Surfactant enhanced surface coverage of CH3NH3PbI3−xClx perovskite for highly efficient mesoscopic solar cells

Accepted Manuscript Surfactant enhanced surface coverage of CH3NH3PbI3-xClx perovskite for highly efficient mesoscopic solar cells Yanli Ding, Xin Yao, Xiaodan Zhang, Changchun Wei, Ying Zhao PII:

S0378-7753(14)01360-3

DOI:

10.1016/j.jpowsour.2014.08.095

Reference:

POWER 19692

To appear in:

Journal of Power Sources

Received Date: 1 July 2014 Revised Date:

19 August 2014

Accepted Date: 20 August 2014

Please cite this article as: Y. Ding, X. Yao, X. Zhang, C. Wei, Y. Zhao, Surfactant enhanced surface coverage of CH3NH3PbI3-xClx perovskite for highly efficient mesoscopic solar cells, Journal of Power Sources (2014), doi: 10.1016/j.jpowsour.2014.08.095. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

J−V curves obtained from the best perovskite solar cells using PVP of 0, 1.6, 3.3 and

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5 wt%, respectively. The inset is the schematic of device architecture.

ACCEPTED MANUSCRIPT Surfactant enhanced surface coverage of CH3NH3PbI3-xClx perovskite for highly efficient mesoscopic solar cells Yanli Ding†a,b, Xin Yao †a, Xiaodan Zhang*a, Changchun Weia and Ying Zhaoa Institute of Photo Electronics Thin Film Devices and Technology of Nankai

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University, Weijin Road 94#, Nankai District, Tianjin 300071, PR China b

Department of Physics and Information Engineering, Shangqiu Normal

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University, Shangqiu 476000, PR China

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Abstract

Controlling the morphology of perovskite absorbers is important for achieving high performance devices. Polymer poly-(vinylpyrrolidone) (PVP) is applied as a

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surfactant to enhance the surface coverage of perovskite. The introduction of this polymer results in the production of perovskite films with smooth surfaces and uniform crystal domains. Sufficient surface coverage not only reduces the amount of

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light passed straight through the cell without absorption, but also reduces charge

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recombination at the interfaces. The cell power conversion efficiency (PCE) is enhanced from 4.51% to 8.74% under the optimum PVP doping ratio of 3.3 wt%. Keywords: Perovskite, Solar cells, Surface coverage, Surfactant

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Corresponding author. Fax: +86 22-23499304; Tel. +86-22-23499304. E-mail: [email protected] †These authors contributed equally to the work.

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ACCEPTED MANUSCRIPT 1. Introduction The silicon technology that has dominated the field of photovoltaic (PV) cells is facing a strong challenge from third generation PV technologies. Among the third

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generation solar cells, mesoscopic solar cells are presently under intense investigation owing to their cost-effective high efficiency solar power conversion and simple fabrication methods [1-4]. Organometallic trihalide perovskites show promise as

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high-performance light harvesters in mesoscopic solar cells [5-8]. A major advantage

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of these organic--inorganic hybrid materials is their high absorption coefficients, good carrier mobility and long-range electron-hole diffusion lengths. Mixed halide perovskite materials, whose electron--hole diffusion length is ten times longer than those only containing iodide [9, 10]. Over the last two years, rapid progress has been

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made in the PV devices based on these materials. Lee et al. showed the highest efficiency of 10.9%, where the mixed-halide methylammonium lead perovskite CH3NH3PbI3-xClx is adsorbed on a mesoporous Al2O3 photoanode by a simple

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spin-coating process [11]. Recently, Liu et al. showed that a 350 nm thick layer of the

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CH3NH3PbI3-xClx leads to solar cells with an efficiency of 15.4% at standard 1sun illumination [12]. The perovskite film can be deposited by vacuum evaporation or solution-processing via spin-coating or dip-coating. Compared with evaporation, solution processing, without the need for vacuum deposition, represents the lowest-cost production method for thin film solar cells. The development of solar cell technologies using such methods with lower production costs is therefore of great interest to the industry. Although high device efficiencies have been obtained for 2

ACCEPTED MANUSCRIPT solution-processed perovskite solar cells, obtaining complete coverage and uniformity of the perovskite films on the substrates by spin-coating is still the main challenge of using this low-cost method.

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Controlling the morphology of perovskite absorbers is important for achieving high performance devices [13, 14]. Kim et al. adopted a mixed solvent system to control the surface coverage and morphology of a CH3NH3PbI3 perovskite layer [15].

