Intergrain Connection of Organometal Halide Perovskites: Formation

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Intergrain Connection of Organometal Halide Perovskite: Formation Mechanism and Its Effects on Optoelectrical Property Hyomin Ko, Seok Joo Yang, Chaneui Park, Dong Hun Sin, Hansol Lee, and Kilwon Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20750 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Applied Materials & Interfaces

Intergrain Connection of Organometal Halide Perovskite: Formation Mechanism and Its Effects on Optoelectrical Property

Hyomin Ko, Seok Joo Yang, Chaneui Park, Dong Hun Sin, Hansol Lee, and Kilwon Cho* Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 37673, Korea

H. Ko, S. J. Yang, C. Park, D. H. Sin, H. Lee, and Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology (POSTECH) Pohang, 37673, Korea E-mail: [email protected]

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Abstract Two-step processes are commonly used for the fabrication of organic-inorganic perovskite solar cells; they convert a PbI2 film to a perovskite film by dipping it in CH3NH3I (MAI) solution or spin-coating the MAI solution onto it. Dipping yields perovskite films with discrete and rough morphologies, whereas spinning yields films with smooth and connected morphologies. The residual MAI solution that remains after spinning is the key factor that governs the smoothness of the resulting morphology; centrifugal force has no influence. A perovskite layer forms as soon as the MAI solution is loaded onto the PbI2 film, then the MAI residues left after spinning dissolve this outermost perovskite layer. The subsequent recrystallization of the dissolved perovskites increases the connectivity and smoothness of the crystals. The final morphology is dependent on the degrees of dissolution and recrystallization, which can be controlled by varying the processing conditions. A post thermal treatment can be applied to induce the additional dissolution of the perovskites, which results in an increase in the final grain size while maintaining good connectivity. Combining these results, we fabricated an optimal film morphology that gives rise to perovskite solar cells with improved efficiency. The optimal perovskite film has a smooth and connected morphology as well as better carrier transport than rough and discrete films. This manuscript provides fundamental understanding of the mechanism of formation during two-step processes of connected perovskite morphologies that can guide the further development of two-step processes for the fabrication of optimal perovskite morphologies.


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1. Introduction Organic-inorganic trihalide perovskites (e.g., CH3NH3PbI3) can be used to fabricate solution-processed thin-film solar cells with tremendous power conversion efficiencies1-9. Such perovskite films should have large grains10-11, full surface coverage12, a smooth top surface13, and good connections between grains14. Various methods have been developed for the preparation of perovskite films with the appropriate morphologies and photovoltaic properties including one-step13,

15-16

and two-step solution processing17-18, vapor-assisted

solution processing19, and vacuum evaporation20-21. The two-step processes involve the sequential depositions of the two precursors, PbI2 and CH3NH3I (MAI)22; these processes are simpler than other methods and enable the production of films with consistent optoelectrical properties, so there have been many studies of the effects of processing conditions14, 23-27 on two-step processes and the perovskite crystal growth mechanism10, 13, 28-31. Two such two-step processes have been developed: dipping17 and spinning18. In both processes, PbI2 is first deposited on a substrate to form closely-packed PbI2 grains with dimensions of approximately 30 nm. In the second step, PbI2 is converted to perovskites (e.g., CH3NH3PbI3) either by sequential dipping into an organic solution (e.g., MAI in isopropyl alcohol (IPA)) (the dipping process) or by coating it with an organic solution through sequential spin coating (the spinning process). In our previous study, we investigated the mechanism of formation of perovskite crystals during the dipping process, and found correlations between the processing conditions and the morphological features of the resulting perovskites, i.e., the average grain size, surface coverage, and surface roughness.25 However, the surface morphologies of the perovskites obtained with the dipping process are discontinuous and rough because each grain originates from a different nucleus and they are arbitrarily distributed. Moreover, the degree of interconnection between the grains, which is also an important requirement for the use of perovskites in photovoltaic devices, cannot be improved by controlling the dipping processing conditions. The spinning process involves the spin coating of MAI solution onto the PbI2 film and can produce perovskites with higher interconnectivity than those produced through dipping14. On average, the perovskites obtained with spinning processes have larger grain sizes and moreconnected and smoother surface morphologies than perovskites obtained with dipping processes. However, the critical factor or mechanism that produces interconnected perovskite grains has not been intensively investigated. Previous reports only inferred that the connected



