for a concentrating photovoltaic mini module subsystem with a specific power of up to 4.1 kW/kg for integration into a space solar power system. Concentrating designs are required to achieve specific power over 1 kW/kg with current high-efficiency III-V multijunction solar cells. The 15 sun, linear concentration concept detailed here reduces the system mass by replacing cell and radiation shield area with ultralight carbon fiber reinforced polymer (CFRP) optics. Reducing the cell size to 1 mm width as well as careful optimization of cell architecture and CFRP material and thickness are critical for maintaining cell temperatures under 100 C despite the concentration. We also describe ultralight multilayer optical coatings to increase the thermal emissivity of the concentrator surfaces and enhance radiative transfer for cell cooling, which is a critical technological component of the total system design.

Radiation hard thin-film solar cell technologies are necessary in order to achieve a step forward in the specific power of solar arrays for space applications. In this article, we analyze the degradation of nanowire (NW) solar cells under high energy particles. GaAs NW solar cells have been irradiated with protons of 100 and 350 keV at different fluences. The radiation hardness of the NW solar cells in all the cases is remarkable in comparison with GaAs planar solar cells and prior literature. Design guidelines to optimize the specific power of NW solar cells for space applications by jointly increasing their efficiency and radiation hardness are presented.

Directed propulsion of lightsails to relativistic speeds by laser radiation pressure demands for passive stabilization, which can be achieved with subwavelength metasurface elements. Our tailored nanophotonic design based on silicon nitride is predicted to give rise to restoring forces and torques upon small perturbations in its dynamics. By measuring the angles and intensities of the diffracted orders, we infer the light-induced pressures on our lightsail prototype. Using Michelson interferometry and laser deflection spectroscopy, we can directly observe the dynamical stability of spring-supported suspended membrane designs. Our results pave the way for lab-scale experiments on optical manipulation of microscopic lightsails.

Phased arrays are multiple antenna systems capable of forming and steering beams electronically using constructive and destructive interference between sources. They are employed extensively in radar and communication systems but are typically rigid, bulky and heavy, which limits their use in compact or portable devices and systems. Here, we report a scalable phased array system that is both lightweight and flexible. The array architecture consists of a self-monitoring complementary metal–oxide–semiconductor-based integrated circuit, which is responsible for generating multiple independent phase- and amplitude-controlled signal channels, combined with flexible and collapsible radiating structures. The modular platform, which can be collapsed, rolled and folded, is capable of operating standalone or as a subarray in a larger-scale flexible phased array system. To illustrate the capabilities of the approach, we created a 4 × 4 flexible phased array tile operating at 9.4–10.4 GHz, with a low areal mass density of 0.1 g cm−2. We also created a flexible phased array prototype that is powered by photovoltaic cells and intended for use in a wireless space-based solar power transfer array. By combining a CMOS-based integrated circuit with flexible and collapsible radiating structures, a scalable phased array architecture can be fabricated that has an areal mass density of only 0.1 g cm−2.

The emergence of perovskite solar cells has brought increased attention to the importance of stability testing and continuous maximum-power-point tracking (MPPT) for the characterization of experimental solar cells, even at early stages of R&D. As many experimental cells are often fabricated in multitude, e.g., as multiple ‘pixels’ on each substrate, it is desirable to have a large number of measurement instruments, each of which can be committed for long periods of time, ranging from days to weeks or longer, to characterize the performance of each individual research device. Here, we present a low-cost measurement circuit capable of performing and recording simultaneous measurements on up to 12 devices, including MPPT, I–V sweeps, fixed-voltage, and fixed-current operation. The circuit is suitable for cells with ISC up to ~150 mA and VOC up to ~3 V. Complete schematics, PCB layouts, and firmware are available online, offered for use and improvement by the greater PV research community.

Next generation space solar cells are required to be efficient, light and low cost. In order to GaAs-based devices keep playing a significant role in space the radiation performance needs to be increased to eliminate the required shielding and the cost needs to be dramatically reduced. GaAs nanophotonic structures have the potential to fulfill both requirements. In order to evaluate the radiation performance of different GaAs architectures a simulation platform has been developed and corroborated with experimental data reported in the literature. Exemplary cases of the irradiation-induced degradation in planar and nanowire architectures are presented.

