Enhanced Performance of Perovskite Solar Cells with Zinc Chloride

Nov 23, 2017 - Perovskite solar cells (PSCs) have attracted extensive attention due to their impressive photovoltaic performance. The quality of perov...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 42875−42882

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Enhanced Performance of Perovskite Solar Cells with Zinc Chloride Additives Junjie Jin,† Hao Li,† Cong Chen,† Boxue Zhang,† Lin Xu,† Biao Dong,† Hongwei Song,*,† and Qilin Dai*,‡ †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China ‡ Department of Physics, Atmospheric Sciences and Geoscience, Jackson State University, Jackson, Mississippi 39217, United States S Supporting Information *

ABSTRACT: Perovskite solar cells (PSCs) have attracted extensive attention due to their impressive photovoltaic performance. The quality of the perovskite layer is very critical to achieve high device performance. Here, we explore the partial substitution of PbI2 by ZnCl2 in the preparation of CH3NH3PbI3 and its effects on perovskite morphology, optical properties, and photovoltaic performance. Consequently, the device with 3% ZnCl2 shows great improvement in power conversion efficiency (PCE) from 16.4 to 18.2% compared to that of the control device. Moreover, the device is more stable than the control device, with only 7% degradation after aging for 30 days. These results are attributed to the increased grain size, improved film morphology, and reduced recombination loss after the partial substitution of PbI2 by ZnCl2 in the perovskite film. This work develops a new approach for morphology control through rational additives in the perovskite film, and paves the way toward further enhancing the device performances of PSCs including PCE and stability. KEYWORDS: perovskite solar cells, ZnCl2 additives, larger grain, nonradiative recombination, stability 2-pyridylthiourea improved the quality of the perovskite film in terms of uniformity and the size of the crystal grains, leading to an enhanced PCE of 18.2%.27 Chen et al. introduced AgI into the CH3NH3PbI3 film, partially substituting Pb2+ by Ag+, leading to improved film morphology, reduced electron recombination, and an enhanced PCE from 16.0 to 18.4%.28 Jahandar et al. used CuBr2 as an additive material to fabricate a perovskite absorber layer and improved the PCE from 13.18 to 17.09%.29 In this study, we explored the partial substitution of PbI2 by ZnCl2 in CH3NH3PbI3 and its effect on the structural and optical properties of the perovskite films and the photovoltaic performance of the PSCs. CH3NH3I(PbI2)1−x(ZnCl2)x (MAI(PbI2)1−x(ZnCl2)x) thin films were prepared by a solutionbased method and used as the light absorber in PSCs. A perovskite film with large grains and few pinholes is formed as the doping concentration is low, indicating that lower level doping reduces the number of defects. The prepared PSCs based on MAI(PbI2) 1−x(ZnCl2) x exhibit less hysteresis compared to that of the control PSCs, which is attributed to the reduced charge recombination caused by fewer defects of

1. INTRODUCTION Organic−inorganic hybrid perovskite materials have attracted tremendous attention in the solar cell field due to their excellent photovoltaic properties such as strong light absorption, good carrier mobility, long exciton lifetime and diffusion length, and solution processability.1−6 The power conversion efficiency (PCE) of solar cells based on these materials have been improved from 3.8% to more than 22% in recent years.7,8 The simple structure and outstanding performance of perovskite solar cells (PSCs) make them promising as next generation photovoltaic devices.9−12 Generally, to obtain excellent performance in PSCs, the most attractive approach to apply is to improve the quality of the perovskite film. It has been reported that high quality perovskite films have the characters of large grain, good surface coverage, less defects, and minimum pinholes.13−16 A variety of strategies have been demonstrated to control the growth of the perovskite film, such as the control of precursor ratio, solvent engineering and deposition route (solution-based or vacuumbased), and the nucleation stage of the perovskite.17−23 In addition, the use of additive materials in the perovskite absorber layer is also an effective way to improve the quality of a perovskite film.24−26 Sun et al. reported a simple process to prepare the perovskite film in ambient air by adding 2pyridylthiourea in the precursor solution. The introduction of © 2017 American Chemical Society

