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All-dielectric nanoarrays have strong electromagnetic resonances with various interesting applications and are tuned by adjusting their geometrical parameters. However, their optical properties are permanently encoded during fabrication. This study presents robust dynamically tunable all-dielectric nanoresonators for controllable, reversible, and reproducible color filtering. Our design uses an array of TiO2 nanodiscs embedded in a transparent, stretchable polydimethylsiloxane (PDMS) membrane and exhibits a narrow spectral response due to Mie magnetic and electric dipole resonances hybridized with the TiO2 nanodiscs lattice modes. By mechanically stretching the PDMS membrane, the pitch of the TiO2 nanodiscs was increased and the spectral location of the resonances was altered. Additionally, an optically asymmetric structure was fabricated by partially embedding TiO2 nanodiscs in PDMS. Thus, the magnitude of the Rayleigh anomaly diffraction, which could interrupt the dipole resonances, was reduced. Our design has sharp, frequency-tunable resonances in the visible spectrum, and we demonstrated dynamic tunability by stretching the metasurfaces.

Flexible solid-state Zn–air batteries with CoS1−δ/SnS2−δ and CBCs demonstrate a high energy density of 256 Wh kgcell−1 including all active and inactive components, high operating rates of 50 to 100 mA cm−2, and a temperature range of +80 to −40 °C.

Structural optimization of microwire arrays is important for the successful demonstration of the practical feasibility of radial junction crystalline silicon (c-Si) solar cells. In this study, we investigated an optimized design of tapered microwire (TMW) arrays to maximize the light absorption of c-Si solar cells, while minimizing the surface recombination, for simultaneously improving the open-circuit voltage and short-circuit current density (Jsc). Finite-difference time-domain simulations confirmed that controlling the spacing between the TMWs at the nanometer scale is more effective for increasing the light absorption than increasing the TMW length. The photogenerated current of a c-Si TMW array with a 200 nm spacing was calculated to be 42.90 mA/cm2, which is close to the theoretical limit of 43.37 mA/cm2 in the 300-1100 nm wavelength range. To experimentally demonstrate the TMW arrays with a nanometer-scale spacing of 200 nm, which cannot be realized by conventional photolithography, we utilized a soft lithography method based on polystyrene beads for patterning a c-Si wafer. The solar cells based on optimized TMW arrays exhibited a Jsc of 42.5 mA/cm2 and power conversion efficiency of 20.5%, which exceed those of the previously reported microwire-based radial junction solar cells.

Silicon microwires are very promising candidates for next-generation solar cells. Si microwires achieve strong broadband light absorption, acting as an antireflection coating layer. In addition to superior optical properties, Si microwires can effectively separate and collect photocarriers at the radial junction owing to a short diffusion length (equal to the microwire radius). This factor can relax the requirement for high-quality materials, enabling a reduction in the cost of producing solar cells. In this report, we review recent progress in vertically aligned Si microwire solar cells with reference to (1) optimized microwire structures to enhance light absorption, (2) uniform radial junctions for effective carrier separation, (3) minimization of surface recombination through surface treatments, and (4) fine metal electrodes for efficient carrier collection. In addition, we present studies on flexible crystalline Si solar cells that use mechanically flexible Si microwires. Furthermore, based on each fabrication process, we discuss the challenges related to the realization of high-efficiency Si microwire solar cells.

Summary Crystalline silicon (c-Si) is not used for developing transparent photovoltaics, owing to its opaque nature. Furthermore, adding optical transparency to a conventional c-Si wafer is one of the most challenging problems in this field. Here, we demonstrate the development of a neutral-colored transparent c-Si substrate using a 200-μm-thick c-Si wafer, which is known to be intrinsically opaque. Transparent c-Si substrates were fabricated by placing microhole-shaped light transmission windows on a bare c-Si wafer. These windows were designed to enable the transmission of all incident visible light through the substrate, resulting in a colorless substrate. A light absorption area was also designed on the wafer to efficiently absorb incident light. As a representative application, solar cells fabricated using the neutral-colored transparent c-Si substrate showed a power conversion efficiency of up to 12.2%. Therefore, our transparent c-Si solar cells present a unique opportunity to develop next-generation colorless transparent photovoltaics.