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morphology should be produced by the centrifugal force present during the spinning process14, but their conclusions were not supported by further experiments. Here, we conducted a precise investigation to identify the mechanism by which the perovskite film forms during spinning. We confirmed that the critical factor for the formation of grain connections is not centrifugal force but rather the presence of residual MAI solution on the film after spinning. Moreover, we developed and verified a new mechanism that explains how the residual MAI solution produces crystal connections during spinning. We conclude that the residual MAI solution becomes highly concentrated because of the evaporation of the solvent, and as a result dissolves pre-formed perovskites, which then recrystallize to produce a smooth and connected morphology. Such dissolution and recrystallization was observed and investigated by performing systematic experiments. The degree of dissolution is strongly affected by the concentration and the amount of residual solution; these quantities can be controlled by adjusting the spin conditions. We also applied a post thermal treatment to increase the degree of dissolution and increase the size and connectivity of the perovskite grains in the films. Finally, we fabricated perovskite solar cells with various morphologies, and it was found that the device with the optimal morphology was fabricated with a spinning process followed by thermal treatment; this device has a higher efficiency and superior optoelectrical properties when compared to the device containing perovskite obtained by dipping. Thus this study has revealed the dynamics of perovskite crystal growth during the spinning process, and thus has answered a basic question in this field, namely the mechanism of formation of connected perovskite morphologies.

2. Results and Discussion 2.1. Critical factors in grain connectivity We fabricated discrete and connected perovskite films by performing dipping and spinning processes

respectively.

PbI2

films

were

coated

onto poly(3,4-

ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) substrates, which are widely used as hole-transport layers in perovskite solar cells. One film was dipped into MAI solution (10 mg mL-1) for 3 min (dipping, discrete), and a small amount (80 μL) of concentrated MAI solution (20 mg mL-1) was dripped onto the other film and immediately removed by spinning (spinning, connected). The samples were then heat-treated at 70°C and 90°C, which are the


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conventional annealing temperatures for perovskite films prepared with dipping and spinning respectively. The two films have obviously different surface morphologies (Figure 1). The perovskite film obtained by dipping contains discrete and irregular grains and large single-crystal nanostructures (nanocubes, nanoplates, and nanorods) (Figure 1a). During the dipping process, perovskite crystals nucleate on the PbI2 surface; each perovskite grain originates from a different nucleus10. As a result, the grains are arbitrarily distributed, so the overall perovskite morphology is irregular and rough. Moreover, many nanostructures form due to the additional reaction between excess MAI and perovskite crystals30. In contrast, the film obtained by spinning contains large and smooth grains that are densely connected (Figure 1b). The cross-sectional views of the two films also show that they have different grain sizes and connections, in agreement with the top-view morphologies. To determine how the smooth and connected morphology forms during spinning, we prepared intermediate states (Figure S1a) and compared them to the final state of the film produced by spinning. A PbI2 film was coated onto a PEDOT:PSS substrate, and 80 L of MAI solution (20 mg mL-1) was dripped onto the film and immediately removed by spinning (2000 rpm, 30 s); then, 1 s, 5 s, or 10 s later, an excess of IPA was poured onto the samples to stop the conversion reaction. The surfaces of these intermediates have discrete morphologies that are similar to that of the film obtained after a few seconds of dipping in 20 mg mL-1 MAI (Figures S1b, c, d). These results indicate that a discrete morphology forms initially, and that these discrete grains subsequently become connected to each other during spinning. We separated the steps of discrete grain formation and the connection of grains to check whether the discrete grains are connected by an additional MAI treatment. For this purpose, PbI2 films were dipped into MAI solution for 1 s to produce an outermost perovskite layer (Figure S2a). Then 20 mg mL-1 MAI solution was spun onto the pre-formed perovskite film. The small and discrete grains became large and connected after the second spinning (Figure S2b). These results confirm that discrete grains form at the beginning of spinning, and then connect during spinning. There are two factors that could affect the connection of the grains during spinning: centrifugal force and residual solution of MAI. The spin speed  is directly related to both conditions: as  increases, the centrifugal force increases and the amount of residual MAI solution decreases. At the slower  (1000, 2000 rpm), the crystals are larger and betterconnected than at the faster speed. However, the fastest  (6000 rpm) was found to yield



