Selenium Nanoparticles Decorated Silicon Nanowires with Enhanced


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Selenium Nanoparticles Decorated Silicon Nanowires with Enhanced Liquid Junction Photoelectrochemical Solar Cell Performance Ankita Kolay, Debanjan Maity, Partha Ghosal, and Melepurath Deepa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00062 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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The Journal of Physical Chemistry

Selenium Nanoparticles Decorated Silicon Nanowires with Enhanced Liquid Junction Photoelectrochemical Solar Cell Performance Ankita Kolay,a Debanjan Maity,a Partha Ghosal, b Melepurath Deepa a,* aDepartment

of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy, Telangana 502285, India

bDefence

Metallurgical Research Laboratory, Defence Research & Development Organisation (DRDO), Hyderabad, Telangana 500058, India

ABSTRACT: Selenium nanoparticles decorated silicon nanowires (Se [email protected] NWs) electrode is applied as a photoanode in a liquid junction photoelectrochemical (PEC) solar cell for the first time. Upon illumination, the Se NPs anchored along the axial length of Si NWs allow fast hole extraction at the radial Se/Si junctions, due to the p-type conduction nature of Se NPs thus enhancing electronhole separation, and simultaneously increase the population of photoexcited electrons in Si NWs through light scattering that amplifies the effective light absorption of Si NWs. These attributes of Se NPs result in a power conversion efficiency (PCE) of 7.03% for the Se [email protected] NWs based liquid junction solar cell encompassing a Br-/Br2 electrolyte and a carbon fabric counter electrode. This PCE is greater by 43% than that of the analogous Si NWs based cell. Se NPs are photo-conducting due to facile hole-propagation that occurs particularly along the c-axis of trigonal Se NPs with a hexagonal crystal structure, and size effects improve the optical path length, factors that lead to a significantly improved performance. Compared to Pt or Au NPs that have been explored previously in combination with Si NWs, where their roles are distinctively different, Se NPs here, are not only more cost effective, and easy to synthesize on a large-scale, but they enable an improvement in PCE of Si NWs, by relying on unique mechanisms. Optical-, structural, PEC-, and impedance- studies furnish a deep understanding of the phenomena involved in yielding a superior performing liquid junction PEC solar cell based on the Se [email protected] NWs photoanode.

Introduction Photovoltaic (PV) technology has the colossal potential to play a crucial role in a sustainable energy future with minimal negative impacts on the environment. However, the main drawback in the widespread implementation of silicon (Si) PV modules is imbedded primarily in the initial cost of the solargrade Si wafer and eventually in the high production cost of the cell. Among several approaches undertaken to minimize the cost of developing Si solar cells, engineering silicon nanowires (Si NWs) based photoelectrochemical (PEC) solar cells has been extremely profitable, for their modest power conversion efficiency (PCE) and low cost.1-3 Si NWs prepared by metalcatalyzed wet electro-less etching (using silver nitrate and hydrofluoric acid) of n-type Si wafer is an anisotropic chemical crystalline orientation dependent etch.4 The one-dimensional nanowire array over Si ensures large active surface area, ultralong axial optical path length for efficient light harvesting ranging till near infrared (NIR) spectral region, superior surface anti-reflectivity, surface defect-induced higher electrical conductivity and affordability.5,6 The substantial escalation in the efficiency of the Si NWs PEC cell is credited to the efficient radial charge collection allowing photoexcited carriers to diffuse through short distances thereby reducing recombination.7 Liquid-junction Si NW based solar cells though offer cost advantages over their solid-state counterparts

but their advancement is limited by surface photo-oxidation and photo-corrosion when in contact with the liquid electrolyte. Thus, PEC cells with a liquid redox electrolyte based on Si NW arrays have ample scope of exploration with very few noteworthy reports in the recent past. According to a study by Peng et al., reported in 2009, a platinum nanoparticle (PtNP) decorated Si NW based PEC solar cell using a liquid electrolyte containing a Br-/Br2 redox couple and Pt mesh as the counter electrode (CE) yielded a high PCE of 8.1%.8 This group in 2011, modified the photoanode to PtNP decorated carbon/silicon core/shell nanowire ([email protected]@SiNW) arrays in the above cell assembly, which exhibited a PCE of 10.86% owing to better surface passivation.9 In 2010, they had attempted for excellent stability of the PEC cell by using PtNP decorated methylated Si NWs in combination with an I-/I3- ionic liquid electrolyte but the PCE was only 4.3% under 100 mW cm-2 solar irradiation.10 The catalytic activity of Pt NPs has been extensively tested over the Si NWs electrode, and Au NPs have also been used.8 However, it must be noted that Pt and Au are electron conductors, and therefore, the possibility of these NPs functioning as electron traps cannot be ignored, whereby the photoexcited electrons are irreversibly injected from Si NWs to these NPs. Consequently, such metal nanoparticles may have a deleterious impact on the solar cell performance over a course of time. Substituting these expensive metallic NPs, with less costly alternates that can also

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improve the PCE of Si NW PEC solar cells, by exploiting not the widely reported electron conductivity, but the hole conductivity of the NP, is an exciting, yet unexplored approach. The efficacy of this approach is realized in the present report, where for the first time, trigonal selenium (t-Se) NPs decorated Si NWs based liquid junction PEC solar cell is developed and studied. Trigonal Se is a unique elemental semiconductor with an indirect bandgap of ~1.6 eV and diverse physical and optical properties, such as a relatively low melting point (~217 °C), high photoconductivity (~0.8  105 S cm-1), catalytic activity toward hydration and oxidation reactions, large piezoelectricity, thermoelectricity and nonlinear optical responses.11,12 It is one of the densest and most thermodynamically stable form of Se with covalently bonded Se atoms arranged helically in infinite chains that are trigonally bound together via van der Waals forces in a hexagonal lattice.13 The electrical conductivity of tSe lies between 10-6 to 10-5 Ω-1cm-1 at room temperature14 and due to this reason, it has captivated immense interest for its photoconductive, photovoltaic, and rectifying response under visible light illumination. In this work, t-Se NPs decorated over Si NW surfaces apart from being light sensitive hole conduits that prevent undesirable recombination losses, also scatter the impinging solar radiation by the virtue of their particle size and favorable visible light absorption, thus increasing the optical path length of light within the photoanode, and providing additional indirect photons for Si NWs to absorb and convert to electrons. For the first time, the usual CE for silicon based liquid junction solar cells (TCO glass substrates or Pt) has been replaced with a cheap and robust carbon fabric, not only due to its low sheet resistance (∼10 Ω cm-2) but also due to (i) the high surface area bestowed by its inter-woven mesh like morphology and (ii) good stability in low pH media like the HBr/Br2 electrolyte15 used here. The superior wettability and electrical conductance (0.62 S) of C-fabric as the CE also serves for the amplified cell performance.

