A Review of the Role of Solvents in Formation of High-Quality Solution

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Review

A Review of the Role of Solvents in Formation of High Quality Solution-Processed Perovskite Films Xiaobing Cao, Lili Zhi, Yi Jia, Yahui Li, Ke Zhao, Xian Cui, Lijie Ci, Daming Zhuang, and Jinquan Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16315 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

A Review of the Role of Solvents in Formation of High Quality Solution-Processed Perovskite Films Xiaobing Cao1, Lili Zhi2, Yi Jia3, Yahui Li1, Ke Zhao1, Xian Cui 1, Lijie Ci 2, Daming Zhuang1, Jinquan Wei 1* 1. State Key Lab of New Ceramic and Fine Processing, School of Materials Science and Engineering, Key Laboratory for Advanced Materials Processing Technology (Ministry of Education), Tsinghua University, Beijing 100084, P.R. China 2. School of Materials Science & Engineering, Shandong University, Jinan 250061, Shandong, P.R. China 3. Qian Xueshen Laboratory of Space Technology, Youyi Road No. 104, Haidian District. Beijing 100094, P.R. China *Corresponding Author. E-mail: [email protected]. Phone: +86-10-62781065.

1 Environment ACS Paragon Plus

ACS Applied Materials & Interfaces 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

ABSTRACT Recently, perovskite solar cells have attracted great attention due to their outstanding photovoltaic performance and ease of fabrication. High quality perovskite films hold a key in getting highly efficient perovskite solar cells. Solution-processed fabrication technique is the most widely adopted for preparing perovskite films owing to its low cost. In the solution-proceed perovskite films, solvents not only play the roles to dissolve the solute, but also participate the crystallization of perovskite. In the one-step method, solvents play key roles in controlling morphology, widening process window and achieving room-temperature crystallization of perovskite films. In Addition, the solvents also play important roles in controlling the nuclei/growth, suppressing volume expansion during the two-step method. Especially, the solvent can induce grain coarsening during annealing process. A deep understanding the multiply of roles during the formation of perovskite films will help understand the formation mechanism of perovskite films. Here, a systematic review on the progress in fabrication of high quality perovskite films by making use of solvent to control the crystallization are presented. Meanwhile, we elucidate the key roles of solvent in fabrication of high quality perovskite films. KEYWORDS: solution-proceed technique; solvent; perovskite film; crystallization; growth mechanism;


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Content 1. Introduction ................................................................................................................................. 4 2. The roles of solvent in one-step method ..................................................................................... 5 2.1 One-step method ...................................................................................................................5 2.2 Solvent engineering...............................................................................................................6 2.3 Solvent-solvent extraction approach ..................................................................................... 11

3. The roles of solvent in two-step method ................................................................................... 15 3.1 Two-step method ................................................................................................................ 15 3.2 Fabrication of mesoporous PbI2 films ................................................................................... 16 3.3 Intramolecular exchange approach ....................................................................................... 20 3.4 Lewis acid-base adduct approach ......................................................................................... 22

4. The roles of solvents in annealing............................................................................................. 25 4.1 External solvent induced solvent annealing ........................................................................... 25 4.2 Intrinsic solvent mediated Ostwald ripening .......................................................................... 27

5. Summary and Perspectives ....................................................................................................... 30

Abbreviations list: MA, methylammonium TBP, 4-tert -butyl-pyridine FA, formamidinium HMPA, hexamethylphosphoric triamide PCE, power conversion efficiency IEP, intramolecular exchange PSCs, perovskite solar cells J sc, short-circuit current density DMF, N,N-dimethylformamide Voc, open-circuit voltage DMSO, dimethyl sulfoxide FF, fill factor GBL, gamma-butyrolactone Sol, solvent NMP, N-Methyl pyrrolidone DN, donor number DMAC, Dimethylacetamide ∆E a, activation energy IPA, 2-propanol N 2, Nitrogen SSE, solvent–solvent extraction CB, chlorobenzene XRD, X-ray diffraction SEM, scanning electron microscope AFM, atomic force microscope FTIR, Fourier transform infrared spectrometer TGA, thermal gravimetric analysis ToF-SIMS, time-of-flight secondary ion mass spectrometry



1. Introduction Organic–inorganic hybrid perovskites, with a formula of APbX3 (A=methylammonium (MA), formamidinium (FA); X=Cl, I, Br), have induced a research boom in materials science due to its promising applications in photovoltaic devices. The last few years have witnessed the unprecedented rapid development of solar cells utilizing lead halide perovskites as light-absorbing layers. The power conversion efficiency (PCE) of the perovskite solar cells (PSCs) have significantly increased from 3.8% to 23.3% within a few years.1-2 High quality perovskite films, characterized by full coverage, smooth surface, large columnar grains with excellent crystallization, play a decisive role in fabricating highly efficient PSCs. In order to obtain highly efficient PSCs, various methods have been developed to prepare perovskite films, such as one-step method,3 twostep method,4-5 dual source co-evaporation,6 vapor-assisted solution process,7 vacuum assisted deposition method,8 and solvent-and vacuum free routing.9 Among these methods, the one-step and two-step methods are the most widely used to prepare perovskite films due to its low cost and ease of fabrication. In the solution-processed methods, the solvents not only dissolve solutes, but also participate in the growth of perovskite. The solvents play multiple roles in controlling nuclei/growth,10-11 retarding reaction rate,12 and coarsening grains 13 during the formation of high quality perovskite films. Although, great progress have been made in fabricating high quality perovskite films via solution-processing method, few researchers pay attention to the roles of solvents in the crystallization process of perovskite. Here, we review the recent progress in fabricating high quality perovskite films through solution-processing, and present fundamental understanding on the functions of solvent in forming perovskite films fabricated from solution spin-coating process. Firstly, we make a review on the progress in one-step method for fabricating high quality


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perovskite films, and elucidate the key roles of solvent in controlling morphology, widening process window and achieving room-temperature crystallization of perovskite films. Then, we discuss on the different strategies to overcome the shortcomings in the traditional two-step method, and elucidate the properties of solvents in controlling the nuclei/growth, suppressing volume expansion and controlling the dissolution-recrystallization process during the annealing. Finally, we reveal the concealed coarsening function of solvent during the transformation from solvate complex to perovskite films during annealing process.

2. The Roles of Solvent in One-Step Method 2.1 One-Step Method In the typical one-step method, a mixture of PbX2 (X=Cl, I, Br) and AX (A=MA, FA) is firstly dissolved into a polar aprotic solvent like N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), gamma-butyrolactone (GBL) or N-Methyl pyrrolidone (NMP) to form a precursor solution. The solution is then spin-coated onto a substrate to form a perovskite precursor film, which subsequently converts into perovskite film by removing the residual solvent through annealing.14 In the typical one-step method, the perovskite films usually exhibit needle-like morphology with incomplete coverage due to the fast growth and low nucleation rate of precursor crystals.15 The incomplete coverage of the perovskite films not only decrease the effective area of the light harvest layers, but also induce direct contact between the hole transform layer and electron transform layer, leading to high recombination rate of carriers in PSCs. Meanwhile, the formation of perovskite fabricated from one-step method is very sensitive to the experiment conditions, such as the annealing time,16 annealing temperature,17 solution concentration,18-19

precursor

composition20-21 and the properties of solvent.22-25 As a result, it is difficult to fabricate highly efficient PSCs through the traditional one-step method. It provides an easy way to fabricate



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perovskite film in the early research on PSCs. However, there are few reports on using the traditional one-step method to fabricate the PSCs by now.

2.2 Solvent Engineering The incomplete coverage problem in traditional one-step method impedes its application in fabricating highly efficient PSCs. In order to overcome the shortcoming, Jen et al26 firstly developed a solvent engineering approach to prepare smooth perovskite films with full coverage. Figure 1a depicts a schematic illustration of the solvent engineering procedures for preparing perovskite films. Typically, PbX2 and MAX (X=I, Br,) are dissolved in a mixed solvent of GBL and DMSO (mother solvent) to prepare perovskite solution. Perovskite films were are deposited on substrates by spin-coating the mixed solution, followed by a toluene drip (washing solvent) while spinning. Toluene drip leads to rapid formation of a homogenous transparent intermediate film via quick removal of the excess DMSO solvent. Then, the intermediate film converts to a perovskite film through annealing treatment. Figure 1b shows the phase transformation in fabricating perovskite films by the solvent engineering approach. It forms an intermediate phase of PbI2-MAI-DMSO after dripping the toluene on the casted precursor film during spin-coating. The intermediates transform to CH3NH3PbI3 (MAPbI3) by removing the solvent through annealing. Yao et al.