We report a design for a microscopic lightsail prototype that allows for passive stabilization in the radiation-pressure dominated regime. Stable dynamics of our silicon nitride structure are predicted for initial tilts of up to ±10°.

We develop a new technique for transferring metal contacts to create ultrathin solar cells from 2D materials., Two-dimensional transition metal dichalcogenides are promising candidates for ultrathin optoelectronic devices due to their high absorption coefficients and intrinsically passivated surfaces. To maintain these near-perfect surfaces, recent research has focused on fabricating contacts that limit Fermi-level pinning at the metal-semiconductor interface. Here, we develop a new, simple procedure for transferring metal contacts that does not require aligned lithography. Using this technique, we fabricate vertical Schottky-junction WS2 solar cells, with Ag and Au as asymmetric work function contacts. Under laser illumination, we observe rectifying behavior and open-circuit voltage above 500 mV in devices with transferred contacts, in contrast to resistive behavior and open-circuit voltage below 15 mV in devices with evaporated contacts. One-sun measurements and device simulation results indicate that this metal transfer process could enable high specific power vertical Schottky-junction transition metal dichalcogenide photovoltaics, and we anticipate that this technique will lead to advances for two-dimensional devices more broadly.

Space power systems require photovoltaics that are lightweight, efficient, reliable, and capable of operating for years or decades in space environment. Current solar panels use planar multijunction, III-V based solar cells with very high efficiency, but their specific power (power to weight ratio) is limited by the added mass of radiation shielding (e.g., coverglass) required to protect the cells from the high-energy particle radiation that occurs in space. Here, we demonstrate that III-V nanowire-array solar cells have dramatically superior radiation performance relative to planar solar cell designs and show this for multiple cell geometries and materials, including GaAs and InP. Nanowire cells exhibit damage thresholds ranging from ∼10-40 times higher than planar control solar cells when subjected to irradiation by 100-350 keV protons and 1 MeV electrons. Using Monte Carlo simulations, we show that this improvement is due in part to a reduction in the displacement density within the wires arising from their nanoscale dimensions. Radiation tolerance, combined with the efficient optical absorption and the improving performance of nanowire photovoltaics, indicates that nanowire arrays could provide a pathway to realize high-specific-power, substrate-free, III-V space solar cells with substantially reduced shielding requirements. More broadly, the exceptional reduction in radiation damage suggests that nanowire architectures may be useful in improving the radiation tolerance of other electronic and optoelectronic devices.

Space-based solar power systems (SSPS) offer a renewable source of energy generation that features higher power production than terrestrial photovoltaics (PV) systems with visibly unobstructed access to direct sunlight. Because of the high launch and deployment costs for SSPS, PV arrays intended for such applications are designed so as to maximize the specific power (W/kg) of the array. Here we present a first analysis for a luminescent solar concentrator (LSC) module designed for SSPS. LSC devices absorb and trap incident sunlight in a planar dielectric waveguide through photon downshifting via absorption and Stokes-shifted emission from embedded luminophores. Emitted photons are trapped within the waveguide and concentrated onto small area solar cells, spectrally matched to the luminophore band edge. Here we investigate the influence of luminophore material and concentration factor on the LSC specific power. We develop an LSC system design that can achieve up to 1.8 kW/kg.

We have developed superstrates for perovskite solar cells that feature increased transparency and conductivity due to the incorporation of effectively transparent contacts (ETCs). They increase the short-circuit current density by more than 1 mA/cm2 compared to standard indium tin oxide (ITO) on glass superstrates. These ETC superstrates are composed of sodalime glass with a thin $( \sim 40 \mu \mathrm {m})$ layer of polydimethylsiloxane (PDMS) that features triangular cross-section microscale grooves, which are infilled with a conductive silver ink and subsequently coated by a thin $( \sim 30$ nm) ITO layer such that high lateral conductivity $( < 5 \Omega /$sq) is achieved without altering the surface properties of standard perovskite superstrates.

