Synergistically Enhanced Performance of Ultrathin Nanostructured

Apr 12, 2017 - Ultrathin silicon solar cells fabricated by anisotropic wet chemical etching of single-crystalline wafer materials represent an attract...
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Synergistically Enhanced Performance of Ultrathin Nanostructured Silicon Solar Cells Embedded in Plasmonically Assisted, Multispectral Luminescent Waveguides Sung-Min Lee,† Purnim Dhar,‡ Huandong Chen,§ Angelo Montenegro,‡ Lauren Liaw,§ Dongseok Kang,§ Boju Gai,§ Alexander V. Benderskii,‡ and Jongseung Yoon*,§,∥ †

School of Materials Science and Engineering, Kookmin University, Seoul 02707, Republic of Korea Department of Chemistry, §Department of Chemical Engineering and Materials Science, and ∥Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, United States

‡

S Supporting Information *

ABSTRACT: Ultrathin silicon solar cells fabricated by anisotropic wet chemical etching of single-crystalline wafer materials represent an attractive materials platform that could provide many advantages for realizing high-performance, low-cost photovoltaics. However, their intrinsically limited photovoltaic performance arising from insufficient absorption of low-energy photons demands careful design of light management to maximize the efficiency and preserve the cost-effectiveness of solar cells. Herein we present an integrated flexible solar module of ultrathin, nanostructured silicon solar cells capable of simultaneously exploiting spectral upconversion and downshifting in conjunction with multispectral luminescent waveguides and a nanostructured plasmonic reflector to compensate for their weak optical absorption and enhance their performance. The 8 μm-thick silicon solar cells incorporating a hexagonally periodic nanostructured surface relief are surface-embedded in layered multispectral luminescent media containing organic dyes and NaYF4:Yb3+,Er3+ nanocrystals as downshifting and upconverting luminophores, respectively, via printing-enabled deterministic materials assembly. The ultrathin nanostructured silicon microcells in the composite luminescent waveguide exhibit strongly augmented photocurrent (∼40.1 mA/cm2) and energy conversion efficiency (∼12.8%) than devices with only a single type of luminescent species, owing to the synergistic contributions from optical downshifting, plasmonically enhanced upconversion, and waveguided photon flux for optical concentration, where the short-circuit current density increased by ∼13.6 mA/cm2 compared with microcells in a nonluminescent medium on a plain silver reflector under a confined illumination. KEYWORDS: photovoltaics, silicon solar cells, upconversion, plasmonic nanostructure, luminescent solar concentrator hin-film (5−20 μm) silicon derived from singlecrystalline wafer materials represents an attractive materials platform that can provide a sustainable and scalable pathway for cost-effective, high-performance photovoltaic (PV) systems due to its earth abundance, superior materials properties, and significantly reduced materials consumption.1−3 Such ultrathin silicon, however, inherently suffers from incomplete absorption of long wavelength photons near its bandgap energy and thus requires appropriate light trapping to achieve competitive cell efficiencies.4,5 In this regard, printing-enabled materials assemblies can offer versatile routes to implement various schemes of photon management to boost the absorption of optically thin silicon solar cells.2,6 For ultrathin, microscale silicon solar cells (i.e., microcells)

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© 2017 American Chemical Society

printed on glass or plastics in a sparse array, a polymeric adhesive layer served as a multimode slab waveguide in conjunction with a back-surface reflector to trap photons incident outside the cell area and guide them to the adjacent microcells, thereby effectively increasing the photocurrent beyond the level achievable with absorption through the top surface of microcells.7−12 In a more advanced design, organic dyes were incorporated into the printing medium to form a “luminescent waveguide”.13−15 A short wavelength solar Received: February 3, 2017 Accepted: April 3, 2017 Published: April 12, 2017 4077

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Figure 1. (a) Schematic illustration of fabrication processes for a solar module comprising ultrathin (∼8 μm), nanostructured silicon microcells printed in hybrid luminescent waveguides that incorporate both upconversion and downshifting luminophores in conjunction with a nanostructured plasmonic reflector. (b, c) Photographic images of a completed solar module of ultrathin nanostructured silicon microcells on a mechanically flexible PET substrate at (b) a flat and (c) bent state, respectively. The inset in (b) shows a magnified view of printed cells. (d) Absorption (red line) and emission (green line, under excitation of 395 nm) spectra of DCM dyes dispersed in a fully cured polyurethane (NOA). (e) Absorption (red line), upconversion (green line, under 968 nm excitation), and downshifting (blue line, under 400 nm excitation) luminescence spectra of NaYF4:Yb3+, Er3+ UCNCs dissolved in toluene (∼3 wt %).

