Indepth Studies on Working Mechanism of Plasmon-Enhanced

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Indepth Studies on Working Mechanism of Plasmon-Enhanced Inverted Perovskite Solar Cells Incorporated with Ag@SiO2 Core-Shell Nanocubes Xiaoqian Ma, Ben Ma, Tianyan Yu, Xin Xu, Liuquan Zhang, Wei Wang, Kun Cao, Lingling Deng, Shufen Chen, and Wei Huang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00346 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Applied Energy Materials

Indepth Studies on Working Mechanism of Plasmon-Enhanced Inverted Perovskite Solar Cells Incorporated with Ag@SiO2 Core-Shell Nanocubes Xiaoqian Ma,† Ben Ma,† Tianyan Yu,† Xin Xu,† Liuquan Zhang,† Wei Wang,† Kun Cao,† Lingling Deng,‡ Shufen Chen,*,†,§ and Wei Huang*,†,§ †Key

Laboratory for Organic Electronics and Information Displays & Jiangsu Key

Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China ‡College of Electronic and Optical Engineering & College of Microelectronics, Nanjing

University of Posts and Telecommunications, Nanjing, 210023, China §Institute

of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU),

127 West Youyi Road, Xi'an 710072, Shaanxi, China KEYWORDS: Ag nanocube, Surface plasmon resonance, Local field, Carrier transfer, Perovskite solar cell

ABSTRACT: Noble metal nanoparticles-induced localized surface plasmon resonance as a useful approach has been widely used in solar cells including perovskite solar cells (PSCs) to improve their light-harvesting. Herein, we synthesize Ag@SiO2 core-shell nanocubes and investigate their application in CH3NH3PbI3-based PSCs due to both the

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large local EM field induced by the nanocube with sharp corners and the effective avoidance of exciton/carrier recombination at the surfaces of Ag nanocubes via covering a ~5 nm ultrathin SiO2 shell. Incorporating an appropriate concentration of Ag@SiO2 nanocubes into the CH3NH3PbI3 PSCs realizes a best-performing efficiency of 17.22% with an enhancement factor of 18.1%. Indepth studies on the plasmonenhanced working mechanism of Ag@SiO2 nanocubes with UV-vis absorption spectra, steady-state and time-resolved transient photoluminescence, and electrochemical impedance spectroscopy characterizations eventually demonstrate both the increasing light harvesting and the improving charge transportation and extraction contribute to better performances of PSCs. INTRODUCTION Since the first implementation of methylammonium lead iodide (CH3NH3PbI3) perovskite in liquid dye-sensitized solar cells (DSSCs) as a light absorber (dye) in 2003, organometal halide perovskites have attracted enormous attention owing to approprite optical band gap, high absorption coefficient and excellent bipolar carrier transport properties.1−9 Power conversion efficiency (PCE) of perovskite solar cells (PSCs) based on organometal halides has boosted from 3.8% to over 23% during the past few years.9−12 Among various organometal halide perovskite materials, CH3NH3PbI3 is still regarded as one of the most excellent light harvesters in PSCs due to its proper bandgap of about 1.55 eV and large absorption coefficient of 105 cm-1.13−17 In general, planar heterojunction-based PSCs without any pre- or post-treatment on CH3NH3PbI3 show a middle PCE of 13-15% and many approaches containing the solution-casting technique,


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e.g., solvent engineering,18,19 vapor-assisted solution deposition,20 hot-substrate casting,21,22 and a number of post-treatments including anti-solvent rinsing treatment,18,23,24 vacuum flash treatment,25,26 and thermal or solvent annealing27−29 have been employed to enhance the efficiency.30 Meanwhile, noble metal nanostructureinduced surface plasmons are considered as another method that has potential for researchers to further enhance PCE of PSCs.31−35 Surface plasmons are electromagnetic (EM) surface waves confined to a metaldielectric interface, which can be either localized by metal nanoparticles (noted with localized surface plasmon resonance, LSPR) or propagate as waves along with planar metal surfaces. When an appropriate frequency of incident photons interacts with metal nanoparticles (NPs) or periodic metal nanoarray (PMN), most energy of the incident light is absorbed by the oscillation mode of the NPs or PMN, with a strong local field generated around the NPs or the PMN.36−38 If the resonant wavelength of metallic NPs/PMN is well matched with the light-absorptiong wave band of a photoactive layer, locating metallic NPs/PMN within/near the photoactive layer is beneficial to promoting its light absorption. This is the well-known plasmon-enhanced phenomenon and it has been universally employed to enhance light-absorptiong ability of polymers and fully improve the PCEs of organic photovoltaic devices.39−41 Similarly, the metallic NPs have also been frequently applied in DSSCs in the last several years.42,43 But related reasearch results reveal a totally different working mechanism in DSSCs, that is, a significantly increased charge transfer instead of increasing light absorption has been observed at the interface of photoactive material (dye) and carrier extraction layer with