We report the design, fabrication, and characterization of ultralight highly emissive structures with a record-low mass per area that emit thermal radiation efficiently over a broad spectral (2 to 30 microns) and angular (0-60°) range. The structures comprise one to three pairs of alternating metallic and dielectric thin films and have measured effective 300 K hemispherical emissivity of 0.7 to 0.9 (inferred from angular measurements which cover a bandwidth corresponding to 88% of 300K blackbody power). To our knowledge, these micron-scale-thickness structures, are the lightest reported optical coatings with comparable infrared emissivity. The superior optical properties, together with their mechanical flexibility, low outgassing, and low areal mass, suggest that these coatings are candidates for thermal management in applications demanding of ultralight flexible structures, including aerospace applications, ultralight photovoltaics, lightweight flexible electronics, and textiles for thermal insulation.

We have fabricated a functional prototype of an ultralight power converter tile; a modular building block for a space-based solar power system. The tile is $\sim 10\mathrm \times 15$ cm in area, and weighs $\sim 1.5$ kg/$\text{m}^{\mathbf {2}}$. It comprises a photovoltaic (PV) solar energy collector, a radio-frequency (RF) power converter, and an array of transmission antennas. The PV collector subassembly utilizes $\sim 15\mathrm {x}$, 1D parabolic trough reflective concentrators with triple-junction (3J) solar cells. It has areal mass of $\sim 0.8$ kg/$\textbf{m}^{\mathbf {2}}$, 74% optical efficiency, and a peak specific power of $\sim 230\mathrm {W} /$kg. We demonstrated wireless power transmission over a distance of $\sim 50$ cm in our lab. Analysis of the sources of mass and inefficiency suggest a path towards achieving dramatically higher performance with future designs.

We demonstrate the development of a prototype lightweight (1.5 kg/m^3) tile structure capable of photovoltaic solar power capture, conversion to radio frequency power, and transmission through antennas. This modular tile can be repeated over an arbitrary area to forma large aperture which could be placed in orbit to collect sunlight and transmit electricity to any location. Prototype design is described and validated through finite element analysis, and high-precision ultra-light component manufacture and robust assembly are described.

Concentrator photovoltaic systems can provide supplementary shielding against high energy particles. In this paper we compare the radiation environment that the same solar cell would experience in a flat-plate module versus in a parabolic mirror concentrator system. We have observed that the shielding provided by the concentrator system is remarkable. In order to obtain an accurate prediction of the overall shield needed in our concentrator system triple-junction space solar cells have been irradiated on the edge with 350-keV protons at a fluence of 1012P + cm−2. A mild degradation of the open circuit voltage was measured (~70 mV).

Lightweight parabolic mirrors for solar concentrators have been fabricated using carbon fiber reinforced polymer (CFRP) and a nanometer scale optical surface smoothing technique. The smoothing technique improved the surface roughness of the CFRP surface from ~3 {\mu}m root mean square (RMS) for as-cast to ~5 nm RMS after smoothing. The surfaces were then coated with metal, which retained the sub-wavelength surface roughness, to produce a high-quality specular reflector. The mirrors were tested in an 11x geometrical concentrator configuration and achieved an optical efficiency of 78% under an AM0 solar simulator. With further development, lightweight CFRP mirrors will enable dramatic improvements in the specific power, power per unit mass, achievable for concentrated photovoltaics in space., Comment: IEEE Photovoltaic Specialist Conference (PVSC), DC, USA, 2017

We report the design, fabrication and characterization of ultrathin metasurfaces that exhibit wideband 300 K thermal emissivity. The emissive behavior of these structures is almost independent of the emission angle. Our ultralight subwavelength-thickness metasurfaces can be fabricated relatively easily and are excellent candidates for radiative cooling in space applications.

We report the effects of high energy electron and proton radiation on perovskite solar cells. For irradiation with 1 MeV electrons, at fluence from 1012 to 1016 cm−2, there are no significant changes in the morphology and the crystal phase in the perovskites, and the perovskite solar cells show only slight degradation in photovoltaic performance and spectral response. The results from Monte Carlo simulations show most of the electrons completely penetrate through all of the layers of solar cells with little scattering. In addition, 50 ke V proton radiation with fluence of 1012−2 has no significant impact on the open-circuit voltage or short-circuit current, and degrades only the fill factor. We have further found that the fill factor can be restored with a vacuum annealing process. The results suggest that perovskites have superior electron and proton radiation tolerance, and thus hold particular promise for space applications.

