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Correlations of optical absorption, charge trapping, and surface roughness of TiO2 photoanode layer loaded with neat Ag-NPs for efficient perovskite solar cells Dongwook Yang, Jaegyu Jang, Joohyun Lim, Jin-Kyu Lee, Sung Hyun Kim, and Jong-In Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07079 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016

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ACS Applied Materials & Interfaces

Correlations of Optical Absorption, Charge Trapping, and Surface Roughness of TiO2 Photoanode Layer Loaded with Neat Ag-NPs for Efficient Perovskite Solar Cells Dongwook Yang, Jae Gyu Jang, Joohyun Lim, Jin-kyu Lee, Sung Hyun Kim,* and Jong-In Hong*

Department of Chemistry, College of Natural Sciences, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 08826, Republic of Korea

KEYWORDS Perovskite solar cells; Silver nanoparticles; localized surface plasmon resonance; charge trapping; surface roughness, optical absorbance

ABSTRACT: We systematically investigated the effect of silver nanoparticles (Ag-NPs) on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Neat, spherical Ag-NPs at loading levels of 0.0 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt% were embedded into the titanium 1 Environment ACS Paragon Plus


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dioxide (TiO2) photoanode layer. The plasmonic effect of the Ag-NPs strongly enhanced the incident light absorption over a wide range of the visible wavelength region, in addition to the inherent absorbance of the perovskite sensitizer. The low conduction energy level of the Ag-NPs compared to that of TiO2 provides trap sites for free charge carriers. Thus, the correlation between the enhancement of the optical absorption and the number of charge traps provided by the Ag-NPs is critical to determine the device performance; especially current density (Jsc) and PCE. This is confirmed by the quantitative comparison of the incident light absorption and the time-resolved photoluminescence decay according to the loading levels of the Ag-NPs in the TiO2 layer. The absorption enhancement from 380 to 750 nm in the UV-Visible spectrum is proportional to the increase in the loading levels of the Ag-NPs. However, the Jsc increases with the device with 0.5 wt% Ag-NPs and gradually decreases with increases in the loading level above 0.5 wt% because of the different contributions to the absorbance and the charge trapping by different Ag- NPs loading levels. In addition, the suppression of the surface roughness with dense packing by the Ag-NPs helps to improve the Jsc and the following PCE. Consequently, the PCE of the PSC with 0.5 wt% Ag-NPs is increased to 11.96%. These results are attributed to the balance between increased absorbance by the localized surface plasmon resonance and the decreased charge trapping, as well as the decreased surface roughness of the TiO2 layer with the Ag-NPs.

1. INTRODUCTION Perovskite solar cells (PSCs) have attracted significant interest in recent research because they show superior light-harvesting ability in the solar spectrum1-3 and outstanding charge carrier mobility,4 leading to higher power conversion efficiency (PCE) relative to typical solar cells. In

2 Environment ACS Paragon Plus

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2009, Miyasaka et al. firstly introduced organometal halide perovskite materials with molecular structure CH3NH3PbX3 (X = I or Br) as visible light sensitizers for photovoltaic cells.5 These halides have been extensively studied because of their relatively high efficiency. Seok et al. achieved an efficiency exceeding 20.0% by combining formamidinium lead iodide as an additive with CH3NH3PbX3 perovskite materials.6 In terms of fabrication process, Julian et al. introduced a sequential deposition method for the formation of the perovskite capping layer to optimize the morphology of the material.7 Kim et al. reported developments in the nanostructural control of the TiO2 photoanode layer to increase the current density and device performance.8,9 In order to improve the efficiency, among the factors decisive to the device performance, increasing the light harvesting capability in the panchromatic region of the solar spectrum is vital. However, this must be achieved without increasing the thickness, which can cause decrease the charge generation efficiency and increase charge recombination, as well as the charge transport resistance. Although low-bandgap perovskite sensitizers have drawn much attention because they can absorb the wide-wavelength region in the solar spectrum, the efficiency of the PSCs still suffers from their relatively weak absorption in the long-wavelength region (600–800 nm),10-12 and increasing the absorbance in this regime can enhance the device performance. PSCs with the highest PCE (~20.0%) are typically achieved by controlling the morphology and structure of the crystalline perovskite by additives such as salts11 or solvents,13 or by controlling the ratio of constituent materials without metal NPs.14 Such morphological changes in the perovskite materials are found to increase the absorbance.11,13,14 For the TiO2 photoanode, the disordered structure of the mesoporous TiO2 layer in dye-sensitized solar cells (DSSCs) hinders charge transport from the dye to the anode because of the short electron diffusion length. Different nanostructures for the TiO2 photoanode layer have been introduced to increase the PCE.15-17