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small grains and several discrete regions (Figure 2a). These results show that the connectivity decreases as  increases. If the centrifugal force is the critical factor governing the formation of connections, the connectivity should increase as the centrifugal force increases. The total duration of spinning also has no effect on the connectivity (Figure S3a, b, c); this result implies that the centrifugal force does not affect the formation of connections. In contrast, varying the amount of solution seems to affect the connectivity. During conventional spin coating, most of the solution loaded onto the film is flung off in a few seconds, but a small amount remains; the amount that remains decreases as  increases. Therefore, it is likely that there is a positive correlation between the connectivity and the amount of residual solution. 2.2. Dynamics of grain connection formation during spinning We then investigated the effects of varying the MAI concentration [MAI] on the connectivity. MAI solutions with [MAI] = 10, 20, 40 mg mL-1 were applied to the PbI2 films under the same spin conditions, and then the films were examined. Grain connections formed at [MAI] = 20 and 40 mg mL-1, but not at 10 mg mL-1 (Figure 2b); this result confirms that the amount of residual MAI solution present after spinning is important, and that completion of the grain-connecting process requires a sufficiently high initial [MAI]. At [MAI] = 40 mg mL-1, several aggregates form, which are presumably due to MAI residues. From these results, we infer that the presence of excess MAI in the residual solution results in some kind of reaction with the pre-formed perovskite layer, and that the degree of this reaction is related to [MAI]. Previous research has demonstrated that the dissolution and recrystallization processes of organic-inorganic perovskite crystals during sequential deposition are as follows30 Dissolution

CH3NH3PbI3 (s) +

I-

(sol)

CH3NH3+ (sol) + PbI42- (sol)

(1)

Recrystallization

The forward reaction is the dissolution of perovskite crystals by excess MAI solution, which results in the decomposition of the perovskites into CH3NH3+ and PbI42- ions, which are soluble in IPA. The dissolution continues until the concentration of PbI42- in the residual MAI solution reaches saturation; the recrystallization of PbI42- and CH3NH3+ ions starts once the PbI42- ions reach supersaturation. We suggest that the presence of excess I- ions in the residual MAI solution results in the dissolution of discrete perovskite grains to form mobile


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CH3NH3+ and PbI42- ions, and that these mobile ions recombine to form perovskite crystals with increased connectivity and smoothness, i.e. the mobile phase fills the empty spaces between the discrete grains. To verify the hypothesis that an excess of I- ions dissolves pre-formed perovskite crystals, we performed the spinning process under processing conditions that produce connected morphologies in an N2-filled glove box, which is the conventional environment of our study, and observed the changes in the films over time. The PbI2 film was coated onto a glass/PEDOT:PSS substrate, and MAI solution was spin-coated onto the film. Initially, just after spin coating, the perovskite film is semi-transparent light brown (Film 1 in Figure 3a), but after 30 min it becomes dark brown (Film 2). This indicates that there is an intermediate phase in Film 1 which becomes saturated after 30 min. The intermediate phase probably contains unreacted precursors (PbI2 and MAI) and perovskites dissolved by the excess of Iions. Therefore, the change in the color of the film to dark brown is due to the reactions of unreacted precursors and the recrystallization of the dissolved phase to form perovskites. To investigate the differences between Film 1 and Film 2, we rinsed both films in IPA to freeze their states at each time and examined their morphologies and crystal structures. The grains in rinsed Film 1 are small and discrete, and several pores are present between the grains (Figure 3b). The grains in rinsed Film 2 are larger and more connected than those in rinsed Film 1 (Figure 3c). The cross-sectional images of the two films also show similar trends of morphologies, and the thickness of rinsed Film 1 is lower than that of rinsed Film 2. We also examined an unrinsed perovskite film obtained by spinning that is fully saturated, and found that its morphology is similar to that of rinsed Film 2 (Figure S4). Moreover, the (001) peak of PbI2 in the X-ray diffraction results for rinsed Film 1 has a much higher intensity than that of rinsed Film 2 and that of the saturated film (Figures 3b,c and Figure S4). From these results, we conclude that intermediate phases such as unreacted MAI solution and dissolved perovskites are present in Film 1, but they are removed by IPA, so the thickness of rinsed Film 1 is reduced. Moreover, the relatively high intensity of the PbI2 peak in the XRD results for the rinsed film is due to the removal of dissolved perovskites. The intermediate phases are saturated in Film 2, so the morphologies and diffraction peaks of rinsed Film 2 and the saturated film are similar. These results confirm the presence of intermediate phases in the perovskite film just after spinning; these phases consist of unreacted MAI and dissolved perovskites that are soluble in IPA.