The Space Solar Power Initiative (SSPI) seeks to enable reliable, cost-effective baseload power generation from large-scale solar power stations in space. We propose an ultralight, modular power station, having specific power in the range of 1–10 kW/kg for the photovoltaic (PV) collection subsystem. The building block of the power station is the ‘tile,’ a self-contained element that performs PV energy collection, conversion to radio frequency (RF), and transmission to earth. To minimize PV mass, we select a 1D, 10-20X parabolic trough concentrator geometry, which provides cooling and radiation shielding for the cells, and which folds flat for deployment. Here, we discuss the design, fabrication, and testing of the initial PV tile prototypes.

Silicon microwire arrays have recently demonstrated their potential for low-cost, high-efficiency photovoltaics and photoelectrochemical fuel generation. A remaining challenge to making this technology commercially viable is scaling up of microwire-array growth. We discuss here a technique for vapor–liquid–solid growth of microwire arrays on the scale of six-inch wafers using a cold-wall radio-frequency heated chemical vapor deposition furnace, enabling fairly uniform growth over large areas with rapid cycle time and improved run-to-run reproducibility. We have also developed a technique to embed these large-area wire arrays in polymer and to peel them intact from the growth substrate, which could enable lightweight, flexible solar cells with efficiencies as high as multicrystalline Si solar cells. We characterize these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structure and fidelity, and we test their energy-conversion properties using a methyl viologen (MV $^{2+/+}$ ) liquid junction contact in a photoelectrochemical cell. Initial photoelectrochemical conversion efficiencies suggest that the material quality of these microwire arrays is similar to smaller (∼1 cm $^2$ ) wire arrays that we have grown in the past, indicating that this technique is a viable way to scale up microwire-array devices.

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

Arrays of oriented, crystalline Si wires are transferred into flexible, transparent polymer films. The polymer-supported Si wire arrays in liquid-junction photoelectrochemical cells yield current-potential behavior similar to the Si wires attached to the brittle growth substrate. These systems offer the potential for attaining high solar energy-conversion efficiencies using modest diffusion length, readily grown, crystalline Si in a flexible, processable form.

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.

Silicon microwire arrays have recently demonstrated their potential for low cost, high efficiency photovoltaics. These high aspect ratio, radial junction wire arrays allow for the absorption of nearly all the incident sunlight while enabling efficient carrier extraction in the radial direction. One of the remaining challenges to make this technology commercially viable is scaling up of the microwire array growth. We discuss here a technique we have developed for vapor liquid solid growth of microwire arrays over full six-inch wafers using a cold-wall RF-heated chemical vapor deposition furnace. This geometry allows for fairly uniform growth over large areas, rapid cycle time, and improved run-to-run reproducibility. We have studied these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structural fidelity and uniformity. We have also developed a technique to embed these large-area arrays in polymer and peel them off the substrate, which could enable lightweight, flexible solar cells with efficiencies as high as crystalline Si solar cells. We have tested the energy conversion properties of these microwire array samples grown using a liquid junction contact and a photoelectrochemical cell. Initial efficiencies measured in this way suggest that the material quality of these microwire arrays is similar to earlier small-area wire arrays that we have grown, meaning that this technique is a viable way to scale up microwire array devices.

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.

Columnar-grained polycrystalline silicon films deposited at low temperatures are promising materials for use in thin-film photovoltaics. We study the effects of recombination at grain boundaries, bulk intragranular recombination, grain size, and doping in such structures with two-dimensional device physics simulations, explicitly modeling the full statistics and electrostatics of traps at the grain boundary. We characterize the transition from grain-boundary-limited to bulk-lifetime-limited performance as a function of intergranular defect density and find that higher bulk lifetimes amplify grain boundary recombination effects in the intermediate regime of this transition. However, longer bulk lifetimes ultimately yield higher efficiencies. Additionally, heavier base doping is found to make performance less sensitive to grain boundary defect density.

