Hole Blocking Layer-Free Perovskite Solar Cells with High Efficiencies

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Hole Blocking Layer-Free Perovskite Solar Cells with High Efficiencies and Stabilities by Integrating Subwavelength-Sized Plasmonic Alloy Nanoparticles Xi Chen, and Min Gu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02145 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Applied Energy Materials

Hole Blocking Layer-Free Perovskite Solar Cells with High Efficiencies and Stabilities by Integrating Subwavelength-Sized Plasmonic Alloy Nanoparticles Xi Chen* and Min Gu* Laboratory of Artificial-Intelligence Nanophotonics, School of Science, RMIT University, Melbourne, Victoria 3000, Australia KEYWORDS Perovskite solar cells, hole blocking layer, stability, alloy, subwavelength-sized, plasmonic, light scattering

ABSTRACT Perovskite solar cells hold great promise as prospective alternatives of renewable power sources. Recently hole blocking layer-free perovskite solar cells, getting rid of complex and hightemperature fabrication processes, have engaged in innovative designs of photovoltaic devices. However, the elimination of the hole blocking layer constrains the energy conversion efficiencies of perovskite solar cells, and severely degrades the stabilities. In this paper a simple approach

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(without energy-consuming and time-consuming procedures) for the fabrication of hole blocking layer-free perovskite solar cells has been demonstrated by an integration of copper-silver alloy nanoparticles, which are synthesized by wet chemical method with controllable diameters and elemental compositions. The rear-side integration of the subwavelength-sized silver-copper alloy particles (200 nm diameter), through a spraying/drying method, realizes a pronounced absorption enhancement of the perovskite layer by effectively light scattering in a broadband wavelength range, and achieves a series resistance decrease of the solar cell due to high electrical conductivities of the alloy particles. The particle integration achieves the highest efficiency of 18.89% due to the significant improvement in both optical and electrical properties of solar cells, making this device one of the highest-performing blocking layer-free perovskite solar cells and plasmonic perovskite solar cells. Moreover, the copper-based nanoparticles prevent the perovskite from diffusing into metal back electrodes. Because the diffusion can lead to a severe corrosion of the Au electrode and thus an efficiency degradation, the alloy nanoparticle integration between the perovskite and the electrode results in 80% and 200% improvements in the long-term stability and the photostability of solar cells, respectively. Through the proposed simple and effective fabrication process, our results open up new opportunities in the manufacturability of perovskite solar cells.

INTRODUCTION Perovskite photovoltaic technologies have been regarded as potentially promising strategies to resolve the issues of energy shortage and environmental pollution, due to their outstanding performance and low-cost processing.1-5 Currently a conventional perovskite solar cell (PSC) consists two interface layers for the separation of photogenerated charges - a hole blocking layer


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(HBL), which is typically a metal oxide thin film, and a hole transport layer (HTL).6,7 However, the HBL fabrication requires an annealing process above 400 oC.8-10 The energy-consuming and time-consuming process impedes the large-scale application of PSCs. Consequently, HBL-free PSCs have been extensively investigated over the past few years as promising manufacturable photovoltaic devices.11-20 Since the HBL suppresses the electron-hole recombination21,22 and maximize the light absorption as a refractive-index matching layer,23 the energy conversion efficiencies of PSCs without HBL layers were significantly lower than those of the conventional PSCs.18,24 Though since their first report in 2014 the efficiency of the HBL-free PSCs has boosted from 13% to 19%, most of the high-efficiency outcomes were based on the involvement of additional fabrication procedures13,18-20 such as electrochemical etching and UV light exposure, which increase energy consumption and are time consuming. The manufacturability benefits of the HBL layer elimination diminish with such complex procedures. It is a tremendous challenge to realize a simple approach (without energy-consuming and time-consuming procedures) for the fabrication of HBL-free PSCs with power conversion efficiencies above 15%.12,17,20 Moreover, the HBL-free PSCs suffer from the low stabilized power output, in which a 20% decrease in the initial efficiency generates over a period of 100 h under the ambient condition (25 °C, 35% RH).14 Because simple fabrication processes of high-efficiency and stable PSCs are highly required, recently plasmonics has gained tremendous interest.21 Plasmonic nanostructures synthesized by wet chemical method, such as gold (Au),25-28 silver (Ag),29-32 aluminium (Al)33-35 and copper (Cu)36,37 can improve optical properties of photovoltaic devices by trapping the incident solar light into the light absorbing layer through a strong light scattering effect, and enhance electrical properties by accelerating the charge transfer.37,38 However, the design of plasmonic-integrated


