Deposition of CsFAPbI3 thin films by single source flash evaporation

Formamidinium-cesium lead iodide (CsFAPbI3) is a promising perovskite material for photovoltaic applications with a suitable bandgap of 1.45 eV and excellent optoelectronic properties. In this work, CsFAPbI3 perovskite thin films were deposited by single-source flash evaporation on glass substrates using presynthesized crystalline powders as the source material in which the source challenges of simultaneously controlling the evaporation of organic and inorganic sources are avoided. The structural properties of the powders were evaluated by X-ray diffraction, thermal properties by TGA analysis and optical properties by UV-Vis absorption. We find that the formation of mixed phases is inevitable in flash evaporation of thin films. This undesirable phase could reduce the optical bandgap and the thermal stability which can affect the performance of the thin films. To obtain the cubic phase, a post-annealing process should be employed. We carried out structural, morphological, and optical characterizations to determine the phase purity in the films. These preliminary results suggest that flash evaporation deposition parameters can be optimized to understand the formamidinium evaporation and condensation dynamics for improve the properties of CsFAPbI3 perovskite thin films.


Introduction
Organic-inorganic halide perovskites with a structural formula ABX3 (A = monovalent cations CH3NH3 (MA + ), HC(NH2)2 (FA + ) or Cs + ; B = divalent cations Pb 2+ or Sn 2+ ; X = halides I -, Cl -or Br -), are very promising materials for optoelectronic applications [1].For example, in a short period, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has exceeded 25% [2].While these efficiencies have been developed on a small scale, the long-term stability of the devices is an important issue that must be addressed prior to commercialization.Currently, PSCs with better efficiencies are based on mixtures of single or multiple cations, since it has been shown that thermal and structural properties of perovskite materials are improved [3].
In particular, the double cation halide CsFAPbI3 perovskite has a narrow bandgap (~1.48 eV) and high thermal stability, which can result in higher PCE and better photostability of the fabricated devices [4].Compared with FAPbI3 perovskite, the incorporation of Cs + can inhibit the transition from photoactive α-cubic phase to photoinactive δ-hexagonal phase, and its moisture resistance is significantly improved [5,6].CsFAPbI3 thin films are mainly deposited by solution processing methods [7], however, it is difficult to obtain uniform and large area films.On the other hand, thermal evaporation in vacuum methods offer an alternative to solution processing methods, because compact and homogeneous thin films with scalable area are often obtained [8].In conventional evaporation methods, precursor powders are sublimed simultaneously from different sources, but it is difficult to control the stoichiometric ratio in mixed cation perovskite thin films because each source must be calibrated to find the evaporation rate of each material for obtain the correct stoichiometric composition [9].
Flash evaporation is an alternative for deposit of perovskites thin films, this method is based on a single source consisting of a metallic filament in which a large current is passed to rapidly evaporate the perovskite precursor and then condenses on the substrate to form a thin film.This simple method was used in 2001 by Mitzi et al., to deposit halide perovskite thin films, who called it single-source thermal ablation technique (SSTA) [10].In 2015, Longo et al., deposited crystalline MAPbI3 thin films whit good surface morphology by flash evaporation in low vacuum (~0.1 mbar) [11].Later, in 2016 Xu et al., deposited large area (36 cm 2 ) FAPbI3 thin films, showed that PbI2 free films can only be obtained by carrying out the deposit on substrates heated to 105 °C and using a molar ratio of 1.5 of FAI for compensate the loss of this material [12].In the literature there are few works of perovskite thin films obtained by flash evaporation, so there are many variables that can be explored.
This work shows our initial findings on the use of single-source flash evaporation to deposit CsFAPbI3 thin films.We first synthesized CsFAPbI3 crystals by inverse temperature crystallization (ITC) method.The obtained crystals were ground into powder and then placed in a graphite crucible.The deposition of the CsFAPbI3 films was carried out by applying a current to the crucible, which generates direct heating of the powder resulting in rapid evaporation.The structural properties of the perovskite powders were evaluated by X-ray powder diffraction (XRD), the thermal properties by Thermal gravimetric analysis (TGA) and the optical properties by ultraviolet visible (UV-Vis) spectroscopy.Finally, the structural characterizations show that the films obtained present mixture of cubic and hexagonal phases.After annealing, the films do not present secondary phases and therefore, the number of structural defects decreases, and the optical properties of the films are improved.These results provide strategies to optimize the development of this method and maximize the formation of thin films with desirable phases for different optoelectronic applications.

