Low-Temperature Soft-Cover-Assisted Hydrolysis Deposition of Large-Scale TiO2 Layer for Efficient Perovskite Solar Modules

Perovskite solar cells with TiO2 electron transport layers exhibit power conversion efficiency (PCE) as high as 22.7% in single cells. However, the preparation process of the TiO2 layer is adopted by an unscalable method or requires high-temperature sintering, which precludes its potential use for mass production of flexible devices. In this study, a scalable low-temperature soft-cover-assisted hydrolysis (SAH) method is presented, where the precursor solution is sandwiched between a soft cover and preheated substrate to form a closed hydrolysis environment. Compact homogeneous TiO2 films with a needle-like structure were obtained after the hydrolysis of a TiCl4 aqueous solution. Moreover, by careful optimization of the TiO2 fabrication conditions, a high PCE of 14.01% could be achieved for a solar module (4 × 4 cm2) prepared using the SAH method. This method provides a novel approach for the efficient scale-up of the low-temperature TiO2 film growth for industrial applications. Electronic supplementary material The online version of this article (10.1007/s40820-018-0203-7) contains supplementary material, which is available to authorized users.

Abstract Perovskite solar cells with TiO 2 electron transport layers exhibit power conversion efficiency (PCE) as high as 22.7% in single cells. However, the preparation process of the TiO 2 layer is adopted by an unscalable method or requires high-temperature sintering, which precludes its potential use for mass production of flexible devices. In this study, a scalable low-temperature softcover-assisted hydrolysis (SAH) method is presented, where the precursor solution is sandwiched between a soft cover and preheated substrate to form a closed hydrolysis environment. Compact homogeneous TiO 2 films with a needle-like structure were obtained after the hydrolysis of a TiCl 4 aqueous solution. Moreover, by careful optimization of the TiO 2 fabrication conditions, a high PCE of 14.01% could be achieved for a solar module (4 9 4 cm 2 ) prepared using the SAH method. This method provides a novel approach for the efficient scale-up of the low-temperature TiO 2 film growth for industrial applications.

Introduction
Perovskite solar cells (PSCs) have been demonstrated to be next-generation photovoltaic devices that meet future energy-generation demands owing to their high power conversion efficiency (PCE), low cost, simple solutionbased preparation, lightweight, and flexibility [1][2][3][4][5][6]. Conventional PSCs using TiO 2 as the electron transport material have exhibited high PCEs [7][8][9][10]. However, the high-temperature processing of the TiO 2 layer makes their manufacture more complex and hampers the development of lightweight and flexible substrates. To overcome this limitation, several deposition techniques to fabricate TiO 2 films at low temperatures have been successfully implemented, such as spin coating (SC) [11][12][13], atomic layer deposition (ALD) [14,15], sputtering [16][17][18], chemical bath deposition [19,20], and electron-beam evaporation [21]. Among these techniques, SC is the principal method for low-temperature TiO 2 film preparation. A PSC based on a TiO 2 layer prepared by SC chlorine-capped TiO 2 colloidal nanocrystal solutions has achieved a PCE of 19.5%, with an active area of 1.1 cm 2 , which is the highest PCE reported for PSCs prepared at low temperatures [22]. Non-SC methods, including ALD, sputtering, chemical bath deposition, and electron-beam evaporation, are also applicable for controllable TiO 2 film deposition and have achieved PCEs up to 19% for active areas smaller than 1 cm 2 [7]. However, it is obvious that neither SC nor above non-SC methods are suitable for a large-scale PSC manufacture owing to their inherent limitations [23,24]. Therefore, the investigation of simple TiO 2 -layer preparation methods involving a large area and low temperatures is necessary for industrial applications.
In this study, we report a simple low-temperature softcover-assisted hydrolysis (SAH) method, where a soft polyimide (PI) film is used to cover a TiCl 4 aqueous solution spread on a preheated substrate. Compact and homogeneous large-area TiO 2 films with a needle-like morphology (SAH-TiO 2 ) were obtained after the hydrolysis. Moreover, a solar module (4 9 4 cm 2 ) with an SAH-TiO 2 layer as an electron transport layer using the SAH method exhibited a PCE of 14.01% in a conventional device configuration at low temperatures. Therefore, the proposed SAH technology provides a novel non-SC route to the deposition of large-area TiO 2 films for industrial applications.