Nanowires have promising applications as photodetectors with superior ability to tune absorption with morphology. Despite their high optical absorption, the quantum efficiencies of these nanowire photodetectors remain low due to difficulties in fabricating a shallow junction using traditional doping methods. As an alternative, we report nonconventional radial heterojunction photodiodes obtained by conformal coating of an indium oxide layer on silicon nanowire arrays. The indium oxide layer has a high work function which induces a strong inversion in the silicon nanowire and creates a virtual p-n junction. The resulting nanowire photodetectors show efficient carrier separation and collection, leading to an improvement of quantum efficiency up to 0.2. In addition, by controlling the nanowire radii, the spectral responses of the In2O3/Si nanowire photodetectors are tuned over several visible light wavelengths, creating a multispectral detector. Our approach is promising for the development of highly efficient wavelength-selective photodetectors.

Summary Transparent photovoltaics (TPVs), which combine visible transparency and solar energy conversion, are being developed for applications in which conventional opaque solar cells are unlikely to be feasible, such as windows of buildings or vehicles. In this paper, we review recent progress in TPVs along with strategies that enable the transparency of conventional photovoltaics, including thin-film technology, selective light-transmission technology, and luminescent solar concentrator technology. From fundamental research to commercialization of the TPV, three main perspectives should be considered: (1) high-power conversion efficiency at the same average visible transmittance; (2) aesthetic factors, which should not detract from applications such as buildings and vehicles; and (3) feasibility for real-world applications, including modularization and stability evaluation. We present the distinct analysis criteria for these main perspectives and discuss their importance. We also discuss possible research directions for the commercialization of TPVs.

In this study, an indium tin oxide (ITO)-free carrier-selective contact (CSC) for crystalline silicon (c-Si) solar cells with a micro-grid metal electrode is reported. The ITO layer is crucial for collecting the carriers separated at the junction between CSC and n-Si because of the relatively low conductivity of CSC. However, previous research investigated the formation of ITO films via sputter deposition, which can lead to performance degradation of solar cells due to the parasitic absorption of the ITO layer and plasma damage at the CSC/Si junction during sputtering. Moreover, the use of ITO is hindered because of the rare indium metal. Herein, we investigate the carrier transport mechanism at the MoOx/n-Si junction to understand the reason for the poor performance of ITO-free devices. A majority of the carriers are limited because of a highly resistive carrier path, with a sheet resistance of 16 kΩ sq−1 during carrier transport, leading to the severe degradation of fill factor (FF) and short-circuit current (Jsc). To minimize the power loss during carrier transport, a micro-grid metal electrode that can effectively collect carriers separated at the MoOx/n-Si junction is applied. With this micro-grid electrode, the electrical losses of ITO-free solar cells can be minimized despite the highly resistive path of the MoOx/n-Si junction. Hence, the best device exhibits a power conversion efficiency of up to 17.0% without the ITO layer.

Much attention has been paid to thin crystalline silicon (c-Si) photovoltaic devices due to their excellent flexibility characteristics, stable efficiency, and possibility of use as highly efficient next-generation flexible photovoltaic devices (FPVs). To fabricate thin c-Si FPVs, it is important to improve their light-absorption properties while maintaining the flexible characteristics. In this study, vertically aligned microwires (MWs) on a 50 μm-thick thin c-Si substrate are designed for novel FPVs. Increasing the length of the MWs enhances the optical properties of the thin c-Si without affecting its flexibility. To maximize the efficiency of the thin c-Si FPVs with MWs, tapered MWs and a localized back-contact structure are devised. This device shows a maximum efficiency of 18.9%. In addition, the proposed thin c-Si FPV with MWs shows high stability without any change in efficiency, even with 1000 bending cycles with a bending radius of 12 mm. Thus, we successfully demonstrate battery-free flexible electronic devices integrated with our thin c-Si FPVs with MWs.

