Electrical Integrated Design of Core–Shell Aluminum-Based

Aug 10, 2017 - Laboratory of Artificial-Intelligence Nanophotonics, School of Science, RMIT University, Melbourne, VIC 3001, Australia. ‡ Institute ...
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Letter pubs.acs.org/journal/apchd5

Optical/Electrical Integrated Design of Core−Shell Aluminum-Based Plasmonic Nanostructures for Record-Breaking Efficiency Enhancements in Photovoltaic Devices Xi Chen,† Jia Fang,‡ Xiaodan Zhang,*,‡ Ying Zhao,‡ and Min Gu*,† †

Laboratory of Artificial-Intelligence Nanophotonics, School of Science, RMIT University, Melbourne, VIC 3001, Australia Institute of Photo Electronic Thin Film Devices and Technology, Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Nankai University, Tianjin 300071, China

‡

S Supporting Information *

ABSTRACT: Recently plasmonics has gained tremendous interest in solar cell research because it is capable of improving sunlight-conversion efficiencies. However, plasmonic photovoltaic nanostructures with both excellent optical properties and high electrical conductivities have not been developed, thus limiting the efficiency breakthrough. In this paper, we present an optical/electrical integrated design for plasmonic photovoltaic nanostructures by synthesizing core−shell nanomaterials: aluminum-coated copper nanoparticles. A copper nanocore was synthesized by chemical methods, and then an aluminum nanoshell was physically deposited on the nanocore surface. Strong light-scattering properties have been demonstrated due to the controllable morphology of the nanoparticles and the UV plasmon response of the aluminum nanoshells. Ultrahigh electrical conductivities have been achieved by the pure metallic nanoshells. Once the aluminum-based core−shell particles were integrated into high-efficiency amorphous silicon solar cells, we demonstrated a tremendous efficiency enhancement of 15.4%, which is 51% higher than that from the state-of-theart plasmonic technique using silver nanostructures. KEYWORDS: core−shell nanostructure, solar cell, plasmonic nanostructure, aluminum, light scattering, conductive

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could achieve low-coverage nanoparticle arrays through a drop casting of the nanostructure water suspensions.7−12 Nevertheless, the electrical conductivities of the chemically synthesized structures could not be as high as those from physical deposition, because organic compounds utilized during the synthesis to control the nanostructure morphology could attach on the structure surface and degrade the conductivities.10,18 Due to the optical and electrical optimization problem, through plasmonic PV nanotechnology it is a great challenge to enhance the energy conversion efficiency by more than 12% on state-of-the art silicon solar cells, which are dominating the PV market.10 Consequently, the realization of an optical/electrical integrated design for plasmonic nanostructures holds the key to achieving an efficiency breakthrough in PV devices. In the current work, we present an entirely new integrated design of core−shell aluminum-based PV nanostructures. The concept of the core−shell nanostructurealuminum-coated copper nanoparticle (ACNP)is illustrated in Figure 1, in which the geometry is composed of a chemically synthesized

lasmonics has been regarded as a promising nanotechnology to achieve photovoltaic (PV) devices with ultrahigh sunlight-conversion efficiencies, especially siliconbased devices dominating the market.1−4 Herein plasmonic nanostructures, produced by either physical methods (thermal evaporation and sputtering) or chemical methods, could improve optical properties of PV devices by scattering sunlight into an absorber5−12 (rather than through near-field enhancement because the plasmonic resonance could degrade the light absorption of solar cells7−12) and enhance electrical properties by accelerating the transfer of photogenerated electrons.10−14 However, currently the large-scale deployment of the plasmonic technology is severely hindered by the limited advances of PV device efficiencies,12,15,16 because of the difficulty in achieving plasmonic nanostructures with both excellent optical properties and superior electrical properties. Regarding the physically deposited structures, the metallic surface could lead to an ultrafast electron transfer. However, the optical scattering properties could not be optimized since it is difficult to achieve plasmonic nanoparticle arrays with surface coverages less than 30%.17 Under high coverages the nanostructures absorb a significant amount of light and thereby reduce the solar cell absorption.10−12 On the other hand, the chemical synthesis © XXXX American Chemical Society

Received: April 18, 2017 Published: August 10, 2017 A

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Figure 1. Structure of core−shell PV nanostructures and the mechanism behind strong light scattering and high electrical conductivity of the nanostructures.