Experimental Chemicals n-type Silicon wafers (CZ, resistivity 5-10 Ω cm) were acquired from Siegert wafer. Selenium dioxide (SeO2), silver nitrate (AgNO3), hydrazine hydrate (H6N2O) and isopropanol were purchased from Aldrich. Sulfuric acid (H2SO4, 98%), nitric acid (HNO3, 69%), hydrogen peroxide (H2O2, 30%), hydrofluoric acid (HF, 40%), hydrobromic acid (HBr, 47%), ethanol and acetone were purchased from Merck. Bromine (Br2) was bought from SDFCL, C-fabric was procured from Alibaba Pvt. Ltd and ultra-pure water with a resistivity of ⁓18.2 MΩ cm was obtained through a Millipore Direct–Q3 UV system. Preparation of trigonal Se SeO2 (0.4 g) was dissolved in 20 mL of distilled water and a homogenous solution of selenous acid was prepared under constant stirring. Hydrazine hydrate (0.5 mL) was added slowly to the above solution at room temperature to attain a brick red color indicating the formation of amorphous selenium (a-Se). This solution was further diluted with 20 mL of deionized water and then transferred to a 50 mL stainless steel Teflon lined autoclave. The hydrothermal reaction was maintained at 120 °C for 24 h to obtain trigonal selenium nanoparticles. The black precipitate was isolated by centrifugation, washed thrice with

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ethanol and distilled water respectively and oven-dried at 60 °C for 6 h.16 Etching of Silicon wafer The 1 cm2 n-Si (100) wafer pieces were initially ultrasonically degreased in acetone and ethanol at room temperature for 10 min. each. The wafer pieces then were immersed in a boiling piranha solution (H2SO4: H2O2 = 5:2 by volume) for 30 min., and thereafter rinsed in excess deionized water. The cleaned wafer pieces were immediately dipped into a solution of 5 M HF and 0.02 M AgNO3 for 60 min. Thick dendritic coating of Ag wrapped the wafer surface. The vertically aligned SiNW arrays were obtained after the wafer was immersed in a bath of concentrated HNO3 for 15 min., to remove the residual Ag layer. A black Si NWs etched silicon wafer was acquired which was then washed in deionized water to remove acid residue followed by the removal of oxide layer with buffered hydrofluoric acid solution (H2O: HF = 50:1 by volume) and dried under ambient conditions.8 Photovoltaic device fabrication As synthesized t-Se nanoparticles were dispersed in isopropanol and deposited over the etched nanowires on Si wafer by drop-casting to customize the photoanode for the solar cell. Highly conducting carbon fabric was chosen as counter electrode and the aqueous electrolyte comprised of 8.6 M HBr and 0.05 M Br2 as the redox mix. (Control cell: Oleylaminecapped Au nanoparticles were synthesized using a previously reported method.17 In brief, a two-phase synthesis was employed wherein 50 mL of a 10 mM HAuCl4 solution in H2O was mixed with equal volume of a toluene solution containing 25 mM TOAB. Oleylamine (1.65 mL) was added into the AuTOAB precursor after discarding the initial water phase. The mixture was stirred vigorously while quickly adding NaBH4 solution (0.283 g in 15 mL of H2O) dropwise. Deep red colour indicated formation of Au nanoparticles. The mixture without further purification was drop-casted over the Si NWs etched wafer, used as the photoanode). Instrumental methods XRD patterns were recorded on a PANalytical, X’PertPRO instrument with a Cu Kα (λ = 1.5406 Å) radiation as the X-ray source. Raman spectra were recorded on a Bruker Senterra dispersive Raman microscope spectrometer, with a 532 nm laser excitation. Surface morphology analysis was performed using field emission scanning electron microscopy (FESEM; Carl Zeiss Supra 40). Transmission electron microscopy (TEM) images were obtained for trigonal selenium on a JEOL 2100 microscope operating at an accelerating voltage of 200kV. AFM topography images of the samples were recorded on a using a Veeco, Multimode 8 with Scan Asyst (Nanoscope 8.10 software) microscope. Si tip with a cantilever of antimony (n)doped Si is used for recording the surface images. The optical absorption spectra were measured in the diffuse reflectance mode and converted to absorbance using the Kubelka−Munk function on a UV−VIS−NIR spectrophotometer equipped with an integrating sphere (Shimadzu UV-3600). Current versus potential (I−V) data of solar cells were measured using a LOTOriel solar simulator coupled with a Metrohm Autolab PSTAT302N. The light source was a 150 W Xenon arc lamp, which delivered a collimated output beam of 25 mm diameter through Air Mass (AM) 1.5 filter, providing a light intensity of 100 mW cm−2 (1 sun). The spatial uniformity of irradiance was confirmed by calibrating with a 2 cm × 2 cm Si reference cell

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The Journal of Physical Chemistry

and reaffirmed with a Newport power meter. IPCE versus wavelength was measured on a Newport machine with a tunable Xe lamp (300 W) as the light source. Electrochemical impedance spectra (EIS) for the cells were recorded on an Autolab PGSTAT 302N equipped with a frequency response analyzer (FRA) and a NOVA 1.11 software, under an AC amplitude of 20 mV over the frequency range of 1 MHz to 0.1 Hz. Photovoltage decay versus time and photocurrent versus time measurements were carried out by using a 1 sun white light source coupled to an Autolab PGSTAT 302N, which recorded the chronopotentiometric as well as chronoamperometric data in dark. Cyclic voltammetry (CV) was performed under three electrode system with Pt as the CE, and Ag/AgCl/KCl used as a reference electrode at a scan rate of 20 mV s-1. Conductance (G) was measured by linear sweep voltammetry (LSV) using two FTO glass electrodes for sandwiching the t-Se powder with a 2 mm wide para-film spacer in between.

