Macroscale and Nanoscale Morphology Evolution during in Situ Spray

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Macro- and Nano-scale Morphology Evolution during in situ Spray Coating of Titania Films for Perovskite Solar Cells Bo Su, Herbert A. Caller-Guzman, Volker Körstgens, Yichuan Rui, Yuan Yao, Nitin Saxena, Gonzalo Santoro, Stephan V. Roth, and Peter Muller-Buschbaum ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14850 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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ACS Applied Materials & Interfaces

Macro- and Nano-scale Morphology Evolution during in situ Spray Coating of Titania Films for Perovskite Solar Cells Bo Su,a Herbert A. Caller-Guzman,a Volker Körstgens,a Yichuan Rui,b Yuan Yao,a Nitin Saxena,a Gonzalo Santoro,c Stephan V. Rothc,d and Peter Müller-Buschbauma,* a. Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748 Garching, Germany. b. College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, P. R. China. c. Deutsches Elektronen-Synchrotron (DESY), Notkestr. 85, 22607 Hamburg, Germany. d. KTH Royal Institute of Technology, Department of Fibre and Polymer Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden * E-mail: [email protected]

KEYWORDS: titania, in situ GISAXS, macro-scale, nano-scale, spray coating simulation,

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ABSTRACT Mesoporous titania is a cheap and widely used material for photovoltaics applications. To enable a large-scale fabrication and a controllable pore size, we combine a block copolymer-assisted sol-gel route with spray coating to fabricate titania films, in which the block copolymer polystyrene-block-polyethylene oxide (PS-b-PEO) is used as a structure-directing template. Both macro-and nano-scale are studied. The kinetics and thermodynamics of the spray depositions processes are simulated on a macro-scale, which shows a good agreement with the large-scale morphology of the spray-coated films obtained in practice. On the nano-scale, the structure evolution of the titania films is probed with in situ grazing incidence small angle X-ray scattering (GISAXS) during the spray process. The changes of the PS domain size depend not only on micellization, but also on solvent evaporation during the spray coating. Perovskite (CH3NH3PbI3) solar cells (PSCs) based on sprayed titania film are fabricated, which showcases the suitability of spray-deposited titania films for PSCs.

INTRODUCTION Titania is widely used in photovoltaics, due to the low-cost, abundance, and environmental friendliness.1-6 Particularly, organic-inorganic perovskite solar cells (PSCs), based on titania as electron transfer layer, have shown a tremendous power conversion efficiency improvement from 3.8 % to 22.1% to date.7-8 In general, the mesoscopic titania scaffold (pore size of 2-50 nm) is crucial for the PSCs’ performance with parameters, such as thickness,9-10 porosity and order of titania films having strong impacts.11 Additionally, titania scaffolds can retard charge 2 ACS Paragon Plus Environment

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recombination and elongate the optical light path in the devices.12-13 However, the randomly oriented pores result in defects at the grain boundaries of the perovskite crystallites.14-16 To overcome these limitations, copolymer assisted sol-gel process is applied to fabricate the titania films.17 Such a wet chemical process was used e.g. by Wiesner and coworkers to control the characteristic length scale of metal oxides having a mesoporous structure by using a diblock copolymer.18 Diblock copolymers comprise two chemically distinct segments linked by a covalent bond, which are incompatible. The functional inorganic precursor selectively interacts with one of the polymer blocks via hydrogen bonding. Driven by entropic and enthalpic interactions between both blocks, the desired morphologies of the metal oxide are formed inside the diblock copolymer. Nanostructured hybrid films are achieved through a microphase separation process.19-23 Via microphase separation, diblock copolymers can form characteristic morphologies, such as spheres, cylinders, lamellae and bicontinuous structures on the nanoscale. The hybrid films are exposed to further treatments for example a high temperature calcination to remove the polymer matrix and obtain the final metal oxide films.24 Therefore, diblock copolymers offer a vast potential for designing nanostructured electron transport layers for photovoltaic applications.25-26 For dye-sensitized solar cells (DSSCs), Grätzel and coworkers demonstrated that oriented anatase nanocrystals enhance power conversion efficiencies by 50% as compared to randomly oriented titania films.27 Therefore, mesoporous titania structures with a controllable pore size synthesized via sol-gel processes are widely used in DSSCs, solid-state DSSCs and PSCs to enhance the power conversion efficiency.28-32 So far, most studies used spin coating for thin film preparation including the deposition of the mesoporous titania films. However, with respect to large scale applications spin coating is only of limited feasibility and alternative deposition techniques such as spray coating appear to be