post hybrid silver nanostructures, both excitation and emission processes of upconversion luminophores were strongly enhanced under nonconcentrated solar illumination by the effects of surface plasmon resonance to not only amplify the light intensity at the excitation wavelengths but also facilitate far-field outcoupling at the emission wavelengths.23−28 In such an integrated solar module with plasmonically assisted upconversion waveguides, the solar-to-electric energy conversion efficiency of embedded microcells improved by ∼13% (relative) compared to devices without upconversion luminophores under otherwise identical conditions.23 Motivated by

radiation was converted to low-energy emission through the process of optical downshifting to produce a concentrated photon flux to the embedded solar cells,16−18 where transfer printing allowed printed microcells to accommodate optimized spatial layouts that can minimize intrinsic optical losses in conventional luminescent solar concentrators.13−15 More recently, research efforts were also made to introduce NaYF 4 :Yb 3+ ,Er 3+ nanocrystals as upconversion luminophores19−22 to the polymeric waveguide to capture weakly absorbed long wavelength photons for ultrathin silicon solar cells.23 Enabled with a plasmonic reflector comprising hole/ 4078

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Figure 2. (a) Schematic illustration of PMMA-coated hole/post hybrid silver nanostructures with key dimensional parameters including period (phole), diameter (Dhole), and height (hhole) of nanoholes as well as diameter (Dpost) and height (hpost) of nanoposts. (b) Contour plot of the calculated quality factor (QF) for hole/post hybrid silver nanostructure covered by PMMA and an ultrathin (8 μm) layer of silicon as a function of the diameter (Dpost) and height (hpost) of nanoposts, where the period (phole), diameter (Dhole), and height (hhole) of nanoholes are fixed at 700, 490, and 550 nm, respectively. (c,d) Contour plots of electric-field intensity (|E ⃗|2 ) profiles for hole/post hybrid silver nanostructures under normally incident illumination at wavelengths of (c) 980 nm, 660 nm, 540 nm, and (d) 410 nm, 430 nm, 450 nm, respectively, at the optimal geometry determined in (b) (i.e., phole = 700 nm, Dhole = 490 nm, hhole = 550 nm, Dpost = 340 nm, and hpost = 260 nm).

these advances in spectrally engineered solar modules, herein we report a design of flexible, multispectral luminescent waveguide for ultrathin (∼8 μm), nanostructured silicon solar cells, which can simultaneously exploit optical processes of spectral upconversion and downshifting in conjunction with a hybrid luminescent waveguide and a nanostructured plasmonic reflector. In this printing-enabled heterogeneous embodiment of nanostructured silicon solar cells, an extra amount of both short and long wavelength photons was additionally captured through processes of dual-mode spectral modification and waveguiding, making a synergistic contribution to the enhanced performance of printed microcells with efficiencies greater than those possible with a single-type of luminescent species. In the following, we present systematic studies of the composite PV system with hybrid luminescent waveguides, including (i)

optical and morphological properties of luminophores and plasmonic reflector, (ii) photovoltaic performance of embedded silicon microcells under various conditions of luminophore incorporation, illumination, and reflector, together with (iii) numerical optical modeling based on a finite-difference timedomain method, to quantitatively describe underlying optical processes and optimal design rules for the integrated solar module.