Results and Discussion Structural analysis Prior to evaluating the photoelectrochemical performances of the electrodes, structural analysis is done. Raman spectra of Si NWs and Se [email protected] NWs are displayed in Figure 1a. An intense peak at 523 cm-1 is assigned to the stretching vibration of Si-O bond in the Si NWs etched wafer.18 For Se [email protected] NWs, an additional resonance peak at 235 cm-1 is observed that is attributed to the A1 symmetric vibrational stretching mode of the Se−Se bonds characteristic of the helical Se chains in the trigonal (t) phase.19 XRD patterns recorded for the Se NPs, Si NWs etched wafer and Se [email protected] NWs are shown in Figure 1b and c. The most intense peak for crystalline t-Se is observed at 2θ = 29.70o followed by peaks at 43.64o, 23.51o, 51.71o, 45.35o, 41.30o, 56.13o, 65.23o, 61.65o, 71.58o, 68.25o aligning with d = 3.00, 2.07, 3.78, 1.76, 1.99, 2.18, 1.63, 1.42, 1.50, 1.32, 1.37 Å corresponding to the (101), (102), (100), (201), (111), (110), (112), (210), (202), (113), (211) planes of the hexagonal crystal structure as per JCPDS card no. 060362. No diffraction peaks of SeO2 or any other impurity signify the purity of t-Se. The peaks indexed to lattice constants: a = 4.366Å and c = 4.953Å, are in concurrence with the standard card values for tSe. A highly intense peak at 2θ = 69.14o observed in the diffraction patterns of Si NWs and Se [email protected] NWs matching with d = 1.36 Å corresponds to the (400) plane of the face centered cubic (fcc) crystal lattice (JCPDS: 892955) which is parallel to the (100) plane of n-Si wafer, thus indicating the unidirectional etching of nanowires. The pattern of Se [email protected] NWs clearly shows peaks corresponding to both pristine t-Se and Si. The TEM image of Se NPs (Figure 1d) reveals the presence of spherical NPs of variable diameters, and the corresponding selected area electron diffraction (SAED) pattern shows spotty concentric rings, confirming the polycrystalline nature of Se. The NPs are clusters of 40-80 nm dimensions. Some of the bright spots are indexed to the (100), (101), (112) and (210) planes of hexagonal Se. Compactly packed arrays of oriented Si NWs are obtained over multiple micron scales of the n-Si wafer via electro-less etching, as is evidenced from the SEM image of Si NWs (Figure 1e). The cross-sectional view displays a dense network of vertically aligned Si NWs (Figure 1f). The diameters of the wires vary from 60 to 250 nm, and their average length is about 10 μm. The top view of the Si NWs is shown in Figure

1g, and it shows bunches of Si NWs, aligned almost vertically with respect to the underlying Si wafer. The image is consistent with previously observed images of Si NWs prepared by an etching process.10 The SEM images of Se [email protected] NWs shows Se NPs decorated over Si NWs; the images show that Se NPs are distributed uniformly across the lengths of Si NWs (Figure 1h-j). The top view of this electrode is different from that of pristine Si NWs, the particle density is much higher, indicating that Se NPs cover the top-ends of the Si NWs, and also tend to fill the gaps between the individual wires (inset of Figure 1j). The HRTEM image of Se [email protected] NWs heterojunction (Figure 1k) shows Se nanoparticles, 10-15 nm in size, anchored to the Si NWs. The SEM image of the C-fabric, which is used as a counter electrode in the liquid junction solar cells that were fabricated using the n-Si based photoanodes, is shown in Figure 1l. It shows overlapping smooth fibers of carbon, which impart the fabric like texture to this free standing flexible electrode. Role of Se NPs Scheme 1a illustrates the growth of the Si NWs over the n-Si wafer and the fabrication of Se NPs decorated Si NWs. The scheme also shows a liquid junction PEC solar cell configuration, where Se [email protected] NWs (or Si NWs or n-Si or [email protected]) electrode serves as the photoanode, an aqueous bromine/bromide redox couple serves as the hole scavenger, and a C-fabric as the counter electrode (Scheme 1b). When solar radiation impinges upon Si NWs, it is absorbed along the elongated axial direction, the minority carriers or the photogenerated holes are collected along the radial direction by the bromide ions present in the electrolyte, and therefore their diffusion lengths are very short. The inter-wire gaps in the Si NWs electrode allow deep electrolyte penetration during the solar cell operation. Photoexcited electrons generated in the Si NWs experience unobstructed transport along the length of the wires, due to the 1D-path of the NW, to the underlying Si, followed by their relay to the external circuit. They reach the oxidized Br3- ions at the C-fabric/electrolyte interface, and the ensuing reduced Br- ions diffuse to the photoanode where they oxidize by capturing the holes from Si NWs. Hole injection into the electrolyte from the Si NWs is further accelerated when Se NPs are flanked along the lengths of the Si NWs. Se NP is a ptype semiconductor, and the photoconductivity shown by Se NPs is due to their ability to conduct holes along the Sen chains that exist in the form of spirals aligned parallel to the c-axis in hexagonal Se. Such chains zig-zag through the Se lattice, and allow facile hole propagation.20,21 Scheme 1b illustrates hole transfer facilitated by Se NPs in the Se [email protected] NWs based liquid junction PEC solar cell. The hole concentration at room temperature in t-Se is reported to be of the order of 1013-1014 cm-3.21,22 The p-type behavior of Se NPs is confirmed from a Mott-Schottky plot shown in Figure 2a; the corresponding 1/(capacitance)2 versus voltage profile yields a negative slope, typical of hole conduction. The equation below, yields the density of holes (N). 1/C2 = 2/eroN [E – Efb – kT/e] (1) The slope is equal to 2/eroN, and by assuming r or dielectric constant of hexagonal Se as 8.1 from an earlier report,23 hole density is found to be 5.06  1016 cm-3, which is close to a value of 1015 cm-3 reported for hexagonal Se.24 Photoconductivity of Se NPs is gauged by comparing the I-V curves obtained under light irradiance and dark in the FTO/Se NPs/FTO sandwich configuration, over a voltage window of −2

