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Bernice Mae Yu Jeco-Espaldon, [Wipakorn Jevasuwan](https://orcid.org/0000-0001-9117-2497), Yoshitaka Okada, [Naoki Fukata](https://orcid.org/0000-0002-0986-8485)

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[Optimal performance of silicon nanowire solar cells under low sunlight concentration and their integration as bottom cells in III–V multijunction systems](https://mdr.nims.go.jp/datasets/53619aeb-66c1-47b2-af27-b87fcc95be1c)

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Optimal performance of siliconnanowire solar cells under lowsunlight concentration and theirintegration as bottomcells in III–Vmultijunction systemsBernice Mae Yu Jeco-Espaldon1,2,3*, Wipakorn Jevasuwan1,Yoshitaka Okada4 and Naoki Fukata11Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science(NIMS), Tsukuba, Japan, 2Center for Advanced New Materials, Engineering, and Emerging Technologies(CANMEET), University of San Agustin, Iloilo, Philippines, 3Department of Electronics Engineering,College of Technology, University of San Agustin, Iloilo, Philippines, 4Research Center for AdvancedScience and Technology (RCAST), The University of Tokyo, Tokyo, JapanNanostructured silicon solar cells are designed to minimize costs throughreduced material usage while enhancing power conversion efficiency viasuperior light trapping and shorter charge separation distances compared totraditional planar cells. This study identifies the optimal conditions fornanoimprinted silicon nanowire (SiNW) solar cells to achieve maximumefficiency under low sunlight concentration and evaluates their performanceas bottom cells in III–Vmultijunction solar cell systems. The findings indicate thatthe SiNW solar cell reaches its peak performance at a concentration factor of7.5 suns and a temperature of 40°C or lower. Specifically, the absolute conversionefficiency under these conditions is 1.05% higher than that under unconcentratedlight. Compared to a planar silicon solar cell under identical conditions, the SiNWsolar cell exhibits a 3.75% increase in conversion efficiency. Additionally, the SiNWsingle-junction solar cell, when integrated in series with a commercial lattice-matched InGaP/GaAs dual-junction solar cell, was tested under unconcentratedsunlight, specifically at one-sun, global air mass 1.5 condition, to assess its viabilityin one-sun multi-junction solar cell applications. The results suggest that a III–Vupper subcell with a smaller active area than that of the SiNW subcell is optimal formaximizing current production, which is favorable to the cost reduction of thedevice. This hybrid configuration is particularly advantageous for terrestrialapplications, such as electric vehicles, which demand lightweight, high-performance multijunction solar cell devices. Although the weight reductionof the characterized SiNW solar cell with a full silicon substrate compared to itsplanar solar cell counterpart is 1.8%, recommendations to increase this reductionto as much as 64.5% are discussed to conclude this paper.KEYWORDSlaser beam-induced current, multijunction solar cell, nanofabrication, solar cellcharacterization, silicon nanowire, III–V /silicon solar cellsOPEN ACCESSEDITED BYKeyla M. Fuentes,Spora Biotech, ChileREVIEWED BYSahil Tahiliani,Applied Materials, United StatesSandip Das,Kennesaw State University, United StatesMihir Kumar Sahoo,Indian Institute of Technology Bombay, India*CORRESPONDENCEBernice Mae Yu Jeco-Espaldon,bmyujeco@usa.edu.phRECEIVED 29 June 2024ACCEPTED 18 September 2024PUBLISHED 02 October 2024CITATIONYu Jeco-Espaldon BM, Jevasuwan W, Okada Yand Fukata N (2024) Optimal performance ofsilicon nanowire solar cells under low sunlightconcentration and their integration as bottomcells in III–V multijunction systems.Front. Nanotechnol. 6:1456915.doi: 10.3389/fnano.2024.1456915COPYRIGHT© 2024 Yu Jeco-Espaldon, Jevasuwan, Okadaand Fukata. This is an open-access articledistributed under the terms of the CreativeCommons Attribution License (CC BY). The use,distribution or reproduction in other forums ispermitted, provided the original author(s) andthe copyright owner(s) are credited and that theoriginal publication in this journal is cited, inaccordance with accepted academic practice.No use, distribution or reproduction ispermitted which does not comply with theseterms.Frontiers in Nanotechnology frontiersin.org01TYPE Original ResearchPUBLISHED 02 October 2024DOI 10.3389/fnano.2024.1456915https://www.frontiersin.org/articles/10.3389/fnano.2024.1456915/fullhttps://www.frontiersin.org/articles/10.3389/fnano.2024.1456915/fullhttps://www.frontiersin.org/articles/10.3389/fnano.2024.1456915/fullhttps://www.frontiersin.org/articles/10.3389/fnano.2024.1456915/fullhttps://www.frontiersin.org/articles/10.3389/fnano.2024.1456915/fullhttps://crossmark.crossref.org/dialog/?doi=10.3389/fnano.2024.1456915&domain=pdf&date_stamp=2024-10-02mailto:bmyujeco@usa.edu.phmailto:bmyujeco@usa.edu.phhttps://doi.org/10.3389/fnano.2024.1456915https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.org/journals/nanotechnology#editorial-boardhttps://www.frontiersin.org/journals/nanotechnology#editorial-boardhttps://doi.org/10.3389/fnano.2024.14569151 IntroductionSilicon photovoltaics (Si PV) represent by far the mostdeveloped and widely explored solar cell technology. The highestsolar cell efficiencies garnered were approximately 27% (Yoshikawaet al., 2017; Slade and Garboushian, 2005), while theShockley–Queisser (SQ) efficiency limit for a 1.1 eV single-junction Si solar cell was calculated to be approximately 32%(Rühle, 2016). With these, key technologies are being exploredand technical challenges are being addressed to finally reach oreven exceed the SQ efficiency limit. There are fundamental reasonswhy this efficiency limit is difficult to reach, although the Si PVtechnology is considered mature. One reason is the angle mismatchand temperature differences between the photon absorption andemission of the solar cell material, also known as Boltzmann loss(Hirst and Ekins-Daukes, 2011). In addition, a planar Si solar cellabsorber typically allows only one photon excitation and requireslonger carrier lifetimes as compared with its low-dimensionalstructure counterparts. A potential solution to this is the use ofnanostructured solar cells. Nanostructured solar cells, like Sinanowires (SiNWs), allow multiple exciton generation (MEG),which then increases carrier generation available for collection(Fukata et al., 2017). Although the first versions of SiNW solarcells achieved an efficiency of less than 1% (Tsakalakos et al., 2007;Stelzner et