Sequential deposition method of TiO2/CH3NH3PbI3 films for solar cell application

Seeking to study innovative solar cell compositions with the goal to reach the highest energy efficiency level attainable, the aim of this study was to develop a route to obtain a solar cell composed by hybrid perovskite (CH3NH3PbI3) using a sequential deposition method through the techniques of spin-coating and immersion. Initially, the deposition of PbI2 thin film of was performed on a FTO/glass substrate coated with TiO2, which was subsequently converted into perovskite crystals through spin coating using a CH3NH3I solution. The influence of the PbI2 layer thickness on the formation of CH3NH3PbI3 crystals was evaluated. The hydrophilic characteristic of TiO2 affects the distribution of the crystals nucleation sites, since PbI2 possesses a non-polar liquid characteristic. The characterization of the perovskite thin films showed that thickness affects directly the bandgap and the surface morphology, revealing the presence of dendritic structures and acicular crystals. Both growth and coverage increased for thinner layers of PbI2. It was also possible to observe an increased uniformity in the film for smaller PbI2 layers.


Introduction
The development of sustainable energy sources has become critical, since most of the global energy still relies on fossil matter, whereas a scenario of an industrial sector considerably dependent on coal and the urban mobility that almost entirely leans on polluting fuels such as gasoline and diesel can be verified. Within this context, solar energy is increasingly becoming an attractive alternative, causing a major increase in research and development of more efficient and inexpensive photovoltaic devices either for industrial plants or, more recently, solar-powered zero-emission vehicles [1][2][3]. Aiming to explore novel and more energy-efficient manufacturing alternatives for solar cells, recent literature have focused on perfecting techniques of deposition of different thin films over the solar cell substrate to allow a higher absoption of light and, consequently, enhanced levels of output energy.
The use of organic-inorganic perovskite halides as an absorbable layer in thin film solar cells started in 2009, resulting in a PV device with 3.1% efficiency [4]. In recent years, the rapid evolution of the device has led to a modification of its structure, with a large increase in efficiency, reaching values above 23.7% [5]. This organometallic material has been widely explored due to the singular  [6], ambipolar load transport [7], and very long diffusion lengths [8].
Deposition of perovskite halide thin films can be performed by vapor phase deposition, solution-gel chemistry approaches, one step deposition, and two-step solution synthesis [9][10][11]. Usually, perovskite is synthesized by combining two precursors, an organic and other inorganic, which can be combined by different deposition routes [12]. Most deposition methods are based on the same principle: the combination of an organic component iodide methylammonium (MAI), with an inorganic component, such as iodide or lead chloride (PbI 2 or PbCl 2 ), to produce a perovskite (CH 3 NH 3 PbI 3 or CH 3 NH 3 PbI 3- x Cl x , respectively) [13].
In this work, we have compared the velocity of deposition of halide perovskite (CH 3 NH 3 Pb 2 I 3 ) using a simple two-step method of deposition, over a mesoporous TiO 2 layer deposited in F doped SnO 2 , to access morphological and optical properties for future use in solar cells.

Preparation of precursor solutions
For the preparation of the CH 3 NH 3 PbI 3 films it was used glass covered with FTO (Fluorine doped Tin Oxide) with measurements of 2.5 cm long and 1 cm wide. The slides were immersed in acetone and placed on the ultrasound over 5 minutes for cleaning. They were washed with distilled water and then subjected to a temperature of 400 °C for 30 min to eliminate any organic matter that was present. A TiO 2 paste was prepared for the deposition of TiO 2 by spin coating. The paste was obtained by grinding 50 g of TiO 2 -P25 (Evonik), 20 g of titanium isopropoxide (97 %, Sigma-Aldrich), 2.5 g of carboxi methyl cellulose (Sigma-Aldrich), 80 mL of terpineol (Sigma-Aldrich), 4 mL of acetyl acetonate (Sigma-Aldrich) and 5 mL of ethanol (99 %, Zeppelin) for a period of 12 h, so the paste was fully homogenized. For the preparation of PbI 2 solution, it was used a two-steps methodology: 1 M solution was obtained dissolving PbI 2 in anhydrous N, N-Dimethylmethanamide under agitation at a temperature of 70 °C. The precursor solution was ready for use when the salt was completely dissolved and with a translucent yellow color. For the second stage of two-step deposition, a solution was prepared composed of 10 mg/mL CH 3 NH 3 I in anhydrous isopropanol, agitated for a period of 10 min, in which all iodine methylammonium was dissolved.

Deposition in thin film
The TiO 2 mesoporous layer was made from a dissolution of the TiO 2 paste in ethanol with a proportion of 2:7. The solution was deposited at 3000 rpm per 20 s. The obtained film was dried at 125 °C for 10 min and then sintered 450 °C for 30 min. The perovskite layer was prepared using sequential two steps method describe by Burschka et al [11], but the second step was modified using the spin coating technique. The PbI 2 solution deposition was performed by spin coating, with different speeds for obtaining different thicknesses: 2000, 3000 and 4000 rpm, with two consecutive depositions. After this step, a 10-minute drying was made on a heating plate at 70 °C. Subsequently, each sample was covered with a solution of MAI in a spin coating process using a speed of 2000 rpm, followed by drying the film at 100 °C for 30 min. The film samples were named as PV-2, PV-3, PV-4 according to their respective deposition speeds: 2000, 3000 or 4000 rpm. The whole experiment was carried out at ambient condition (room temperature and open atmosphere).

