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The cross-sectional and top-view SEM images of Si NWA layers fabricated with (a) 20 mM, (b) 30 mM, and (c) 50 mM AgNO 3 . The FF is
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The nanowire array (NWA) layers with controlled structure profiles fabricated by maskless galvanic wet etching on Si substrates are found to exhibit extremely low specular reflectance (<0.1%) in the wavelengths of 200–850 nm. The significantly suppressed reflection is accompanied with other favorable antireflection (AR) properties, including omnidi...
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We present a facile technique that only uses conventional micro-techniques and two size-reduction steps to fabricate wafer-scale silicon nanowire (SiNW) with widths of 200 nm. Initially, conventional lithography was used to pattern SiNW with 2 μm width. Then the nanowire width was decreased to 200 nm by two size-reduction steps with isotropic wet e...
Silicon nanowire (SiNW) arrays were fabricated by using top-down fabrication approach via atomic force microcopy (AFM) lithography on silicon-on-insulator (SOI) (100) substrate. Local anodic oxidation (LAO) technique carried out by AFM lithography to draw oxide pattern on top of the SOI substrate. The next steps to fabricate silicon nanowires invol...
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... The tapered structure provides a gradient of refractive index between the air and substrate and consequently results in significant absorption in SiNWs compared to the bare structure. 25,26 The kinked and straight SiNWs also show gradient porous microstructure, 12,27 but the amount of porosity in each wire varies depending on the fabrication condition. ...
We have experimentally demonstrated that slanted, kinked and straight silicon nanowire arrays form a broadband omnidirectional light‐harvesting structure. The unique design of kinked and slanted nanowires allows them to trap more light effectively across a wide wavelength range than straight silicon nanowires (SiNWs). We report that the light absorption in slanted, kinked and straight wires is enhanced by controlling their geometrical parameters. The p‐type SiNWs have less reflection than n‐type SiNWs due to their larger porosity gradient structure. Aperiodic and tapered slanted, kinked and straight SiNWs are remarkably photoactive and promising low‐cost materials for photoelectrochemical water splitting applications. The aperiodic‐tapered‐kinked straight silicon nanowires (SiNWs) is fabricated by two‐step metal‐assisted chemical etching. The kinked SiNWs show high absorption and good solar‐driven photoelectrochemical water splitting. The multiple reflection or optical path length of the light inside the nanowires is controlled by the morphology of the nanowires.
... 4(a) and 4(b), Fan et al. pioneered a nanopillar-array CdTe/CdS photovoltaics with a 3D geometric configuration [28] . The 3D device structure affords effective radial charge collection and light absorption [10,13,55] . The experimental result Fig. 4(c) and simulation calculations Fig. 4(d) show that absorption is improved in 3D nanodevice. ...
Flexible solar cells are important photovoltaics (PV) technologies due to the reduced processing temperature, less material consumption and mechanical flexibility, thus they have promising applications for portable devices and building-integrated applications. However, the efficient harvesting of photons is the core hindrance towards efficient, flexible PV. Light management by nanostructures and nanomaterials has opened new pathways for sufficient solar energy harvesting. Nanostructures on top surfaces provide an efficient pathway for the propagation of light. Aside from suppressing incident light reflection, micro-structured back-reflectors reduce transmission via multiple reflections. Nanostructures themselves can be the absorber layer. Photovoltaics based on high-crystallinity nanostructured light absorbers demonstrate enhanced power conversion efficiency (PCE) and excellent mechanical flexibility. To acquire a deep understanding of the impacts of nanostructures, herein, a concise overview of the recent development in the design and application of nanostructures and nanomaterials for photovoltaics is summarized.
... The scattered light reflection from the rear back of the module can still be absorbed by solar cells and contribute to both objectives at once. Several light-trapping methods have been implemented over the last few years to enhance the light absorption within the solar cell that is much smaller compared to the material's intrinsic absorption length [11][12][13][14][15][16]. The average efficiency of solar panels during this period has increased but mainly because of higher performance solar cells. ...
Photovoltaic (PV) systems directly convert solar energy into electricity and researchers are taking into consideration the design of photovoltaic cell interconnections to form a photovoltaic module that maximizes solar irradiance. The purpose of this study is to evaluate the cell spacing effect of light diffusion on output power. In this work, the light absorption of solar PV cells in a module with three different cell spacings was studied. An optical engineering software program was used to analyze the reflecting light on the backsheet of the solar PV module towards the solar cell with varied internal cell spacing of 2 mm, 5 mm, and 8 mm. Then, assessments were performed under standard test conditions to investigate the power output of the PV modules. The results of the study show that the module with an internal cell spacing of 8 mm generated more power than 5 mm and 2 mm. Conversely, internal cell spacing from 2 mm to 5 mm revealed a greater increase of power output on the solar PV module compared to 5 mm to 8 mm. Furthermore, based on the simulation and experiment, internal cell spacing variation showed that the power output of a solar PV module can increase its potential to produce more power from the diffuse reflectance of light.