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Liang et al. incorporated 1,8-diiodooctane into the precursor solution as an additive to

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modulate thin film formation [16]. Eperon and Dualeh studied the effect of the annealing temperature on the formation of perovskite film [17, 18]. In this study, CH3NH3PbI3-xClx is used as absorber material due to its long-range electron-hole diffusion lengths. Here we present a new and very effective method to enhance the

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surface coverage of perovskite using spin-coating methods. To the best of our knowledge, this is the first example of the adoption of PVP to modulate thin film formation leading to significant efficiency enhancement. The power conversion

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to 8.74%.

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efficiency (PCE) greatly increased from 4.51% (with no PVP in the precursor solution)

2. Experimental

2.1 Materials and preparation of the substrate Methylammonium

iodide

(CH3NH3I),

lead

chloride

(PbCl2)

and

poly-(vinylpyrrolidone) (PVP K30) were purchased from Sigma-Aldrich and used without further purification. Anhydrous N,N-dimethylformamide (DMF) was purchased from Tianjin Guangfu Fine Chemical Research Institute. 3

ACCEPTED MANUSCRIPT A transparent conducting fluorine-doped SnO2-coated glass substrate (FTO) was ultrasonically cleaned sequentially in detergent and deionized water and finally dried under a flow of clean air. We employed a chemical bath deposition process to deposit

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a hole-blocking layer of compact TiO2 onto the FTO-glass substrate via hydrolysis of TiCl4 in water at 70 °C for 30 min. This step was followed by an annealing at 500 °C for 40 min. A mesoporous TiO2 (m-TiO2) layer composed of 20-nm-sized particles

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was then deposited by spin coating using a commercial TiO2 paste diluted in ethanol

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(1:4, weight ratio) and annealed at 500 °C for 40 min. Spin-coating was carried out at 5000 rpm for 60 s. 2.2 Device fabrication

It is noted that all device fabrication steps were carried out under ambient

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conditions with humidity below 30% at room temperature. A DMF solution (38 wt%) of CH3NH3I and PbCl2 (3:1 M ratio) was deposited onto the m-TiO2 films by spin-coating and the perovskite (CH3NH3PbI3-xClx) was formed after annealing at

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140 °C for 50 min. Films containing perovskite and PVP were similarly prepared

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using the perovskite/PVP solutions. These solutions were prepared by adding PVP (0-5 wt% with respect to perovskite weight) to the perovskite precursor solution. Subsequently, the spiro-OMeTAD-based hole-transfer layer (HTM) was deposited by spin coating at 5000 rpm for 60 s. The HTM solution consists of 80 mg spiro-OMeTAD,

28.5

µL

4-tertbutylpyridine

and

17.5

µL

lithium-bis

(trifluoromethane)sulfonyl imide (Li-TFSI) solution (520 mg Li-TFSI in 1 mL acetonitrile), dissolved in 1 mL chlorobenzene. Finally, a 200 nm thick Ag layer was 4

ACCEPTED MANUSCRIPT deposited on the HTM layer by thermal evaporation. 2.3 Characterization X-ray diffraction (XRD) analysis was performed on a Rikaku, ATX-XRD with Cu

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Ka radiation (λ=1.5405 Å) in the 2θ range from 10° to 80°. The absorption spectra of CH3NH3PbI3-xClx adsorbed on the TiO2 film were obtained using a Cary spectrophotometer (Cary 5000 UV–vis–NIR) in the wavelength ranging from 300 to

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800 nm. The morphologies of the samples were observed using NanoNavi-SPA400

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atomic force microscopy (AFM). The SEM images were recorded on a Jeol JSM-6700F scanning electron microscopy. The photocurrent density−voltage (J−V) curves of the solar cells were measured at 25 °C under AM 1.5, 100 mW cm-2 light illumination. The quantum efficiency (QE) measurements were also performed to

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evaluate the spectral response of the fabricated solar cells. We note that all of the above measurements including I-V and QE were conducted under ambient conditions. 3. Results and discussion

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PVP is chosen as a surfactant to control the surface coverage and morphology of the CH3NH3PbI3-xClx perovskite layer due to its non-toxic, good film-forming

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properties and stability. The crystalline quality of the resulting perovskite films is demonstrated in Fig. 1(a), which displays the XRD spectra of a perovskite film fabricated using 0, 1.6, 3.3 and 5 wt% of PVP with respect to perovskite weight, respectively. The main diffraction peaks in the XRD spectra at 14.02°, 28.38°, and 44.4° are assigned to the 110, 220 and 330 peaks, respectively, indicating a high level of phase purity. These peaks match those reported for CH3NH3PbI3-xClx crystallized in the tetragonal perovskite structure [19]. The doping of the perovskite layer with PVP