27, 28

studied the

crystal structure of the intermediate formed in the solvent engineering. They found that the intermediates are ribbon-like chains of [Pb3I8]2-, which form from edge-shared lead iodide octahedra (PbI6). The [Pb3I8]2− ribbons can be depicted as excised ribbons from the layered PbI 2 structure with widths of 3 octahedral PbI6 units. The methylammonium (MA+) and DMSO molecules are located in spaces between the ribbons. Therefore, the intermediates can be expressed as MA2Pb3I8∙2DMSO. There are three characteristic diffraction peaks centered at 6.61°, 8.12°and


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9.63°in XRD curves of the intermediates, corresponding to (002), (021) and (022) planes of the intermediates, respectively. Solvent engineering is a key breakthrough in fabricating high quality perovskite films and highly efficient PSCs. Compared with those perovskite films fabricated from the traditional onestep method, the perovskite films prepared from solvent engineering exhibit uniform, full coverage and smooth compact morphology with grain size ranging from 100 to 500 nm (see Figure 1c). The root mean square roughness for the perovskite film is only 8.3 nm as shown in atomic force microscope (AFM) image (see Figure 1d). As a result, the PSCs basing on the high quality perovskite films result in a certificated efficiency of 16.2%. Figure 1e shows comparison of the morphology of perovskite films fabricated from different solvent system. It clearly shows that the perovskite films exhibit large grains with incomplete coverage when the DMSO is excluded from the precursor solutions (see upper in Figure 1e). It is noted that the incomplete coverage morphology is also observed in the perovskite films fabricated from the precursor solution with DMSO, but without toluene drips during spinning process (see bottom left in Figure 1e). Dense perovskite films with full coverage can be obtained when the DMSO is introduced into perovskite precursor solution, and toluene drips are applied during the spinning process (see bottom right in Figure 1e). The as-casted films fabricated from precursor solutions with DMSO shows transparent color. However, the casted film fabricated from pure GBL shows a brown color, which is close to the color of perovskite films. The difference in color indicates that the role of DMSO in MAI-PbI2-DMSO is to retard the reaction rate between MAI and PbI2 during the spinning. These results indicate that a rational selection for the mother solvent and washing solvent holds a key for controlling the morphology of perovskite films. Three basic rules should be considered to select solvent in solvent engineering approach. Firstly, the ligand



solvent (e.g. DMSO) should have strong coordination ability to form a stable intermediate film, because the ligand solvent in intermediate film can retard the reaction rate between MAI and PbI2. Additional, the ligand solvent should have low solubility in wash solvent, which helps to obtain accurate composition of intermediate film with good reproducibility. Thirdly, the non-ligand solvent (e.g. GBL) should have a high solubility in wash solvent (e.g. toluene). The high miscibility ensure the fast extraction of GBL by toluene in a very short time, resulting in a uniform and transparent intermediates layer. To clearly understand the key roles of solvent in controlling the morphology of perovskite films in the solvent engineering approach, it needs to deeply investigate the nucleation/growth process of the perovskite. Here, we use the Lamer model, which is usually used to describe the nucleation and growth process of crystals, 11, 29-31 to clarify the formation process of perovskite grains in solvent engineering approach. Figure 1f is a Lamer model curve, which describes the concentration change of perovskite precursor solution as the spinning-coating processed. The Lamer curve describes the formation process of crystal with three different stage: (I) Prenucleation; (II) Burst-nucleation; (III) Growth by diffusion. In the stage I, no nuclei generate from the solution because they should overcome the energy barrier for spontaneous homogeneous nucleation. In the stage II, some stable nuclei generate accompanied with the growth of the nuclei at the same time when the concentration of solution reaches to a critical value (C c) to overcome the energy barrier for nucleation. As the solvent evaporate continuously, the concentration of solution increases, which promote the growth of nuclei and regeneration of new nuclei. At the same time, the concentration of the solution decreases due to its continuous consumption of solute during nucleation and growth process. As a result, the concentration decrease to C c, at which no nuclei generate in the solution. Below the concentration Cc, the system enters to the stage III, in


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which no new nuclei generate and the existing nuclei keep growing as long as the concentration is above Cs . In solvent engineering approach, a typical mixture of GBL and DMSO used as mother solvent to dissolve MAI and PbI2. Toluene is used as washing solvent to remove the mother solvent due to its miscibility with both DMSO and GBL, but insolubility with the perovskite precursor. In the perovskite solution, the perovskite precursors are generally dispersed as colloidal particles in mother solvent. The colloidal particles may acts as nucleation seeds during the formation of perovskite films.

32-35

For the perovskite films prepared from solvent engineering approach, the

liquid perovskite solution condensed to be film as spinning processed. Meanwhile, the solution concentration will increase due to the evaporation of solvent (stage I). When the concentration increases to Cc, the solution system enters stage II. In this stage, some perovskite intermediate crystals nucleate on the substrate. When toluene drips onto the concentrated films, it leads to a fast supersaturating state in the cast films due to the fast extraction of the extra DMSO by toluene. With the assistance of toluene, the concentration is always maintained above C c, resulting in more crystals nucleate on the substrate until they fully cover the substrate (see Figure 1g). As the consumption of solute, the concentration of solution drops to the region below Cc, the existing nuclei only grow up, and no new nuclei generate (stage III). As a result, densely packed perovskite films with full coverage are obtained via solvent engineering (Figure 1c). In the traditional onestep method, after the nuclei are generated, the nuclei will grow rapidly, leading to fast solute consumption. Because there is no extraction process of DMSO by washing solvent, the concentration reduced to Cc but large than Cs , as shown in the blue part in Figure 1h. In the later stage, there are no new nuclei generated but the grains will keep grow. Therefore, the amount of nuclei in the traditional one-step method is much less than that in solvent engineering approach



(Figure 1h). As a result, the perovskite films fabricated from traditional one-step method usually exhibit large grains with incomplete coverage (see left column in Figure 1e). As discussed above, the formation process of perovskite films demonstrates that the washing solvent (e.g. toluene) and mother solvent (e.g. DMSO) have important roles in increasing the nuclei number and retarding the reaction rate during the formation of perovskite films in the solvent engineering approach. As the mother solvent is extracted by the washing solvent, the concentration of solution always maintains at above Cc during spin process. The high concentration above Cc provides chance to generate new nuclei. Owing to the introduction of DMSO into the precursor solution, the obtained film after dropping by toluene shows transparent color rather brown color, indicating that DMSO retards the reaction rate between MAI and PbI 2 due to its strong coordination with PbI2. Therefore, the full-covered perovskite films fabricated from engineering approach can be ascribed to the increase in nuclei number and the reduction in growth rate of perovskite grains. In the solvent engineering approach, it is noted that the washing solvent (toluene) can wash not only GBL but also DMSO due to its miscibility between toluene and DMSO. The extraction of DMSO from intermediates of MAI-PbI2-DMSO makes it difficult to decide the accurate composition of MAI-PbI2-DMSO. As a result, the random composition of the intermediates reduce the reproducibility during the fabrication of perovskite films. In order to obtain the same intermediates reproducibility, Park`s group36 dissolved PbI2 and MAI into mixture solvent of DMF and DMSO with accurate mole ration. They used diethyl ether as wash solvent in the solvent engineering during spinning. They obtained an intermediate of MAI∙PbI2∙DMSO with accurate stoichiometric ratio of 1:1:1 because diethyl ether dissolves only DMF. As a result, they achieved


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high efficient perovskite solar cells with an average PCE of 18.3% and the best PCE of 19.7% reproducibly. The solvent engineering approach is a widely adopted laboratory-scale spin-coating method to prepare high quality perovskite films in the past several years due to its fast fabrication process. However, the time window for adding the washing solvent (e.g. toluene) is narrow, which impedes its application for scalable highly efficient PSCs. It also need an annealing process to remove the solvent, which increases the cost for the commercialization of PSCs.