The intermediates obtained by rinsing indicate that dissolved perovskites are present in the perovskite film just after spinning, but it was impossible to observe changes in the film over time by conserving their states without damage. We performed atomic force microscopy (AFM) and X-ray diffraction (XRD); neither approach requires vacuum conditions, and both yield results in a few minutes, so they are appropriate for observing the changes in Film 1 without any damage. However, even though both AFM and XRD are performed under ambient condition, the perovskite film obtained by spinning in the ambient condition is apparently different from the film spin coated in the glove box. In the ambient air, the perovskite film obtained by spinning turns brown as soon as spin coating is complete, and its color does not change further over time (Figure S5a). Thus there is no intermediate phase or the intermediate phase is quickly saturated in the ambient air. We set up a small chamber with a nitrogen environment containing a spin coater, and performed the spinning process. The color of the film after spinning in the chamber is the same light brown (Figure S5b) as that spin-coated in the glove box. We immediately removed the film into the ambient air, and then its color became slightly darker brown within one minute, which is much less than the time this process takes in the glove box, i.e. 30 min. Thus intermediate phases are definitely present in the film just after spinning, and these phases are saturated faster in the ambient air than in the N2-filled chamber (glove box). According to our hypothesis, the saturation process must be the recrystallization of dissolved perovskites. The evaporation rate of IPA in the ambient air is faster than in the N2-filled chamber (glove box) because it is an open system, so the recrystallization of the dissolved phases occurs more rapidly. The perovskite film grown in the ambient air has smaller grains and lower crystallinity than the film grown in the N2-filled chamber (glove box) under the same processing conditions (Figures S5a,b); these characteristics are believed to be due to the rapid saturation of the intermediate phase. Using the difference between the times for saturation of the intermediate phase in the ambient air and in the glove box, it is possible to obtain time-resolved intermediates for the perovskite film after spinning in the closed system (e.g. glove box) without any damage. If the perovskite film is present in the glove box for a certain period of time tgb, and is then exposed to ambient air, the resulting film will be an intermediate that has undergone a slow saturation process in the glove box for tgb and is then frozen by the fast evaporation of IPA in the ambient air. Film 1 (0 min) and Film 2 (30 min) were prepared in the glove box and then exposed to the ambient air. The AFM images of these frozen films show clearly that they


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have different surface morphologies. Film 1 contains many small grains between large grains, which are presumed to be the result of the fast quenching of dissolved perovskites (Figure 3d), whereas Film 2 contains large and connected grains (Figure 3e). We also prepared intermediates by applying the air-frozen method for 0, 5, 10, 30, and 60 min, and determined their diffraction peaks. As the holding time increases, the intensity of the (110) peak of perovskite increases (Figure 3f). The coherence length of the (110) peak of CH3NH3PbI3 at 14.1°, which is proportional to the average grain size, increases rapidly after 5 min, and becomes saturated at 30 min (Figure 3f, Figure S6). These results indicate that the dissolved perovskites recrystallize after spinning, and slow process in the glove box induces a more smooth and connected morphology. The proposed process that arises after spinning is illustrated schematically in Figure 4. When the spinning process has just finished, most of the PbI2 crystals have been converted to small and discrete perovskite grains. The PbI2 film used in this experiment is porous (Figure S7a), so MAI solution can diffuse to the bottom surface of the PbI2 film24. As a result, conversion occurs simultaneously throughout the entire film. The pre-formed perovskite grains begin to dissolve in the highly concentrated residual MAI solution. The concentration of the residual solution is initially its original concentration [MAI], but increases as the solvent evaporates during spinning. The recrystallization of the dissolved species CH3NH3+ and PbI42- becomes dominant when the concentration of dissolved PbI42- ([PbI42-]) is supersaturated in the residual MAI solution. A high [PbI42-] induces the reverse of the reaction in eq (1), which results predominantly in the recombination of dissolved phases to form perovskites. During spinning, residual MAI solution is present on the surface of the film and in the empty spaces between discrete grains, so the recrystallization of dissolved species results in increases in the connectivity and smoothness of the perovskite film. The effects of varying the processing conditions [MAI] and spin speed shown in Figure 2 can be explained by this mechanism. As [MAI] increases, the degree of dissolution increases, so the connectivity increases. As the spin speed increases, the amount of residual MAI solution decreases, so the evaporation of IPA occurs more rapidly. In this case, there is insufficient time for the recrystallization of dissolved perovskites, so the resulting connectivity decreases. The dissolution of pre-formed perovskites by I- ions and the recrystallization of dissolved species due to the supersaturation of PbI42- ions are indispensable to the enhancement of connectivity during spinning, and the spontaneous transition from dissolution to



