Improved Moisture Stability of 2D Hybrid Perovskite (HOOCâCH2

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Improved Moisture Stability of 2D Hybrid Perovskite (HOOC-CH2NH3)2PbI4 by Dehydration Condensation between Organic Components Qizhong Zhang, Jiangtao Zhao, Zhenhong Xiao, Jingtian Zhou, Bin Hong, Zhenlin Luo, Jun Bao, and Chen Gao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00199 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Energy Materials

Improved Moisture Stability of 2D Hybrid Perovskite (HOOC-CH2-NH3)2PbI4 by Dehydration Condensation between Organic Components

Qizhong Zhang†, Jiangtao Zhao†, Zhenhong Xiao†, Jingtian Zhou†, Bin Hong†, Zhenlin Luo*,†,‡, Jun Bao*,†,‡, and Chen Gao†,‡

†

National Synchrotron Radiation Laboratory, Collaborative Innovation Center of Chemistry for

Energy Materials, University of Science and Technology of China, Hefei 230029, China. ‡

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and

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

ABSTRACT: In this work, a one-step solution-processing approach was purposed and demonstrated for the dehydration condensation between the organic components of 2D hybrid perovskite (HOOC-CH2-NH3)2PbI4 (abbreviated as GLy2PbI4). The novel product obtained after condensation (labeled as GLy2PbI4-DCC) shows greatly-enhanced

stability

without

any

obvious

degradation

in

the

accelerated moisture-tolerance test. XPS reveals the strong covalent bonds formed between the adjacent -COOH and -NH3+ terminal groups. XRD, SEM, TEM and moisture-tolerance test results show that the change from non-covalent interactions to covalent interactions helps the 2D hybrid perovskite to resist the intrusion of moisture, as well as facilitates the growth of smooth and uniform films. This work sheds light on the moisture stability problem of organic-inorganic hybrid perovskites. KEYWORDS: 2D hybrid perovskite, moisture stability, dehydration condensation, organic component, chemical shift INTRODUCTION Organic-inorganic lead-based hybrid perovskites APbX3 (A = CH3NH3+ or HC(NH2)2+, X = Cl-, Br-, or I-), have attracted recently considerable interests

due to

their outstanding properties for photovoltaic applications, including broad absorption 1 / 25

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ranges up to near infrared (near-IR)1, low trap-state density2, long electron-hole diffusion length3, cheap and facile preparation techniques4-7. The power conversion efficiency (PCE) of solar cells using these materials as visible-light sensitizer has soared from 3.8%8 to 22.10%9 in the past eight years from 2009 to 2017. This developing trend is much faster than other-type solar cells, such as multicrystalline silicon (mc-Si), cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) solar cells10,11. Although gratifying breakthroughs have been achieved for these perovskite solar cells (PSCs), two major issues, which are long-term instability and toxicity of lead, have not yet been addressed satisfactorily12 and became the hugest bottlenecks for the practical applications13-15. In order to resolve the latter problem, some researchers have tried to seek lead-free perovskites by substituting lead with other environment friendly metals (such as Sn, Bi)16,17, but the PCE of the final devices dramatically decreased. Actually, the toxic harmfulness is majorly derived from the degradation of lead-based perovskites in humid environment or at higher temperatures, where PbX2 are irreversibly formed due to the loss of AX18-21. Therefore, these two bottleneck issues might be addressed via improving the perovskite stability once and for both. The poor moisture stability of the hybrid perovskite may originate from its intrinsic ionic properties22 and relatively lower formation energy23, which makes it easy to be attacked by water molecules and eventually collapse into separate parts of organic and inorganic components. Taking into account the hybrid perovskite material itself, there are two approaches to improve the stability, searching for new-types of hybrid perovskites with relatively high stability and further modifying or enhancing the stability of 3D hybrid perovskites. The recently emerging 2D hybrid perovskites24-27, with relatively higher formation energy than their initial 3D counterparts, may be more appropriate for revolving this stability problem. These 2D hybrid perovskites adopt the crystalline structure of Ruddlesden-Popper (RP) type, with a general chemical formula of (R-NH3)2(A)n-1PbnX3n+1 (n is an integer), where R is an alkyl or aromatic moiety, A is an organic cation, and X is a halide28-30. Particularly, when n = 1, the formula becomes (R-NH3)2PbX4, which is the simplest 2 / 25