al., 2008), as of this article, multiple studies have alreadyexplored how to optimize SiNW solar cells (Kumar et al., 2011;Huang et al., 2012; Jung et al., 2013). SiNW solar cells, whendesigned with periodicity on the same order as the photonwavelength, can demonstrate high light-trapping efficiencies(Garnett and Yang, 2010; Yu et al., 2016), thereby allowing morephotons to excite electrons in the conduction band and producecurrent. Another possible way is to concentrate sunlight on the Siabsorber. Although there have been studies on planar Si singlejunction solar cells (1JSCs) (Yoshikawa et al., 2017; Campbell andGreen, 1986; Sinton and Swanson, 1987; Xing et al., 2015; Sintonet al., 1986; Green et al., 1986), there are no studies yet on SiNW1JSCs for concentrator systems to the best of the authors’ knowledge.Such studies were probably unpursued due to the unwanted effectsof Auger recombination at high temperatures brought about by longexposure to high solar irradiance (Vossier et al., 2010).Currently, the highest solar cell efficiency is achieved bymultijunction solar cells (MJSCs) made entirely of III–Vcompound semiconductors, reaching an efficiency of 47.6%under a sunlight concentration of 665 suns (Fraunhofer Institutefor Solar Energy Systems, 2022), where 1 sun is 100 mW/cm2.Despite their superior performance, these cells are prone to thermalrunaway over time (Zimmermann, 2013; Bett and Yarema, 2022;Nakamura et al., 2023; Algora, 2007), which can significantly reducetheir lifespan. Additionally, the limited availability of group IIImaterials poses substantial challenges for their widespreadcommercialization (Fitchette and Freundlich, 2016). To addressthese issues, replacing the long-wavelength absorber with a morecost-effective material, such as silicon, in concentrator III–V-basedMJSCs could lower the overall cost of this technology. AlthoughIII–V on planar Si MJSCs have been extensively studied forterrestrial applications (Yamaguchi et al., 2024; Essig et al., 2017;Schygulla et al., 2022), the current record efficiency for these devicesis held by wafer-bonded, two-terminal III–V//Si triple-junction solarcells (3JSCs), which have achieved an efficiency of 36.1% underconcentrated sunlight conditions (Schygulla et al., 2023). Althoughthere are several studies already on nanostructured III–V on SiMJSCs (Tong, 2023; Mi and Chang, 2009; Zhao et al., 2008), specificstudies on III-V on nanostructured Si MJSCs have no publications todate with no studies published to date. Meanwhile, perovskite-on-Sitandem solar cells have demonstrated efficiencies (Liu et al., 2023) ofapproximately 34%. Despite their promising performance, thesecells are susceptible to degradation under prolonged sunlightexposure, which can lead to phase changes in the perovskitelayer. This vulnerability raises concerns about their performancestability, particularly when exposed to concentrated sunlight overextended periods, where the degradation may be accelerated (AzkarUl et al., 2024; Deng et al., 2018; Hou et al., 2020; Schulze et al., 2020;Köhnen et al., 2021).From the gaps identified, it may be worth exploring at whichamount of sunlight concentration (in suns) will the SiNW solar cellsdeliver the optimal efficiency and determine their suitability as bottomcells for III–V-based MJSCs. Hence, we explored the performance ofnanoimprinted SiNW solar cells at sunlight concentration factorsbetween 1 and 20 suns and at various cell operating temperaturesbetween 25°C and 100°C. Investigating these effects is critical foroptimizing solar cell designs for real-world applications, particularlyin varying environmental conditions. Additionally, we obtained thecurrent–voltage (J-V) characteristics of an SiNW 1JSC electricallyconnected to a commercial lattice-matched (LM) InGaP/GaAs dual-junction solar cell (2JSC) to determine the performance of the SiNW asa bottom cell for III–V-based MJSCs. This was explored to leverage thehigh efficiency of III–V cells while potentially reducing costs throughthe use of SiNW technology. This hybrid approach is particularlyrelevant for terrestrial applications, such as electric vehicles, wherecost-effective and efficient energy solutions are increasingly necessary(Yamaguchi et al., 2021; Stauch, 2021; Alanazi, 2023).2 Materials and methods2.1 SiNWnanoimprint on 2” (100) n-Si wafersSiNWs on a 2″ n-type Si (100) substrates were fabricated bynanoimprinting, followed by the Bosch process, as done byJevasuwan et al. (2016), with some modifications. First, a 300-nm-thick commercial photoresist (NIAC) was spin-coated ontothe samples at 4,000 rpm for 60 s. Following the spin-coating,the samples were baked at 70°C for 20 s to ensure that the photoresistwas adequately dried. Next, a 30-nm-thick magnesium oxide (MgO)layer was deposited on the photoresist. This layer serves as a maskfor the subsequent patterning of the nanowires. The patterning wasachieved using UV nanoimprint lithography, employing a circularmask file to define the desired features. The 30-nm-thick MgO layerprovides sufficient coverage for effective masking during theimprinting process. After imprinting, the wafers were baked for5 min. Next, the photoresist was lifted off using an N-methyl-2-pyrrolidone (NMP) solution. After this, the wafers with thenanoimprinted wires were subjected to the Bosch process.Specifically, this step employed deep Si etching for 3 min usingSF6 and C4F8 plasma. These gases were kept at a flow rate of 35 sccmunder 0.75 Pa. The RF power of the plasma was set at 100 W. TheFrontiers in Nanotechnology frontiersin.org02Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915etching depth was confirmed after deep Si etching using scanningelectron microscopy (SEM). Finally, the 30-nm MgO layer wasetched out using a 1% H3PO4 solution. The process flow of theSiNW nanoimprinting lithography described is shown in Figure 1.The SiNW pitch and height are both 500 nm, and the diameter is200 nm, as illustrated in Figure 2A, and the arrays of SiNW viewed inSEM are shown in Figure 2B.2.2 SiNW solar cell fabricationAfter nanoimprinting the SiNW on n-Si substrates, solar cellfabrication was done as follows. First, the samples were cut into asize of 15 mm × 15 mm and then pre-treated with 2%hydrofluoric (HF) acid for 90 s. Next, the junction was grownby chemical vapor deposition (CVD) at 750°C for 3 min, whileSiH4 and B2H6 gases were flown at 19 and 0.5 sccm, respectively.The thickness of the p-type Si in this condition wasapproximately 70 nm. The back surface underwent dry etchingfor 40 min using SF6 gas at a flow rate of 20 sccm. Then, thesamples were treated with 2% HF acid