the use of metallic NPs in DSSCs.42 Since the first report of plasmon-induced performance enhancements by Zhang et al,44 different shapes of metallic NPs have been successfully used into PSCs. But these research groups held on different opinions on plasmon-enhanced principles of their PSCs. For example, Kakavelakis et al. demonstrated that the doping of Al NPs into poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) induced an absorption enhancement of CH3NH3PbI3, along with a 13.5% enhancement factor for PCE.34 In contrast, Fu and Yuan et al. has recently demonstrated that the dominant contribution to the enhanced performance of plasmonic PSCs is the hot electron injection from Au NPs to TiOx, rather than the absorption enhancement induced by plasmon,31,32 since the plasmonic enhancement effect becomes less effective or nonexistent in the zone where the original absorption of the photoactive layer is already very significant.45,46 In our recent work, we demonstrate the coexistence of increasing light absorption and charge transfer capability in our plasmonic PSCs with Au tetrahedra NPs as surface plasmons.33 To boost the PCE, it is necessary to further explore the key working mechanism of plasmon-enhanced PSCs through analysis on different shapes and components of NPs. In this work, unique metallic nanocubes (NCs) structures are synthesized and employed into planar heterojunction PSCs to investigate metal plasmons-triggered performance enhancements. Ag NCs are chosen because of the following reasons. First of all, referring to previous literature report,47−49 unique “hot spots” can be generated at sharp corners of metallic NPs and they will induce more intense EM fields around the


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corners of these metallic NPs than conventional shapes. Based on such a design mind, we choose metallic nanocubes as surface plasmons in this work mainly owing to their intrinsic sharp corners and potential strong resonance intensities. Herein, instead of using Au nanocubes, we select Ag nanocubes because of its resonance peak at around 450 nm. From Hsiao’s opinion, it is particularly helpful to enhance solar cell’s light absorption and PCE through using metallic NPs with their resonant band located within the weak absorption region of the photoactive layer.41 Thus, the absorption in the nearUV wavelength range of less 400 nm is possibly very useful in our case since the photon-to-electron conversion efficiency for as-used CH3NH3PbI3 perovskite is extremely low in such a region.50 Improving perovskite’s light-absorption ability to the incident solar illumination or increasing excitons’ transfer rate in these region can improve cell’s conversion efficiency, which contributes to the eventual short-circuit current density (Jsc) and PCE of PSCs. In addition, the wide absorption band of the Ag NCs covering from near-UV to near-infrared region is beneficial to promoting the light absorption of the peroveskite layer over a broad wavelength range. Second, direct contact of the photoactive film with metallic NPs will inevitably leads to serious exciton quenching and free carriers recombination.51−54 To slove this problem, a ~5 nm ultrathin SiO2 shell is covered onto the Ag NCs to form a Ag@SiO2 NCs core-shell structure via a Stober method. Finally, instead of directly doping into the perovskite precursor, locating Ag@SiO2 NCs under the CH3NH3PbI3 perovskite film not only reduces the negative influence of its ethanol solvent on CH3NH3PbI3 but also fully employs the local field generated by Ag surface plasmon resonance, which eventually yields Jsc and



PCE values of 22.58 mA cm-2 and 17.22%, with enhancement factors of 11.7% and 18.1% over those of reference device, respectively. EXPERIMENTAL SECTION Chemicals. Sodium hydrosulfide hydrate (NaHS·xH2O, 68 wt% in water), silver nitrate (AgNO3, ≥99%), polyvinyl pyrrolidone (PVP, M.W.=29000), ethylene glycol (EG, ≥99.5%), tetraethyl orthosilicate (TEOS, ≥99%), ammoniawater (NH3(aq), 28%), lead iodide (PbI2, 99.999%), and methylammonium iodide (MAI, 99.9%). All abovementioned chemicals were obtained from Sigma-Aldrich and used as received without any purification. PEDOT:PSS (AI 4083) and PC61BM were purchased from Heraeus Materials Technology Co. Ltd. and Nano-C, respectively. Synthesis of Ag NCs. PVP solution was prepared by adding 80.5 mg of PVP into 4 mL of EG solution. The EG solution was stirred at 150 °C for 1 h, and then 40 μL of NaHS in EG solution (2.97 mM) was dropped into the heated solution. After stirring for 9 min, 0.75 mL of as-prepared PVP solution was added into the reaction, followed by the addition of 0.25 mL of AgNO3 in EG solution (0.54 M). The mixture was stirred for 35 min and then transferred to a 25 °C water bath. Synthesis of Ag@SiO2 NCs. The as-prepared Ag NCs was coated with a thin silica layer (~5 nm) through a Stober method. First, 0.5 mL of Ag NCs was added into 10 mL of ethanol, which needs an ultrasonic treatment for 15 min. Then, 150 mL of NH3 (aq) was swiftly dropped into the above-mentioned mixture. After stirring for 10 min, 0.5 µL of TEOS solution was added. The mixed solution was stirred overnight and storaged in a light-free environment. The mixture was washed with deionized water and ethanol