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ACCEPTED MANUSCRIPT does not lead to the formation of other phases. Further investigation indicates that the full width at half maximum (FWHM) values gradually increase as the amount of PVP increases and then decrease as the amount of PVP further increases, as shown in Fig. 1(b). The magnified XRD spectra of the [110] diffraction peaks are shown in the inset

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of Fig. 1(b). The [110] FWHM values of samples 0%, 1.6%, 3.3% and 5% first increase from 0.16, 0.25 to 0.32, and then decrease to 0.21. The average crystalline sizes can be estimated from Scherrer's equation: D = 0.89 λ/βcosθ, where D is the

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crystallite size, λ is the wavelength of the X-ray, β is the corrected half width of the diffraction peak, and θ is the diffraction angle. Factor 0.89 is the shape factor

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characteristic of a particle. This indicates that the introduction of PVP makes the grains smaller.

To optimize the performance of the perovskite film, the effects of different

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amounts of PVP on the perovskite thin film and device performance are investigated. The AFM images of the CH3NH3PbI3-xClx perovskite films with and without PVP are shown in Fig.2. Images correspond to the perovskite films deposited on m-TiO2 films

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containing 0–5 wt% PVP, respectively. These images clearly show that the

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morphologies of the formed perovskite film are significantly influenced by the surfactant. For sample 0%, which does not contain PVP, no homogeneous film is formed; instead, large islands of CH3NH3PbI3-xClx are observed on the surface of the m-TiO2 film, and a significant portion of the substrate is exposed without CH3NH3PbI3-xClx coverage. Film coverage in this sample is very low and has the highest root-mean-squared surface roughness (RMS) of 177 nm among four films. When a small amount of PVP (1.6 wt%) is added to the CH3NH3PbI3-xClx precursor

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ACCEPTED MANUSCRIPT solution, there is a drastic change in the appearance of the film formed. The areas of “exposed m-TiO2” decrease in size and the coverage of the m-TiO2 is found to be much higher than that without PVP. The RMS of this film is reduced to 71 nm. As the

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amount of PVP increases to 3.3 wt%, the islands of exposed m-TiO2 either disappear or become less obvious. The morphology in this case exhibits smaller crystallites with the lowest RMS of 45 nm, which is consistent with results drawn from the XRD

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spectra. These smaller crystallites form a densely interconnected network that

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enhances the surface coverage of perovskite and results in a much smoother and more continuous surface. As the amount of PVP increases further, the film becomes less uniform and the RMS increases. The optimized blending ratio of PVP in the precursor solution is around 3.3 wt% with respect to perovskite weight used in this experiment.

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These results are consistent with the SEM topography images, as shown in Supplementary Fig. S1. PVP not only acts as surfactant [20, 21], but also as a chelating agent to control the crystal growth [22]. In these perovskite films, we

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believe that PVP performs both these roles. The influence of chelation encourages

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homogenous nucleation and likely modifies the interfacial energy favorably, which ultimately alters the kinetics of crystal growth. The difference in the surface coverage of perovskite films on the m-TiO2 substrate

will likely affect the device characteristics. A series of efficient FTO/compact TiO2/m-TiO2/CH3NH3PbI3-xClx/spiro-OMeTAD/Ag solar cells were fabricated. The structure of the solar cells is shown schematically in Fig. 3(a). Photovoltaic performances of these typical small-area (0.07 cm2) perovskite solar cells without and 7

ACCEPTED MANUSCRIPT with PVP are measured under AM 1.5, 100 mW cm-2 light illumination. The photocurrent density−voltage (J−V) curves of the solar cells without and with the incorporation of PVP are presented in Fig. 3(b), the corresponding photovoltaic

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parameters of which are summarized in Table 1. The cell with PVP achieved the highest PCE of 8.74%, whereas the cell without PVP achieved a PCE of only 4.51%. Compared with the device without PVP, the cell with PVP showed significant

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enhancement in the open-circuit voltage (Voc), short-circuit current density (Jsc) and

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fill factor (FF). The obtained improvement in the efficiency is a mainly a consequence of the better perovskite film coverage. Sufficient coverage by the perovskite film can reduce the light passing through the cells without absorbing it; as light absorption in the perovskite active layer is improved, the available photocurrent

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(Jsc) is increases. Sufficient coverage also reduces the number of “shunt paths” that allow contact between the spiro-OMeTAD and TiO2 compact layers. Consequently, charge recombination is reduced and the shunt resistance is increased, increasing the

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Voc and FF, and accordingly the PCE. The trend in the Jsc is reflected in the UV-vis