Materials and Reagents
All chemicals were used as received. PbI 2 (99%), N,Ndimethylformamide (anhydrous), dimethyl sulfoxide, and chlorobenzene were purchased from Sigma Aldrich. Methylammonium iodide (98%) was obtained from Tokyo Chemical Industry Co. Ltd. Titanium tetrachloride (TiCl 4 ) was purchased from Alfa Aesar. A low-temperature TiO x SC solution was purchased from Shanghai MaterWin New Materials Co., Ltd.

Preparation of an SAH-TiO 2 Layer
The substrates were cleaned by a detergent followed by sequential ultrasonic washing in deionized water, ethanol, and acetone (for 30 min in each of them); they were then dried under nitrogen gas. After 15 min of oxygen plasma treatment, the substrates were preheated at 75°C for 10 min on a heating plate. Different concentrations (0.1-0.6 M) of 25 lL cm -2 TiCl 4 aqueous solution were added at the centers of the substrates, and a piece of a soft film, used as the soft cover, was immediately placed on the precursor. The covered film was peeled off after the hydrolysis for 20 min, followed by washing with deionized water and ethanol; the obtained film was then dried in air.

Fabrication of PSCs and Modules
A perovskite precursor solution (90 lL) comprising 1.3 M PbI 2 and CH 3 NH 3 I (1:1/n:n) in N,N-dimethylformamide and dimethyl sulfoxide (4:1/v:v) was spread on the SAH-TiO 2 film using a consecutive two-step SC process at 1000 and 5000 rpm for 12 and 30 s, respectively. Chlorobenzene (500 lL) was dropped on top of the substrates during the second SC step, 20 s before the end of the procedure, followed by annealing at 100°C for 10 min. A precursor solution of the hole transport layer (HTL) was prepared by dissolving 72.3 mg spiro-OMeTAD, 28.8 lL 4-tert-butylpyridine, and 17.5 lL lithium bis(trifluoromethylsulphonyl)imide acetonitrile solution (520 mg mL -1 ) into 1 mL chlorobenzene. The HTL was then deposited on perovskite by SC at 3000 rpm for 30 s. An 80-nm-thick Au electrode was then thermally evaporated on top of the device to form the back contact. The active area of the device was fixed to 1.02 cm 2 .
For the solar module, the fabricating process was similar to that of small solar cells; the laser scribing patterning process was consistent with that reported previously [25]. In brief, 400 lL TiCl 4 precursor solution was used for a 6 9 6 cm 2 substrate. For standard control samples, 1.5 mL TiO x solution was spin-coated onto a cleaned fluorinedoped tin oxide (FTO) substrate with the same area at 3000 rpm for 30 s. The laser scribing process included scribing on a 4 9 4 cm 2 FTO/SAH-TiO 2 layer using a 1064-nm laser. A pulse laser with energy of 30 lJ, spot size of 25 lm, pulse frequency of 30 kHz, and low scribing speed of 50 mm s -1 was used to scribe FTO/SAH-TiO 2 substrates with a line width of 100 lm. We then used pulse energy of 10 lJ, spot size of 25 lm, pulse frequency of 20 kHz, and scribing speed of 20 mm s -1 to scribe perovskite/TiO 2 and HTL with a line width of 300 lm. Finally, pulse energy of 10 lJ, spot size of 25 lm, pulse frequency of 35 kHz, and scribing speed of 80 mm s -1 were applied to scribe the Au electrode with a line width of 100 lm.

Measurement and Characterization
Current-voltage characteristics were measured using a solar simulator (Oriel Class A, 91195A; Newport) and source meter (2400 series; Keithley) at 100 mW cm 2 and AM 1.5 G illumination. The simulated light intensity was calibrated with a silicon photodiode. The J-V curves were measured in the reverse (from 1.2 to -0.2 V) or forward (from -0.2 to 1.2 V) scanning modes [26]. The voltage step was fixed at 10 mV, and the delay time (delay at each voltage step before the measurement of the current) was fixed at 50 ms. Monochromatic incident photon-to-current conversion efficiency spectra were measured using a monochromatic incident light (1 9 10 16 photons cm -2 ) in the direct-current mode (CEP-2000BX; Bunko-Keiki). The morphologies and thicknesses of the films were investigated using a field-emission scanning electron microscope (SEM) (JSM-7800F; JEOL). The microstructure of the product was investigated by a field-emission transmission electron microscope (TEM) (JEM-2100F, JEOL). X-ray diffraction (XRD) patterns of the samples were recorded using an X-ray diffractometer (Ultima IV; Rigaku) with Cu K radiation of 1.54 Å and speed of 21°min -1 . Ultravioletvisible (UV-Vis) absorption spectra were recorded using a spectrophotometer (UV-2450; Shimadzu) in a wavelength range of 200-800 nm at room temperature. X-ray spectroscopy measurements were performed using an ESCA-LAB 250Xi spectrometer (Thermo Scientific).