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PSCs has not been optimised. Regarding the nanostructures size, the current research focuses on nanosized diameters (below 100 nm),15,16 which are easily synthesized in aqueous solutions through reactions between metallic salts and reducing agents (such as sodium citrate and sodium borohydride).25-27,29 Theoretically larger nanostructures with subwavelength-sized diameters show relatively stronger scattering effects compared with absorption effects, while nanosized particles exhibit more intensive absorption effects.39 The strong parasitic absorption, which is generated from near-field surface plasmon enhancement, leads to a decrease in the light absorption of the perovskite layer.39-41 Next, regarding the integration location of the nanostructures, in many attempts these nanostructures are incorporated at the front side of the light absorbing layer.25,26,2931,36

In this case the incorporation blocks out a significant amount of light and thereby reduces the

light absorption of solar cells.42 Finally, regarding the plasmonic material, an efficiency/stability integrated design has not been developed. Cu prevents the perovskite diffusion into the metal back electrodes, and thus prevent the performance degradation of PSCs.43-45 However, Au and Ag nanostructures, which have been mainly used to boost PSC efficiencies,21 do not provide considerably lower stability improvements than those from Cu nanostructures.33 In this paper, we propose and demonstrate a novel design of HBL-free PSCs, based on integration of subwavelength-sized Cu-Ag alloy nanoparticles (CANPs) with 200 nm diameters inside the rear-side HTL layer. Due to the optimized scattering properties and the high electrical conductivities, the use of the alloy nanoparticles yields a remarkable highest efficiency of 18.89%. In addition, considerably increased lifespans (defined as the duration when solar cells produce at least 80% of the initial efficiencies) under the ambient condition or the sunlight illumination have been demonstrated.


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METHODS Synthesis of CANPs. CANPs were synthesized by a controllable wet chemical method. First, 0.2 M CuSO4 and 0.2 M AgNO3 were added drop wise into 5 ml of 0.02 M ascorbic acid and polyvinyl alcohol (PVA) aqueous solution in 80 oC under vigorous stirring. The elemental compositions of CANPs can be adjusted by the amount of CuSO4 and AgNO3 solutions, and the various CANP diameters can be achieved through the different PVA concentrations. Next, the solution was centrifuged at 600 rpm for 10 minutes and then the CANP suspension can be obtained under vigorous sonication. For comparison, Ag, Cu and Au nanoparticles were prepared by wet chemical methods according to literature procedures.32,37,46 Al nanoparticles were synthesized by a thermal evaporation method.34 Fabrication of CANP-integrated HBL-free PSCS. FTO glass substrates were etched with 1 M hydrochloric acid and Zn powder, and then cleaned by the following procedure: deionized water under 5 min sonication, acetone under 15 min sonication, 2-propanol under 15 min sonication and oxygen/argon plasma for 5 min. Next, methylammonium iodide (MAI) was synthesized by reacting methylamine with hydroiodic acid. MAPbI3 were prepared by mixing 461 mg PbI2 with 159 mg MAI at a 1:1 molar ratio in anhydrous N,N-dimethylformamide (DMF).14 The perovskite solution was spin coated on the FTO substrates following a two-step protocol, which consisted of a first step of 1000 rpm for 10 s followed by a second step of 4000 rpm for 30 s. Subsequently, the samples were annealed at 100 oC for 2 h. After the perovskite deposition, a 10 nm HTL layer containing 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino-amine)-9,9′-spirobifluorene (spiroMeOTAD) in chlorobenzene, lithium bis(trifluoromethanesulfonyl)imide solution in anhydrous acetonitrile and tert-butylpyridine was spin coated at 3000 rpm for 30 s. CANPs were sprayed from aqueous suspensions onto the HTL layer using an Iwata airbrush. The CANP surface


