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Efficient and Stable Perovskite Solar Cells Prepared in Ambient Air Based on Surface-Modified Perovskite Layer Chang Liu, Wenhui Ding, Xianyong Zhou, Jishu Gao, Chun Cheng, Xing-Zhong Zhao, and Baomin Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00847 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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The Journal of Physical Chemistry

Efficient and Stable Perovskite Solar Cells Prepared in Ambient Air Based on Surface-Modified Perovskite Layer †‡

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Chang Liu , , Wenhui Ding , Xianyong Zhou , Jishu Gao , Chun Cheng , Xingzhong Zhao and Baomin Xu*,

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†Department of Materials Science and Engineering, South University of Science and Technology of China, Shenzhen, Guangdong Province 518055, China ‡Deptment of Physics, Wuhan University, Wuhan, Hubei Province 430072, China ABSTRACT: Among many photovoltaic conversion technologies, perovskite solar cells have received significant research interests as effective photovoltaic materials owing to their high solar conversion efficiencies and low cost. However, the performance of perovskite solar cells is limited by the instability of CH3NH3PbI3 to water and ambient moisture. To address this issue, in this study, we introduced a new fundamental approach that utilizes 4-tert-butylpyridine (tBP) as the surface modification agent to enhance the performance and stability of CH3NH3PbI3-based perovskite solar cells fabricated in ambient air. The tertiary butyl group in tBP is highly hydrophobic, leading to the formation a hydrophobic layer on the surface of CH3NH3PbI3, thus increasing the moisture stability of perovskite solar cells. With this strategy, the performance of perovskite solar cells prepared at even >50% RH in ambient air was tremendously enhanced by as much as 200% compared to that without tBP additive. Besides, the stability of perovskite solar cells in ambient air was also markedly improved.

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1. Introduction As a result of the rapid industrial development, the worldwide energy crisis is the most critical problem challenging human life.1, 2 Solar energy stimulated photovoltaic conversion is one of the most important areas of research, as it is closely related to the global energy and environmental issues. A variety of technologies based on different fundamental mechanisms have been extensively developed to address these issues, 3–7 among which photovoltaic conversion is one of the most effective fundamental processes to solve these problems by utilizing solar energy.8,

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Perovskite solar cells have received significant

research interests as effective photovoltaic materials owing to their high solar conversion efficiencies and low cost. In the past 5 years, the efficiencies of CH3NH3PbI3-based perovskite solar cells have increased from 3.8% to over 20%.10–15 In spite of high quantum efficiency in electron–hole pair generation by CH3NH3PbI3, the actual application of perovskite solar cells is essentially limited by the instability of CH3NH3PbI3 to water and ambient moisture.16, 17 In most cases, perovskite films have to be processed in an inert atmosphere and pristine devices cannot survive long in air, hampering the mass production and real applications of the perovskite solar cells. Several different strategies have been developed to improve the stability of perovskite solar cells. Halide ion doping, such as Br and Cl doping, has been demonstrated to enhance the stability of perovskite solar cells and is attributed to the stronger interaction between halide ions and CH3NH3+.18, 19 Interfacial modification layer coating between the perovskite and the hole transport layer can also be used to improve the stability of perovskite solar cells; however, the intrinsic resistance of perovskites to moisture remains unchanged.20,

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Nevertheless, so far most of the high

performance perovskite solar cells are still needed to be prepared in a glovebox to avoid contacting moisture. Therefore, it is desperately needed to explore new pathways and mechanism to improve the stability of perovskite solar cells in ambient air in a cost-effective approach. In this study, by taking CH3NH3PbI3-based perovskite solar cells as a representative example, we proposed and experimentally demonstrated a novel strategy of using 4-tert-butylpyridine (tBP) to significantly improve the performance of CH3NH3PbI3-based perovskite solar cells prepared in the