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in turn for three times to obtain the eventual products of Ag@SiO2 NCs. Device Fabrication. Firstly, ITO glasses (17 mm × 17 mm) were washed with acetone, ethanol and deionized water in sequence, followed by ultrasonic cleaning with deionized water, acetone, and ethanol for 15 min. After drying with high purity N2 and vacuum drying in an oven, the ITO substrates were treated with UV-ozone for 15 min. Then, PEDOT:PSS was spin-coated onto the patterned ITO glass substrates at 3000 rpm for 30 s and dried at 120 °C for 30 min, forming a 30 nm thick film. After that, Ag@SiO2 NCs with concentrations of 1, 3, 5 and 7 vol% were respectively spin-coated onto the PEDOT:PSS film at 3000 rpm for 30 s. After a 120 °C annealing for 20 min, the Ag@SiO2 NCs-covered PEDOT:PSS and the pristine PEDOT:PSS film were transferred into the glove-box to spincoat the MAPbI3 layer via a two-step method. First, 30 µL of PbI2 in DMF/DMSO solvent (4:1 (v/v)) was spin-coated onto the substrates at 6000 rpm for 30 s, with a fast annealing of 100 °C for 20 s. Then 60 µL of CH3NH3I in isopropanol (60 mg mL-1) was deposited onto the PbI2 layer with a rotation speed of 3000 rpm and a time of 30 s, followed by a drying process under 100 °C for 30 min. Once cool, 30 µL of PC61BM chlorobenzene solution (30 mg mL-1) was spin-coated onto the MAPbI3 layer at 3000 rpm for 60 s, which was then room-temperature remained for 9 min. Finally, 9 nm of BCP and 100 nm of Ag were sequentially deposited by thermal evaporation under a pressure of 1.4 × 10−4 Pa. The active area for all devices is 0.1 cm−2. Characterization. The characterizations of shape and size for the Ag@SiO2 NCs were used with a transmission electron microscope (TEM, Hitachi, HT7700). X-ray



diffraction (XRD) patterns were carried out using a Bruker D8 ADVANCE XRD equipment. The surface morphology images of the Ag@SiO2 NCs-coated PEDOT:PSS films and the perovskite layer were obtained by using a Hitachi S-4800 field emission scanning electron microscopy (SEM). The UV-vis absorption spectrum was measured with a PerkinElmer Lambda 650S spectrophotometer. Electrochemical performances were studied by an electrical impedance spectroscopy (EIS) on the VMP3 Electrochemical Workstation (Bio-logic). The current density–voltage characteristics were carried out using Keithley 2400 under AM 1.5G illuminations (100 mW cm-2) from a solar simulator (Oriel Sol3A Class Solar Simulator (94023A)). RESULTS AND DISCUSSION The Ag@SiO2 NCs were successfully synthesized with a conventional Stober method described in recent literature.55 Since the structural and thermal stability of the bare Ag NPs is proven to be extremely poor in ambient air conditions, SiO2 insulating shells were introduced to protect the as-prepared Ag NCs from damage of Ostwald ripening.56−58 More importantly, this SiO2 insulating shell can suppress exciton quenching or free carriers recombination inside devices through avoiding direct contact of metallic Ag with the CH3NH3PbI3 perovskite. Transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDX) were used to confirm the morphology and component of the Ag@SiO2 nanostructures. From the TEM image in Figure 1a, we observe the NPs are completely composed of cube shape, whose average size length is 50 nm, as the dimension statistics shown in Figure S1. Figure 1b illustrates


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an EDX map of Ag@SiO2 nanocube which clearly shows a red silver core surrounded by a green SiO2 shell with an average thickness of ~5 nm. The surface plasmonic absorption spectrum of the as-prepared Ag@SiO2 NCs is shown in Figure 1c, which was measured by dispersing these Ag@SiO2 NCs in ethanol solvent. A major resonance peak at 456 nm is obviously observed together with a weak shoulder of 350 nm, coming from the dipole plasmon resonance mode and plasmon high-order mode of Ag NCs, respectively.59−61