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and well-studied compounds in 2D perovskites. The band-gap of 2D hybrid perovskites can be flexibly tuned by altering the n values29 and the natural self-assembly feature31 during film deposition is beneficial to form an ultrahigh surface coverage. In addition, the highly oriented lead-iodide layers perpendicular to substrate surface found in the films with n ≥ 3 is beneficial to the transport of photo-generated carriers32,33. Therefore, 2D hybrid perovskites had been considered as a perfect alternative to the 3D-types for high efficiency PSCs34,35, and has acquired a PCE of 12.52% and a relatively long-term stability of 2,250 h (about 94 days) in (BA)2(MA)n-1PbnI3n+1 family25 (BA = CH3(CH2)3NH3+, MA = CH3NH3+). Another effective method to enhance the moisture stability of hybrid perovskites is to utilize organic molecules with bi-functional groups, like an alkylphosphonic acid ammonium36 or 5-aminovaleric acid (5-AVA)37, as an additive during the synthesis process. These molecules were introduced into the interfacial regions between neighboring grains to enhance the cross-linking effect by strong hydrogen bonding, examining the “anchoring” position which is touched the inorganic lead-iodide parts of 3D perovskite grains36. These organic cations can also improve the adhesion of perovskite with substrate38,39 and act as a growth-controlling agent during fabrication31,37. In summary, although such bi-functional group introduction has obtained a considerable moisture stability of the 3D PSCs, the perovskite grains still encounter degradation when faced with the worse moisture condition. This is because only the contact of neighboring perovskite grains is enhanced but the bonding type within the crystalline grains is not changed at all and the bifunctional organic cations located at the 12-fold coordinated holes40 among the PbI6 octahedra are separated from each other. Hence, how about modifying the chemical bonds by entering into the crystal interior and enhancing the contacts among organic cations? Considering the unique layered structure of 2D hybrid perovskite, these operations could be easily realized. That is to say, strengthening or altering the chemical bonds40-43 inside the 2D hybrid perovskites may be a practical method to resolve the moisture stability problem of PSCs. Obtaining a high-n member of 2D perovskite compounds with enhanced 3 / 25



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moisture stability is our goal, which can be divided into two steps: 1) obtaining a 2D perovskite compound with higher n value; 2) improving its moisture stability. Since RP perovskites (n > 1) could be derived from the simplest 2D (n = 1) by adding the number of metal cation layers28,47, this work only focuses on how to enhance the moisture stability of the n = 1 case. In 2D hybrid perovskites (n = 1), considering that the Coulomb interaction between organic and inorganic components41 and the weak hydrogen bonding between amine cation and halide anion40,42 are intrinsic and difficult to be changed, the modifying the chemical bonds of organic-organic parts may be a viable way. We propose a strategy to alter the bonding types of the organic parts into strong covalent interaction while remain the inorganic framework primitive. Specifically, bifunctional organic molecules should be introduced into a 2D perovskite structure and then cross-linked with each other by using condensation between the terminal functional groups of the organic molecules. In general, amino group (-NH2) is essential for the formation of 2D hybrid perovskite. Therefore, bifunctional organic molecules with one terminal of -NH2 and another terminal condensable with -NH2 are our choices. Inspired by the biological formation process of polypeptides or proteins44, Glycine (Gly) was chose as the additive organic precursor in this work. The potential benefits of selecting glycine are threefold: 1) it is α-amino acid with bi-functional groups of -NH2 and carboxyl (-COOH), therefore it could form a dipeptide through dehydration condensation45, converting the non-covalent bonds into covalent bonds; 2) -COOH also can act as an anchoring group to help forming an oriented and compact deposition film31,46; 3) -COOH as good hydrogen donor / acceptor will afford hydrogen bonds between each other47. To achieve this goal, the common condensation reagent

