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Photocurrent Enhancement of Perovskite Solar Cells at the Absorption Edge by Electrode-Coupled Plasmons of Silver Nanocubes Gyu Min Kim, and Tetsu Tatsuma J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 7, 2017

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The Journal of Physical Chemistry

Photocurrent Enhancement of Perovskite Solar Cells at the Absorption Edge by Electrode-Coupled Plasmons of Silver Nanocubes Gyu Min Kim and Tetsu Tatsuma* Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 1538505, Japan

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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ABSTRACT: The power conversion efficiency of efficient solar cells would be further improved by enhancement of light absorption at the absorption edge region by plasmonic nanoparticles. Here we achieved this for perovskite solar cells (PVSCs), by taking advantage of "electrode-coupled" plasmons. We developed planar PVSCs with plasmonic silver nanocubes coupled electromagnetically with a silver back electrode. The plasmonic peak wavelength is adjusted to the absorption edge of the perovskite active layer (600−800 nm) by tuning the thickness of an electron-transport layer inserted in between the nanocubes and the electrode. The far-field scattering and optical near field from the nanocube face closest to the perovskite layer are also enhanced extensively by the coupling. The electrode-coupled plasmons enhance the photocurrents at the absorption edge. The average power conversion efficiency is also enhanced from 11.9% to 13.3%. The enhancement effect is more prominent for a thinner perovskite layer.


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INTRODUCTION To date, various different types of solar cells have been developed. A new type is invented, its power conversion efficiency (PCE) is improved, and finally the efficiency is saturated. A universal way to further improve the efficiency is red-shifting of the absorption edge, or absorption enhancement at the edge region. The latter can be achieved by using plasmonic nanoparticles. Plasmonic nanoparticles have been exploited for enhancement of photocurrents and thereby PCE of solar cells. In particular, solution-processed solar cells are compatible with the plasmonic enhancement; dye-sensitized solar cells,1,2 organic polymer solar cells,3,4,5 and colloidal quantum dot solar cells6 have received the benefit from plasmonic enhancements. This is also the case for perovskite solar cells (PVSCs),7-9 which are one of the cells receiving the most attention recently for their high efficiency and processability.10-12 Snaith et al. improved the PCE of a PVSC from 10.7 to 11.4%7 and Kim, Hong et al. from 10.96 to 11.96%.9 For further enhancement, the absorption enhancement at the absorption edge region would be important as described above. We recently achieved the photocurrent enhancement at the absorption edge region of PbS colloidal quantum dot solar cells (800−1200 nm), by using silver nanocubes (AgNCs).13 Plasmonic metal nanocubes are often used for plasmonic enhancement4,14 because of their strong far-field scattering and optical near field generated at edges and vertices,15 both of which contribute to the enhancement effect.16-20 Although plasmonic metal nanorods6,21 and coupled nanoparticles22 are used for photocurrent enhancement at long wavelengths, those nanoparticles resonate with light of a specific polarization angle preferentially. Here we achieved the photocurrent enhancements of PVSCs at the absorption edge region by taking advantage of "electrode-coupled" plasmons. It is known that film-coupled plasmons,23-25 which are based on the nanoparticles placed close, but not attached, to a metal film,


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exhibit strong absorption and that their resonance wavelength is easy to control by tuning the gap between the nanoparticles and the metal film. In this work, we enhance the photocurrents of PVSCs in the absorption edge region by the plasmons of AgNCs coupled with a silver back electrode (Figure 1a, b). The resonance wavelength of the "electrode-coupled" plasmons is adjusted to the absorption edge region by tuning the gap between the AgNCs and the back electrode. A small gap enhances the far-field scattering and optical near field generated from the AgNC face closest to the perovskite layer. Intrinsically high photocurrents of PVSCs are improved further, resulting in enhancement of their PCE from 11.9% to 13.3%.

EXPERIMENTAL Synthesis of AgNCs. The AgNCs were synthesized by a method reported elsewhere26 with slight modifications. A 5.1-mL of ethylene glycol (EG) in a 20-mL glass vial was stirred with the cap opened for 5 min at 145 °C. NaSH (1.5 mM in EG, 60 µL), HCl (3 mM in EG, 0.5 mL), and poly(vinylpyrrolidone) (20 mg mL-1 in EG, 1.25 mL) were injected into the solution successively. CF3COOAg (62.3 mg mL-1 in EG, 0.4 mL) was injected after a 2-min interval. The solution was stirred for 180 min and quenched in cold water, followed by washing with acetone and water. The 70-nm AgNCs thus obtained were dispersed in methanol (5 mL).


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Figure 1. (a) Structure and (b) energy diagram of the PVSC with the electrode-coupled AgNCs. (c) Experimentally measured absorption spectrum of 240-nm-thick-perovskite layer (A) and scattering spectra of AgNC (70 nm) on a PCBM (10 nm)-coated glass plate (B) and glass/PCBM(10 nm)/AgNC(70 nm)/BCP(10 nm)/Ag(100 nm) (C).