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coverage can be adjusted by the suspension concentration. Finally, another 250 nm spiroMeOTAD HTL layer was spin coated on the top of the nanomaterials followed by a thermal evaporation of an Au back contact. The configuration of the HBL-free PSC is glass/700 nm FTO/300 nm MAPbI3/260 nm spiro-MeOTAD with CANPs/60 nm Au electrode. The area of the as-fabricated solar cell is 0.1 cm2. For comparison, the bare solar cells without the nanostructure integration were fabricated under the same conditions. Material and solar cell characterization. Commercially available software of finite difference time domain (FDTD) solutions, Lumerical Co. Ltd. was used to simulate Jsc of HBL-free PSCs, in which the optical constants of perovskite and CANP were measured by the same methods as published in the literatures.47,48 Scanning electron microscope (SEM) measurements of CANPs and PSCs were characterized by a Philips XL30 Scanning Electron Microscope. The surface coverages of CANPs were calculated from the SEM images. Energy-dispersive X-ray (EDX) mapping tests of CANPs were conducted using JEOL 2100F FEGTEM. A spectrometer (Perkin Elmer, Lambda 1050) was employed to measure the extinction spectra of the nanoparticles under the same concentration. The sheet resistances of the CANP-embedded HTL layers were measured by JANDEL RM3000 system. The photoluminescence (PL) spectra were obtained using a Horiba Scientific FluoroMax-4 spectrophotometer with an excitation wavelength of 488 nm. Timeresolved photoluminescence spectra were measured by PicoQuant using 405 nm pulse laser. The performances and stabilities of the solar cells were characterized through an I-V test (Oriel-Sol 3A-94023) and an External quantum efficiency (EQE) measurement (PV Measurement QEX10). The I-V curves were expressed at Standard Test Conditions (spectral irradiance AM1.5G, 1000 W/m2). Electrical impedance spectra and statistic resistances were measured on a Gamry Interface 1010 electrochemical workstation. The electrical impedance spectroscopy (EIS) tests were


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conducted under 1 sun AM 1.5 illumination at open circuit and a 10 mV voltage perturbation in the frequency range from 0.1 to 1M Hz.

RESULTS AND DISCUSSION The geometry of the proposed HBL-free PSC is demonstrated in Figure 1a, in which a methylammonium lead iodide (MAPbI3) layer, a spiro-MeOTAD HTL layer integrated with CANPs and an Au contact were subsequently deposited on the top of a fluorine-doped tin oxide (FTO) layer. The CANPs are synthesized and integrated via a simple fabrication approach (Figure 1b). Wet chemical method is simply used to prepare the aqueous suspension of CANPs with controllable diameters and elemental compositions. Herein Ag+ and Cu2+ are reduced by ascorbic acid in the presence of PVA. Then the CANPs are sprayed above a spiro-MeOTAD layer, in which the CANP surface coverage could be adjusted by the suspension concentration. Next, another spiro-MeOTAD layer is spin-coated and thus the nanoparticles are embedded into the PSC.

Figure 1. (a) Structure of hole blocking layer (HBL)-free perovskite solar cell (PSCs) integrated with Cu-Ag alloy nanoparticles (CANPs), which boost PSC performances through light scattering


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effect and electrical conductivity improvement. (b) Synthesis and integration procedures of CANPs.

