Elucidating the Key Role of a Lewis Base Solvent in the Formation of

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Elucidating the key role of Lewis base solvent in formation of perovskite films fabricated from Lewis adduct approach Xiaobing Cao, Lili Zhi, Yahui Li, Fei Fang, Xian Cui, Youwei Yao, Lijie Ci, Kongxian Ding, and Jinquan Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07216 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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ACS Applied Materials & Interfaces

Elucidating the key role of Lewis base solvent in formation of perovskite films fabricated from Lewis adduct approach Xiaobing Cao1, Lili Zhi2, Yahui Li1, Fei Fang1,3, Xian Cui1, Youwei Yao3, Lijie Ci2, Kongxian Ding4, Jinquan Wei1* 1. State Key Lab of New Ceramic and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China 2. School of Materials Science & Engineering, Shandong University, Jinan 250061, Shandong, P.R. China 3. Institute of Advanced Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, P.R. China. 4. Shenzhen Jiawei Solar Lighting Co., Ltd., New Industrial Zone No. 1-4, Fuping Road, Longgang District, Shenzhen 518112, Guangdong, P.R. China

ACS Paragon Plus Environment

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ABSTRACT High quality perovskite films can be fabricated from Lewis acid-base adducts through molecule exchange. Substantial work is needed to fully understand the formation mechanism of the perovskite films, which help to further improve their quality. Here, we study the formation of CH3NH3PbI3 perovskite films by introducing some dimethylacetamide into PbI2/N,Ndimethylformamide solution. We reveal that there are three key processes during the formation of perovskite films through the Lewis acid-base adduct approach: molecule intercalation of solvent into PbI2 lattice, molecule exchange between the solvent and CH3NH3I, and dissolutionrecrystallization of the perovskite grains during annealing. The Lewis base solvents play multiple functions in the above processes. The properties of solvent, including Lewis basicity and boiling point, play key roles in forming smooth perovskite films with large grains. We also provide some rules for choosing Lewis base additives in order to prepare high quality perovskite films through the Lewis adduct approach.

KEYWORDS: perovskite film; Lewis acid-base adduct; Lewis basicity; formation mechanism; solar cell;


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1. INTRODUCTION Since the solid state perovskite solar cells with efficiency exceeding 9% was reported,1 organic–inorganic hybrid perovskite solar cells (PSCs) have attracted much attention due to its excellent photovoltaic performance. In parallel with fabrication of high efficient PSCs, some independent groups revealed that solvate complex played a key role in fabricating high quality perovskite films.2,3 Up to now, some groups have successfully fabricated high quality perovskite films from Lewis acid-base adducts via one-step method.

4-6

Recently, we successfully prepared

high quality perovskite films and efficient perovskite solar cells by combining the Lewis adduct approach with the traditional two-step method.

7, 8

Shortly after, we fabricated perovskite films

with grain size exceeding 1 µm by optimizing the strength of Pb-O bonds in the PbI2-based Lewis adducts via adding some strong Lewis base solvents, such as methyl-2-pyrrolidone (NMP), dimethyl sulphoxide (DMSO), and hexamethylphosphoramide (HMPA) into PbI2/N, Ndimethylformamide (DMF) solutions. 9 These results suggested that the Lewis base solvents play important roles in formation of perovskite films. However, it is still inadequate to understand the formation mechanism of the perovskite films, which impedes further improving performance of the solar cells. However, these Lewis base solvents have different molecular structure, chemical properties, and amount in the PbI2-based Lewis adducts,10, 11 which make it difficult to elucidate their roles in controlling the formation of the perovskite films. Dimethylacetamide (DMA), a homologue of DMF, was also used to fabricate efficient perovskite solar cells through one-step method.

12, 13

which is slightly higher than that of DMF (26.6).

14

The DMA has a donor number of 27.8,

Both of the DMA and DMF can form PbI2-

based Lewis adducts through dative Pb-O bonds, where the solvents act as Lewis base, and Pb2+ acts as Lewis acid.10 It forms solvate complexes when PbI2 was dissolved in DMF and DMA


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solution, and lead (II) binds six solvent molecules in the solvate complexes. The bond lengths of Pb-O in the solvate complexes are 2.55 and 2.48 Å for the DMF and DMA, respectively. However, the solvate complex is hemidirected in DMF solution, and holodirected in DMA solution.15 It was also reported that some transition metal ions in the mixed solvents of DMF and DMA exhibit solvation steric effect.