2.3 Solvent-Solvent Extraction Approach One of the shortcoming of solvent engineering approach is that it requires a thermal annealing treatment to remove the solvent during the transformation from the intermediate to perovskite film. However, the thermal annealing process is a time-consuming and energy-costing process, which make solvent engineering approach to be an uneconomical method. In Addition, thermal annealing will induce lattice strain deriving from the mismatch between perovskite and substrates during the heating and cooling treatment, which accelerates the degradation of perovskite solar cells.37 In order to simplify the fabrication process and reduce the production cost of the perovskite solar cells, Padture et al. 38 proposed a solvent-solvent extraction (SSE) conception to induce fast crystallization of high quality perovskite films at room temperature. Figure 2a shows a schematic illustration of the solvent-solvent extraction process. They prepared a perovskite solution by dissolving the PbI2 and MAI (molar ration=1:1) in pure NMP solvent. The solution was then spincoated onto a substrate to form perovskite precursor films, followed by immediately immersing into a diethyl ether bath for 2 min. Then, the film was then taken out of the bath, and dried at room temperature without further thermal annealing. They obtained a uniform coverage and smooth



perovskite films with grains size of ~100 nm via solvent-solvent extraction process (see Figure 2b). By optimizing the thickness of perovskite films, they fabricated PSCs with an efficiency of 15.2%. Figure 2c shows the evolution of XRD curves of the films fabricated from the solventsolvent extraction process at different stages. The XRD curve of the as-spin-coated film exhibits no evident diffraction peaks, indicating that there are some residual NMP embedded in the film. Upon contacting with diethyl ether for only 2 s, the characteristic peaks of perovskite were detected, indicating the formation of perovskite after NMP-diethyl ether extraction. As the elongation of extraction time to 2 min, the characteristic peaks of perovskite films become sharp, indicating the improvement in crystallization of perovskite films. The extraction of NMP in the perovskite precursor films was also confirmed by the disappearance of C=O stretching band in FTIR spectrum (located at 1700 cm-1) when the film was immersed into a diethyl ether bath for 2 min (see Figure 2d). In their work, they also proposed a possible mechanism for the perovskite films fabricated from solvent-solvent extraction process, as shown in Figure 2e. When the perovskite precursor films were dipped into the diethyl ether bath, the residual NMP was rapidly extracted from the perovskite precursor films due to its high miscible in diethyl ether. It is believed that the extraction of NMP first happened at the localized patches of films, where there were variations in composition of the films and it most likely to in contact with the diethyl ether. The direct contact between NMP and diethyl ether triggered a rapid crystallization of perovskite films, and the crystallization spreads to the whole films as the continuous extraction of NMP by diethyl ether in a short time. Since the perovskite is insoluble in diethyl ether solvent, the perovskite film was well kept at room temperature. Finally, diethyl ether volatilize easily at room temperature due to its low boiling point (35 °C). Ding et al.39 studied the co-solvent effects on the formation of perovskite films. They dissolved perovskite precursors into mixture of DMAC/NMP, and fabricated PSCs with a high


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efficiency of 17.09% without annealing, which is very close to those through annealing (17.38%). These results demonstrate that it is feasible to prepare highly efficient PSCs without annealing process via choosing appropriate solvent system. Obviously, the high miscibility between mother solvent (e.g. NMP) and extraction solvent (e.g. diethyl ether) enable rapid crystallization of perovskite films. The easy volatility of extract solvent (e.g. diethyl ether) eliminates the annealing treatment for removal of excess solvent. The annealing free advantage make it is potentially suitable for roll-to-roll scalable technology for flexible PSCs. Another challenge for solvent engineering is the narrow time for adding the washing solvent, which makes it difficult to be compatible with the scalable methods. In order to make the solventsolvent extraction approach to be compatibility with the scalable deposition technology, it is necessary to widen processing window for the scalable highly efficient solar cells with good reproducibility. Zhu et al.40 extended the processing window to several minutes (~8 min) by tuning the solvent compositions for preparing solutions. They prepared various precursor solutions by dissolving the solute into pure DMF, DMF/DMSO mixture (8:9 v:v), DMF/NMP mixture (8:9 v:v), respectively. The perovskite precursor films were fabricated by spinning or blading coating onto substrates from different mixed solvents. The perovskite precursor films were transferred to diethyl ether bath for 30 s to finish solvent-solvent extraction process. After drying by nitrogen flow, the films were annealed at 150 °C to form the final perovskite films. They studied the effect of delaying time, which is defined as the time between the precursor film being blade-coated/spin-coated and the solvent bathing step, on the average absorbance (450-700 nm) of the films. For the pure DMF, the average of absorbance started to increase almost immediately after the coating step. This is consistent with rapid color change from being clear film to haze, which is close to the color of perovskite films. However, for the mixture solvent of DMF/DMSO and DMF/NMP, the processing



window times for the precursor films are extended to 2 min and 8 min, respectively (see Figure 2f). Figure 2g shows the effect of the delaying time on the PCE of the solar cells fabricated from different solvent composition. It is obvious that the PCE of solar cells is insensitive to the delaying time in a wide range from 0.5 min to 6 min when the DMF/NMP mixture is used. This wide processing window time provide a possibility for scalable fabrication of high quality perovskite films with good reproducibility. In contrast, the PCE depend strongly on the delaying time by using other two mixed solvents. The PCE drops quickly (especially for pure DMF) as the elongation of delaying time, which is consistent with the time window before the optical properties of precursor films undergo a drastic changes (scaled by the average absorbance). Obviously, the evaporation rate of solvent strongly depends on the vapor pressure of solvent. Among the common used solvents, DMF has the highest vapor pressure at room temperature (2.7 mmHg). 40 Therefore, DMF evaporates quickly during/after the spin-coating, which explains the narrow time of wet-film stage from pure DMF. However, DMSO and NMP have relative low vapor pressure of only 0.42 and 0.29 mmHg, respectively.40 The introduction of DMSO and NMP into DMF can retard the evaporation of the solvent during/after spin-coating, extending the processing windows. Therefore, the low vapor pressure induced a slow evaporation rate should be responsible for the wide processing window in the mixture solvent system. In this work, they also demonstrated that a fast annealing process (~1 min) with flexibility (1~10 min) can also be achieved by added 15% excess MACl into the perovskite solution. These results demonstrate that an appropriate selection of solvent along with composition tuning in perovskite solution enable a wide processing window and flexibility annealing process in solvent-solvent extraction method. These advantages make this method is attractive for future scalable device fabrication during the commercialization of PSCs.


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3. The Roles of Solvents in Two-Step Method 3.1 Two-Step Method Two-step method is widely adopted in fabricating high quality perovskite films due to its full coverage and excellent reproducibility. In the typical two-step method, PbI2 films are first deposited onto substrates by spin-coating from solution of PbI2/DMF followed by annealing at high temperature (first step). The PbI2 films are then transformed to MAPbI3 by reacting with MAI (second step).4-5 The formation of perovskite film can be regarded as a dynamic intercalation of MAI into the PbI2 lattices.41 After intercalation, the volume of unit cell is expanded from 124 Å3 to 248 Å3, which is about two times larger than that of PbI2.42 The volume expansion is further confirmed by the density of PbI2, MAI, and MAPbI3, which is 6.26, 4.29, and 4.16 g cm-3, respectively.43 During the interaction, some small grains might extrude to the surface of the perovskite films, leading to a rough surface. 44 Meanwhile, the transformation of PbI2 to MAPbI3 accomplishes through a solid-liquid diffusion reaction.45 In the two-step method, MAI diffuses and intercalates into the PbI2 lattice from surface of PbI2 films. The MAPbI3 might obstruct the diffusion of MAI in the inner of the PbI2, which restrain the transformation from PbI2 to MAIPbI3 gradually. As a result, there are usually some residual PbI2 in the perovskite films, which is harmful for efficiency, reproducibility,46 and stability 47-48 of PSCs. Schlipf et al. 49 investigated the inner structure of perovskite films during the transformation from dense PbI2 to perovskite by grazing incidence small-angle X-ray scattering. They demonstrated that the perovskite grains mainly grow along vertical direction due to lateral confinement by substrates. The anisotropic growth would lead to significant strain within crystals, which leads to cracking of grains near the substrates. As a result, there are much smaller grains and grain boundary inside the perovskite films. The volume



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expansion, residual PbI2 and the strain occurred in two-step method, making it difficult to prepare high quality perovskite films and highly efficient PSCs.