RESULTS AND DISCUSSION Figure 1a schematically illustrates fabrication processes of a flexible solar module for ultrathin (∼8 μm) nanostructured silicon microcells printed in hybrid luminescent waveguides that incorporate both upconversion and downshifting luminophores. As a first step to realize the composite PV system that 4079

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Figure 3. (a) Tilt- and top-view SEM images of hole/post hybrid silver nanostructure fabricated on a silicon substrate. (b) Corresponding total (diffuse + specular) reflectance spectra of the silver nanostructure shown in (a) at various materials configurations including in air (red line), with PMMA (blue line), and with PMMA and UCNCs (pink line), measured on a spectrophotometer equipped with an integrating sphere using a silver mirror as a 100% reflectance standard. Calculated spectra appear as thin lines (orange line: in air, green line: with PMMA). (c) Upconversion luminescence spectra of UCNCs coated on plain and nanostructured silver substrates with transparent encapsulation layers of PMMA/SU-8. A 968 nm continuous-wave (CW) laser was illuminated at an incidence angle of 5°, and the upconversion luminescence was collected by a fiber-optic-connected spectrometer. (d) Downshifting luminescence spectra of UCNCs in (c) under the excitation of 400 nm.

polyurethane (NOA). Figure 1b,c shows photographic images of a completed flexible solar module of ultrathin nanostructured silicon microcells on a PET substrate at a flat and bent state, respectively, incorporating both upconversion and downshifting luminescent species in conjunction with a nanostructured plasmonic reflector. The yellow fluorescent emission resulted from the DCM-doped downshifting medium under the ultraviolet (∼395 nm) illumination. Absorption and emission spectra of the DCM in a fully cured NOA were characterized using a spectrophotometer and fluorometer, respectively, as depicted in Figure 1d. The peak wavelengths for emission and absorption spectra are 662 (at the excitation wavelength (λex) of 480 nm) and 462 nm, respectively. Figure 1e shows upconverting (green line, λex = 968 nm) and downshifting (blue line, λex = 400 nm) luminescence spectra of NaYF4:Yb3+,Er3+ dissolved in toluene (∼3 wt %), where the center wavelengths of upconversion luminescence are ∼660, ∼540, and ∼520 nm, matching with literature values.21,24−27 As previously reported,23,29−33 these upconversion nanocrystals (UCNCs) can also function as a downshifting luminophore by absorbing highenergy photons (red line) and reemitting at longer wavelengths with a peak emission at ∼500 nm (blue line). For the multispectral luminescent waveguide reported here, we chose PMMA as a superstrate medium of UCNCs coated on a plasmonic substrate to allow improved thermal and chemical stabilities over the previously reported system that employed a silicone-based polymer.23,34 Given that the spectral response of a plasmonic nanostructure changes sensitively with the refractive index of a surrounding material, we performed numerical calculations based on the finite-difference time-

exploits processes of dual-mode spectral modification, a nanostructured plasmonic reflector that can amplify the upconversion luminescence under nonconcentrated (e.g., 1 Sun) solar illumination was fabricated by adopted procedures whose details are available elsewhere.23 Briefly, the process began with the formation of cylindrical nanoholes on a thermally curable polymer (SU-8) coated on a poly(ethylene terephthalate) (PET) substrate via softimprint lithography, where the height (hhole), diameter (Dhole), and periodicity (phole) of hexagonally periodic nanoholes were 300, 570, and 700 nm, respectively. Angled (∼15°) electron beam evaporation of metals (Cr (15 nm)/Ag (550 nm)) over the polymeric relief resulted in a “hole/post hybrid” silver (Ag) nanostructure consisting of truncated nanocones (“posts”) that reside at the center of cylindrical nanocavities (“hole”).23 Subsequently, a solution of hexagonal-phase NaYF4 nanocrystals doped with Yb3+ and Er3+ in toluene (∼3 wt %) was spincoated on top of the plasmonic nanostructure as an upconverting luminescent medium (thickness of ∼100 nm), followed by the successive lamination of poly(methyl methacrylate) (PMMA) and epoxy (SU-8) as a nonluminescent (NL) encapsulating medium (thickness: ∼3 μm/∼2 μm). The 8 μm-thick nanostructured (i.e., hexagonal array of front-surface nanoposts with near optimal geometries; period = 500 nm, diameter = 350 nm, and height = 140 nm)9,23 silicon microcells fabricated from a (111) single-crystalline silicon wafer were printed on top of the PMMA/SU-8 layer using a downshifting printing medium (thickness: ∼30 μm) containing an organic dye (DCM, (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl4H-pyran), ∼0.19 wt %) dispersed in photocurable 4080