We demonstrate here an embedded metal electrode for highly efficient organic–inorganic hybrid nanowire solar cells. The electrode proposed here is an effective alternative to the conventional bus and finger electrode which leads to a localized short circuit at a direct Si/metal contact and has a poor collection efficiency due to a nonoptimized electrode design. In our design, a Ag/SiO2 electrode is embedded into a Si substrate while being positioned between Si nanowire arrays underneath poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), facilitating suppressed recombination at the Si/Ag interface and notable improvements in the fabrication reproducibility. With an optimized microgrid electrode, our 1 cm2 hybrid solar cells exhibit a power conversion efficiency of up to 16.1% with an open-circuit voltage of 607 mV and a short circuit current density of 34.0 mA/cm2. This power conversion efficiency is more than twice as high as that of solar cells using a conventional electrode (8.0%). The...

We created a polymer-embedded TiO2 nanodiscs device with a robust tunable color filters that preserve veiy sharp spectral bands through spatial variations of the pitch of the meta array.

The combination of energy generation and energy storage systems is the ultimate solution to meet the ever-increasing demand for high-energy-density power sources. Here, we demonstrate a new class of monolithically integrated, photo-rechargeable portable power sources based on miniaturized crystalline Si photovoltaics (c-Si PVs) and printed solid-state lithium-ion batteries (LIBs). A solid-state LIB with a bipolar cell configuration is fabricated directly on the aluminium electrode of a c-Si PV module through an in-series printing process, which enables the seamless architectural/electrical connection of the two different energy systems. The single-unit PV–LIB device shows exceptional electrochemical performance that lies far beyond those achievable by conventional PVs or LIBs alone: it displays fast, low-light-intensity and high-temperature photo-charging; a photo-electric conversion/storage efficiency of 7.61%; a sustainable cycling performance; and continuous discharging at an extremely high current density of 28C under sunlight illumination. This study opens a facile and scalable route for the development of single-unit, photo-rechargeable mobile high-performance batteries that are required for the future era of ubiquitous electronics.

The spectral absorption range of polymer solar cells can be efficiently increased by molecular compatibility and energy level control in the energy transfer system. However, there has been limited research on energy transfer materials for both amorphous and highly crystalline polymer active materials. For the first time, we developed customized iridium (Ir(III) complexes that are incorporated into the active materials, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7, amorphous) or poly(3-hexylthiophene) (P3HT, high crystalline) as energy donor additives. The Ir(III) complex with the 2-phenyl quinolone ligand increased the power conversion efficiency of the corresponding devices by approximately 20%. The enhancements are attributed to the improved molecular compatibility and energy level matching between the Ir(III) complex and the active material, long Forster resonance energy transfer radius, and high energy down-shift efficiency. Overall, we reveal Ir(III) complex additives for amorphous and highly crystalline polymer active materials. These additives would enable efficient energy transfer in polymer solar cells while retaining the desirable active layer morphology, thereby improving the light absorption and conversion.

We optimize the thickness of a transparent conducting oxide (TCO) layer, and apply a microscale mesh-pattern metal electrode for high-efficiency a-Si/c-Si heterojunction solar cells. A solar cell equipped with the proposed microgrid metal electrode demonstrates a high short-circuit current density (JSC) of 40.1 mA/cm(2), and achieves a high efficiency of 18.4% with an open-circuit voltage (VOC) of 618 mV and a fill factor (FF) of 74.1% as result of the shortened carrier path length and the decreased electrode area of the microgrid metal electrode. Furthermore, by optimizing the process sequence for electrode formation, we are able to effectively restore the reduction in VOC that occurs during the microgrid metal electrode formation process. This work is expected to become a fundamental study that can effectively improve current loss in a-Si/c-Si heterojunction solar cells through the optimization of transparent and metal electrodes.

We demonstrated through a series of electromagnetic simulations and FTIR measurements a proof of concept for using vertically aligned arrays of InSb nanostructures as devices for selective SWIR and MWIR light absorption.