3.2 Fabrication of Mesoporous PbI2 Films Fabrication of mesoporous PbI2 is an effective way to overcome the problems in two step method. The pores in the PbI2 films not only provide diffusion channels for MAI, but also accommodate the volume expansion and strain during the transformation from PbI 2 to MAPbI3. Therefore, the pore in PbI2 film facilitate getting smooth perovskite films in a short reaction time. Up to now, various methods have been proposed to obtain mesoporous PbI2 films.48,

50-54

For

example, Wong et al.48 synthesized porous PbI2 films from a mixture of Pb(CH3OO)2 and MAI with a molar ration of 2:1. Abundant pores form during the reaction between Pb(CH 3OO)2 and MAI by releasing methylammonium acetate. The pores and grain size of the PbI2 film can be modulated by the annealing temperature. Li et al.51 prepared porous PbI2 films on the PbPc films, which is used as hole transport layer in the inverted perovskite solar cells. Mesoporous PbI2 films can also be fabricated by controlling nucleation/growth process during the crystallization of PbI2 film. It forms a complex of PbI2∙xSol when PbI2 solution is spin-coated from solutions.55-57 In the case of DMF, it forms a PbI2∙xDMF film as the DMF evaporate during spin-coating.56,

58-59

Because there are some unbounded solvent in the fresh films, we use x (x>1)

rather the precise stoichiometric ratio to express the complex films before annealing in the following discussion. When the solvate complex of PbI2∙xDMF are annealed at high temperature, it transforms to PbI2 by releasing the DMF molecules. There are dissolution-recrystallization process accompanied with volume shrink during annealing. The volume shrink can be evidenced by large difference in unit cell volume and density between solvate complex of PbI2∙xDMF and PbI2. For the solvent of DMF, the density increase from 3.718 g cm-3 to 6.260 g cm-3 accompanied


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with the unite cell volume shrinking from 954.2 Å3 to 124 Å3 when PbI2∙xDMF transforms to PbI2.56-57 It is noted that solvent embedded in the solvate complex can intrigue a dissolutionrecrystallization during the annealing, which is proved to be a solvent-mediated Ostwald ripening process in our previous works.60, 61 Due to the existence of DMF in the PbI2∙xDMF films, the PbI 2 grain will be dissolved by DMF. It is noted that smaller grain is more energetically unstable than the large grain owing to its higher chemical potentials. Therefore, the concentration of the dissolved components near the smaller grains is higher than those near the large grains (namely dissolution). The dissolved component will transport from the small grains to the large grains via mass transportation (namely recrystallization process). The dissolution-recrystallization process leads to obvious growth of PbI2 grains. There are good grounds for believing that it leads to some cracks and/or voids in PbI 2 films if the films are random stacked by large gains, which results in mesoporous PbI2 films. It is noted that DMF molecule escape quickly during the spinning and annealing process due to its low boiling point (152°C) and weak coordination interaction ability with PbI2.62-63 The quick evaporation of DMF lead to small grains and relative compact morphology. In order to obtain larger PbI 2 grains, Liu et al.64 developed a time-dependent controlled growth of mesoporous PbI2 film approach. They first prepared PbI2∙xDMF film onto substrates by spin-coating the PbI2/DMF solution, and then kept the PbI2∙xDMF film in a closed Petri dish for different times ranging from 0 min to 9 min, as illustrated in Figure 3a. The surface morphology evolution of the PbI2 films for different time is shown in Figure 3b. It clearly shows that both grain size and the void in the films became larger as the annealing time elongates. The random stack of the large PbI2 grains leads to more voids in the films. The mean size of the voids increases from 12 nm to 113 nm, and the fraction of voids in the whole films increase from 3.1% to 15.8% when the growth time elongates from 0 min to 7 min.



The large size and fraction of voids in the mesoporous PbI2 films provide efficient contact between MAI and PbI2 when the PbI2 films are exposed to MAI/IPA solution, resulting in accelerating the formation rate of perovskite films and enhancement of photovoltaic performance of PSCs. Chen et al. adopted two-step method to produce mesoporous PbI2 films. In the time-dependent controlled growth of mesoporous PbI2 film, this process is similar to the DMF solvent induced the growth of MAPbI3 as proposed by Huang`s group.65 A closed Petri dish acts as a container to restrain the escape of DMF. The restrained DMF induces an obvious solvent annealing effect, resulting in the growth of the large PbI2 grains. The preparation of mesoporous PbI2 is an effective way to fabricate efficient planar PSCs. Mesoporous PbI2 films also can be prepared by using solvent–solvent extraction (SSE) method.56,

66-69

Figure 3c illustrates a typical SSE method for fabricating mesoporous PbI2 films.

PbI2∙xDMF films are deposited onto the substrates from PbI2/DMF solution, followed by IPA drops while the substrates is spinning. The as-prepared film turns from pale yellow (PbI2∙xDMF) to dark yellow (PbI2) immediately, indicating rapid formation of PbI2 films due to the fast extraction of DMF by IPA in a very short time. The morphology of PbI2 film fabricated from SSE are given in Figure 3d and 3e. It clearly shows that the PbI2 films exhibit mesoporous features from surface to inside of the film along the thickness direction. In the solvent–solvent extraction method for mesoporous PbI2 films, because DMF molecule have a very high solubility in IPA, it easily diffuses to the IPA when IPA drops onto the PbI2∙xDMF film. As discussed previously, there are obvious volume shrink when PbI 2∙xDMF transforms to PbI2 film. The volume shrink during the transformation will lead to some cracks on the surface of PbI2 films. The cracks facilitates the penetration of IPA into the films. Therefore, the solventsolvent extraction proceeds iteratively until IPA penetrates into the bottom of the films (see Figure


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3f). Basing on the formation process of mesoporous PbI2 fabricated by SSE methods, some general rules can be summarized for the selection of solvent in this approach. Firstly, there are high miscibility between the precursor solvent (e.g. DMF, DMSO) and extraction solvent (e.g IPA, chlorobenzene (CB)), which enable the extraction process finish in a very short time. Secondly, PbI2 has low solubility in the extraction solvent, which eliminates disappearance of PbI 2 films during the extraction process. Thirdly, the precursor solvent for the dissolution of PbI 2 should have relative high boiling point, which prevent the solvent evaporation induced crystallization of PbI 2 during the spin coating. Additive engineering is another effective way to fabricate mesoporous PbI2.47,

57, 70-73

Some

researchers fabricated mesoporous PbI2 films by introducing DMSO into PbX2/DMF solution (X=I or Br).57 Figure 3g shows a schematic illustration for fabricating high quality perovskite films from the mesoporous PbI2 films. The morphologies of PbI2 and the resultant perovskite films are shown in Figure 3h to Figure 3j. It mainly forms PbI2∙xDMSO (x>1) rather than really PbI2 films when the solution was spin-coated onto the substrates, which is characterize by the peaks in the low diffraction angle from 5ºto 10ºin XRD curves (Figure 3k). Because DMSO has higher boiling point and stronger coordination ability than DMF, 62-63 DMSO remains in the wet precursor films after spin coating. The residual DMSO leads to a solvent annealing effect during annealing, resulting in coarsening the PbI2 grains. The random stack of large of PbI2 grains leads to lots of voids in the films. Smooth perovskite films with large columnar grains can be obtained by dipping the mesoporous PbI2 films in a hot solution of MAI (10 mg mL-1) for 2 min (see Figure 3j and Figure 3l). Based on the above discussion, if an additive with high boiling point is introduced into PbI2/DMF solution, it may strengthen the coarsening process of PbI2 grains during annealing, resulting in mesoporous PbI2 films. This conclusion is also confirmed by the previous reports, in



which high boiling point additives, such as, TBP (236 °C)47,

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73

and HMPA (233 °C),72 was

introduced into PbI2/DMF solution to prepare mesoporous PbI2 films and highly efficient PSCs.

3.3 Intramolecular Exchange Approach Fabrication of mesoporous PbI2 is an effective way to accommodate the volume expansion and strain occurred in the two-step method. Another strategy to solve these problems is to preexpand the lattice of PbI2 before the intercalation of MAI. PbI2 crystals has a typical twodimensional layered hexagonal structure. The two adjacent layers are bonded by van der Waals interaction with a large layer distance of 6.98 Å. Consequently, the foreign molecule can intercalate into the PbI2 lattice, resulting in lattice expansion along c axis.74-75 Zhu et al.

