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Enhanced performance and stability of perovskite solar cells using NH4I interfacial modifier Hai-Ying Zheng, Guozhen Liu, Liang-Zheng Zhu, Jia-Jiu Ye, Xu-Hui Zhang, Ahmed Alsaedi, Tasawar Hayat, Pan Xu, and Songyuan Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12721 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 29, 2017

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ACS Applied Materials & Interfaces

Enhanced performance and stability of perovskite solar cells using NH4I interfacial modifier Haiying Zheng,ab Guozhen Liu,ab Liangzheng Zhu,ab Jiajiu Ye,ab Xuhui Zhang,ab Ahmed Alsaedic, Tasawar Hayatce, Xu Pan*a and Songyuan Dai*acd a

Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied Technology, Hefei

Institutes of Physic Science, Chinese Academy of Sciences, Hefei 230031, China. bUniversity

cNAAM

of Science and Technology of China, Hefei 230026, China.

Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia. dState

Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China

Electric Power University, Beijing 102206, China. eDepartment

of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan.

KEYWORDS: perovskite solar cells, NH4I, interfacial modifier, electron extraction, humidity and UV light stability

ABSTRACT: Despite organic–inorganic hybrid perovskite solar cells have rapid advances in power conversion efficiency in recent years, their serious instability of the device under practical working conditions is the current main challenge for commercialization. In this study, we have successfully inserted NH4I as an interfacial modifier between the TiO2 electron transport layer and perovskite layer. The result shows that it can significantly improve the quality of the perovskite films and 1

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electron extraction efficiency between the perovskite and electron transport layer. The devices with NH4I are obtained an improved power conversion efficiency of 18.31% under AM 1.5G illumination (100 mW cm-2). More importantly, the humidity and UV light stability of the devices are greatly improved after adding NH4I layer. The uncoated devices only decrease by less than 15% of its original efficiency during 700-h stability tests in a humidity chamber (with a relative humidity of 80%) and the efficiency almost maintains 70% of its initial value over 20 h under UV light stress tests. This work provides a potential way by interfacial modification to significantly improve photovoltaic performance and stability of perovskite solar cells.

■ INTRODUCTION In the past few years, organometal halide perovskites have attracted enormous interest due to outstanding performance in photovoltaic application. In 20091, Miyasaka et al. first introduced perovskite materials to dye-sensitized solar cells applications achieved 3.8% power conversion efficiency (PCE), and then the perovskite materials started to be used as the light absorber in photovoltaic devices. However, the problems on the decomposition of MAPbI3 in liquid electrolyte have impacted perovskite materials further development. Until 20122, Gratzel and Park achieved all solid-state mesoscopic solar cell with PCE of over 9% using solid-state hole transport material of spiro-MeOTAD. Since then, perovskite solar cells (PSCs) showed the rapid increasing in PCE, rising from 3.8% to 22.1%3-8. Although PSCs have high PCE, they undergo poor stability in the presence of moisture, solvents, UV irradiation and light exposure which seriously hindered the further applications9-13. To solve the instability problem of PSCs, many studies have been made14-19. For example, Chen et al.14 2


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have improved stability under ambient conditions (RH: 30–40%) by the addition of NH4SCN into FAPbI3 films. Saliba et al.15 have enhanced PCE and overall stability of PSCs by incorporation of rubidium cations. Bella et al.16 have improved PCE and stability by the coating of photocurable fluoropolymers on the contact side of PSCs. In addition, some other factors that affect the stability of PSCs are also widely studied and solved, such as the perovskite/ TiO2 electron transport layer (ETL) interface20-24, perovskite/HTM interface25-26 and gold migration at the HTM/electrode interface27. Especially, many efforts have been devoted to the interfacial modification between TiO2 ETL and perovskite layers for enhancing the performances and stability28-30. Yang et al.28 have introduced multifunctional fullerene derivative for interface engineering in PSCs resulting in a 20.7% improvement in PCE and stability testing showed the devices lifetime have enhanced under ambient conditions. Lei et al.29 have greatly improved photovoltaic performance and light stability of planar PSCs via introducing an amine based fullerene as interfacial modifier. Li et al.30 have developed efficient and stable inverted solar cells on flexible substrates by inserting PEI between perovskite layers and the s-VOx. It indicated that the interfacial modification is very important for the performance and stability of PSCs31-34. Therefore, an appropriate interface modifier is very critical to gain high performance and enhanced stability. In the past report, NH4I and NH4+ have been added directly to the perovskite as additive35-38, which can improve the performance of PSCs. In this work, high concentration of NH4I is introduced as an interfacial modifier between the TiO2 ETL and perovskite layer for the first time. As some NH4I keeps on the surface of TiO2, the rest enters the perovskite, making it both an interfacial modifier and an additive. By introducing the NH4I, the perovskite films show preferential crystal 3