nanostructure with a 200 nm diameter could provide a higher efficiency enhancement than that from the structure above 200 nm.10,12 To calculate the optimized Al nanoshell thickness with the strongest light-scattering properties, we simulated the scattering efficiencies of ACNPs of various thicknesses by the finite difference time domain (FDTD) method (FDTD solutions, http://www.lumerical.com). Within the wavelength range from 500 to 800 nm in which light trapping from plasmonic nanostructures is effective for the efficiency enhancement of silicon solar cells (incident light below 500 nm could be fully absorbed by the state-of-the-art cells),10,12 the highest scattering efficiency is achieved when the Cu nanocore diameter is 170 nm (see Figure S1, Supporting Information). Consequently, for the realization of the strongest light scattering, it is crucial to synthesize ACNPs with total diameters of around 200 nm and Al nanoshell thicknesses of around 15 nm. In the ACNP synthesis, the particle diameters can be tailored by changing the viscosity of the reaction solution during the Cu nanocore synthesis via adjusting the poly(vinyl alcohol) (PVA) concentrations.21 As shown in Figure 2b−d, it was observed from the scanning electron microscope (SEM) images that the PVA concentrations of 1, 3, and 5 g/L lead to ACNPs with total diameters of 100, 200, and 300 nm, respectively. The thickness of the Al nanoshell could be controlled by the duration of Al thermal evaporation.11 The nanostructures were characterized by transmission electron microscope (TEM) images (Figure 2e−g). proving the formation of a core−shell nanostructure. The thicknesses of the nanoshells are 25, 15, and 5 nm, respectively. According to the size and the shell thickness distribution graphs of the 200 nm ACNPs (see Figure S2, Supporting Information), the absolute deviations of the particle size and the shell thickness are 14 and 3 nm, respectively. For comparison, ACNPs with different morphologies were also synthesized. A SEM image of 200 nm multifaceted ACNP nanostructures with average Al nanoshell thicknesses of 15 nm is shown in Figure S3a, Supporting Information. Herein the Cu nanocores were prepared by ascorbic acid reduction of CuSO4 solutions without the PVA utilization. The absolute deviations of the Al shell thickness are 5 nm (Figure S3b, Supporting

copper (Cu) nanocore and a physically deposited aluminum (Al) nanoshell. The morphology of the nanocore could be controlled by the parameter adjustment in the chemical synthesis, to maximize the optical scattering properties. Moreover, ultrahigh conductivities could be achieved through the fast electron transfer in the pure metallic nanoshell deposited by physical procedures (Figure 1). Once the particles are integrated into silicon-based solar cells, we demonstrated an efficiency enhancement significantly higher than that from the state-of-the-art plasmonic PV technology using silver (Ag) nanostructures, and one of the highest-performing singlejunction amorphous silicon solar cells has been fabricated. Since Al and Cu are much cheaper than Ag, the “better and cheaper” solution could pave the way toward new-generation PV techniques competitive with fossil-fuel power techniques. The fabrication method of the novel nanomaterial is illustrated in Figure 2a. Herein Cu nanoparticles were synthesized by the chemical reduction of a CuSO4 aqueous solution. Then the Cu nanoparticles were deposited above a glass substrate thermally coated by an Al thin film and were used as substrates in the thermal evaporation of another Al thin film followed by a heat treatment. Next, under sonication treatment the ACNPs were suspended in water. We sprayed the suspension on solar cells and easily tailored the surface coverage of the ACNPs, especially the low coverage below 30%,11 by simply changing the deposition amount of the nanoparticles. The realization of a controllable fabrication method is crucial for the material optimization. The light-scattering properties of ACNPs, which could enhance the PV device efficiencies through the nanostructure integration, are determined by the particle sizes and the Al nanoshell thicknesses.5,6,19,20 Theoretically larger plasmonic particles indeed exhibit a stronger scattering than the smaller ones.20 However, for PV applications in amorphous silicon solar cells, the nanostructures are located inside a rear-side dielectric layer.10,12 When the thickness of the dielectric layer (usually around 100 nm) is much less than the ACNP diameter, the integrated solar cells can not exhibit the expected high efficiency enhancement due to some contact loss in the layer structure.10,12 Therefore, a PV B

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Figure 2. (a) Synthesis mechanism of ACNPs. SEM images of 100 nm (b), 200 nm (c), and 300 nm (d) ACNPs. Scale bar: 100 nm. TEM images of 200 nm ACNPs with 25 nm (e), 15 nm (f), and 5 nm (g) Al nanoshells. Scale bar: 20 nm. (h) EDX spectrum of 200 nm ACNPs with 170 nm Cu nanocores. (i) Absorption spectra of 200 nm ACNP water suspensions. The diameters of the Cu nanocores are 150, 170, and 190 nm.

is only in the UV range.11,23,24 In contrast, the visible peak from the Cu plasmonic resonance is found when the Cu core diameter is 190 nm (Figure 2i).25,26 The results demonstrate that the core−shell nanostructure could eliminate the visible parasitic absorption and has great potential to significantly enhance PV device efficiencies. To demonstrate PV applications of ACNPs, first we measured the optical scattering properties of the plasmonic structures. Here the reflectances of Al-doped ZnO dielectric layers embedded by ACNPs with different sizes and coverages are presented in Figure S5, Supporting Information. The reflectances of the ZnO layer with 200 nm ACNPs (170 nm Cu cores) at 650 nm are dramatically higher than those with 100 and 300 nm ACNPs under surface coverage ranges from 5% to 15%, because 100 nm ANCPs exhibit a small scattering cross-