Received: October 9, 2017 Accepted: November 23, 2017 Published: November 23, 2017 42875

DOI: 10.1021/acsami.7b15310 ACS Appl. Mater. Interfaces 2017, 9, 42875−42882

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic device architecture of MAI(PbI2)1−x(ZnCl2)x perovskite solar cells. (b−f) SEM surface images of the MAI(PbI2)1−x(ZnCl2)x perovskite film with different ZnCl2 (x = 0, 1, 3, 5, and 10%) content. (g) Average grain size of MAI(PbI2)1−x(ZnCl2)x perovskite film vs x.

the perovskite film. At the optimal condition (x = 3%), the MAI(PbI2)0.97(ZnCl2)0.03 based PSCs yield a PCE of 18.2% with less hysteresis. More importantly, the stability of PSCs based on MAI(PbI2)0.97(ZnCl2)0.03 at ambient atmosphere is also increased compared to that of the control device. A total of 93% of the initial PCE was maintained after aging in an ambient atmosphere for 30 days.

current conversion efficiency (IPCE) spectra were recorded by using a Solar Cell Scan 100. Electrochemical impedance spectroscopy (EIS) was performed by a CHI630E Electrochemical Analyzer (ChenHua, China).

3. RESULTS AND DISCUSSION The device configuration of our PSCs is presented in Figure 1a, the optimized MAI(PbI2)1−x(ZnCl2)x perovskite absorber layer is sandwiched between a compact TiO2 (c-TiO2) layer and Spiro-OMeTAD layer, where the electrons and holes are transported to c-TiO2 and Spiro-OMeTAD, respectively. The corresponding film thicknesses of c-TiO 2 , MAI(PbI2)0.97(ZnCl2)0.03, Spiro-OMeTAD, and Au are 60, 410, 260, and 100 nm, respectively (Figure S1). Energy-dispersive spectroscopy (EDS) was used to identify the presence of zinc chloride in the perovskite film (Figure S2). Notably, both Zn and Cl distribute homogenously with Pb and I throughout the entire film. Figure 1b−f shows the scanning electron microscopy (SEM) images of the MAI(PbI2)1−x(ZnCl2)x film with different ratios (x = 0, 1, 3, 5, and 10%). The average grain size versus ZnCl2 was determined by image analysis and is plotted in Figure 1g. It can be observed that the average grain size of the control perovskite film with 0% ZnCl2 doping is around 120 nm, which is comparable to the results reported in recent literature reports.27,30,31 In addition, a larger size is obtained by ZnCl2 doping via our fabrication techniques, which indicates that doping with ZnCl2 leads to a larger perovskite grain size without changing other experimental parameters. At lower ZnCl2 doping (1−3%), the perovskite film has a large grain size with the average value of ∼350 nm, and the film become more dense and compact. This large grain size means that the density of grain boundaries becomes much lower, which is advantageous to the transport and collection of photogenerated charges. However, the cracks and pinholes caused by ZnCl2 high doping (5−10%) shown in Figure 1e,f lead to reduced coverage of the perovskite film on the substrate, which decreases device performance. We believe that the crystal defects will be reduced during perovskite film growth as ZnCl2 is incorporated into the film. In addition, to determine that the crystal grain size is mainly associated with Zn or Cl, we prepared perovskite films doped with ZnI2. Figure S3 shows that the perovskite film prepared with ZnI2 has an increased crystal grain size, which is similar to ZnCl2. This confirms that