for 60 s. The back surfacewas spin-coated with a p-dopant solution at 5,000 rpm for 60 s.The samples with back surface p-dopant were then subjected tothermal annealing at 850°C for 25 min to allow phosphorusdiffusion. After this, the samples were treated with 5% HFacid for 2 min to remove the residual dopant solution at thebackside and reduce surface oxides. Next, the Ti/Ag back contactmultilayer was deposited by reactive sputtering. The Ti layer wasFIGURE 1Process flow diagram of SiNW nanoimprinting lithography.FIGURE 2(A) Cross-section illustration (not to scale) of SiNWs with dimensions indicated. (B) Tilted (30°) cross-section SEM image of nanoimprinted SiNWsbefore junction growth viewed at 25.0 k magnification, 10.0 kV. The scale bar is 2.00 µm.Frontiers in Nanotechnology frontiersin.org03Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915deposited at an RF power of 200 W for 16 min, while the Ag layerwas deposited at a DC power of 170 W for 4 min. These backelectrode layers were grown to a size of 50 nm and 200 nm,respectively. Both steps were done while 15 sccm Ar gas wasflowing in the sputter chamber. Then, on the front side, a 250-nmAg layer was deposited by reactive sputtering at a DC power of170 W for 4 min, and Ar gas was also flown at 15 sccm. Astainless-steel mask (SUS304) was used to selectively deposit thefront electrode pattern. Finally, an approximately 120-nm-thickindium–tin oxide (ITO) layer was deposited on the front surfaceof the device by reactive sputtering. The sputtering process wascarried out at an RF power of 100 W for approximately 14 min.During the deposition, Ar and O2 gases were introduced into thesputtering chamber at flow rates of 19.2 sccm and 0.8 sccm,respectively. The ITO layer serves as a transparent conductiveelectrode that aims to enhance carrier collection from the device.The same process was used for fabricating the planar Si solarcells. The simplified cross-section schematics of the solar cells areshown in Figure 3.After ITO layer sputtering, the samples were manually cut with adiamond pen within the spans of the front electrode’s busbars andgrid. Thus, the effective active area of each sample was expected to bedifferent. The effective cell areas were determined by the ratio andproportion of the cell’s short-circuit current, ISC, in amperes withand without a 0.16-cm2 mask on the cell during measurement. Themeasured effective active areas of the best planar Si and SiNW solarcell samples were 0.33 and 0.43 cm2, respectively.2.3 Optoelectronic characterization2.3.1 Current–voltage (J–V) characteristicmeasurementsThe DC J–V characteristic measurements were acquired fromplanar Si and SiNW solar cells. There were two setups used for thesemeasurements. One was a continuous 150-W Xe white bias lamp(Bunkoukeiki Co., Ltd.) simulating the 1-sun, global air mass 1.5(AM 1.5G) standard condition. It has an illumination area of20 mm × 20 mm, JIS C8912/C8933 class A spectral match, and anon-uniformity irradiance of ±5%. Its light intensity stability iswithin ±3%. On the other hand, the pulsed J–V measurement setupthat simulates variable sunlight concentration factors employed aflash lighting system (Sugawara Laboratories Inc.: ESD-VF2M-U2 strobe driver and SLA-153-U1 lamp housing). The flashlamp’s flashing range, duration, and delay time are 0.22 Hz,50 µs to 2 m, and 10 µs, respectively. Both lamps were irradiatedthrough AM 1.5 G filters and were calibrated with a commercialcrystalline Si photodetector diode with known 1-sun currentproduction (Bunkokeiki, 2012).Right after electrode fabrication, the 1-sun J–Vcharacteristics of the planar Si and SiNW solar cells weremeasured at room temperature using the continuouslamp. Then, prior to pulsed J–V characteristic curvemeasurements, the back contacts of the bare cells weremounted on different Cu plates using In paste at 180°C. Aftercooling down, the front electrodes were bonded using the Al wireto the other side of the Cu plate, which is electrically isolatedfrom where the back contact was mounted. The actual images ofthe mounted samples are shown in Supplementary Figure S1.The pulsed J–V curves were obtained at various concentrationfactors between 1 and 20 suns. Using a Peltier stage, pulsemeasurements were also obtained at various cell temperaturesbetween 25°C and 100°C.We also measured the current–voltage characteristic curvesfrom the p-on-n SiNW 1JSC using an InGaP/GaAs 2J filter, acommercial n-on-p InGaP/GaAs 2JSC and series-connectedcommercial n-on-p InGaP/GaAs 2JSC and SiNW 1JSC wefabricated to emulate a three-terminal, triple-junction solar cell(3JSC), hereby referred to as InGaP/GaAs//SiNW 3JSC. Theseries-connected InGaP/GaAs//SiNW 3JSC was measured usingtwo independent, AM 1.5G 1-sun calibrated solar simulators. Aschematic of the dual lamp setup is shown in Figure 4.The series resistance, RS, and shunt resistance, RSH, wereapproximated usingR � dVdJ�∑w−1u�1Vu+1−VuJu+1−Ju∣∣∣∣∣ ∣∣∣∣∣( )w, (1)where Vu+1 and Vu are adjacent voltages; Ju+1 and Ju are the adjacentcurrent densities corresponding to Vu+1 and Vu, respectively; and wFIGURE 3Cross-section schematic illustration of (A) planar Si and (B) SiNW solar cells.Frontiers in Nanotechnology frontiersin.org04Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915is the total number of data points within the voltage range chosen.The voltage ranges used in RS and RSH calculations were from VOC toVOC + 0.04 V and from 0 to 0.2 V, respectively, in which theseparasitic resistances are most pronounced.2.3.2 External quantum efficiency measurementsExternal quantum efficiency (EQE) measurements wereobtained to quantify the photon-to-electron conversion of theplanar Si and nanoimprinted SiNW solar cells. A choppedmonochromated light was used to illuminate the samples duringmeasurements. The monochromated light sources are 500-W Xeand 400-W halogen lamps calibrated at AM 1.5G conditions using acrystalline Si photodetector diode with known 1-sun currentproduction (Bunkokeiki, 2012). The lamps have JIS C8912/C8933 Class A standard spectral match, irradiation intensities of5–50 μW/cm2 between 300 and 1,200 nm, positional non-uniformityof within ±2.5%, wavelength intensity constancy within ±3%, and amaximum wavelength variability of 20 nm. The illumination area iswithin 20 mm × 20mm. Using a Peltier stage, EQEmeasurements atvarious cell temperatures were carried out. The measurements wereacquired at short-circuit conditions, i.e., at V = 0 V. Then, thecurrent density was derived using the relationJAM1.5G � qhc∫λULλLLλ × EQE λ( ) × IAM1.5G λ( )dλ, (2)where λUL and λLL are the absorption range limits of the