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ambient air, and also greatly enhance the stability of the prepared solar cells. By simply adding tBP into the precursor solution of PbI2, high quality perovskite films can be readily prepared in ambient air even when the average relative humidity (RH) exceeds 40%. Compared to the typical perovskite solar cells prepared from PbI2 precursor without tBP additive, the photoelectric conversion efficiency (PCE) of perovskite solar cells prepared in ambient air was improved by 200% and achieved 12.6 %. Based on this fundamentally new mechanism, we further demonstrated that the stability of perovskite solar cells without encapsulation exposed in ambient air can be significantly improved, clearly illuminating the practical applicability of this strategy. Because this significant enhancement of the photoelectric conversion efficiency through tBP addition is universally applicable to any material based perovskite solar cells, the new mechanism and approach will lead to a revolutionary advance of the field towards the goal to fabricate efficient and stable perovskite solar cell devices under ambient atmospheric conditions. 2. Experimental Section 2.1. Materials. Mesoporous TiO2 paste (Dyesol) and methylammonium iodide (MAI; powder) were purchased from Xi’an Polymer Light Technology Corp.), and 2,2’,7,7’-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9’spirobifluorene (Spiro-OMeTAD) was purchased from Feiming Chemical Limited. All of the other salts and anhydrous solvents were purchased from Sigma-Aldrich, including PbI2 powder, lithium bis[(trifluoromethyl)sulfonyl]imide salt (Li-TFSI), titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropyl alcohol), N,N-dimethylformamide (DMF), ethanol, isopropyl alcohol, tert-butylpyridine (tBP), chlorobenzene, acetonitrile, 1-butanol, diethyl ether, and dimethyl sulfoxide (DMSO). All the above chemical products were used directly without further purification or other treatment. 2.2. Device Fabrication Fluorine-doped tin oxide (FTO) glasses (Nippon Sheet Glass) were cleaned with detergent, deionized water, and acetone and sonicated with ethanol in an ultrasonic bath for 30 minutes. After that, FTO glasses were treated in a UV cleaner for 30 minutes. The cleaned FTO glasses were coated with 0.15 M



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titanium diisopropoxide bis(acetylacetonate) in 1-butanol solution by the spin-coating method at 2000 rpm for 60 s, followed by heating at 125 °C for 5 minutes. The films were cooled down to room temperature and a 0.15 M titanium diisopropoxide bis(acetylacetonate) solution in 1-butanol was spin coated again to make a pin hole free dense TiO2 film. And then the substrates were calcined in a box furnace at 450 °C for 30 minutes. Mesoporous TiO2 (mp-TiO2) layer was deposited on the dense TiO2 layer by spin-coating the TiO2 ethanol solution containing 14.3 wt % TiO2 paste at 4000 rpm for 20 seconds, which was then calcined at 500 °C for 0.5 hour. The 460 mg of PbI2 (Aldrich, 99.9985%) was dissolved in 1 mL DMF and then 100 µL tBP was added in the mixed solution, which was heated at 70°C for 2 hours under magnetic stirring, and the mixture was then spin coated on the as-prepared TiO2 film at 3000 rpm for 30 seconds in ambient air. The films were dropped with a solution of CH3NH3I in 2propanol (30 mg/mL) and spin-coated at 4000 rpm for 30 seconds. Afterward, the as-prepared films were heated at 90°C for around 30 min until the color changed to dark red. The hole-transporting layer (HTL) was prepared by spin-coating an HTM solution, which was prepared by dissolving Spiro-OMeTAD in chlorobenzene (76 mg/1 ml) with 28.8 µl tBP solution and 17.5 µl Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI)/acetonitrile (520 mg/1 ml). Devices were then left overnight in air. Finally, a 120 nm thick silver (Ag) layer was thermally evaporated under vacuum of 4×10−5 Torr, at a rate of ∼0.05 nm/s, to finish the device fabrication. 2.3. Measurements and Characterization. The morphology and structure of the films were characterized by Field Emission Scanning Electronic Microscope (FESEM, ZEISS Merlin), at a 5 kV accelerating voltage. X-ray diffraction (XRD) patterns of the samples were obtained using a diffractometer (D8 Advance ECO) with Cu Ka radiation at a scan rate of 5°/min under operation condition of 20 kV and 30 mA. The optical properties of samples were measured on a UV/Vis spectrophotometer (PerkinElmer Lambda 950). Photocurrent density-voltage (J-V) curves were measured under AM 1.5G one sun illumination (100 mW/cm2) with a solar simulator