Preparation of PVSCs. An indium-tin oxide (ITO) electrode was precleaned by sonication in deionized water, acetone, and 2-propanol in sequence, followed by an oxygen plasma treatment to obtain a hydrophilic surface. Poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) (PEDOT:PSS, Clevios AI 4083, 50 µL) was spin-coated at 5000 rpm for 30 s, followed by annealing at 150 °C for 10 min. The layer thus obtained was coated with a perovskite layer by two different ways, the short spinning and vacuum drying (SSVD)27 and the conventional


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methods. In the former, a perovskite precursor ink, i.e. dimethylformamide (DMF) containing CH3NH3I and PbCl2 (molar ratio is 3:1, 40 µL, 36 wt% for 240-nm-thick and 28 wt% for 180nm-thick perovskite layers), was spin-coated on the PEDOT:PSS layer at 4000 rpm within 5 s in air at relative humidity under 40%. The ink-coated substrate was promptly transferred to a vacuum oven (~0.05 kPa). The substrate was left at room temperature for 15 min until the ink was fully dried, followed by annealing at 100 °C for 5 min. In the conventional method, spincoating time for the perovskite ink was 35 s. The ink-coated substrate was transferred to a hot plate in ambient air, and was annealed at 95 °C for 90 min without pre-drying. The perovskite layer was coated with phenyl-C61-butyric acid methyl ester (PCBM, 1.8 wt% in chlorobenzene, 30 µL) at 3000 rpm for 45 s. A 15-µL aliquot of the AgNC suspension was dynamically dispensed on the PCBM layer while the substrate was spinning at 3000 rpm. The dynamic dispense of the small amount of suspension is critical to prevent the suspension from penetrating through the PCBM layer and damaging the perovskite layer beneath PCBM. The spin-coating of AgNCs was conducted at 4000−5000 rpm for 1, 2, or 4 times to control the AgNC coverage. Bathocuproine (BCP, ~10 nm thick) was evaporated at the rate of 0.01 nm s-1 on the PCBM layer either with or without AgNCs, followed by evaporation of a silver back electrode (100 nm thick). Characterization. Optical properties of the samples were measured by using a spectrophotometer (Jasco V-670) equipped with an integrating sphere. We obtained photocurrent action spectra using Hamamatsu Photonics OSG (photon flux = 5 × 1015 photons cm-2 s-1). The current density-voltage characteristics were measured by scanning the voltage at 100 mV s-1 with a source meter (Keithley 2612B) under AM1.5G irradiation (100 mW cm-2) from a solar simulator (Bunkoukeiki, BSS-150T).


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Simulation. Optical spectra of AgNCs and electric field distributions were simulated by a finite-difference time-domain (FDTD) method using FDTD Solutions (Lumerical Solutions). The simulation domain (1 × 1 × 1 µm) consisted of 4 nm cubic cells and the central region (190 × 190 × 190 nm) around a AgNC was further meshed with a three-dimensional grid of 1 nm spacing. Backward scattering from the nanocube was monitored by a 340 × 340 nm square screen set 120 nm apart from the AgNC-PCBM interface. The dielectric functions of Ag and glass were extracted from the literature data.28 Refractive index values of PCBM and BCP were 2 and 1.7, respectively.

RESULTS AND DISCUSSION Tuning of Resonant Wavelength on the Basis of Electrode-Coupled Plasmons. Previously, film-coupled plasmons have been observed typically for nanoparticles deposited on a metal film coated with a thin dielectric layer.23-25 In the present work, however, a silver back electrode is mounted after deposition of AgNCs (Figure 1a). We therefore examined possible film-coupled plasmons in a system with AgNCs coated successively with a thin dielectric layer and silver, on the basis of simulation via a FDTD method. In the calculation model, a glass substrate is coated with a PCBM layer, on which a AgNC (70 nm) is placed. The substrate is further coated with 5-, 10-, 15-, or 20-nm-thick BCP and finally with 30-nm-thick silver. The PCBM and BCP layers transport electrons from the perovskite layer to the back silver electrode in the PVSC prepared in sections below (Figure 1a, b). Here we deposited PCBM directly on a glass plate to assess the optical properties of AgNCs coupled with the silver back electrode, without a perovskite layer that absorbs light extensively.