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orientation, smoother, preferable surface coverage and pore-free. It can significantly enhance the absorption intensities, carrier extraction and electron mobility. The devices with NH4I layer display a high PCE of 18.31% under AM 1.5G illumination owing to the improvement of FF and Jsc and have negligible hysteresis. Most importantly, the uncoated devices with NH4I layer can maintain over 85% of the initial PCE after 700-h stability tests in a humidity chamber with a relative humidity over 80% and the modified PSCs retain over 70% of the original PCE over 20 h under UV light stress tests. After adding NH4I, the devices greatly improve humidity and UV light stability.

■ RESULTS AND DISCUSSION Here, we selected (FAPbI3)0.85(MAPbBr3)0.15 as perovskite layer which exhibits high PCE and stability. NH4I layer was prepared via spin-coating high concentration of NH4I ethanol solution on TiO2 mesoscopic layer and then the perovskite layer was spin-coated on NH4I layer after annealing on a hotplate. The schematic illustration of the fabrication process for perovskite films is shown in Figure 1a and the preparation details are given in the experimental section. Figure 1b depicts the schematic structure of mesoscopic PSCs with interfacial modification of NH4I. To further clarify and confirm the existence of NH4I on TiO2 after spin-coating the perovskite layer, Raman spectroscopy was adapted here. Raman spectra of TiO2, TiO2 with NH4I and TiO2 with NH4I after soaking in DMF and DMSO mixed solvent for 2 s are shown in Figure 1c. An observed peak at 144 cm-1 can be attributed to the characteristics of the anatase TiO2 phase in TiO2 without NH4I modification. Whereas, for TiO2 with NH4I and TiO2 with NH4I after soaking in mixed solvent for 2 s, the observed peaks at about 113, 146 and 172 cm-1 are corresponding with anions I3-, anatase TiO2 and I2. It is also observed that the peak of anatase TiO2 with NH4I 4


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modification shifts to higher wavenumbers as the existence of NH4I39-43. These results all indicates that the NH4I exists and inserts into the interface of TiO2 and the perovskite layer. In Figure S2, the corresponding reflectance spectra and XRD patterns are further given. It can be found from the reflectance spectra, the optical response of TiO2 with NH4I emerges red shift due to the presence of NH4I on TiO2, and the red shift of TiO2 with NH4I reduces after soaking in solvent for 2 s. The optical response red shift of TiO2 with NH4I indicates the existence of NH4I and the incorporation into the interface of TiO2 and the perovskite layer. Compared with the XRD patterns of TiO2 films, the XRD patterns of TiO2 with NH4I and TiO2 with NH4I after soaking in solvent for 2 s not only displays the characteristic peaks of TiO2 but also the characteristic peaks of NH4I. The results of reflectance spectra and XRD patterns are consistent with the Raman spectra and further prove the presence of NH4I on the TiO2 surface.

c

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(220)