Dicyclohexylcarbodiimide

(DCC)48-50

is

selected

for

dehydration

condensation. EXPERIMENTAL SECTION Materials. FTO glass were purchased from NSG Group, Japan. Glycine (HOOC-CH2-NH2, GLy) AR (99.5~100.5%), Lead iodide (PbI2) (98%), aqueous hydriodic acid (HI) (45% w/w), N,N-Dimethylformamide (DMF) AR (99.5%), 4 / 25


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Hypophosphorous acid (H3PO2) (30.0~35.0%), and N,N'-Dicyclohexylcarbodiimide (DCC) (95%) were all purchased from Sinopharm. HOOC-CH2-NH3I (GLyI) AR (99%) was purchased from MaterWin. All chemicals were used as received. Syntheses. GLy2PbI4 crystal was synthesized from a stoichiometric reaction between GLy and PbI2 in concentrated aqueous HI (45% w/w). The detailed synthesis procedure referred to the single crystal growth technique were previously reported28 for 2D hybrid perovskites (BA)2(MA)n-1PbnI3n+1 (n = 1). Under the atmosphere, 1500 mg (20 mmol) GLy was neutralized with 5mL HI 45% w/w under constant magnetic stirring for 2 h at room temperature, giving a clear pale yellow solution. In a separate beaker, 4610 mg (10 mmol) PbI2 was dissolved in a mixture of 10.0 mL HI 45% w/w and 1.7 mL aqueous H3PO2 by heating to 100 °C under constant magnetic stirring for 10 min, which resulted a bright yellow solution. Addition of the GLyI solution to the PbI2 solution, heating the combined solution to 100 °C. The stirring was then discontinued after evaporating half of the solvent, and the solution was left to cool down to room temperature during which time allowed the slow start to formation of pale yellow needle-like crystals. The needle-like crystals were isolated by suction filtration and washed with cold diethyl ether, then thoroughly dried in vacuum chamber at 80 °C over night. The scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) elemental analyses confirmed that the synthesized compound was GLy2PbI4. Film Fabrication. The already dried GLy2PbI4 crystals (4000 mg, 4.6 mmol) were dissolved in DMF (6.4 mL) solvent under constant magnetic stirring for 2 h at 60 °C, then affording a yellow 40 wt% precursor solution of GLy2PbI4. The precursor was divided into two halves, half for preparing the film via spin-coating method, and the other for the next reaction. DCC (950.0 mg, 4.6 mmol) was added to the GLy2PbI4 precursor (3.2 mL) under constant magnetic stirring for 2 h at 90 °C, the color of the solution gradually changed from bright yellow to dark brown, forming the GLy2PbI4-DCC precursor. The dark brown solution was placed in a refrigerator overnight and then filtered, collecting the filtrate for later film preparation. As substrate for film preparation, the FTO glass was cleaned using successively toluene, 5 / 25