Molecularly engineered Ir(III) complexes can transfer energy from short-wavelength photons (λ 500 nm), which can enhance the otherwise low internal quantum efficiency (IQE) of crystalline Si (c-Si) nanowire solar cells (NWSCs) in the short-wavelength region. Herein, we demonstrate a phosphorescent energy downshifting system using Ir(III) complexes at short wavelengths (300–450 nm) to diminish the severe surface recombination that occurs in c-Si NWSCs. The developed Ir(III) complexes can be considered promising energy converters because they exhibit superior intrinsic properties such as a high quantum yield, a large Stokes shift, a long exciton diffusion length in crystalline film, and a reproducible synthetic procedure. Using the developed Ir(III) complexes, highly crystalline energy downshifting layers were fabricated by ultrasonic spray deposition to enhance the photoluminescence efficiency by increasing the radiative decay. With the optimized energy downshifting layer, our 1 cm2 c-Si NWSCs with Ir(III) complexes exhibited a higher IQE value for short-wavelength light (300–450 nm) compared with that of bare Si NWSCs without Ir(III) complexes, resulting in a notable increase in the short-circuit current density (from 34.4 mA·cm−2 to 36.5 mA·cm−2).

We developed a unique nano- and microwire hybrid structure by selectively modifying only the tops of microwires using metal-assisted chemical etching. The proposed nano/micro hybrid structure not only minimizes surface recombination but also absorbs 97% of incident light under AM 1.5G illumination, demonstrating outstanding light absorption compared to that of planar (59%) and microwire arrays (85%). The proposed hybrid solar cells with an area of 1 cm(2) exhibit power conversion efficiencies (Eff) of up to 17.6% under AM 1.5G illumination. In particular, the solar cells show a high short-circuit current density (Jsc) of 39.5 mA cm(-2) because of the high light-absorbing characteristics of the nanostructures. This corresponds to an approximately 61.5% and 16.5% increase in efficiency compared to that of a planar silicon solar cell (Eff = 10.9%) and a microwire solar cell (Eff = 15.1%), respectively. Therefore, we expect the proposed hybrid structure to become a foundational technology for the development of highly efficient radial junction solar cells.

We report on highly efficient flexible inverted organic solar cells (IOSCs) fabricated by low-temperature solution process on polyethylene terephthalate (PET) substrate. In general, IOSCs have been required to use an annealed (>200 degrees C) zinc oxide (ZnO) as an electron transport layer. However, any twisting of the flexible substrate during heat treatment leads to poor device performance. To overcome this issue, we developed a novel low temperature process for flexible IOSCs using an alcohol-/water-soluble conjugated polymer, namely poly [(9,9-bis(3'-(N, N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) instead of the ZnO. Using this low-temperature process, we successfully demonstrate highly efficient flexible IOSCs that are proven to be capable of the power conversion efficiency (PCE) of 6.17% which retain 96% of its efficiency at a bending radius of R approximate to 5 mm or less. To the best of our knowledge, this PCE 6.17% is the best result among the reported values so far for flexible OSCs fabricated on PET substrate. (C) 2015 Elsevier Ltd. All rights reserved.

This chapter provides an overview of fabrication of silicon nanowires by top-down etching methods. The focus will be on wet and dry etching methods to achieve vertically aligned silicon nanowires. We will start by building an understanding of the dry etching process. Then we will delve deeper into the mechanisms, benefits, and drawbacks of these dry etching processes. This is followed by wet etching methods, primarily metal-assisted chemical etching methods. Finally, we will provide a guideline to fabricate vertically aligned Si nanowire arrays with both dry and wet chemical processes.

Transparent conducting electrodes (TCEs) are considered to be an essential structural component of flexible organic solar cells (FOSCs). Silver nanowire (AgNW) electrodes are widely used as TCEs owing to their excellent electrical and optical properties. The fabrication of AgNW electrodes has faced challenges in terms of forming large uniform interconnected networks so that high conductivity and reproducibility can be achieved. In this study, a simple method for creating an intimate contact between AgNWs that uses cold isostatic pressing (CIP) is demonstrated. This method increases the conductivity of the AgNW electrodes, which enables the fabrication of high-efficiency inverted FOSCs that have a power conversion efficiency of 8.75% on flexible polyethylene terephthalate with no short circuiting occurring as the CIP process minimizes the surface roughness of the AgNW electrode. This allows to achieve 100% manufacturing yield of FOSCs. Furthermore, these highly efficient FOSCs are proven to only be 2.4% less efficient even for an extreme bending radius of R ≈ 1.5 mm, compared with initial efficiency.