16, 17

The bond length of the transition metal-oxygen was

shortened in the DMF/DMA binary mixed solvents due to the solvation steric effect,

18

which

plays a key role in the growth of the perovskite grains.19 DMA and DMF have similarity in structure and difference in Lewis basicity, which provide convenience to investigate the key role of Lewis base solvent in controlling formation of the perovskite films. Here, we study the interaction between Pb2+ and solvent molecules by adding some DMA into the PbI2/DMF solution, and reveal the formation mechanism of the MAPbI3 films fabricated from the Lewis adducts.

2. EXPERIMENTAL SECTION 2.1 Fabrication of perovskite films The FTO-coated glasses were subsequently cleaned by deionized water, acetone and isopropanol, and then exposed to UV ozone cleaner for 15 min before deposition of TiO2 films. A compact TiO2 layer was firstly deposited on the cleaned FTO by spin-coating from titanium isopropoxy solution in ethanol at 2000 rpm for 30 s. Mesoporous TiO2 was coated on the compact TiO2 by spin-coating at a speed of 4500 rpm for 30 s from a diluted TiO2 paste (Dyesol18NRT, Dyesol) in ethanol (2:7, weight ratio). The TiO2 coated FTO substrates were annealed at 500 °C for 30 min. The perovskite films were fabricated by a modified two-step method, which has been described in details in our previous work.

9

Here, we added some DMA into the

PbI2/DMF (1.2 M) solution, and stirred at 100 °C for overnight to form a precursor solution. PbI2-based Lewis adduct films were deposited on the mesoporous TiO2 substrate by spin coating


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at a speed of 3000 rpm for 30 s from precursor solution. The PbI2-based precursor films were exposed to a solution of mixed CH3NH3I:CH3NH3Cl (weight ration=10:1) in 2-propanol with a concentration of 30 mg mL-1 for 60 s to prepare perovskite precursor films. The perovskite precursor films were then annealed at 100 ºC for 60 min to remove the residual solvent to form perovskite films. After cooling to room temperature, a hole transport layer (HTL) was deposited by spin-coating from Spiro-OMeTAD solution at the speed 3000 rpm for 30 s. The SpiroOMeTAD solution was prepared by dissolving 100 mg Spiro-OMeTAD in 1 mL chlorobenzene with additive of 40 µLTBP, 36.3 µL TFSI (520 mg mL-1 in acetonitrile) and 6 0 µLFK102 (300 mg mL-1 in acetonitrile). Finally, a gold layer (60 nm) was deposited on the HTL to form a complete solar cell. The active area is fixed at 0.06 cm2 by using a shadow mask during the photovoltaic characterization. 2.2 Characterization The perovskite films were characterized by field-emission scanning electron microscope (SEM, MERLIN VP Compact), X-ray diffraction (XRD, D8-Advance), atomic force microscopy (AFM5500, Agilent), and Fourier transform infrared spectroscopy (VERTEX 70v). The PbI2based Lewis adducts were evaluated by thermal gravimetric analysis (TGA/Q5000IR) from 50 to 400 ℃.

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C-NMR spectra of the solutions were characterized by 600-MHz NMR spectrometer

(JNM-ECA600). Incident photon-to-current efficiency (IPCE) was characterized by using a QEX10 solar cell quantum efficiency measurement system (QEX10, PV measurements, USA). The current−voltage (J-V) curves were recorded by using a Keithley 4200-SCS under illumination with a solar simulator (AM 1.5G, 100 mW cm-2, 91195, Newport). The devices were measured from -0.1 V to 1.2 V at a scan speed of 5 mVs-1.

3. RESULTS AND DISCUSSION


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Figures 1a to 1c are SEM images of the perovskite films fabricated from PbI2/DMF solutions adding with different amount of DMA. The average sizes are 274, 324 and 283 nm for the perovskite films fabricated from 0%, 5%, 10% DMA, respectively (see Figures 1g to 1i). It indicates that the grain size depends on the amount of DMA in the PbI2/DMF solution. There are some faceted precipitates on the surface of the films, which are chlorine-rich grains.20 The roughness of the perovskite films also decreases as DMA is added into the PbI2/DMF solutions (see Figure S1). Figures 1d to 1f show the corresponding cross-sectional SEM images of the perovskite films. All of the films have compact structure. It is noted that just one perovskite grain penetrates the whole film along thickness direction when 5% DMA is introduced into PbI2/DMF solution (see Figure 1e), which is desired for high efficient PSCs for reducing non-radiative charge carrier recombination at grain boundaries. 21 All of the perovskite films contain some residual PbI2, as shown by XRD patterns in Figure S2. It has been demonstrated that Lewis base solvents (including DMF and DMA) can form Lewis acid-base adducts (PbI2·xDMF and PbI2·xDMA) through Pb–O bonds.