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to +2 V (Figure 2b). The I-V profiles are linear affirming an Ohmic behavior and the conductivity () of Se NPs, is deduced from the slopes of the linear fits, as per the relation:  = I/V (d/a), where d is the thickness of the spacer or the sample and a is the sample’s cross-sectional area. The dark conductivity and the photoconductivity are 41.1 and 381.6 μS cm-1 under illumination. The ⁓10 times increment in the electrical conductivity of the as-synthesized Se NPs justifies their photoconductivity to an extent and serves as evidence for the improved hole-conduction under irradiance, thus amplifying the solar cell performance. While hole mobility is controlled by the acceptor states lying just above the valence band of Se, hole conduction is governed by the short range order of the Sen chains, and carrier density is dependent on the arrangement of these chains.25 Illumination is reported to increase photoconduction by decreasing the activation energy for hole conduction,22 which is favorably impacted by the aforesaid factors. Si NWs demonstrate strong light trapping properties in comparison with planar n-Si, as shown in Figure 2c. The average reflectivity of planar Si wafer is ⁓37% whereas that of Si NWs and Se [email protected] NWs are only ⁓5% and ~3% respectively over the 450–1000 nm wavelength range. The steep drop in reflectivity for the Si NW array specifies the light trapping ability due to its anti-reflection surface. The reflectivity light ratio is dramatically suppressed in the Si NWs structure, which concurs with the performance of Si NWs fabricated by an alternative, but more complex method, such as a vapor-liquid-solid growth process. The absorption spectra of Si NWs, Se NPs and the Se [email protected] NWs photoanode are shown in Figure 2d. Se NPs exhibit an absorption band in the 400 to 700 nm wavelength span and the band gap is calculated to be 1.69 eV. The absorption band of Si NWs is broad extending over the visible and NIR region from 400 to 1000 nm, and then it tapers off. From the absorption edge, the band gap of the n-Si semiconductor is calculated to be 1.16 eV. Se [email protected] NWs absorbs over both visible and NIR regions, with an enhanced absorption compared to pristine Si NWs, particularly over the 400 to 650 nm span. Over this region, the light scattered off the Se NPs is redirected into the Si NWs, increasing the optical path-length, thus increasing the effective absorption cross-section. Besides hole conduction, Se NPs also increase the absorption of Si NWs through light scattering effects. Impact of Se NPs on solar cell performance Si solar cells were assembled with planar n-Si, Se [email protected] n-Si, Si NWs and Se [email protected] NWs based photoanodes, a HBr (8.6 M)/Br2 (0.05 M) aqueous electrolyte and C-fabric as the CE. Current density (J) versus voltage (V) plots recorded under 1 sun irradiance (100 mW cm-2, AM 1.5 spectrum) are shown in Figure 3a and the parameters are summarized in Table 1. Optimization of Se NPs content is achieved through J-V studies (Table S1, supporting information). The solar cell with the planar n-Si wafer as photoanode gives a power conversion efficiency (PCE) of 0.048%. Deposition of Se NPs over planar Si (Se [email protected]) almost doubled the cell efficiency to 0.092% with small increase in all three parameters, open circuit voltage (VOC), short circuit current density (JSC) as well as fill factor (FF). The NWs etched Si wafer based cell, Si NWs/Br-/Br3-/Cfabric delivers a PCE of 4.91% with VOC almost doubling to 734 mV compared to the planar Si, and JSC escalating almost 30

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times to 13.88 mAcm-2. This improvement in cell efficiency, almost 100 times that of planar Si is due to: i) increased effective surface area (from BET analysis, surface area per unit mass obtained for planar n-Si is 0.13 m2/g and that of etched Si NWs is 36.74 m2/g), ii) enhanced optical absorption due to manifold light scattering and suppressed reflection, iii) improved charge separation at the Si NWs-electrolyte interface and iv) facile unidirectional charge carrier transport along the length of the nanowires. The PCE of this cell with Si NWs photoanode escalated to 7.03% when the Se NPs were deposited over the nanowire arrays. The significant rise in the JSC to 17.12 mA cm-2 for the Se [email protected] NWs/Br-/Br3-/C-fabric solar device is attributed to: i) the photoconductivity of Se NPs which favors rapid hole injection to the tri-bromide ions, thus increasing electron-hole separation compared to sole Si NWs, and ii) the particle size of Se clusters (40-80 nm) that enables the absorbed radiation to be scattered into the far field, thereby increasing the effective light path in the Si NWs photoanode. These effects translate into an elevated VOC of 790 mV, signifying increased transfer of the photogenerated holes at the charge separating Si NW/Se NP junction. Scheme 1 shows e--h+ separation, and hole transport via Se NPs to the Br- ions. This efficient hole transfer to Se NPs, reduces surface recombination dramatically, giving a 43% boost in the photovoltaic performance (compared to Si NWs alone). Since the recombination pathway is predominantly unavailable in the presence of Se NPs, the majority carriers or electrons immediately travel through the 1D pathways that prevail in Si NWs to the external circuit. The whole energy levels of Si NWs, Se NPs, Br-/Br2 electrolyte and C-fabric (Scheme 2) give a clearer insight to the working mechanism. The energy level positions of Si NWs and Se NPs were calculated from optical band gap and cyclic voltammetric studies. Details are provided in supporting information as Figure S1 and Table S2. When light impinges on the Se [email protected] NWs electrode, photoexcited electrons are injected from Si NWs to the external circuit, and the photogenerated holes are scavenged by the suitably positioned valence band (VB) of Se NPs, which is less negative than that of Si NWs (versus vacuum level). It is this hole extraction capability of Se NPs, which improves the charge separation in Si NWs, and imparts the cell with a high PCE. The energy level diagram shows that the conduction band (CB) of Si NWs is more negative than that of Se NPs. Furthermore, electrons from the VB of Se NPs can also undergo excitation upon irradiance, and the excited electrons cascade into the CB of Si NWs, and are then extracted by the external circuit. Solar cell parameters and their average for 5 different cells of each configuration are summarized in Table S3 to confirm the reproducibility of the result. A comparison of the Se [email protected] NWs based device against a control architecture which incorporates electronconducting Au metal nanoparticles in the Si NWs/Br-/Br3-/Cfabric cell was performed. Oleylamine capped Au NPs were prepared using a phase transfer method. The absorption peak of Au NPs is observed at 550 nm, which is attributed to surface plasmon resonance (Figure S2a). Au NPs act as electrocatalysts but their inclusion in the cell configuration does not improve the cell parameters to the extent that Se NPs does as is evident from the Table S4 in supporting information. Liquid junction PEC solar cell based on Si NWs photoanode yields a PCE of 4.91%, and with the addition of Se NPs to the photoanode, the PCE increment is almost 1.5 times whereas for Au NPs the rise