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more promising.33 So far, reports about making use of spray coating for the preparation of mesoporous titania films are rather rare. For example, Raj et al. used chemical spray pyrolysis of an aerosol of titanyl acetylacetonate to make anatase phase titanium dioxide thin films and homogeneous films were achieved at 450 °C.34 Recently, Song et al. reported about spray coating being a cost-effective and scalable technique to fabricate titanium dioxide films for solidstate DSSCs.35 A block copolymer assisted sol-gel synthesis with and without crystalline titania nanoparticles was used. The spray protocol was designed in a way that dry films were deposited (in literature sometime referred to as airbrush). In contrast, Bo et al. reported an alternative spray protocol, in which wet droplets were deposited (in literature called spraying) giving rise to an improved porous titania structure with less defects.36 Both spray coating methods differ in their underlying mechanisms of structure formation. In case of the dry spray condition, the droplets are already solidified before deposition. In contrast, in case of the wet spray condition, the droplets contain solvent and additional processes such as solvent evaporation and solidification happen after arriving on the substrates. As a consequence, an improved pore structure was reported.36 It was demonstrated that the weight fraction of the titanium precursor titanium tetraisopropoxide (TTIP) controlled the pore size. Moreover, it was reported that (101) facets were more favorable for low TTIP content, causing a blue shift of the bandgap of the titania films.36 Today one major focus of current perovskite solar cell development is related to scalability of device fabrication. In this respect, the development of a spray process which enables fabrication of perovskite solar cells with reasonable efficiency values is an important step. As a consequence, it is crucial to further study the hybrid titania films’ structure evolution during the spray process. In case of the wet spray condition, the structure evolution during the solvent


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evaporation step needs to be well understood. Such fundamental understanding of the spray deposition process will enable to achieve a full control of the titania structure, being of utmost importance for applications in PSCs. In particular, it is crucial to understand the titania film morphology on both, macro- and nano-scale, since spray coating can produce also macroscopic structures besides the nanoscopic structures introduced by the diblock copolymer assisted sol-gel chemistry. In situ grazing incidence small angle X-ray scattering (GISAXS) was shown to give valuable structure information on the nanoscale during spray coating of titania films.35 In the present investigation, we study the evolution of hybrid titania films prepared via a similar sol-gel recipe using a block copolymer for templating and TTIP. To deposit wet hybrid films instead of dry films the substrate temperature is lowered to 50 °C and the time of each spray shot is doubled as compared to earlier work.35 With a longer wait time a dry film is achieved after each spray step. Moreover, modelling of the spray process is performed to address the macro-scale structures. With combining GISAXS and modelling we achieve structure information on the nano- and macro-scale for the first time. Furthermore, we demonstrate the usability of the sprayed titania films in PSCs based on pure MAPbI3 as a model perovskite layer. Based on sprayed titania layers, devices with power conversion efficiencies (PCE) of 10.0 % are manufactured, which shows an improvement as compared to spin coated titania films based on a commercial TiO2 paste (efficiencies for the same device architecture reach 8.3 %).