evaluatedcell, IAM1.5G(λ) is the wavelength-dependent AM 1.5G irradiancebased on ASTMG-173 standard (ASTM Standard Reference SpectraG173-03, 2012), q is the electron charge, h is the Planck’s constant,and c is the speed of light.2.3.3 Laser beam-induced current mappingOne can employ laser beam-induced current (LBIC) mapping toobserve the quality of the cell’s active area (Bajaj et al., 1987;Honsberg and Bowden, 2010) and quantify its currentproduction uniformity. In this work, a modulated 785-nm laserwas used to excite the Si solar cells. Modulating a laser enablesenhanced signal-to-noise ratio (SNR) and improved spatialresolution, which results in better quality of LBIC map images.The measured laser power density, switching frequency, scanningfrequency, and duty cycle of the laser are 3,550 mW/cm2, 2000 Hz,2 Hz, and 50%, respectively. The Peltier stage was also used to varythe cell operating temperature between 25°C and 100°C (Yu Jecoet al., 2019a; Yu Jeco et al., 2018a).To quantify the homogeneity of carrier collection, the currentuniformity was calculated using standard deviation, σJ, normalizedwith average LBIC, JLBIC,ave, which is then given as (Yu Jeco et al.,2019a; Yu Jeco et al., 2018a; Yu Jeco-Espaldon et al., 2020; Yu Jecoet al., 2019b; Yu Jeco et al., 2016; Yu Jeco et al., 2017a; Yu Jeco et al.,2018b; Yu Jeco et al., 2017b)σJJLBIC,ave�������������������1N−1∑Ni�1 Jl − JLBIC,ave∣∣∣∣ ∣∣∣∣2√JLBIC,ave, (3)whereN is the total number of LBICmap points and Jl is the currentat spot l. Here, a lower σJ/JLBIC,ave value means better currentproduction uniformity and hence better cell quality. Althoughgenerally only σJ is calculated, normalizing with JLBIC,aveeliminates the differences in carrier collection efficiency amongthe samples, thereby yielding a fair comparison of their currentuniformity. In addition, the perimeter region was excluded from thecalculation to suppress the influence of perimeter recombination.FIGURE 4Schematic illustration of dual AM 1.5G, 1-sun-calibrated lamp setup for measuring the J–V characteristics of InGaP/GaAs//SiNW 3JSC. Themeasured InGaP/GaAs 2JSC is an n-on-p device, while the series-connected SiNW 1JSC is a p-on-n device.Frontiers in Nanotechnology frontiersin.org05Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.14569153 Results and discussion3.1 Planar Si and SiNW 1JSCs at 1-sun, roomtemperature3.1.1 J–V characteristic curveThe J–V characteristic curves of planar Si and SiNW solar cells weremeasured to determine their electrical performance in pristineconditions, particularly after ITO layer deposition. Figure 5 shows theJ–V characteristic curves of the best-performing planar Si and SiNWsolar cells at 1-sun, AM1.5G, room temperature condition. The electricalparameters of the solar cells derived from 1-sun J–V characteristic curvesare listed in Table 1. By visual inspection, it can be inferred that theperformance of the SiNW solar cell is better than that of the planar Sisolar cell. Such inference is reflected numerically in their short-circuitcurrents, JSC, open-circuit voltages, VOC, and conversion efficiencies, η.Approximately 4.1% higher η than the η of the planar structure wasachieved using the SiNW structure for solar cells. However, the fill factor(FF) of SiNW is less than the FF of the planar Si solar cell. This isattributed to the SiNW solar cell having a significantly larger JSC thanplanar Si while having a small difference in VOC. The reason the VOC ofthe SiNW1JSC did not increase significantly compared to theVOC of theplanar Si 1JSC can be explained as follows. Their energy gaps,E.g., did notvary because the NW height (500 nm) and diameter (200 nm) are stilltoo high to induce the quantum confinement effect. In past literature, theNWdiameter, dNW, that can cause a noticeable increase in E.g., should bein the range of 1–10 nm (Xia and Cheah, 1997; Li and Wang, 2004).Using the relation between the change in E.g., ΔEg, and dNW (Li andWang, 2004), we haveΔEg � βNWdαNWNW,where βNW and αNW for Si were empirically determined to be1.53 and the unit of βNW is eV × (nm)α; the estimated ΔEg for a200-nm SiNW is 0.46 meV. If the E.g., offset,WOC, is approximately400 meV for Si (King et al., 2011); then, based on the definition ofWOC, which relates E.g., and VOC,WOC � Egq− VOC.A 0.46 meV increase in E.g., does not significantly change VOC as itwould if dNW is approximately 1–10 nm. If dNW was either 1 nm or10 nm, ΔEg would have been 1.53 eV and 45.2 meV, respectively.3.1.2 EQE measurementsEQE measurements were obtained from Cu-mounted planarand SiNW 1JSC. Figure 6 shows the EQE response of the solar cells.Between 350 and 450 nm, the EQE response of the planar Si solar cellwas higher than that of the SiNW. Since the SiNW has a largersurface area than planar Si, more surface defects may form, therebydegrading the EQE response at shorter wavelengths. On the otherhand, beyond 450 nm, the EQE response of the SiNW solar cell ismuch higher than that of the planar Si solar cell. Surfacerecombination has less impact on longer wavelengths as they areabsorbed deeper in the bulk silicon, away from the surface defects.Carriers generated deeper in the bulk by longer wavelengths have ahigher probability of being collected despite surface defects (Ohet al., 2012). One way to mitigate this is to coat the SiNWwith Ir(III).FIGURE 5One-sun, AM 1.5G J–V characteristic measurements obtainedfrom planar Si and SiNW 1JSCs after ITO layer deposition.TABLE 1 Planar Si and SiNW solar cell performance at 1-sun, AM 1.5Gillumination, and room temperature.Parameter Planar Si solar cell SiNW solar cellJSC (mA/cm2) 20.62 29.28VOC (V) 0.524 0.533FF 0.57 0.65η (%) 6.13 10.21FIGURE 6EQE measurements obtained from planar Si and SiNW solar cellsat room temperature. The dashed–dotted lines are placed at 670 nm(≈Eg,InGaP = 1.87 eV) and 870 nm (≈Eg,GaAs = 1.42 eV).Frontiers in Nanotechnology frontiersin.org06Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915Our device did not have an Ir(III) coating (Kim et al., 2018); hence,compared with the planar 1JSC, the light-to-electricity conversion inSiNW 1JSC was only improved from 450 nm and beyond.As for the potential use of the SiNW as a lower-bandgap subcellin an MJSC device, one must select an upper subcell that will enablethe SiNW subcell to absorb wavelengths beyond 450 nm. To explorethis further, the JSC values at two wavelength ranges were derivedfrom the EQE measurements. The ranges of 670–1,200 nm and870–1,200 nm were chosen to determine how much JSC the SiNWsubcell may produce when stacked with InGaP only and InGaP/GaAs absorbers, respectively. These materials are typically used