10,22,23

Here, the

interaction between Pb2+ and solvent molecules in PbI2·xDMF and PbI2·xDMA are further confirmed by Fourier transform infrared (FTIR) transmittance spectra. Fingerprint peaks located at 1670 and 1644 cm-1 are identified for DMF and DMA, respectively. In comparison to the pure solvents, the fingerprint peaks of the PbI2·xDMF and PbI2·xDMA separately shift to 1627 and 1635 cm-1, as shown in Figure 2a and 2b. The shifts of the fingerprint peaks indicate the formation of Lewis acid-base adducts.4,5,7,8,10 Figure 2c shows TGA spectra of the Lewis adducts. The PbI2·xDMA has higher decomposition temperature, which indicates that the Pb-O bond in the PbI2·xDMA is thermally stronger than that in the PbI2·xDMF. The stronger Pb-O bond is ascribed to the stronger Lewis basicity of DMA (DN=26.6 for DMF and 27.8 for DMA).14


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To further investigate the roles of additive in PbI2 solutions, we characterize

13

C nuclear

magnetic resonance (NMR) at 600 MHz for the pure DMF, DMA, and PbI2/DMF solution with 5% DMA additive. The enlarged 13C sp2 spectra are shown in Figure 2d (the full spectra are given in Figure S3). The main NMR peak locates at 164.5 p. p. m. for the pure DMF, corresponding to the methyne group (CH) linked to oxygen (see dark line).24 For comparison, a chemical shift of ∆δ=~0.39 p.p.m. in the PbI2/DMF solution is identified (see blue line). The chemical shift derives from forming PbI2·xDMF through Pb-O bond,23, 25 which is consistent with the FTIR spectra (Figure 2a) and previous report.25 However, the characteristic peak of the DMA shifts from δ=~169.8 to δ=~172.3 p.p.m., when 5% DMA is added into the PbI2/DMF solution (see red line). The trend of the chemical shift is similar to that of the PbI2/DMF solution.25 The chemical shift in the NMR spectra suggest that there is interaction between PbI2 and DMA through Pb-O bonds. Meanwhile, this characteristic peak of the methyne group in DMF shifts from δ=~164.49 to δ=~164.8 p.p.m. with a split peak is also observed. This chemical shift in the DMF might derive from partially replacement of the DMF molecule with DMA molecules due to their strong electron-pair donating ability. The molecular replacement weakens the influence of C-O bonds in DMF molecules. Therefore, the average bond strength of Pb-O bond in the Lewis adduct increases due to the strong coordination between PbI2 and DMA. Combining with our previous works,

7-9

herein, we discuss the formation mechanism of

MAPbI3 films modulated by Lewis base solvents. The MAPbI3 films fabricated from Lewis adducts can be divided to three processes: molecule intercalation of the Lewis base solvent into PbI2, molecule exchange between solvent and MAI, and dissolution-recrystallization during annealing. As demonstrated by NMR spectra, the DMF molecules coordinated with the PbI2based Lewis adducts are partially replaced by DMA due to its stronger electron-pair donating


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ability. Therefore, the average bond strength of the Pb-O between PbI2 and solvent increases. As a result, the increasing Pb-O bond retards the molecule exchange between solvent and MAI, which reduces the conversion rate of the PbI2-based Lewis adducts to MAPbI3 film. According to Ko’s report,26 the finally perovskite grain size (R) is inversed proportional to the cubic root of overall reaction rate (r) (R∝ r

-1/3

) in the sequential deposition method. Thus, the increasing

grain size can be ascribed to the reduction of the conversion rate due to increase of the Pb-O bond strength when introduce some strong Lewis base additives into the PbI2/DMF solutions (see Figure 1b). However, too much strong Lewis additives, such as 10% DMA, will dissolve perovskite grains during the following dissolution-recrystallization process, resulting in reduction of grains size (see Figure 1c).The low conversion rate can also be confirmed by the XRD patterns of the perovskite films fabricated from different solution at the same reaction time (Figure 3a). When more DMA is added to the PbI2/DMF solution, the (001) peak of PbI2 is getting stronger, which indicates that more residual PbI2 exist in the perovskite films. The residual PbI2 further confirm the key roles of DMA in retarding the conversion of the PbI2based Lewis adducts to perovskite. Figure 3b shows the evolution of XRD patterns during the formation of MAPbI3 films at different crystallization stages. There are two obvious characteristic peaks at low angle in PbI2·xDMF (red line in Figure 3b). The peaks at 9.03º and 9.56º separately correspond to the (011) and (020) planes of the PbI2·xDMF.