An effective photocathode adopting a tapered Si nanohole (SiNH) array has been developed for photoelectrochemical water splitting. The tapered feature of SiNH photocathodes resulted in a gradation of the refractive indices between Si and air, such that the mismatching of optical impedance was alleviated and light absorption was enhanced. Adjusting the depth of the SiNHs successfully simulated the number of dielectric layers, optimizing the destructive interference for an antireflective coating (ARC). Only a 200 nm-thin NH array was required to absorb ∼96% of solar spectral irradiance for photoelectrochemical applications. This thickness also minimized the undesirable surface recombination loss. When compared to a similar system using a planar technology, the formation of NHs was observed to cause an increase in the optical bandgap. This could generate a surface-passivation effect, resulting in a lowering of dark current and an increase in photovoltage, which could be utilized for an anodic shift of the onset voltage. Due to the addition of tapered SiNHs, the photogenerated current was improved by ∼30% (∼33 mA cm−2) compared to a planar counterpart (∼25 mA cm−2), while the overpotential required for H2 evolution was reduced.

Formation of a selective emitter in crystalline silicon solar cells improves photovoltaic conversion efficiency by decoupling emitter regions for light absorption (moderately doped) and metallization (degenerately doped). However, use of a selective emitter in silicon nanowire (Si NW) solar cells is technologically challenging because of difficulties in forming robust Ohmic contacts that interface directly with the top-ends of nanowires. Here we describe a self-aligned selective emitter successfully integrated into an antireflective Si NW solar cell. By one-step metal-assisted chemical etching, NW arrays formed only at light-absorbing areas between top-metal grids while selectively retaining Ohmic contact regions underneath the metal grids. We observed a remarkable ∼40% enhancement in blue responses of internal quantum efficiency, corresponding to a conversion efficiency of 12.8% in comparison to the 8.05% of a conventional NW solar cell.

We suggest a design guideline that optimally combines light absorptance and electrical performance in silicon nanowire (SiNW) solar cells. The design was experimentally developed using a tradeoff relation between wire length and space-filling ratio. Increasing (or decreasing) the wire length decreases (or increases) the wire filling ratio required to achieve the same amount of light absorption. The superior antireflection characteristics of SiNW arrays stem from optical impedance matching. A wire length of 1.2 μm suppressed light reflection enough to obtain 99% absorptance at a filling ratio of 38%. A short length (∼500 nm) design using nanopillars rather than nanowires effectively diminished the surface and bulk recombinations to achieve a short circuit current of ∼30 mA/cm2 while maintaining good light absorptance.

Traditional Pt counter electrode in quantum-dot-sensitized solar cells suffers from a low electrocatalytic activity and instability due to irreversible surface adsorption of sulfur species incurred while regenerating polysulfide (Sn2–/S2–) electrolytes. To overcome such constraints, chemically synthesized Cu2ZnSn(S1–xSex)4 nanocrystals were evaluated as an alternative to Pt. The resulting chalcogenides exhibited remarkable electrocatalytic activities for reduction of polysulfide (Sn2-) to sulfide (S2–), which were dictated by the ratios of S/Se. In this study, a quantum dot sensitized solar cell constructed with Cu2ZnSn(S0.5Se0.5)4 as a counter electrode showed the highest energy conversion efficiency of 3.01%, which was even higher than that using Pt (1.24%). The compositional variations in between Cu2ZnSnS4 (x = 0) and Cu2ZnSnSe4 (x = 1) revealed that the solar cell performances were closely related to a difference in electrocatalytic activities for polysulfide reduction governed by the S/Se ratios.