acetone, alcohol and deionized water. Then the GLy2PbI4 and GLy2PbI4-DCC solution was dynamically spin-coated respectively at 3500 rpm for 60 s. After annealing at 100 °C for 15 min on a heating table, the GLy2PbI4 and GLy2PbI4-DCC films were formed. Characterization. Powder and film X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab X-ray diffractometer with Cu-Kα radiation (1.5406 Å). Film XRD patterns were recorded in a 1D measurement mode by a D/teX Ultra detector. Optics alignment, detector center correction and sample alignment were performed prior to general measurements. Diffraction patterns in the range of 3-70° were recorded using a scan rate of 0.02°/s. Ultraviolet-visible-Near Infrared (UV-vis-NIR) absorption spectra were acquired using a SOLID UV-vis-NIR 3700 spectrometer (Shimadzu, Japan). SEM images were obtained by a field-emission scanning electron microscope (FEI Quanta 200), equipped with an Oxford EDS detector. SEM-EDS mapping was obtained by using of a finely focused beam to guide electrons to bombard a region of interest. Transmission electron microscopy (TEM) images were acquired on a JEM-2100F field emission transmission electron microscopy (JEOL, Japan). XPS data were carried out on a Thermo ESCALAB 250 instrument with a monochromatic Al-Kα X-ray radiation (1486.6 eV). Grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed on the BL14B beamline of Shanghai Synchrotron Radiation Facility (SSRF) with photo energy of 10 keV (λ = 1.2398 Å). The GIWAXS intensity was recorded using a 2D charge-coupled device (CCD) detector with 3072 × 3072 pixels. Raman spectra were obtained in a confocal raman microscopy (LabRamHR) equipped with a 514.5 nm excitation laser. The spectra are registered in the 50-300 cm-1 range, particularly sensitive the Pb-I modes. The accelerated moisture-tolerance tests. Humidity studies on the films were conducted above the top opening of glass container containing saturated aqueous sodium chloride solution at the bottom. The bottle was preheated at 60 °C for a while to accelerate the evaporation of water vapor. The samples were held at each position for about 30 s and then dried on a hot plate at 80 °C. Optical photoes and powder 6 / 25


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XRD patterns were obtained to monitor the changes of color and structure of the perovskite films.

RESULTS AND DISCUSSION Structural characterization. Figure 1a shows the powder XRD pattern of the synthesized yellow needle-like crystals in comparison with those of GLy and PbI2. It reveals that the XRD peaks (2θ) for PbI2 locate at 12.7°, 25.9°, 34.3°, 38.7 and 39.5°, while the peaks at 14.8°, 19.0°, 20.2°, 23.9° and 29.9° belong to GLy. All these peaks disappeared and were substituted by a series of new diffraction peaks located at 8.8°, 9.3°, 11.6°, 15.7° and 18.7°, indicating that a new compound was successfully synthesized. This XRD pattern is similar to the patterns of 2D perovskites such as (HO-(CH2)2-NH3)2PbI451 and (HOOC-(CH2)3-NH3)2PbI447, suggesting they possess similar 2D layered perovskite structure. More, both the XPS and SEM-EDS elemental analyses verify that the I/Pb atomic ratio in the compound is very close to the the expected value of 4 in the stoichiometric crystal (Supporting Information, Table S1 and Table S2). All the above results implies the chemical formula of the new compound is “GLy2PbI4”. Subsequently, to fulfil the final purpose, the crystals were dissolved in DMF solvent and then DCC was added for condensation. The color of the solution gradually changes from bright yellow to dark brown (Supporting Information, Figure S1), indicating that a chemical reaction occurs and which is confirmed by the conversion of DCC to DCU48,52,53 (Supporting Information, Figure S2). The possible reaction mechanism can be written as by equation：

The precursors before and after adding DCC, that is to say the GLy2PbI4 and GLy2PbI4-DCC precursors were then separately spin-coated on FTO substrates for following analysis and testing. In the XRD pattern of the Gly2PbI4 film (Figure 1b), a series of strong and sharp diffraction peaks appeared at 8.6°, 16.6°, 25.1°, 33.7°, 42.5° and 51.5°, indicating that the film exhibits a remarkable orientation preference and which is consistent with the feature of the 2D halide perovskite film26,54,55. 7 / 25


In


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contrast, the peaks at 8.6° and 16.6° did not appear but the peak at 11.4° remained in the XRD pattern of the GLy2PbI4-DCC film. Other weaker peaks at 25.3°, 29.6°, 32.0° and 33.7° were also found in the XRD patterns of GLy2PbI4 film (Supporting Information, Figure S3a). These indicate that the the crystalline orientation of the GLy2PbI4-DCC film has changed and the crystallinity was lower.

However, this

point can be improved by increasing the annealing temperature (Supporting Information, Figure S3b). When the temperature reached 180 ℃, strong diffraction peaks at 8.6° and 16.6° reappeared.