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(Enlitech SS-F7-3A) equipped with 300 W Xenon lamp and a Keithley 2400 source meter. The light intensity was adjusted by an NREL-calibrated Si solar cell. During measurement, the cell was covered with a mask having an aperture (0.1 cm2). The external quantum efficiency (EQE) was measured with an EQE system (Enlitech QE-R) containing a xenon lamp, a monochromator, a Si detector and a dualchannel power meter. The FT-IR spectrometer measurements of the devices were carried out by using a Nicolet NEXUS 870 FT-IR spectrometer to collect the FT-IR spectral data in the 700 cm-1-3600 cm-1 range. Drop shape analysis was done using a drop shape analyzer (AST VCA Optima XE) with 18 MΩ water and a 4 µL dispense volume. Drops were measured at 6 different places across the substrate at room temperature 25 °C. 3. Results and discussion

Figure 1. (a) Structure of tBP functional group; (b) The sketch to illustrate the mechanism to enhance the hydrophobicity and moisture stability of perovskite solar cells with tBP; (c) Schematic diagram showing the fabrication process of perovskite solar cell devices in ambient air. It was demonstrated that the degradation of CH3NH3PbI3 in ambient air is mainly caused by the hydrolyzation of CH3NH3PbI3 in the presence of moisture.22 The instability of CH3NH3PbI3 in humidity environment raises the difficulty of fabricating perovskite solar cells in ambient air,



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which hinders the mass production and practical applications of perovskite solar cells. Therefore, it is urgent to find a low-cost and simple way to improve the performance of CH3NH3PbI3based perovskite solar cells prepared in ambient air, and enhance their stability. Previous reports have proved that the moisture stability of CH3NH3PbI3 can be markedly improved by increasing its hydrophobicity.23-25 Based on that, we proposed here to use tBP as a surface modification agent to augment the hydrophobicity of CH3NH3PbI3. As shown in Figure 1a, tBP has a tertiary butyl group on one end, exhibiting high hydrophobicity. In the meantime, the strong coordination between the nitrogen atom on the other end of tBP and the Pb2+ on PbI2 can result in numerous pyridyl groups to be anchored on the surface of PbI2, surrounded with the hydrophobic tertiary butyl groups surround on the outside.26, 27 After PbI2 reacts with CH3NH3I to form CH3NH3PbI3, segmental tBP molecules stay on the surface of CH3NH3PbI3, forming a hydrophobic layer (Figure 1b). Hence tBP molecule could be an ideal candidate to promote the moisture stability of perovskite solar cells through being anchored on the surface of CH3NH3PbI3. Based on this strategy, the experimental design for the fabrication of surface-modified CH3NH3PbI3-based perovskite solar cells is schematically depicted in Figure 1c. The devices were made with two-step spin coating of precursor solutions containing equivalent CH3NH3I and PbI2 onto mesoporous TiO2. The fine perovskite films were readily prepared in our laboratory even when the average relative humidity (RH) exceeds 40%, suggesting excellent moisture stability of the perovskite. Detailed information on the perovskite precursor solution preparation, perovskite thin-film deposition, and the fabrication of devices is described in the experimental section. For comparison, CH3NH3PbI3-based perovskite solar cells without tBP additive were also prepared with exactly the same procedure.


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Figure 2. SEM images of (a,b) PbI2 and CH3NH3PbI3 films prepared without tBP additive and (c,d) PbI2 and CH3NH3PbI3 films prepared with tBP additive. Figure 2 shows the morphologies of the PbI2 and CH3NH3PbI3 films prepared with and without tBP additive, labeled as PbI2 W/O tBP, CH3NH3PbI3 W/O tBP, PbI2 W/ tBP, and CH3NH3PbI3 W/tBP, respectively. Without tBP additive, a large fraction of the mesoporous TiO2 remained exposed, and only a part was covered by the PbI2 grains with a size of ∼200 nm, as shown in Figure 2a. After the reaction with CH3NH3I solution, in Figure 2b, the top-view SEM image shows that lots of pinholes formed in the interval of the CH3NH3PbI3 grains with ∼400 nm size. On the other hand, when the tBP was used, as we expected, a highly compact PbI2 film composed by the PbI2 grains with sizes ranging from 50 nm to 100 nm was obtained by the same route (Figure 2c). The CH3NH3PbI3 perovskite was formed on the surface of mesoporous TiO2 film composed by closely packed CH3NH3PbI3 crystals with sizes ranging from 100



