Revealing the detailed path of sequential deposition for metal … · Revealing the detailed path...

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Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and En-gineering, École Polytechnique Fédérale de Lausanne, Station 6, CH-1015 Lausanne,Switzerland.*Corresponding author. Email: [email protected]

Ummadisingu and Grätzel, Sci. Adv. 2018;4 : e1701402 2 February 2018

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Revealing the detailed path of sequential deposition formetal halide perovskite formationAmita Ummadisingu and Michael Grätzel*

Sequential deposition has been extensively used for the fabrication of perovskite solar cells. Nevertheless, fundamen-tal aspects of the kinetics of methylammonium lead iodide perovskite formation remain obscure. We scrutinize theindividual stages of the reaction and investigate the crystallization of the lead iodide film, which occurs before theintercalation of methylammonium iodide commences. Our study identifies the presence of mixed crystalline aggre-gates composedof perovskite and lead iodide during intercalation and structural reorganization. Furthermore, Ostwaldripening occurs in the film for reaction times beyond the point of conversion to perovskite. Using cross-sectional con-focal laser scanningmicroscopy for the first time, we reveal that lead iodide in the over-layer and at the bottomof themesoporous layer converts first. We identify unreacted lead iodide trapped in the mesoporous layer for samples ofcomplete conversion. We acquire kinetic data by varying different parameters and find that the Avrami models bestrepresent them. The model facilitates the rapid estimation of the reaction time for complete conversion for a varietyof reaction conditions, thereby ascertaining amajor factor previously determined by extensive experimentation. Thiscomprehensive picture of the sequential deposition is essential for control over the perovskite film quality, whichdetermines solar cell efficiency. Our results provide key insights to realize high-quality perovskite films for opto-electronic applications.

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INTRODUCTIONIn the search for superior materials for use as light harvesters in me-soscopic heterojunction solar cells, organic-inorganic lead halideperovskites have emerged as strong contenders (1, 2). Numerous re-searchers have been investigating solar cells based on methylammo-nium lead iodide (CH3NH3PbI3), which has remarkable optical andelectronic properties (2). Various methods for the preparation of theperovskite films have been proposed, including single-step deposi-tion (3), sequential deposition (1), and solvent engineering (4), amongothers.

Sequential deposition is one of the main methods used for de-positing perovskite films for various applications. It was optimized byBurschka et al. (1) for the fabrication of solar cells. In this method, theformation of a CH3NH3PbI3 film is achieved through two steps. First, alead iodide (PbI2) solution is infiltrated into a mesoporous TiO2 film,and then the film is dried. Second, the PbI2 film is dipped in a solution ofmethylammonium iodide (MAI) in 2-propanol for a few seconds, re-sulting in the formation of the perovskite.

Research on this method has been focused on improving photo-voltaic performance by controlling the perovskite morphology throughoptimization of the processing conditions (5, 6) or via the use of ad-ditives (7, 8). Some work focused on particular stages of the perovskiteformation. For instance,Wu et al. (9) and Tu et al. (10) have looked intocontrolling the morphology and crystallinity of the PbI2 film to achievebetter solar cell efficiency. Others have controlled the concentrationof reactants, namely, PbI2 (11) and MAI (12), to optimize the perov-skite cell performance. Despite the effort invested in this method,given the complexity of the reaction, it is difficult to control selectedaspects of the deposition process and achieve reproducible results,while details of the remaining stages of the reaction are poorly un-derstood. Thus, it is critical to scrutinize the various stages of perov-

skite formation and obtain a comprehensive picture of the path ofsequential deposition.

In particular, fundamental aspects of the kinetics of CH3NH3PbI3formation, such as the evolution of the filmwith the reaction coordinate(that is, with time), remain obscure. Characterization tools and techni-ques with the capacity to identify different chemical components needto be used to study the surface of the film during perovskite formation.In addition, there is little insight available into the directionality of thereaction in the cross section of the film during or after the dipping. Fur-thermore, the role of unreacted PbI2 in the film has been well studiedbecause it affects solar cell performance (13, 14). However, the residualPbI2, likely to be trapped at some regions in the film, has not been im-aged so far.

Knowledge about these aspects of perovskite film formation wouldallow better control over perovskite film quality, thereby enhancing thesolar cell performance (15). A convenient way of determining the opti-mal reaction parameters that are currently determined through rigorousexperimentationwould aid in achieving reproducible filmquality. Thus,systematic kinetic studies and a robustmodel that represents the kineticdata for a variety of reaction conditions are necessary.