Information), indicating that the nanoshells formed on the multifaceted surfaces are not as even as those on the spherical surfaces. Energy-dispersive X-ray (EDX) spectroscopy of 200 nm ACNPs with 15 nm nanoshells is shown in Figure 2h, indicating the formation of an Al layer on the surface of a Cu nanocore. For the same diameter of ACNPs, the EDX spectra verify that a thicker nanoshell could exhibit a stronger Al peak (see Figure S4, Supporting Information). In PV applications of plasmonic nanostructures the parasitic absorption must be taken into account, because it could reduce light absorption in solar cells.22 In the absorption spectra of 200 nm ACNPs with 150 and 170 nm Cu nanocores (Figure 2i), no obvious absorption peak can be observed in the visible wavelength range because the resonance of the Al nanoshells C

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Figure 3. (a) Total reflectance of Al-doped ZnO layers embedded with 200 nm ACNPs (170 nm Cu cores), 200 nm Ag nanoparticles, and 200 nm Au nanoparticles at 650 nm under different surface coverages. Total reflectance (b) and diffuse reflectance (c) curves of Al-doped ZnO layers embedded with 200 nm ACNPs (170 nm Cu cores), 200 nm Ag nanoparticles, and 200 nm Au nanoparticles. Note: The light-trapping effect can work only above the wavelength of 500 nm, indicated by the pink dashed line. (d) Sheet resistances of an ITO layer embedded with 200 nm ACNPs, 200 nm chemically synthesized Ag nanoparticles, and physically deposited Ag nanoparticles under different coverages.

section20 and 300 nm ACNPs could destroy the dielectric layer structure.10,12 The highest reflectance with 200 nm ACNPs could be observed under a 10% surface coverage, since a 5% coverage seems to be insufficient to scatter incident light and the 15% surface coverage leads to a high absorption from the plasmonic structures, reducing the reflection.11,12 Moreover, 200 nm ACNPs with 170 nm Cu nanocores could demonstrate the highest total reflectance among ACNPs with different Cu nanocore diameters (see Figure S6, Supporting Information), due to the strongest light scattering (see Figure S1, Supporting Information). Next, we compared the scattering properties of 200 nm ACNPs with two typical plasmonic materials: 200 nm Ag nanoparticles and 200 nm gold (Au) nanoparticles (Figure 3a). Both Ag and Au nanoparticles have the same average diameters as and similar distribution statistics to those of ACNPs (see Figure S7, Supporting Information). The 200 nm Ag nanoparticles have been verified to exhibit a broadband plasmon response to scatter light12 and were reported to make a device with the highest-performing plasmonic amorphous silicon solar cells.16 The maximum optical reflectance of the ZnO layer with ACNPs is 88.3%, 17% higher than that of Au (75.3%) at 650 nm due to the strong visible

light absorption of Au nanoparticles.27 A 2.2% stronger reflectance could also be measured compared with that embedded by Ag nanoparticles (86.3%). As shown in Figure 3b regarding the reflection spectra of the ZnO layer with the three types of PV nanostructures, within the wavelength range between 600 and 800 nm ACNPs could reflect more light than Ag nanoparticles. More importantly, the core−shell structure could scatter more light into an angle, as shown in Figure 3c, of the diffuse reflection spectra. The diffuse reflectance from ACNPs is also dramatically higher than other reported dielectric reflectors.28,29 Below the plasmonic resonance of metallic nanostructures, a negative influence named the Fano effect can be generated from the parasitic absorption, causing the degradation of solar cell absorption.11,30 Among plasmonic materials the monopolar plasmonic modes of Au nanoparticles and the dipolar plasmonic modes of Ag nanoparticles are within the visible wavelength range,11,20,31 while the plasmon resonance of Al nanoparticles appears in the ultraviolet range, where the intensity of solar irradiance is negligible.31 Therefore, PV devices integrated with Al-based nanostructures could avoid the negative influence from the Fano effect, dissimilar to the devices integrated by Au and Ag nanoparticles. D

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Figure 4. (a) Structure of an amorphous silicon solar cell with ACNPs integrated in the back dielectric layer, and schematic illusion of the lighttrapping and the electron transfer mechanisms of ACNPs. (b) Efficiency enhancements of a-Si solar cells with 100, 200, and 300 nm ACNP integration under 10% coverage. The Cu nanocore diameters are 85, 170, and 255 nm, respectively. (c) J−V characteristic for the cells without and with 200 nm ACNPs (170 nm Cu cores) under 10% coverage. (d) EQE curves of a bare a-Si cell and a cell with 200 nm ACNPs under 10% coverage. Inset: Absorption enhancement of an a-Si solar cell by the ACNP integration. (e) Relationship between the enhancements of solar cell efficiencies and the coverages of 200 nm ACNPs, 200 nm Ag nanoparticles, and 200 nm Au nanoparticles.

resistances of the ACNP-integrated and Ag-integrated ITO layers are 42 and 53 Ω/sq under 10% coverage, respectively. On the other hand, we utilized a thermal evaporation method to fabricate Ag nanoparticles into the ITO layer and found that the resistances are much lower than those integrated by the Ag nanoparticles synthesized by a chemical method (Figure 3d). The results demonstrate a huge conductivity gap between the plasmonic nanostructures prepared by chemical synthesis and physical evaporation, which is driven by organic compounds on the nanoparticle surface formed through chemical syn-