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is negligible. The comparative J-V plots are shown in Figure S2b. Similarly, relative impedance spectra of the Se [email protected] NWs based cell against a Au based device are shown in Figure S2c. The recombination resistance is lower for Se NPs based cell (1.69 ), compared to the Au NPs (2.18 ) one. A comparison of PV parameters of PEC cells based on Si NWs from literature is presented in Table S5 (supporting information). In most of the promising studies on Si NWs based liquid junction PEC solar cells, optical reflectance of Si NW arrays is reported to be < 5% but JSC values vary drastically (~0.9 to 37 mA cm-2, Table S5). While low reflectance is indirectly suggestive of enhanced e--h+ pair separation or improved JSC, but it isn’t necessarily so. This implies, that in this study, not all photons absorbed by Si NWs are converted to e--h+ pairs. But the VOC here is very high unlike other reports (0.3-0.73 V, Table S5) and this is attributed to Se NPs, which serve as hole scavengers and contribute to the charge build-up in the photoanode. IPCE versus wavelength plots for the two photoanodes, measured with C-cloth as the counter electrode and 0.05 M Br2 in 8.6 M HBr solution as the electrolyte are displayed in Figure 3b. This plot is also shown below for ready reference. The Si NWs/Br-/Br3-/C-fabric cell exhibits a maximum IPCE of 51.7% at λ = 600 nm. By the inclusion of Se NPs, the IPCE of the cell is increased to 62.2%. Both the curves taper off to zero at λ = 1100 nm. IPCE is controlled by electron injection and collection efficiencies, and light absorption by photoanode. Upon illumination, hole propagation in Se NPs is enhanced which results in better charge-separation or reduced recombination for the Se [email protected] NWs electrode, which is reflected in the increased IPCE for this cell, over the entire wavelength range under consideration. In terms of light harvesting, Se NPs exhibit a broad absorption peak over 350 to 630 nm range. Scattering can be defined as a combination of excitation + re-radiation. Thus, scattering and absorption are not mutually independent processes. The diffuse reflectance and absorbance spectra of Se NPs are shown in Figure S3 (supporting information). Though in the 350-630 nm, the reflectance is comparatively lower than that at longer wavelengths, but it is significant enough to enhance IPCE in this region. IPCE is therefore increased via two modes: (1) light which is (diffuse) reflected by Se NPs directly (this is equivalent to scattering of unabsorbed light) and (2) light which is absorbed and re-radiated. The average cluster of Se NPs is large enough (it is 40-80 nm), and ideally it should be > 50 nm to enable process (2). This light is now available for Si NWs for undergoing photoexcitation. Further, to quantify the absorption and re-radiation capability of Se NPs,  or absorption coefficient change induced by the inclusion of Se NPs in the Si NWs photoanode, was calculated by using the expression: () = ()Se [email protected] NWs  ()Si NWs, where  = 1/d  log(1/T). In the latter, R and T are the reflectance and transmittance respectively, and d is the thickness of the photoactive material. The variation of () versus wavelength is shown as an inset of Figure 3b. The magnitude of  is positive and finite till ~630 nm, which agrees with the enhanced IPCE over the 350-630 nm wavelength domain. At wavelengths > 630 nm, the diffuse reflectance of Se NPs is substantially high, and this accounts for the increased IPCE in the 630-900 nm wavelength range. Besides light absorption, photoconductivity of Se NPs also contributes to the IPCE increment at   630 nm.

To understand the reason for the high VOC shown by the Si NWs based cells prepared in this study, transient photovoltage decay plots are measured in the dark after attaining a stable voltage under a white light irradiance of 100 mW cm-2 (Figure 3c). The best cell based on Si NWs reported till date delivered a PCE of 10.86% under 1 sun irradiance, and has the following configuration: [email protected]@SiNWs/Br2/Br-/Pt mesh.9 The VOC of this cell was 0.53 V, which is lower than the VOC of 0.79 V achieved for the solar cell reported in this study (Se [email protected] NWs/Br2/Br-/C-fabric).9 While the PCE of the cell reported here is lower (7.03%), the VOC is higher due to the following reasons. Unlike [email protected] employed by Wang et al.,9 which is an electron conductor, the Se NPs anchored to the Si are hole conductors and this conductivity is further enhanced under illumination. The magnitude of VOC is directly proportional to the hole transfer capability of the photoanode to the oxidized redox species in the electrolyte (Br3-). Faster the hole transfer rate, higher is the VOC. Contrary to Pt NPs which can only serve as conductors for the photoexcited electrons generated by Si NWs, here the Se NPs, accept the photo-generated holes from Si NWs and transfer them rapidly to Br3- by the virtue of their excellent hole conduction characteristics. This advantage is not available to the [email protected]@SiNWs/Br2/Br-/Pt mesh based cell, and consequently, the VOC is lower. Even for our control cell without Se NPs (Si NWs/Br2/Br-/C-fabric), the VOC is high (0.73 V), due to the good internal charge build-up afforded by the Si NWs. This is clearly evidenced from Figure 3c. Greater the VOC, longer is the photovoltage decay duration. This duration is a measure of the charge build-up in the photoanode that is equivalent to the in-built potential. It is also representative of the rate of back electron transfer to the oxidized Br3- species of the redox couple. In the Si NWs based liquid junction PEC solar cell, the normalized photovoltage decay time is 30 ms, whereas the cell based on the Se [email protected] NWs electrode takes 42 ms to drop by the same extent. Firstly, this long decay time observed for Si NWs, translates to the high VOC of 0.73 V. Further increment in VOC (reaching 0.79 V) by incorporation of Se NPs, is an outcome of the hole (photo)conductivity benefit imparted by Se NPs (again reflected in the longer photovoltage decay span of 42 ms). In case of SiNWs, under illumination electronic excitation takes place and hole is generated in valence band. This light generated hole can recombine with the oxidized electrolyte. Due to this effect, Voc drops but when a hole scavenging agent like Se NPs are flanked along the axial length of SiNWs then recombination is reduced and open circuit voltage increases. The photovoltage drop was also measured at 2 more illumination intensities (Figure S4). It is 14.5 ms under 0.75 sun and 4.5 ms under 0.5 sun for Si NWs whereas the cell based on the Se [email protected] NWs electrode takes 20 ms under 0.75 sun and 7 ms under 0.5 sun irradiance. With the lowering of the light intensity, the photovoltage drop is faster in both cells, indicating a reduction in the charge build-up in the photoanode. However, at a given illumination intensity, the photovoltage decay time is longer for the cell with Se NPs. This reiterates that Se NPs improve the photovoltage response of the Si NWs cell. The open circuit voltage of a Schottky barrier can be mathematically expressed as function of short circuit current26 as, VOC = kbT/q (ln (ISC/Io) +1) (2)