Results and Discussion
Schematics of the steps involved in the SAH are shown in Fig. 1. A certain amount of the TiCl 4 precursor solution was dropped onto the preheated substrate at 75°C (Fig. 1a), and a piece of the soft film was used to cover the liquid precursor (Fig. 1b). The precursor solution spreads out into a liquid film through the capillary attraction between the precursor solution and soft film (Fig. 1c). A relatively closed environment was formed to prevent rapid evaporation of the solvent into the air. The soft film was then peeled off by a programmed mechanical hand after 20 min of hydrolysis; consequently, a raw TiO 2 film was obtained (Fig. 1d). The deposited substrate was rinsed with water and ethanol successively to remove any loosely bound materials or unreacted precursor solution. According to the features of solution-based underlayer preparation techniques, the flatness of the TiO 2 film in the SAH significantly depended on the uniform liquid film formed during the precursor spreading. This would be related to the wettability of the capped soft film to the precursor solution. A larger wettability implies a larger capillarity between the soft film and solution, which helped form a thin uniform liquid film [27]. The aqueous solution was considered to be the main component used for the hydrolysis; therefore, the candidate soft film should have good wettability with the aqueous solution. Some of the available soft films, such as polyethylene (PE), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and PI films were selected and their contact angles were measured by dropping the aqueous solution onto their surfaces at room temperature in ambient air. As shown in Fig. S1, the average contact angles of the PE, PET, PVC, and PI films were 86°, 81°, 78°, and 52°, respectively. The contact angle of the PI film was significantly smaller than those of the other films; therefore, PI exhibited an excellent wettability with the aqueous solution and was selected as the soft cover for the SAH.
SEM images of the TiO 2 layers on the FTO substrates formed by the SAH using a 0.4-M TiCl 4 aqueous solution are shown in Fig. 2a. The needle-like TiO 2 completely covered the surface of the FTO substrate to form a continuous TiO 2 layer; the resulting SAH-TiO 2 film showed good optical transparency with transmittance larger than  Fig. 1 Schematics of the steps in the SAH 70% in the visible region (Fig. S2) and uniform current distributions (Fig. S3). Moreover, the size and morphology of the as-prepared TiO 2 product scratched from the TiO 2 film were analyzed by TEM, as shown in Fig. S3. The TEM images revealed that several nanoneedles were stacked together to form a flower-like structure. The TiO 2 film was also analyzed by atomic force microscopy (AFM) to provide further insights and obtain quantitative information about the film roughness (Fig. 2b). The average root-meansquare roughness for an area of 5 9 5 lm 2 was 14.7 nm, which was propitious for the deposition of perovskite [28]. The morphology of the perovskite films based on the SAH-TiO 2 film is presented in Fig. S4. The surface exhibited a uniform morphology with dense grains. The entire film was composed of a homogeneous well-crystallized perovskite layer, with crystalline grain lengths on the order of hundreds of nanometers. This may have been induced by the smooth morphology of the SAH-TiO 2 film, which was beneficial for the growth of perovskite layers. More details about the crystal structure of the product are shown in Fig. 2c [29]. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical composition of the TiO 2 film. The Ti 2p 3/2 , Ti 2p 1/2 , and O 1s characteristic peaks of the TiO 2 film are shown in Fig. 2d. The Ti 2p spectra were identical to the Ti 2p 3/2 and 2p 1/2 spectra, with peaks centered at binding energies of 458.9 and 464.7 eV, respectively, typical for Ti 4? states. The corresponding O 1s spectra showed two major peaks at 529.9 and 532.6 eV, assigned to the lattice oxygen and surface bridging oxygen, respectively [30]. In general, the thicknesses of the resulting films are affected by the concentration of the precursor solution [31]. Figure 3 shows the cross-sectional SEM images of the TiO 2 layers formed by SAH using different concentrations of a TiCl 4 aqueous solution (0.2-0.6 M). The TiO 2 nanoneedles were well developed on the FTO substrate, forming thin films with thicknesses of 20, 45, and 80 nm for TiCl 4 aqueous solution concentrations of 0.2, 0.4, and 0.6 M, respectively. The changes in the corresponding surface morphology are shown in Fig. S5. It is well known that the TiO 2 layer plays an important