Because pure metallic nanoshells are fabricated by a thermal evaporation method, the electrical conductivity of ACNPs could be extremely high, contributing to the PV efficiency enhancement. To verify this, we synthesized 200 nm ACNPs, Ag nanoparticles and Au nanoparticles. The particles were integrated into an indium tin oxide (ITO) layer. The sheet resistances of the conductive layer are shown in Figure 3d and Figure S8, Supporting Information. For surface coverages between 5% and 30%, ACNPs result in higher electrical conductivities than those of Ag and Au nanoparticles. The sheet E

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thesis.10,18 The insulated substance leads to a decrease in the electron transfer speed.32 In contrast, because the Al nanoshells of ACNPs were deposited by the physical evaporation method, photogenerated electrons could pass through the nanostructure quickly. The electrical conductivity of ACNPs could be higher than that of chemically synthesized Ag nanoparticles (physically deposited Ag nanoparticles are not suitable for PV applications due to their high surface coverage10,12). We also synthesized 200 nm Al nanoparticles by a thermal evaporation method using NaCl powders as the deposition substrates11 and integrated them into the ITO layer. The sheet resistances of the ITO layer with Al nanoparticles are much higher than those of ACNPs (Figure S9, Supporting Information). During the synthesis of the Al nanoparticles a large amount of the NaCl powders could be utilized and the NaCl insulator attaches on the Al surface, leading to a conductivity degradation compared with pure Al nanoshells on the ACNP surface.11 The results could further support that the high electrical conductivities of ACNPs are generated from the physically deposited metallic nanoshells. Because the ACNPs could exhibit strong light-scattering properties and high conductivities, the novel Al-based core−shell nanostructures have great potential in PV applications to provide a better and cheaper solution than the state-of-the-art plasmonic PV technique using Ag nanostructures. To demonstrate this, we integrated the as-prepared ACNPs into single-junction amorphous silicon solar cells, in which the silicon layer was deposited by plasma-enhanced chemical vapor deposition (PECVD) as the light-absorbing layer (Figure 4a). ACNPs were embedded inside the Al-doped ZnO dielectric layer (Figure S10, Supporting Information). The rear-side dielectric layer could act as a back reflector to strongly scatter light and thereby increase the solar cell absorption. In the meantime the transfer of photogenerated electrons inside the dielectric layer is accelerated by the high electrical conductivities of the plasmonic nanostructures. The efficiencies of ACNP-enhanced amorphous silicon solar cells were measured for different ACNP sizes and Al nanoshell thicknesses. The highest efficiency enhancement can be achieved through the integration of 200 nm ACNPs (Figure 4b), and the optimum thickness of the Al nanoshells is 15 nm (Figure S11, Supporting Information). In this case the solar cell efficiency rises by 15.4%, increasing considerably from 10.0% to 11.6% (Figure 4c and Table 1) and making this device one of the highest-performing single-junction amorphous silicon solar cells reported to date.15,33 The improvement of short-circuit photocurrent density (Jsc) results from the external quantum efficiency (EQE) enhancement between 500 and 800 nm,

compared with that of the bare cell (Figure 4d). This EQE enhancement is further validated by the measurement of solar cell absorption (inset of Figure 4d), which consistently shows an enhancement in this long-wavelength range. For comparison, Ag nanoparticles and Au nanoparticles were integrated into the amorphous silicon solar cells. The plasmonic PV technology using 200 nm Ag nanoparticles has been reported to realize the world’s most efficient plasmonic amorphous silicon solar cell.16 The relationship between the surface coverages of plasmonic PV structures and the efficiency enhancements is shown in Figure 4e. For the 10% surface coverage 200 nm ACNPs with 170 nm Cu nanocores provide the highest PV performance enhancement (15.4%). The efficiency enhancement for the 5% coverage is not as high as that for the 10% coverage due to fewer integrated particles, while the 15% surface coverage leads to a high plasmonic absorption and thereby a reduction of the light absorption in the silicon layer.10−12 On the other hand, it has been found that the spherical morphology of the ACNPs could lead to a high efficiency enhancement. Once the multifaceted ACNPs were integrated into amorphous silicon solar cells, due to the uneven Al nanoshells (see Figure S3b, Supporting Information), the maximum efficiency enhancement is only 1.2%, much less than that from spherical ACNPs. The maximum efficiency enhancement from the spherical ACNPs indeed is dramatically higher than those from Ag and Au (Figure 4e) and Al nanoparticles (Figure S12, Supporting Information). The enhancement from the state-of-the-art plasmonic PV technique17 using 200 nm Ag nanoparticles is 10.2% under the 10% surface coverage (Table 1), while the highest enhancement from ACNPs (15.4%) is 51% higher than it. Because during the efficiency measurement multiple samples were tested and the mean absolute deviation was 0.06%, the results undoubtedly verify that ACNP could provide a higher PV efficiency gain compared with the state-of-the-art plasmonic technique. Because Al is subject to a quick oxidation, an Al2O3 layer with a thickness of a few nanometers could be formed on the ACNP surface.34 The oxidation layer could lead to an enhancement in the plasmonic scattering properties and the performances of the ACNP-integrated PV devices. To verify this, in the PVD 75 System we deposited Al nanoshells on the surface of Cu nanoparticles (located above a dielectric layer) and immediately sputtered another dielectric layer, in order to embed oxide-free ACNPs into the dielectric layer. As shown in Table S1, Supporting Information, the reflectance values of Al-doped ZnO layers embedded with 200 nm oxide-free ACNPs (170 nm Cu cores) are lower than those of ACNPs with a-fewnanometers-thick oxide layers, due to the red-shift of the scattering response.35−37 Once the oxide-free particle was integrated into amorphous silicon solar cells, the efficiency enhancement is less than that from ACNPs with oxide layers (Table S1, Supporting Information). Since the oxide layer on the ACNP surface could avoid corrosion and further oxidation,38 the ACNP integration could not lead to a degradation of the solar cell stability. Through 120 h light soaking the efficiency degradation was 21%, dramatically lower than that from Ag nanoparticles (26%). Consequently, the low-cost core−shell nanostructure could provide an innovative solution to realize record-breaking efficiency enhancements in PV devices.