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As shown in the XRD patterns, DMF and DMA

intercalate into PbI2lattice by forming PbI2-based Lewis adducts. The weak peak located at 7.25º might derive from the intercalation of DMA into PbI2 lattice due to its large molecular size. The PbI2 has a lattice distance of 6.98 Å along c axis (Scheme 1a). The intercalation of solvent into the PbI2 crystal leads to lattice expansion along c axis 28 (stage I in Scheme 1). The (001) peak of


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the PbI2-based Lewis adducts moves to 7.25º. Therefore, the lattice distance along the c axis expands to 12.21 Å calculated by Bragg’s law (Scheme 1b). When the PbI2-based Lewis adducts are exposed to MAI/IPA solution, they form MAI-PbI2-Sol complex as characterized by the three peaks between 5º and 10º in the XRD patterns29 in Figure 3b. The (001) peak of MAI-PbI2-Sol complex locates at 6.47º with a lattice distance of 13.68 Å (Scheme 1c). The lattice expansion can be ascribed to the difference in molecule dimension between MAI and solvent molecule. The expansion of the PbI2 lattice reduces the volume expansion rate during the formation of MAPbI3, which is responsible for the smooth surface of the MAPbI3 films when DMA is added to the PbI2/DMF solution

30

(see Figure S1). The expanded lattice of the PbI2 also provides channels

for MAI in the following molecule exchange. When the PbI2-based Lewis adducts are exposed to the MAI/IPA solution, the solvent molecule will be partially replaced by MAI due to its stronger electron donating ability of I¯ via molecule exchange process as expressed by the following formula (stage II in Scheme 1). PbI 2 ⋅ Sol + MAI → MAPbI 3 + Sol It is noted that the reaction products obtained from the molecule exchange are MAI-PbI2-Sol complex rather than MAPbI3. There is still some residual solvent molecule embedded in the films, which is confirmed by the XRD patterns of in Figure 3b. The residual solvent plays key roles in the dissolution-recrystallization process during the annealing treatment (stage III in Scheme 1). The MAI-PbI2-Sol grains will be partially dissolved by the residual solvent when they escape from the Lewis adducts. As a result, the small perovskite grains dissolve, recrystallize, and connect with each other to form large grains by annealing. In this process, the amount of residual solvent holds a key to obtain large perovskite grains. Excess residual solvent in the Lewis adduct films will dissolve the perovskite film


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seriously, which is harmful to the following recrystallization process. As a result, the mean grain size of the perovskite reduces inevitably. It is noted that the amount of residual solvent depends on the chemical properties and boiling point of the solvent. The stronger Lewis basicity (scaled by donor number) and higher boiling point, the more residual solvent in the MAI-PbI2-Sol film. In order to verify the dissolution-recrystallization process, we investigate the effects of additive concentration and annealing temperature on the perovskite grain sizes. Due to its high boiling point and strong Lewis basicity, the amount of residual DMA embedded in the MAIPbI2-Sol films increases as the increase of DMA concentration in the PbI2/DMF solution. Too much DMA embedded in the MAI-PbI2-Sol film will dissolve the small perovskite grains. The grains cannot grow up in the annealing treatment. Therefore, films consist of small perovskite grains at high DMA concentration (see Figure 4c). In the case of too little residual DMA embedded in the MAI-PbI2-Sol film, the solvent is insufficient for dissolution-recrystallization process during the annealing. The small grains cannot merge with each other, also resulting in small perovskite grains (see Figure 4a). At appreciate amount of residual DMA, it can balance the dissolution-recrystallization process during annealing, resulting in large perovskite grains (see Figure 4b). The amount of residual solvent embedded in the MAI-PbI2-Sol films can also be modulated by annealing temperature and annealing time. The perovskite grain size increases as the increase of annealing temperature and annealing time (see Figures 4d to 4i). At low annealing temperature, the solvent molecule escapes from the MAI-PbI2-Sol films slowly. The perovskite grains are partially dissolved into tiny grains by the solvent (see Figures 4d and 4e). At high annealing temperature, the solvent molecule escapes from the MAI-PbI2-Sol films quickly. The dissolution process is weakened in annealing process. Therefore, the recrystallization process


