Physical Review B

Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diameter and the length of the top n-segment were critical for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nanometer-diameter nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 volt exceeds that of its planar counterpart, despite about 30 times higher surface-to-volume ratio of the nanowire cell.

https://science.sciencemag.org/content/sci/339/6123/1057.full.pdf

InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit

Porous anodic aluminum oxide: anodization and templated synthesis of functional nanostructures.

Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of "bottom-up" growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.

/pdf/25th-anniversary-article-semiconductor-nanowires-synthesis-4ccslz11ke.pdf

25th Anniversary Article: Semiconductor Nanowires – Synthesis, Characterization, and Applications

Light management is of great importance in photovoltaic cells, as it determines the fraction of incident light entering the device. An optimal p–n junction combined with optimal light absorption can lead to a solar cell efficiency above the Shockley–Queisser limit. Here, we show how this is possible by studying photocurrent generation for a single core–shell p–i–n junction GaAs nanowire solar cell grown on a silicon substrate. At 1 sun illumination, a short-circuit current of 180 mA cm –2 is obtained, which is more than one order of magnitude higher than that predicted from the Lambert–Beer law. The enhanced light absorption is shown to be due to a light-concentrating property of the standing nanowire, as shown by photocurrent maps of the device. The results imply new limits for the maximum efficiency obtainable with III–V based nanowire solar cells under 1 sun illumination.

/pdf/single-nanowire-solar-cells-beyond-the-shockley-queisser-200ib66540.pdf

Single-nanowire solar cells beyond the Shockley-Queisser limit

The mechanisms of Ga-assisted GaAs nanowires grown by molecular beam epitaxy are addressed. The axial and radial growth rates as a function of the Ga rate and As pressure indicate that on the opposite of what is observed in thin film epitaxy, the growth rate of the nanowires is arsenic limited. As a consequence, the axial growth rate of the wires can be controlled by the ${\mathrm{As}}_{4}$ pressure. Additionally, due to the small ${\mathrm{As}}_{4}$ pressure leading to nanowire growth, the deposition on the facets is very slow, leading to a much lower radial growth rate. Finally, we present a model that is able to accurately describe the presented observations and predicts a maximum length of nontapered nanowires of $40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$.

Ga-assisted catalyst-free growth mechanism of GaAs nanowires by molecular beam epitaxy

Gallium arsenide p-i-n radial junctions were fabricated by molecular beam epitaxy. The current-voltage characteristics of single nanowires were measured in the dark and under various illumination conditions including 1.5 AM. The total efficiency was 4.5%. Spatially resolved and power dependent photocurrent measurements indicate that the p-i-n junction is homogeneous along the nanowire. Electroluminescence measurements show an emission peak at about 1.4 eV, further corroborating the good quality of the nanowire. These results constitute an important progress for the use of nanowires in photovoltaic applications.

/pdf/gallium-arsenide-p-i-n-radial-structures-for-photovoltaic-22n879q1pq.pdf

Gallium arsenide p-i-n radial structures for photovoltaic applications

Molecular beam epitaxy Ga-assisted synthesis of GaAs nanowires is demonstrated. The nucleation and growth are seen to be related to the presence of a SiO2 layer previously deposited on the GaAs wafer. The interaction of the reactive gallium with the SiO2 pinholes induces the formation of nanocraters, found to be the key for the nucleation of the nanowires. With SiO2 thicknesses up to 30nm, nanocraters reach the underlying substrate, resulting into a preferential growth orientation of the nanowires. Possibly related to the formation of nanocraters, we observe an incubation period of 258s before the nanowires growth is initiated.

/pdf/nucleation-mechanism-of-gallium-assisted-molecular-beam-2lel2u1kdu.pdf

Nucleation mechanism of gallium-assisted molecular beam epitaxy growth of gallium arsenide nanowires

The effect of surfaces on the optical properties of GaAs nanowires is evidenced by comparing nanowires with or without an AlGaAs capping shell as a function of the diameter. We find that the optical properties of unpassivated nanowires are governed by Fermi-level pinning, whereas, the optical properties of passivated nanowires are mainly governed by surface recombinations. Finally, we measure a surface recombination velocity of 3 x 10(3) cm s(-1) one order of magnitude lower than values previously reported for (110) GaAs surfaces. These results will serve as guidance for the application of nanowires in solar cell and light emitting devices.

Impact of surfaces on the optical properties of GaAs nanowires

The dielectric functions of amorphous and polymorphous silicon films prepared under various plasma conditions have been deduced from UV-visible spectroscopic ellipsometry measurements. The measured spectra have been firstsimulated by the use of the Tauc-Lorentz dispersion model and then the compositions of the films have been obtained by the use of the tetrahedron model combined with the Bruggeman effective medium approximation. This approach allows us to determine the hydrogen content, the crystalline fraction, and the void fraction of the films. This is particularly important in the case of polymorphous films in which the low crystalline fraction (below 10%) can only be detected when an accurate description of the effects of hydrogen on the dielectric function through the tetrahedron model is considered. The hydrogen content and film porosity deduced from the analysis of the spectroscopic ellipsometry measurements are in excellent agreement with the hydrogen content and film density deduced from combined elastic recoil detection analysis and Rutherford backscattering spectroscopy measurements. Moreover, despite their high hydrogen content (∼15%-20%) with respect to hydrogenated amorphous silicon films deposited at the same temperature (8%), polymorphous silicon films have a high density, which is related to their very low void fraction.

A. Fontcuberta i Morral

Papers

Ga-assisted catalyst-free growth mechanism of GaAs nanowires by molecular beam epitaxy

Gallium arsenide p-i-n radial structures for photovoltaic applications

Nucleation mechanism of gallium-assisted molecular beam epitaxy growth of gallium arsenide nanowires

Impact of surfaces on the optical properties of GaAs nanowires

Structure and hydrogen content of polymorphous silicon thin films studied by spectroscopic ellipsometry and nuclear measurements