Table 1. Solar Cell Characteristics for Amorphous Silicon Solar Cells and Solar Cells Integrated with 200 nm ACNPs (170 nm Cu Nanocore) and 200 nm Ag Nanoparticles under 10% Coverage

a-Si cell a-Si cell with ACNPs a-Si cell with Ag nanoparticles

Jsc (mA/cm2)

Voc (V)

fill factor

efficiency (%)a

16.12 17.63 17.13

0.92 0.93 0.93

0.68 0.70 0.69

10.0 11.6 11.0

a Six samples were fabricated. The efficiency was calculated by averaging three highest values of the samples, in which the mean absolute deviation of the three efficiencies was 0.06%.

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CONCLUSIONS In summary, we have presented a novel core−shell design of Al-based plasmonic nanostructures to dramatically enhance the efficiencies of solar cells. An innovative nanostructure composed of a Cu nanocore and an Al nanoshell was synthesized. The chemical synthesis of the copper nanocores and the UV plasmon response of the Al nanoshells lead to strong light scattering in the visible wavelength range, and the controllable thermal evaporation of the Al nanoshell results in ultrahigh electrical conductivities. The ACNP integration could realize innovative ultra-high-efficiency PV devices with higher performances and lower costs, compared with those using the integration of the state-of-the-art PV nanostructures, Ag nanoparticles. The results could pave the way toward solar power techniques with price-performance ratios competitive with fossil-fuel power techniques in the world.

another 80 nm Al-doped ZnO layer was sputtered on top of the nanomaterials followed by the evaporation of a 250 nm Ag back contact. The area of the as-fabricated solar cell is 0.25 cm2. For comparison, the bare solar cells without the integration of plasmonic nanostructures were fabricated under the same conditions. Material and Solar Cell Characterization. ACNPs were characterized by a Tecnai F20 TEM/EDX system and a ZEISS Supra 40 SEM system. A spectrometer (PerkinElmer, Lambda 1050) was employed to measure the absorption spectra of the nanomaterials and the total reflectance and diffuse reflectance of Al-doped ZnO layers embedded with various plasmonic nanomaterials. The PV performances of the solar cells were characterized through an I−V test (Oriel-Sol 3A-94023) and an EQE measurement (PV Measurement QEX10). The I−V curves were expressed at standard test conditions (and spectral irradiance AM1.5G, 1000W/m2). The test cell temperature was controlled during the measurement so as to maintain a junction temperature in the range 25−27 °C. A K-type thermocouple was positioned adjacent to the cell tested on the back side of the substrate to monitor the temperature of the device. The sheet resistances of the embedded ITO layers were measured by a JANDEL RM3000 system.