2. EXPERIMENTAL SECTION 2.1. Device Fabrication. Fluorine-doped tin oxide (FTO) glass substrates were first etched with zinc powder and 30% HCl, then ultrasonicated in deionized water, acetone, and ethanol, successively, followed by oxygen plasma treatment. Compact TiO2 (c-TiO2) was deposited onto the cleaned FTO glass by spin-coating 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropyl alcohol) in 1-butanol solution at 2000 rpm for 30 s and annealing in air at 500 °C for 30 min. The MAI(PbI2)1−x(ZnCl2)x perovskite solution was prepared by mixing methylammonium iodide (MAI), PbI2, and ZnCl2 powders at a molar ratio of 1:x:1 − x (x = 0, 1, 3, 5, and 10%) in dimethyl sulfoxide solution. Then, the perovskite film was fabricated on the as-prepared c-TiO2 layer in a nitrogen atmosphere through a two-step method at 1000 and 4000 rpm for 10 and 30 s, respectively. During the second step, 100 μL of chlorobenzene was kept on the spinning substrate for 20 s prior to the end of the spinning program. After spin-coating, the films were annealed at 100 °C for 5 min. Then, the 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD) solution (50 mg of 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD), 22.5 μL of 4-tert-butylpyridine, and 22.5 μL of acetonitrile solution containing 170 mg/mL lithium bis(trifluoromethylsulfonyl)imide in 1 mL of chlorobenzene) was spin-coated on the perovskite layer at 1500 rpm for 30 s. Finally, the Au electrode with a thickness of 100 nm was deposited by thermal evaporation in a vacuum chamber (9 × 10−4 Pa). 2.2. Device Characterization. A Sirion field-emission scanning electron microscope was employed to evaluate the surface morphology of the films and cross-sectional view of the device. The energydispersive spectroscopy (EDS) spectra were recorded on a Nova_NanoSEM430. X-ray diffraction (XRD) for perovskite films was carried out on a Rigaku D/max 2550 X-ray diffractometer, using a monochromatized Cu target radiation source at a scanning rate of 4°/ min. The absorption spectra were recorded using a UV-1800 spectrometer. The photoluminescence (PL) characterization was carried out using a fluorescence luminescence spectrometer. The J− V characteristics of the devices were measured under simulated 100 mW/cm2 AM 1.5 G irradiation using an Abet Sun 2000 solar simulator calibrated with a reference silicon cell (RERA Solutions RR-1002), and Keithley Model 2400 as a digital source meter. Incident photo-to42876

DOI: 10.1021/acsami.7b15310 ACS Appl. Mater. Interfaces 2017, 9, 42875−42882

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) XRD patterns of the perovskite films with different doping concentrations. (b) UV−vis absorption spectra. (c) The UPS spectra. (d) The energy band diagram. (e) The PL spectra and (f) the time-resolved photoluminescence (TR-PL) spectroscopy of MAI(PbI2)1−x(ZnCl2)x perovskite films with different ZnCl2 concentrations (x = 0, 1, 3, 5, 10%).

energy band diagram of the MAI(PbI2)1−x(ZnCl2)x perovskite film with different ZnCl2 concentrations.33,34 It was noticed that the valence band edge of the MAI(PbI2)1−x(ZnCl2)x perovskite films and the corresponding conduction band (estimated by using the valence band and the optical bandgap values (Figure S6)) shift downward slightly. It can be observed that the conduction/valence band positions do not change that much when the ZnCl2 doping concentration is less than 3%, whereas a large shift can be caused by higher doping concentrations, which may be another reason for the decreased device performance. The photoluminescence (PL) spectra of the MAI(PbI2)1−x(ZnCl2)x perovskite films are shown in Figure 2e. All perovskite films were prepared on cleaned glass to avoid the influence of charge injection among the absorber layer and electron transfer layer.35−37 As shown in Figure 2e, the PL intensity of the films increases as the ZnCl2 concentration increases from 1 to 3%, then decreases when the ZnCl2 content increases further, leading to the strongest PL when the ZnCl2 concentration is 3%. The PL intensity of the perovskite material based on 10% ZnCl2 is weaker than that of the pristine film. The PL intensity enhancement implies that ZnCl2 doping reduces the traps or defects in the perovskite layer. In addition, there is a slight red shift (∼3 nm) for the PL peak, which agrees with the variation of bandgap. The time-resolved PL (TR-PL) decay curves of the ZnCl2-incorporated perovskite films are displayed in Figure 2f, and the decay parameters were obtained from a biexponential decay function as below