... The reflectance loss of light is one of the problems that restricts the ability of solar cell devices to capture incident photons [155]. Nano/micro structures based on anti-reflection (AR) schemes such as nano/micropyramids [99,156,157], nanowires [158,159], nanopillars [160][161][162], nanocones [163][164][165], nanodomes [163], and nanospheres [166,167] have been proved to be a simple and effective method for suppressing the reflection on the front surface of the device [35,168,169]. Recently, the use of anodized aluminum (AAO) as a template to prepare these nanomaterials has been widely reported [2,170]. ...
Due to the unique optical and electrochemical properties, large surface area, tunable properties, and high thermal stability, nanoporous anodic aluminum oxide (AAO) has become one of the most popular materials with a large potential to develop emerging applications in numerous areas, including biosensors, desalination, high-risk pollutants detection, capacitors, solar cell devices, photonic crystals, template-assisted fabrication of nanostructures, and so on. This review covers the mechanism of AAO formation, manufacturing technology, the relationship between the properties of AAO and fabrication conditions, and applications of AAO. Properties of AAO, like pore diameter, interpore distance, wall thickness, and anodized aluminum layer thickness, can be fully controlled by fabrication conditions, including electrolyte, applied voltage, anodizing and widening time. Generally speaking, the pore diameter of AAO will affect its specific application to a large extent. Moreover, manufacturing technology like one/two/multi step anodization, nanoimprint lithography anodization, and pulse/cyclic anodization also have a major impact on overall array arrangement. The review aims to provide a perspective overview of the relationship between applications and their corresponding AAO pore sizes, systematically. And the review also focuses on the strategies by which the structures and functions of AAO can be utilized.
... In order to elucidate the effect of SiNWAs on the light trapping capability which is useful for solar cells performance, we have measured UV-Vis spectra as shown in Fig. 2(a), it is observed that the SiNWAs etched for prolonged time of 15 min effectively suppresses R tot to less than 4.5% against the polished Si substrate over a full broadband range [25]. The significant reduction in the reflectance is due to the gradual change in the high index from air to SiNWs to Si substrate and the existence of subwavelength structures for additional light scattering in the nanowires which is discussed in our earlier reports [24,26]. The two peaks as observed in all the graphs at 270 and 367 nm, come from the inter-band tran-sitions of Si. ...
... The two peaks as observed in all the graphs at 270 and 367 nm, come from the inter-band tran-sitions of Si. These reduction is also believed to occur due to the destructive interferences along the nanowire projection [25,26]. ...
In the present work, we have explored the passivation mechanism of Graphene based schottky junction photodiodes. The graphene sheet was developed by low cost camphor as a solid source of carbon to avoid toxic gaseous carbon precursor. Moreover, coupling with Si or SiNWAs makes it a promising candidate for large-scale and cost effective optoelectronic application. The 8-10 μm nanowire arrays in the present case, demonstrates excellent optical light trapping properties with the reflectance of 4% over a wide range of wavelength (200-1200 nm). On the other hand, large scale graphene sheet developed by camphor contributes no defect D band at (~1360 cm-1) with 2D/G ratio of 2.43. It is found that, the performance of Gr/SiNWAs schottky junction is restricted by strong carrier recombination and relative low schottky barrier height (SBH). We have modified the surfaces of nanowires by H-passivation in order to improve the electrical performance of the device. Passivation of SiNWAs showed the capability to effectively suppress the surface carrier recombination up to certain extent, leading to a more electron transfer and an improvement in the JSC values from 0.3 mA/cm2 to 2.7 mA/cm2 typically at -1 V.
... These relatively high values are due to the smooth silicon surface and the absence of incident light-trapping structures (Figure 8a). The peaks at 274 and 367, as observed in the graph of Si wafer, arise from the inter-band transitions of Si [41,42]. After the formation of vertical SiNWs, a strong suppression in the reflectance to about 10% throughout the entire wavelength range occurred, which is consistent with the black surface appearance against the grey and polished Si substrate. ...