EXPERIMENTAL SECTION Materials and methods



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The block copolymer named polystyrene-block-polyethylene oxide (PS-b-PEO) was obtained from Polymer Source Inc., Canada, with a polydispersity index of Mw/Mn = 1.02. Its number average molecular weight was 20.5 kg/mol with fPS of 0.72. After dissolving it in 1,4-dioxane (≥99.5 %, extra pure, Carl Roth), polytetrafluoroethylene filters (PTFE, Carl Roth GmbH) with 0.2 µm pore size were used for removing large particles. An appropriate amount of titanium(IV) isopropoxide (97% TTIP, Sigma Aldrich) and hydrochloric acid (37% HCl, extra pure, Carl Roth) were diluted by 1,4-dioxane, then injected into the copolymer solution by a speed of 0.25 ml/min. The concentration of the copolymer was 16.09 mg/ml in final solution, and the weight ratios of 1,4-dioxane, TTIP, and HCl were 0.92, 0.07 and 0.01, respectively. Moreover, the microfluidic method was used for mixing during the sol-gel synthesis, which controls the reaction speed when adding hydrochloric acid (HCl) and titanium tetraisopropoxide (TTIP). More details about the microfluidic method were described in the work of Rawolle et al.37 The final solution was stirred for 45 min before use. All the materials were used as received without further purification. Silicon substrates (Si 100, n-type, Silchem) were cut into 2 cm by 2 cm, followed by immersing in an acid bath (consisting of 200 ml H2SO4, 70 ml H2O2, and 130 ml deionized water (DI)) at 80 °C for 15 min. Afterwards, silicon substrates were rinsed by DI water and dried by N2 gas. Titania compact films were spin coated on the pre-cleaned silicon substrates following Hua’s description38, and calcined at 450 °C for 30 mins. Solar cell preparation: Fluorine-doped tin oxide (FTO, NSG Tec 7, Pilkington, 80% - 82% of visible transmittance, 6 - 8 Ω / □ )) glass substrates were etched with zinc powder and hydrochloric acid in the desired pattern. Then, the substrates were cleaned by Alconox solution, ethanol, acetone and 2-propanol under sonication, respectively. After 10 min of plasma


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treatment, a compact titania layer was dynamically at 2000 rpm for 45 s spin-coated on the FTO substrate.39 The mesoporous titania films were prepared under the same conditions as the one from the in situ spray GISAXS experiment. For comparison, the commercial titania paste (Dyesol 18NRT) was diluted in ethanol with the weight ratio of 2:7 to achieve the same thickness of sprayed samples. They were spin coated on the precleaned FTO substrates with 5000 rpm for 30 s. Subsequently, the substrates were dried at 125oC for 15 min. Both sprayed and spin-coated samples were calcined at 450 oC for 30 min. The PbI2 (99.9985%, Alfa Aesar) was dissolved in DMF (1 M) for 1 hour under 100oC. MAI (methylammonium iodide, Solaronix) was dissolved in 2-propanol with a concentration of 40 mg/ml for 1 hour at 70oC. After filtration with a 0.45 µm diameter filter, the solution was cool down to room temperature. The PbI2 solution was spin coated at 3000 rpm for 30 s and afterwards kept at room temperature for 10 min. Next, the MAI solution was spin coated at 3000 rpm for 20 s. The substrates were annealed at 100 oC for 1 hour in a nitrogen filled glove box. After the perovskite annealing, a spiro-OMeTAD solution (70 mM in chlorobenzene) was spin coated at 4000 rpm for 30 s. The spiro-OMeTAD solution was doped with bis (trifluoromethylsulfonyl) imide lithium salt (Li-TFSI, Sigma-Aldrich), tris(2-(1H-pyrazol-1-yl)4-tert-butylpyridine)- cobalt(III) tris(bis(trifluoromethylsulfonyl)imide) (FK209 Co(III) TFSI salt, Dyesol) and 4-tert-Butylpyridine (TBP, Sigma-Aldrich), and the molar ratios of additives for spiro-OMeTAD were: 0.5, 0.03 and 3.3 for Li-TFSI, FK209 and TBP respectively. The gold counter electrode was deposited by thermal deposition under high vacuum.