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These structures promoted sufficient dye uptake and fast electron pathway. In addition, the dense packing of the TiO2 photoanode layer is advantageous for efficient charge transport.18,19 Thus, we expect that the addition of Ag-NPs can change the morphology of the TiO2 photoanode and positively affect the corresponding efficiency. Light absorption by the localized surface plasmon resonance (LSPR) of metal nanoparticles (NPs) is often used in the various types of organic solar cells such as DSSCs and organic photovoltaic cells (OPVs) in order to increase the amount of absorbed photons without increasing the thickness of the device.17,20 A strong local electromagnetic field near the NPs, induced by the collective and coherent oscillations of the electrons in resonance with the incident light frequency, enhances photon absorption. Additionally, the NPs with relatively large average diameters are advantageous in increment of the optical path length by increasing the light scattering effect, which helps the efficient absorption of incident solar light. Numerous studies have shown increases of the PCE by the incorporation of the NPs in typical organic solar cells. Recently Gong et al. reported a PCE of 10.3% owing to the enhanced absorption offered by the introduction of nano-popcorn-shaped silver-gold nanocomposites in the TiO2 photoanode layer of PSCs.21 However, no further studies have addressed the detailed relationships among the absorption enhancement, the scattering effect, the charge trap, and the aggregation behavior of NPs. In another case, Zhu et al. quantitatively investigated the scattering and plasmonic enhancement in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester OPVs with silver NPs (Ag-NPs) in the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate buffer layer.22 Generally, Ag-NPs are known to induce a larger scattering effect but a smaller LSPR effect than those of gold NPs. However, this effect only occurs when the average diameter of the Ag-NPs is over 40 nm, which strengthens the local field by the interference of the incident light. It is well-


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known that Ag-NPs with diameters below 40 nm show relatively limited scattering with respect to the corresponding absorbance.23 There are relatively few reports on the correlation between the absorbance and the formation of charge traps24 for the contribution to the efficiency of PSCs. Even though Ag-NPs can enhance the optical absorption, free charge carriers can be trapped on the NPs due to their lower conduction energy level compared to that of TiO2. In addition, the surface roughness of the TiO2 photoanode layer with the Ag-NPs, which induce variations in the electrical properties, is must also be investigated for effects on the device performance. Here, we systematically studied the quantitative effects of the optical absorbance, charge trap formation, and aggregation behavior by Ag-NPs loaded at different levels on the PSC performance. Neat, spherical Ag-NPs of ~25 nm in diameter loading levels of 0.0 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt% were incorporated in the TiO2 photoanode layer of the PSCs. The highest device efficiency of 11.96% was achieved for 0.5 wt% Ag-NPs due to the enhanced absorbance and the suppressed charge trapping on the Ag-NPs. In addition, the surface roughness was suppressed by the dense packing of TiO2 loaded with 0.5 wt% Ag-NPs, which contributed to the improvement of the charge transport as well as of the PCE.

2. EXPERIMENTAL SECTION Synthesis of Ag nanoparticles First of all, 10 g of polyvinylpyrrolidone (PVP; 10,000 g/mol, TCI, Japan) was dissolved in 50 mL of ethylene glycol (EG; Samchun Chemical, Korea). The reaction temperature was raised to 120 °C. Then, 0.8 g of silver nitrate (AgNO3; Sigma-Aldrich, Belgium) was dissolved in 5 mL of EG, and the previous mixture was added to the PVP solution. After 2 h of stirring, the dark-brown product was purified by centrifugation to remove the excess