A key requirement for the development of highly efficient silicon nanowires (SiNWs) for use in various kinds of cutting-edge applications is the outstanding passivation of their surfaces. In this vein, we report on a superior passivation of a SiNWs surface by bismuth nano-coating (BiNC) for the first time. A metal-assisted chemical etching technique was used to produce the SiNW arrays, while the BiNCs were anchored on the NWs through thermal evaporation. The systematic studies by Scanning Electron Microscopy (SEM), energy dispersive X-ray spectra (EDX), and Fourier Transform Infra-Red (FTIR) spectroscopies highlight the successful decoration of SiNWs by BiNC. The photoluminescence (PL) emission properties of the samples were studied in the visible and near-infrared (NIR) spectral range. Interestingly, nine-fold visible PL enhancement and NIR broadband emission were recorded for the Bi-modified SiNWs. To our best knowledge, this is the first observation of NIR luminescence from Bi-coated SiNWs (Bi@SiNWs), and thus sheds light on a new family of Bi-doped materials operating in the NIR and covering the important telecommunication wavelengths. Excellent anti-reflectance abilities of ~10% and 8% are observed for pure SiNWs and Bi@SiNWs, respectively, as compared to the Si wafer (50–90%). A large decrease in the recombination activities is also obtained from Bi@SiNWs heterostructures. The reasons behind the superior improvement of the Bi@SiNWs performance are discussed in detail. The findings demonstrate the effectiveness of Bi as a novel surface passivation coating, where Bi@SiNWs heterostructures are very promising and multifunctional for photovoltaics, optoelectronics, and telecommunications.
... This is due to the bundle-like structures of the wires in prolonged etching times. Chang et al. [32] reported a similar variation in VFR, however, as a function of AgNO 3 concentrations. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 19 Vertically aligned SiNWs with good uniformity were clearly seen in the cross-section images (Figure 3m-r). ...
... The sharp suppression in the reflectance is ascribed to three important factors: (i) The gradual variation towards the refractive index from air (n ≈ 1) to SiNWs and to Si substrate (n ≈ 3.42), (ii) the tapered morphology of silicon wires leading to light trapping due to the multiple reflections back and forth in the inner surface and (iii) the sub-wavelength structures for additional light trapping within the NWs [17,33,45]. The observed peaks of Si wafer at 275 and 367 nm come from the interband transitions of Si [32,45]. In addition, as readily seen in the figure, all the reflectance values of SiNW arrays with various lengths are almost close and low (9-15%) owing to the tapered NWs structure. ...
... The sharp suppression in the reflectance is ascribed to three important factors: (i) The gradual variation towards the refractive index from air (n ≈ 1) to SiNWs and to Si substrate (n ≈ 3.42), (ii) the tapered morphology of silicon wires leading to light trapping due to the multiple reflections back and forth in the inner surface and (iii) the sub-wavelength structures for additional light trapping within the NWs [17,33,45]. The observed peaks of Si wafer at 275 and 367 nm come from the inter-band transitions of Si [32,45]. In addition, as readily seen in the figure, all the reflectance values of SiNW arrays with various lengths are almost close and low (9-15%) owing to the tapered NWs structure. ...
In this work, vertically aligned silicon nanowires (SiNWs) with relatively high crystallinity have been fabricated through a facile, reliable, and cost-effective metal assisted chemical etching method. After introducing an itemized elucidation of the fabrication process, the effect of varying etching time on morphological, structural, optical, and electrical properties of SiNWs was analysed. The NWs length increased with increasing etching time, whereas the wires filling ratio decreased. The broadband photoluminescence (PL) emission was originated from self-generated silicon nanocrystallites (SiNCs) and their size were derived through an analytical model. FTIR spectroscopy confirms that the PL deterioration for extended time is owing to the restriction of excitation volume and therefore reduction of effective light-emitting crystallites. These SiNWs are very effective in reducing the reflectance to 9–15% in comparison with Si wafer. I–V characteristics revealed that the rectifying behaviour and the diode parameters calculated from conventional thermionic emission and Cheung’s model depend on the geometry of SiNWs. We deduce that judicious control of etching time or otherwise SiNWs’ length is the key to ensure better optical and electrical properties of SiNWs. Our findings demonstrate that shorter SiNWs are much more optically and electrically active which is auspicious for the use in optoelectronic devices and solar cells applications.
... The use of advanced nanophotonic light trapping approaches can contribute to both objectives simultaneously. Various light trapping techniques have been implemented over the last few years to increase the light absorption within the semiconductor layer that is much smaller compared to the material's intrinsic absorption length [4][5][6][7][8][9][10][11]. Enhancing the optical absorption also allows for decreasing the active absorber layer thickness, which in turn decreases the production costs through the use of significantly less materials. ...