High-resolution TEM (HRTEM) images reveal

the condensation process more clear from the morphology. The GLy2PbI4 film (Figure 1c) shows obvious ordered lattice fringes, while the GLy2PbI4-DCC film (Figure 1d) presents disordered lattice fringes ascribed to varying degrees of bending56,57. The randomly distributed sinuous lattice fringes presented in the latter image correspond to the distorted inorganic lead-iodine layers and could be attributed to the influence of interconnected organic components. In addition, these results also imply that a cross-linking effect36 occurs in the internal lattices rather than merely between adjacent grain surfaces, because the lattice fringes will not distort in the latter case.

Figure 1. Diffraction data and the HRTEM images. (a) XRD patterns of PbI2, Gly and GLy2PbI4 powder. (b) XRD patterns of GLy2PbI4 and GLy2PbI4-DCC films on FTO glass, # indicates the FTO peak. HRTEM images of (c) GLy2PbI4 film and (d) GLy2PbI4-DCC film. To further confirm the orientation difference between the GLy2PbI4 and GLy2PbI4-DCC films, GIWAXS analysis were performed by using synchrotron 8 / 25


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radiation25,58. For the GLy2PbI4 film (Figure 2a), a series of sharp and discrete Bragg spots (marked with ∗) indicates the preferred crystal planes parallel to the substrate surface. By contrast, for the GLy2PbI4-DCC (Figure 2b), the above-marked Bragg spots disappear, while a Bragg spot (marked with white arrow) appear instead of the Debye-Scherrer ring in the pattern of the GLy2PbI4 film. Such results verify that the GLy2PbI4-DCC film has a different orientation preference compared to the GLy2PbI4 film. The GIWAXS patterns of the GLy2PbI4-DCC film after annealing at 180 ℃ also shows sharp and discrete Bragg spots (Figure 2c), the ∗ marked Bragg spots appeared again, indicating a 2D halide perovskite structure analogous to that of GLy2PbI4. This structural similarity were also confirmed by Raman measurements. Similar to the Raman spectra of the 2D hybrid perovskites BA2PbI459 and (HOOC(CH2)4NH3)2PbI460, four bands appear in the range of 50-300 cm-1 (Figure 2d, e). According to previous assignments, the band at 88 cm-1 can be attributed to the Pb-I stretching mode. The remaining three bands are assigned to vibrational modes of the organic components coupled with the inorganic Pb-I parts. The signals at 108 and 135 cm-1 are ascribed to libration modes, and the band at 167-168 cm-1 is ascribed to torsional modes. After annealing at 180 ℃, the Raman spectrum of the GLy2PbI4-DCC film also showed four bands at 88, 106, 133, 168 cm-1 (Figure 2f), indicating that the structure of the inorganic Pb-I moieties in the three films were almost the same.

9 / 25



Figure 2. GIWAXS patterns and Raman spectra. GIWAXS images for (a) GLy2PbI4 film, (b) GLy2PbI4-DCC film and (c) GLy2PbI4-DCC film after annealing at 180 ℃. Raman spectra for (d) GLy2PbI4 film, (e) GLy2PbI4-DCC film and (f) GLy2PbI4-DCC film after annealing at 180 ℃.

Morphological analysis. TEM and SEM measurements are applied to unravel morphological differences of the two films. The TEM image in Figure 3a indicates that GLy2PbI4 are elongated needle-like crystals with smaller grain size and aligned not in close contact with each other. In contrast, GLy2PbI4-DCC crystals are tulle-like smooth

with

much larger sizes (Figure 3b). The improvement of the crystal quality

of GLy2PbI4-DCC probably resulted from the cross-linking occurred between the internal lattices. The difference in microscopic morphology leads to differences in the coverage of the two 2D perovskite films. The SEM image presented in Figure 3c indicates that the hazy pale-yellow GLy2PbI4 film presents poly-dispersed pinholes isolated by interconnected micron-sized island-like regions36, leaving large areas where the film is completely absent. In stark contrast, the GLy2PbI4-DCC film shares an extremely uniform and transparent appearance, completely covering the FTO, indicating a much denser coverage as shown in Figure 3d and Figure S4 in the Supporting Information. The SEM-EDS mappings confirm the uniform distribution of Pb, I, N elements in the GLy2PbI4-DCC film (Figure 3e) and GLy2PbI4 film 10 / 25


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(Supporting Information, Figure S5). Meanwhile, the results clearly reveal that the denser coverage of the GLy2PbI4-DCC film than that of the GLy2PbI4 film.