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PVP once at 4000 rpm for 10 min in acetone, and three times at 17,000 rpm for 10 min in ethanol (EtOH), and the final Ag-NPs were dispersed in EtOH. Solar cell fabrication Patterned Fluorine-doped tin oxide (FTO) glass plates (Pilkington, TEC-8, Japan) were cleaned in a detergent solution using an ultrasonic bath for 10 min, and then rinsed with water and EtOH. The fabrication process of the TiO2 photoanode layer consists of three steps: (i) a spray-pyrolysis step to prepare a compact TiO2 film as a blocking layer, (ii) a hydrothermal step to form TiO2 from TiCl4, and (iii) a spin-coating step to prepare a mesoporous TiO2+n wt% Ag-NPs layer. In order to prevent direct contact between FTO and the perovskite sensitizer, a titanium diisopropoxide bis(acetylacetonate) solution was deposited on the FTO substrate by spray-pyrolysis at 450 °C to form the compact TiO2 film. The FTO glass with the compact TiO2 film were immersed in a 40 mM aqueous titanium tetrachloride (TiCl4) solution at 70 °C for 30 min and subsequently sintered at 500 °C for 30 min to improve the Jsc by enhancing the dye absorption on the surface25 and increasing the electron transport rate26 as well as the charge separation efficiency.27,28 Finally, the mesoporous TiO2+n wt% Ag-NPs layer fabricated to adsorb the perovskite sensitizer was constructed by the spin-coating (at 5000 rpm for 30 s) of a diluted TiO2 paste (Dyesol 18NR-T, 1 : 3.5 w/w diluted in EtOH) with the addition of n wt% of Ag-NPs (n=0.0, 0.5, 1.0, and 2.0). The Ag-NPs were dispersed in EtOH, which mixed well with the diluted TiO2 EtOH solution.29 The formed TiO2 photoanode layer with n wt% Ag-NPs was sintered at 500 °C. To form perovskite structure on the TiO2 photoanode, a 1.0 M lead iodide (PbI2) solution in dimethylformamide (DMF) was dropped on the TiO2/FTO substrate by spincoating at 6500 rpm for 30 s. The substrates were dried on a hot plate at 70 °C for 30 min. After cooling, the layer was dipped into an 8 mg/mL methylammonium iodide (CH3NH3I) in 2propanol for 25 s, and dried at 70 °C for 15 min. The thickness of the TiO2 photoanode+n wt%


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Ag-NPs layer combined with the perovskite sensitizer was kept at ~530 nm without the loading level of the NPs. For a deposition of hole-transporting material (HTM) layer, a 60 mM solution of 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) in chlorobenzene

was

prepared.

Two

additives

of

7.0

mL

lithium-bis(tri-

fluoromethanesulfonyl)imide (Li-TFSI) in acetonitrile (520 mg/mL) and 8.0 mL 4-tertbutylpyridine (tBP) were added to the HTM solution. The HTM layer was spin-cast onto the FTO/TiO2/methylammonium lead iodide (CH3NH3PbI3) at 3000 rpm and the thickness was 180 nm. Finally, the device was pressurized to 10-5 Torr and gold counter electrode of 60 nm in thickness was deposited on top Measurement The solar cells efficiencies were measured under simulated 100 mW/cm2 AM 1.5G irradiation using a xenon (Xe) arc lamp with an AM 1.5G. The simulator irradiance was characterized using a calibrated spectrometer and the intensity of illumination was set using a silicon diode with an integrated KG1 optical filter certified by National Renewable Energy Laboratory (NREL): The spectral mismatch factor of each device was calculated to be less than 5%. Short circuit currents (Jsc) were also found to be within 5% of the values calculated using the integrated external quantum efficiency (EQE) spectra and the solar spectrum. The EQE was measured using a reflective microscope in order to focus the light output from a 75 W Xe lamp, monochromator, and optical chopper; the photocurrent was characterized using a lock-in amplifier and the absolute photon flux was determined by a calibrated silicon photodiode. The Ultraviolet-Visible

(UV-Vis)

spectra

were

recorded

using

a

Beckman

DU

650

spectrophotometer. The device performance was measured under AM 1.5G illumination (100 mW/cm2) using a solar simulator (Peccell, Japan) and a solar cell IPCE measurement system (McScience, K3100, Korea). The light intensity at each wavelength was calibrated using a