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Figure 4. Representative current density−voltage (J−V) curves of individual nanostructured silicon microcell printed on a (a) plain and (b) nanostructured silver (i.e., Ag hybrid) reflector at various materials configurations including cells with a nonluminescent (NL; red line), upconversion (UCNCs; green line), downshifting (DCM; blue line), and hybrid (DCM/UCNCs; orange line) luminescent medium.

images of the hybrid silver nanostructures, where the diameter (Dhole) and height (hhole) of the nanoholes are ∼490 and ∼550 nm, and the diameter (Dpost) and height (hpost) of the nanoposts are ∼340 and ∼270 nm, respectively. The corresponding total (i.e., diffuse + specular) reflectance spectra measured on a spectrophotometer using a silver mirror as a 100% calibration standard exhibited a reasonable agreement with the calculated spectra in the position of resonant frequencies (Figure 3b), where the coupling of incident light with SPPs distinctively appears as the reflectance dips near the absorption and emission wavelengths of UCNCs. The observed discrepancy in the magnitude of reflectance is mainly caused by the nonideal features in the fabricated nanostructures including rounded edges and uneven surfaces. The efficacy of the fabricated plasmonic reflector to enhance the upconversion luminescence was verified by comparing the luminescence spectra on a plain (red line) and nanostructured (green line) silver reflector coated with upconversion (NaYF4:Yb3+,Er3+, ∼ 100 nm) and NL (PMMA/SU-8, ∼ 5 μm) layers under the excitation of 968 nm (Figure 3c). The intensity of upconversion luminescence on the nanostructured silver substrate was strongly augmented at 660, 540, and 520 nm, which is accredited to the effects of plasmonically amplified light intensity at the excitation wavelength in the region where UCNCs are deposited as well as the efficient far-field outcoupling of upconverted photons. As discussed in Figure 1e, the UCNCs can also serve as a downshifting luminophore upon the absoprtion of high-energy photons. Figure 3d shows the fluorescence spectra of UCNCs under the excitation of 400 nm on a plain (red line) and nanostructured (blue line) silver reflector. The spectra of downshifting fluorescence on the nanostructured reflector exhibited strong enhancement over a broad wavelength range with a peak wavelength of ∼420 nm. We postulate that this amplified fluorescence results from the effect of surface plasmon resonance to facilitate the outcoupling of emitted photons,23,28,37 which is also consistent with the calculated field profiles that suggest existence of SPP modes supported by the plasmonic nanostructure (Figure 2d). We studied photovoltaic performance of printed nanostructured silicon solar cells under the simulated AM 1.5G solar illumination (1000 W/m2) at various materials configurations of the composite solar module (Figure 4, Table S1). Figure 4a,b shows representative current density−voltage (J−V) curves of individual nanostructured silicon solar cells integrated on a plain and nanostructured silver (i.e., Ag hybrid) reflector, respectively. When a dye-doped printing medium was employed with a plain silver reflector (Figure 4a; blue line), the short-circuit current density (Jsc) increased by ∼4.0 mA/

domain (FDTD) method to optimize the geometric design of PMMA-coated hybrid silver nanostructures that can maximize the emission intensity of upconversion luminophores by the effects of surface plasmon resonance to not only amplify the absorption at the excitation wavelength but also facilitate farfield outcoupling at the emission wavelengths.23−28,35,36 Following procedures adopted from our previous report, a quality factor (QF) as defined below was evaluated over structural parameters of hole/post hybrid silver nanostructures in a model PV system that incorporates an ultrathin (8 μm) silicon integrated on a PMMA-coated plasmonic substrate (Figure 2a).23 ⎡ I ⎤ AM1.5G(λabs)AAg (λabs)TSi(λabs) ⎥ QF = ⎢ 1100nm ⎢⎣ ∑ ⎥⎦ I ( ) A ( ) T ( ) λ λ λ Ag Si λ= 400nm AM1.5G × [AAg (λabs)TSi(λabs)] × [∏ AAg (λem, i)] i