Here, we studied the individual stages of the reaction using variousspectroscopic and analytical techniques, such as x-ray diffraction (XRD),scanning electron microscopy (SEM), SEM-cathodoluminescence (CL),and confocal laser scanning fluorescencemicroscopy (CLSM).We dem-onstrate that the stages of the reaction are the unexpected crystallizationof partially amorphous PbI2 followed byMAI intercalation and structuralreorganization, endingwithOstwald ripening.Wedemonstrate the use ofSEM-CL to study perovskite films, specifically looking at mixed PbI2-perovskite crystals. We show the innovative application of CLSM tolook at cross sections of perovskite films at different stages of the reac-tion and obtain new insights into the various layers that compose thefilm. Last, we examine kinetic data obtained under different reactionconditions and identify that Avrami models best represent the reac-tion kinetics. This model allows prediction of the dipping time for aparticular degree of conversion, including the point of complete con-version, as required for solar cell fabrication.

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RESULTS AND DISCUSSIONStages of the reactionCrystallization of PbI2To begin with, we use XRD to examine PbI2 films dipped in MAIsolution (6 mg ml−1) for different times between 2 and 400 s. PbI2films were prepared by spin coating from a 1.3 M solution in N,N′-dimethylformamide at 70°C onto a fluorine-doped tin oxide (FTO)–coated glass substrate covered with a mesoporous Al2O3 layer, followedby heating at the same temperature for 10 min. XRD spectra of thesefilms are presented in Fig. 1A.We find that the unreacted PbI2 film is ofthe 2Hpolytype and the (001) reflection at 12.66° is themost prominent(1, 16). The perovskite is detected at 14.01° and 14.07°, corresponding tothe (002) and the (110) reflections of the tetragonal phase of the perov-skite, respectively (17, 18).

Remarkably, the perovskite formation is delayed, with the peaksdetected only after 25 s. Instead, the (001) reflection of PbI2 under-goes changes. To investigate the observations further, we analyzed theXRD data in detail. The full width at half maximum (FWHM) fromthe Voigt function fits of the (001) reflection of PbI2 and the (110) reflec-tion of the perovskite is shown in Fig. 1B. We observe a narrowingof the (001) reflection of PbI2 up to 25 s, which indicates that thereaction commences with an increase in the crystallinity of PbI2. After25 s, we observe a slight broadening in the PbI2 reflection coincidingwith the formation of the perovskite, whose reflections are detectedfrom 25 s onward, indicating that the broadening of the PbI2 reflectionis due to the consumption of the crystalline PbI2 as it forms perov-skite. Thus, upon dipping into MAI, the crystallinity of PbI2 increasesbefore perovskite formation (depicted in Fig. 2A). This conclusion isconsistent with suggestions reported by Harms et al. (19) and with ourprevious report (20) where photoluminescence (PL) imaging indicatedthat PbI2 crystallization is the first step in the sequential deposition,occurring before MAI intercalation commences.

We considered the possibility that the observed changes in the XRDpatterns attributed to crystallization could be due to solvent effects suchas a lower solubility of PbI2 in 2-propanol that increases crystallinityupon dipping. However, this possibility has been eliminated throughfurther XRD analysis. The analysis also ruled out the possibility that ob-servations were due to reorientation of the PbI2 crystals in the film upondipping (details presented in the Supplementary Materials).


Crystallization of the partly amorphous PbI2 film, rather than thedirect formation of the crystalline perovskite, is explained by Ostwald’sstep rule. It states that the final stable phase is formed through con-secutive steps with increasing thermodynamic stability (21). In our case,the PbI2 is the phase that nucleates first, as depicted in Fig. 2. Thenucleation mechanism in the PbI2 film was investigated in detail inour previous report (20).Intercalation and structural reorganizationAt this point, we look at the progress of the reaction using SEM imagesof PbI2 films dipped in a MAI solution for different periods. The un-reacted PbI2 film is planar as seen in Fig. 3A. The growth of nuclei inthe hexagonal platelet structure characteristic of the 2H polytype ofPbI2 (19, 22, 23) is visible in Fig. 3B, which shows a sample dipped inMAI for 2 s. After the partial crystallization of PbI2, the reaction ofthe crystalline PbI2 proceeds via the intercalation of MAI between thelayered PbI2 and structural reorganization to form the perovskite, asmentioned in the literature (1, 19). Ahmad et al. (24) investigated thephenomenon of intercalation and showed that controlling reactant con-centrations is crucial for optimal intercalation. The report by Liu et al.(25) indicates that the intercalation of MAI starts at grain boundariesand defects in the crystalline PbI2. This onset and the subsequent struc-tural reorganization to form the perovskite are shown schematically inFig. 2B.