asupper subcells of III–V-based MJSCs (Cotal et al., 2009), where thebandgaps of InGaP and GaAs are 1.87 and 1.42 eV (Kazuaki andTakaaki, 2011), corresponding to absorption band edges ofapproximately 670 nm and 870 nm, respectively. The calculatedJSC values from EQE at different wavelength ranges using Equation 2are summarized in Table 2. Based on the calculated JSC values, thecurrents required for matching the SiNW as a bottom cell with anInGaP top cell and an InGaP/GaAs 2J tandem cell are approximately15.74 and 5.40 mA/cm2, respectively. Practically, the JSC of III–V-based individual and tandem cells is approximately 10–14 mA/cm2at 1 sun (Cotal et al., 2009; Takamoto et al., 2005; Lueck et al., 2006).Thus, the estimated current mismatch for InGaP and InGaP/GaAson SiNW systems would be 1.74–5.74 mA/cm2 and 4.6–8.6 mA/cm2,respectively. Nevertheless, there are possible approaches to augmentthis issue. The simplest approach would be to implement a three-terminal configuration in which a third conductive electrode isplaced between the III–V and the SiNW subcells (Schnabel et al.,2020). Meanwhile, if the SiNW, as a lower-bandgap subcell, does notlimit the monolithic or wafer-bonded MJSC current, i.e., in theInGaP//SiNW tandem case, one may try to select a much shorter orlarger NW array periodicity or a shorter NW height to reduce itscurrent production (Elrashidi, 2022). In the case of SiNW being thecurrent-limiting bottom cell (InGaP/GaAs//SiNW case), one mayuse a photo-assistive layer to enhance SiNW carrier collection by theluminescent coupling effect (Yu Jeco-Espaldon et al., 2020). Incurrent-mismatched MJSCs, the luminescent coupling effect isthe reabsorption of photons in a lower-bandgap subcell (Bauret al., 2007; Lee et al., 2011; Derkacs et al., 2012; Friedman et al.,2013; Steiner et al., 2012; Friedman et al., 2014). Specifically, thesephotons were emitted from a higher-bandgap subcell or a photo-emissive layer toward a lower-bandgap subcell.3.1.3 Laser beam-induced current mappingTo observe the current uniformity of planar Si and SiNW solarcells, LBIC measurements were acquired after ITO layer sputtering.Figures 7A, B show the LBIC maps of planar Si and SiNW solar cells,respectively. Their average LBIC and normalized currentuniformities calculated using Equation 3 are listed in Table 3.The low-current regions are the busbar and the grid fingers ofthe solar cells, while the high-current regions are the active cell areas.Comparing the active regions of the cells, SiNW was found to yieldlarger current production than the planar Si. This agrees with the JSCof the SiNW solar cell acquired from J–V characteristics and EQEmeasurements, being larger than those of the planar Si solar cell.Although the maps were acquired at zero-bias (V = 0) conditions, itshould be noted that the LBIC is much less than the JSC because theLBIC is the current measured per laser spot area and only uses asingle-wavelength light source. SiNW LBIC yielded lowerσJ/JLBIC,ave, indicating that it produced a more uniform current.In addition, although the perimeter was excluded from σJ/JLBIC,avecalculation, there are still sources of errors, such as the valleycurrents obtained from the front grid pattern whose width is inthe same order as that of the 785 nm excitation laser spot diameterand the difference in cell active area, since the size of the perimeterregion assumed was the same for both sample measurements.Nevertheless, the LBIC map colors of the active regions of thesamples in Figure 7 qualitatively show that the calculated σJ/JLBIC,avecan be considered reliable.3.2 Planar Si and SiNW solar cells at variousconcentration factors and operatingtemperaturesAfter 1JSC measurements at 1 sun, room temperature, pulsedJ–V characteristics, EQE and LBIC measurements were done atvarious temperatures. To allow fair comparison and to beaccommodated by the contact probes of the pulsed J–Vcharacteristic measurement setup, each of the best-performingplanar Si and SiNW solar cells was mounted on a Cu plate.3.2.1 Pulsed J–V characteristic measurementsPulsed J–V characteristic curves of Cu-plate-mounted planar Siand SiNW solar cells are shown in Supplementary Figure S2. Thesecurves were obtained at various concentration factors and celltemperatures. Although SiNW solar cell performance is superior at1 sun, shunt resistances (RSH) became more severe with concentrationfactors of 15 suns and higher, compared to planar Si (SupplementaryTables S1, S2). These results suggest that the increased concentrationfactor aggravates surface recombination in SiNWs. To solve this, surfacepassivation or two-step H2 annealing (Jevasuwan et al., 2017) of SiNWsolar cells is recommended. Moreover, although the RSH trend generallydecreased with an increasing concentration factor, some data pointsdeviated from this trend, particularly at 2.5 suns and 25°C and 40°C forplanar 1JSC and at 7.5 suns and 25°C and 2.5 suns and 40°C for SiNW1JSC, as shown in Supplementary Figures S3A, B. This is attributed tothe flashing duration of the pulsed lamp used, which ranges between50 µs and 2 m, as described in Section 2.3.1. This can cause somefluctuations during current measurement at each voltage appliedbecause of the variability in the Xe lamp used (De Rooij, 2024;Chawla, 2024), which then affects the RSH calculation usingEquation 1. On the other hand, it is difficult to comment on theevolution of series resistance (RS) with temperature since these valuesare within the same order of magnitude (Supplementary Tables S3, S4;TABLE 2 Planar Si and SiNW1JSC JSC derived from 1 sun EQEmeasurementsat various wavelength ranges.Wavelength range (nm) JSC from EQE (mA/cm2)Planar Si 1JSC SiNW 1JSC305 to 1,200 (1J) 18.07 24.47670 to 1,200 (2J, InGaP//SiNW) 11.19 15.74870 to 1,200 (3J, InGaP/GaAs//SiNW) 4.00 5.40Frontiers in Nanotechnology frontiersin.org07Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915Supplementary Figures S3C, D). Nevertheless, the calculatedRS for bothsamples at any temperature decreased with increasing concentrationfactor. Because the flash lamp irradiated thewhole area of the cell, theRStrend with increasing concentration factor may be attributed to thephoton healing effect occurring within the cell perimeter, therebyincreasing lateral transport efficiency (Ramspeck et al., 2007; Trupkeet al., 2007; Hinken et al., 2007; Kampwerth et al., 2008; Xu et al., 2019).However, the photon healing effect was found insufficient to reducesurface recombination, most especially with SiNW solar cellscharacterized at higher concentration factors.Figures 8A, B and Supplementary Tables S5, S6 show theevolution of