Figure 1. (a) The TEM images for Ag nanocubes and (b) the EDX map of an Ag@SiO2 nanocube showing an Ag core (red) and a SiO2 shell (green). Inset is the high magnification image of Ag@SiO2 nanocube. (c) The absorption spectrum of the Ag@SiO2 nanocubes. These Ag@SiO2 NCs were then directly spin-coated onto the PEDOT:PSS hole extraction layer (HEL) to fabricate PSCs with device structures of ITO/PEDOT:PSS (30 nm)/Ag@SiO2 NCs/CH3NH3PbI3 (300 nm)/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM, 30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, 9 nm)/Ag (100 nm), as shown in Figure 2a. Here, the doping concentrations of Ag@SiO2 NCs are varied from 0 to 7 vol%. And ITO, PEDOT:PSS, PC61BM, BCP and Ag are used as anode, hole buffer layer, electron transport layer, electron buffer layer and



cathode, respectively. In general, it is essential to avoid direct contact between metallic NPs and photoactive layer in plasmon-enhanced solar cells,32,62 because that unavoidably leads to charge recombination and thus exciton quenching loss at those surfaces of metallic nanostructures.44,63,64 Herein, we coated SiO2 shells onto these Ag NCs to sufficiently solve this problem by substantially preventing from the charge recombination on these metallic surfaces. The schematic illustration of the Ag@SiO2 NCs-incorporated CH3NH3PbI3 film fabrication process is shown in Figure 2b and the preparation details are described in Experimental section (Supporting Information). As the scanning electron microscope (SEM) images of NCs-covered PEDOT:PSS shown in Figure 2c, the density distribution of Ag@SiO2 NCs increases with its doping concentration. As the Ag@SiO2 NCs concentration exceeds 5%, it occurs some agglomeration of NPs, which might affect device performances to some extent. In order to investigate the possible influence of these NPs on subsequent films and device performances, we first compared the crystallization of the CH3NH3PbI3 films with and without Ag@SiO2 NCs. The coating of Ag@SiO2 NCs on PEDOT:PSS hardly alters the grain size of the subsequent perovskite, indicative of ignorable effect of metallic NPs on the formation and crystallization of the perovskite film (Figure S2), which is consistent with our previous result.33,40 And the XRD patterns of CH3NH3PbI3 films were also measured to verify this point by coating the perovskite layer on the PEDOT:PSS layer with and without the insertion of Ag@SiO2 NCs (Figure 2d). The main diffraction peaks located at 14.1°, 28.4° and 31.7° can be assigned to (110), (220) and (310) planes of CH3NH3PbI3, respectively. Here different CH3NH3PbI3 films have


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extremely similar characteristic peaks, demonstrating no change occurs in perovskite crystal structure.

Figure 2. (a) The structural diagram of our PSCs. (b) The film fabrication process. (c) The SEM images of Ag@SiO2 NCs with 1-7 vol% concentrations respectively spincoated onto the PEDOT:PSS layer. (d) The XRD patterns of the CH3NH3PbI3 films on



the pure PEDOT:PSS and Ag@SiO2 NCs-covered PEDOT:PSS with different NCs concentrations of 1-7 vol%. To further study the influence of NPs on the inside EM field intensity of PSCs, a finite difference time domain (FDTD) analysis software was utilized to calculate the local field distribution around Ag@SiO2 NCs. Note that the plasmon-enhanced device in our simulation work uses an optimal doping concentration of 3 vol% for Ag@SiO2 NCs and here the NPs are supposed to be periodically dispersed onto the PEDOT:PSS film with a repetition period of 2355 nm (referring to the statistic data in Figure S3). As shown in Figure 3a, all films are located within XY plane along Z axis, with ITO, PEDOT:PSS, CH3NH3PbI3, PC61BM, BCP and Ag covering Z values of (-220)−(-40), (-40)−0, 0−(+280), (+280)−(+310), (+310)−(+318) and (+318)−(+418) nm in sequence. Figure 3b shows the simulation field intensity distributions on the XZ plane at Y=0. Some representative wavelengths of 463, 665 and 808 nm were extracted from the plasmonic device with Ag@SiO2 NCs and the counterpart without any NPs. Obviously, the simulation results confirm a significantly enhanced local field inside CH3NH3PbI3 perovskite film in our plasmonic PSCs, which is caused by the localized surface plasmon resonance of Ag@SiO2 NCs with the incident solar light. As shown in the Figure 3c−h, with the use of the Ag@SiO2 NCs, an extremely strong local field is generated on the XZ plane with intensity exceeding 920 at the plasmonic resonance wavelength of 463 nm (Figure 3f, a redshift in resonance/absorption peak from 456 to 463 nm is due to increase in refractive index of surrounding dielectric), four orders of magnitude higher than that (~0.5, Figure 3c) without NPs. Similarly, another two