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nm to 300 nm, as shown in Figure 2d. These results indicate that the presence of tBP can affect the morphology of the spin-coated CH3NH3PbI3 perovskite films. This morphological effect of tBP can be attributed to the high hydrophobicity of the tertiary butyl groups on the tBP, thus enhancing the moisture resistance of CH3NH3PbI3 perovskite films. Hence, the corrosion of CH3NH3PbI3 caused by the moisture in ambition air reduced, and the pinholes on the surface of CH3NH3PbI3 perovskite films also decreased. A less amount of pinholes leads to lower charge carrier recombination, enhancing the opencircuit voltage of the solar cell.28, 29

Figure 3. XRD patterns of the (a) PbI2 W/tBP and PbI2 W/O tBP,；(b) CH3NH3PbI3 W/tBP, and CH3NH3PbI3 W/O tBP； (c) UV–Vis absorbance spectra of CH3NH3PbI3 W/ tBP and CH3NH3PbI3 W/O


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tBP films and (d) FTIR spectrums of PbI2 W/tBP, PbI2 W/O tBP, CH3NH3PbI3 W/tBP, and CH3NH3PbI3 W/O tBP films. In order to determine the phase compositions of the films prepared at the different steps, X-ray diffraction (XRD) analysis was performed. Figures 3a and b show the XRD patterns of the PbI2 W/tBP, PbI2 W/O tBP, CH3NH3PbI3 W/tBP and CH3NH3PbI3 W/O tBP films. As shown in Figure 3a, PbI2 W/tBP and PbI2 W/O tBP films both annealed at 70°C for 30 minutes were measured. For the PbI2 W/O tBP sample, all the characteristic diffraction peaks assigned to the pure PbI2 are detected, indicating that no PbI2 and DMF complex was formed.30 For the PbI2 W/ tBP sample, in addition to the diffraction peaks of PbI2, the characteristic diffraction peaks located at 6.8° and 13.6° were observed, indicating that tBP and PbI2 have strong interaction and form PbI2-tBP complex. This result is well consistent with those in literature.26 It should be noted that it is possible to form PbI2-DMF-xtBP before annealed, while after annealing at 70 ℃ for 30 min, most of DMF were evaporated due to the lower boiling point. In that case, as shown in XRD pattern, only peaks that belong to the PbI2-tBP complex exist indicating that tBP and PbI2 have strong interaction. Compared to the sharp diffraction peaks of PbI2 W/O tBP sample, the diffraction peaks of PbI2 W/tBP sample are considerably broadened, indicating that the sizes of these PbI2 nanoparticles are relatively small. This result is consistent with the results shown in the SEM images. Figure 3b shows the XRD patterns of CH3NH3PbI3 W/tBP and CH3NH3PbI3 W/O tBP films. For both samples, most of the characteristic peaks corresponded well to CH3NH3PbI3, and only the weak characteristic peak for the PbI2 (001) plane still exists, suggesting that most of PbI2 apparently converted to CH3NH3PbI3 via these two different processes. Figure 3c shows ultraviolet–visible (UV–vis) absorption of the perovskite films. The smooth and compact morphology of the CH3NH3PbI3 W/tBP films led to a much stronger absorbance as well as darker color (Figure S1) than the CH3NH3PbI3 W/O tBP sample in the range from 400 to 750 nm.31 In order to confirm whether tBP still existed after the formation of the CH3NH3PbI3 perovskite film, the interactions of tBP as well as DMF with both PbI2 and



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CH3NH3PbI3 were also verified by FTIR spectroscopy, and the results are shown in Figure 3d. For the PbI2 W/O tBP and PbI2 W/tBP samples, the four peaks appearing at 1670, 1400, 1250, and 1100 cm−1 result from the stretching vibration of the C=O, C–H, C–N, and C–N sp3, respectively, which are all the typical stretching vibration modes of DMF.32 Compared to PbI2 W/O tBP sample, there are four other peaks appearing at 1610, 1210, 1000, and 820 cm−1 for the PbI2 W/tBP sample, attributed to the stretching vibration of the C=C and C–C, the in-plane bending vibration of C–H and out-of-plane bending vibration of C–H, indicating that the PbI2-tBP complex formed in PbI2 W/tBP sample. For the CH3NH3PbI3 W/O tBP sample, with the transformation of PbI2 to CH3NH3PbI3 by reacting with CH3NH3I, the vibrational mode of DMF almost disappeared and the stretching vibration of N–H and C–H bond appeared at 3500 and 3100 cm-1 wavenumber, suggesting the formation of CH3NH3PbI3 perovskite.33 Except the typical bonds of CH3NH3PbI3, the CH3NH3PbI3 W/tBP sample shows two weak C=C and C–H bonds at ∼1610 and 820 cm−1 from tBP, indicating that a certain portion of tBP is still adsorbed on the surface of CH3NH3PbI3 perovskite. The FTIR spectroscopy results confirmed that tBP was not fully removed after the formation of the CH3NH3PbI3 perovskite, and the adsorbed tBP can be used as hydrophobic protectant to enhance the moisture resistance CH3NH3PbI3-based perovskite solar cell prepared in ambient air.