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standard silicon solar cell as a reference. The current density–voltage (J–V) curves were measured with a Keithley 2400 source measurement unit. A Hitachi-7600 transmission electron microscope (TEM; Hitachi, Japan) was used to obtain the microscopic image. A DektakXT surface profiler (Bruker, USA) was used to measure the thickness of each layer in the devices, which the average thickness were obtained by ten samples of each layer. A Nanoscope IV controller (Veeco Instruments, USA) was used to analyze the atomic force microscopy (AFM) images. A time-resolved photoluminescence (PL) spectrometer (Becker & Hickl, SPC-150, Germany) was used to measure the PL decay with a time channel of 12.2 ps. A femtosecond laser (MaiTai HP, Spectra-Physics, USA) was pumped with a photonic crystal fiber (NKT Photonics, femtoWHITE-800, Denmark) to generate a supercontinuum light, from which the excitation wavelength of 630 ± 5 nm was extracted using a bandpass filter (Thorlabs Inc., Newton, USA). Dynamic light scattering (DLS; Malvern, Zetasizer Nanoseries, USA) was conducted to measure the aggregated mean particle diameter. X-ray diffraction (XRD; PANalytical, X’pert Pro, Netherland) measurements were performed by using Cu Kα1, and Kα2 radiation sources (λ = 1.54056 Å, and 1.54443 Å) at 45 kV and 40 mA, respectively. Field emission scanning electron microscope (FE-SEM; Carl Zeiss, SIGMA, United Kingdom) images were performed at a 15 kV accelerating voltage.


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3. RESULTS AND DISCUSSION

Figure 1. (a) Schematic device structure, energy level diagram, and average thickness values for each layer of the PSC (The red and blue arrows represent electron- and hole-transport pathway). (b) TEM image of Ag-NPs ( ~25 nm in diameter). The device structure consisting of FTO/TiO2+n wt% Ag-NPs/CH3NH3PbI3/spiro-OMeTAD/Au and the corresponding energy level diagram are shown in Figure 1a. CH3NH3PbI3 is used as a perovskite sensitizer and n is varied with 0, 0.5, 1.0 and 2.0. The conduction band of TiO2 is lower than that of the perovskite material, which promotes electron transfer from the sensitizer into the TiO2 photoanode layer. For a TiO2 photoanode layer embedded with the Ag-NPs, the NPs can act as charge-trap sites because the conduction energy level of the Ag-NPs is between those of the TiO2 and the sensitizer. Generally, LSPR by Ag-NPs can improve the absorbance in conventional organic solar cells.3,20,22 Thus, we can assume that the management between the absorbance and the charge trapping according to the loading levels of the Ag-NPs determines the strength of the photocurrent in the device operation.30 We used the Ag-NPs as shown in Figure 1b; these were synthesized by a typical reduction method of AgNO3 using a reducing agent according to the procedure reported in the experimental section.20 The Ag-NPs were dispersed in EtOH to promote mixing with the TiO2 NPs for the photoanode layer. In general, the optical properties of Ag-NPs are tuned by the particle size, shape, and density, which in turn influence



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the device performance of typical organic-based solar cells.23,31-33 In particular, Ag-NPs increase the light absorption in the visible wavelength range by LSPR.3 The size of the NPs used in this study (~25 nm in diameter) is small enough to suppress the scattering effect (Figure S1),23,34 which is consistent with our hypothesis.

Figure 2. (a) J-V characteristics of perovskite solar cell with the device structure of FTO/TiO2+n wt% Ag-NPs/CH3NH3PbI3/spiro-OMeTAD/Au. The n represents the loading levels of Ag-NPs and varies from 0.0 wt% to 2.0 wt%. (b) Relative enhancements of the average device performances with n wt% Ag-NPs. Table 1. Perovskite Solar Cell Performancea Parameters

Loading levels of Ag-NPs in TiO2 layer

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

0.0 wt% Ag-NPs

17.85 ± 0.68

0.88 ± 0.02

69.46 ± 0.62

10.96 ± 0.43

0.5 wt% Ag-NPs

18.91 ± 0.42

0.90 ± 0.02

70.20 ± 0.70

11.96 ± 0.37

1.0 wt% Ag-NPs

18.45 ± 0.54

0.90 ± 0.02

70.01 ± 0.84

11.65 ± 0.36

2.0 wt% Ag-NPs

17.87 ± 0.62

0.89 ± 0.01

69.27 ± 0.63

11.07 ± 0.49

a

Total 50 devices were measured and calculated.