76

incorporated small amount of MAI into PbI2/DMF solution to pre-expand the layered PbI2 by forming a PbI2∙xMAI film (x: 0.1~0.3). Then, the PbI2∙xMAI precursor films were dipped in the MAI/IPA solutions to form high quality MAPbI3 films. The pre-expanded lattice of PbI2 leads to fast MAPbI3 formation without residual PbI2 during the intercalation of MAI. Meanwhile, the preexpansion of PbI2 by MAI reduced the final volume expansion ration, resulting in smooth perovskite films with controllable morphology. Actually, the polar aprotic organic solvent, such as DMF, NMP and DMSO, can also intercalate into the Pb-I-Pb layer by forming solvate complexes. The intercalation of these solvents into PbI2 lattice produce an expansion of interlayer space along c-axis.74, 77 Seok et al.78 developed intramolecular exchange process (IEP) to fabricate high quality perovskite films, and achieved highly efficient solar cells with a certificated efficiency exceeding 20%. In their research, they used PbI2(DMSO) complex dissolved into DMF to prepare solutions. Then, they prepared PbI2(DMSO) films via spin-coating process. Figure 4a shows XRD curves of the complexes with different mole ratios of PbI2 and DMSO. It is noted that there are a diffraction peak located at ~9.8º


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in XRD pattern of PbI2(DMSO) film, which derives from the shift of (001) plane of PbI2 due to the intercalation of DMSO.75,

77

The shift of (001) diffraction peak indicates the distance along c-

axis increased from 6.98 Å to 9.1 Å.75 The expanded lattice provides diffusion channels for diffusion of FAI, which is beneficial to eliminate residual PbI2 in perovskite film. When the PbI2(DMSO) film was exposed to FAI solutions, the DMSO molecules embedded in the PbI2 framework will be replaced by FAI due to its stronger affinity. The intermolecular exchange between DMSO and FAI is schematically illustrated in Figure 4b. Due to the preexpansion of the PbI2 lattice by DMSO, the final volume expansion rate was suppressed during the formation or perovskite films. Seok et al.78 found that the thickness of PbI2(DMSO) film increases only from 510 nm to 560 nm after transforming to FAPbI3 films through intermolecular exchange due to similarity in molecular size between DMSO and FAI. For comparison, they also prepared PbI2 films with a thickness of 290 nm, which was double to 570 nm for the FAPbI3 films by direct reaction of PbI2 with FAI. These results indicate that the volume expansion is obvious suppressed by pre-expanding the PbI2 lattice with DMSO, which facilitate fabricating high quality perovskite films. Figure 4c and Figure 4d show the surface morphology of the perovskite films fabricated from IEP method and conventional method, respectively. Comparing to the perovskite films fabricated from conventional method, the FAPbI3 film fabricated from IEP method exhibit a columnar structure with lager grains, which are extremely desired for highly efficient solar cells. Figure 4e is XRD curves of the perovskite films fabricated from different methods. In the perovskite films fabricated from conventional method, there are some residual PbI 2 in the final perovskite films, and the annealing process at high temperature is required for the transformation from PbI2 to perovskite films. However, there are no residual PbI2 in perovskite film fabricated



from IEP, and the annealing process only play roles in improve the crystallization of perovskite films. The elimination of PbI2 in perovskite films can be ascribed to the fast diffusion of FAI due to the pre-expansion of DMSO in the first step, which provide wide diffusion channels for the FAI in the second step. Figure 4f is the efficiency distribution of PSCs fabricated from different methods. It clearly shows that the device fabricated by intermolecular exchange exhibit superior efficiency with smaller deviation to those fabricated from conventional methods. The best solar cells fabricated from intermolecular exchange methods exhibit performance of Jsc=24.7 mA cm-2, Voc=1.06 V, FF=77.5%, PCE=20.4%.

3.4 Lewis Acid-Base Adduct Approach In the traditional two-step method, it first to deposit PbI2 films on the substrate from solution, followed by annealing to remove the solvent; and then the PbI2 films are exposed to MAI/IPA solution to fabricate perovskite films through intercalation reaction. 41 It unconsciously filters the multiple functions of solvents during the formation of perovskite films. In order to take the advantage of the residual solvent during the crystallization perovskite films, we developed a Lewis acid-base adduct approach to prepare high quality perovskite films. 55, 58, 79 It uses Lewis acid-base adducts of PbI2∙xSol rather than PbI2 films to react with MAI/IPA to form perovskite precursor films in such an approach. Then, the perovskite precursor films transform to high quality perovskite films by annealing at high temperature. Actually, the Lewis acid-base adduct of PbI2∙xSol is the PbI2 precursor film before annealing, which is a product after PbI2 solution spincoated onto substrates. In the Lewis acid-base adduct of PbI2∙xSol, Pb2+ act as Lewis acid, solvents act as Lewis base, and Pb2+ and solvent is connected through Pb-O bond.80


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We found that the quality of perovskite films can be tuned by the Lewis basicity of additive solvents adding into the PbI2/DMF solutions.79 The Lewis basicity of solvent is quantitative scaled by donor number (DN) of solvent. DMF is typical Lewis basicity solvent having a DN of 26.6. In order to investigate the effects of Lewis basicity on the perovskite quality, strong Lewis basicity additives of NMP, DMSO, HMPA, with DN values separately of 27.3, 29.8 and 38.8, were introduced into the PbI2/DMF solutions to modulate the formation process of perovskite films. The grain size of perovskite films strongly depends on the Lewis basicity evidently. The average grains size are 197, 237, 340 and 146 nm for pure DMF, 10% NMP, 8% DMSO and 2% HMPA, respectively. Large grains exceeding 1 μm are observed in the sample adding with 8% DMSO. Meanwhile, the grains evolves from small grains to columnar grain when 8% DMSO was introduced into PbI2/DMF solutions. Figure 5b is the TGA curves of the corresponding Lewis acid-base adducts. The initial decomposition temperature Ti, which is defined as the temperature at the weight of 95%.81 The Ti reflects the interaction strength between the Pb 2+ and solvent. The higher value of Ti, the stronger Pb-O bonds in the Lewis adducts of PbI2∙xSol. Figure 5c describes the relationship between the decomposition temperature/mean grain size and DN of solvent. I t shows that the strength of Pb-O bonds in the Lewis adducts of PbI2∙xSol increase as the increase of Lewis basicity of solvent (scaled by donor number). It exhibits a non-monotonic dependence between mean grain size of perovskite films and DN. The perovskite films fabricated from DMSO (DN=29.8) has the largest grain size, which demonstrates that the Lewis basicity of the solvent plays a key role in controlling the grains size of perovskite films. In light of the similarity in the molecular structure and difference in Lewis basicity between DMF and DMA, we clarified the formation mechanism of perovskite films fabricated from Lewis acid-base adduct by introducing DMA (DN=27.8) into PbI2/DMF solution.60 The formation



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process of perovskite films fabricated from Lewis acid-base adduct are illustrated in Figure 5d. There are three key process during the formation of perovskite films, which are all mediated by solvents in every stage. First, the mother solvent (DMF) and additives (e.g. DMA, DMSO, NMP, and HMPA) intercalate into the PbI2 lattice by forming Lewis adducts of PbI2∙xSol, resulting in the lattice expansion along c axis. The expanded lattice provides large diffusion channels for MAI. Second, the solvent in PbI2∙xSol will be partial replaced by MAI through exchange reaction due to its stronger affinity with PbI2 when PbI2∙xSol films expose to MAI solutions. The exchange product is solvent-included perovskite precursor films of MAI-PbI2-Sol complex. Third, MAIPbI2-Sol complex transforms to MAPbI3 films by removing the solvent at high temperature through annealing, accompanying with a solvent-mediated dissolution-recrystallization process. Due to pre-expansion of the PbI2 lattice, the Lewis adducts of PbI2∙xSol reduce the finally volume expansion rate in the formation of perovskite films, resulting in smooth perovskite films. The final grains size is commonly controlled by the exchange reaction rate between MAI and solvent and dissolution-recrystallization process during annealing. Loo et al.82 found that solvent with high DN coordinate more strongly with PbI2, which inhibits the iodide coordination and retards perovskite crystallization. The perovskite grain size (R) is inversely proportional to the cubic root of the overall reaction rate (r) in the two-step method.