In situ measurements and Film Characterization



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The in situ GISAXS spray experiment was carried out at the PETRAIII beamline P03 at DESY with a photon energy of 13 keV, which corresponds to a wavelength of 0.957 Å. The sample was placed on a heating stage (DHS 1100, Anton Paar) at a constant temperature of 50 °C. The Grafo T3 (Harder & Steenbeck) spray setup was operated with a nozzle size of 0.4 mm. The sample-tonozzle distance was 16 cm and the pressure of the N2 carrier gas was 1 bar. A Pilatus 300k was used as a 2D detector to record the scattering data probed at an incidence angle of 0.245°, which was chosen to be well above the critical angles of PS (0.09°), PEO (0.10°) and TiO2 (0.18°). To avoid beam damage of the hybrid films, the sample stage was scanned along the y-direction (perpendicular to the x-ray beam direction) during the experiment over 6 mm in 0.05 mm steps. The data acquisition time for each frame was 0.1 s. 2 s of spray were followed by 5 s wait time in one so-called spray cycle. In total 10 of such spray cycles were done (see figure S1). Scanning electron microscopy (SEM) images were measured with a field emission SEM (Zeiss NVision 40) which was operated at an accelerating voltage of 5 KV. Low working distances of 1 to 3 mm were chosen. An Axiolab A microscope (Carl Zeiss) was used in combination with a CCD camera for the optical characterization. The crystal structures were characterized with high resolution transmission electron microscopy (HRTEM) using a JEM-2100F (JEOL Co., Japan). The X-ray diffraction experiments were performed with a Bruker D8 diffractometer operating at an X-ray wavelength of 1.54 Å (copper anode at 40 kV). The software named DIFFRAC.SUITE was used to collect the data. A 2θ range from 20° to 60° was scanned with a step size of 0.01° using a counting time of 0.1 s. The reference peaks for TiO2 were taken from the Joint Committee on Powder Diffraction Standards (JCPDS). The solar cells were measured using a solar simulator (SolarConstant by K. H. Steuernagel Lichttechnik GmbH) under AM 1.5G. The light intensity was calibrated with a silicon-based


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calibration solar cell (WPVS Reference Solar Cell Typ RS-ID-3 by Fraunhofer ISE). Currentvoltage characteristics of the perovskite solar devices were obtained by applying bias voltage with Keithley 2400. No device preconditionings were applied before performing the measurements. The solar cell active areas were 0.165 cm2.

RESULTS AND DISCUSSION a)

Macro-scale structure To study the evolution of the morphology on the macro-scale, a simulation of the spray coating

process is performed. We investigate the evolution of the hybrid film morphology during spray coating, based on the theory provided by Filipovic and coworkers, which neglects surface tension.40 The equations used in the simulation are shown in the supporting information. The motion equations are coupled to the diffusion equation to obtain the evolution of the droplet size, using a finite difference scheme to solve the motion equations. In this method, we use small time steps (5 × 10-7 s) to solve the equation. The size of droplets in the simulation is influenced by the hot zone regime (more details are shown in supporting information). Figure 1 shows the simulation results compared with optical microscopy images taken for sprayed films. A characteristic patchy film surface is found, which originates from the superposition of individual droplets during the spray coating. In images from the simulation, we set the transparency to a value of 0.3 for each droplet to show droplet overlap. Such overlap cannot be seen in low magnification optical microscopy images of the compact films, which causes deviations in the images obtained from the optical microscopy and simulation. The color intensity provides information about the height of the layered film. The darker the color, the 9 ACS Paragon Plus Environment


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more droplets overlap at that position in the simulation. In the simulated images, the size distribution matches very well with the observations of the optical microscopy images. On the macro-scale, the spray coated film is almost uniform over the entire surface. Zooming-in, the individual droplets become visible. However, due to neglecting surface tension in the modelling not all features from the optical images are fully covered. For example, the turquoise color area in Figure 1c is not present in the simulation results. It results from the Marangoni effect, which is caused by surface tension gradients.