In Fig. 3C, we observe that platelets are visible after dipping for4 s, indicating that intercalation has not commenced. The texturingseen in Fig. 3D, on a sample dipped for 8 s, is investigated further sub-sequently. We see that areas in the image, which are localized aroundthe crystals, are devoid of PbI2, and the mesoporous scaffold is ex-posed (indicated with arrows). This points to the transport of PbI2during its crystallization through the MAI solution or along the sam-ple surface, in addition to in situ crystallization. This finding is in agree-ment with the nucleation and growth mechanism proposed in ourprevious work (20).

Looking at a sample dipped for 10 s, distinct clusters are seen onthe surface of the crystal in Fig. 4A. SEM-CL measurements weremade on the same sample to identify the composition of these clustersthat evolved from the textured crystals seen in Fig. 3D. CL spectraobtained at two different positions on the sample are shown in Fig. 4B.The emission between 480 and 550 nm, attributed to PbI2, is assigned

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Fig. 1. XRD analysis of PbI2 crystallization and perovskite formation. (A) XRD spectra of PbI2 samples dipped in MAI for 2, 4, 8, 25, 50, 200, and 400 s, showing the (001)reflection of the 2H polytype of PbI2 and the (002) and (110) reflections of the tetragonal perovskite. (B) FWHM of the (001) reflection of PbI2 and of the (110) reflection of theperovskite versus time for the spectra shown in (A). a.u., arbitrary units.

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green, and the emission between 720 and 810 nm, attributed to perov-skite, is assigned red. In Fig. 4C, we see that both red and green emis-sions originate from the textured crystals, showing that these aremixed crystalline aggregates composed of perovskite and PbI2 formedas a result of partial MAI intercalation and structural reorganization.These mixed crystalline aggregates have, so far, never been reported.Green emission, indicated with the arrows in Fig. 4D, identifies theclusters on the crystal as PbI2, whereas the red emission from the under-lying crystal shows that it is perovskite (see the Supplementary Materialsfor a sample of nearly complete conversion and further discussionon the CL signal strength).Ostwald ripeningIt is interesting to look into samples dipped in a MAI solution for ex-tended periods of time, long after most of the PbI2 has been convertedto perovskite. The presence of residual PbI2 is investigated later. Cuboidsof perovskite lying over the mesoporous layer make up the cappinglayer (26), and SEM images in fig. S3 show the capping layer of suc-cessive samples of long dipping times of 400 and 800 s. Comparingthese images, we observe an increase in the surface coverage of thecapping layer, although dipping times are well beyond the point ofcomplete PbI2 conversion. These findings support the hypothesis ofOstwald ripening taking place in the film, as previously suggested inthe literature (1, 13). This thermodynamically driven phenomenon in-volves the transport of matter from smaller crystals to larger ones.During the initial stages of Ostwald ripening, depicted in Fig. 2C, pe-rovskite formed in the mesoporous layer is transported to add to min-ute crystals on the surface, thereby increasing the surface coverage.

Upon dipping even longer (that is, for thousands of seconds), smallcrystals in the capping layer dissolve to add to the growth of the largerones as shown in Fig. 2D. The disappearance of the smaller crystals


would decrease the surface coverage. Cao et al. (13) investigated Ostwaldripening for very long dipping times and showed that the large crystalswith low surface coverage were obtained as a result of the extensiveripening phenomenon, and this decreased the solar cell efficiency. Thus,dipping beyond the point of complete conversion of PbI2 to perovskite,which results inOstwald ripening, is unfavorable. Further discussion onthis aspect will be taken up subsequently.