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Figure 4. Contact angle measurements were taken of water droplets on the films of (a) PbI2 W/O tBP and (b) PbI2 W/tBP, (c) CH3NH3PbI3 W/O tBP, and (d) CH3NH3PbI3 W/tBP. In order to verify the addition of tBP enhances the hydrophobicity of the samples, water contact angle measurements were performed upon the four different films used for the perovskite solar cells in this study. As shown in Figures 4a and b, our results indicate an increase in the contact angle from 67.3° to 77.4° in the presence of tBP as the additive in the PbI2 spin-coating solution, demonstrating that the presence of the tBP leads to a more hydrophobic PbI2 layer. Figures 4c and d show that CH3NH3PbI3 W/tBP sample has a contact angle of 56.8° exhibiting an obviously improved hydrophobicity when compared to CH3NH3PbI3 W/O sample which has a contact angle of 34.7°. As such, the hydrophobic properties of the perovskite layer can be easily tuned by introducing tBP into the perovskite layer, which leads to the possibility to prepare high performance perovskite solar cells in ambient air and improve the stability of the cells. In addition, the molecular structure of this type of perovskite layer, makes it especially simple to contact with the HTM (hole transport material), which also contain tBP as the additive.34 In conclusion, it is proved that the hydrophobicity of the perovskite films as well as the morphology itself can be strongly affected by the presence of tBP. Some previous reports have demonstrated that such hydrophobic properties are extremely beneficial to solar cell stability.35,

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perovskite layer without tBP as surface modifying agent will be more readily wet by water, and thus more susceptible to moisture than the one composed of hydrophobic tBP. In addition, the resistance to liquid water and water vapor permeating will also depend on other factors such as pinhole density, which also can be improved by the tBP additive.



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Figure 5. (a) Typical J–V characteristics and (b) Corresponding EQE of CH3NH3PbI3 W/tBP and CH3NH3PbI3 W/O tBP-based solar cells prepared in ambient air. According to the surface modification induced by the tBP additive design, a perovskite solar cell with a high PCE was expected to be fabricated in ambient air, because of the formation of a hydrophobic layer onto the surface of CH3NH3PbI3 film. To validate the effectiveness of the fabrication strategy used, the photovoltaic performances of CH3NH3PbI3 W/O tBP and CH3NH3PbI3 W/tBP-based solar cells prepared in ambient air were systematically evaluated. Figure 5a illustrates the J–V curves of the prepared devices based on the PSCs fabricated. As shown in Figure 5a, CH3NH3PbI3 W/O tBP-based perovskite solar cells were fabricated in the air with an RH over 50%, and the device shows the PCEs of 6.03% and 4.45% in the reverse scan and forward scan, respectively. The detailed photovoltaic parameters of CH3NH3PbI3 W/O tBP and CH3NH3PbI3 W/tBP-based solar cells are summarized in Table S1. In contrast, the CH3NH3PbI3 W/tBP-based solar cell show the PCE of 12.62% at reverse scan and 12.13% at forward scan under the same conditions. The average PCE of 10 devices prepared under the same conditions is 12.21±1.01%, suggesting an excellent moisture stability of CH3NH3PbI3 W/tBP-based perovskite solar cells. The above results demonstrate that the preparation of perovskite solar cells is more moisture resistive with tBP additive than that without tBP additive. This is probably because the tBP additive got into the perovskite layer, effectively reducing the moisture intrusion into the CH3NH3PbI3 layer under a


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high humidity condition. Moreover, considering that tBP was also used as an additive in HTM, the incorporation of tBP may increase the adhesion at the CH3NH3PbI3/HTM interface, allowing the protective HTMs to strongly adhere onto the CH3NH3PbI3 layer, which significantly decrease the direct exposure of CH3NH3PbI3 to moist air.