The rear-side located CANPs can effectively scatter the incident light in a broad-spectrum range, thereby increasing the optical path length in the perovskite layer. Consequently, the light absorption of PSCs can be enhanced, as shown in Figure 1a. The innovative design of the subwavelength-sized particles avoids the particle parasitic absorption that cannot contribute to photocurrents and thus limits the PSC efficiencies. In the meantime, the CANPS exhibit significantly higher electrical conductivities than that of the particle-free HTL layer. The series resistances of the PSCs are minimized and thereby the electrical properties are improved (Figure 1a). Light scattering properties of CANPs are determined by the particle diameters and the elemental compositions. Based on the FDTD method we simulated short-circuit photocurrent density (Jsc) values in CANP-integrated solar cells. The simulation is based on the common HBL-free PSC geometry (FTO: 700 nm, MAPbI3: 300 nm, Spiro-MeOTAD: 260 nm and Au contact: 60 nm).13,14,49 The CANPs are located inside the spiro-MeOTAD layer with a distance of 10 nm between the particle and the MAPbI3 layer (Figure S1). Various diameters and Ag contents of CANPs were applied to optimize the plasmon resonance for the achievement of the best optical performance. It can be observed that high Jsc values are achieved by the integration of the subwavelength-sized particles.50 A 23.2 mA/cm2 of Jsc exhibits when 200 nm Ag9Cu alloy nanoparticles are integrated, with a 14.3% relative enhancement compared with that of the nanoparticle-free cell. The strong scattering effect of CANPs can be verified by the simulation results shown in Figure S2a-c. Though at 400 nm the 100 nm Ag nanoparticle demonstrates a maximum scattering efficiency due to the plasmon resonance, the resonance peak is red-shifted


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with the increase of the particle diameter and the decrease of the Ag contents.50,51 The optimum scattering efficiencies at both 550 and 700 nm, in which light trapping from plasmonic nanostructures is effective for the efficiency enhancement of PSCs,52,53 are generated by the 200 nm Ag9Cu alloy nanoparticles. Experimentally the optimum diameter of the alloy nanoparticles can be achieved in controllable synthesis procedures. Herein CANPs are reduced from an aqueous solution of AgNO3 and CuSO4, in which PVA has been added for the control of the the nanoparticle nucleation rate. The particle diameter can be tailored by the adjustment of the PVA concentrations.54 SEM images of these Ag9Cu nanoparticles are shown in Figures 2a-d. It was observed that the concentrations of 0.5 g/l, 1 g/l, 3 g/l and 5 g/l lead to the formation of Ag9Cu nanoparticles with diameters of 100 nm, 150 nm, 200 nm and 250 nm, respectively. According to the size distribution graphs of the alloy nanoparticles (Figure S3a-d), the average absolute deviations of the particle sizes are 15 nm, 19 nm, 20 nm and 22 nm, respectively. Another parameter determining light scattering properties of CANPs - the elemental compositions can be tailored by the concentration ratio between CuSO4 and AgNO3. An EDX spectroscopy of 200 nm Ag9Cu nanoparticles was demonstrated in Figure 2e, indicating the existence of Ag and Cu atoms. Figure 2f and 2g display the elemental mapping images of the particle. It can be observed that Cu and Ag elements have a homogeneous distribution. For the same diameter of CANPs, the EDX spectra verify that higher Cu contents exhibits relatively weaker Ag peaks (Figure S4a-b). The optical properties of CANPs with different diameters and elemental compositions were measured. For the 200 nm Ag9Cu nanoparticle a broadband peak within the visible wavelength range is generated by the plasmonic scattering of the alloy nanoparticle (Figure 2h).55 In the spectra