Confocal laser scanning fluorescence microscopyAt this point, we delve further into the directionality of the reaction andlimitations that result in the presence of residual PbI2 in the film. Tospatially resolve the conversion of PbI2 withinmesoporous layers in ad-dition to the capping layer, we used CLSM innovatively to obtain cross-sectional images. Cross-sectional CLSM images allow us to study thedifferent layers in vertical cross sections of solar cells, not just the perov-skite film surface. Figure 5A shows the different parts of a sample thatcan be mapped using both standard surface imaging and our cross-sectional imaging technique together to obtain a complete descriptionof the sample. This cross-sectional CLSM imaging for perovskite filmswill be demonstrated here for the first time.

Samples with a larger Al2O3mesoporous layer, approximately 2.5 mmthick, were imaged because standard samples could not be imaged dueto the resolution limit of the technique. Nevertheless, insights obtainedhere, such as any directionality of the conversion reaction, would beapplicable to standard samples in a similar manner. The difference inthe reaction for these two samples would possibly be that more time isneeded to fully convert a thicker PbI2 layer as demonstrated below.Moreover, our results are particularly relevant for preparing solar cellsin the architecture proposed byMei et al. (27). Their cells have a doublelayer of mesoporous TiO2 and ZrO2, amounting to about 3 mm, as a

Fig. 2. Schematic depicting the stages of the reaction in sequential deposition. Dashed arrows indicatemass transfer. (A) Nucleation and growth of PbI2. (B) Intercalation ofMAI and structural reorganization to form CH3NH3PbI3 perovskite. (C) Ostwald ripeningwhere perovskite from themesoporous layer is transported to the capping layer.(D) Further Ostwald ripening at longer dipping times where perovskite from the small crystals in the capping layer is transported to larger ones. (E) Gibbs free energyshown as a function of the reaction coordinate.

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scaffold infiltrated with perovskite and covered by a porous carbon filminstead of a hole conductor.

Images of our thick sample, dipped for 60 s, resulting in intermediateconversion, are shown in Fig. 5 (B and C). Emission collected between500 and 550 nm is attributed to PbI2 and represented as green, whereasthe emission between 700 and 800 nm is attributed to perovskite andassigned red (20). In the surface view shown in Fig. 5B, unconvertedPbI2, indicated with arrows, is visible inside the mesoporous layer,which is exposed between the perovskite crystals of the cappinglayer. PbI2 underneath the perovskite capping layer is not detectabledue to the strong absorption of the perovskite. However, it is seen inthe cross-sectional image in Fig. 5C. This is feasible because the planechosen for cross-sectional imaging is perpendicular to that used forobtaining surface images (Fig. 5A), and the perovskite crystals of thecapping layer are not in the way to hamper detection of the PbI2emission. Thus, our novel cross-sectional CLSM imaging techniquehas enabled the mapping of unreacted PbI2 in the mesoporous layerfor the first time (Fig. 5C). Obtaining this information was not feasiblewith standard surface imaging, where layers lying beneath another areoften inaccessible.

The image is analyzed by horizontally averaging the PbI2 and perov-skite emission, followed by normalization of the two curves plotted as a


function of depth in Fig. 5D. The consumption of PbI2 in the over-layerresults in the formation of the highly emissive perovskite capping layer.Strikingly, the PbI2 at the bottomof themesoporous scaffold converts toperovskite, before the PbI2 located directly below the perovskite cappinglayer. These observations are consistent with the proposal in the reportby Harms et al. (19) that the perovskite forms a compact layer, denyingthe MAI solution access to the underlying PbI2 for conversion.

As mentioned earlier, several reports (13, 28) have demonstratedthat residual PbI2 is beneficial for solar cell performance. However,residual PbI2 has, so far, never been imaged or localized. Figure 5E showsa sample of long dipping time, expected to have nearly complete conver-sion. We see that the unconverted PbI2 is visible in green, trapped in themesoporous layer. This finding addresses long-standing questions regard-ing the locationof theunreactedPbI2 in the film(13). In the cross-sectionalimages shown in Fig. 5 (C and E), low emission from the PbI2 or perov-skite in the mesoporous layer may be due to the small size of the crystals,because they are restricted by the pores of the mesoporous layer.

In situ isothermal kinetic monitoring and solid-statekinetic modelTo quantify the kinetics of perovskite formation, we conducted in situisothermal experiments at reaction temperatures of 10°, 15°, and 25°C.

