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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.

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.

Solar-driven reactive oxygen species (ROS) generation is an attractive disinfection technique for cell death and water purification. However, most photocatalysts require high stability in the water environment and the production of ROS with a sufficient amount and diffusion length to damage pathogens. Here, a ROS generation system was developed consisting of tapered crystalline silicon microwires coated with anatase titanium dioxide for a conformal junction. The system effectively absorbed >95% of sunlight over 300-1100 nm, resulting in effective ROS generation. The system was designed to produce various ROS species, but a logistic regression analysis with cellular survival data revealed that the diffusion length of the ROS is ∼9 μm, implying that the most dominant species causing cell damage is H2O2. Surprisingly, a quantitative analysis showed that only 15 min of light irradiation on the system would catalyze a local bactericidal effect comparable to the conventional germicidal level of H2O2 (∼3 mM).

Solar-driven reactive oxygen species (ROS) generation is an attractive disinfection technique for cell death and water purification. However, most photocatalysts require high stability in the water environment and the production of ROS with a sufficient amount and diffusion length to damage pathogens. Here, a ROS generation system was developed consisting of tapered crystalline silicon microwires coated with anatase titanium dioxide for a conformal junction. The system effectively absorbed95% of sunlight over 300-1100 nm, resulting in effective ROS generation. The system was designed to produce various ROS species, but a logistic regression analysis with cellular survival data revealed that the diffusion length of the ROS is ∼9 μm, implying that the most dominant species causing cell damage is H

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.

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.

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.

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.