Figure 6. (a) Structures of four types of molecules used as the surface modification agent for perovskite layer with the same structure except terminal groups marked as hydrophilic and hydrophobic groups. (b) Corresponding J–V characteristics of modified-CH3NH3PbI3-based solar cells prepared in ambient air. To further investigate the effect of tBP consisting of hydrophobic group accounting for the enhancement of efficiency and stability of perovskite solar cells prepared in ambient air, four types of molecules were selected as the surface modification agent for the perovskite layer. Figure 6a shows that these four molecules have the same structural unit, except terminal groups which are amino, hydroxyl, tertiary butyl, and ethyl groups. As we know, amino and hydroxyl groups are typical hydrophilic groups, whereas the tertiary butyl and ethyl groups have high hydrophobicity. If the surface of the CH3NH3PbI3 layer was decorated with the hydrophilic groups, the CH3NH3PbI3 perovskite layer can more easily adsorb the moisture in the air, thus significant decomposition of CH3NH3PbI3 and lower efficiency of



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perovskite solar cells. In contrast, the surface modification of CH3NH3PbI3 layer with the hydrophobic groups can enhance the moisture resistance of CH3NH3PbI3 layer, leading to the higher efficiency and stability of perovskite solar cells prepared in ambient air. As expected, Figure 6b shows that perovskite solar cells with hydrophilic groups modification exhibits lower PCE (5.81% for amino group and 5.15% for hydroxyl group) compared to the original perovskite solar cell (6.02%); however, after modification with hydrophobic groups, both tertiary butyl and ethyl groups modified-perovskite solar cells showed improved PCE from 6.02% to 12.62% and 10.63%, respectively. Our results demonstrate that the enhancement of efficiency and stability of perovskite solar cells prepared in ambient air can be attributed to the hydrophobic group modification for the surface of perovskite layer, which significantly improves the moisture resistance of CH3NH3PbI3.

Figure 7. (a) Evolution of the PCEs of CH3NH3PbI3 W/ tBP and CH3NH3PbI3 W/O tBP-based solar cells upon ageing in air without encapsulation (average values of 10 devices fabricated under identical conditions). (b) J–V characteristics (measured under AM 1.5 illumination) of CH3NH3PbI3 W/tBP-based solar cells prepared under different humidities of 40%–50%, 50%–60%, 60%-70%, and 70%–80%. Surface modification of perovskite layer induced by tBP additive not only remarkably enhanced the photovoltaic performance of perovskite solar cells but also significantly improved the stability for perovskite solar cells exposed in air. Since excellent moisture resistance has been observed in


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CH3NH3PbI3 W/tBP-based solar cells during the fabrication process in ambient air, they are expected to achieve a good long-term ambient stability.37–39 As shown in Figure 7a, the PCE of CH3NH3PbI3 W/tBPbased solar cells without encapsulation decreases by about 20% after being stored in open air with the average RH level above 50% for over 7 days. In contrast, the CH3NH3PbI3 W/O tBP-based solar cells almost entirely lost performance under the same experimental conditions. Previous studies revealed that the hydrophobicity of CH3NH3PbI3 perovskite film may significantly affect its stability in air.31, 38 With the highly reliable and simple introduction of tBP additive into the perovskite layer, the hydrophobicity of CH3NH3PbI3 perovskite film markedly improved, enhancing the stability and recyclability of perovskite solar cells. On the other hand, because the CH3NH3PbI3 W/tBP film shows more uniform and compact surface morphology than the CH3NH3PbI3 W/O tBP film prepared in air, the better morphology of the former is also a possible reason for the improved ambient stability of the devices.40–42 In order to further test the moisture resistance of surface-modified perovskite solar cells with tBP, we fabricated the solar cells under different RH levels. As shown in Figure 7b, the device fabricated under the humidity in the ranges 40%–50% exhibits an excellent average PCE of 12.62%. As the environment humidity increases to 50%–60%, the PCE of the fabricated device reduced to 10.88%. When the humidity was further increased to 60%–70%, the PSCs still maintained a favorable average PCE of 9.24 %. Even the humidity was further increased to 70%–80%, the perovskite solar cells can still exhibit an average PCE of 7.21%. These results indicate that the fabrication process of perovskite solar cell with the introduction of tBP showed a high moisture resistance even when the RH is >40%. Conclusions A facile strategy to fabricate efficient and stable perovskite solar cells in ambient air was developed, in which an amphiphilic additive tBP was introduced into the perovskite layer as the surface-modifying molecule. With this strategy, the performance of tBP modified perovskite solar cells prepared at even >50% RH was significantly enhanced by as much as 200% compared to that without tBP additive. Moreover, the CH3NH3PbI3 W/tBP-based perovskite solar cells maintained 80% of its initial performance for 7 days