83

It indicates that it is an effective

way to increase the mean grain size by slowing down the reaction rate. In the Lewis acid-base adduct approach, the stronger of Pb-O bonds in the Lewis adduct of PbI2∙xSol (scaled by DN), the slower molecular exchange rate between MAI and solvent, which is beneficial to obtain larger grains. Additionally, the amount of residual solvent in MAI-PbI2-Sol play a key role in controlling the grain size of the final perovskite films. Excess solvent in MAI-PbI2-Sol will dissolve the grains seriously, which is harmful for the formation of large grains through recrystallization process. For


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example, when 2% HMPA is introduced into PbI2/DMF solution, the mean perovskite grain size reduces significantly (see Figure 5a). For HMPA, it is difficult to escape before annealing due to its high boiling point and strongest interaction between HMPA and Pb2+. Too many HMPA embedded in the film of MAI-PbI2-Sol complex, which dissolve the small grains seriously. As a result, the finally perovskite films composed of small grains when 2% HMPA was introduced into PbI2/DMF solutions. Basing on the above discussion, we provided three rules to select additives to prepare high quality perovskite films via Lewis acid-base adduct approach. First, the molecular size of the additive should be comparable with MAI. The comparable molecular size will suppress the volume expansion during the formation perovskite films, which facilitates to obtain smooth perovskite film. Second, the Lewis basicity of the solvent should be stronger than DMF. The solvent with strong Lewis basicity will slow down the exchange rate between MAI and solvent, which is beneficial to increase the grains size of perovskite films. Third, the additive should have higher boiling point than DMF. The relative high boiling point of additive will lead to some amount residual solvent in the perovskite precursor of MAI-PbI2-Sol, which is beneficial to induce the recrystallization for large perovskite grain during the annealing process.

4. The Roles of Solvents in Annealing 4.1 External Solvent Induced Solvent Annealing In solution-processed perovskite films approach, thermal annealing treatment is a necessary procedure to improve morphology, crystallization, and grain size of perovskite films. Usually, the perovskite films are annealed under protective atmosphere (e.g. N 2) to improve the quality of perovskite films. In 2014, Huang’s group firstly reported a solvent annealing method to increase



the grain size and crystallinity of the perovskite films.65 They first prepared all solid-state PbI2 films and MAI films by spin-coating. Then, the films were annealed in a closed petri dish with or without external DMF at 100 °C for 1h (see Figure 6a). The DMF in the petri dish provides a wet solvent environment, so that the ions and molecules diffused more effectively when compared with the all solid-state thermal annealing, which promoted the grains growth rate and result in large grains (see Figure 6b and 6c). Compared with the traditional thermal annealing method, the perovskite films employed by solvent annealing exhibit columnar grains with high crystallization, low grain boundary density, low defect density. These features have efficiency enhancement in solar cells due to the improvement in carrier mobility and long diffusion length. It means that posttreatment on perovskite films is also an effective way to improve its grain size and crystallinity through the solvent annealing method. In later research, Yang et al.84 explored the distribution of solvent in the films during the solvent annealing process. They studied the growth process of the MAPbI 3 film under N2 and DMSO atmosphere by varying the annealing time. The distribution of MA 2Pb3I8(DMSO) 2 was characterized by the time-of-flight secondary ion mass spectrometry (ToF-SIMS) and elemental mapping images of the perovskite films annealed under DMSO atmosphere for 1h. They found that the sulfur formed a network, which is similar to the grain structure of MAPbI3 in SEM images. However, the distribution of uniform CN element is uniform in the same areas. The element distribution demonstrated that DMSO molecule can react with MAPbI3 to form an intermediate of MA2Pb3I8(DMSO) 2 at grain boundaries due to their extreme heterogeneity. Meanwhile, Yang et al.84 also investigated kinetics of grain-coarsening under different atmosphere. They found that the coarsening rate increases significantly when the perovskite films are annealed in the DMSO atmosphere (see Figure 6d), and fitted the results by an exponential


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model. They calculated the activation energy (∆Ea) under different atmosphere. The activation energy for grain boundary in the DMSO atmosphere is 0.13 eV, which is lower than that under N 2 atmosphere (Figure 6e). The lower activation energy enables the perovskite grains for growing faster under DMSO atmosphere than that in N2 atmosphere. Figure 6f is brief picture of MAPbI 3 grains grows under different atmosphere. Once the wet perovskite precursor films were heated to 95 °C, it quickly transformed to a stagnant stage with small perovskite grains. When the perovskite films are annealed under N2 atmosphere, further annealing have litter effects on perovskite grains size due to the relative high activation energy. However, when the perovskite films annealed under DMSO atmosphere, it forms intermediates of MA2Pb3I8(DMSO)2 at the grain boundaries, which reduces the activation energy. Due to the low activation energy, the grain boundary move from the large grain to the small grain, resulting in grain-coarsening process. The low activation energy for grain boundary migration also makes it possible to fabricate uniform perovskite films with large grains at high temperature in a short time. 85-86

4.2 Intrinsic Solvent Mediated Ostwald Ripening In the solution-processed approach, many efforts have been made to understand the multiply roles of solvent in fabricating high quality perovskite films. However, most of works focus on the ligand functions of solvent in forming a solvate complex for fabrication of high quality perovskite films, rather than on the annealing treatment. It is noted that the coarsening function of the solvent during annealing received little attention due to its fast evaporation at high temperature. The solvent annealing method is an important method to coarsen perovskite grains by making use of the external solvent. However, this method is very sensitive to the experiment conditions, such as temperature,84 the distribution and amount of solvent, 87 annealing time,88-90 solvent selection,91 making it difficult to obtain high quality perovskite films with good reproducibility.



In the solution-processed approach, perovskite film usually obtained from intermediates by releasing solvent via annealing treatment. During the releasing of solvent, it might also intrigue solvent annealing effects, resulting in the coarsening of perovskite grains. However, the fast evaporation of solvent in intermediates weaken the solvent annealing effects for the coarsening of grains. In order to magnify the concealed intrinsic solvent annealing effects, we developed a sandwich structure for annealing to retard the escape of the solvent. 61, 92 Figure 7a is the schematic illustration of the fabrication process of the sandwich structure. Figure 7b is the evolution of XRD curves of the films during the fabrication process. When PbI2/DMF solution is deposited onto the substrates, it forms Lewis acid-base adducts of PbI2∙xDMF, which is identified by the two diffraction peaks at 9.03 º,9.56 ºin XRD curves, corresponding to the plane of (011) and (002) of PbI2∙xDMF, respectively.56 It forms solvent embedded intermediates of MAI-PbI2-DMF when PbI2∙xDMF is exposed to MAI/IPA solutions, which is evidenced by three diffraction peaks located from 5ºto 10ºin XRD patterns (see blue line in Figure 7b). In order to retard the escape of the residual DMF, a PC61BM layer is immediately deposited onto the intermediates of MAI-PbI2-DMF to form a sandwich structure of FTO/MAI-PbI2-DMF/PC61BM, and then annealing at 100 °C for 30 min to form perovskite films. For comparison, the intermediates of MAI-PbI2-DMF without PC61BM layer directly annealed at 100 °C for 30 min. Figure 7c and 7d are the top morphology of perovskite films fabricated from different method. It clearly shows that the grain size significantly increased by adopting the sandwich structure during annealing. The average grain size increases from 194.8 nm to 718.6 nm, and the largest grain exceeding 1μm can be observed in the films annealed by sandwich structure. Meanwhile, the microstructure of grains evolves from equiaxed structure to columnar grains, which were featured by the invisible grain boundary along the thickness direction (Figure 7 e and 7f). Figure 7g is the plots of the mean grain size of films