Figure 1. a-c) Optical microscopy images of the films after spray coating taken with different magnifications. d-f) Simulated structures shown of same scale as explained in the text. In the simulated images some degree of transparency is used to better show the droplet overlap.


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The characteristic droplet structure of the spray coated hybrid film results from a layer-bylayer deposition. After the first spray deposition step, the initial droplets partially cover the substrate in a dry state due to the applied parameters (1 bar of N2 pressure, 16 cm of sample-tonozzle distance and 50 °C of substrate heating). Fresh droplets arriving on the sample during the following spray cycles dissolve the pre-deposited film due to the residual solvent in these droplets and thereby create a wet film again. In the hybrid film, the polymer network with its precursor loading rearranges due to the solvent induced mobility. During the wait time between two subsequent spray steps, the solvent evaporates and the film arrests via drying. After several spray cycles, a fully covered substrate can be obtained due to the random deposition of the droplets. Concerning application in solar cells, the randomly distributed round shape textures introduced by the spray coating process can enhance the light scattering properties, due to an increase of the optical path length of the active material. In literature, a light scattering layer of TiO2 with a few micrometer thicknesses was successfully applied in DSSCs.41-42 However, such thick layer would not be suitable for the architecture of thin film PSCs. Therefore, alternative approaches were found, for example a moth-eye TiO2 layer, for which the optical absorption was significantly enhanced at the interface between the TiO2 layer and the perovskite layer.43 In our study, an alternative route arises from the macro-scale spray morphology, which is easily realized due to the used deposition method without the need of additional fabrication steps.

b)

Nano-scale structure

Besides the macroscopic structures of the hybrid films prepared via spray coating, also the nano-scale structure evolution is of crucial importance for photovoltaic applications. Real-space



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imaging techniques are typically limited to probing surface structures and can also suffer from a low time resolution on the nano-scale.33 To probe the temporal evolution of nano-scale structures, in situ x-ray scattering experiments have been proven to be powerful methods. With x-ray scattering the inner film morphology can be probed and using synchrotron radiation, a subsecond time resolution can be realized.44-45 Among the used x-ray scattering methods, in particular GISAXS is perfectly suited to probe inner structures down to the nanometer scale in a non-destructive manner.46 Since data modelling is required for the GISAXS data treatment, a complementary ex situ observation of the surface morphology with real space imaging techniques, such as optical microscopy and scanning electron microscopy (SEM) is carried out. In the present study, the in situ GISAXS technique is combined with a spray setup.35,47 The in situ scattering experiments are performed at a synchrotron to enable a subsecond time resolution, providing excellent statistics for the scattering signals. By modeling line cuts from the 2D GISAXS data, the key structural parameters such as film density, structure sizes and spatial correlations of adjacent particles can be extracted at the nano-scale.48-49 Figure 2 shows a schematic view of the used GISAXS geometry. The sample is placed on the heating stage at 50 °C in order to ensure continuous drying of the films. The spray ink consists of 94 wt% 1,4dioxane, 1 wt% HCl, 5 wt% TTIP, and 16.1 mg/ml PS-b-PEO. The PS-b-PEO acts as structure guide, and TTIP is the titania precursor. HCl is a selective solvent, which induces the microphase separation due to the increased surface energy and low solubility of the PS blocks in HCl. A spray shot of 2 s is triggered by a software-controlled spray gun, using N2 gas as the carrier gas under 1 bar. The first spray shot is started at time t=0 s. More details about preparation of sol-gel solution and the spray parameters can be found in the experimental section, and the spray protocol is pictured in the supporting information (Figure S1). All parameters for the wet spray


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coating are optimized based on our experiments with the target to achieve smooth titania films which can be used in PSCs.

Figure 2. Schematic view of the GISAXS geometry. The incident angle of incoming X-ray beam (denoted ki) is αi and the exit angle of scattered X-ray beam (denoted kf) in the scattering plane is αf. A beam stop is placed at the specular beam position. The Yoneda peak is located at the position of the critical angle of the film.