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of AgCu and Ag7Cu3 nanoparticles the scattering peaks are weaker than that of Ag9Cu, in accordance with the simulation results (Figure S2b-c). In contrast, narrow peaks are shown around 450 and 510 nm in the spectra of 100 and 150 nm nanoparticles, respectively (Figure 2i). The smaller plasmonic particles exhibit a stronger resonance absorption.55 Considering that CANPs are located inside the HTL layer, the light absorption in the perovskite layer cannot be boosted through the resonance absorption generated from plasmonic absorptions. The results demonstrate that the integration of small nanoparticles is not a good choice for the achievement of high-performance HBL-free PSCs (The resonance absorption is only effective for solar cell performance enhancement once the small particles are in the middle of the perovskite layer. However, in this case the particle surfaces operate as recombination centres for the photogenerated carriers and thus the electrical property of the PSCs is degraded severely).21 Besides the optical contribution, the integration of alloy nanoparticles can improve the electrical properties of PSCs through the enhancement of charge transportation rates (Figure 1a). To verify this, we integrated 200 nm Ag9Cu nanoparticles into a spiro-MeOTAD layer. The electrical conductivity of the alloy nanoparticles is much higher than that of spiro-MeOTAD. Consequently, the sheet resistances of the HTL layer decrease substantially with the particle coverage increase, from 560 Ω/sq of the particle-free layer to 279 Ω/sq under the 20% coverage (Figure 2j). The results indicate that the CANP incorporation accelerates the transfer of photogenerated charges in PSCs. On the other hand, because Ag electrical conductivity is a bit higher than that of Cu, the sheet resistances of the CANP-embedded spiro-MeOTAD layer decrease with the increase of Ag contents (Figure S5).


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Figure 2. Scanning electron microscope (SEM) images of (a) 100, (b) 150, (c) 200 and (d) 250 nm Ag9Cu nanoparticles. Scale bar: 100 nm. (e) Energy-dispersive X-ray (EDX) spectra of 200 nm Ag9Cu nanoparticles. EDX mapping of Cu (f) and Ag (g) elements on the nanoparticle. Scale bar: 100 nm. (h) UV-vis extinction spectra of 200 nm CANP water suspensions with different element compositions. (i) UV-vis extinction spectra of Ag9Cu nanoparticle water suspensions with different diameters. (j) Sheet resistances of a 260 nm spiro-MeOTAD layer embedded with 200 nm Ag9Cu nanoparticles under different surface coverages.


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To demonstrate photovoltaic applications of CANPs, the CANPs with various diameters and elemental compositions were embedded into a spiro-MeOTAD layer, located at the rear side of a HBL-free PSC. In this case the particle integration can avoid the sunlight blocking.34,47 Figure S6 displays a cross-section image of the CANP-integrated HBL-free PSC. The average thicknesses of the perovskite layer and the spiro-MeOTAD layer are 300 and 260 nm, respectively. Then we measured the performances of the HBL-free PSCs. Figure 3a exhibits energy conversion efficiencies of PSCs integrated with 200 nm Ag9Cu nanoparticles under different surface coverages. This implies that a 10% surface coverage provides optimum photovoltaic performances. The coverage lower than the optimum is insufficient for a significant efficiency enhancement, whereas a higher coverage leads to a high parasitic absorption and thereby a reduction of the light absorption in the perovskite layer.32,37,50 Under the 10% coverage the PSC delivers a Jsc of 22.25 mA/cm2, an open-circuit voltage (Voc) of 1.116 V, and a fill factor (FF) of 0.713, leading to an efficiency of 17.72% under standard AM1.5G illumination (Figure 3b). The efficiency is 29.5% higher than that of the nanoparticle-free PSC with an efficiency of 13.68%. The performance histograms measured for 50 CANP-integrated and 50 CANP-free devices display in Figure S7. Most of the integrated devices show efficiencies between 17% and 19%, revealing a high reproducibility. The best efficiency of 18.89% has been achieved with a Jsc of 22.96 mA/cm2, a Voc of 1.124 V, and an FF of 0.732 (Figure 3c), leading to one of the highest-performing HBL-free PSCs14,18 and plasmonic PSCs21,56. The high performance verifies that the CANP integration realizes a simple fabrication approach for the efficiency improvement of HBL-free PSCs from 15%12,17,20 to the value approaching that of conventional PSCs.24 Moreover, it has been observed