Fig. 3. Perovskite formationwith time studied using SEM. (A) Unreacted PbI2 film. (B) Sample dipped for 2 s showing the growth of nuclei as hexagonal platelets of PbI2.(C) Sample dipped for 4 s showing hexagonal platelets of PbI2 on the crystal surfaces. (D) Sample dipped for 8 s showing the textured crystals. Scale bars, 0.5 mm (A to C) and1 mm (D).

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StandardPbI2 samples,made using a 1.3Msolution asmentioned earlier,were used. The absorbance of the perovskite at 700 nm was monitoredduring conversion (see Materials and Methods). The ratio of the ab-sorbance at time t and that after complete conversion is defined asthe conversion fraction [a(t); 0 ≤ a(t) ≤ 1], which represents thefraction of perovskite formed from the PbI2 at time t. The remnantPbI2 that cannot be converted, despite further dipping as discussedearlier, is neglected in the present analysis. Smoothened data, repre-sented in terms of the conversion fraction (a) versus time, arepresented in Fig. 6A.

The kinetic data were fitted to a generalized equation proposed byKhawam and Flanagan (29), representing various solid-state kineticmodels. The generalized model equation was taken up in its integralform to determine the appropriate model for the perovskite formation.Data fitting by means of nonlinear regression showed that the Avramimodels were the most suitable to describe the experimental data. Theirgeneral form under isothermal conditions is

kt ¼�� lnð1� aÞ

�1=nð1Þ


where n is the Avrami exponent, k is the effective reaction rate constant,a is the conversion fraction, and t is time.

We assume that the model parameters are constant through-out the reaction. The values of n for these reactions were foundto be very close to 1. The value of k is expected to increase with tem-perature, and these values were found to be 0.8, 2.2, and 3.5 min−1

for increasing reaction temperatures of 10°, 15°, and 25°C, re-spectively. The change in the isothermal reaction rate (da/dt) withthe conversion fraction (a) is represented in Fig. 6B. It is seen thatthe reaction is accelerating in the initial stages and deceleratingin the final stages, with a maxima at an intermediate conversion.This is a characteristic feature of sigmoidal models, such as theAvrami models, thereby verifying the aptness of the choice (30).The advantage of identifying the appropriate model is that upondetermining the model parameters once for a temperature ofchoice, the reaction time can be rapidly determined using the modelequation for any degree of conversion. The implications of de-termining the reaction time are discussed further below. The appli-cability of this model for a variety of reaction conditions will be takenup next.

Fig. 4. CL study on films of intermediate conversion. (A) SEM image of sample dipped for 10 s with clusters visible on the crystals. Scale bar, 0.5 mm. (B) CL spectra takenat two different points on the same sample. PbI2 emission between 480 and 550 nm is assigned green, and perovskite emission between 720 and 810 nm is assigned red.(C) Pseudocolor CL image overlay on a SEM image of the same sample showingmixed crystalline aggregates composed of perovskite and PbI2. Scale bar, 0.5 mm. (D) PseudocolorCL image overlay on a SEM image. The crystal is perovskite, whereas the clusters (indicated with arrows) on it are identified as PbI2. Scale bar, 0.5 mm.

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Fig. 5. CLSM of samples with a 2.5-mm mesoporous Al2O3 layer. Emission between 500 and 550 nm attributed to PbI2 was assigned green, and emission between700 and 800 nm attributed to perovskite was assigned red. The color saturation scales with the emission intensity. (A) Schematic showing planes parallel to the samplesurface and the cross section, showing the different parts of the sample being imaged. (B) Composite pseudocolor image of the surface of a sample of intermediateconversion dipped for 60 s. Scale bar, 5 mm. (C) Composite pseudocolor cross-sectional image of the same sample showing the perovskite capping layer over theunconverted PbI2 in the mesoporous layer. Scale bar, 1 mm. (D) Normalized emission intensity showing horizontally averaged values of PbI2 emission and perovskiteemission across the depth of the sample in (C). (E) Composite pseudocolor cross-sectional image of a sample of nearly complete conversion showing trapped, un-converted PbI2 in the mesoporous layer. Scale bar, 2 mm.

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In situ kinetic monitoring in the dark and under illuminationIn our previous work (20), we demonstrated the effect of light on theperovskite formation and the resulting film morphology. To quantifythe impact of illumination on the perovskite formation, we conductedin situ kinetic experiments at room temperature, in the dark, and underillumination (see Materials and Methods).