the addition of Zn in the perovskite precursor has a crucial impact on the nucleation and subsequent crystal growth processes.15 To further study the effects of ZnCl2 substitution on crystal structure, we measured the X-ray diffraction patterns of the MAI(PbI2)1−x(ZnCl2)x (x = 0, 1, 3, 5, and 10%) perovskite films (Figure 2). All perovskite films were prepared under the same conditions with a thickness of ∼400 nm. Figure 2a shows that all diffraction patterns are in good agreement with that of the tetragonal perovskite structure and the intensity of the (110) diffraction peak increases significantly as the content of ZnCl2 is increased.32 In addition, their corresponding full width at half-maximum (FWHM) of the (110) peak of all samples was compared, and the values are plotted in Figure S4. As presented, the FWHM decreases with the increase of ZnCl2 content, indicating better crystallinity caused by ZnCl2. These results are consistent with the SEM results, which confirm that the crystal grain size is gradually increased with the content of ZnCl2 in MAI(PbI2)1−x(ZnCl2)x. Figure S5 shows the enlarged region of the (110) peak. A peak shift toward a higher angle is observed in the figure, which is attributed to the smaller atomic radius of Zn compared to that of Pb.15 Figure 2b shows the ultraviolet−visible (UV−vis) spectra of the MAI(PbI2)1−x(ZnCl2)x perovskite films. All films were prepared with the same conditions except ZnCl2 content. Compared to that of the pristine film, the absorbance of the doped films increases with the increase of Zn doping content when the doping level is less than 3%, which is attributed to the increased crystallinity and improved surface morphology due to Zn doping. With further increases of the ZnCl2 incorporated concentration to 5 and 10%, there is a significant decrease of absorbance due to the poor quality of the prepared film because of the high doping concentration. Figure 2c,d shows the ultraviolet photoelectron spectroscopy (UPS) spectra and the

F(t ) = A1 exp( −t /τ1) + A 2 exp(−t /τ2)

where A1 and A2 are the time independent coefficients of the amplitude fraction, respectively; τ1 and τ2 are the fast decay time and slow decay time, respectively. The average decay time was calculated using the following equation38,39 42877

DOI: 10.1021/acsami.7b15310 ACS Appl. Mater. Interfaces 2017, 9, 42875−42882

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ACS Applied Materials & Interfaces

Figure 3. (a) J−V curves measured under reverse scan mode for the devices based on MAPbI3 and MAI(PbI2)0.97(ZnCl2)0.03. (b) The steady-state power output at the maximum power output values for optimized control and MAI(PbI2)0.97(ZnCl2)0.03 devices. (c) IPCE of control and MAI(PbI2)0.97(ZnCl2)0.03 devices. (d) Recombination resistance (Rrec) is plotted vs applied voltage. (e) VOC vs light intensity plot. (f) Dark J−V curves of devices based on MAPbI3 and MAI(PbI2)0.97(ZnCl2)0.03.

Table 1. Device Parameters of the PSCs Based on Pristine MAPbI3 and MAI(PbI2)0.97(ZnCl2)0.03 Perovskite Films device MAPbI3 MAI(PbI2)0.97(ZnCl2)0.03

τave =

scan direction

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

SPO (%)

reverse forward reverse forward

21.09 20.53 22.04 21.94

1.05 1.04 1.09 1.09

74.05 68.84 75.76 73.59

16.4 14.7 18.2 17.6

16.1 18.0

prepared by the same conditions with Gaussian fitting is presented in Figure S7.2 It shows that all PSCs based on MAI(PbI2)0.97(ZnCl2)0.03 exhibit higher PCEs than those of the control devices. In addition, the photovoltaic performance of the devices with different ZnCl2 incorporation concentrations is presented in Figures S8 and S9. Noticeably, the device performance increases significantly as the ZnCl2 concentration increases from 1 to 3%, because the ZnCl2 doping results in the increased crystallinity and improved surface morphology. However, with a further increase of ZnCl2 concentration from 5 to 10%, the devices exhibit a low JSC and VOC. The decreased photovoltaic performance can be explained by the obvious pinholes in the perovskite films with 5 and 10% ZnCl2 (Figure 1e,f). Hysteresis of J−V curves is an important issue for determining the actual PCE, which is affected by the defect states or the band bending due to ion migration in PSCs.40,41 It is noted that the MAI(PbI2)0.97(ZnCl2)0.03-based device has 3% PCE hysteresis, which is lower than that of the control device (10%). Thus, the hysteresis effect is suppressed, which might be associated with the large grain formation caused by the partial substitution of PbI2 with ZnCl2, leading to enhanced film quality and reduced film defects. To further confirm the lower amount of hysteresis for the device with the addition of ZnCl2, the stability of power output was plotted at a constant bias at its near maximum power output value, as shown in Figure 3b. For