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absorption spectra and incident-photon-to-current conversion efficiency (IPCE) spectra measured for samples 0%, 1.6%, 3.3%, and 5%. As shown in Fig. 4(a), all films show absorption onsets at 800 nm with a broad absorption in the visible to near-IR region. The sample 3.3% film has a higher optical density than the other films owing to its higher perovskite coverage than the other samples. The device with PVP is found to have relatively higher IPCE values than that without PVP in the entire measured wavelength, as shown in Fig. 4(b). These improvements in the IPCE and 8

ACCEPTED MANUSCRIPT absorption are mainly attributed to the enhanced surface coverage of perovskite films and are in agreement with the J–V measurements. 4. Conclusion The usefulness of PVP is as a surfactant to enhance the perovskite film coverage

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is reported for the first time. Light absorption in the perovskite active layer is improved and recombination loss at the interfaces is reduced by improving the interfacial contacts and reducing the voids between the perovskite and m-TiO2 layer.

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The PCE is enhanced remarkably from 4.51% to 8.74% under the optimum PVP

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doping ratio of 3.3 wt%. Our strategy offers a simple and effective method to develop high performance organic-inorganic hybrid solar cells.

Acknowledgments

The authors gratefully acknowledge the supports from Science and Technology

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Support Program of Tianjin (12ZCZDGX03600), Major Science and Technology Support Project of Tianjin City (No. 11TXSYGX22100), Specialized Research Fund for the PhD Program of Higher Education (20120031110039) and the Ph.D.

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Candidate Research Innovation Fund of Nankai University (68140001).

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[22] Z.Q Li, Y. Zhang, Angew. Chem. 118 (2006) 7896 –7899.

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PVP

Polymer poly-(vinylpyrrolidone)

PV

photovoltaic

FF

fill factor

FTO

fluorine-doped SnO2

FWHM full width at half maximum hole-transfer material

IPCE

incident-photon-to-current conversion efficiency

J−V

photocurrent density−voltage

Jsc

short-circuit current density

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m-TiO2 mesoporous TiO2

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HTM

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Abbreviations

power conversion efficiency

QE

quantum efficiency

TFSI

(trifluoromethane)sulfonyl imide

Voc

open-circuit voltage

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PCE

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ACCEPTED MANUSCRIPT Figure captions Fig.1. (a) XRD patterns of CH3NH3PbI3-xClx perovskite film using PVP of (a) 0, (b) 1.6 wt%, (c) 3.3 wt%, (d)and 5 wt%; (b) FWHM variation versus the doping ratio of

XRD spectra of the [110] diffraction peaks.

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PVP with respect to the weight of perovskite, the inset of Fig. 1(b) is the magnified

Fig.2. AFM images of CH3NH3PbI3-xClx perovskite film using PVP of (a) 0, (b) 1.6

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wt%, (c) 3.3 wt%, (d)and 5 wt%.

Fig.3. (a) Schematic of device architecture (b) J−V curves obtained from the best perovskite solar cells without and with the PVP.

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Fig.4. (a) UV−vis absorption spectra of CH3NH3PbI3-xClx (b) IPCE curves of solar

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cells without and with PVP.

ACCEPTED MANUSCRIPT Table 1. Detailed photovoltaic parameters of the devices made without and with the PVP Jsc (mA cm-2)

Voc (V)

FF

η (%)

0%

13.12

0.787

43.7

4.51

1.6%

14.96

0.823

48.5

5.97

3.3%

17.54

0.848

58.8

8.74

5%

16.56

0.837

36.8

5.10

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Samples

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Figure 1

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Figure 2

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Figure 3

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PVP is first applied as surfactant to enhance perovskite surface coverage.




Smooth surface and uniform crystal domains of perovskite films are obtained.




The optimum weight ratio of PVP/perovskite precursor is determined as 3.3

The cell power conversion efficiency is enhanced from 4.51% to 8.74%.

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wt%.

ACCEPTED MANUSCRIPT Supporting Information for: Surfactant enhanced surface coverage of CH3NH3PbI3-xClx perovskite for highly efficient mesoscopic solar cells

a

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Yanli Ding†a,b, Xin Yao †a, Xiaodan Zhang1a, Changchun Weia and Ying Zhaoa Institute of Photo Electronics Thin Film Devices and Technology of Nankai

University, Weijin Road 94#, Nankai District, Tianjin 300071, PR China

Department of Physics and Information Engineering, Shangqiu Normal

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University, Shangqiu 476000, PR China

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b

Figure S1. SEM images of CH3NH3PbI3-xClx perovskite film using PVP of (a) 0, (b) 1.6 wt%, (c) 3.3 wt%, (d) 5 wt%.

1

Corresponding author. Fax: +86 22-23499304; Tel. +86-22-23499304. E-mail: [email protected] †These authors contributed equally to the work.