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that the ratio between the forward- and reverse-scan efficiencies of CANP-integrated HBL-free PSCs is 0.88, as shown in Figure S8 of the PSC hysteresis behaviors. Through the 200 nm Ag9Cu integration the integrated Jsc measured by EQE test increases from 19.18 mA/cm2 to 22.14 mA/cm2 (Figure S9). The substantial Jsc gain is resulted from a broadband EQE enhancement from 400 nm to 800 nm (Figure 3d). This EQE enhancement is validated by the absorption measurement of the PSC by the CANP integration (Figure 3e and S10), which consistently shows an improvement in such wavelength range.

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Ag9Cu nanoparticles under the 10% coverage, which are closest to the average efficiencies. The average efficiencies were confirmed for 50 Ag9Cu-integrated and 50 particle-free devices. (c) Performance for the best CANP-integrated HBL-free PSC. (d) External quantum efficiency (EQE) curves of the PSCs without and with 200 nm Ag9Cu nanoparticles under the 10% coverage. (e) Absorption enhancement of the PSCs by the CANP integration.

The 200 nm Ag9Cu nanoparticles provide a better photovoltaic performance than those embedded by CANPs with other diameters and elemental compositions (Figure 4a). We have investigated the influence of the CANP integration on the Jsc relative enhancement. Figure 4b displays that for 200 nm Ag7Cu3, Ag9Cu and Ag nanoparticles a 10% surface coverage provides an optimum Jsc gain, while an optimum coverage of 5% can be found under the integration of AgCu nanoparticles. Moreover, we synthesized 200 nm Au and Cu nanoparticles through wet chemical method37,46 and 200 nm Al nanoparticles through thermal evaporation method34. The Jsc enhancements from the integration of these particles are significantly lower than those from 200 nm Ag9Cu nanoparticles (Figure S11), indicating that the CANP integration provides an optimised solution to light trapping inside PSCs. On the other hand, the diameter of the subwavelength-sized alloy nanoparticles has been optimised. As demonstrated in Figure 4c, Jsc enhancements from Ag9Cu nanoparticles with diameters of 100, 150 and 250 nm are lower than those from the 200 nm one, due to the weaker light scattering properties demonstrated in the simulation results (Figure S1-2) and the extinction spectra (Figure 2i). When the coverages are larger than 10%, the Jsc enhancement percentages decrease with the coverage increase. It is due to strong parasitic absorption from the plasmonic nanostructures and, for the 200 and 250 nm particles, some direct particle/Au contacts.


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Figure 4. (a) Efficiencies of HBL-free PSCs integrated by CANPs with elemental compositions and diameters. (b) Relationship between the short-circuit photocurrent density (Jsc) enhancements and the integration coverages of 200 nm Ag, Ag9Cu, Ag7Cu3 and AgCu nanoparticles. Mean absolute percentage errors of the Jsc enhancements are within 10%. (c) Relationship between the Jsc enhancements and the integration coverages of 100 nm, 150 nm, 200 nm and 250 nm Ag9Cu nanoparticles. Mean absolute percentage errors of the Jsc enhancements are within 10%. (d) Series resistance decreasement from the integration of 200 nm Ag9Cu nanoparticles under different coverages.