∑ Aiτi 2/∑ Aiτi

The corresponding parameters of PL lifetime are summarized in Table S1. The average PL lifetime (τave) increases from 36.9 ns for 0% ZnCl2 doping to the maximum of 84.3 ns for 3% ZnCl2 doping, which is attributed to the enlarged grain size and the improved surface morphology, but at the higher ZnCl2 concentrations (5−10%), the film shows cracks and pinholes, which lead to the decreased τave (28.3 ns). These results indicate that lower ZnCl2 doping can reduce the defect density in the perovskite film and thus increase the charge transport efficiency, but higher doping can introduce nonradiative recombination channels and lead to a decrease in the PL lifetime. J−V curves of the devices based on MAI(PbI2)0.97(ZnCl2)0.03 and MAIPbI2 measured with reverse and forward scans are shown in Figure 3a, and the relevant photovoltaic parameters are listed in Table 1. The control device measured under the reverse scan has a short-circuit current density (JSC) of 21.09 mA/cm2, an open-circuit voltage (VOC) of 1.05 V, a fill factor (FF) of 74.05%, and a resulting PCE of 16.4%. The device performance improves significantly as 3% ZnCl2 is doped into the perovskite films, and the corresponding JSC, VOC, FF, and PCE obtained under the reverse scan are 22.04 mA/cm2, 1.09 V, 75.76%, and 18.2%, respectively. In addition, the statistical PCE histogram of 60 PSC devices with and without ZnCl2 42878

DOI: 10.1021/acsami.7b15310 ACS Appl. Mater. Interfaces 2017, 9, 42875−42882

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ACS Applied Materials & Interfaces

Figure 4. Normalized JSC, VOC, FF, and PCE for the MAPbI3 and MAI(PbI2)0.97(ZnCl2)0.03 PSCs stored in an ambient environment of 25−28 °C with 30−55% humidity without any encapsulation.

voltage, as shown in Figure 3d. The Rrec of the MAI(PbI2)0.97(ZnCl2)0.03-based device is substantially larger than that of the control device under the same applied voltage, which indicates the enhanced carrier mobility in the device based on MAI(PbI2)0.97(ZnCl2)0.03, leading to significantly improved VOC and FF. To obtain more detailed information about charge recombination processes, we measured VOC as a function of light intensity for the control and MAI(PbI2)0.97(ZnCl2)0.03-based devices according to the formula42,43

the MAI(PbI2)0.97(ZnCl2)0.03-based device, it quickly increases to a steady state with a maximum PCE of 18.0%, whereas the control device takes more time to achieve a stabilized PCE of 16.1%. To understand the origin of the improved device performance, photoelectric and electrochemistry measurements were carried out. Figure 3c shows the incident photo-to-current conversion efficiency (IPCE) spectra of the device based on MAI(PbI2)0.97(ZnCl2)0.03 and the control device. Obviously, the IPCE of the MAI(PbI2)0.97(ZnCl2)0.03-based device is higher than that of the control device in the region from 400 to 800 nm, which is attributed to the higher absorption intensity of the MAI(PbI2)0.97(ZnCl2)0.03-based device, leading to higher photocurrent. In addition, the integrated JSC values estimated from the IPCE spectra are 21.03 and 22.00 mA/cm2 for the control and MAI(PbI2)0.97(ZnCl2)0.03-based devices, respectively. These are consistent with the values obtained from the J−V curves. Figure S10 shows the electrochemical impedance spectroscopy (EIS) spectra of the two devices with or without ZnCl2 doping measured in dark conditions under a bias of 0.8 V. The simulated equivalent circuit is shown in the inset of Figure S10, where RS, Rrec, and CPE are the series resistance of the electrode, the recombination resistance, and the stray capacitance, respectively. As reported in the literature, Rrec is inversely proportional to the recombination rate.13 From the fitting results in Table S2, Rrec increases from 202.4 to 910.9 Ω after ZnCl2 has been incorporated, indicating the decreased recombination of the perovskite layer and the electron transporting layer. In addition, Rrec also depends on the applied