recrystallization is a feature that occurs only in the spinning process. The perovskite film obtained by dipping does not contain connected areas (Figure 1a), even though this process results in a greater excess of MAI and provides a longer dissolution time than spinning. During the conventional dipping process, the dissolved PbI42- ions cannot become supersaturated because the amount of MAI solution is much higher than that of the residual MAI solution in the spinning process. Moreover, in the dipping process the dissolved species (CH3NH3+ and PbI42- ions) are present throughout the entire MAI solution, so are not present only between discrete grains. Therefore, the recrystallization of dissolved species is not dominant during the dipping process, so grain connection does not occur. Instead of forming connections, nanostructures such as nanocubes, nanoplates, and nanorods form as a result of the dissolution and recrystallization processes that arise in the film obtained by dipping30. The recrystallization of dissolved species occurs in only a few locations at which the local [PbI42-] reaches saturation, which results in the formation of nanostructures. These nanostructures have shapes that are very similar to those of other crystals (alumina, BaTiO4) produced by abnormal grain growth (AGG), also known as discontinuous grain growth or secondary recrystallization32-34. AGG occurs under conditions different from the critical conditions of normal grain growth or crystallization. During AGG, a few energetically favored grains grow rapidly by consuming other grains. Representative characteristics of AGG are the presence of secondary-phase inclusions, the high anisotropy of the solid-liquid interface, and grain boundary energy. In the case of the dipping process, the abnormally high local [PbI42-] could be a strong contributor to AGG, so many nanostructures are observed. In summary, the process by which the discrete perovskite grains become smooth and connected occurs via the dissolution of the pre-formed discrete perovskites by the residual high concentration solution of MAI. The dissolved perovskites start to recrystallize when the dissolved PbI42- ions become supersaturated in the residual MAI solution, and as a result of this recrystallization process, the connectivity and smoothness of the grains increase. The degree of dissolution has a positive effect on the connectivity, and is in turn affected by [MAI] and the spin conditions during processing. 2.3. Thermal treatment and optoelectrical properties The final perovskite film obtained after spinning contains densely connected grains. The grain size increases as the degree of dissolution increases. A good strategy to increase the


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degree of dissolution is to reduce the spin speed and increase [MAI], but both of these adjustments have limits. Both low speeds and high [MAI] induce the formation of a mottled film surface with aggregates, and leave unreacted MAI solutes that are not beneficial to the electrical properties of perovskite films. The perovskite films obtained by spinning without these side effects have small grains (Figure 5a). Usually the perovskite grain size required for solar cell applications is similar to the thickness of the film, so the grain size should be increased while maintaining connectivity. Thermal annealing is a common post-treatment that has been used in previous research into the two-step spinning process11, 18. These studies suggested that the thermal energy of annealing induces the diffusion of MAI molecules into the perovskite and the further reaction of unreacted PbI2 and MAI. Furthermore, the grain size and crystallinity of perovskites also increase by the thermal treatment after the two-step spin process40. The high conversion and crystallinity of perovskite resulting from this application of thermal energy improve the power conversion efficiency of the associated perovskite solar cell. We could also obtain perovskite films with the increased grain size after thermal treatment. We performed post thermal-treatment on a perovskite film fabricated under the conditions of Figure 5a, i.e. 90°C for 1 h; the grain size increases and becomes similar to the thickness of the film, and the crystallinity also increases (Figure 5b). Cross-sectional transmission electron microscopy (TEM) images also indicate that the crystallinity increases after thermal treatment. Highly aligned stripes, similar to the arrangement of a crystal lattice, are only evident in the thermally treated film. Moreover, the thermally treated film has a fast Fourier transform (FFT) pattern that is consistent with that of tetragonal perovskite crystal, whereas the untreated film has a different FFT pattern, which is presumed to be due to a mixture of tetragonal perovskite and hexagonal PbI2. We suggest that the increase in the grain size after thermal treatment should be due to the increase of degree of dissolution. Immediately after spinning, the perovskite film (Film 1 in Figure 3) contains unsaturated phases: dissolved perovskites, unreacted PbI2, residual MAI solution, and precipitated MAI solutes. Thermal energy not only induces the further conversion of the unreacted precursors but also increases the degree of dissolution of the perovskites. This dissolution is an endothermic reaction35 (eq (1)), so the degree of dissolution increases as the temperature increases. Moreover, the precipitated MAI solutes can also participate in the dissolution of perovskites41. We prepared a PbI2 film, added MAI