Bi et al. (11) demonstrated that increasing the concentration of thePbI2 solution would lead to better pore filling of the mesoporous scaf-fold, resulting in higher PbI2 loading. Using PbI2 solutions of concen-trations greater than 1 M gives such high PbI2 loading that it results inthe formation of an over-layer of PbI2. This over-layer of PbI2 forms acapping layer of perovskite upon dipping in aMAI solution (11). Thus,differences between perovskite formation in the mesoporous and thecapping layer can be studied using samples of different PbI2 loading.Moreover, we expect thicker PbI2 layers to take longer to convert to pe-rovskite, which indicates that the kinetic data collected for the two casesare likely to be quite different. Thus, we also include samples of twodifferent levels of PbI2 loading in this experiment to ensure that thesediverse conditions are investigated in the study. We are interested inverifying whether their kinetics can be represented well by the modelidentified earlier.

Samples of low PbI2 loading were prepared using a PbI2 solutionwith a very low concentration (0.25M, details inMaterials andMethods)to ensure that the PbI2 is present only in the mesoporous layer and insmall quantities. After dipping such a sample in a MAI solution, theCH3NH3PbI3 formed remains restricted to the mesoporous layer. Onthe other hand, samples with high PbI2 loading, prepared using a high-concentration PbI2 solution (1.3 M, details in Materials and Methods),have an additional over-layer of PbI2. This results in the presence ofperovskite within the mesoporous scaffold and as a capping layer upondipping in a MAI solution. These samples of high PbI2 loading are thestandard samples used throughout this report. The smoothened data,represented as the conversion fraction, are shown in Fig. 6C.

For the samples of high PbI2 loading, the time to achieve 99% con-version was found to be around 2.1 under light bias and 2.9 min in thedark. The analogous values for the samples of low PbI2 loading are 0.3and 1.2 min, respectively. As expected, we found that under identicalillumination conditions, samples of high PbI2 loading take longer toconvert to perovskite compared to those of low PbI2 loading. Remark-ably, the conversion is reached in less time upon biasing the sampleswith light, compared to the reaction in the dark, for both samples. Thus,


light accelerates the perovskite formation within the mesoporous scaf-fold, as well as in the capping layer. Upon fitting the data for samples ofhigh PbI2 loading with the Avrami models, the value of nwas obtainedas 1.4 and 1.1 for the dark and light bias cases, respectively. A highervalue of k (1.8 min−1) was obtained for the reaction under light bias,as compared to the value of 1.0min−1 in the dark. This is consistentwiththe observations in our earlier report (20), where the reaction underlight was rapid compared to that in the dark.

We identified that the Avramimodels are themost suitable to repre-sent the in situ kinetic data for the reaction. They appropriately repre-sent the reaction for various PbI2 loading levels and light intensities, inaddition to our previous demonstration for different reaction tempera-tures, indicative of robustness.

The model facilitates the estimation of the reaction time for a givenconversion at different reaction conditions, including the point of com-plete conversion. The loss in solar cell performance due to Ostwaldripening, as discussed earlier, can be eliminated by identifying the pointof complete conversion for a given set of reaction conditions andchoosing that as the dipping time. Thus, the model presents a conve-nient and rapid option for ascertaining the reaction time for completeconversion, a previously uncertain parameter in sequential deposition.This helps address a major source of irreproducibility in perovskite filmformation.

CONCLUSION AND BROADER CONTEXTAlthough the sequential deposition method is widely used, studies havefocused on optimizing the photovoltaic performance, rather than exam-ining the pathway of the reaction and its kinetics. Our present investi-gation allowed unraveling the salient features of this intriguing andwidely applied conversion process. Contacting the PbI2 film depositedon a mesoscopic oxide support with a MAI solution first induces thecrystallization of the PbI2 film. This is followed by the intercalation ofMAI into the PbI2 crystal lattice and structural reorganization, formingmixed crystalline aggregates of perovskite and PbI2 as intermediates.

Cross-sectional CLSM imaging revealed the directionality of theconversion of the PbI2 film, showing that the PbI2 in the mesoporousscaffold, underneath the capping layer, is the last to convert and thatunreacted PbI2 is trapped there even after long reaction times.Our iden-tification of trapped, unreacted PbI2 in the film addresses open ques-tions raised in the past (13, 14) regarding the location of residual PbI2

Fig. 6. In situ kinetic data of perovskite formation. (A) Isothermal kinetic data at 10°, 15°, and 25°C shown as the conversion fraction (a). (B) Isothermal reaction rate(da/dt) shown as a function of conversion fraction (a) for the data in (A). (C) Kinetic data for samples of low and high PbI2 loading dipped under light and in the dark,shown as the conversion fraction (a).