Figure 3. Microscopy images. TEM images of (a) GLy2PbI4 film and (b) GLy2PbI4-DCC film. The surface SEM images of (c) GLy2PbI4 film and (d) GLy2PbI4-DCC film. (e) From left to right, SEM-EDS mapping of the Pb, I and N components in GLy2PbI4-DCC film.

Moisture stability test. In order to verify whether the moisture stability of GLy2PbI4-DCC has been improved compared to GLy2PbI4, an accelerated moisture-tolerance test61,62 was performed and the results are presented in Figure 4. First, the samples were held for 30 s at the top opening of a bottle containing saturated aqueous sodium chloride solution (as shown in Figure 4a). Then, photos were taken at 11 / 25



different stages to reflect the states of degradation. It can be seen from Figure 4b that the pale yellow GLy2PbI4 film immediately degraded with the color changed to yellow, while the bright yellow GLy2PbI4-DCC film almost remained original through 30 s of infiltration. XRD patterns reveal this discrepancy more clearly. For the GLy2PbI4 film (Figure 4c), a new diffraction peak appears at 12.7° after infiltration, indicating the hydrolysis product of PbI227. For the GLy2PbI4-DCC film, the pattern remains original with no PbI2 peak appeared (Figure 4d). This strongly suggests that the condensation process with DCC indeed improves the moisture stability of GLy2PbI4.

Figure 4. The accelerated moisture-tolerance tests data. (a) The photograph of the operation of the accelerated moisture-tolerance test. (b) The color change of GLy2PbI4 film (left) and GLy2PbI4-DCC film (right) at each stage: before the moisture, infiltrating and after drying. The XRD pattern comparison of original film and suffered moisture erosion film of (c) GLy2PbI4 film and (d) GLy2PbI4-DCC film, the gray dashed line indicates the position of PbI2 peak and # indicates the FTO grass peak.

Binding energy analysis. The changes in film morphology and moisture 12 / 25


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stability are believed to originate from the dehydration condensation process between organic components. To explore how the process occurred, XPS measurements have been performed. The relevant XPS peak is calibrated against the C1s signal of contaminant carbon with a binding energy (BE) of 284.80 eV. The overview spectra of the two specimens (Supporting Information, Figure S6) indicate the presence of carbon, nitrogen, oxygen, lead and iodide elements. And the following core level analysis in detail reveals the nature of chemical bonds. In the two samples, the spin-orbit split between Pb 4f7/2 and Pb 4f5/2 levels is 4.85 eV while the split between I 3d5/2 and I 3d3/2 levels is 11.50 eV, showing these values are equal to the corresponding value for PbI263. For GLy2PbI4-DCC, in comparison with GLy2PbI4, the Pb 4f and I 3d peaks shift -0.36 eV and -0.45 eV respectively (Figure 5a,b), implying that a certain relaxation occurs between the organic groups and inorganic lead-iodine layers. In particular, the chemical shift of I 3d is lower than the shift of Pb 4f in GLy2PbI4→GLy2PbI4-DCC while the chemical shift of I 3d is higher than the shift of Pb 4f in PbI2→GLy2PbI4 (Supporting Information, Figure S7a,b). This result could be attributed to the hydrogen bonds generated between organic cation and inorganic iodine anion during PbI2→GLy2PbI4, and these bonds were weakened in GLy2PbI4→GLy2PbI4-DCC due to the protonation of NH2 and the loss of H atom from NH3+. It will be further described below. The C 1s XPS spectra of GLy2PbI4 presented three peaks (Figure 5c) belonging to amorphous carbon64, C-NH3+65, and the carbon of -COOH66, and their core levels locate at 284.80 eV, 286.71 eV and 289.29 eV, respectively. The C-NH3+ peak in GLy2PbI4 significantly shifted ~0.51 eV toward higher BE compared to the C-NH267 peak in GLy (Supporting Information, Figure S8a). This huge chemical shift was caused by the protonation of the amino68 and the effect of lead-iodine octahedra on the amino of organic parts. The -COOH peak shifted ~0.83 eV toward higher BE in GLy→GLy2PbI4, which could be explained from three aspects: 1) the formation of 2D layered perovskite structure; 2) the induced effect of C-NH2→C-NH3+; 3) the effect of hydrogen bonds among -COOH47. The C 1s spectrum of GLy2PbI4-DCC also consists of three peaks (Figure 5c) located at 284.80 eV, 286.26 eV and 288.89 eV. The peak at 13 / 25