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Figure 1 (a) Schematic illustration of the fabrication process for perovskite films with NH4I modification. (b) Schematic structure of mesoscopic PSCs with NH4I modification. (c)Raman spectra of TiO2, TiO2 with NH4I and TiO2 with NH4I after soaking in DMF and DMSO mixed solvent for 2 s and (d) XRD patterns of perovskite thin films with and without NH4I modification. Crystallinity quality of the perovskite films has strongly impact on the ultimate performance of PSCs. Thus, we measured the X-ray diffraction (XRD) patterns of perovskite thin films with and without NH4I modification on the TiO2. As depicted in Figure 1d, the main diffraction peaks, located at about 14.2, 20.0, 24.5, 28.4, and 31.8°, are corresponding to (110), (112), (202), (220), and (310) crystal planes of the perovskite crystal structure. Obviously, the perovskite thin films with NH4I modification have stronger diffraction intensity, indicating that a better crystallinity of (FAPbI3)0.85(MAPbBr3)0.15 film at the same experimental conditions. The NH4I modification on TiO2 would facilitate the volume increase and uniform growth of the perovskite film resulting in the improvement of diffraction signal. The cross-sectional SEM images are shown in Figure 2a and b. It is clearly clarified that the modified perovskite layer with 573 nm, is much thicker than that in the unmodified device of 468 nm. The perovskite layer with NH4I is formed smooth and grain boundaries are effectively reduced, which can reduce the carrier recombination centers and be benefit to the performance improvement44, 45. The top view SEM and AFM images of perovskite thin films with and without NH4I modification are shown in Figure 2c, d and Figure S3. It can be found that the roughness of perovskite films with NH4I (RRMS=17.2 nm) is significantly lower than that of perovskite films without NH4I (RRMS=22.7 nm) which indicates that the high-quality modified film shows smooth, preferable surface coverage and pore-free owing to the NH4I modification. 6


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Figure 2 (a), (b) Cross-sectional and (c), (d) top view SEM images of perovskite thin films on top of mesoporous TiO2 layer with and without NH4I modification. Figure 3a shows the UV−vis absorption spectra of perovskite films with and without NH4I modification on top of mesoporous TiO2 layer. The perovskite (FAPbI3)0.85(MAPbBr3)0.15 films show an absorption peak at 780 nm and the absorption intensities are evidently enhanced with NH4I modification. We suppose that it is contributed to the smoother perovskite films with improved thicknesses and preferential crystal orientation. The steady-state photoluminescence (PL) spectra of perovskite thin films with and without NH4I modification are shown in Figure 3b. The PL spectra of the perovskite (FAPbI3)0.85(MAPbBr3)0.15 have an emission peak at 780 nm, in reasonable agreement with the UV−vis absorption peak. The PL intensity of the perovskite with NH4I modification is evidently reduced. The corresponding time-resolved PL (TRPL) decay spectra are shown in Figure S5 and the TRPL curves are fitted with a single exponential decay function. It can 7



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be found that perovskite films with NH4I (t2=1.77 ns) displays shorter TRPL lifetime, which indicates favored electron injection owing to the modification of NH4I. Due to the faster electron injection and pinhole-free structure of the perovskite film, the PL intensity is significantly quenched. Transient absorption (TA) techniques and electrochemical impedance spectroscopy (EIS) were used to investigate the influence of the NH4I layer toward charge transfer and carrier recombination between the TiO2 ETL and perovskite layers. Figure 3c and d show TA and EIS of perovskite films on TiO2 with and without NH4I layer. Here, TA are characterized in a 760-nm emission window and 500 nm as the detecting light. As shown in Figure. 3c, the TA decay can be fitted very well. It can be seen from the fitting, compared to the t1=116 ns of unmodified film, the TA lifetime (t2) of the film with NH4I layer is 185 ns. The longer TA lifetime reflects the enhancement of charge recombination lifetime of the electron in TiO2 and hole in perovskite, which indicates that the NH4I layer can effectively restrain the charge recombination between TiO2 and perovskite. EIS of the PSCs with and without NH4I modification was measured at V=0.9 V in the dark, as shown in Figure 3d. In the high frequency range, the value of intercept on the horizontal axis represents the ohmic resistance (Rs) and the radius of the left semicircle reflects the charge transfer resistance (Rtr). In the low frequency range, the right incomplete semicircle is attributed to recombination resistance Rrec between the TiO2 film and the perovskite layer. From the results, the modified PSCs exhibit reduced Rtr and increased Rs and Rrec, compared with unmodified devices. It demonstrates that NH4I-modified PSCs display a smaller charge transfer resistance, lower carrier recombination and faster carrier transfer property.