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in perovskite solar cells. These insights obtained from our pioneeringcross-sectional imaging technique were otherwise inaccessible throughthe traditional surface imaging technique. On a broader note, our inno-vative demonstration of this technique’s potential opens the door forunderstanding the properties of perovskites in cross sections of solar cells,not just the perovskite surface, as currently presented in the literature (31).

Dipping beyond the point of complete conversion results inOstwaldripening, which is detrimental to solar cell performance (13). Thus,identifying the dipping time corresponding to the point of completeconversion is warranted. We found that the Avrami models are bestsuited to represent the kinetic data, and we verified its robustness underdifferent reaction conditions of temperature, film thickness, and illumi-nation. The model allows rapid estimation of the dipping time for anyconversion, eliminating the need for extensive experiments.We therebyaddress critical problems in batch-to-batch reproducibility in perovskitefilm fabrication.

This comprehensive picture of the sequential deposition is essentialfor control over perovskite film quality, which is critical because it de-termines device performance (15). Our findings provide the foundationfor the improvement of perovskite film fabrication through sequentialdeposition for various applications.

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MATERIALS AND METHODSMaterialsMaterials used in the studywere purchased fromSigma-Aldrich orAcrosOrganics.MAIwasprepared as reported in the report by Im et al. (3). TheAl2O3 paste of particles (diameter, 23 nm) was prepared in-house. Filmsfor the in situ kinetic experiments were made using an FTO-coated glasssubstrate (TEC 15, Pilkington), whereas Nippon sheet glass was used forthe remaining experiments.

Sample preparationFilms used for the experiments, with the exception of CLSM, had amesoporous layer of 300-nm thickness made using the Al2O3 nano-particles. The mesoporous layer was deposited and processed as de-scribed by Burschka et al. (1). A thicker mesoporous Al2O3 layer ofapproximately 2.5 mmwas deposited for samples for CLSM. For stan-dard samples, a 1.3 M solution of PbI2 in N,N′-dimethylformamide,kept at 70°C, was deposited by spin coating at 6500 rpm for 20 s. Thesamplewas then heated at 70°C for 10min. A low concentration ofMAIsolution in 2-propanol (6 mg ml−1) was used in all the kinetic experi-ments presented, rather than 8 or 10 mgml−1, as used for solar cell fab-rication (1), to keep the reaction rate sufficiently low so that kineticfeatures could bemonitored and analyzed effectively. Samples of specificreaction times were prepared by dipping PbI2 films in MAI for the re-quired period, followed by washing in 2-propanol to halt the reaction.They were then heated at 70°C for 10 min.

XRD measurementsTheXRDmeasurements were performed on anX’PertMPDPRO fromPANalytical operated in the Bragg-Brentano geometry. It has a ceramictube (Cu anode, l = 1.54060 Å), a secondary graphite (002) monochro-mator, and a real-timemultiple strip X’Celerator detector. The automaticdivergence slit and beammask were adjusted to the dimensions of thefilms. A step size of 0.002° and an acquisition time of up to 5 min perdegreewere chosen for the scan in Fig. 1A,whereas a step size of 0.008°and an acquisition time of up to 1.5min per degreewere chosen for thescan in fig. S1.


SEM-CLThe SEM-CL measurements were conducted at room temperatureusing an Attolight Rosa 4634 CL microscope, with a beam probe of5 nm, an accelerating voltage of 2 kV, and a beam current of 20 nA.The CL microscope tightly integrates a high–numerical aperture (0.72)achromatic reflective lens within the objective lens of a field emissiongun SEM (FEG-SEM). The focal plane of the light lens matches theFEG-SEM optimum working distance. The CL collection efficiency isconstant over a 300-mm field of view so that the CL emission can becompared quantitatively between two distinct points. The CL emissionwas spectrally resolved with a Czerny-Turner spectrometer (focal length,320 mm; grating, 150 grooves/mm) and detected by an ultraviolet–visible charge-coupled device camera.