We demonstrate the controlled exfoliation of a heavily n -doped mesoporous Si double layer prepared using two-step electrochemical etching, which can be further utilized in Si-epitaxial growth for layer transfer. The first step of electrochemical etching was to apply a lower anodic current density to form self-organized mesoporous silicon with a small pore diameter (≤4.5 nm). For the second step, a high anodic current density was applied to create a layer with a higher porosity beneath the first layer, which enables easier detachment from the substrate. Exfoliating the porous double layer was controllable only for heavily n -doped silicon under specific conditions of anodic current density and etching time. The differences in pore morphologies, which depended upon wafer doping levels, were analyzed using two different modes of electrical breakdowns, avalanche and Zener.

We found that multi-layer coating of a Si substrate with SiO2 dielectric nanoparticles (NPs) was an effective method to suppress light reflection by silicon solar cells. To suppress light reflection, two conditions are required for the coating: 1) The difference of refractive indexes between air and Si should be alleviated, and 2) the quarter-wavelength antireflection condition should be satisfied while avoiding intrinsic absorption loss. Light reflection was reduced due to destructive interference at certain wavelengths that depended on the layer thickness. For the same thickness dielectric layer, smaller NPs enhanced antireflectance more than larger NPs due to a decrease in scattering loss by the smaller NPs.

Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowires during vapor-liquid-solid growth using Au as a catalyst within a specific range of chlorine-to-hydrogen atomic ratio. The mechanism of nanodisk formation has been investigated by changing the source gas ratio of SiCl4 to H2. We report that an over-supply of silicon into the Au-Si liquid alloy leads to highly supersaturated solution and enhances the precipitation of Au in the silicon nanowires due to the formation of unstable phases within the liquid alloy. It is shown that the gold precipitates embedded in the silicon nanowires consisted of a metastable gold silicide. Interestingly, faceting of gold silicide was observed at the Au/Si interfaces, and silicon nanowires were epitaxially grown on the top of the nanodisk by vapor-liquid-solid growth. High resolution transmission electron microscopy confirmed that gold silicide nanodisks are epitaxially connected to the silicon nanowires in the direction of growth direction. These gold silicide nanodisks would be useful as nanosized electrical junctions for future applications in nanowire interconnections.

The electrochemical etching behaviors of GaAs on smooth and textured surfaces were systematically investigated in aqueous KOH electrolytes. When the applied potential was higher than the pore formation potential (PFP), the electrochemical etching of the smooth surface could be categorized into three regions: pore/wire formation, texturing, and electropolishing regions. Triangular GaAs nanowires were observed at low potentials while pronounced lateral etching occurred at high potentials. In the case of the textured surface, which consisted of groove arrays, more complicated etching behavior was demonstrated. The pore growth direction changed from 〈1 1 1〉B to 〈0 0 1〉 at a high current density while accompanying a drastic change in the pore morphologies such as tetrahedron-like pores along 〈1 1 1〉B transforming into 200 μm deep pores propagated along the 〈0 0 1〉 direction.

A periodic array of silicon pillars was photoelectrochemically fabricated using the two-step etching process with a n-type Si (1 0 0) substrate. Two key factors, backside illumination and anodic bias, were required to obtain a high-aspect ratio macropore array of silicon. It was found that the initial pore could be separated into two different pores when the applied anodic bias was greater than a certain critical value. The pore size of the macroporous silicon with a high porosity was increased by anisotropic etching in an alkaline solution. Due to destruction of the pore sidewalls, KOH etching allowed for the fabrication of silicon pillars on a large-scale wafer with an improved uniformity. The anisotropic etching behavior of KOH solution led to necking of the silicon pillars when the etching time exceeded 60 s.