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Figure 2a depicts the J-V curves of the four devices with different Ag-NPs loading levels. The average device performances such as the Jsc, open-circuit voltage (Voc), fill factor (FF), and PCE are summarized in Table 1. Compared to the control device with no Ag-NPs, the three devices with different loading levels of the NPs shows little difference in Voc and FF, but distinguishing feature in Jsc.35 The Jsc of the device with 0.5 wt% Ag-NPs is 18.91 mA/cm2 compared to 17.85 mA/cm2 of the control device. The improved Jsc in the PSC containing 0.5 wt% Ag-NPs can be attributed to the absorption enhancement by LSPR.3 Therefore, we conclude that the incorporation of 0.5 wt% Ag-NPs improved the PCE by enhancing the Jsc. However, the NPs loading levels above 0.5 wt% in the TiO2 photoanode layer cause decreases in the current density, which can be ascribed to the increase in the charge traps on the Ag-NPs and in the surface roughness. The Ag-NPs are believed to trap the photogenerated electrons and hinder charge transport due to the energy level mismatch between TiO2 and the Ag-NPs.36 The relative enhancement of the average device performance in Figure 2b shows almost equivalent tendencies in the Jsc and the PCE, indicating that the increased efficiency is mainly dependent on the variation in the Jsc. The highest value of the Jsc is observed in the PSC with 0.5 wt% Ag-NPs, and decreases with increases in the loading level of the Ag-NPs. The PCE with 0.5 wt% Ag-NPs is superior to those of the PSCs incorporated with metal NPs in previously reported studies.21 Despite the increased trapping of photoinduced charges, the absorption enhancement improved the PCE. We believe that the charge trap and the absorbance are closely related in determining the device performance, as is explained later (see Figure 4 and 5).



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Figure 3. IPCE spectra (from 380 to 800 nm) of the PSCs with different loading levels of the Ag-NPs in TiO2 photoanode layer. Figure 3 illustrates the incident photon-to-electron conversion efficiencies (IPCEs) and relative enhancement of IPCE to confirm the effect of the LSPR on the enhanced PCE as a function of Ag-NPs loading levels. The device with 0.5 wt% Ag-NPs indicates the highest enhancement of the IPCE in the spectral range of 380-800 nm, whereas relatively low enhancements are observed with the NPs loading levels greater than 0.5 wt%. The IPCEs of the devices with the Ag-NPs represents broad increases over the panchromatic wavelength range compared to that of the sample without the NPs due to the absorption enhancement by LSPR of the Ag-NPs.21 Regardless, Ag-NPs loading levels above 0.5 wt% in the devices generate only small increases compared to that of the control device with no Ag-NPs. This may be related with charge trapping on the NPs, which decrease electron transfer and the corresponding Jsc; finally the IPCE is decreased due to the undesired transport of electrons to the NPs, where the charge carriers are trapped by the potential barrier between the Ag-NPs and TiO2.


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Figure 4. (a) UV-Vis spectra of FTO/TiO2+n wt% Ag-NPs in film and the Ag-NPs only in solution. (b) Normalized absorbance of FTO/TiO2+n wt% Ag-NPs/CH3NH3PbI3. (n=0.0, 0.5, 1.0, and 2.0; each histogram in the inset represents the relative increase ratio with different loading levels of the Ag-NPs.)

Figure 4a shows the UV-Vis absorption spectra of FTO/TiO2 + n wt% Ag-NPs in film and the absorbance of the Ag-NPs only in solution in which their intensities in the range of 400–500 nm depend only on the absorption of the different loading levels of the Ag-NPs. The absorption spectrum around 470 and 700 nm indicates oscillations caused by interference effects.37 The plasmonic absorption peak at 410 nm in a solution of the Ag-NPs red-shifted to ~450 nm when the NPs are incorporated in the TiO2 photoanode layer because of the reduced distance between the NPs.38 The absorbance of the samples is proportional to the increases in the loading levels of the Ag-NPs in the TiO2 layer, which indicates the interaction between the oscillating free electrons of the Ag-NPs and the incident light. The absorbance enhancement in the sample with FTO/TiO2+n wt% Ag-NPs/CH3NH3PbI3 is depicted in Figure 4b. Note that the enhancement is not pronounced for wavelengths shorter than 400 nm, because the TiO2 film has relatively high absorption in this region (Figure S2b).