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The VOC difference between Si NWs and Se [email protected] in all the batches are less than or equal to 60 mV (Table S3, supporting information). Since Se NPs decorated Si NWs showed higher short circuit current (Isc) compared to Si NWs, thus the Se [email protected] NWs device will show greater open circuit voltage(Voc) as the dark saturation current (Io) in both the devices are almost same. Therefore, although this difference may not be statistically large, but this difference cannot be ignored and it is attributed to the hole transporting and light scattering properties of Se NPs which causes a rise in ISC under illumination. The photocurrent versus time transients under 1 sun illuminated and dark conditions are recorded (Figure 3d). The highest photocurrent delivered by the Si NWs based cell is 14 mA cm-2 which rapidly decays under initial irradiation time. The stabilized photocurrent then attained after a few seconds is ⁓7 mA cm-2. The Se [email protected] NWs cell shows a maximum response of 17 mA cm-2 which also drops to a stable value of ~11 mA cm-2, but the decay is comparatively slower than that of Si NWs cell. The initial decay in the photocurrent is due to the oxidation of the silicon surface induced by the redox electrolyte when in contact thereby increasing the series resistance of the device. The adhering of the Se NPs over the Si NWs surface increases its surface roughness which in turn minimizes the series resistance by considerably suppressing the oxidizing effect of the electrolyte to stabilize the photocurrent. AFM topography images of Si NWs and Se [email protected] NWs, and the corresponding section profiles are shown in Figure S5 (supporting information). Si NWs show smooth nanowires (Figure S5a), and Se NPs tethered to Si NWs can be observed in Figure S5b. The root mean square surface roughness for the two samples are 56.45 and 90.32 nm. Se NPs impart a higher roughness to the electrode. The EIS curves for the Si solar cells with similar photoanodes that are tested for the solar cell parameters are recorded to assess the resistances offered by the systems to charge transfer and transport as seen in Figure 3e and f. The Nyquist plots were generated over a frequency range of 1 MHz to 0.1 Hz, by superimposing an ac potential of 20 mV over the open circuit potential in dark. The Nyquist plots display a skewed semicircle, followed by an incomplete arc, and these plots were fitted into the [R(RQ)(RQ)] circuit shown in Figure S6. The EIS parameters obtained from the fits are tabulated in the Table S6 of supporting information. Resistances: R1, R2, and R3 correspond to the bulk resistance of the electrolyte, charge transfer resistance at the C-fabric CE/electrolyte interface, and the recombination resistance at the photoanode/electrolyte interface. Yo1/N1 and Yo2/N2 are the two corresponding constant-phase elements. The overall charge transfer resistance in the planar n-Si photoanode based cells is fairly high (> 30 kΩ) compared to the cells with Si NWs. Recombination resistance, Rrec is inversely proportional to the recombination rate of electrons in Si with the acceptor Br3- species in the electrolyte at the Si/electrolyte interfaces. The diameter of the skewed semicircle in the mid- to low frequency range represents Rrec. The Rrec value for Se [email protected] NWs/Br-/Br3-/C-fabric cell is 1.69 kΩ and for bare Si NWs, the value drops to 1.09 kΩ which implies Se NPs in the former reduce the charge recombination between the electron from the CB of Si to the Br3- (oxidized electrolyte) species at the interface. Rrec being sensitive to surface processes, exhibits a strong dependence on

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illumination owing to the presence of photogenerated charge carriers (Figure 3g). Though the Rrec value for the Se [email protected] NWs/Br-/Br3-/C-fabric solar cell lowers to 0.97 kΩ but the total transfer and transport resistances at all the interfaces (CE/electrolyte and photoanode/electrolyte) and across the bulk of the electrodes also diminish due to the visible light responsive electrical conductivity of Se NPs. The stability of the liquid junction photoelectrochemical cell was tested wherein the effect of continuous 1 sun illumination on a cell with Se [email protected] NWs/Br-/Br3-/C-fabric configuration was monitored. The variation in solar cell parameters as a function of illumination time as shown in Figure 3h and the parameters are listed in Table S7 of supporting information. Generally, solid state and ionic liquid-based cells showed less degradation compared to cells with aqueous electrolytes.27 Withstanding the constraints posed by aqueous Br-/Br2 electrolyte, here, the stability drop of the aforementioned cell is considerably slow in the initial hours. Here, JSC and PCE drop by 51.7% and 66.4% respectively after 6 h of continuous exposure to 1 sun irradiance. A similar but a more drastic decay in JSC was also observed in a previous study of Si NWs solar cell, where the normalized JSC dropped dramatically by 90% in a Br-/Br2 electrolyte after 4 h of prolonged exposure to 1 sun irradiance.10