We demonstrate novel all-back-contact Si nanohole solar cells via the simple direct deposition of molybdenum oxide (MoOx) and lithium fluoride (LiF) thin films as dopant-free and selective carrier contacts (SCCs). This approach is in contrast to conventionally used high-temperature thermal doping processes, which require multistep patterning processes to produce diffusion masks. Both MoOx and LiF thin films are inserted between the Si absorber and Al electrodes interdigitatedly at the rear cell surfaces, facilitating effective carrier collection at the MoOx/Si interface and suppressed recombination at the Si and LiF/Al electrode interface. With optimized MoOx and LiF film thickness as well as the all-back-contact design, our 1 cm(2) Si nanohole solar cells exhibit a power conversion efficiency of up to 15.4%, with an open-circuit voltage of 561 mV and a fill factor of 74.6%. In particular, because of the significant reduction in Auger/surface recombination as well as the excellent Si-nanohole light absorption, our solar cells exhibit an external quantum efficiency of 83.4% for short-wavelength light (∼400 nm), resulting in a dramatic improvement (54.6%) in the short-circuit current density (36.8 mA/cm(2)) compared to that of a planar cell (23.8 mA/cm(2)). Hence, our all-back-contact design using MoOx and LiF films formed by a simple deposition process presents a unique opportunity to develop highly efficient and low-cost nanostructured Si solar cells.

A systematic study was conducted into the use of metal-assisted chemical etching (MacEtch) to fabricate vertical Si microwire arrays, with several models being studied for the efficient redox reaction of reactants with silicon through a metal catalyst by varying such parameters as the thickness and morphology of the metal film. By optimizing the MacEtch conditions, high-quality vertical Si microwires were successfully fabricated with lengths of up to 23.2 μm, which, when applied in a solar cell, achieved a conversion efficiency of up to 13.0%. These solar cells also exhibited an open-circuit voltage of 547.7 mV, a short-circuit current density of 33.2 mA/cm2 and a fill factor of 71.3% by virtue of the enhanced light absorption and effective carrier collection provided by the Si microwires. The use of MacEtch to fabricate high-quality Si microwires therefore presents a unique opportunity to develop cost-effective and highly efficient solar cells.

We address a development of efficient solar energy harvesting systems based upon Si nanostructures with a study of the optical and electrical properties of Si nanostructures, and suggest a basic design guideline for integrating nanostructures on Si surface.

A cost-effective process is reported for fabricating flexible transparent conducting electrodes (TCEs) through the use of a reusable silicon master mold with a microgrid patterned Ag/Cr–metal double layer. Flexible TCEs can be simply produced via metal electroplating on the master mold followed by metal transfer onto a flexible polymer substrate. Nickel (Ni), an inexpensive metal, is used as the TCE material in this study, allowing low-cost TCE fabrication. The fabricated flexible Ni TCEs exhibit excellent optical and electrical properties (sheet resistance of 6.1 Ω sq−1 with 96% transmittance) along with outstanding mechanical flexibility with no significant change in sheet resistance after 1000 bending cycles at a bending radius of 4 mm. A touch screen panel is also successfully demonstrated using an Ni microgrid TCE to confirm the practical applicability of these TCEs. The results indicate that the proposed flexible Ni microgrid TCEs have potential for rapid commercialization because of their high optoelectronic performance, low cost, and facile fabrication process.

There is an exponentially growing need for well-oriented, vertical silicon nano/micro-structure arrays, particularly in high-density integrated electronic devices. Here, we demonstrate that precisely controlled vertical arrays of silicon wires and cones can be fabricated by a combined treatment strategy of electrochemical and chemical etchings. First, a periodically ordered array of silicon wires was readily fabricated at microscale by simple electrochemical etching in which the current density played a critical role in determining the wire diameter and interspacing. The microstructures fabricated by electrochemical etching were more precisely tuned by further chemical etching, thereby transforming into cone arrays with extremely sharp tips where the cone height was controlled by the etching time. This approach could have broad utility in many electronics requiring miniaturization and high-density integration such as field emitters, photovoltaic and thermoelectric devices.

The optimal hybridization of photovoltaic (PV) and thermoelectric (TE) devices has long been considered ideal for the efficient harnessing solar energy. Our hybrid approach uses full spectrum solar energy via lossless coupling between PV and TE devices while collecting waste energy from thermalization and transmission losses from PV devices. Achieving lossless coupling makes the power output from the hybrid device equal to the sum of the maximum power outputs produced separately from individual PV and TE devices. TE devices need to have low internal resistances enough to convey photo-generated currents without sacrificing the PV fill factor. Concomitantly, a large number of p-n legs are preferred to drive a high Seebeck voltage in TE. Our simple method of attaching a TE device to a PV device has greatly improved the conversion efficiency and power output of the PV device (~30% at a 15°C temperature gradient across a TE device).