role in the charge collection within PSCs, where a pinhole-free layer of appropriate thickness is desirable for an effective charge extraction. A simple method to evaluate the density of pinholes within the SAH-TiO 2 layers is shown in Fig. S6 [32]. The resistance value gradually increased with the TiCl 4 precursor solution concentration owing to the increased thickness of the TiO 2 layer. Moreover, the difference between the resistance values for the Ag paste and Ag vapor treatments of the TiO 2 layers were significantly larger when the TiCl 4  Table 1. The PCE initially increased and then decreased with the increase of the SAH-TiO 2 layer thickness. With the 0.1-M TiCl 4 precursor solution, the coverage of TiO 2 on the FTO substrate was unsatisfactory (Fig. S5a), which increased the risk of contact between the FTO substrate and perovskite, leading to a low photovoltaic performance. With the increase of the concentration of the TiCl 4 precursor solution to 0.4 M, the coverage on the FTO substrate improved and pinholes gradually disappeared (Figs. S5b-d and 3d). Therefore, all three photovoltaic device parameters, shortcircuit current density (J sc ), open-circuit potential (V oc ), and fill factor (FF), increased during this evolution process, suggesting an efficient suppression of the charge recombination on the FTO surface. The SAH-TiO 2 films (* 45 nm) prepared by treating the FTO substrate with 0.4 M TiCl 4 exhibited the highest PCE of 17.09% with a J sc of 21.96 mA cm -2 , V oc of 1.069 V, and FF of 0.728. Furthermore, the integrated photocurrent from the incident photon-to-current conversion efficiency was 21.91 mA cm -2 , which is close to that from the J-V measurement (Fig. S8). However, a further increase in the concentration of TiCl 4 led to a deterioration of the device performance owing to the lower electron transport in the thicker TiO 2 layers and prolonged transport path, increasing the series resistance [33]. This could be confirmed by the apparent decrease in the J sc and FF values.
It is well known that SC is only suitable for a small-area deposition simply, as the spinning rates at the core and edges significantly differ, leading to a poor uniformity of the layers in a large-area deposition. To verify this, we attempted to deposit a TiO 2 layer on a large-area substrate (approximately 6 9 6 cm 2 ) by SC; a reference sample prepared by SAH was used for comparison. The large coated substrates were divided into 9 small pieces (2 9 2 cm 2 ) along the white dashed lines shown in the inset of Fig. 5a. The UV-Vis light absorption spectra of these TiO 2 films deposited by SC and SAH are shown in Fig. 5a, b, respectively. The light absorption at 320 nm was chosen as the index to determine the film uniformity as the absorption was proportional to the TiO 2 film thickness. The variation in the absorption of the SAH-TiO 2 film at 320 nm was only 6%, whereas that of the SC-TiO 2 film was 17%, which suggested that SAH was a more efficient deposition system than SC to fabricate uniform TiO 2 films. Undoubtedly, the uniformity of the TiO 2 layer influences the device performance. To illustrate this point, perovskite devices were fabricated with small TiO 2 pieces obtained, as mentioned above. The statistical PCE distributions of 45 samples in 5 batches of PSCs based on SC-TiO 2 and SAH-TiO 2 films are shown in Fig. 5c, d, Fig. S9 and Table S1. The fluctuations in PCE for the SAH-TiO 2 -based PSCs in different sub-areas were very small, indicating a higher reproducibility.
The proposed SAH method for TiO 2 film growth exhibited a remarkable uniformity over large surface areas. A 4 9 4 cm 2 sub-module comprising 6 cells connected in series was considered to demonstrate the consistency of the  SAH method. A photograph of the module and its J-V characteristics are shown in Fig. 6. The module exhibited V oc of 6.12 V, J sc of 3.48 mA cm -2 , remarkably high FF of 0.658, and overall conversion efficiency of 14.01% with hysteresis. The obtained results of the module using SAH-TiO 2 as the electron transport layer are encouraging for the development of perovskite photovoltaic technologies at low temperatures.

Conclusion
A simple SAH method was introduced for large-scale deposition of TiO 2 films at low temperatures. Using this method, we achieved compact homogeneous TiO 2 films with a needle-like morphology. A solar module fabricated using SAH-TiO 2 films exhibited a PCE of 14.01% with hysteresis. The results indicated that SAH is a convenient and versatile novel approach for the deposition of largearea TiO 2 films, demonstrating a large potential for practical applications in the future.