Even though LSPR by Ag-NPs is occurring, the

absorption at a short-wavelength of ~400 nm does not show a distinct increase because the 13 Environment ACS Paragon Plus


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absorption is saturated due to the intrinsically high extinction coefficient of the perovskite sensitizer. However, the Ag-NPs do enhance the light absorption in the panchromatic wavelength range (400–800 nm), which can be attributed to the absorption increased by LSPR in addition to the absorbance of the perovskite sensitizer. We can rule out the scattering effect because of the small size of the Ag-NPs (~25 nm).23,39 Therefore, only LSPR by the Ag-NPs contributes to the enhancement of optical absorption. In this regard, we can deduce that the absorption enhancement in the sample with FTO/TiO2+Ag-NPs/CH3NH3PbI3 leads to the improvement of the Jsc and the corresponding increase of the PCE. The relative enhancements, based on the sample with no Ag-NPs are represented in FTO/TiO2+n wt% Ag-NPs and FTO/TiO2+n wt% AgNPs/CH3NH3PbI3, respectively (Figure 4a and 4b inset). The relative absorption enhancement ratio between FTO/TiO2+n wt% Ag-NPs (Figure 4a inset) and FTO/TiO2+n wt% AgNPs/CH3NH3PbI3 (Figure 4b inset) is not similar. In the case of FTO/TiO2+n wt% Ag-NPs, the relative absorption increase is proportional only to the increase in the loading level of the AgNPs. However, Figure 4b inset mainly indicates the additional absorbance increase of the perovskite sensitizer by the LSPR effect of the Ag-NPs in the long-wavelength region (400–800 nm), which is consistent with the previous researches.21, 40 The small increase of the relative absorbance enhancement in Figure 4b inset is dependent on the intrinsic absorption ability of the perovskite material. The absorbance of the material is strong in the short-wavelength region (~400 nm) and relatively weak in the long-wavelength region (600–800 nm). Thus, the perovskite material limits the absorbance in the short-wavelength region but receives the enhancement from the plasmonic effect of the Ag-NPs in the long-wavelength region. As a result, a relatively small increase in the absorbance enhancement ratio of FTO/TiO2+n wt% AgNPs/CH3NH3PbI3 is shown in Figure 4b inset. The values of relative increase ratio were


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calculated by integrating the area under each UV-Vis spectrum based on the loading level of the Ag-NPs, which could be quantitatively related to the Jsc. In the device with 0.5 wt% Ag-NPs, the increase ratio of 5.0% in the absorbance correlates to the enhancement in Jsc of 5.9% and the PCE of 9.1%. The relative increase ratio of the PCE is higher than that of the Jsc while the relative UV absorption improvement is similar to the increase ratio of the Jsc. The origin of the higher relative increase ratio in the PCE seems to indicate limited charge trapping with additional contribution from the decreased resistance by suppressing the surface roughness of TiO2 film when 0.5 wt% Ag-NPs are embedded which will be discussed later with AFM images (see Figure 6).41 However, the other devices with loading levels of 1.0 wt% and 2.0 wt% Ag-NPs reveal the increase ratios of 5.1% and 10.9% for absorbance but 3.4% and 0.1% for Jsc, 6.3% and 1.0% for PCE, respectively. These results are attributed the increase in the surface roughness and the numbers of charge trap. It is known that the performance of PSCs varies with the difference in thickness of the capping layer.42,43 We measured the thickness of the perovskite capping layer with n wt% Ag-NPs (n=0.0, 0.5, 1.0, and 2.0) using a surface profiler. Ten samples of each one with different loading levels of the Ag-NPs were analyzed. The average thickness of ~13 nm is depicted in Figure S3. It indicates that the addition of the Ag-NPs with n wt% loading has little effect on the thickness of the capping layer.44 Consequently, the effect of the thickness of the capping layer with n wt% Ag-NPs on the corresponding device performance can be ruled out. The crystal size of the perovskite material, which also changes the absorbance and the thickness of the capping layer, is important in determining the device performance of PSCs. Under our experimental conditions, the concentration and composition of the perovskite material and the processing conditions are



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identical, thus the crystal size in the FE-SEM images is nearly identical despite the different loading levels of the Ag-NPs as shown in Figure S4.