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when the MAI-PbI2-DMF annealing at 100 °C for different time. It clearly shows that the mean grain size increase significantly. The coarsening rate decreases as the elongation of annealing time. We proposed an Ostwald ripening model to understand the grain coarsening caused by the intrinsic solvent annealing.61According the Ostwald ripening model, 93 the smaller particle is always more unstable than the larger particle due to the difference in the chemical potential. The grain coarsening process are schematically illustrated in Figure 7h to Figure 7k, which is a solvent mediated dissolution-recrystallization process. When the perovskite precursor film annealing at high temperature, the grains with different radius will be dissolved by the solvent in different degrees. The small grain are dissolved easily by solvent due to its relative higher chemical potential. The concentration of dissolved components near the smaller grains is always higher that near the large grains. Therefore, it forms a concentration gradient from small grains to large grains, which results in the mass transportation from the small grains to the large grains. As a result, the large grain coarsening occurs by absorbing the small grains via mass transportation. As the elongation of annealing time, the dissolution and mass transportation process stopped due to the evaporation of the solvent in the intermediates, resulting in the stop of coarsening of grains (see Figure 7g). The PC61BM layer in the sandwich structure plays roles in retarding the evaporation of the solvent in the films, so Ostwald ripening effect is enhanced. As a result, the mean grains size increased significantly when the MAI-PbI2-DMF is covered by PC61BM during annealing. Basing on the analysis above, the sandwich annealing method is a time-saving and effective method to improve performance of PSCs along with reduction in fabrication process. High quality perovskite films with large grain can be obtained by elongating the Ostwald ripening process during annealing. The Ostwald ripening process can be elongated by employing face-down annealing method,94-9 5



introducing solvent into MAI/IPA solutions, 96-99 using homogeneous cap-mediated crystallization, 100-101

choosing new Lewis solvent with high boiling point. 102

5. Summary and Perspectives We summarized the recent progress on fabrication of high quality perovskite films by taking advantage of solvent to modulate the formation process of perovskite. Fabrication methods, including one-step and two-step method were discussed in details. It showed that solvent engineering approach was a best choice for laboratory-scale spin-coating method to prepare high quality perovskite films due to its advantages of easy process and time saving. Solvent-solvent extraction approach exhibited advantages of wide processing window and compatibility with the scalable techniques, making it is attractive for future scalable device fabrication during the commerliartion of perovskite solar cells. Especially, the sandwich annealing method not only make fabrication step become much simpler, but also can improve the quality of perovskite films. Therefore, sandwich annealing method become a good choice for development of high performance and low-cost perovskite solar cells. In parallel with the discussion of fabrication methods, we also clarified the multiply roles of solvent in the formation of perovskite films. Except for the ligand function of solvent, the solvent play multiply roles in retarding the reaction rate, controlling the nuclei/growth, coarsening grains for preparing high quality perovskite films. The properties of solvent, such as boiling point, vapor pressure, Lewis basicity, molecular size and miscibility, have important effects on the formation process of perovskite. This work clarify the formation mechanism of perovskite films fabricated via solution-proceed techniques. It provides some important guidelines to filtrate the non-toxic solvent to prepare high quality perovskite films, and fabricate lead-free perovskite films by making use of the interaction between solvent and solute.


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

AUTHOR INFORMATION

Corresponding Author *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 National Natural Science Foundation of China (51472019), Tsinghua University Initiative Scientific Research Program (20161080165), and Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics (KF201704).



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Solar Cells. ACS Appl. Mater. Interfaces, 2017, 9, 26937-26947. (97) Yao, Z.; Jones, T. W.; Grigore, M.; Duffy, N. W.; Anderson, K. F.; Dunbar, R. B.; Feron, K.; Hao, F.; Lin, H.; Wilson, G. J. Tunable Crystallization and Nucleation of Planar CH3NH3PbI3 Through Solvent-Modified Interdiffusion. ACS Appl. Mater. Interfaces, 2018, 10, 14673-14683. (98) Mao, P.; Zhou, Q.; Jin, Z.; Li, H.; Wang, J. Efficiency-Enhanced Planar Perovskite Solar Cells via an Isopropanol/Ethanol Mixed Solvent Process. ACS Appl. Mater. Interfaces, 2016, 8, 23837-23843. (99) Chiang, C.H.; Nazeeruddin, M. K.; Grätzel, M.; Wu, C. G. The Synergistic Effect of H2O and DMF Towards Stable and 20% Efficiency Inverted Perovskite Solar Cells. Energy Environ. Sci., 2017, 10, 808- 817. (100)Zhu, W.; Bao, C.; Lv, B.; Li, F.; Yi, Y.; Wang, Y.; Yang, J.; Wang, X.; Yu, T.; Zou, Z. Dramatically Promoted Crystallization Control of Organolead Triiodide Perovskite Film by a Homogeneous Cap for High Efficiency Planar-Heterojunction Solar Cells. J. Mater. Chem. A, 2016, 4, 12535-12542. (101) Wang, Y.; Li, J.; Li, Q.; Zhu, W.; Yu, T.; Chen, X.; Yin, L.; Zhou, Y.; Wang, X.; Zou, Z. PbI2 Heterogeneous-Cap-Induced Crystallization for an Efficient CH3NH3PbI3 Layer in Perovskite Solar Cells. Chem. Commun., 2017, 53, 5032-5035. (102) Zhu, L.; Xu, Y.; Zhang, P.; Shi, J.; Zhao, Y.; Zhang, H.; Wu, J.; Luo, Y.; Li, D.; Meng, Q. Investigation on the Role of Lewis Bases in the Ripening Process of Perovskite Films for Highly Efficient Perovskite Solar Cells. J. Mater. Chem. A, 2017, 5, 20874-20881.


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 Figure (a)

(b)

(c)

(e)

(d)

(f)

(g) (h) Figure 1 (a) The illustration of solvent engineering for fabrication of perovskite films.(b) Scheme for the formation of perovskite films via intermediate phase. (c) The top SEM images of perovskite films fabricated from solvent engineering. (d)AFM images of perovskite films fabricated from solvent engineering. (e) Comparison of morphology of films deposited from solvent combination. (f) Lamer plot: the concentration change of the perovskite precursor solution as a function of time. The schematic illustration of the materials nucleation/growth with (g) and without (h) washing solvent. Reprint with permission. 26 Copyright 2018, Springer Nature.



Figure 2 (a) The schematic illustration of the solvent-solvent extraction(SSE) process for fabrication of perovskite film at room temperature.(b) The surface SEM images of perovskite films fabricated from SSE process.(c) The evolution of XRD patterns of films with different extraction time in NNP.(d) The corresponding FTIR spectrum of the films with different extraction time. (e) The schematic illustration of SSE deposition mechanism. (f) The average absorbance of the perovskite precursor films as a function of the processing delaying time. (g) The effect of delaying time on the PCE of corresponding solar cells. Reprint with permission. 38, 40 Copyright 2018, Royal Society of Chemistry & Springer Nature.


Page 44 of 58

Page 45 of 58 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


(c)

(a)

(b) 3.1%

4.4%

9.7%

11.9%

15.8%

15.5%

(d)

(e)

(f)

(k)

(g)

(h)

(i)

(l)

(j)

Figure 3 (a) The schematic illustration of the time-dependent growth of mesoporous PbI2. (b) The morphology evolution of PbI2 films with controlled growth time. (c) The schematic illustration of the solvent-solvent extraction (SSE) process for fabrication of mesoporous PbI 2 films.(d) and (e)are the morphology of PbI2 films fabricated from SSE methods. (f) Schematic illustration of crystallization of PbI2 films fabricated from SSE method. (g) Schematic illustration of mesoporous PbI 2 films and corresponding high quality perovskite films by using additive engineering. (h) The morphology of mesoporous PbI 2 films fabricated by introducing DMSO into PbI2/DMF solutions. (i), (j) are the morphology of the perovskite films basing on mesoporous PbI2 films before and after washed by IPA, respectively. (k) The XRD patterns of the PbI2 based solvent complex. (l) The cross-sectional images of a complete solar cells fabricated from mesoporous PbI2 films. Reprint with permission. 56,64,57 Copyright 2018, Wiley-VCH,. Royal Society of Chemistry & American Chemical Society



(a)

(c)

(b)

(e)

(d) (f)

Figure 4 (a) The XRD curves of the complexes with different ration of PbI 2 and DMSO. (b) The schematic illustration of the perovskite films fabricated from intramolecular exchange.(c) and (d) are the comparison of SEM images of perovskite films fabricated from different approach. The inset is the cross-sectional images of a complete solar cells fabricated from intramolecular exchange. (e) The evolution of XRD curves the films from different approach. (f) The distribution of the efficiency of solar cells fabricated from different approach. Reprint with permission. 78 Copyright 2018, American Association for the Advancement of Science.


