Lieber. 2012. Tuning light absorption in core/shell silicon nanowire photovoltaic devices through morphological design. Nano Letters 12(9): 4971-4976.Published Version

Abstract:Over the past decade extensive studies of single semiconductor nanowire and nanowire array photovoltaic devices have explored the potential of these materials as platforms for a new generation of efficient and cost-effective solar cells. This feature review discusses strategies for implementation of semiconductor nanowires in solar energy applications, including advances in complex nanowire synthesis and characterization, fundamental insights from characterization of devices, utilization and control of the unique optical properties of nanowires, and new strategies for assembly and scaling of nanowires into diverse arrays that serve as a new paradigm for advanced solar cells. 2 Table of Contents Entry:Advanced synthetic control over the electronic and optical properties of semiconductor nanowires enables testing new paradigms for advanced solar cells. Broader Context Box:The solar power received by the earth dwarfs global power demands by several orders-ofmagnitude. Photovoltaics convert light to electrical energy and have the potential to partially replace current energy technologies that rely on carbon-based fuels. At present, however, a lack of infrastructure and the high costs of photovoltaics prevent this. New ideas and materials are being explored to develop next-generation solar cells that could operate more efficiently and cheaply. Nanowires have emerged as one promising platform to explore such new concepts.Their small dimensions allow for efficient charge separation and light absorption properties unique as compared to bulk materials. Furthermore, the synthesis and fabrication of nanowire devices differs significantly from traditional wafer-based technologies, thus presenting new opportunities such as use of less abundant materials or cheaper substrates. Here, we discuss the benefits and remaining challenges of nanowires for PV and review progress towards understanding and optimizing the electrical and optical performance of nanowire devices. We focus on single nanowire studies that can define limits for what is achievable when multiple nanowires are assembled. Challenges and initial progress towards scaling are presented, and, for the first time, we articulate unique capabilities of solar cells derived from multiple, distinct NWs. 6

We report that > 80% of the photons generated inside a photonic crystal slab resonator can be funneled within a small divergence angle of ±30• . The far-field radiation properties of a photonic crystal slab resonant mode are modified by tuning the cavity geometry and by placing a reflector below the cavity. The former method directly shapes the near-field distribution so as to achieve directional and linearly-polarized far-field patterns. The latter modification takes advantage of the interference effect between the original waves and the reflected waves to enhance the energydirectionality. We find that, regardless of the slab thickness, the optimum distance between the slab and the reflector closely equals one wavelength of the resonance under consideration. We have also discussed an efficient far-field simulation algorithm based on the finite-difference time-domain method and the near-to far-field transformation.

Recent investigations of semiconductor nanowires have provided strong evidence for enhanced light absorption, which has been attributed to nanowire structures functioning as optical cavities. Precise synthetic control of nanowire parameters including chemical composition and morphology has also led to dramatic modulation of absorption properties. Here we report finite-difference time-domain (FDTD) simulations for silicon (Si) nanowire cavities to elucidate the key factors that determine enhanced light absorption. The FDTD simulations revealed that a crystalline Si nanowire with an embedded 20-nm-thick amorphous Si shell yields

The nano-imprint lithography method was employed to incorporate wide-area (375 x 330 mum(2)) photonic-crystal (PC) patterns onto the top surface of GaN-based LEDs. When the 280-nm-thick p-GaN was partly etched to ~140 nm, the maximal extraction-efficiency was observed without deteriorating electrical properties. After epoxy encapsulation, the light output of the PC LED was enhanced by 25% in comparison to the standard LED without pattern, at a standard current of 20 mA. By three-dimensional finite-difference time-domain method, we found that the extraction efficiency of the LED tends to be saturated as the etch-depth in the GaN epitaxial-layer becomes larger than the wavelength of the guided modes.