To achieve a high time resolution in the in situ spray GISAXS experiments, 0.1 s exposure time is chosen for each frame. Selected 2D GISAXS data are shown in the Figure S2 for



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different times of the spray deposition. Lateral structure information is extracted from horizontal line cuts of the 2D GISAXS data taken at the Yoneda peak of PS-b-PEO. These horizontal line cuts at different times of the spray deposition are comprised into a contour plot (Figure 3a). In the initial 10 s (before the first spray step, which means before t= 0 s), only the compact titania layer is observed. When the spray deposition starts (t=0 s), the intensity distribution in horizontal direction changes immediately, indicating the emergence of lateral structures. A peak at around qy = 0.21 nm-1 (emphasized by the red solid rectangle in Figure 3a) appears after 5 s. This peak is attributed to distances between neighboring PS domains, since micelles with PS cores form in a PEO matrix containing TTIP. In addition, there is a high scattering contrast between PEO containing TTIP (δ= 3.09 × 10-05) and PS (δ= 9.13 × 10-06). The intensity of the peak increases with spray time, indicating that a hybrid film with such features increases in thickness on the substrate. This nano-scale structure is formed by a solidification process. Figure 3b shows selected horizontal line cuts. The bottom most curve in Figure 3b represents the structure information of the initial compact titania layer before the spray deposition. The peak is seen at around qy = 0.21 nm-1 from the second curve on, i.e. with the onset of the spray deposition. It shifts to slightly larger qy values, corresponding to a decrease in the PS domain distance (see Figure S3).


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Figure 3. a) Mapping of horizontal line cuts from the in situ 2D GISAXS data cut at qz = 0.35 nm-1. The first 10 s probe the bare compact titania layer before the spray deposition. Spray deposition starts at t = 0 s. The red rectangle emphasizes that the peak around qy = 0.21 nm-1 becomes more pronounced with time. b) Selected horizontal line cuts (solid dots) and fits (red solid lines). The curves are shifted along the y-axis for clarity. The plots from the bottom to the top show data measured at -8 s (prior to spray deposition), 2 s, 8 s, 14 s, 20 s, 26 s, 32 s, 38 s, 44 s, 50 s, 56 s, 62 s, 68 s, and 86 s, respectively. Each curve is obtained by integrating 10 frames.

We integrate 10 frames of 0.1 s exposure time to have horizontal line cuts with good statistics for modeling of the data. The horizontal line cuts are analyzed within the framework of the distorted-wave Born approximation (DWBA), using the effective interface approximation (EIA) and the local monodisperse approximation (LMA).17,

50-51

Form factor (domain radius) and

structure factor (domain center-to-center distance) are extracted from the analysis. We assume a cylindrical shape of the domains. To describe the data two populations of objects with different



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distances are required. The small objects are PS domains in the PEO matrix with TTIP and the large objects resemble clusters. The resulting parameters are plotted in Figure 4. During the spray process being operated in the wet film deposition regime, the morphology of the hybrid films is mainly governed by micelle deposition and solvent evaporation, which causes differences to a dry spray deposition as studied by Song et al.35 The initial form factor (radius of PS domains rPS) and structure factor (distance between PS domains dPS) are 6.9 nm and 29 nm, respectively. During the first waiting period, residual solvent from the deposited film evaporates and the PS domain size decreases (rPS = 6.6 nm). The structure factor remains unchanged during solvent evaporation. In the second spray step, the structure factor of PS domains decreases, because sol-gel droplets dissolve the already deposited film and stack in vertical direction, which compacts the film and ensures the formation of a bicontinuous titania phase after calcination. Additionally, the form factor of the PS domains decreases slightly during the drying step. For each spray cycle, the same trend is observed. PS domain distances decrease with each spray step from 29 nm after 0 s to 25 nm after 65 s and the PS domain sizes continuously decrease from 7 nm after 0 s to 5.9 nm after 65 s. After 10 spay cycles the characteristic structures stay constant within the error bars. For the larger size clusters, which are formed due to local defects of the hybrid films during solvent evaporation, only the form factor decreases during the spray cycles, whereas the structure factor of the clusters stays constant throughout the whole spray coating process. After the last spray step, the form factor and structure factor are also constant. This implies that the morphology of the dry film has equilibrated. The final dry films exhibit more compact structures as compared to the initially deposited wet material. For the selected conditions, 10 spray cycles are needed to achieve an equilibrated morphology.