where IAM1.5G (λ), AAg (λ), and TSi (λ) are standard solar irradiance (AM 1.5G; ASTM G-173), absorptance of a plasmonic substrate, and transmittance of silicon, respectively. The plasmonic nanostructure supporting a large value of the QF enables a strong coupling of normally incident solar radiation with surface plasmon polaritons (SPPs) in both absorption (λ abs ) and emission (λ em ) wavelengths of upconversion luminophores, while minimizing parasitic optical losses in irrelevant spectral bands. Figure 2b shows a contour plot of the calculated QFs as a function of diameter (Dpost) and height (hpost) of nanoposts under the predetermined layout of cylindrical nanoholes (i.e., phole = 700 nm, Dhole = 490 nm, hhole = 550 nm), from which we can readily identify the optimal design (Dpost = 340 nm and hpost = 260 nm) of nanoposts. The excited SPPs on the optimized silver nanostructure effectively confine the normally incident solar radiation in a tight spatial volume at the metal/dielectric interface so that its intensity is strongly amplified at resonant frequencies, consistent with the calculated electric-field intensity (Figure 2c) at absorption (980 nm) as well as emission (660, 540 nm) wavelengths of UCNCs. It is also noteworthy that there exists SPP modes supported by the optimized plasmonic nanostructure within the spectral range (i.e., 400−500 nm) of downshifting fluorescence of UCNCs (Figure 2d), which can contribute to the enhanced Stokes emission of UCNCs as will be discussed subsequently. Based on these outcomes of the numerical study, we fabricated a plasmonic reflector comprising hole/post hybrid silver nanostructures with near-optimal geometries. Figure 3a depicts top- and tilt-view scanning electron microscope (SEM) 4081

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Figure 5. (a) Schematic illustration of lithographically defined metallic aperture (Cr/Au) implemented to produce a spatially confined illumination for printed nanostructured silicon microcells, where the area (∼65 × 1020 μm2) of illumination is slightly larger than the area (∼46 × 970 μm2) of a single microcell, thereby limiting the contribution of waveguided photon flux to the photocurrent. (b) Measured Jsc increase (ΔJsc = Jsc − Jsc,baseline) of nanostructured silicon microcells under various conditions of luminophore incorporation (i.e., NL, DCM, UCNCs, both DCM and UCNCs), illumination (i.e., unconfined, confined), and reflector (i.e., plain, nanostructured), where the short-circuit current density of microcells in a NL matrix on a plain silver reflector served as a baseline (Jsc,baseline). Measured ΔJsc of nanostructured silicon microcells under unconfined, narrow band (fwhm = 10 nm) illumination with a center wavelength of (c) 551 nm and (e) 978 nm at various configurations of luminophores and reflectors. Schematic illustration of corresponding optical processes under the (d) 551 nm and (f) 978 nm illumination at various conditions of luminophore incorporation on a nanostructured silver reflector.

cm2 compared with cells embedded in a NL matrix (Figure 4a; red line) due to the effect of luminescent concentration.13−15 Organic dyes (i.e., DCM) dispersed in the printing medium capture the short-wavelength (λ < ∼700 nm) solar radiation and reemit at longer wavelengths (∼600 nm < λ < ∼800 nm), which can be trapped in the multimode slab waveguide and absorbed through the sidewall and bottom surface of the adjacent microcells.13 As expected, there was little effect of incorporating upconversion luminophores with a plain silver reflector (Figure 4a; green line) mainly because of the lack of

SPP coupling and correspondingly negligible upconverted emission from UCNCs under 1 Sun illumination. Due to similar reasons, the performance of microcells in the hybrid luminescent waveguide containing both DCM and UCNCs (Figure 4a; orange line) was comparable to that only with DCM. When a nanostructured reflector was employed instead of plain silver, the overall performance of printed silicon microcells was improved in all cases (Figure 4b). Oblique angle scattering at the surface of a nanostructured reflector 4082