Figure 5. Time-resolved PL of FTO/TiO2+n wt% Ag-NPs/CH3NH3PbI3.

We performed a series of time-resolved PL measurements on the PSCs with different Ag-NPs loading levels in order to confirm the charge trapping behavior by electron lifetime decay, as shown in Figure 5. The decay in the fluorescence intensity (I) with time (t) was fitted with the bicomponent exponential decay analysis which delivered two lifetime components following the equation

where the slower decay time (τ1) represents the recombination

of a strongly localized electron-hole pair or exciton and faster decay time (τ2) is that of a weakly localized pair.45 The constants A1 and A2 are the respective amplitudes of these components. Interestingly, all devices showed increased absorbance as the loading levels of the Ag-NPs increased, and the lifetimes of the excited electrons showed the same tendency. The reference TiO2 film with no Ag-NPs showed the shortest lifetimes of 4.57 and 1.14 ns. The PL lifetimes gradually increased with increasing the loading level of the Ag-NPs, which are 5.70 and 1.27 ns for 0.5 wt% Ag-NPs, 6.06 and 1.40 ns for 1.0 wt% Ag-NPs, and 6.26 and 1.59 ns for 2.0 wt% Ag-NPs. According to the average lifetime (τaverage), the increase ratio was 22.0%, 30.0%, and


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37.4% in sequential order of the Ag-NPs loading levels. These values could represent that free charge carriers might be trapped in the lower conduction energy level of the Ag-NPs, hindering the charge transport. The correlation between the charge trapping and the Jsc is inversely proportionate. The increase ratio in the charge trapping and Jsc is 22.0% and 5.9% for 0.5 wt% Ag-NPs, 30.0% and 3.4% for 1.0 wt% Ag-NPs, and 37.4% and 0.1% for 2.0 wt% Ag-NPs, respectively. These results could explain why the addition of more than 0.5 wt% Ag-NPs to the TiO2 layer improved the incident light absorption, but suppressed the PCE. By charge trapping, the Ag-NPs hampered charge transport from the perovskite sensitizer to the photoanode. Consequently, the Jsc was not proportional to the absorbance enhancement because of the increased number of charge traps as shown by the relatively longer lifetime of free charge carriers in TiO2+Ag-NPs/CH3NH3PbI3.



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Figure 6. AFM topography images of FTO/TiO2+n wt% Ag-NPs; (a) n=0.0, (b) n=0.5, (c) n=1.0, and (d) n=2.0.

The Ag-NPs tended to aggregate into clusters as the loading level was increased in the solution (Figure S1 and S2) because of van der Waals attraction.46 We analyzed AFM images of the films to study the surface roughness and the aggregation behaviors in FTO/TiO2+Ag-NPs according to the loading levels of the NPs. In Figure 6, the TiO2+Ag-NPs clusters by the aggregation of TiO2 NPs with the Ag-NPs were created through the addition of the Ag-NPs in TiO2 photoanode layer because of electrostatic charge-charge interaction between them.47 The TiO2 layer with no Ag-


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NPs shows a root mean square (RMS) roughness of 21.1 nm, while the films incorporating 0.5 wt%, 1.0 wt%, and 2.0 wt% Ag-NPs display 18.7, 18.6, and 26.5 nm, respectively. Mimetic diagrams show to depict the aggregation behavior according to the loading level of the Ag-NPs in the inset of Figure 6. At the 0.5 wt% Ag-NPs, the surface roughness of the film is decreased by 13.4% (Figure 6a). This is attributed to the effect of the dense packing of the TiO2 photoanode layer, resulting from the electrostatic interaction between the negatively charged TiO2 and the positively charged Ag-NPs. In other words, the formation of TiO2+Ag-NPs clusters, which influences the density of packing in the TiO2 layer, reduced the surface roughness. However, when a high loading level of 2.0 wt% Ag-NPs are embedded, excessive numbers of clusters are formed and the surface roughness is increased by 25.6% as depicted in Figure 6d.