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Figure 3. (a) One Ag@SiO2 NC sample is in a three-dimensional rectangular coordinate system. Here, incident light propagation and polarized directions of this incident light beam are along Z and X axes, respectively. (b) The cross-section schematic of Ag@SiO2 NCs with partial solar cell structure (XZ plane at Y = 0 nm). (c−e) correspond to the field intensities at 463, 665 and 808 nm without Ag@SiO2 NCs and (f−h) correspond to the field intensities at 463, 665 and 808 nm with Ag@SiO2 NCs. groups of data at the longer wavelength of 665 nm (Figure 3g vs Figure 3d) and the cut-



off wavelength of 808 nm (Figure 3h vs Figure 3e) also exhibit Ag@SiO2 NCs-induced strong local field inside the CH3NH3PbI3 film, fully demonstrating it is efficient over a wide wavelength range from near-UV to near-infrared. All these results indicate that the LSPR excited at the interface of Ag@SiO2 NCs and CH3NH3PbI3, particularly at the corner of Ag@SiO2 NCs, causes a strong field enhancement in the perovskite film and this enhancement is beneficial to a further promotion of the absorption of the perovskite active layer. To accurately quantify the amplitude of the local field in PSCs with Ag@SiO2 NCs, electric field intensities along X axis satisfying Y=0 nm and Z=5 nm (dot black line in Figures 3b) were extracted. As a result, Figures 4a−c describe the X-axis electric field strength at 463, 665 and 808 nm and the insets of Figures 4a−c show the local field only entering into the perovskite layer. Given the boundaries of X=±30 nm, significantly enhanced electric field intensities are observed at all wavelengths and the enhancement factor at 463 nm is particularly obvious, up to 33.3. The local field intensities at other position (e.g., along X axis at Y=30 nm) or with different polarized directions of the incident light beam (along with the diagonal line of the nanocube’s bottom plane) are also calculated and extracted, as illustrated as Figure S4 and Figure S5. And the enhanced electric field intensities are similarly observed despite of slightly different strength values. To systematically investigate the performance alterations of PSCs with the introduction of Ag@SiO2 NCs, over 50 solar cells were fabricated with a series of concentration conditions (from 0 to 7 vol%). Here, the typical current density-voltage


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Figure 4. The X-axis electric field intensity at (a) 463, (b) 665 and (c) 808 nm satisfying Y=0 and Z=5 nm. Inset figures are the magnified field distributions in the perovskite layer. (J-V) characteristics under a standard illumination of 100 mW/cm2 are shown in Figure 5a, while the statistical analysis on the photovoltaic characteristics of devices with representative doping concentrations of 0, 1, 3, 5 and 7 vol% are depicted in Table S1 and Table S2. Compared to the reference device without Ag@SiO2 NCs, the performances of the devices incorporated with Ag@SiO2 NCs increase firstly and then decline with the continuously increasing NCs concentrations, and the optimized doping concentration is 3 vol%. For the optimized concentration condition, the typical parameters of the device with Ag@SiO2 NCs are 22.58 mA cm-2, 1.01 V, 75.5% and 17.22% for Jsc, open-circuit voltage (Voc), fill factor (FF) and PCE, respectively, of which Jsc, Voc, FF and PCE show dramatical enhancements compared with the reference counterparts without NPs (20.21 mA cm-2, 74.4%, 0.97 V and 14.58%). The aforementioned results suggest that the incorporation of the Ag@SiO2 NCs with appropriate distribution density could remarkably improve the performances of PSCs via increasing both Jsc, Voc and FF. More attractively, the plasmon-enhanced devices with Ag@SiO2 NCs show less hysteresis than the reference ones, as shown in Figure 5b and summarized hysteresis index in Table S2. From previous analysis,65,66 the



negligible hysteresis is supposed as the effect of interfacial trap passivation, resulting in the restrained ion migration and more balanced charge mobility. Here, it may be closely related to faster charge transfer and transport with the use of Ag@SiO2 NCs, as discussed later. To reveal the factors governing the Jsc enhancement, further studies were carried out by measuring the optical absorption properties of the thin perovskite films with different densities of Ag@SiO2 NCs below these perovskites. As shown in Figure 5c, the CH3NH3PbI3 photoactive layers exhibit strong light extinction in the high energy region, particularly in the range of

Indepth Studies on Working Mechanism of Plasmon-Enhanced

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