288.89 eV shifted ~0.40 eV toward lower BE compared to the -COOH peak in GLy2PbI4, indicating that the shielding effect of the outer electrons of the C atom is enhanced. This enhancement could be attributed to the formation of C-CO-NH2+ bond during the dehydration condensation process between the •OH in -COOH and •H in -NH3+ groups. Analogous, the peak at 286.26 eV shifted ~0.45 eV toward lower BE compared to the C-NH3+ peak in GLy2PbI4, implying that the change from OOC-C-NH3+ to OC-C-NH2+. The comprehensive effects of -COOH→-CO-NH2+ and -NH3+→-NH2+-CO ultimately caused the chemical shifts of C 1s toward lower BE.

Figure 5. High-resolution XPS spectra and valence-band spectra. High-resolution XPS spectra of (a) Pb 4f, (b) I 3d, (c) C 1s, (d) N 1s. Valence-band spectra of (e) GLy2PbI4 and GLy2PbI4-DCC film. The N 1s XPS spectra of the two samples are totally different (Figure 5d). For GLy2PbI4, the spectrum presents a broad peak that can be fitted in two main components. The peak with higher BE at 402.14 eV could be ascribed to protonated amines -NH3+69, and a very small peak (∼8% area of the total N 1s) at 400.41 eV 14 / 25


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should be assigned to a small amount of free amines -NH270,71. On the contrary, although the N 1s spectrum of GLy2PbI4-DCC also consists of similar two peaks at this region, the area ratio of these two peaks switches over 1/4. The peak with higher BE becomes smaller while the peak with lower BE become stronger. Here, the peak at 400.43 eV should be assigned to C-NH2+-CO bond, which is confirmed by the chemical shift of N 1s core level in GLyI→GLyI-DCC. As shown in Figure S8b in the Supporting Information, the N 1s spectra of GLyI consist of three peaks, and the peak at 401.94 eV with the highest BE should be protonated amines -NH3+. But the GLyI-DCC only contains one peak at 400.36 eV that is C-NH2+-CO bond. The N 1s core level of -NH3+→C-NH2+-CO shifted ~1.58 eV toward lower BE in GLyI→GLyI-DCC. Similarly, considering the influence of the inorganic components, the chemical shifts of NH3+→C-NH2+-CO in GLy2PbI4→GLy2PbI4-DCC should be higher than 1.58 eV. Therefore, the peak at 400.43 eV in GLy2PbI4-DCC is attributed to C-NH2+-CO bond rather than free amines -NH2. The chemical shift of NH3+→C-NH2+-CO is ~1.71 eV in GLy2PbI4→GLy2PbI4-DCC, which is in line with the anticipations. The organic components are connected together through the dehydration condensation, and thus further affecting the inorganic components, resulting in the chemical shifts of the Pb 4f and I 3d core levels. Besides the photoelectron lines, the valence-band spectra were also found different and therefore were used to explore the changes of the valance band maximum (VBM). For 2D hybrid perovskite, the VBM is formed of an anti-bonding Pb 6s/I 5p combination72,73. It can be determined by linear extrapolation of valance band onset subtracted to the background around Fermi level65. As a result, the VBM of the two films locate at 1.22 eV and 1.02 eV below the Fermi level, respectively (Figure 5e). That is to say the value is reduced by ~0.20 eV. As references, the VBM of PbI2/PbI2-DCC and GLyI/GLyI-DCC have been determined (Supporting Information, Figure S9a,b). The VBM of PbI2 systems are 1.18 eV and no change before and after adding DCC, while the VBM of GLyI systems are 1.79 eV before condensation and 1.47 eV after condensation. All of the above XPS analyses results strongly suggest that the condensation did occur between the -COOH and -NH3+ 15 / 25



groups of the neighbouring organic cations, this process is schematically showed in Figure S10 in Supporting Information.