Results and Discussion
For morphological characterization, scanning electron microscopy (SEM) was used to analyze the layer thickness influence of the PbI 2 films. The comparisons of the results obtained under the effect of the variation of deposition parameters are presented in Figure 1. The analyses revealed that the samples are homogeneous (without cracks) and with uniform appearance throughout the whole surface. It can be seen in Figure 1 (a), (c) and (e) that as the PbI 2 deposition velocity was increased,  [14,15] possibly affects the distribution of the nucleation sites of the crystals originating from the PbI 2 . Figure 1 (e) revealed the presence of dendritic structures and needle-shaped crystals. There is the formation of perovskite grains for the speeds of 2000 and 3000 rpm (Figure 1 a to d). When 4000 rpm was used (Figure 1 f) there is no presence of these grains, and only dendritic structures are formed. This behavior of dendritic formation can be explained by the rapid cooling of the PbI 2 solution due to its higher velocity of deposition. It is also possible to observe the mesoporous layer of TiO 2 below the grains and dendrites of formed perovskites. The optical bands for CH 3 NH 3 PbI 3 /TiO 2 /FTO films were determined by diffuse reflectance measurements (Figures 2 a, b and c). The measure of the extinction coefficient , which is proportional to F(R), is calculated using the reflectance data according to the equation Kubelka-Munk [16,17] 4 energy and n is the power index that is related to the optical absorption, which in this case is related to a indirect transition and have the value of 1/2. According to other studies, the band gap of the TiO 2 determined based on the indirect transition is between 3.17 and 3.28 eV [18], due to the higher presence of the anatase phase. The CH 3 NH 3 PbI 3 bandgap is equivalent to 1.5 eV [6], which is formed between the orbital Pb and I. The calculated values for the optical band gaps are 3.15; 3.3 and 2.7 eV respectively, for PV-4, PV-3, PV-2 samples. It was observed that for higher deposition speeds, the bandgap is directly related to the presence of TiO 2 , because the perovskite layer is thinner. For the deposition in lower velocity, the band gap was estimated between the values 1.5 and 3.2 eV due to the greater presence of perovskite. The UV-vis absorption spectra of CH 3 NH 3 PbI 3 films deposited on TiO 2 were depicted in Figure 3a. As can be seen in the diagram, the absorption spectra in the UV region have prominent peaks at 280 nm for the PV-2 , for PV-3 around 282 nm and for PV-4 around 281 nm indicating a perovskite formation [19]. The film prepared at 3000 rpm presents a greater absorption compared to the other films corroborating the results of SEM where it is possible to observe a greater formation of perovskite. In the other films, there is no significant difference in the absorption spectrum.  The X-ray diffractogram for PV-3 is shown in Figure 3b. The diffractograms of the other samples were very similar to the PV-3 spectra. It can be noted that the diffraction signal related to the formation of perovskite CH 3 NH 3 PbI 3 in 2 = 14° is the most intense, and confirms the high crystallinity of the synthesized film. In addition, a weak signal at 2 = 12.4 °, related to the precursor PbI 2 , can be seen, indicating that the majority of PbI 2 precursor has been transformed into perovskite. Usually, the presence of PbI 2 is related to the degradation of the perovskite in the presence of moisture [20]. The XRD results indicates that the perovskite film has a good stability because only a residual presence of the precursor was identified. Finally, the perovskite planes (202) and (220), in 2 = 24.4° and 2 = 28.3°, respectively, can be identified, in addition to the anatase peaks at 2 = 25.2° and rutile at 2 = 26.4°, belonging to TiO 2 film from the substrate [21]. To calculate the average grain size of the 10 20 30 (220) perovskite, the Scherrer equation was used, considering the value of fwhm for the peak associated with the plane (110). Analysis of this data provided an average grain size of approximately 91 ± 3 nm.

Conclusion
In this study, the influence of the thickness of PbI 2 layers on FTO/TiO 2 /CH 3 NH 3 PbI 3 films was investigated. The surface morphology of the films showed that with the increase of deposition velocity, there is a variation in the formation of perovkskite crystals, which can be explained by the rapid cooling of the PbI 2 solution in the FTO substrate. The PV-3 sample was the one with the better homogenous distribution of perovskite. There was a variation in the optical band gap for the lower velocity layer due to the greater presence of deposited material (PbI 2 ). The sample PV-3 had a more expressive absorption due to the greater amount of perovskite formed. Little crystallographic variation was observed, confirming the perovskite formation for all samples. The thin films have been synthesized successfully indicating that it is possible to employ this film in a future use for solar cells. The methodology allows fast production of thin films with lower cost due to their possible production in ambient conditions.