CLSM and image processingCLSM images were captured using the Leica Application Suite X soft-ware on a confocal laser scanningmicroscope (Leica TCS SP8) using anHCPLAPOoil objective (63×/1.40). A pulsed diode laser (440 nm)wasused for excitation. The excitation power and the gain were chosen tooptimize the dynamic range of the detector (photomultiplier tube, LeicaHyD, or Leica HyD SMD).

Single-plane 512 × 512 images were acquired at 25°C from a un-idirectional scan of 400-Hz speed. Figure 5 (B and C) was imaged usinga pinhole size of 1 airy unit. A pinhole size of 0.628 airy unit was usedto improve signal strength for capturing Fig. 5E. Figure 5B was acquiredat a resolution of 72 nm in xy, whereas it was 46 nm for Fig. 5 (C and E).Image bit depth was 8. Fiji was used for image processing: The two com-ponents mapped to different channels were pseudocolored. Linear bright-ness and contrast adjustments were applied to the entire image for eachchannel. For display, the image in Fig. 5B was cropped to 36.9 mm ×18.45 mm, whereas the images in Fig. 5 (C and E) were cropped to showan area of 8 mm × 8 mm and 14 mm × 14 mm, respectively.

SpectroscopyThe isothermal kinetic data were recorded through absorbance spec-troscopy in transmission mode in a Varian Cary 5 spectrophotometer.The amount of perovskite formedwas quantified bymonitoring the ab-sorbance of the perovskite at a 700-nm wavelength.

The kinetic data of the perovskite formation, in the dark and underlight bias, were recorded through absorbance spectroscopy in transmis-sion mode (also at 700 nm) using an integrating sphere of 110-mm di-ameter in the same spectrophotometer. Dark conditions are as dark asachievable with attenuated probe light, whereas light biasing wasachieved using awhite light-emitting diode that supplemented themax-imum intensity of the probe light.

A 1.3 M solution of PbI2 in N,N′-dimethylformamide was used toprepare samples of high PbI2 loading. These samples were similar to thestandard samples used throughout the report. Thus, these samples wereused to conduct the isothermal kinetic study aswell. A 0.25MPbI2 inN,N′-dimethylformamide solution was used to prepare samples of lowPbI2 loading for some of the in situ kinetic experiments.

For both the experiments, mesoporous films infiltrated with PbI2were placed in a cuvette.Using pipettes,MAI solutionwas added to startthe reaction, and the reactionwas stopped by rapidly replacing theMAIsolution in the cuvette with 2-propanol.

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Supplementary Textfig. S1. XRD spectra of unreacted PbI2 sample and sample dipped in MAI solution for 4 s in thedark.fig. S2. CL study on a sample dipped for 60 s for nearly complete conversion.fig. S3. SEM images of perovskite films with increasing dipping times showing Ostwaldripening.table S1. FWHM of the (001) reflection of the 2H polytype of PbI2 and the correspondingheight of the platelet from the Scherrer equation.Reference (32)

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Acknowledgments: We acknowledge D. Gachet (Attolight AG) for collecting the CL images.We thank N. Pellet for experimental help and assistance with SEM. We acknowledgeK. Schenk, L. Bozzo [BioImaging and Optics Platform (BIOP)–École polytechnique fédérale deLausanne (EPFL)], and R. Guiet (BIOP-EPFL) for help with XRD analysis, CLSM, and imageprocessing, respectively. We thank W. Tress and R. Humphry-Baker for discussions. Funding:Financial support was provided by the Swiss NSF through grant no. 200021-157135/1.Author contributions: A.U. conceptualized the study, conducted the experiments, analyzedthe data, and prepared the manuscript. M.G. directed the study and commented on themanuscript. Competing interests: The authors declare that they have no competing interests.Data and materials availability: All data needed to evaluate the conclusions in the paperare present in the paper and/or the Supplementary Materials. Additional data related to thispaper may be requested from the authors.

Submitted 1 May 2017Accepted 5 January 2018Published 2 February 201810.1126/sciadv.1701402

Citation: A. Ummadisingu, M. Grätzel, Revealing the detailed path of sequential deposition formetal halide perovskite formation. Sci. Adv. 4, e1701402 (2018).

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Revealing the detailed path of sequential deposition for metal halide perovskite formationAmita Ummadisingu and Michael Grätzel

DOI: 10.1126/sciadv.1701402 (2), e1701402.4Sci Adv

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