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METHODS Synthesis of ACNPs. ACNPs were synthesized by a thermal coating procedure on the surface of Cu nanoparticles. First, Cu nanoparticles were prepared by a wet-chemical method under ambient conditions. A 0.5 mL amount of 0.2 M CuSO4 was added dropwise into 5 mL of 0.02 M ascorbic acid and PVA solution in 80 °C under vigorous stirring. The solution was centrifuged at 600 rpm for 10 min, and the precipitate was redispersed in deionized water. Next, an Al thin film was deposited on a glass substrate by thermal evaporation under vacuum (10−4 Torr), and the suspension of Cu nanoparticles was sprayed on it using an Iwata airbrush. Then Al thin films with different thicknesses were thermally deposited on top of the particles followed by a heat treatment at 200 °C for 3 h. Finally the suspension of ACNPs can be obtained by vigorous sonication in water. For comparison, Ag and Au nanoparticles were prepared by a wet-chemical method according to literature procedures,12,39 and Al nanoparticles were synthesized by a modified thermal evaporation method using NaCl powders as the deposition substrates.11 FDTD simulation was used to calculate the scattering cross-sections of various plasmonic nanostructures. The optical constants as a function of wavelength for each of the materials used in the simulation were measured in our laboratory by ellipsometry software WVASE32 and then fit to the FDTD data. ACNP-Enhanced Solar Cell Fabrication. ACNPs were integrated into the rear dielectric layer of single-junction amorphous silicon solar cells. A boron-doped zinc oxide layer was deposited on a glass by metal organic chemical vapor deposition with the root-mean-square roughness of 90 nm being used as the front electrode. The solar cell was fabricated by PECVD at a substrate temperature of 210 °C, with a mixture of hydrogen (H2) and silane (SiH4) as the precursor gas and carbon dioxide (CO2) as the oxygen source. A power density of 76.4 mW/cm2 was used for deposition, but was increased to 347 mW/cm2 for the subsequent H2 plasma treatment of the films, with a pressure of 2 Torr being used for both processes. The structure of the absorbing layer was 12 nm p-type microcrystalline SiOx/6 nm i-type amorphous SiOx/300 nm itype amorphous silicon/25 nm n-type microcrystalline SiOx.40−42 Then a 20 nm Al-doped ZnO layer was sputtered. Before the nanomaterial integration, the solar cell samples were subjected to a 5 min exposure to an ethanol solution under sonication. Next, ACNPs were sprayed from water suspensions onto the Al-doped ZnO layer using an Iwata airbrush. Finally,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00396. Figures of scattering efficiencies of ACNPs; size and shell thickness distributions of 200 nm ACNPs; SEM image and shell thickness distributions of 200 nm multifaceted ACNPs; EDX spectra of ACNPs; total reflectance of dielectric layers with ACNPs; size and shell thickness distributions of Au and Ag nanoparticles; sheet resistances of an ITO layer with plasmonic nanostructures; SEM image of ACNPs on a dielectric layer; the relationship between the enhancements of solar cell efficiencies and ACNP coverages; relationship between the enhancements of solar cell efficiencies and Al nanoparticle coverages; total reflectance of Al-doped ZnO layers embedded with ACNPs; table of the efficiency enhancement of amorphous silicon solar cells integrated with ACNPs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaodan Zhang: 0000-0002-0522-5052 Min Gu: 0000-0003-2343-6475 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.G. and X.C. acknowledge the financial support from the Science and Industry Endowment Fund (RP04-024) and the technical support from Centre for Micro-Photonics, Swinburne University of Technology, Australia. Dr. Sergey Rubanov is acknowledged for his assistance with TEM measurements. J.F., G

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X.Z., and Y.Z. acknowledge the financial support from the International Cooperation Project of the Ministry of Science and Technology (2014DFE60170), National Natural Science Foundation of China (61474065, 61674084), Tianjin Research Key Program of Application Foundation and Advanced Technology (15JCZDJC31300), Key Project in the Science & Technology Pillar Program of Jiangsu Province (BE20141473), and the 111 Project (B16027).