Figure 6. Optical absorption in GLy2PbI4 and GLy2PbI4-DCC film. (a) The absorbance versus wavelength. and indicate the positions of the primary and second absorption edges in GLy2PbI4 film, while , , and stand for the primary, the second and the additional third absorption edge in GLy2PbI4-DCC film. (b) The (αhν)2 versus energy plot to determine the optical band-gap. (inset) zoomed version of the third absorption edge in GLy2PbI4-DCC film. Absorption characteristics. At the end, the moisture stability of GLy2PbI4 film has been improved by condensation, but how about its absorption characteristics? The UV-vis-NIR absorption spectra (Figure 6a) show that GLy2PbI4 film has two absorption edges, including the primary absorption edge and secondary absorption edge, consistent with other 2D (n = 1) hybrid perovskites26,33. Its primary optical band-gap (Eg) is determined to be 2.95 eV by Tauc plot method (Figure 6b) and the second peak (main exciton absorption peak) is calculated to be 3.32 eV. On another hand, in the GLy2PbI4-DCC film, besides the two normal absorption edges, an additional third absorption edge (inset in Figure 6b) appears at a longer wavelength. The Eg is 2.84 eV with 0.11eV blue shifts compared with GLy2PbI4 film, the second edge locates at 3.32 eV and the third absorption edge locates at 2.43 eV. Why the third absorption edge appears after adding DCC? The reason might be that an additional impurity level is introduced in the forbidden energy gap as the result of the distorted inorganic lead-iodine layers during the condensation between adjacent organic groups. 16 / 25


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The condensation seems expand the material’s absorption range.

CONCLUSIONS We have demonstrated a one-step solution-processing strategy of adding DCC into GLy2PbI4 precursor to synthesize organic-inorganic hybrid perovskite compounds GLy2PbI4-DCC with greatly-enhanced moisture stability. Our results strongly support that condensation did occur between the -COOH and -NH3+ terminal groups located in the neighboring organic cations. The resulting GLy2PbI4-DCC film didn’t appear any obvious degradation when resisted the intrusion of moisture. The interconnected organic polymeric chain acts as a barrier, just like a careful-weaved cage, to wrap up the inorganic lead-iodine layer, to resist the intrusion of moisture, as well as facilitating the growth of smooth and uniform films. Additional, the randomly distorted inorganic lead-iodine layer and condensed organic polymeric chains might introduce an impurity level into the forbidden energy gap. Based on this work, further studies, like applying this strategy in RP (n > 1) PSCs should be performed, in order to acquire PSCs with both good moisture stability and excellent PCE value.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Photographs of different precursors, XRD patterns of DCC and filtered residue, XRD patterns of GLy2PbI4-DCC film at different annealing temperatures, SEM images and EDS mapping, XPS and valence-band spectra data. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Zhenlin Luo). ORCID: 0000-0002-7196-6537 *E-mail: [email protected] (Jun Bao). 17 / 25



Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the Key Lab of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences (PECL2017K002), the National key Basic Research Program of China (2016YFA0300102), the National Natural Science Foundation of China (11675179, 11434009) and the Key Program of the Chinese Academy of Sciences (CX3430000001). This work was partially supported by the Fundamental Research Funds for the Central Universities and was partially carried out at the USTC center for Micro and Nanoscale Research and Fabrication. Thanks to Dr. H.L.Huang and Dr. W.Wen on beamline BL14B of SSRF for assistance with GIWAXS.

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