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at >50% RH without any encapsulation, suggesting that the stability of perovskite solar cells was also significantly improved. The effects of tBP modification for CH3NH3PbI3 based perovskite solar cells was systematically investigated and found that the formation of a hydrophobic layer on the surface of CH3NH3PbI3 induced by tBP is a key factor to promote the moisture stability of perovskite solar cells. This simple but very effective approach paves the way to fabricate efficient and stable perovskite solar cells in ambient air condition, which could be of great utility to the mass production and commercialization of perovskite solar cells. Supporting Information. The digital photos of perovskite films and photovoltaic parameters of perovskite solar cells based on perovskite films fabricated with and without tBP additive. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author: Baomin Xu; Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the startup funding from the Southern University of Science and Technology (Grants Nos. 25/Y01256112 and 25/Y01256212), the Peacock Team Project funding from Shenzhen Science and Technology Innovation Committee (Grant No. KQTD2015033110182370), and the National Key Research and Development Project funding from the Ministry of Science and Technology of China (Grants Nos. 2016YFA0202400 and 2016YFA0202404), and the China Postdoctoral Science Foundation (Grant No. 2016M602347) REFERENCES


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Figure 1. (a) Structure of tBP functional group; (b) The sketch to illustrate the mechanism to enhance the hydrophobicity and moisture stability of perovskite solar cells with tBP; (c) Schematic diagram showing the fabrication process of perovskite solar cell devices in ambient air. 81x41mm (300 x 300 DPI)



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Figure 2. SEM images of (a,b) PbI2 and CH3NH3PbI3 films prepared without tBP additive and (c,d) PbI2 and CH3NH3PbI3 films prepared with tBP additive. 120x120mm (300 x 300 DPI)


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Figure 3. XRD patterns of the (a) PbI2 W/tBP and PbI2 W/O tBP,；(b) CH3NH3PbI3 W/tBP, and CH3NH3PbI3 W/O tBP； (c) UV–Vis absorbance spectra of CH3NH3PbI3 W/ tBP and CH3NH3PbI3 W/O tBP films and (d) FTIR spectrums of PbI2 W/tBP, PbI2 W/O tBP, CH3NH3PbI3 W/tBP, and CH3NH3PbI3 W/O tBP films. 158x117mm (300 x 300 DPI)



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Figure 4. Contact angle measurements were taken of water droplets on the films of (a) PbI2 W/O tBP and (b) PbI2 W/tBP, (c) CH3NH3PbI3 W/O tBP, and (d) CH3NH3PbI3 W/tBP. 81x60mm (300 x 300 DPI)


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Figure 5. (a) Typical J–V characteristics and (b) Corresponding EQE of CH3NH3PbI3 W/tBP and CH3NH3PbI3 W/O tBP-based solar cells prepared in ambient air. 83x32mm (300 x 300 DPI)



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Figure 6. (a) Structures of four types of molecules used as the surface modification agent for perovskite layer with the same structure except terminal groups marked as hydrophilic and hydrophobic groups. (b) Corresponding J–V characteristics of modified-CH3NH3PbI3-based solar cells prepared in ambient air. 382x185mm (300 x 300 DPI)


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Figure 7. (a) Evolution of the PCEs of CH3NH3PbI3 W/ tBP and CH3NH3PbI3 W/O tBP-based solar cells upon ageing in air without encapsulation (average values of 10 devices fabricated under identical conditions). (b) J–V characteristics (measured under AM 1.5 illumination) of CH3NH3PbI3 W/tBP-based solar cells prepared under different humidities of 40%–50%, 50%–60%, 60%-70%, and 70%–80%. 83x32mm (300 x 300 DPI)



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Efficient and Stable Perovskite Solar Cells ... - ACS Publications

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