10

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completes in a short time, leading to large perovskite grains (see Figure 4f). The perovskite grain sizes also increase as the annealing time elongates (see Figures 4g to 4i). The short annealing time leads to more solvent embedded in the MAI-PbI2-Sol films. The dissolution dominates in the annealing process, leading to small perovskite grains (see Figure 4g and 4h). We also observe the dissolution-recrystallization process in the MAI-PbI2-Sol films prepared by using DMSO additive. The concentration of DMSO in the PbI2/DMF solution is preoptimized in our previous work.9 Since DMSO has stronger Lewis basicity and higher boiling point than DMA, the effects of DMSO on controlling the grain size become more obvious (see Figure S4 and Figure S5). These results also confirm the key roles of Lewis solvent in controlling perovskite grain size. The PSCs fabricated from the DMSO additive exhibit high performance due to large perovskite grains.9 Basing on the above discussion, the Lewis base solvent plays multiple functions in fabricating perovskite films. Firstly, the solvent molecule expands the PbI2 lattice through forming Pb-O bonds, which provides not only diffusion channels for MAI, but also space to accommodate volume expansion due to forming perovskite. Secondly, the Lewis base additive can modulate the bond strength of Pb-O. The strengthened Pb-O bonds can retard the molecule exchange, which is beneficial to obtain large perovskite grains. Thirdly, the strength of the Pb-O bonds and boiling point of the Lewis base additive affect the amount of residual solvent embedded in the MAI-PbI2-Sol films. Appropriate amount of the residual solvent can balance the dissolution and recrystallization during the annealing treatment, which help to form large perovskite grains. Here, we propose some rules for choosing additives to fabricate smooth MAPbI3 films with large grains through Lewis adduct approach.


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1) The additive has comparable molecular size with MAI. The solvent molecule expands the PbI2 lattice by forming Lewis adducts, which provides not only diffusion channels for MAI, but also reduces the final volume expansion after formation of MAPbI3. The suppression of volume expansion occurred in molecule exchange is beneficial to obtain smooth perovskite films and high efficient solar cells. 2) The additive has stronger Lewis basicity than that of DMF. The strong interaction between additive and Pb2+ will retard the molecule exchange process, which helps to grow large perovskite grains. 3) The additive should have higher boiling point than that of DMF. The solvent with high boiling point will be retained in the MAI-PbI2-Sol films, which is beneficial to merge for small grains during the annealing treatment, and forming uniform MAPbI3 film with large grains. As shown in Figure 5a, we also fabricate PSCs with a device configuration of FTO/TiO2/MAPbI3/Spiro-OMeTAD/Au. Figure 5b shows J-V curves of the best PSCs fabricated from different solutions. The corresponding photovoltaic parameters are listed in Table 1. The PSCs fabricated from 5% DMA exhibits the best performance with short-circuit current density (Jsc) of 22.49 mAcm-2, open-circuit voltage (Voc) of 1.02 V, fill factor (FF) of 0.70 and power conversion efficiency (PCE) of 16.05%, which is higher than those fabricated from the pure DMF (Jsc=20.41 mAcm-2, Voc=0.99 V, FF=0.61, PCE=12.32%). The statistic results based on a series of devices also show similar tendency to the best PSCs (see Figure S6). Figure 5c is stabilized output at the maximum power point of the PSCs fabricated from different solutions. The PSCs fabricated from 5% DMA show higher photovoltaic performance than those fabricated from the pure DMF. It is noted that the stabilized output values are lower than those


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obtained from the reverse scan. This phenomenon is widely observed in the PSCs with evident hysteresis behaviors

31,32

(also see Figure S7). The typical incident photon-to-current efficiency

(IPCE) of PSCs fabricated from solutions is shown in Figure 5d. It further confirms that the PSCs fabricated from 5% DMA exhibits better light-quantum yield ability than the PSCs fabricated from the pure DMF, which is consistent with the photovoltaic performance in Figure 5b. The integrated Jsc from IPCE spectrum is lower than the value obtained from reverse scan. This confusing discrepancy may be ascribed to the slow response of Jsc 5 as can be seen in the time-dependent current density (see Figure 5c) or the presence of surface defects on TiO2 layer.33

4. CONCLUSIONS In summary, Lewis base solvent plays multiple functions in the formation of MAPbI3 films. The solvent with strong Lewis basicity can retard the molecule exchange process. Additionally, solvent with strong Lewis basicity and high boiling point can modulate the amount of residual solvent embedded in the MAI-PbI2-Sol films. The appropriate amount of residual solvent can balance the dissolution and recrystallization in the annealing treatment. This work provides not only a vision to understand the roles of additive in controlling the crystallization of perovskite, but also some basic rules for choosing additive to fabricate high quality perovskite films.

ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. SEM,

AUTHOR INFORMATION

Corresponding Author


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*E-mail: [email protected] (J.W.). Tel: +86-10-62781065. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was financially supported by Shenzhen Jiawei Photovoltaic Lighting Co. Ltd., and Tsinghua University Initiative Scientific Research Program (20161080165).


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REFERENCES (1) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.;

Humphry-Bake. R.; Yum, J. H.; Moser, J. E.; Grätzel, M.; Park, N. G.

Lead iodide

perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2012, 2, 591-597. (2) Jeon, N. J.; Noh, J. K.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering

for High-Performanc Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. Mater., 2014, 13, 897-903. (3) Conings, B.; Babayigit, A.; Klug, M. T.; Bai, S.; Gauquelin, N.; Sakai, N.; Wang, J. T. W.;

Verbeeck, J.; Boyen, H. G.; Snaith, H. J. A Universal Deposition Protocol for Planar Heterojunction Solar Cells with High Efficiency Based on Hybrid Lead Halide Perovskite Families. Adv. Mater., 2016, 28, 10701–10709. (4) Ahn, N.; Son, D. Y.; Jang, I. H.; Kang, S. M.; Choi, M.; Park, N. G. Highly Reproducible

Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide. J. Am. Chem. Soc., 2015, 137, 8696−8699. (5) Lee, J. W.; Kim, H. S.; Park, N. G. Lewis Acid −Base Adduct Approach for High Efficiency

Perovskite Solar Cells. Acc. Chem. Res., 2016, 49, 311 – 319. (6) Pham, N. D.; Tiong, V. T.; Chen, P.; Wang, L.; Wilson, G. J.; Bell, J; Wang, H. Enhanced

Perovskite Electronic Properties via a Modified Lead (II) Chloride Lewis Acid –Base Adduct and Their Effect in High-Efficiency Perovskite Solar Cells. J. Mater. Chem. A, 2017, 5, 5195 –5203.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

(7) Cao, X. B.; Li, Y. H.; Fang, F.; Cui, X.; Yao, Y. W.; Wei, J. Q. High Quality Perovskite

Films Fabricated from Lewis Acid –Base Adduct through Molecular Exchange. RSC Adv., 2016, 6, 70925–70931. (8) Cao, X. B.; Li, C. L.; Li, Y. H.; Fang, F.; Cui, X.; Yao, Y. W.; Wei, J. Q. Enhanced

Performance of Perovskite Solar Cells by Modulating the Lewis Acid–Base Reaction. Nanoscale, 2016, 8, 19804–19810. (9) Cao, X. B.; Li, C. L.; Zhi, L. L.; Li, Y. H.; Cui, X.; Yao, Y. W.; Ci, L. J.; Wei, J. Q.

Fabrication of High Quality Perovskite Films by Modulating the Pb-O Bonds in Lewis AcidBase Adducts. J. Mater. Chem. A, 2017, 5,8416–8422. (10) Wharf, I.; Gramstad, T.; Makihja, R.; Onyszchuk, M. Synthesis and Vibrational Spectra of

Some Lead (II) Halide Adducts with O-, S-, and N-donor Atom Ligands. Can. J. Chem., 1976, 54, 3430–3438. (11) Zhang, Y.; Gao, P.; Oveisi, E.; Lee, Y.; Jeangros, Q.; Grancini, G.; Paek, S.; Feng, Y.;

Nazeeruddin, M. K. PbI2−HMPA Complex Pretreatment for Highly Reproducible and Efficient CH3NH3PbI3 Perovskite Solar Cells. J. Am. Chem. Soc., 2016, 138, 14380−14387. (12) Shen, D.; Yu, X.; Cai, X.; Peng, M.; Ma, Y.; Su, X.; Xiao, L.; Zou, D. Understanding the

Solvent-Assisted Crystallization Mechanism Inherent in Efficient Organic–Inorganichalide Perovskite Solar Cells. J. Mater. Chem. A, 2014, 2, 20454–20461. (13) Lv, M.; Dong, X.; Fang, X.; Lin, B.; Zhang, S.; Ding J.; Yuan, N. A Promising Alternative

Solvent of Perovskite to Induce Rapid Crystallization for High-efficiency Photovoltaic Devices. RSC Adv., 2015, 5, 20521–20529.