powders to the film, and applied thermal energy (90°C, 1 h). The regions in direct contact with the MAI powders were converted into perovskites (Figure S8a, b). Thus solid-phase MAI can also be reactive in the presence of sufficient thermal energy and precipitated MAI solutes can participate in the additional dissolution of perovskites. To assess the effects of the combination of thermal energy with MAI solution and solute, we performed thermal treatments on a saturated film (Film 2 in Figure 3) and on a film obtained by dipping. Neither film contained a MAI residue, and thermal treatment had no noticeable effect (Figure S9a, b). This failure to react means that the increase in the grain size during thermal treatment is mainly due to the increase in dissolution that is a result of the combined effects of the MAI residues and thermal energy. We fabricated perovskite solar cell devices with films that had discrete (dipping, device 1) or connected (spinning (w/o TA) (device 2) (w/ TA) (device 3)) surface morphologies. These three perovskite films were successfully fabricated; the average grain sizes of films from dipping and spinning (w/ TA) are similar, and the size of a film from spinning (w/o TA) is smaller than the others (Figure S10a,b,c). In addition, X-ray diffraction results of these devices also show the highest crystallinity of Device 3 followed by Device 1 and Device 2 (Figure S10d). The devices consisted of ITO glass, PEDOT:PSS as the hole pathway, and PC60BM and a LiF/Al electrode as an electron pathway36 (see the inset in Figure 6a). Device 1 was fabricated by using the conventional procedure, and Device 2,3 were fabricated by optimizing [MAI], the spinning conditions, and the conditions of post-treatment (see the Experimental section in the Supporting Information). Device 3 has the highest power conversion efficiency (PCE). Device 2 had the lowest PCE, possibly because this film has a small grain size (Figure 6a). The PCE of the device 3 was 11.14%, which is 28% higher than that of the device 2 (Figure 6b, Table 1). The open circuit voltage VOC is the highest for the device 3, is the next highest in the device 1, and the lowest for the device 2. VOC is related to the recombination dynamics in the perovskite film; VOC increases as the charge carrier recombination in the perovskites decreases. In the perovskite film, grain boundaries are major carrier recombination sites. Device 2 has the small grain size, so there are many grain boundaries. Therefore, VOC of device 2 is the smallest due to the highest recombination. In addition, Device 1 and 3 have similar grain sizes, but VOC is higher for device 3 than for device 1, so this result demonstrates that the perovskite film with more connected morphology exhibits the reduced charge-carrier recombination. The short-circuit current


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density JSC is also higher for device 3, possibly as a result of the higher connectivity of the perovskite film. The final values of VOC ~ 0.97 V and JSC ~17.78 mA cm-2 of the optimized device 3 are lower than current records, but are similar to the results of several previous studies that used the same PEDOT:PSS / PC60BM device architecture16, 37-38. We changed the hole transport material to 1,1-bis[(di-4-tolyamino)phenyl]cyclohexane (TAPC), and obtained an improved PCE. The HOMO level of TAPC is deeper than that of PEDOT:PSS and similar to that of perovskites. Therefore, TAPC can more efficiently extract and transport hole carriers from perovskites39; as a result, VOC and FF increased, and a PCE of 13.8% was achieved (Figure S11). The decay profiles in Figure 6b obtained with time-resolved photoluminescence measurements (Tr-PL) for the three devices are also different. Glass/PEDOT:PSS substrates were used to maintain film properties in the devices. The profiles are fitted to bi-exponential curve, and there are two time constants. A short lifetime 𝜏1 is the shortest in Device 3 (0.70 μs), followed by Device 1 (1.50 μs) and Device 2 (1.80 μs). The shorter of 𝜏1, the faster quenching of carriers to PEDOT:PSS. The results of steadystate PL (Figure S12) support the tendency; the intensity of Device 2 is the highest, followed by Device 1 and Device 3. A longer lifetime (𝜏2) indicates that intrinsic properties of perovskite films; the lower defects sites in films. 𝜏2 is the longest in Device 3 (11.44 μs), followed by Device 2 (9.39 μs) and Device 1 (7.24 μs). It means that connected films have lower defects than a discrete film.