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effect of plasmonic scattering to guide and capture photons incident outside the cell in a NL matrix. The UCNC-doped waveguide with a nanostructured silver reflector and confined illumination (blue square) increased the Jsc even higher than the DCM-doped waveguide, confirming the effectiveness of plasmonically enhanced upconversion to additionally capture weakly absorbed photons by optically thin silicon solar cells.23 Notably, the increase of Jsc from incorporating both DCM and UCNCs was almost negligible under the confined illumination (i.e., compared to the cases only with DCM or UCNCs), implying there exists synergistic interplay and contribution of upconversion and downshifting processes for promoting the absorption of waveguided photons in the reported system. We also studied the wavelength-selective response of printed microcells to further elucidate underlying optical processes in the hybrid luminescent waveguide, where band-pass filters having the center wavelengths of 551 or 978 nm with a full width half-maximum (fwhm) of 10 nm were employed to narrow the spectral band of incident light.23 Under 551 nm illumination, which is relevant to the downshifting luminescence of DCM and UCNCs (Figure 5c and S4), the overall trend of Jsc increase was similar to that with a broadband illumination except the case with UCNCs and nanostructured reflector (orange diamond). As illustrated schematically in Figure 5d, plasmonic scattering supported by the nanostructured reflector effectively increased the absorption of silicon microcells by the waveguided photon flux in a NL matrix (Figure 5d; upper left). The downshifting medium helped to capture more strongly the short wavelength photons (Figure 5d; upper right), although some portion of reemitted photons was lost by plasmonic absorption on the nanostructured reflector. Notably, the Jsc increase with UCNCs (compared to that with DCM) was smaller than in broadband illumination, owing to the limited spectral range of incident light that can be involved in the downshifting process of UCNCs (Figure 5d; lower left). When both DCM and UCNC were incorporated together on a nanostructured reflector (Figure 5d; lower right), the Jsc increased even higher, which is attributed to the collective contribution of downshifting process by both types of luminophores as well as the decrease of plasmonic absorption loss of emitted photons by DCM with the aid of the layer of UCNCs coated on a plasmonic reflector. Under 978 nm illumination, which is relevant to the upconversion process by UCNCs (Figures 5e,f and S5), the oblique angle scattering by the nanostructured reflector did not make meaningful improvement of absorption because of the low absorption coefficient of silicon for such long wavelength photons (Figure 5f; upper left). There was little effect of incorporating DCM on the performance due to the mismatch of absorption band (Figure 5f; upper right). As expected, UCNCs on a nanostructured reflector boosted the Jsc by plasmonically enhanced upconversion as well as the waveguiding of upconverted photons (Figure 5f; lower left). Remarkably, co-integration of both types of luminophores in conjunction with a nanostructured reflector further increased the photocurrent, which is attributed to the efficient capturing of upconverted photons that can otherwise escape from the module by the processes of absorption and reemission of the overlying DCM-doped downshifting medium (Figure 5f; lower right). Under the confined illumination, such synergistic effects to promote the absorption of waveguided photons are restricted, leading to the significantly smaller Jsc increase (Figure S6).

provided a similar effect to the diffuse reflector to facilitate capturing of photons incident outside the cell area, thus leading to the higher Jsc in a NL matrix (Figure 4b; red line).12 However, introduction of organic dyes to a module on a nanostructured silver reflector (Figure 4b; blue line) yielded a smaller net increase of Jsc (ΔJsc = Jsc,DCM − Jsc,NL = ∼1.9 mA/ cm2) than a module on a plain silver reflector (ΔJsc = ∼4.0 mA/ cm2) due to intrinsic optical losses accompanied by nonradiative absorption of surface plasmon resonance that can degrade the waveguiding efficiency for downshifted photons (Figure S1). By contrast, incorporating UCNCs (Figure 4b; green line) resulted in noticeable improvement of the Jsc by ∼3.6 mA/cm2 over the baseline (i.e., a NL matrix; Figure 4b; red line), where both upconverting and downshifting emission of UCNCs account for the increased photocurrent. The UCNCs directly underneath the silicon microcell absorb long wavelength (around 980 nm) photons not fully captured by the overlying silicon and produce plasmonically enhanced upconverted emission, while those in open areas between cells are accessible to the broadband solar spectrum and can undergo both upconversion and downshifting processes.23 Notably, when the downshifting layer was additionally incorporated into the module with UCNCs (Figure 4b; orange line), the Jsc increased by ∼6.5 mA/cm2 compared to the NL module (Figure 4b, red line), which is even higher than the simple sum (∼5.5 mA/cm2) of ΔJsc obtained from UCNCs (∼3.6 mA/cm2) and DCM (∼1.9 mA/cm2), suggesting possible synergistic interplay between co-integrated luminescent species. Similar trends were also obtained from bare silicon microcells without front-surface nanostructures at otherwise same conditions (Figure S2). To provide further insight into underlying optical processes in the reported solar module with hybrid luminescent waveguides, we extended our study of cell performance under spatially confined and/or narrow-band illumination, which can minimize the contribution of waveguided photon flux and provide wavelength-selective responses, respectively. For measurements with a confined illumination area, an electronbeam evaporated metal aperture (Cr/Au = 10 nm/200 nm) was lithographically defined to allow the simulated solar light to enter the module through the top surface of a microcell and a narrowly opened region in its direct vicinity (Figure 5a). Figure 5b summarizes the increment (ΔJsc = Jsc − Jsc,baseline) of shortcircuit current density for microcells under various conditions of (i) luminophore incorporation (i.e., NL, DCM, UCNC, both DCM and UCNC), (ii) illumination (i.e., unconfined, confined), and (iii) reflector (i.e., plain, nanostructured), where the short-circuit current density of microcells in a NL matrix on a plain silver reflector served as a baseline (Jsc,baseline) (Figure S3, Table S2). On a plain silver reflector, the overall trend of Jsc improvement between measurements with confined (red square) and unconfined (green diamond) illumination was analogous. In both cases, the downshifting luminescence by DCM was effective to increase the photocurrent, while the optical concentration by waveguided photons played an important role to produce higher Jsc in the module with unconfined illumination (ΔJsc = ∼ 4.0 and ∼1.3 mA/cm2 for unconfined and confined illumination, respectively). On the other hand, the net increase of Jsc between DCM-doped and NL waveguides on a nanostructured reflector was fairly small (compared to the case with a plain silver reflector) under confined illumination (blue square) due to the limited contribution of waveguided photons as well as the pre-existing 4083