δVOC = n(KBT /q) ln(I ) + constant

where n is the ideality factor, KB is the Boltzmann constant, T is absolute temperature, q is elementary charge, and I is light intensity. As shown in Figure 3e, by linearly fitting VOC versus log-scaled light intensity, a slope of 1.49KBT/q and 1.27KBT/q were obtained for the control and MAI(PbI2)0.97(ZnCl2)0.03based devices, respectively. The smaller slope indicates that fewer trap sites exist in the MAI(PbI2)0.97(ZnCl2)0.03 film, which results in the reduced nonradiative recombination loss and a higher VOC.44,45 Furthermore, the dark J−V curve of the MAI(PbI2)0.97(ZnCl2)0.03-based device has a lower leakage current density than that of the control device (Figure 3f), which indicates the reduced recombination losses in the MAI(PbI2)0.97(ZnCl2)0.03-based device. We evaluated the long-term stability of the control device and the MAI(PbI2)0.97(ZnCl2)0.03-based device, which were stored in an ambient environment of 25−28 °C with 30−55% 42879

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ACS Applied Materials & Interfaces humidity without any encapsulation.46,47 Figure 4 shows the normalized JSC, VOC, FF, and PCE as a function of time. The MAI(PbI2)0.97(ZnCl2)0.03-based device shows excellent stability against degradation in ambient air, the PCE of the MAI(PbI2)0.97(ZnCl2)0.03-based device maintains 93% of its initial value. However, the PCE of the control device decreased to 44% of its initial value. Similarly, the JSC, VOC, and FF values for the control device suffer a substantial decrease during the aging process. After ZnCl2 incorporation, the MAI(PbI2)0.97(ZnCl2)0.03 film shows a larger crystal grain size and fewer surface defects than those of the control film, as shown in Figure 1. The better morphology is a possible reason for the improved air stability of the PSCs based on MAI(PbI2)0.97(ZnCl2)0.03. As shown in Figure S11, a diffraction peak at 2θ = 12.49° appears in the XRD pattern for the control perovskite after 30 days of aging in an ambient atmosphere, which is assigned to the (001) diffraction peak of PbI2, indicating the formation of PbI2 during decomposition of the perovskite.48,49 However, the MAI(PbI2)0.97(ZnCl2)0.03 film shows a very tiny peak at 2θ = 12.49°, indicating the slower decomposition process of the MAI(PbI2)0.97(ZnCl2)0.03 film compared to that of the control film. Therefore, a more stable perovskite structure can be produced as ZnCl2 is incorporated into the perovskite lattice. We attribute this to the substitution of Pb2+ by Zn2+, which reduces the crystal defects during the perovskite material growth, leading to better quality perovskite films and better stability. In addition, we also compared the thermal stability of the control device and the MAI(PbI2)0.97(ZnCl2)0.03-based device, as shown in Figure S12. It was noticed that, in practical applications, the outdoor temperature can reach as high as 85 °C, such as in the desert, because of the direct exposure of solar cells to sunlight and local heating.50 Hence, the thermal stability of the devices was measured in air at 85 °C without encapsulation. We kept the devices at 85 °C for 180 h, and the MAI(PbI2)0.97(ZnCl2)0.03based device shows remarkable thermal stability with 70% of the initial PCE, whereas the control device maintains only 30% of its initial PCE. This is because the ion migration in the MAI(PbI2)0.97(ZnCl2)0.03 film could be largely suppressed due to the larger crystal grain size and fewer defects in the MAI(PbI2)0.97(ZnCl2)0.03 film compared to those of the control film.51,52 In addition, the Cl-containing perovskite lattice inherently has a stronger electrostatic attraction with MA+, which is beneficial to better chemical stability. To explore a more in-depth understanding of the better thermal stability, we measured the UV−vis spectra of the control, MAI(PbI2)0.97(ZnCl2)0.03, and MAI(PbI2)0.97(ZnI2)0.03 films before and after annealing at 85 °C for 180 h, as shown in Figure S13. It was noticed that the MAI(PbI2)0.97(ZnCl2)0.03 and MAI(PbI2)0.97(ZnI2)0.03 films show similar stability after annealing, but the control film shows a serious decrease in stability compared to that of the other two films.