JSC with an increasing concentration factor atvarious operating temperatures of planar Si and SiNW solar cells,respectively. Here, the error bars indicate the standard deviation ofthree measurement trials. The SiNW solar cell yielded a larger JSCthan the planar Si. Based on the shape of the J–V curves, the largerJSC in SiNW solar cells is partially due to the leakage current broughtabout by surface defects and, hence, not necessarily a desirablecurrent increase. This agrees with the lower RSH values calculated athigher concentration factors, as discussed earlier. The trend ofplanar Si and SiNW solar cells’ VOC with increasing celltemperature and concentration factor showed no difference, asplotted in Figures 8C, D and listed in Supplementary Tables S7,S8. On the other hand, the planar Si 1JSC VOC values were found tobe generally larger than those of the SiNW 1JSC, which is contrary tothe observation made before they were mounted on a Cu plate, asshown in Figure 5 and Table 1. This contrasting observation thensuggests that the contact quality of the back electrode of the planar Si1JSC with the In paste and the Cu plate is better than that ofthe SiNW 1JSC.Figure 8E and Supplementary Table S9 show that an optimal FFis achieved at approximately 5 suns for the planar Si solar cell. Onthe other hand, the FF of the SiNW solar cell degraded with anincreased concentration factor at any cell operating temperature(Figure 8F; Supplementary Table S10). Moreover, the decrease in FFof the SiNW solar cell with increasing cell temperature is sharper ascompared to the FF of the planar Si solar cells. Aside from having alower-quality Cu plate contact, these observed degradations can beexplained as follows. As temperature increases, the rate ofelectron–hole recombination increases. This phenomenon isparticularly pronounced in SiNW structures due to their highsurface-to-volume ratio, which leads to more surface states thatcan act as recombination centers. In addition, as light concentrationincreases, the number of charge carriers generated in the SiNW solarcell also increases significantly. This leads to higher recombinationrates, especially in the presence of defects or impurities in the SiNW.The increased recombination reduces the effective RSH, which thendegrades the FF as more carriers are lost before contributing to theoutput current. These findings further confirm the detrimental effectof surface defects in the SiNW solar cell with increasingconcentration factor. Consequently, the conversion efficiency, η,of the planar Si 1JSC (Figure 8G; Supplementary Table S11) at ahigher concentration factor is generally superior to that of the SiNW1JSC (Figure 8H; Supplementary Table S12).The η values in Figures 8G, H were adjusted by +0.77% and+4.53% to eliminate the parasitic resistance effects caused by Cuplate mounting using the In paste. The calculation of η valueadjustments, Δη, can be found in Supplementary Material,Section 1.2. The optimal concentration factor that yields thehighest conversion efficiencies for solar cells is approximately7.5 suns at cell temperatures of 40°C. In particular, the absoluteconversion efficiency under these conditions is 1.05% higher thanthat under 1 sun. Upon comparing to a planar Si 1JSC under thesame condition, the SiNW 1JSC achieved a 3.75% absoluteincrease in conversion efficiency. In CPV systems, there existsan optimal concentration factor at which a solar cell operatesTABLE 3 Average LBIC, JLBIC,ave, and normalized current uniformities, σJ/JLBIC,ave, of planar Si and SiNW 1JSCs.Structure Average LBIC, JLBIC,ave (mA/cm2) Normalized current uniformity, σJ/JLBIC,avePlanar 1JSC 2.35 0.2734SiNW 1JSC 3.65 0.2180FIGURE 7LBIC maps measured from (A) planar Si and (B) SiNW 1JSCs at room temperature and zero-bias (V = 0 V) conditions.Frontiers in Nanotechnology frontiersin.org08Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915FIGURE 8(A–B) JSC, (C–D) VOC, (E–F) FF, and (G–H) η of the best performing planar Si and SiNW 1JSCs mounted on the Cu plate, respectively, at various celloperating temperatures and concentration factors. The error bars indicate the standard deviation derived from three trials ofmeasurements. The JSC,VOC,and FF values plotted in (A–F)were directly obtained from raw J–V characteristics shown in Supplementary Figure S2, while the η values plotted in (G, H)were adjusted by +0.77% and +4.53% to eliminate the parasitic resistance effects caused by Cu plate mounting using the In paste. The calculation ofη value adjustments, Δη, is discussed in Supplementary Material, Section 1.2.Frontiers in Nanotechnology frontiersin.org09Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915most efficiently. If the concentration factor exceeds this optimallevel, thermodynamic principles indicate that a portion of theconcentrated sunlight will be lost as heat, leading to reducedefficiency. Therefore, SiNW solar cells would be more suitable forlow-concentration photovoltaic applications rather than forhigh-concentration scenarios.3.2.2 One-sun EQE measurements at different celloperating temperaturesThe EQE responses of Cu plate-mounted planar Si and SiNWsolar cells are shown in Figures 9A, B, respectively. The redshiftof EQE tails with increasing cell operating temperatures indicatesbandgap reduction and is a typical behavior for a semiconductordevice (Varshni, 1967; Tobnaghi et al., 2013; Schlangenotto et al.,1974; Lautenschlager et al., 1985; Alex et al., 1996; Loureno et al.,2004; Ishitani et al., 1994). As one may infer from the JSC valuesderived from EQE measurements using Equation 2 (Table 4), thisredshift can be beneficial if the SiNW is used as a lower bandgapsubcell in an MJSC structure. This can be inferred from thehigher JSC calculated at higher temperatures for the870–1,200 nm range.On the other hand, the reduction in the EQE response withincreasing cell temperature is more abrupt in the SiNW than thatof the planar Si, particularly at wavelengths between 450 nm and950 nm. This suggests that more events of carrier recombinationcould happen in the SiNW at high operating temperatures, whichcan happen at higher concentration factors due to thermalrunaway. This agrees with the sharp decrease in FF withincreasing concentration factor and cell operatingtemperature, as discussed in Section 3.2.1. Therefore, whenusing the SiNW solar cell for CPV applications, a coolingsystem should be included in the system design to helpmaintain lower operating temperatures. In addition, we canincrease the p-shell growth time during CVD (Adachi et al.,2013) to possibly achieve EQE tail redshift in the SiNW, whichwill be favorable