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continuous-wave laser (intensity = 12 W/cm2) and 400 nm pulsed laser (pulse width: 150 fs, repetition rate: 250 kHz, average intensity = 7.6 W/cm2) as excitation light sources, respectively, where the excitation light was obliquely incident ((θ = 5°) on the sample through a multimode optical fiber with a collimating lens (EFL = 10 mm). The emitted light was collected by fiber-optic-connected spectrometer (USB 4000, Ocean Optics). Current−voltage characteristics of silicon microcells under simulated AM 1.5G illumination (1000 W/m2) were obtained on a full-spectrum solar simulator (94042A, Oriel) and a semiconductor parameter analyzer (4156C, Agilent Technologies). Photovoltaic measurements under narrow-band illumination were conducted using band-pass filters (Thorlabs, Inc.) that have center wavelengths of 551 and 978 nm, respectively, where the illumination intensity from the solar simulator was adjusted to make the filtered light intensity at a peak wavelength close to that without the filter at 1 Sun. Numerical Optical Modeling of Nanostructured Plasmonic Reflectors. Optical simulation for nanostructured plasmonic reflectors was performed using the FDTD method (FDTD Solutions 8, Lumerical Solutions) as described in our previous report,23 where the refractive index of PMMA was fixed at 1.49 based on the measurement with a spectroscopic ellipsometer (VASE, J. A. Woollam).

CONCLUSION In summary, we demonstrated an integrated PV system of nanostructured, ultrathin silicon solar cells that exploit multispectral luminescent waveguides to enhance their light absorption and photovoltaic performance beyond the level possible with the solar cell itself. The 8 μm-thick nanostructured silicon microcells were integrated on a luminescent medium containing NaYF4:Yb3+,Er3+ and DCM as upconverting and downshifting luminophores, respectively, coated on a plasmonic reflector comprising hole/post hybrid silver nanostructures. The ultrathin nanostructured silicon microcells embedded in the composite luminescent waveguides exhibited a more strongly augmented photocurrent (∼40.1 mA/cm2) and energy conversion efficiency (∼12.8%) than devices with only a single type of luminescent species, owing to the synergistic contribution of optical downshifting and plasmonically enhanced upconversion as well as waveguided photon flux for optical concentration, where the short-circuit current density increased by ∼13.6 mA/cm2 compared with microcells in a NL medium on a plain silver reflector under a confined illumination. Materials design and fabrication strategies presented here for the integrated flexible solar module with multispectral luminescent waveguides are compatible with other types of luminophores and solar cell systems that can benefit from improved quantum efficiencies of weakly absorbed or belowthe-bandgap photons as well as reduced areal coverage of solar cells through concentrated photon flux.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00777. I−V curves of printed silicon microcells, extracted photovoltaic device characteristics, and reflectance spectra of PMMA-coated plain and nanostructured silver reflectors (PDF)