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Except for the light scattering effect, the alloy nanostructure induces electrical conductivity enhancements in PSCs and thus significant FF boosts. As shown in Figure S12, the FF enhancements generated by Ag9Cu and Ag nanoparticles are substantially more obvious than those from Ag7Cu3 and AgCu nanoparticles, due to the extremely high electrical conductivity of Ag. It is observed that 250 nm alloy nanoparticle leads to a less enhancement than that of the 200 nm one. In this case the thickness of the spiro-MeOTAD layer is similar to the particle diameter. The electrical conductivity of the HTL layer is degraded through some contact loss on the particle surfaces.42,47 The highest FF enhancement from 200 nm Ag9Cu nanoparticles can be supported by the reduction of the series resistance (Rs) through the particle integration, consistent with the results that the alloy nanoparticles decrease the sheet resistances of the HTL layers (Figure 2j). As shown in Figure 4d, the 10% surface coverage leads to an optimum Rs decrease. The coverages larger than 10% cannot provide further Rs reductions due to the contact loss. The electrical property improvement of PSCs from the CANP integration can be attributed to the suppression of charge recombination, and thus the charge transfer promotion among the solar cell layers. As seen in Figure 5a, the 300 nm MAPbI3/200 nm spiro-MeOTAD film embedded with 200 nm Ag9Cu nanoparticles exhibits relatively lower PL intensities than that of the particlefree film. Moreover, through the time-resolved PL decay curves shown in Figure 5b a declined decay lifetime can be observed through the 200 nm CANP integration. These results are in accordance with the recombination resistance decrease, observed by EIS investigations shown in Figure 5c.


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Figure 5. (a) Steady-state photoluminescence (PL) spectra for perovskite/spiro-MeOTAD films with and without 200 nm Ag9Cu nanoparticles under 10% coverage. (b) Time-resolved PL decays for perovskite/spiro-OMeTAD films with and without 200 nm Ag9Cu nanoparticles under 10% coverage. (c) Nyquist plots measured under illumination of HBL-free PSCs with and without the integration of 200 nm Ag9Cu nanoparticles under 10% coverage. Poor stability of PSCs occurring is a barrier to the large-scale application of PSCs. Herein the effect of the alloy nanoparticle integration on the PSC stability has been explored. It is observed in Figure 6a that in ambient air (25 oC, 35% RH) the PSC with 200 nm Ag9Cu nanoparticles exhibits a much better long-term stability than that of the particle-free PSC. The nanoparticle


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incorporation into the spiro-MeOTAD layer improves the lifespan of PSCs under the ambient conditions from 100 h to 180 h. On the other hand, the higher Cu content leads to a better stability, as demonstrated in Figure S13a-c. Except for the 80% long-term stability enhancement in ambient air, the PSC with Ag9Cu nanoparticles is also more stable in high temperature and humidity (50 oC,

75% RH), as shown in Figure S13b. Moreover, the CANP integration provides a high

photostability under AM1.5 simulated sunlight illumination. The efficiency remains 80% after a 540 min illumination. The lifespan is 3 and 2.5 times as much as that of the particle-free HBL-free PSC (Figure 6b) and the Ag-integrated HBL-free PSC (Figure S13c), respectively. The stabilities of the CANP-integrated PSCs are better than published results of HBL-free PSCs17,57 and plasmonic PSCs33.

1.0


0.8 0.6 0.4 0.2

0

50 100 150 200 250 300 350 400

(b)

Normalized efficiency

(a)

Normalized efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0


0.9 0.8 0.7 0.6 0.5

Time (h)

0

100

200

300

400

500

600

Time (min)

Figure 6. Comparison of (a) the long-term stability in the ambient condition (25 oC, 35% RH) and (b) the photostability of HBL-free PSCs under AM1.5 simulated sunlight, with and without the integration of 200 nm Ag9Cu nanoparticles (under the 10% coverage). Data were collected from 10 devices and the average absolute deviation of the normalized efficiencies is within 0.03. No samples are encapsulated. Brown line indicates the lifespan of PSCs, in which solar cells produce at least 80% of the initial efficiencies.