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Figure 4. Structure evolution from the detailed analysis of the in situ GISAXS data (shown in Figure 3): a) Radii of PS domains rPS, b) center-to-center distances of PS domains dPS, c) radii of clusters rcluster and d) center-to-center distance clusters dcluster. The structure present before spray deposition is shown with a black triangle. The data during the spray steps is shown with black squares and data during the drying steps with red squares.

Based on the information retrieved from the GISAXS data analysis, Figure 5 shows a schematic of the film formation on the nano-scale during the applied spray protocol. The macroscale spray droplets contain a nano-scale substructure caused by the micellar type of ink used for the spray coating. Evaporation of the solvent 1,4-dioxane and morphology transition towards equilibrium during the drying step cause a shrinkage of the nano-scale structure related to the individual packing of the PS core and PEO/TTIP shell micelles. PS domains remain at the same position and shrink two-dimensionally during the heating interval step, which results in a



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constant dPS. Thus, the temporal evolution of the nano-scale morphology in the hybrid films is different from previous reports on ZnO nanostructure formation out of zinc acetate dihydrate sprayed out of N,N-dimethylformamide. Sarkar et al. observed with in situ GISAXS studies that an initially deposited large ZnO cluster structure is filled with small ZnO nanoparticles until a homogenous film is achieved.48 Moreover, the temporal evolution on the nano-scale is also different from titania hybrid films deposited via dry spary coating process due to the presence of solvent after each spray shot.35

Figure 5. Illustration of film evolution during wet spray deposition. Light and dark blue cylinders represent PS domains with and without solvent, respectively. Green cylinders represent PEO domains with titania nanoparticles. Characteristic stages are shown for a) after the first 2 s spray cycle (initial spray shot), b) after the following 5 s heating interval (initial drying phase), c) after the second 2 s spray cycle (second spray shot) and d) after the whole spray protocol.


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To investigate the final film morphology in real space after finishing the spray protocol, SEM measurements are performed. Figure 6a shows a representative SEM image of the sample after spray coating. The PS domains, which have low conductivity as compared to the PEO domains containing titania nanoparticles, appear as dark areas in the SEM images. The average PS domain size is 5.8 ± 0.2 nm, which is in good agreement with the GISAXS results. Large-sized structures called clusters in the GISAXS data fitting are highlighted with white circles in Figure 6a and resemble clusters in the nano-scale structure, due to the local arrangement of the hybrid films during solvent evaporation caused by the Marangoni effect. The clusters mainly appear in droplet overlap regimes, which have a turquoise color in Figure 1c. These features are mainly attributed to different evaporation rates and surface tension on the whole droplet-air interface. In order to remove the diblock copolymer and crystalize the titania in the films, the sample is calcined at 450 °C for 30 minutes (Figure 6b, Figure S4). A porous and bicontinuous titania nanostructure is formed. From cross-sectional SEM images (Figure 6c) the average film thicknesses of the mesoporous and of the compact titania layers are extracted, which are 185 nm and 96 nm, respectively. Furthermore, the pore sizes are equal in vertical direction, which shows a difference as compared to porous titania films prepared by block copolymer assisted sol-gel method via spin coating. Titania films deposited with spin coating have a more ordered surface part as compared to the mesoporous structure inside the films.20 The spray-coated titania films (tetragonal; space group, 141/amd; a=3.785, b=3.785, c=9.514) are also investigated by high resolution transmission electron microscopy (HRTEM) to have a higher real space resolution (Figure 6d). Lattice spacing of 0.36 nm can be assigned to the (101) planes of anatase titania phase in agreement to XRD data (Figure S5).