to the current-matching within III–V on SiNWMJSCs. However, if the p-shell is too thick, the SiNW 1JSC mayexperience increased recombination due to defect formation(Adachi et al., 2013), doping imbalances, and increasedresistance (Kendrick et al., 2017). Therefore, the p-shell mustbe optimized to achieve EQE tail redshift without compromisingon the SiNW 1JSC device quality.3.2.3 LBIC mapping at various cell temperaturesFigure 10A–E and Figure 10F–J show the LBIC maps of Cuplate-mounted planar Si and SiNW 1JSCs, respectively, at celltemperatures between 25°C and 100°C, and Table 5 provides asummary of their averaged LBIC, JLBIC,ave, and normalizedFIGURE 9One-sun EQE response of Cu-mounted (A) planar Si and (B) SiNW solar cells at various cell operating temperatures.TABLE 4 Planar Si and SiNW solar cell JSC calculated from 1 sun EQE measurements at various wavelength ranges and operating temperatures.T(°C)Planar Si JSC (mA/cm2) SiNW JSC (mA/cm2)305–1,200 nm(1J)670–1,200 nm(2J)a870–1,200 nm(3J)b305–1,200 nm(1J)670–1,200 nm(2J)a870–1,200 nm(3J)b25 18.07 11.19 4.00 24.47 15.74 5.4040 18.48 11.55 4.21 26.07 16.92 6.0060 18.54 11.71 4.42 25.93 17.00 6.2480 18.43 11.77 4.59 25.38 16.81 6.37100 17.92 11.56 4.65 24.30 16.25 6.35aEstimate for the InGaP//SiNW, 2JSC, where the absorption band edge of InGaP is approximately 670 nm (Eg = 1.87 eV).bEstimate for the InGaP/GaAs//SiNW, 3JSC, where the absorption band edge of GaAs is approximately 870 nm (Eg = 1.42 eV).Frontiers in Nanotechnology frontiersin.org10Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915current uniformity, σJ/JLBIC,ave. The JLBIC,ave of the SiNW 1JSC wasconsistently higher than that of the planar Si at any cell temperature.However, as discussed earlier, a portion of the SiNW current mayincrease due to shunt leakage, which is not necessarily desirable.Meanwhile, current degradation with increasing cell temperaturewas observed in both solar cells. This observed decrease in current isattributed to an increase in nonradiative recombination events athigher temperatures brought about by elevated energy of chargecarriers in the device (Schubert et al., 2018; Shaker et al., 2024). Onthe other hand, σJ/JLBIC,ave has no definite trend in both cells. As itdoes not vary by more than hundredths, it may be inferred that theσJ/JLBIC,ave did not vary with cell temperature, as previously observedin III–V solar cell materials (Yu Jeco et al., 2019a; Yu Jecoet al., 2018a).3.3 Characterizing the SiNW as a bottom cellfor III–V MJSCAs mentioned earlier, SiNWs are attractive candidates as thebottom cell material for III–V-basedMJSCs because they have betterlight trapping and a shorter charge separation distance in thejunction than their planar counterparts. In this section, wecharacterized the EQE of SiNW 1JSC with and without theInGaP/GaAs 2J filter, and each active layer of the InGaP/GaAs2JSC was measured. As shown in Figure 11, adding an SiNW as abottom cell to the III–V 2JSC extended the absorption range to theinfrared region, from approximately 850 nm to 1,200 nm. InFigure 11, the EQE of planar 1JSC was added for ease ofcomparison. Comparing the SiNW EQE as 1JSC under theInGaP/GaAs 2J filter, it can be observed that the filtered SiNWhas a lower EQE than the unfiltered SiNW 1JSC at850 nm–1,200 nm. This could be due to optical losses, such asreflection and destructive light interference, in the 2J filter.Based on the derived ISC from independent EQEmeasurements of InGaP/GaAs 2JSC and SiNW 1JSC with andwithout the 2J filter (Table 6), the latter will be limiting thecurrent of the InGaP/GaAs//SiNW 3JSC. This is also consideringthat the InGaP/GaAs 2JSC, measured independently, has anactive area, Acell, of 1.00 cm2 and that it is made up of directbandgap III–V absorbers, which can deliver much higher powerconversion efficiency than Si. Thus, for the 1-sun J–Vcharacteristic measurement, we applied a laser-cut metal maskwith a 0.20 cm2 aperture on top of the 1.00-cm2 InGaP/GaAs2JSC to reduce the current production on it. The resultant J–Vcurves and the derived electrical performances are shown inFIGURE 10LBIC measurements acquired from Cu-mounted (A–E) planar Si and (F–J) SiNW solar cell at cell operating temperatures between 25°C and 100°C.TABLE 5 Average LBIC current, JLBIC,ave, and normalized current uniformity, σJ/JLBIC,ave, of Cu-mounted planar Si and SiNW 1JSCs at different celltemperatures.T (°C) Average LBIC, JLBIC,ave (mA/cm2) Normalized current uniformity, σJ/JLBIC,avePlanar Si SiNW Planar Si SiNW25 2.25 3.10 0.2851 0.229340 2.24 3.11 0.2808 0.227960 2.23 3.03 0.2851 0.231580 2.17 2.93 0.2862 0.2309100 2.07 2.76 0.2828 0.2309Frontiers in Nanotechnology frontiersin.org11Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915Figure 12 and Table 7, respectively. With a 0.20-cm2 mask, theInGaP/GaAs 2JSC became the current-limiting cell. This suggeststhat there is an optimal active area that will current-match theSiNW bottom cell. This observation implicitly agrees with that ofa recent theoretical study (Yu Jeco-Espaldon and Okada, 2023),in which a GaAs/Si tandem absorber was conjectured to deliverthe optimal efficiency if the Si bottom cell area is 1.5 times largerthan the upper III–V cell area. Thus, for a 0.39-cm2 SiNW 1JSCwith the ITO conductive layer, the optimal active area of theupper InGaP/GaAs 2JSC could be approximately 0.26 cm2. Thisapproach is also favorable to the device cost because the optimalarea suggested reduces the amount of the III–V material needed.It can be noticed that there is a discrepancy between the ISCderived from the EQE and the one measured under 1-sun J–Vcharacteristics. This can be attributed to differences in the lightsources used in these independent measurements. The EQEmeasurement typically employs a monochromatic light source,whereas the 1-sun J–V characteristic measurement uses a solarsimulator lamp, which aims to replicate the solar spectrum.Variations in the spectral output and intensity distributionbetween these light sources can lead to differences in the ISCvalues (Sheng et al., 2021; Park et al., 2023; National RenewableEnergy Laboratory, 2024).On a further note, the SiNW 1JSC characterized was fabricatedwithout NW passivation and H2 annealing. To further boost theconversion efficiency of III–V//SiNW MJSCs, one may add surfacepassivation andH2 annealing steps (Jevasuwan et al., 2017; Yang andZeng, 2021; Wang et al., 2014; Li et al., 2013). These are alreadyknown to reduce surface defects, which is one of the main challengesin fabricating a nanostructured solar cell. If we then improve thequality of SiNW bottom cells, we may be able to further