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Enhanced synthetic control of the morphology, crystal structure, and composition of nanostructures can drive advances in nanoscale devices. Axial and radial semiconductor nanowires are examples of nanostructures with one and two structural degrees of freedom, respectively, and their synthetically tuned and modulated properties have led to advances in nanotransistor, nanophotonic, and thermoelectric devices. Similarly, developing methods that allow for synthetic control of greater than two degrees of freedom could enable new opportunities for functional nanostructures. Here we demonstrate the first regioselective nanowire shell synthesis in studies of Ge and Si growth on faceted Si nanowire surfaces. The selectively deposited Ge is crystalline and its facet position can be synthetically controlled in situ. We use this synthesis to prepare electrically-addressable nanocavities into which solution soluble species such as Au nanoparticles can be incorporated. The method furnishes multi-component nanostructures with unique photonic properties and presents a more sophisticated nanodevice platform for future applications in catalysis and photodetection.Semiconductor nanowires (NWs) represent a diverse class of nanomaterials whose synthetically-tunable structural, electronic, and optical properties 1-3 have enabled active nanodevices including high-performance field-effect transistors, 4 ultra-sensitive biological probes, 5-7 and solar cells and photonic devices with tunable optical spectra. [8][9][10][11][12] NWs can be classified according to the number of degrees of freedom (DoF) they possess, which represent fundamental physical coordinates along which their structure can be manipulated. Axial and radial (core/shell) modulated NWs have 1 and 2 DoF, respectively, and have been extensively studied and characterized. 2,[13][14][15][16][17][18][19] Nevertheless, the properties of nanostructures possessing greater complexity and anisotropy have not been determined.A nanostructure with 3 DoF and higher can be realized by breaking the rotational symmetry of conventional radial shell growth ( Figure 1A). A high-resolution scanning electron micrograph (SEM) of a faceted core/shell Si NW ( Figure 1B) reveals well-defined surfaces that were previously indexed 9 as {111}, {011}, and {113}. NWs with this same morphology and set of surface facets serve as the faceted templates from which all subsequent nanostructures in this study are grown. Following chemical vapor deposition (CVD) synthesis of the SiNW templates, 9 introduction of GeH 4 and H 2 at lower tem- perature and pressure into the same reactor (Supporting Information) yields a new product featuring selective material deposition on the {111} and {011} Si surface facets ( Figure 1B). Energy dispersive x-ray spectroscopy (EDS) performed on the nanostructure ( Figure 1C) confirms the elemental identity of the deposited material as Ge and reveals that facet selectivity is preserved along the length of the nanostructure. A planview transmission electron micrograph (...

Radiative coolers spectrally tailored at mid-infrared wavelengths (5−30 μm) achieve passive and efficient heat dissipation, thereby working together with existing conduction and convection channels. However, radiative coolers developed to date are structurally too complex to be scalable, too thick for monolithic integration, visibly opaque or reflective, or thermally unstable, particularly when integrated into optoelectronic devices. Here, we report wafer-scale, submicron-thick, on-chip radiative coolers designed for high-temperature concentrated solar energy devices. A hexagonally arranged SiO 2 /AlO x (500/300 nm) doubleshell-covered array on a wafer exhibited an emissivity larger than 0.8 at omnidirectional incidence via optical-resonance coupled photon-tunneling effects, that is, uniform absorption distribution and reduced surface reflectivity. Daytime experiments showed that the double-shell hollow cavity film lowered the Si wafer temperature by 10 °C at one-sun intensity, concurrently boosting the Si absorptivity by 19% compared to a bare Si wafer. To reveal the feasibility of applying our developed film to concentrated solar energy devices, we performed concentrated solar simulator experiments at various illumination intensities up to twenty-sun. The radiative cooling performance became progressively significant by increasing the illumination intensity; at twenty-sun intensity, the hollowcavity film yielded a 31 °C temperature drop when the Si wafer was heated at 200 °C. These economically viable, on-chip radiative coolers will help overcome the saturated quantum efficiencies of current optoelectronic devices that are fundamentally limited by thermal effects.