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.

Silicon Microwires as Photocathodes Solar hydrogen generation will require the development of photocathodes with high surface area, durability, and efficiency. Silicon microwire arrays, which allow for greater light penetration, could achieve this goal if the carrier mobilities are sufficiently high so that surface reactions occur before charges recombine. Boettcher et al. (p. 185 ) report the electronic properties on positively doped silicon microwire arrays that were grown with copper catalysts and used in a methyl viologen redox system. Although equivalent efficiencies for normal solar fluxes were only 2 to 3%, the high internal efficiencies and low use of the available optical flux suggest that further improvements are possible.

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.

Knowledge of the catalyst concentration within vapor-liquid-solid (VLS) grown semiconductor wires is needed in order to assess potential limits to electrical and optical device performance imposed by the VLS growth mechanism. We report herein the use of secondary ion mass spectrometry to characterize the Au catalyst concentration within individual, VLS-grown, Si wires. For Si wires grown by chemical vapor deposition from SiCl 4 at 1000 degrees C, an upper limit on the bulk Au concentration was observed to be 1.7 x 10(16) atoms/cm(3), similar to the thermodynamic equilibrium concentration at the growth temperature. However, a higher concentration of Au was observed on the sidewalls of the wires.

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.

Radial pn junctions are of interest in photovoltaics because of their potential to reduce the materials costs associated with cell fabrication. However, devices fabricated to date based on Au-catalyzed vapor-liquid-solid growth have suffered from low open-circuit voltages (to our knowledge the highest reports are 260 mV in the solid state and 389 mV in solid-liquid junctions). Herein we report on the potential of low-cost catalysts such as Cu and Ni to fabricate Si wire arrays with potentially higher minority-carrier lifetimes than is possible with a Au catalyst, as well as on the use of reactive ion etching to fabricate high-purity analogs to vapor-liquid-solid grown arrays.

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.

In an effort to develop low-cost solar energy conversion techniques, high uniformity vertically oriented silicon wire arrays have been fabricated. These arrays, which allow for radial diffusion of minority charge carriers, have been measured in a photoelectrochemical cell. Large photovoltages (∼400 mV) have been measured, and these values are significantly greater than those obtained from the substrate alone. Additionally, the wire array samples displayed much higher current densities than the underlying substrate, demonstrating that significant energy conversion was occurring due to the absorption and charge-carrier transport in the vertically aligned Si wires. This method therefore represents a step toward the use of collection-limited semiconductor materials in a wire array format in macroscopic solar cell devices.

Arrays of n-Si microwires have to date exhibited low efficiencies when measured as photoanodes in contact with a 1-1′-dimethylferrocene (Me_2Fc^(+/0))–CH_3OH solution. Using high-purity Au or Cu catalysts, arrays of crystalline Si microwires were grown by a vapor-liquid-solid process without dopants, which produced wires with electronically active dopant concentrations of 1 × 10^(13) cm^(−3). When measured as photoanodes in contact with a Me_2Fc^(+/0)–CH_3OH solution, the lightly doped Si microwire arrays exhibited greatly increased fill factors and efficiencies as compared to n-Si microwires grown previously with a lower purity Au catalyst. In particular, the Cu-catalyzed Si microwire array photoanodes exhibited open-circuit voltages of ~0.44 V, carrier-collection efficiencies exceeding ~0.75, and an energy-conversion efficiency of 1.4% under simulated air mass 1.5 G illumination. Lightly doped Cu-catalyzed Si microwire array photoanodes have thus demonstrated performance that is comparable to that of optimally doped p-type Si microwire array photocathodes in photoelectrochemical cells.

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.

Arrays of vertically oriented Si wires with diameters of 1.5 µm and lengths of up to 75 µm were grown over areas >1 cm^2 by photolithographically patterning an oxide buffer layer, followed by vapor-liquid-solid growth with either Au or Cu as the growth catalyst. The pattern fidelity depended critically on the presence of the oxide layer, which prevented migration of the catalyst on the surface during annealing and in the early stages of wire growth. These arrays can be used as the absorber material in novel photovoltaic architectures and potentially in photonic crystals in which large areas are needed.