Thin film solar cells are one of the important candidates utilized to reduce the cost of photovoltaic production by minimizing the usage of active materials. However, low light absorption due to low absorption coefficient and/or insufficient active layer thickness can limit the performance of thin film solar cells. Increasing the absorption of light that can be converted into electrical current in thin film solar cells is crucial for enhancing the overall efficiency and in reducing the cost. Therefore, light trapping strategies play a significant role in achieving this goal. The main objectives of light trapping techniques are to decrease incident light reflection, increase the light absorption, and modify the optical response of the device for use in different applications. Nanostructures utilize key sets of approaches to achieve these objectives, including gradual refractive index matching, and coupling incident light into guided modes and localized plasmon resonances, as well as surface plasmon polariton modes. In this review, we discuss some of the recent developments in the design and implementation of nanostructures for light trapping in solar cells. These include the development of solar cells containing photonic and plasmonic nanostructures. The distinct benefits and challenges of these schemes are also explained and discussed.
... This is due to the reduction in the n eff value and multiple scattering of light from the inhomogeneous surface of the S 4 sample. 39 The reflectivity value of less than 3.5% is observed from the top and the cross-sectional surface of all the samples with different nanowire lengths for both polarization states of incident light, which signifies the scalability of broadband omnidirectional anti-reflectivity. ...
... Therefore, our study can be utilized in the design and fabrication of SiNWs based photonic devices with a controlled n eff profile which can significantly eliminate the surface reflection over the broad wavelength range and maximize the photon collection efficiency. 39 The polarization-and direction-independent near-zero reflection of light constitutes the true two-dimensional omnidirectional anti-reflector. The results presented here have manifold applications in applied physics, especially in disorder-induced light emission and localization due to the highindex contrast of the SiNWs samples. ...
Controlling the light reflection using vertically-aligned nanowires has great importance in fundamental research with interesting applications in photonic devices. Here, we discuss the spatial- and polarization-dependent reflectivity measurements from the cross-sectional as well as from the top surface of vertically-aligned disordered silicon nanowires. The gradient variation in an effective refractive index along the nanowire length is estimated using the cross-sectional reflectivity measurements. We have studied the gradient variation of an effective refractive index profile and its tunability with the nanowire length. The reflectivity is measured to be as low as 5% irrespective of spatial directions and the polarization of incident light in a broad wavelength range. This constitutes the signature of a broadband omnidirectional anti-reflector that is scalable with the nanowire length. The reflectivity measurements are in good agreement with theoretical calculations. Such omnidirectional anti-reflection in a broad wavelength range is useful for applications such as photon management in photovoltaic devices and disorder-induced light scattering.
... However, there is still up to 20% reflection loss over the broadband wavelengths and angles of incident by using micropyramidal structures, which severely limits the photovoltaic performance [11]. Subwavelength structures have been considered as a promising alternative candidate to eliminate the reflection over broadband wavelength regions [12][13][14][15]. Their light-harvesting properties have been extensively studied by optimizing the morphologies of nanostructures [12,15,16]. ...
... Subwavelength structures have been considered as a promising alternative candidate to eliminate the reflection over broadband wavelength regions [12][13][14][15]. Their light-harvesting properties have been extensively studied by optimizing the morphologies of nanostructures [12,15,16]. In addition, since the feature sizes of nanostructures are in the same range as the diffusion length of minority photocarriers, structural designs such as a radial junction can minimize the carrier recombination due to short transport distances and then facilitate photocarrier collection, resulting in increased short-circuit current density (JSC) and open-circuit voltage (VOC) [17]. ...
... Incident light strikes on the subwavelength structures as if it comes across a thin AR layer with an intermediate neff between refractive index indices of air and Si, increasing the amount of light entering the structures. Hierarchical structures with long NWs provide a smoother neff transition from air to Si micropyramids compared with that with short NWs, aiding more incident photon entering to the substrates, which lowers reflectance losses and therefore enhances absorptance [7,12,25]. Second, when incident light encounters the micropyramids, these microscaled structures lead to effective multiple scattering, increasing the probability of light absorption and prolonging the optical path in the solar devices. The scattering behavior can be determined by haze ratio (Fig. 2b). ...
Hierarchical silicon structures consisting of micropyramids and nanowire arrays are fabricated by two-step chemical etching processes aimed at achieving cost and time effectiveness constraints without using any expensive vacuum system or complicated lithography process. The hierarchical structures can suppress the average reflectance to as low as 4.3% from 300 to 1100 nm without causing poor minority carrier lifetimes, exhibiting excellent broadband light-harvesting abilities with minimal recombination losses, which is the key point to design high performance nanostructured solar cells. By utilizing hierarchical structures in practical solar cells application, the short-circuit current density (JSC) shows a significant enhancement from 21.5 to 28.7 mA/cm2, and the conversion efficiency is enhanced by a factor of 35%. Such a significant enhancement is attributed not only to the superior light harvesting achieved by hierarchical structures but also to the benefit of small electrical losses in the solar cells. Thus, the concept and technique presented in this study open avenues for developing high-performance structure solar devices.