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The model used for the calculation and calculated scattering and absorption spectra are shown in Figure 2a. The scattering peaks are higher than the absorption peaks because the nanocube volume is large; it is known that plasmonic scattering intensity depends strongly on the particle volume.29 The scattering peak of the AgNC is split into three main peaks (Peaks A−C), and the degree of peak splitting increases as the gap between the AgNC and the silver electrode, namely the BCP thickness in the present model, decreases, suggesting that the splitting is caused by the AgNC-electrode coupling. Note that the BCP thickness in the experiments described below is not completely uniform. In order to assign those peaks, the electric field distributions were calculated at the peak wavelengths. The results for the model with 20-nm-thick BCP are shown in Figure 2d. The electron oscillation is localized at the proximal face of the AgNC, which is the closest face to the silver electrode, for Peaks A and C, and at the distal face, which is in contact with PCBM, for Peak B. The electron oscillation of Peak B, which is localized at the AgNC/PCBM interface, is advantageous for introducing far-field scattering light to the perovskite layer. In addition, perovskite can be excited by the optical near field from the distal face if it penetrates through the PCBM layer. We therefore calculated the electric field distributions at the PCBM/glass interface, which corresponds to the PCBM/perovskite interface in the PVSC, under 500- and 750-nm light. The BCP thickness is 10 nm. The results in Figure 2b (B) shows that strong near field reaches the perovskite layer at around Peak B. In the meantime, the 10-nm-thick PCBM layer between the perovskite and AgNCs prevents back energy transfer from the excited perovskite to AgNCs. The optimum gap between the active layer and the plasmonic metal nanoparticles is typically ~10 nm.20,30,31 Therefore, if Peak B is adjusted to the absorption edge of perovskite, the AgNC


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coupled with the electrode would enhance the excitation of perovskite and thereby photocurrents via the far-field scattering and near-field nanoantenna effects.

Figure 2. (a, c) Simulated scattering and absorption spectra, (b) lateral electric field distributions at the glass-PCBM interface at Peaks A and B, and (d, e) cross-sectional electric field distributions at Peaks A−C of the (a, b, d) glass/PCBM(10 nm)/AgNC(70 nm)/BCP(5−20


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nm)/Ag and (c, e) glass/PCBM(10 nm)/AgNC(70 nm)/BCP/flat Ag systems. BCP thickness = (b) 10 or (d) 20 nm.

If the silver is flat like the conventional film-coupled plasmon systems (Figure 2c, e), the scattering Peak B is much weaker and less red-shifted. These results indicate that the indentation of the AgNC into the silver electrode in the present "electrode-coupled" system enhances the splitting and the intensity of Peak B, likely because of stronger coupling with the electrode. Effects of the AgNC size on their optical properties were also examined (Figure 3). The scattering peaks red-shift as the particle size increases, because the relative BCP thickness in comparison with the particle size decreases. The size increase also enhances the scattering/absorption ratio. Among the AgNC sizes and BCP thicknesses examined, 70-nm AgNC, which is relatively easy to synthesize than larger ones, with 10-nm-thick BCP shows high scattering and moderate absorption at the absorption edge region of perovskite (700−800 nm, Figure 1c, A). We therefore use 70-nm AgNCs coated with 10-nm-thick BCP in the experiments below. Experimental Characterization of Electrode-Coupled Plasmons. The structure optimized by the FDTD calculation was characterized experimentally. When 70-nm AgNCs were synthesized and deposited on a PCBM-coated glass plate, the single main scattering peak was observed at 525 nm (Figure 1c, B). This peak overlaps the strong absorption band of perovskite (Figure 1c, A), so that significant photocurrent enhancements would be difficult. The AgNCs on the PCBM layer was further coated with 10-nm-thick BCP and 100-nm-thick silver (Figure 1c, C). As expected from the FDTD simulation described above, the sample exhibit two scattering peaks at 520 and 790 nm. The peak at 790 nm, which is ascribed to the electrode-coupled


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plasmons at the distal face, covers the absorption edge region of the perovskite. In addition, the scattering intensity was improved by a factor of 3 even at 520 nm, even though direct reflection from the sample was cut-off by an integrating sphere, indicating a strong enhancement of the plasmon resonance by the coupling with the silver electrode.


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Figure 3. Simulated scattering and absorption spectra of the glass/PCBM (10 nm)/AgNC (30−90 nm)/BCP (10 nm)/Ag system.