Figure 7. XRD patterns of FTO/TiO2+n wt% Ag-NPs.

In Figure 7, the XRD patterns show strong diffraction peaks at 25.4° and 48.0° which were assigned as the distinct anatase phase of TiO2.48 The intensity of the peak at 25.4° changes according to the different loading levels of the Ag-NPs; this is attributed to the difference in the size of the TiO2 crystallites, which can be inferred from the different aggregation behaviors of



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TiO2+n wt% Ag-NPs.49 The crystallite size is calculated with the Scherrer equation50 2 = / where B is the full width at half maximum, K is Scherrer constant, is wavelength, and L is crystallite size. The crystallite sizes are 10.8 nm, 11.5 nm, 12.1 nm, and 8.5 nm for 0.0 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt% of the Ag-NPs loading levels, respectively. This trend in the crystallite size as the aggregation behavior51 is consistent with the tendency of the roughness observed in the AFM images (Figure 6). Generally, a smooth, homogeneous TiO2 layer promotes charge transport and the following current density.42,52 At the small loading level of 0.5 wt% AgNPs, the increased absorbance and the relatively smooth surface as well as the increased in the number of the clusters leads to the enhancement of the Jsc. Even though the surface roughness of TiO2 film and aggregation behavior with 1.0 wt% Ag-NPs was similar to those of the film with 0.5 wt% Ag-NPs, the increased charge traps in the film with 1.0 wt% Ag-NPs hampered to exceed the Jsc as much as it had in the sample with 0.5 wt% Ag-NPs. Furthermore, the highest level of 2.0 wt% promoted large numbers of the aggregated TiO2+Ag-NPs clusters. The increase of both the surface roughness and the charge trapping with excessive numbers of the clusters caused a decrease of Jsc regardless of the increased absorbance.

4. CONCLUSION We demonstrated the correlations among the absorbance, the charge trapping, and surface roughness for efficient PSCs using neat Ag-NPs. The device with 0.5 wt% Ag-NPs showed a PCE of 11.96% in which this value is the highest efficiency from a PSC using metal NPs in TiO2 photoanode layer. The enhancement of the optical absorption through the LSPR of the Ag-NPs was proportional to the loading level of the Ag-NPs in the panchromatic wavelength region of the PSC without the contribution of the scattering effect. The density of charge traps increased


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with increasing the loading level of the Ag-NPs. However, the densely packed TiO2 layer with 0.5 wt % Ag-NPs induced the suppression of the surface roughness. Therefore, the relative balance between the absorbance and the number of charge traps is the origin of the high Jsc and the corresponding PCE. In addition, the reduced surface roughness promoted by the dense packing of the TiO2 layer through the formation of the TiO2+Ag-NPs clusters also contributes to the increase in the device performance, in addition to the optical enhancement by the Ag-NPs. Consequently, the absorbance, the charge trapping, and the aggregation behavior in the TiO2 photoanode layer, which are determined by the loading level of the Ag-NPs, are decisive factors in increasing the efficiency and to understand the charge transport. We believe that introducing the Ag-NPs in TiO2 photoanode in PSCs can improve the device performance not only by the enhancement of the absorbance in the long-wavelength region, but also by the increase in the charge transport capability.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Supplementary data for observing diameter of aggregated Ag-NPs with DLS measurement (Figure S1), the absorption of (a) n wt% Ag-NPs and (b) TiO2 only with UV-Vis spectroscopy (Figure S2), average thickness values of the perovskite capping layer with n wt% Ag-NPs by a surface profiler (Figure S3), and crystal morphology of the perovskite material with n wt% AgNPs in FE-SEM images; (a) n=0.0, (b) n=0.5, (c) n=1.0, and (d) n=2.0 (Figure S4).



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] *Tel: +82-2-880-6682 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the NRF grant (No. 2013R1A1A2074468) funded by the MSIP, and Mid-career Research Program (NRF-2015R1A2A2A04006172) through NRF grant funded by the Ministry of Education, Science and Technology (MEST, Korea). We thank Dr. Jaejung Ko, Hyeju Choi for kind help in solar cell fabrication.


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