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Evaporation of AgAl Alloy Nanostructures into the Hole-Transport Layer. ACS Appl. Mater. Interfaces 2016, 8, 26098−26104. (18) Gao, J. W.; Krzysztof, K.; Giesig, M.; Akinoglu, E. M.; Han, B.; Li, R. P. Physics of Transparent Conductors. Adv. Phys. 2016, 65, 553−617. (19) Mandal, P.; Sharma, S. Progress in Plasmonic Solar Cell Efficiency Improvement: a Status Review. Renewable Sustainable Energy Rev. 2016, 65, 537−552. (20) Temple, T. L.; Bagnall, D. M. Broadband Scattering of the Solar Spectrum by Spherical Metal Nanoparticles. Prog. Photovoltaics 2013, 21, 600−611. (21) Chen, X.; Jia, B. H.; Saha, J. K.; Stockes, N.; Qiao, Q.; Wang, Y. Q.; Shi, Z. R.; Gu, M. Strong Broadband Scattering of Anisotropic Plasmonic Nanoparticles Synthesized by Controllable Growth: Effects of Lumpy Morphology. Opt. Mater. Express 2013, 3, 27−34. (22) Lassiter, J. B.; Sobhani, H.; Fan, J. A.; Kundu, J.; Capasso, F.; Nordlander, P.; Halas, N. J. Fano Resonances in Plasmonic Nanoclusters: Geometrical and Chemical Tunability. Nano Lett. 2010, 10, 3184−3189. (23) Sobhani, A.; Manjavacas, A.; Cao, Y.; McClain, M. J.; de Abajo, F. J.; Nordlander, P.; Halas, N. J. Pronounced Linewidth Narrowing of an Aluminum Nanoparticle Plasmon Resonance by Interaction with an Aluminum Metallic Film. Nano Lett. 2015, 15, 6946−6951. (24) Zhang, Y. N.; Stokes, N.; Jia, B. H.; Fan, S. H.; Gu, M. Towards Ultra-thin Plasmonic Silicon Wafer Solar Cells with Minimized Efficiency Loss. Sci. Rep. 2015, 4, 4939. (25) Stuart, H. R.; Hall, D. G. Absorption Enhancement in Siliconon-insulator Waveguides using Metal Island Films. Appl. Phys. Lett. 1996, 69, 2327. (26) Luo, M.; Ruditskiy, A.; Peng, H. C.; Tao, J.; Figueroa-Cosme, L.; He, Z. K.; Xia, Y. N. Penta-Twinned Copper Nanorods: Facile Synthesis via Seed-Mediated Growth and Their Tunable Plasmonic Properties. Adv. Funct. Mater. 2016, 26, 1209. (27) Wang, Z. J.; Cao, D. W.; Yong, L. Y.; Cui, R.; Obergfell, M.; Mi, Y.; Zhan, Z. B.; Nasori, N.; Demsar, J.; Lei, Y. Manipulation of Charge Transfer and Transport in Plasmonic-ferroelectric Hybrids for Photoelectrochemical Applications. Nat. Commun. 2016, 7, 10348. (28) Mendes, M. J.; Morawiec, S.; Simone, F.; Priolo, F.; Crupi, I. Colloidal Plasmonic Back Reflectors for Light Trapping in Solar Cells. Nanoscale 2014, 6, 4795−4805. (29) Wang, E. C.; Mokkapati, S.; White, T. P.; Soderstrom, T.; Varlamov, S.; Catchpole, K. R. Light Trapping with Titanium Dioxide Diffraction Gratings Fabricated by Nanoimprinting. Prog. Photovoltaics 2014, 22, 587−592. (30) Lombardi, A.; Grzelczak, M. P.; Pertreux, E.; Crut, A.; Maioli, P.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Vallee, F.; Del Fatti, N. Fano Interference in the Optical Absorption of an Individual Gold−Silver Nanodimer. Nano Lett. 2016, 16, 6311−6316. (31) Cortie, M. B.; McDonagh, A. M. Synthesis and Optical Properties of Hybrid and Alloy Plasmonic Nanoparticles. Chem. Rev. 2011, 111, 3713−3735. (32) Akimov, Y. A.; Koh, W. S. Design of Plasmonic Nanoparticles for Efficient Subwavelength Light Trapping in Thin-film Solar Cells. Plasmonics 2011, 6, 155−161. (33) Ma, J.; Bai, H.; Zhang, J. J.; Yuan, Y. J.; Ni, J.; Zhang, K. L. Sizecontrolled nc-Si:H/a-SiC:H Quantum Dots Superlattice and its Application to Hydrogenated Amorphous Silicon Solar Cells. Sol. Energy Mater. Sol. Cells 2016, 157, 923−929. (34) Cabrera, N.; Mott, N. F. Theory of the Oxidation of Metals. Rep. Prog. Phys. 1948, 12, 163−184. (35) Bisio, F.; Zaccaria, R. P.; Moroni, R.; Maideccni, G.; Alabastri, A.; Gonella, G.; Giglia, A.; Andolfi, L.; Nannarone, S.; Mattera, L.; Canepa, M. Pushing the High-energy Limit of Plasmonics. ACS Nano 2014, 8, 9239−9247. (36) Maidecchi, G.; Gonella, G.; Zaccaria, R. P.; Moroni, R.; Anghinolfi, L.; Giglia, A.; Nannarone, S.; Mattera, L.; Dai, H. L.; Canepa, M.; Bisio, F. Deep Ultraviolet Plasmon Resonance in Aluminum Nanoparticle Arrays. ACS Nano 2013, 7, 5834−5841.