Conclusion In summary, a novel assembly for Si NWs based liquid junction PEC solar cell was fabricated with t-Se NPs decorated Si NWs as the photoanode and C-fabric as the CE retaining the usual Br/Br2 redox aqueous electrolyte. The Se [email protected] NWs/Br-/Br3/C-fabric cell delivers a high PCE of 7.03% that validates strong light scattering and absorption, superior hole transfer and transport and unhindered charge-transfer/transport at the radial Si NW/Se NP junction, compared to its Si NW counterpart cell. In the Si NW liquid junction solar cell, the photoexcited charge carriers generated in the Si NW array are separated by the builtin field, where electrons are directed to the n-type Si region and holes are driven to the Si NW surface where they are scavenged by the Br- electrolyte species. The Br3- produced in turn diffuses to the C-fabric where it gets reduced back to Br-. When the Se NPs decorated Si NWs is used as the photoanode: (i) the p-type conduction behavior of Se NPs, and the photoconductivity of Se NPs allow facile hole transfer at the Se NP/Si NW interface thus minimizing recombination and improving charge separation, and (ii) light scattering by Se NPs in the visible region dictated by their particle size, increases the effective light absorption capability of Si NWs, thereby increasing the number of majority charge carriers or photoexcited electrons, resulting in improved PCE, and contrasting significantly with the performance of the cell devoid of Se NPs. The PEC analysis data concurs with the PCEs of the solar cells, thus recommending well-tailored uniquely decorated Si NW array as a promising electrode for economical and scalable photovoltaic devices.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Detailed experimental method and characterization techniques, a table of solar-cell parameters for the optimization of Se NPs

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deposited over Si NWs, energy level diagram, CV plots, and table for energy levels, statistical J-V data for different cells, characterization of Au NPs and cells, a table of solar cell parameters for comparison between literature reports and this work, AFM images of Si NWs and Se [email protected] NWs, equivalent circuit diagram, stability data for a Se [email protected] NWs/Br-/Br3-/C-fabric cell.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: +91-40-23016024. Fax: +91-4023016003. ORCID Melepurath Deepa: 0000-0001-7070-5100

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Author A.K. is thankful to University Grants Commission (UGC) for the grant of a junior research fellowship. IPCE measurements were performed on an instrument obtained through a Solar Energy Research Initiative-Department of Science & Technology (DST/TM/SERI/2K12-11(G)) grant.

REFERENCES (1) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Coaxial Silicon Nanowires as Solar Cells and Nanoelectronic Power Sources. Nature 2007, 449, 885–890. (2) Peng, K.; Xu, Y.; Wu, Y.; Yan, Y.; Lee, S.-T.; Zhu, J. Aligned Single-Crystalline Si Nanowire Arrays for Photovoltaic Applications. Small 2005, 11, 1062–1067. (3) Liu, R.; Wang, J.; Sun, T.; Wang, M.; Wu, C.; Zou, H.; Song, T.; Zhang, X.; Lee, S. T.; Wang, Z. L.; Sun, B. Silicon Nanowire/Polymer Hybrid Solar Cell-Supercapacitor: A SelfCharging Power Unit with a Total Efficiency of 10.5%. Nano Lett. 2017, 17, 4240–4247. (4) Peng, B. K.; Lu, A.; Zhang, R.; Lee, S.-T. Motility of Metal Nanoparticles in Silicon and Induced Anisotropic Silicon Etching. Adv. Funct. Mater. 2008, 18, 3026–3035. (5) Peng, K.; Wang, X.; Lee, S.-T. Silicon Nanowire Array Photoelectrochemical Solar Cells. Appl. Phys. Lett. 2008, 92, 163103. (6) Shen, X.; Xia, Z.; Chen, L.; Li, S.; Zhao, J. Optical and Electrical Enhancement for High Performance Hybrid Si/Organic Heterojunction Solar Cells Using Gold Nanoparticles. Electrochim. Acta 2016, 222, 1387–1392. (7) Dalchiele, E. A.; Martín, F.; Leinen, D.; Marotti, R. E.; RamosBarrado, J. R. Single-Crystalline Silicon Nanowire Array-Based Photoelectrochemical Cells. J. Electrochem. Soc. 2009, 156, 77– 81. (8) Peng, K. -Q.; Wang, X.; Wu, X. -L.; Lee, S.-T. Platinum Nanoparticle Decorated Silicon Nanowires for Efficient Solar Energy Conversion. Nano Lett. 2009, 9, 3704–3709. (9) Wang, X.; Peng, K. -Q.; Pan, X. -J.; Chen, X.; Yang, Y.; Li, L.; Meng, X. -M.; Zhang, W. -J.; Lee, S.-T. High-Performance Silicon Nanowire Array Photoelectrochemical Solar Cells through Surface Passivation and Modification. Angew. Chem. Int. Ed. 2011, 50, 9861–9865.