reduce theactive area of the upper III–V subcell for optimal current matchingin a III–V//SiNW MJSC device.Aside from III–V upper subcell area reduction, anotherapproach that can be explored to achieve low payload, flexible,and high-efficiency III–V//SiNW MJSCs for electric vehicleapplications is reducing their thickness. In particular, substrateremoval by epitaxial lift-off (ELO) (Bauhuis et al., 2014; Konagaiet al., 1978) and substrate thickness reduction by wet chemicaletching (Duda et al., 2012), dry etching (Yoo, 2010), or a combinedapproach of continuous-plasma CVD and contact-free laser transferprinting technique (Li et al., 2024; Xiong et al., 2018; Sheng et al.,2014; Lumb et al., 2014) can be applied. In this work, no substratethinning was done for the SiNW 1JSC. Thus, compared to its planarcounterpart, the percent weight reduction of Si, %ΔwSi, is only 1.8%FIGURE 11Measured EQE of planar and SiNW 1JSC without the 2J filter (dotted lines), SiNW 1JSC with the 2J filter (solid line), InGaP/GaAs 2JSC (dashed line),and InGaP/GaAs 2JSC in series with the SiNW 1JSC (dash–dotted line).TABLE 6 JSC of the InGaP/GaAs 2JSC and SiNW 1JSC with and without theInGaP/GaAs 2J filter calculated from 1 sun EQE measurements at roomtemperature.Parameter ISC from EQE (mA)2J–InGaP (Acell = 1.00 cm2) 11.772J–GaAs (Acell = 1.00 cm2) 11.89SiNW, no 2J filter (Acell = 0.39 cm2) 9.54SiNW, with the 2J filter (Acell = 0.39 cm2) 1.95Frontiers in Nanotechnology frontiersin.org12Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915for an SiNW fabricated in a 280-µm-thick substrate. However, if wefabricate an SiNW 1JSC on a 100-µm-thick Si substrate, %ΔwSi ofapproximately 64.5% can be achieved. The detailed calculation canbe found in Supplementary Material and Section 1.3.4 ConclusionThis study demonstrated that nanoimprinted SiNW solar cellsachieve optimal efficiency under low sunlight concentration attemperatures of 40°C or lower and a concentration factor of7.5 suns, resulting in a 1.05% increase in conversion efficiencycompared to unconcentrated light and a 3.75% increase overplanar Si solar cells. The integration of SiNW 1JSC with acommercial InGaP/GaAs dual-junction solar cell shows promisefor multijunction applications as a smaller active area in the III–Vupper subcell enhances current matching, thereby potentiallyreducing costs. Additionally, while the SiNW solar cell exhibits aweight reduction of 1.8% compared to its planar counterpart,strategies to further improve this reduction to 64.5% and beyondcan be achieved through SiNW solar cell substrate thinning,highlighting the potential of SiNW solar cells for flexible,lightweight, high-performance applications in electric vehiclesand other terrestrial applications.FIGURE 12Measured AM 1.5G, 1-sun J–V characteristic curves of SiNW 1JSC, InGaP/GaAs 2JSC, and InGaP/GaAs 2JSC in series with the SiNW 1JSC. Themeasurements with the InGaP/GaAs 2JSCwere obtained, while a laser-cut metal mask with a 0.20-cm2 square aperture was placed on top of the 2JSC inan attempt to match the current generation of the SiNW 1JSC.TABLE 7 Summary of electrical performance of the SiNW 1JSC, InGaP/GaAs 2JSC, and InGaP/GaAs 2JSC in series with the SiNW 1JSC under 1-sun, AM 1.5G,room temperature.Parameter InGaP/GaAs 2Ja(A = 1.00 cm2)InGaP/GaAs 2J(A = 0.20 cm2)SiNW 1J, filtered(A = 0.39 cm2)Series-connected(InGaP/GaAs//SiNW 3JSC)Short-circuit current, ISC (mA) 13.07 2.81 4.19 2.81Open-circuit voltage, VOC (V) 2.37 2.25 0.35 2.60Fill factor, FF 0.83 0.81 0.62 0.81Conversion efficiency, η (%) 25.83 25.67 2.33 15.15aadded for InGaP/GaAs 2JSC, performance without mask.Frontiers in Nanotechnology frontiersin.org13Yu Jeco-Espaldon et al. 10.3389/fnano.2024.1456915https://www.frontiersin.org/journals/nanotechnologyhttps://www.frontiersin.orghttps://doi.org/10.3389/fnano.2024.1456915Data availability statementThe original contributions presented in the study are included inthe article/Supplementary Material, further inquiries can be directedto the corresponding author.Author contributionsBYJ-E: conceptualization, data curation, formal analysis,funding acquisition, investigation, methodology, software,validation, visualization, writing–original draft, andwriting–review and editing. WJ: data curation, formal analysis,investigation, methodology, validation, and writing–review andediting. YO: conceptualization, formal analysis, fundingacquisition, project administration, resources, supervision,validation, and writing–review and editing. NF: conceptualization,formal analysis, funding acquisition, project administration,resources, supervision, validation, and writing–review and editing.FundingThe author(s) declare that financial support was received for theresearch, authorship, and/or publication of this article. This workwas supported by the National Research and Development Agency,New Energy and Industrial Technology Development Organization(NEDO), the Ministry of Economy, Trade and Industry (METI),Japan, under Contract #2000942-0, the Japan Society for thePromotion of Science (Grant-in-Aid for ChallengingResearch—Exploratory 20K21135), and the Department ofScience and Technology–Philippine Council for Industry, Energy& Emerging Technology (DOST-PCIEERD) Grant-in-AidNo. 1212673.AcknowledgmentsThe authors would like to like thank Yumiko Sawabe, MariInoue, Susumu Soeya, and Yukinaga Arai (Namiki Foundry,National Institute for Materials Science) for the highly dedicatedsupport they extended during the fabrication of SiNW solar cells.BYJ-E would like to thank the Department of Science andTechnology-Balik Scientist Program (DOST-BSP) for receivingfinancial support. The Perplexity AI uses OpenAI’s ChatGPT3.5 model.Conflict of interestThe authors declare that the research was conducted in theabsence of any commercial or financial relationships that could beconstrued as a potential conflict of interest.Publisher’s noteAll claims expressed in this article are solely those of the authorsand do not necessarily represent those of their affiliatedorganizations, or those of the publisher, the editors and thereviewers. Any product that may be evaluated in this article, orclaim that may be made by its manufacturer, is not guaranteed orendorsed by the publisher.Supplementary materialThe Supplementary Material for this article can be found onlineat: https://www.frontiersin.org/articles/10.3389/fnano.2024.1456915/full#supplementary-materialReferencesAdachi, M. M., Anantram, M. P., and Karim, K. S. (2013). Core-shell silicon nanowiresolar cells. Sci. Rep. 3 (1), 1546. doi:10.1038/srep01546Alanazi, F. (2023). 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