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Figure 3 (a) UV-vis spectra and (b) PL spectra of perovskite thin films on top of mesoporous TiO2 layer with and without NH4I modification. (c) Normalized TA responses of TiO2/perovskite film without (1) and with (2) NH4I modification. (d) Nyquist plots of PSCs with and without NH4I modification at V=0.9 V in the dark. The equivalent electrical circuit is also shown. To further study the impact of NH4I on the photovoltaic performances, we systematically investigated the devices of (FAPbI3)0.85(MAPbBr3)0.15 with different concentrations of NH4I ranging from 40 to 70 mg mL-1. The current-voltage (J-V) curves and photovoltaic parameters of perovskite devices are shown in Figure 4a and Table S1, respectively. The champion device with NH4I modification has a high PCE of 18.31% when the NH4I concentration is 50 mg mL-1. It can be found that the high PCE attributes to the improvement of FF and Jsc. The high quality of perovskite films with large grain size and the improvement of the preferential crystal orientation can decrease the bulk recombination resulting in the improved FF. After the insertion of NH4I layer, the enhanced 9



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absorption and sufficient electron extraction from the perovskite to the TiO2 ETL lead to the enhancement of Jsc The photovoltaic performances of devices are influenced by the hysteresis phenomenon of PSCs46-49. Thus, we then evaluated hysteresis behavior via measuring the J–V curves of the devices with different scan directions. Figure. 4b and Table S2 show the corresponding J-V and photovoltaic parameters. The devices with NH4I modification demonstrate excellent reproducibility and have negligible hysteresis which is in good agreement with different sweep directions. It is supposed that the hysteresis phenomenon is effectively eliminated due to the significant passivation of the trap states at the TiO2 surface by the modification of NH4I. To confirm the improvement of Jsc, the incident photo to current conversion efficiency (IPCE) spectra of the devices have been carried out. Figure. 4c shows the IPCE spectra of the devices with and without NH4I. The IPCE of PCSs without NH4I treatment is below 80%. Whereas, the IPCE spectrum of the modified device exhibits a high incident photo to current conversion efficiency of over 80% and it is remarkably improved range from 300 to 750 nm. The calculated Jsc slightly improves from 20.44 to 22.20 mA cm-2 which are consistent with the corresponding values measured under sun simulator. Figure 4d presents the PCE histogram fitted with a Gaussian distribution (red line) histogram from over 30 measured devices. It shows that the modified devices exhibit an average PCE about 16.8%, compared with 14.5% for the unmodified devices. This result illustrates that the devices with NH4I modification have high evident improvement PCE and reproducibility.

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b 20

Current Density (mA cm-2)

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Figure 4 (a) J−V curves of PSCs with different concentrations NH4I. (b) J–V curves of best PSCs with NH4I modification under reverse and forward scan directions. (c) Incident photo to current conversion efficiency (IPCE) spectra of PSCs with and without NH4I modification. (d) The PCE histogram fitted with a Gaussian distribution of the devices with and without NH4I modification over 30 measured devices. Recently, stability is most concerned issue for PSCs

50-52,

therefore we further study the

humidity and UV illumination stability of unsealed PSCs with and without NH4I modification. Figure 5 shows the normalized efficiency variation curves of unsealed PSCs with and without NH4I modification under 80% relative humidity and continuous UV irradiation. As demonstrated in Figure 5a, all devices were stored in closed container with a humidity of about 80% at room temperature. The PCE of modified device shows slight improvement in initial 200-h storage and maintains over 85% of the initial value after 700-h storage. In contrast, the PCE of devices without 11



NH4I modification declines seriously and loses over 70% of their original efficiency. It can be seen from the Figure 5b, PSCs with and without NH4I modification all have decreased PCE under continuous UV irradiation, while the PCE of modified PSCs slowly declines and retains over 70% of the starting value. The unmodified PSCs leave over only 30% of PCE. These results demonstrate that NH4I layer as a novel interfacial modifier between the TiO2 ETL and perovskite layers can significantly improve humidity resistance and the UV stability of PSCs.