METHODS Fabrication of Silicon Solar Module Embedded in a Hybrid Luminescent Waveguide on a Nanostructured Silver Reflector. A plasmonic reflector consisting of hole/post hybrid silver nanostructures was fabricated through adopted processes reported in our previous work.23 A hexagonal array of nanoholes (phole = ∼700 nm, Dhole = ∼570 nm, hhole = ∼300 nm) on a 1.5 μm-thick epoxy (SU8 2, Microchem) was formed on a PET substrate (thickness: ∼50 μm) by softimprint lithography, followed by angled (∼15°) electron-beam deposition (Temescal) of Cr/Ag (15/550 nm) to produce hole/post hybrid silver nanostructures (phole = ∼700 nm, Dhole = ∼490 nm, hhole = ∼550 nm, Dpost = ∼340 nm, and hpost = ∼260 nm). Synthesized hexagonal-phase NaYF4:Yb3+,Er3+ upconversion nanocrystals38 dispersed in toluene (∼3 wt %) were then spin-coated (1500 rpm, 10 s) on the plasmonic reflector and baked (95 °C, 2 min). Subsequently, a solution of PMMA (Mn = 996 000 g/mol, Sigma-Aldrich, 13.7 wt % in toluene) and SU-8 (SU-8 2, Microchem) were successively spincoated (3000 rpm, 30 s) and baked (120 °C, 5 min) to form a NL encapsulating layer. Nanostructured ultrathin silicon solar microcells fabricated from a (111) silicon wafer9,23 were then transfer-printed on the prepared plasmonic reflector using a downshifting printing medium consisting of an organic dye (DCM, 0.19 wt %, Exciton) dispersed in photocurable polyurethane (NOA61, Norland Products Inc.).13 To form a metallic aperture to confine an illumination area, Cr (10 nm) and Ag (200 nm) were deposited by electron-beam evaporation and lithographically patterned using a wet chemical etchant (CR-7, KMG Chemicals). Characterization of Optical and Photovoltaic Properties. Total (i.e., specular + diffuse) reflectance (incidence angle (θ) = 8°) was measured by a spectrophotometer (Lambda 950, PerkinElmer) equipped with an integrating sphere, where a silver mirror (Thorlabs, Inc.) was used as a calibration standard for 100% reflectance. Measurements of absorption (θ = 0°) and emission (θ = 50°, λexc = 480 nm) spectra of DCM were performed using spectrophotometer (Lambda 950, PerkinElmer) and fluorometer (FluoroMax-3, Horiba), respectively. Upconverting and downshifting emission spectra from NaYF4 :Yb 3+,Er 3+ nanocrystals were measured using 968 nm

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Sung-Min Lee: 0000-0001-9446-9122 Alexander V. Benderskii: 0000-0001-7031-2630 Jongseung Yoon: 0000-0001-5342-1181 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS S-M.L., H.C., L.L., D.K., and J.Y. gratefully thank the National Science Foundation (ECCS-1202522, ECCS-1509897) for support. S-M.L. acknowledges support from the Leading Foreign Research Institute Recruitment Program (2013K1A4A3055679) and Nano-Material Technology Development Program (2009-0082580) through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning. The authors thank the Center for Energy Nanoscience at USC for the use of spectrophotometer, Donghai Zhu and John Curulli for help using facilities at Keck photonics laboratory and center for electron microscope and microanalysis (CEMMA) at USC, respectively. REFERENCES (1) Baca, A. J.; Ahn, J. H.; Sun, Y. G.; Meitl, M. A.; Menard, E.; Kim, H. S.; Choi, W. M.; Kim, D. H.; Huang, Y.; Rogers, J. A. Semiconductor Wires and Ribbons for High-Performance Flexible Electronics. Angew. Chem., Int. Ed. 2008, 47, 5524−5542. 4084

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DOI: 10.1021/acsnano.7b00777 ACS Nano 2017, 11, 4077−4085