Much attention has been paid to thin crystalline silicon (c-Si) due to excellent flexible characteristics, stable performance, and possibility of high efficiency as a means for developing next generational flexible solar cells. Here, we confirm that the 50 µm thin c-Si with various length of microwire arrays (MWA) shows stable flexible characteristics regardless of MWA length. In our result, the fabricated flexible solar cells using 50 µm thin c-Si with 20 µm MWA exhibit power conversion efficiencies of up to 15.2% under AM 1.5G illumination with an open-circuit voltage of 576 mV and a short-circuit current density of 34.1 mAcm-2. Furthermore, to overcome the light reflection of flat-top-area of MWA, tapered MWA that can reduce surface reflection to around 1% is fabricated by simple wet-etching process. Applying the 20 µm tapered MWA on 50 µm thin c-Si, our flexible solar cell shows maximum efficiency of 16.8% and stable photovoltaic performance during bending tests up to 1000 times at the bending radius of 10 mm. Therefore, we believe that our proposed flexible solar cell can be a next-generation solar energy harvesting devices through high efficiency and stable flexibility.

The effects of isopropyl alcohol (IPA) on the dispersion of bimodal silver nanoparticles inside polyaniline emeraldine base (Ag-PANIEB) as nanocomposite were investigated and discussed. Firstly, the bimodal Ag nanoparticles were synthesized with the help of precipitation method by semi-batch reactor and then ultrasonically blended inside the PANIEB matrix in the presence/absence of IPA followed by various characterization of Ag-PANIEB nanocomposite. It was observed that the bimodal dispersion of Ag nanoparticles inside PAIEB thin film was significantly improved in the presence of isopropyl alcohol (IPA). Such bimodal dispersion of Ag nanoparticles in the presence of IPA also enhances the electrical response of the Ag-PANIEB nanocomposite. A mechanism explaining the improvement in electrical response of bimodal Ag-PANIEB nanocomposite in the presence of IPA is proposed and discussed.

A novel stamped hybrid solar cell was proposed using the stamping transfer technique by stamping an active PEDOT:PSS thin layer onto the top of silicon nanowires (SiNWs). Compared to a bulk-type counterpart that fully embeds SiNWs inside PEDOT:PSS, an increase in the photovoltaic efficiency was observed by a factor of 4.6, along with improvements in both electrical and optical responses for the stamped hybrid cell. Such improvements for hybrid cells was due to the formation of well-connected and linearly aligned active PEDOT:PSS channels at the top ends of the nanowires after the stamping process. These stamped channels facilitated not only to improve the charge transport, light absorption, but also to decrease the free carriers as well as exciton recombination losses for stamped hybrid solar cells. (Some figures may appear in colour only in the online journal)

Optimizing the antireflection coatings of porous silicon (PS ARC) is found to be sensitive to their thicknesses. The optimal reflectance for a 400-nm-thin, PS ARC is calculated to be as low as 1.55%.

A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the reflection spectra of freestanding SiNW arrays is presented in this Letter. At a fixed nanowire diameter, three different wire lengths reproducibly formed a cross point in their reflectance curve plots. The cross point wavelength corresponded to the optical bandgap, as verified by the classical Tauc's model. The optical bandgap of the SiNW arrays (112 nm in average diameter) was measured to be ~1.19 eV, which is larger than the ~1.08 eV bandgap of bulk Si. Further decreasing the wire diameter to 68 nm caused an increase of the bandgap to ~1.24 eV, which is closer to the optimal bandgap (~1.40 eV) required to achieve the highest conversion efficiency in single-junction photovoltaic devices. Our method suggests that the multijunction tandem structure can be realized via control of the diameter of SiNW arrays.