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The high stability of CANP-integrated PSCs is attributed to the fact that Cu prevents the contact between the perovskite layer and the Au electrode within a long time frame.43-45 Hydrogen iodide, the decomposition product of MAPbI3 diffuses across the HTL layer and then reacts severely with gold atoms, resulting in a corrosion of the Au electrode and thus a severe degradation of the PSC efficiencies.44 The Au corrosion can be verified through the resistance measurement of the PSCs treated by an annealing process. The statistic resistance of the particle-free PSC increases by 98% after a heating at 80oC for 100 h. On the other hand, Cu exhibits excellent corrosion resistance in hydrogen iodide. The CANP-integrated PSC only has a 6% increase in the statistic resistance under the same annealing conditions (Figure S14). The result supports that CANPs (which cannot be oxidized by oxygen or moisture because of the embedding inside a HTL layer) can retard the Au corrosion and reduce charge trap states on the electrodes.43-45

CONCLUSIONS In summary, we have presented a novel nanostructure design of subwavelength-sized Cu-Ag alloy nanoparticles to simply fabricate PSCs with high efficiencies and stabilities. The alloy nanoparticles, synthesized through a solution-processed and controllable method, exhibit outstanding light scattering capabilities and excellent electrical conductivities. The nanoparticle integration into HBL-free PSCs enables a substantial increase in the Jsc and the FF. A remarkable 18.89% efficiency has been obtained along with considerable stability improvements. This value is significantly higher than that of the state-of-the-art HBL-free PSCs fabricated by simple approaches (without energy-consuming and time-consuming procedures). Since the rear-side integration does not involve morphology changes in the perovskite layer and the front-side semitransparent electrode, the proposed nanotechnology can be compatible with the existing


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pathways optimizing PSC performances, and thus has great potential to achieve high performances and excellent stabilities in many types of PSCs.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. FDTD simulation results; size distributions and EDX spectra of CANPs; sheet resistances of a Spiro-MeOTAD layer embedded with CANPS; a cross-sectional SEM image, performance histograms and J–V hysteresis of solar cells; integrated Jsc curves; solar cell absorption spectra; relationship between the Jsc enhancements and the integration coverages of alloy nanoparticles; FF enhancement through CANP integration; stability; the statistic resistance change of annealed PSCs. AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected] Notes The authors declare no competing financial interest. Author Contributions


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X.C. proposed the idea and the strategy for material and solar cell design. M.G. directed the study. X.C and M.G completed the writing of the paper. X.C. conducted the material synthesis and integration, the fabrication and the characterization of solar cells. ACKNOWLEDGMENT The authors 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. REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Tsai, H.; Asadpour, R.; Blancon, J. C.; Stoumpos, C. C.; Durand, O.; Strzalka, J. W.; Chen, B.; Verduzco, R.; Ajayan, P. M.; Tretiak, S.; Even, J.; Alam, M. A.; Kanatzidis, M. G.; Nie, W. Y.; Mohite, A. D. Light-induced Lattice Expansion leads to High-efficiency Perovskite Solar Cells. Science 2018, 360, 67-70. (3) Tsai, H. H.; Nie, W. Y.; Blancon, J. C.; Toumpos, C. C. S.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; Pedesseau, L.; Even, J.; Alam, M. A.; Gupta, G.; Lou, J.; Ajayan, P. M.; Bedzyk, M. J.; Kanatzidis, M. ; Mohite, A. D. High-efficiency Two-dimensional Ruddlesden-Popper Perovskite Solar Cells. Nature 2016, 536, 312-316. (4) Petrus, M. L.; Schlipf, J.; Li, C.; Gujar, T. P.; Giesbrecht, N.; Müller-Buschbaum, P.; Thelakkat, M.; Bein, T.; Hüttner, S.; Docampo, P. Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells. Adv. Energy Mater. 2017, 7, 1700264.


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Table of contents Hole Blocking Layer-Free Perovskite Solar Cells with High Efficiencies and Stabilities by Integrating Subwavelength-Sized Plasmonic Alloy Nanoparticles

Keywords Perovskite solar cells, hole blocking layer, stability, alloy, subwavelength-sized, plasmonic, light scattering Xi Chen and Min Gu

For Table of Contents Only


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