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0.8 0.6

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Figure 5 Normalized efficiency variation curves of unsealed perovskite solar cells without and with NH4I modification (a) under 80% relative humidity. (b) under continuous UV irradiation. We supposed that there are two reasons for resulting in the enhancement of humidity and UV stability of PSCs. On the one hand, NH4I as an interfacial modifier and additive is beneficial to promote more orderly deposition of perovskite and improves the perovskite quality, thus enhancing the humidity and UV resistance of perovskite layer21-23. On the other hand, when the modified devices exposed to the high humidity condition, NH4+ which exists in perovskite structure firstly combines with water molecule on the perovskite surface, and NH4+ continually undergoes cation exchange with FA+ into the perovskite layer surface to replenish depleted NH4+. Therefore, it will prevent further damaging the perovskite structure and decomposing the FA+, leading to high moisture resistance. Meanwhile, some NH4+ on the TiO2 surface inserts into the TiO2 to fill up the 12


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oxygen vacancies and reduces the deep traps which leads to the recombination loss of photo-generated charges and the instability of perovskite under UV light28,29. What’s more, due to the using of high concentration of NH4+ and the continuous cation exchange of NH4+ into the TiO2 surface, the decomposition of NH4+ under UV light is neglected. Hence, the decrease of surface defects due to the existence of NH4+ on TiO2 can enhance the UV stability. The crystal structure of anatase TiO2 before and after NH4I modification, and the schematic process of NH4I modified TiO2 and perovskite layer are shown in Figure 6.

Figure 6 (a) Crystal structure of anatase TiO2 before and after NH4I modification. (b) Schematic process of NH4I modified TiO2 and perovskite layer.

■ CONCLUSIONS To summarize, NH4I layer as a novel interfacial modifier is introduced between the TiO2 ETL and perovskite layer in PSCs, which can promote photovoltaic performance, suppress hysteresis and enhance moisture resistant and UV light stability of the devices. After inserting NH4I layer, the 13



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improved preferential crystal orientation, smoother perovskite films and enhanced absorption intensities are obtained. Accordingly, the carrier extraction enhances and carrier recombination reduces from perovskite to TiO2 ETL, resulting in the improvement of photovoltaic performance. The devices exhibit a high PCE over 18% and excellent stability under 80% RH and UV light soaking conditions. These results offer a new option with inserting NH4I layer to solve the serious unstable problem of PSCs for the technology repeatable.

■ EXPERIMENTAL SECTION Materials and preparation. Formamidine iodide (FAI) was prepared by stoichiometrically reacting hydroiodic acid with formamidine acetate and they were stirred in the ice bath for 2 h. The white precipitate was obtained from evaporation of the solvent at 60 °C using rotary evaporation under reduced pressure. Then it was purified by dissolving in ethanol and collected by filtration. The purification procedure was repeated twice to get pure FAI. The final product was completely dried at 60 °C. Other materials were purchased from Alfa without further purification.

Device fabrication. FTO-coated glass was rinsed by sonication in detergent, and respectively cleaned three times with ultrapure water and ethanol. A compact TiO2 layer was deposited on the FTO by spray pyrolysis at 460 °C and the precursor solution is composed with 0.6 mL titanium diisopropoxide and 0.4 mL bis(acetylacetonate) in 7 mL isopropanol. A mesoporous TiO2 layer was spin-coated on the compact layer with a speed of 4000 rpm for 20 s, from a 30 nm TiO2 paste which is diluted with ethanol (TiO2: ethanol =1:5.5). After the spin coating, mesoporous TiO2 were then slowly annealed by sintering from room temperature to 510 °C for 3h on flattening oven. The NH4I interfacial modification solutions with a concentration 40, 50, 60, 70 mg/mL in ethanol was spin-coated on the top of mesoporous TiO2 layer with 4000 rpm for 15 s and then the substrate was 14


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heated at 105 °C for 10 min on a hotplate. The 1.4 M Pb2+ of (FAPbI3)0.85(MAPbBr3)0.15 precursor solutions were obtained from dissolving the corresponding perovskite powders in DMF and DMSO mixed solvent (DMF: DMSO=85%:15% by volume) with stirring at 60 °C for 30 min. Then, the precursor solutions were spin-coated at firstly 1100 rpm for 15 s, secondly 4200 rpm for 35 s in an air flowing glovebox. About 120 μL of chlorobenzene was drop-casted on the substrate during the spin coating step 30 s before the end of the procedure. The substrate was then annealed at 105 °C for 60 min on a hotplate. The HTM solution with 73 mM spiro-OMeTAD, 4-tert-butylpyridine, Li+ salt and cobalt(III) salt in chlorobenzene solvent was spin-coated onto the perovskite layer at 3000 rpm for 20 s. Finally, 60 nm of Au were deposited via thermal evaporating on top of the HTM layer.