REFERENCES

(1) Koenderink, A. F.; Alu, A.; Polman, A. Nanophotonics: Shrinking Light-based Technology. Science 2015, 348, 516−521. (2) Zhou, L.; Tang, Y. L.; Wang, J. Y.; Xu, W. C.; Yuan, Y.; Cai, W. S.; Zhu, Z. N.; Zhu, J. 3D Self-assembly of Aluminium Nanoparticles for Plasmon-enhanced Solar Desalination. Nat. Photonics 2016, 10, 393−398. (3) Zhou, L.; Yu, X. Q.; Zhu, J. Metal-Core/Semiconductor-Shell Nanocones for Broadband Solar Absorption Enhancement. Nano Lett. 2014, 14, 1093−1098. (4) Gu, M.; Ouyang, Z.; Jia, B. H.; Stockes, N.; Chen, X.; Fahim, N.; Li, X. P.; Ventura, M. J.; Shi, Z. R. Nanoplasmonics: a Frontier of Photovoltaic Solar Cells. Nanophotonics 2012, 1, 235−248. (5) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (6) Lee, Y. H.; Lee, T. K.; Song, I.; Yu, H.; Lee, J.; Ko, H.; Kwak, S. K.; Oh, J. H. Boosting the Performance of Organic Optoelectronic Devices Using Multiple-Patterned Plasmonic Nanostructures. Adv. Mater. 2016, 28, 4976−4982. (7) Zhang, Y. N.; Du, Y. P.; Shum, C.; Cai, B. Y.; Le, N. C. H.; Chen, X.; Duck, B.; Fell, C.; Zhu, Y. G.; Gu, M. Efficiently-cooled Plasmonic Amorphous Silicon Solar Cells Integrated with a Nano-coated Heatpipe Plate. Sci. Rep. 2016, 6, 24972. (8) Jia, B. H.; Chen, X.; Saha, J. K.; Qiao, Q.; Wang, Y. Q.; Shi, Z. R.; Gu, M. Concept to Devices: from Plasmonic Light Trapping to Upscaled Plasmonic Solar Modules. Photonics Res. 2013, 1, 22−27. (9) Zhang, Y. N.; Chen, X.; Ouyang, Z.; Lu, H. Y.; Jia, B. H.; Shi, Z. R.; Gu, M. Improved Multicrystalline Si Solar Cells by Light Trapping from Al Nanoparticles Enhanced Antireflection Coating. Opt. Mater. Express 2013, 3, 489−495. (10) Chen, X.; Jia, B. H.; Cai, B. Y.; Fang, J.; Chen, A.; Zhang, X. D.; Gu, M. Graphenised Carbon Nanofiber: a Novel Light-Trapping and Conductive Material to Achieve an Efficiency Breakthrough in Silicon Solar Cells. Adv. Mater. 2015, 27, 849−855. (11) Chen, X.; Jia, B. H.; Zhang, Y. N.; Gu, M. Exceeding the Limit of Plasmonic Light Trapping in Textured Screen-printed Solar Cells using Al Nanoparticles and Wrinkle-like Graphene Sheets. Light: Sci. Appl. 2013, 2, 48. (12) Chen, X.; Jia, B. H.; Saha, J. K.; Cai, B. Y.; Stockes, N.; Qiao, Q.; Wang, Y. Q.; Shi, Z. R.; Gu, M. Broadband Enhancement in Thin-film Amorphous Silicon Solar Cells Enabled by Nucleated Silver Nanoparticles. Nano Lett. 2012, 12, 2187−2192. (13) Gao, Y.; Jin, F. M.; Su, Z. S.; Zhao, H. F.; Luo, Y. S.; Chu, B.; Li, W. L. All Thermal-evaporated Surface Plasmon Enhanced Organic Solar Cells by Au Nanoparticles. Org. Electron. 2016, 39, 71−76. (14) Chen, J. Y.; Wu, H. C.; Chiu, Y. C.; Chen, W. C. PlasmonEnhanced Polymer Photovoltaic Device Performance Using Different Patterned Ag/PVP Electrospun Nanofibers. Adv. Energy Mater. 2014, 4, 1665. (15) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 49). Prog. Photovoltaics 2017, 24, 3−13. (16) Villamar, L. Australian Solar Industry Research Develops Most Efficient Thin-film Solar Cells, http://www.ibtimes.com.au/australiansolar-industry-research-develops-most-efficient-thin-film-solar-cells1293610 (accessed Feb 2012). (17) Wang, J. F.; Jia, X. K.; Zhou, J. P.; Pan, L. K.; Huang, S. M.; Chen, X. B. Improved Performance of Polymer Solar Cells by Thermal H

DOI: 10.1021/acsphotonics.7b00396 ACS Photonics XXXX, XXX, XXX−XXX

ACS Photonics

Letter

(37) Langhammer, C.; Schwind, M.; Kasemo, B.; Zoric, I. Localized Surface Plasmon Resonances in Aluminum Nanodisks. Nano Lett. 2008, 8, 1461−1471. (38) Sun, W.; Wang, L. D.; Wu, T. T.; Zhu, T. Z.; Xie, H. B.; Pan, Y. Q.; Liu, G. C. A Catalyst-Based Self Sufficient System with Durable Self-Healing Functionality. Adv. Eng. Mater. 2016, 6, 923−931. (39) Fahim, N. F.; Ouyang, Z.; Jia, B. H.; Zhang, Y. N.; Shi, Z. R.; Gu, M. Enhanced Photocurrent in Crystalline Silicon Solar Cells by Hybrid Plasmonic Antireflection Coatings. Appl. Phys. Lett. 2012, 101, 261102. (40) Liu, H. J.; Huang, Q.; Hou, G. F.; Jiao, B. C.; Wang, G. C.; Zhang, W.; Zhang, D. K.; Zhao, Y.; Zhang, X. D. Cost-effective Hollow Honeycomb Textured Back Reflector for Flexible Thin Film Solar Cells. Sol. Energy Mater. Sol. Cells 2016, 155, 128−133. (41) Fang, J.; Chen, Z.; Hou, G. F.; Wang, F. Y.; Chen, X. L.; Wei, C. C.; Wang, G. C.; Sun, J.; Zhang, D. K.; Zhao, Y.; Zhang, X. D. Highquality Hydrogenated Intrinsic Amorphous Silicon Oxide Layers Treated by H2 Plasma used as the p/i Buffer Layers in Hydrogenated Amorphous Silicon Solar Cells. Sol. Energy Mater. Sol. Cells 2015, 136, 172−176. (42) Fang, J.; Chen, Z.; Wang, N.; Bai, L. S.; Hou, G. F.; Chen, X. L.; Wei, C. C.; Wang, G. C.; Sun, J.; Zhao, Y.; Zhang, X. D. Improvement in Performance of Hydrogenated Amorphous Silicon Solar Cells with Hydrogenated Intrinsic Amorphous Silicon Oxide p/i Buffer Layers. Sol. Energy Mater. Sol. Cells 2014, 128, 394−398.

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DOI: 10.1021/acsphotonics.7b00396 ACS Photonics XXXX, XXX, XXX−XXX