16

Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Gritzner, G. A Critical View on the Lewis-Donor (Nucleophilic) Properties of Solvents. J.

Mol. Liq., 1997, 73, 487-500. (15) Persson, I.; Lyczko, K.; Lundberg, D.; Eriksson, L.; Placzek, A.; Coordination Chemistry

Study of Hydrated and Solvated Lead (II) Ions in Solution and Solid State. Inorg. Chem., 2011, 50, 1058–1072. (16) Umebayashi, Y.; Matsumoto, K.; Watanabe M.; Ishiguro S. Individual Solvation Number of

First-row Transition Metal (II) Ions in Solvent Mixtures of N, N -Dimethylformamide and N, N –Dimethylacetamide–Solvation Steric Effect. Phys. Chem. Chem. Phys., 2001, 3, 54755481. (17) Umebayashi, Y.; Matsumoto, K.; Mekata, I.; Ishiguro, S. Solvation Structure of

Lanthanide(III) Ions in Solvent Mixtures of N, N-Dimethylformamide and N, NDimethylacetamide Studied by Titration Raman Spectroscopy. Phys. Chem. Chem. Phys., 2002, 4, 5599–5605. (18) Ishiguro, S. i.; Umebayashi, Y.; Kato, K.; Takahashi, R.; Ozutsumi, K. Strong and Weak

Solvation Steric Effects on Lanthanoid(III) Ions in N, N-dimethylformamide -N, NDimethylacetamide Mixtures. J. Chem. Soc., Faraday Trans., 1998, 94, 3607-3612. (19) Song, X.; Wang, W.; Sun, P.; Ma, W.; Chen, Z. K. Additive to Regulate the Perovskite

Crystal Film Growth in Planar Heterojunction Solar Cells. Appl. Phys. Lett., 2015, 106, 033901. (20) Liao, H. C.; Guo, P.; Hsu, C. P.; Lin, M.; Wang, B.; Zeng, L.; Huang, W.; Soe, C. M. M.; Su,

W. F.; Bedzyk, M. J.; Wasielewski, M. R.; Facchetti, A.; Chang, R. P. H.; Kanatzidis, M. G.; Marks, T. J. Enhanced Efficiency of Hot-Cast Large-Area Planar Perovskite Solar


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

Cells/Modules Having Controlled Chloride Incorporation. Adv. Energy Mater., 2017, 7, 1601660. (21) Marco, N. D.; Zhou, H.; Chen, Q.; Sun, P.; Liu, Z.; Meng, L.; Yao, E.-P.; Liu, Y.; Schiffer,

A.; Yang, Y. Guanidinium: A Route to Enhanced Carrier Lifetime and Open-Circuit Voltage in Hybrid Perovskite Solar Cells. Nano Lett., 2016, 16, 1009−1016. (22) Jungbanger,

M. A.; Curran, C. N-C=O Bending Vibration in Complexes of

Dimethylformamide with Metal Halides. Nature, 1964, 202, 290. (23) Wakamiya, A.; Endo, M.; Sasamori, T.; Tokitoh, N.; Ogomi, Y.; Hayase, S.; Murata, Y.

Reproducible Fabrication of Efficient Perovskite-Based Solar Cells: X-ray Crystallographic Studies on the Formation of CH3NH3PbI3 Layers. Chem. Lett., 2014, 43, 711–713. (24) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR Chemical Shifts of Common Laboratory

Solvents as Trace Impurities. J. Org. Chem., 1997, 62, 7512-7515. (25) Kim, B. J.; Kim, D. H.; Kwon, S. L.; Park, S. Y.; Li, Z.; Zhu, K.; Jung, H. S. Selective

Dissolution of Halide Perovskites as a Step towards Recycling Solar Cells. Nat. Commun., 2016,7,11735. (26) Ko, H.; Sin, D. H.; Kim, M.; Cho, K. Predicting the Morphology of Perovskite Thin Films

Produced by Sequential Deposition Method: A Crystal Growth Dynamics Study. Chem. Mater., 2017, 29, 1165−1174. (27) Zheng, H.; Wang, W.; Yang, S.; Liu, Y.; Sun, J. A Facile Way to Prepare Nanoporous PbI2

Films and Their Application in Fast Conversion to CH3NH3PbI3. RSC Adv., 2016, 6, 1611– 1617.