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Figure 6. SEM and TEM images of the titania films after the in situ spray experiment: a) Top view SEM of the hybrid film as deposited, b) top view SEM of the film after calcination, c) cross-sectional SEM of the film after calcination and d) HRTEM image of films after calcination.


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To demonstrate usability of the spray coated titania films in PSCs, devices are fabricated using pure MAPbI3 as a model perovskite layer. PSCs based on the block copolymer-assisted sol-gel synthesized mesoporous titania (S-TiO2 in Figure S7) and a commercial titania paste (C-TiO2 in Figure S7) are compared. To enable a comparison with equal thickness of 185 nm, the commercial titania paste (Dyesol 18NRT) needs to be diluted in ethanol. Example current– voltage curves and related photovoltaic parameters are shown in Figure S7. Solar cells based on a sprayed titania film using the sol-gel route obtain PCE values of 10.0%, which is slightly larger than the devices based on the commercial titania paste (PCE of 8.3 %). It should be noted that the PSCs are not optimized towards reaching champion efficiencies. However, the improvement in PCE appears promising for further studies and confirms practicability of the used spray protocol for PSCs. Because it was reported that a well-defined crystallinity, the feature size of perovskites, and the charge mobility benefits from the presence of ordered porous titania structures,52-55 we assume that the increase in PCE originates from these effects. A mode detailed analysis of the influence of the titania layer morphology on the perovskite structure is beyond the scope of the present manuscript and will be subject of future research.

CONCLUSION We investigate the structure evolution of hybrid films during spray coating on the macro- and on the nano-scale to get a detailed fundamental understanding of the complex processes governing film formation. On the macro-scale, the applied spray coating protocol results in overlapping droplets. Our simulation of the spray process on the macro-scale matches well with the optical microscopy images of the sprayed films. It implies that the present hot zone regime



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not only dries the films, but also influences the droplet size. The nano-scale structure is revealed with in situ GISAXS during spray deposition. As compared with spray deposition of dry films a different film formation is found. Micelle deposition, evaporation of the solvent and transformation of the morphology towards equilibrium influence the structure features on the nano-scale. The heating interval step is crucial for the wet spray process, in which solvent could evaporate and also the ordered titania structure is formed. An equal distribution of the pore size in the vertical film direction is seen in SEM images. The PSC results show the importance of controlling the pore size of the TiO2 layer, and demonstrate great potential to deposit mesoporous titania films using the wet spray coating method for PSCs applications. Therefore, scalability of device fabrication, which is one major focus of current perovskite solar cell development, will be possible based on the reported spray coating conditions.

ASSOCIATED CONTENT Supporting Information: Spray protocol; selected 2D GISAXS data; horizontal line cuts from 2D GISAXS data; TEM image of titania film; XRD data; simulation of spray coating; and J-V curves of perovskite solar cells.

ACKNOWLEDGMENT This work was supported by funding from TUM.solar in the context of the Bavarian Collaborative Research Project Solar Technologies Go Hybrid (SolTech), the Excellence Cluster


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Nanosystems Initiative Munich (NIM), the Center for NanoScience (CeNS) and the International Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS). B. S. and Y. Y. acknowledge the China Scholarship Council (CSC). A.C-G. thanks the Erasmus Mundus MaMaSELF program and V.K. thanks the Bavarian State Ministry of Sciences, Research and Arts for funding this research work via project Energy Valley Bavaria. We thank Professor Alexander Holleitner and Peter Weiser for the chance to carry out SEM measurements. Portions of this research were carried out at the synchrotron light source PETRA III at DESY. DESY is a member of the Helmholtz Association (HGF).

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Macroscale and Nanoscale Morphology Evolution during in Situ Spray

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