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10% NMP

Pure DMF

(a)

400 nm

400 nm (f)

Au Spiro-OMeTAD

2% HMPA

8% DMSO

400 nm (g)

(d)

400 nm

(h)

MAPbI3 Porous TiO2 FTO 400 nm

(b)

200 nm

200 nm

400 nm

400 nm

200 nm

400 nm

(c)

Figure 5 (a) The surface (top) and cross-sectional (bottom) morphology of perovskite films fabricated from different solution. (b)Thermal gravimetric analysis (TGA) spectra of the Lewis Adducts. (c) The plots of initial decomposition temperature of the Lewis adduct (red) and mean grain size of the corresponding perovskite (blue) depending on the donor numbers of the Lewis bases. (d) The formation process of perovskite films fabricated from Lewis acid-base adduct approach. Reprint with permission. 2018, Royal Society of Chemistry & American Chemical Society.


60 ,79 Copyright


(a)

(b)

(c)

(d)

(e)

(f)

Figure 6 (a) The schematic illustration of solvent annealing-induced grain size increase. (b) and (c) are the cross-sectional images of perovskite films fabricated from thermal annealing and solvent annealing, respectively.(d) The fitting results between the means grains size and annealing time. (e)The energy diagram of atoms diffusion at grain boundary between perovskite grains under different atmosphere.(f) The morphology evolution of grain under different atmosphere. Reprint with permission. 65,84 Copyright 2018, Wiley-VCH.& American Chemical Society.


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(a) ()

(g) 210

(b)

Grain size (nm)

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


With DMF

Without DMF

180 150 120 90 0

(c)

(d)

400 nm

400 nm

400 600 Time (s)

(h)

(i)

(j)

(k)

800

1000

400 nm

(f)

(e)

200

400 nm

Figure 7 (a) Schematic illustration of fabrication process of sandwich structure for annealing. (b) Evolution of XRD patterns for different films during the formation of perovskite films. The morphology of perovskite films fabricated from different annealing method, (c)(e) are from traditional annealing method, (d)(f) are from sandwich structure. (g) The evolution of mean grain size when the MAI-PbI2-DMF films annealing at 100 °C for different time. The schematic illustration dynamic coarsen process of the perovskite grains as the elongation of annealing time based on Ostwald ripening model.(h)initial stage,(i) early stage, (j)middle stage, (k) later stage. Reprint with permission. 61 Copyright 2018, American Chemical Society.



TOC

DMF

N

NMP

S

O

DMSO

C

H

Perovskite


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 Figure (a)

(b)

(c)

(e)

(d)

(f)

(g) (h) Figure 1 (a) The illustration of solvent engineering for fabrication of perovskite films.(b) Scheme for the formation of perovskite films via intermediate phase. (c) The top SEM images of perovskite films fabricated from solvent engineering. (d)AFM images of perovskite films fabricated from solvent engineering. (e) Comparison of morphology of films deposited from solvent combination. (f) Lamer plot: the concentration change of the perovskite precursor solution as a function of time. The schematic illustration of the materials nucleation/growth with (g) and without (h) washing solvent. Reprint with permission. 26 Copyright 2018, Springer Nature.



Figure 2 (a) The schematic illustration of the solvent-solvent extraction(SSE) process for fabrication of perovskite film at room temperature.(b) The surface SEM images of perovskite films fabricated from SSE process.(c) The evolution of XRD patterns of films with different extraction time in NNP.(d) The corresponding FTIR spectrum of the films with different extraction time. (e) The schematic illustration of SSE deposition mechanism. (f) The average absorbance of the perovskite precursor films as a function of the processing delaying time. (g) The effect of delaying time on the PCE of corresponding solar cells. Reprint with permission. 38, 40 Copyright 2018, Royal Society of Chemistry & Springer Nature.


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(c)

(a)

(b) 3.1%

4.4%

9.7%

11.9%

15.8%

15.5%

(d)

(e)

(f)

(k)

(g)

(h)

(i)

(l)

(j)

Figure 3 (a) The schematic illustration of the time-dependent growth of mesoporous PbI2. (b) The morphology evolution of PbI2 films with controlled growth time. (c) The schematic illustration of the solvent-solvent extraction (SSE) process for fabrication of mesoporous PbI2 films.(d) and (e)are the morphology of PbI2 films fabricated from SSE methods. (f) Schematic illustration of crystallization of PbI2 films fabricated from SSE method. (g) Schematic illustration of mesoporous PbI2 films and corresponding high quality perovskite films by using additive engineering. (h) The morphology of mesoporous PbI2 films fabricated by introducing DMSO into PbI2/DMF solutions. (i), (j) are the morphology of the perovskite films basing on mesoporous PbI2 films before and after washed by IPA, respectively. (k) The XRD patterns of the PbI2 based solvent complex. (l) The cross-sectional images of a complete solar cells fabricated from mesoporous PbI2 films. Reprint with permission. 56,64,57 Copyright 2018, Wiley-VCH,. Royal Society of Chemistry & American Chemical Society



(a)

(c)

(b)

(e)

(d) (f)

Figure 4 (a) The XRD curves of those complexes with different ration of PbI 2 and DMSO. (b) The schematic illustration of the perovskite films fabricated from intramolecular exchange.(c) and (d) are the comparison of SEM images of perovskite films fabricated from different approach. The inset is the crosssectional images of a complete solar cells fabricated from intramolecular exchange. (e) The evolution of XRD curves the films from different approach. (f) The distribution of the efficiency of solar cells fabricated from different approach. Reprint with permission. 78 Copyright 2018, American Association for the Advancement of Science.


Page 54 of 58

Page 55 of 58 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


10% NMP

Pure DMF

(a)

400 nm

400 nm (f)

Au Spiro-OMeTAD

2% HMPA

8% DMSO

400 nm (g)

(d)

400 nm

(h)

MAPbI3 Porous TiO2 FTO 400 nm

(b)

200 nm

200 nm

400 nm

400 nm

200 nm

400 nm

(c)

Figure 5 (a) The surface (top) and cross-sectional (bottom) morphology of perovskite films fabricated from different solution. (b)Thermal gravimetric analysis (TGA) spectra of the Lewis Adducts. (c) The plots of initial decomposition temperature of the Lewis adduct (red) and mean grain size of the corresponding perovskite (blue) depending on the donor numbers of the Lewis bases. (d) The formation process of perovskite films fabricated from Lewis acid-base adduct approach. Reprint with permission. 2018, Royal Society of Chemistry & American Chemical Society.


60 ,79 Copyright


(a)

(b)

(c)

(d)

(e)

(f)

Figure 6 (a) The schematic illustration of solvent annealing-induced grain size increase. (b) and (c) are the cross-sectional images of perovskite films fabricated from thermal annealing and solvent annealing, respectively.(d) The fitting results between the means grains size and annealing time. (e)The energy diagram of atoms diffusion at grain boundary between perovskite grains under different atmosphere.(f) The morphology evolution of grain under different atmosphere. Reprint with permission. 65,84 Copyright 2018, Wiley-VCH.& American Chemical Society.


Page 56 of 58

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(a) ()

(g) 210

(b)

Grain size (nm)

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


With DMF

Without DMF

180 150 120 90 0

(c)

(d)

400 nm

400 nm

400 600 Time (s)

(h)

(i)

(j)

(k)

800

1000

400 nm

(f)

(e)

200

400 nm

Figure 7 (a) Schematic illustration of fabrication process of sandwich structure for annealing. (b) Evolution of XRD patterns for different films during the formation of perovskite films. The morphology of perovskite films fabricated from different annealing method, (c)(e) are from traditional annealing method, (d)(f) are from sandwich structure. (g) The evolution of mean grain size when the MAI-PbI2-DMF films annealing at 100 °C for different time. The schematic illustration dynamic coarsen process of the perovskite grains as the elongation of annealing time based on Ostwald ripening model.(h)initial stage,(i) early stage, (j)middle stage, (k) later stage. Reprint with permission. 61 Copyright 2018, American Chemical Society.



TOC

DMF

N

NMP

S

O

DMSO

C

H

Perovskite


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A Review of the Role of Solvents in Formation of High-Quality Solution

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