3. Conclusion We have identified the factors that enable the preparation with two-step processes of perovskite films with smooth and connected morphologies. The connected film forms from the originally discrete perovskite layer that is present immediately after the loading of the MAI solution. The presence of residual MAI solution is the critical factor. The connections form as a result of continuous dissolution and recrystallization. The pre-formed perovskite layer is dissolved by the excess I- ions in the residual MAI solution, then the dissolved perovskites recrystallize to increase the smoothness and connectivity of the film. The spontaneous transition from dissolution to recrystallization during spinning is the key process that encourages the formation of connections. When the dipping process is used, a significant amount of MAI solution is present until the end of the reaction, so recrystallization cannot occur; instead, large nanostructures form as a result of abnormal grain growth. Post-



treatments with thermal energy can increase the degree of dissolution and enable the formation of large perovskite grains with good connectivity. By combining these results, we have optimized the two-step process for the fabrication of perovskite solar cells. The connected morphology that results from spinning improves the device efficiency and exhibits lower carrier recombination than the discrete morphology obtained by dipping. These results identify the critical factors producing connectivity and explain the effects of most of the conventional processing conditions of the spinning process on the final perovskite morphology.

Acknowledgements This work was supported by a grant (Code No. 2011-0031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science and ICT, Korea. The authors thank the Pohang Accelerator Laboratory for providing the synchrotron radiation sources at the 3C, and 5A beamlines used in this study.


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Experimental 1. Synthesis of methylammonium iodide(MAI) Methylamine iodide (MAI) was synthesized by reacting methylamine, 40 wt% in H2O (Sigma-Aldrich) with hydroiodic acid (HI), 57 wt% in H2O (Sigma-Aldrich). HI was added dropwise at 0 oC under N2 Environment and reacted with MA for 2 h in a round bottomed flask. White MAI powder was obtained using rotary evaporator and washed three times using diethyl ether. Cleaned powder was dried in 60 oC vacuum oven for overnight. 2. Fabrication of perovskite films Substrates(Si Wafer or Glass) are cleaned with detergent, DI water, acetone, and IPA in order with ultra-sonication. After UV-ozone treatment for 20 min, PEDOT:PSS(AI4083, Bayer AG) was spin-casted on the substrates, thermally annealed in 120 oC oven for 30 min, and transferred into glove-box. 1 M PbI2 (Aldrich, 99%) (460 mg ml-1) was prepared in DMF at 70 oC, and X mg ml-1 MAI solution was prepared in IPA at room temperature for 1 h with stirring. PbI2 film was spin-casted at 4000 rpm at for 15 s. 80ul of MAI solution was dripped on the PbI2 films and spin coated with various processing conditions; spin speed, spin time, and MAI concentration ([MAI]) (Figure 2). Thermal annealing was conducted at 90 oC for 1 h (Figure 5b). To get time-dependent intermediates of the spin process, excess amount IPA was poured on the films at the appointed time during the spinning (Figure S1a). On the other hand, PbI2 film was dipped into the X mg ml-1 MAI for several minutes, which is the perovskite films from the dipping process (Figure 1a). 3. Solar cell device fabrication ITO coated glass (ITO glass) was cleaned with detergent, DI water, acetone, and IPA in order with ultra-sonication. PEDOT:PSS was coated on the ITO glass as a hole transport layer. A perovskite layer was sequentially deposited on the PEDOT:PSS. 1, 1-Bis[(di-4tolyamino)phenyl]cyclohexane (TAPC) is dissolved in the chlorobenzene at 50 oC for 1 h, and spin coated on the ITO glass. PCBM (Nano-C, Inc.) was spin-casted from 20 mg ml-1 PCBM solution in CB at 1000 rpm for 20 s. Al (120nm) cathode was thermally deposited under high vacuum (

Intergrain Connection of Organometal Halide Perovskites: Formation

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