30−55% humidity without any encapsulation after 30 days. Ninety three percent of its initial PCE remained for the PSC based on ZnCl2 doping compared to 44% for the control device. The thermal stability was also improved after aging at 85 °C for 180 h. Our results prove that the partial substitution of PbI2 by ZnCl2 is a promising strategy to enhance the photovoltaic performance and stability of PSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15310. SEM cross-sectional image, energy-dispersive spectroscopy maps for the MAI(PbI2)0.97(ZnCl2)0.03-based device and the FWHM, bandgaps, photovoltaic performance, and stability for the MAPbI3 and MAI(PbI2)0.97(ZnCl2)0.03-based devices (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.S.). *E-mail: [email protected] (Q.D.). ORCID

Lin Xu: 0000-0001-5831-430X Hongwei Song: 0000-0003-3897-5789 Qilin Dai: 0000-0001-8680-4306 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program (2016YFC0207101), the Major State Basic Research Development Program of China (973 Program) (No. 2014CB643506), the National Natural Science Foundation of China (Grant Nos. 61674067, 11674127, 11674126, 61775080, 11374127, and 11504131), the Jilin Province Natural Science Foundation of China (Nos. 20150520090JH and 20170101170JC), and the Jilin Province Science Fund for Excellent Young Scholars (Nos. 20170520129JH and 0170520111JH).

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REFERENCES

(1) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.-j.; Sarkar, A.; Nazeeruddin, M. K.; et al. Efficient inorganic−organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors. Nat. Photonics 2013, 7, 486−491. (2) Shao, Y.; Yuan, Y.; Huang, J. Correlation of energy disorder and open-circuit voltage in hybrid perovskite solar cells. Nat. Energy 2016, 1, No. 15001. (3) Wang, Q.; Shao, Y.; Dong, Q.; Xiao, Z.; Yuan, Y.; Huang, J. Large fill-factor bilayer iodine perovskite solar cells fabricated by a lowtemperature solution-process. Energy Environ. Sci. 2014, 7, 2359−2365. (4) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542−546. (5) Tan, H.; Jain, A.; Voznyy, O.; Lan, X.; de Arquer, F. P. G.; Fan, J. Z.; Quintero-Bermudez, R.; Yuan, M.; Zhang, B.; Zhao, Y.; et al. Efficient and stable solution-processed planar perovskite solar cells via contact passivation. Science 2017, 355, 722−726. (6) Ahn, N.; Son, D.-Y.; Jang, I.-H.; Kang, S. M.; Choi, M.; Park, N.G. Highly reproducible perovskite solar cells with average efficiency of

4. CONCLUSIONS In conclusion, we have demonstrated that the partial substitution of PbI2 by ZnCl2 in perovskite films can increase the grain size and improve the film morphology, which is beneficial to enhancing the performance of the PSCs. By optimizing the ZnCl2 incorporation concentration, the device exhibits an efficiency of 18.2% with less hysteresis compared to those of the control device (16.4%). In addition, the device with ZnCl2 exhibits better air stability compared to that of the control device when aged under ambient air at 25−28 °C with 42880

DOI: 10.1021/acsami.7b15310 ACS Appl. Mater. Interfaces 2017, 9, 42875−42882

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