Uniform Loading of AgNCs. Next we prepared PVSCs in which AgNCs were inserted in between the PCBM and BCP layers (Figure 1a). To take full advantage of the AgNCs, we should disperse AgNCs uniformly on the PCBM surface. We coated an ITO electrode with a holetransport layer consisting of PEDOT:PSS, and further with a CH3NH3PbI3 perovskite layer prepared from CH3NH3I3 (MAI) and PbCl2. A 10-nm-thick PCBM film as the electron-transport layer was formed on the perovskite surface by spin-coating, followed by deposition of 70-nm AgNCs, 10-nm-thick BCP, and 100-nm-thick silver. When we process the perovskite layer by a conventional method (see Experimental), AgNCs are aggregated as confirmed by the micrographs (Figure 4a, c). This stems from the low coverage of PCBM and rough surfaces. It is known that perovskite layers prepared by a conventional method with MAI and PbCl2 tends to form sharply faceted domains, resulting in rough surfaces, and that a special treatment such as applying a heat flow during spin coating is necessary to obtain smooth surfaecs.32 The PCBM layer on the perovskite layer cannot mitigate the rough surface because the PCBM thickness is smaller than the perovskite roughness (mean roughness Ra ~ 60 nm). Thus, AgNCs tend to aggregate in the surface voids during the spin-coating process. We previously reported a method called short spinning and vacuum drying (SSVD), by which ultrasmooth and flat perovskite films can be fabricated without special treatments.27 Actually the Ra value of the PCBM layer spin-coated on perovskite prepared by the SSVD method is ~1 nm. It was found that AgNCs can be loaded uniformly on the PCBM layer as shown in the micrographs (Figure 4b, d).


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Figure 4. (a, b) SEM and (c, d) AFM images of AgNCs deposited on an ITO/PEDOT:PSS/perovskite/PCBM substrate with the perovskite layer processed by (a, c) the conventional and (b, d) the SSVD methods. (a, b) Scale bar is 1 µm.

Photocurrent and PCE Enhancements of PVSCs by Electrode-Coupled Plasmons. We examined the current density-voltage (J-V) curves of the PVSCs with different amounts of AgNCs under AM 1.5G light (100 mW cm-2) (Figure 5a). The perovskite layer (~240 nm thick) was processed by the SSVD method. When the coverage of AgNCs on PCBM is 5% or higher, the short-circuit photocurrent density (Jcs) and the open-circuit photovoltage (Voc) values are significantly reduced (pink and blue lines), resulting in low performances compared to the cell without AgNCs (black line). The inferior performances may be explained by the following two factors. (i) The perovskite layer might be damaged by methanol in which AgNCs are dispersed, because the spin-coating process is repeated several times on the thin PCBM layer for a high AgNC coverage. (ii) Densely packed AgNCs might work as trap sites for electrons, and increase serial resistances.


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Figure 5. (a, b) J-V curves of the PVSCs with different coverages of AgNCs. (c, d) IPCE spectra of the cells with and without 2% AgNCs. (e, f) Increment of IPCE upon introduction of 2% AgNCs and backward scattering from the glass/PCBM/AgNC/BCP/Ag substrate (data from Figure 1c). Perovskite thickness: (a, c, e) 240 or (b, d, f) 180 nm.

Enhanced photocurrents are observed when the coverage is ~2% (red line). The Jcs value was enhanced from 19.5 ± 0.46 to 21.4 ± 0.45 mA cm-2 (mean ± standard deviation of at least 6 cells), implying that the AgNCs can enhance the photocurrents without severe degradation of Voc and


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fill factor FF. As a result, the PCE value was also enhanced from 11.86 ± 0.88 to 13.3 ± 1.12%. Hysteresis is negligible both for the cells with and without AgNCs (Figure 6a, b). The parameters and performances of the PVSCs are listed in Table 1.

Figure 6. Forward and reverse scans for the cell (240-nm-thick-perovskite layer) with 2% AgNCs, (b) the cell (240-nm-thick-perovskite layer) without AgNCs and (c) the cell (180-nmthick-perovskite layer) with 2% AgNCs.


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Table 1. Performances of the PVSCs with and without AgNCs under AM 1.5G light irradiation.a Perovskite Coverage of layer Jsc, mA cm-2 Voc, V AgNCs, % thickness, nm

FF

PCE, %

11

4.14 ± 0.35

0.73 ± 0.03

0.51 ± 0.02

1.58 ± 0.19

5

15.51 ± 0.31

0.87 ± 0.02

0.58 ± 0.04

7.89 ± 1.47

2

21.4 ± 0.45

1.0 ± 0.02

0.62 ± 0.04

13.3 ± 1.12

0

19.5 ± 0.46

1.0 ± 0.01

0.61 ± 0.05

11.86 ± 0.88

2

17.1 ± 0.09

1.02 ± 0.02

0.62 ± 0.03

11.08 ± 0.43

0

15.2 ± 0.05

1.03 ± 0.01

0.61 ± 0.02

9.67 ± 0.36

240

180 a

All the values (mean ± standard deviation) were evaluated for at least 6 cells. The light intensity was 100 mW cm-2.

To scrutinize the relationship between the photocurrent enhancements and the electrodecoupled plasmons, incident photon to current conversion efficiency (IPCE) was measured for the PVSCs with AgNCs (2% coverage) at different irradiation wavelengths (Figure 5c). The IPCE value is enhanced by the introduction of AgNCs in 560−790 nm range while slight decreases are observed in the shorter wavelength range at 720 nm, where the absorption of perovskite is low and the plasmon resonance is strong. In contrast, at

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