Characterization. Raman spectra was measured on Laser Raman Spectrometer (LabRamHR). The measurement wavelength was from 100 to 1800 nm and the wavelength of the excitation laser for Raman spectroscopy was 1064 nm. Atomic force microscopy (AFM) were tested by a MultiMode V (Veeco) viewer and analyzer. XRD patterns of the perovskite films were recorded on an X’Pert MPD PRO (PANalytical). The data were confirmed in the 2θ range 5–70° at room temperature. Films morphology was studied by a high-resolution scanning electron microscope with a Schottky Field Emission gun. Absorption spectra and reflectance spectra were recorded on an ultraviolet−vis (UV−vis) spectrophotometer (U-3900H, HITACHI, Japan). Steady-state PL spectra were tested by a spectrofluorometer (photon technology international) and analyzed by the software Fluorescence. The exciting wavelength was 473 nm and excited by a standard 450 W xenon CW lamp. J–V curves were tested by using a solar simulator (Newport, Oriel Class A, 91195A) with a source meter (Keithley 2420) at 100 mW/cm2 illumination AM 1.5G. The active area for each device was 0.09 cm2 by masking a black mask. Incident photon to current efficiency (IPCE) were collected as a function of wavelength from 300 to 900 nm (PV Measurements, Inc.), with dual Xenon/quartz halogen light source, measured in DC mode with no bias light used. The setup was calibrated with a certified silicon solar cell (Fraunhofer ISE) prior to measurements. Transient 15



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absorption spectra (TAS) was measured on LKS (Applied photophysics). The pump light wavelength was 500 nm and a probe light wavelength was 760 nm. Repetition rate of 5 Hz, the energy of laser device was 150 μJ/cm2. Electrochemical impedance spectroscopy (EIS) was record at -0.9 V under dark, in the frequency range of 1 Hz to1 MHz by using an Autolab analyzer (Metrohm, PGSTAT 302N, Switzerland). The humidity stability test was tested in a container with 80% relative humidity. The container was kept in the dark and the temperature was remained about 20 °C. The UV stability test was performed in a container with UV irradiation. The container was remained less than 15% RH and the temperature around the sample was kept at 20 to 30 °C by a temperature control and cooling system. The UV light source with the wavelength of 360 nm (UV-Hg-2000, Beijing Lighting Research Institute) generated 3-4 times the amount of UV light in AM1.5G spectrum when the sample was kept 50 cm from the light source.

■ ASSOCIATED CONTENT *Supporting Information XRD patterns of perovskite thin films with different concentrations of NH4I modification; Reflectance spectra and XRD patterns of TiO2, TiO2 with NH4I and TiO2 with NH4I after soaking in DMF and DMSO mixed solvent for 2 s; AFM images of perovskite thin films on top of mesoporous TiO2 layer without and with NH4I modification; UV-vis spectra and PL spectra of perovskite thin films on top of mesoporous TiO2 layer with different concentrations of NH4I modification; TRPL decay spectra of perovskite thin films on top of mesoporous TiO2 layer with and without NH4I modification; Normalized TA responses of TiO2/ perovskite film with different concentrations of NH4I modification; Nyquist plots of PSCs with different concentrations of NH4I modification at V = 0.9 V in the dark; J–V curves of PSCs without NH4I modification under reverse and forward scan 16


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directions; Short-circuit current density (Jsc), open circuit voltages (Voc) and fill factor (FF) histogram fitted with a gaussian distribution of the devices without and with NH4I modification over 30 measured devices; Normalized efficiency variation curves of unsealed perovskite solar cells without and with different concentrations NH4I modification under 80% relative humidity and continuous UV irradiation; Photovoltaic parameters of perovskite solar cells with different concentrations of NH4I; Photovoltaic parameters of perovskite solar cells with and without NH4I modification under reverse and forward scan directions.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was financially supported by the National High Technology Research and Development Program of China under Grant No.2015AA050602, the Science and Technology Support Program of Jiangsu Province under Grant No. BE2014147-4.

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