18

Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Wu, C.; Zheng, X.; Yang, Q.; Yan, Y.; Sanghadasa, M.; Priya, S. Crystallization of

HC(NH2)2PbI3 Black Polymorph by Solvent Intercalation for Low Temperature Solution Processing of Perovskite Solar Cells. J. Phys. Chem. C, 2016, 120, 26710−26719. (29) Bae, S.; Han, S. J.; Shin, T. J.; Jo, W. H. Two Different Mechanisms of CH3NH3PbI3 Film

Formation in One-Step Deposition and Its Effect on Photovoltaic Properties of OPV-Type Perovskite Solar Cells. J. Mater. Chem. A, 2015, 3, 23964 –23972. (30) Zhang, T.; Yang, M.; Zhao, Y.; Zhu, K. Controllable Sequential Deposition of Planar

CH3NH3PbI3 Perovskite Films via Adjustable Volume Expansion. Nano Lett., 2015, 15, 3959− 3963. (31) Noel, N. K.; Abate, A.; Stranks, S. D.; Parrott, E. S.; Burla, V. M.; Goriely, A.; Snaith, H. J.

Enhanced Photoluminescence and Solar Cell Performance via Lewis Base Passivation of Organic Inorganic Lead Halide Perovskites. ACS Nano, 2014, 8, 9815–9821. (32) Zhang, Y.; Liu, M.; Eperon, G. E.; Leijtens, T. C.; McMeekin, D.; Saliba, M.; Zhang, W.;

Bastiani, M.; Petrozz a, A.; Herz, L. M.; Johnston, M. B.; Lin, H.; Snaith, H. J. Charge Selective Contacts, Mobile Ions and Anomalous Hysteresis in Organic–Inorganic Perovskite Solar Cells. Mater. Horiz., 2015, 2, 315–322. (33) Li, L.; Chen, Y.; Liu, Z.; Chen, Q.; Wang, X.; Zhou, H. The Additive Coordination Effect

on Hybrids Perovskite Crystallization and High-Performance Solar Cell. Adv. Mater., 2016, 28, 9862–9868.


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Figures

Figure 1. Surface (top), cross-sectional (middle) SEM images and the distribution of grains size (bottom) of the perovskite films fabricated from PbI2/DMF solution with different concentration of DMA. (a), (d), and (g) are without DMA; (b), (e), and (h) are with 5% DMA; (c), (f), and (i) are with 10% DMA.


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Figure 2. FTIR spectra of the DMF (a), DMA (b) solvents and their corresponding PbI2-based Lewis acid-base adducts. (c) TGA curves of the Lewis adducts of PbI2 ·xDMF and PbI2·xDMA. (d) NMR spectra of pure DMF, pure DMA, and PbI2/DMF solutions with 5%DMA additives.

Figure 3. (a) XRD patterns of the perovskite films fabricated from different solutions. (b) Evolution of the XRD patterns of the MAPbI3 film at different crystallization stages.


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Scheme 1. Illustration of the Lewis base solvent modulating crystallization of the MAPbI3 at different stages. (a) PbI2 crystals, (b) PbI2-based Lewis adducts, (c) MAI-PbI2-Sol complex, (d) MAPbI3.


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Figure 4. SEM images of the perovskite films fabricated from different conditions. Top is from solutions with different DMA additives, (a) 0% DMA (b) 5% DMA, (c) 10% DMA; middle is from solution with 5% DMA and annealed for 10 min at different temperature, (d) 40°C, (e) 80°C, (f) 120°C; bottom is from solution with 10% DMA and annealed at 120 °C for different time. (g) 0s, (h) 30s, (i) 120s. Insets are distribution of grain size in the corresponding samples.


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Figure 5. (a) Cross-sectional SEM images of the typical perovskite solar cell (PSC) fabricated from 5% DMA. (b) J-V curves of the best PSCs fabricated from different solutions. (c) The stabilized outputs of the PSCs at the maximum power points. (d) Typical IPCE spectra of the PSCs.


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Table 1. The photovoltaic parameters of best PSCs obtained from J-V curves Sample

PCE

Rs

RSH

(%)

(Ω cm2)

(Ω cm2)

0.61

12.32

6.8

1200

1.02

0.70

16.05

3.9

1200

1.02

0.63

13.60

6.4

1500

Jsc

Voc

(mA cm-2)

(V)

0% DMA

20.41

0.99

5% DMA

22.49

10% DMA

21.10

FF


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Table of Contents


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Elucidating the Key Role of a Lewis Base Solvent in the Formation of

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