(10) Shen, X.; Sun, B.; Yan, F.; Zhao, J.; Zhang, F.; Wang, S.; Zhu, X.; Lee, S. High-Performance Photoelectrochemical Cells from Ionic Liquid Electrolyte in Methyl-Terminated Silicon Nanowire Arrays. ACS Nano 2010, 4, 5869–5876. (11) Gates, B.; Yin, Y.; Xia, Y. A Solution-Phase Approach to the Synthesis of Uniform Nanowires of Crystalline Selenium with Lateral Dimensions in the Range of 10-30 nm. J. Am. Chem. Soc. 2000, 122, 12582–12583. (12) Li, X.; Li, Yan.; Li, S.; Zhou, W.; Chu, H.; Chen, W.; Li, I.; Tang, Z. Single Crystalline Trigonal Selenium Nanotubes and Nanowires Synthesized by Sonochemical Process. Cryst. Growth Des. 2005, 5, 911-916. (13) Zhang, R.; Tian, X.; Ma, L.; Yang, C.; Zhou, Z.; Wang, Y.; Wang, S. Visible-Light-Responsive t-Se Nanorod Photocatalysts: Synthesis, Properties, and Mechanism. RSC Adv. 2015, 5, 45165– 45171. (14) Mort, J. Transient Photoconductivity in Trigonal Selenium Single Crystals. J. Appl. Phys. 1968, 39, 3543–3549. (15) Johnson, E.L. The Texas Instruments Solar Energy System Development. Proc. 16th Intersoc. Energy Convers. Eng. Conf. 1981, 1, 798-804. (16) Zhang, B.; Dai, W.; Ye, X.; Hou, W.; Xie, Y. Solution-Phase Synthesis and Electrochemical Hydrogen Storage of Ultra-Long Single-Crystal Selenium Submicrotubes. J. Phys. Chem. B 2005, 109, 22830–22835. (17) Choi, H.; Chen, W. T.; Kamat, P. V. Know Thy Nano Neighbor. Plasmonic versus Electron Charging Effects of Metal Nanoparticles in Dye-Sensitized Solar Cells. ACS Nano 2012, 6, 4418–4427. (18) Ko, A. C.-T.; Hewko, M.; Sowa, M. G.; Dong, C. C. S.; Cleghorn, B.; Choo-Smith, L.-P. Early Dental Caries Detection Using a Fibre-Optic Coupled Polarization-Resolved Raman Spectroscopic System. Opt. Express 2008, 16, 6274–6284. (19) Zhang, S.-Y.; Liu, Y.; Ma, X.; Chen, H.-Y. Rapid, Large-Scale Synthesis and Electrochemical Behavior of Faceted SingleCrystalline Selenium Nanotubes. J. Phys. Chem. B. 2006, 110, 9041–9047. (20) Hyman, R. A. The Electrical Conductivity of Hexagonal Selenium. Proc. Phys. Soc. 1956, 69, 743–747. (21) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. Synthesis and Characterization of Uniform Nanowires of Trigonal Selenium. Adv. Funct. Mater. 2002, 12, 219–227. (22) Murphy, K. E.; Wunderlich, B. B.; Wunderlich, B. Effect of Structure on the Electrical Conductivity of Selenium. J. Phys. Chem. 1982, 86, 2827–2835. (23) Sicha, M.; Studnicka, J.; Prosser, V.; Gruber, B. The Permittivity Tensor of Hexagonal Selenium in the 3.3 cm Wave Region. Phys. Stat. Solidi 1964, 7, 1045–1050. (24) Fuhs, W.; Stuke, J. Hopping Recombination in Trigonal Selenium Single Crystals. Phys. Stat. Solidi 1968, 27, 171–184. (25) Spear, W.E. The Hole Mobility in Selenium. Proc. Phys. Soc. 1960, 76, 826–832. (26) Kamat, P.V.; Tvrdy, K.; Baker, D.R.; Radich, J.G. Beyond Photovoltaics: Semiconductor Nanoarchitectures for Liquidjunction Solar Cells. Chem. Rev. 2010, 110, 6664–6688. (27) Shen, X.; Chen, L.; Li, J.; Zhao, J. Silicon Microhole Arrays Architecture for Stable and Efficient Photoelectrochemical Cells Using Ionic Liquids Electrolytes. J. Power Sources 2016, 318, 146–153.

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FIGURES

Figure 1. (a) Raman spectra of Se [email protected] and Si NWs. XRD patterns of (b) Se NPs, and (c) Se [email protected] NWs; inset is an enlarged view of the low 2 region. (d) TEM image of Se NPs and their SAED pattern is shown as an inset. (e) SEM image of Si NWs; inset shows the image of a n-Si wafer (scale bar = 10 μm). (f) Cross-sectional view and (g) top view of Si NWs. (h-j) SEM images (inset of j is top-view, scale bar = 10 μm); and (k) HRTEM of Se [email protected] (l) SEM image of C-fabric.

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Figure 2. (a) Mott-Schottky plot of Se NPs recorded in dark. (b) J-V plots of Se NPs recorded in dark and under 1 sun illumination. (c) Variation of reflectivity with wavelength for Si NWs and n-Si wafer. (d) Absorbance spectra of Se NPs, Si NWs and Se [email protected]

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Figure 3. (a) J-V characteristics of liquid junction PEC solar cells with different photoanodes, C-fabric as the CE, and Br-/Br2 as the electrolyte, under 1 sun irradiance (AM 1.5); inset shows enlarged views. (b) IPCE versus wavelength curves for the solar cells with Se [email protected] and Si NWs photoanodes. () versus wavelength plot is shown as an inset. (c) Photovoltage versus time and (d) photocurrent versus time transients of solar cells with the two photoanodes. Nyquist plots of (e,f) solar cells with photoanode-Br-/Br2C-fabric configurations with varying photoanodes in dark, over a frequency range of 106 to 0.1 Hz at VOC and (g) a solar cell with the Se [email protected] photoanode under dark and under irradiance (50 mW cm-2) at VOC. (h) Effect of 1 sun (100 mW cm-2) exposure on cell parameters: VOC, JSC, FF, and PCE of a Se [email protected] NWs/Br-/Br3-/C-fabric solar cell on continuous illumination for 6 h.

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SCHEMES Scheme 1. (a) Illustration of fabrication of Si NWs, Se [email protected] and Se [email protected] photoanodes through photographs. (b) Schematic of the preparation of Se [email protected] and the liquid junction photoelectrochemical solar cell.

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Scheme 2. Energy level diagram of the Se [email protected] NWs-Br-/Br2-C-fabric solar cell showing all possible electron transfer modes upon illumination.

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TABLE Table 1. Solar cell parameters of cells with 8.6 M HBr/0.05 M Br2 aqueous solution electrolyte and C-fabric counter electrode under 1 sun illumination (100 mW cm-2) with the listed photoanodes (exposed cell area ⁓0.5 cm2). Photoanode configuration

VOC (mV)

JSC (mA cm-2)

FF (%)

best (%)

n-Si

426

0.41

27.42

0.048

Se [email protected]

479

0.69

28.05

0.092

Si NWs

734

13.88

48.23

4.912

Se [email protected] NWs

790

17.12

51.96

7.028

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TOC GRAPHIC

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