CsFAPbI3 crystals growth
CsFAPbI3 single crystals were synthesised by inverse temperature crystallization (ITC) method introduce by Saidaminov et al [13].The schematic procedure and crystals are shown in Figure 1.The stoichiometry of perovskite obtained is .This structure was obtained dissolving FAI, CsI and PbI2 in 2 ml of gamma butyrolactone ( -GBL) as solvent, at room temperature under stirring.The solution was filtered using PTFE filters with a 0.2 μm pore size.The filtered solution was put in a 4 ml vial, and this was placed in oil bath at 80 °C.The temperature was gradually increase to 115 °C and the time of crystallization take around 2 h.When mixing the precursors in -GBL and heating the solution, leads to perovskite crystallisation due to the retrograde solubility of CsFAPbI3.The cultivate crystals were dry and transfers to a clean vial and keep in vacuum until use.Then, crystals were ground in an agate mortar until powder and used as source material.

CsFAPbI3 thin film evaporation
The schematic diagram of the flash evaporation system used to deposit CsFAPbI3 thin films is shown in Figure 2. Thin films were fabricated on glass substrates of cm 2 previously cleaned with sonication treatment in 2% Hellmanex solution, acetone, and isopropanol, followed by ultraviolet-ozone treatment for 15 min.The clean substrates were dry with a nitrogen flow and 4 substrates were placed in a sample holder, that was mounted in the evaporation chamber at a vertical distance of 6 cm from the evaporation source.Then, 30 mg of CsFAPbI3 crystal powder was put in a graphite crucible which was clamped between two electrodes.When the pressure in the chamber was below 1×10 −6 Torr, the crucible working current increased from 0 to 8 A, and the source temperature increased rapidly to the point where the powder evaporates completely in a few seconds.The total deposition process was approximately 3 minutes and then the current was lowered to 0 A. The thickness of the CsFAPbI3 thin film was 300 nm and can be controlled by adjusting the amount of powder in the source.

Characterization
The X-ray diffraction patterns of the powders and thin films were measured with a PANalytical Empyrean equipment using Cu radiation (1.5406 Å).Absorbance spectra were acquired using a Cary 5000 UV-vis/NIR spectrophotometer in the wavelength range from 350 to 900 nm.Photoluminescence spectra were performed with a Horiba spectrofluorometer, NanoLog model, using a pulse Xenon Lamp, with an excitation wavelength of 380 nm.The thickness of the films was measure using a DEKAT 150 profilometer.The thermal stability properties of CsFAPbI3 powders were obtained using the Discovery Simultaneous Thermogravimetry (TGA) and Differential Scanning Calorimetry (DSC) system in an N2 atmosphere.The SEM image was taken with a Hitachi SU-8230 field emission scanning electron microscope, and the average grain size was obtained with ImageJ software.
The color of CsFAPbI3 crystals gradually changed from black to yellow after 24 h regardless of vacuum storage, indicating the transition from cubic -CsFAPbI3 phase to hexagonal -CsFAPbI3 phase.The diffraction peaks at 11.75°, 16.28°, 26.20°, 30.54°, 32.87° and 41.66° correspond to perovskite -CsFAPbI3 hexagonal phase.However, the -CsFAPbI3 phase completely disappeared when the powders were annealed at 150 °C and the -CsFAPbI3 cubic phase was obtained again.
The lattice parameters were determined with the X´Pert Highscore Plus software PANalytical and the crystallite size was calculated from XRD patterns of the powders by using Debye-Scherrer: (1) where, is the crystallite size, is the full width half maximum (FWHM) of the reflection peak that has the maximum intensity in the diffraction pattern, is the wavelength of X-rays and is the diffraction angle of the X-rays.
The crystallite size of -CsFAPbI3 powders was calculated in the range of 88.67 nm with lattice parameter a= 6.3522 consist with the cubic -CsFAPbI3 phase.For -CsFAPbI3 powders the crystallite size was 85.07 nm with lattice parameters a=5.8939 , b=9.6313 and c= 11.2344 corresponding with hexagonal -CsFAPbI3 phase, similar results was presented by J.A Vigil et al [15].
The annealed powders showed prolonged stability at room temperature.This could be because the incorporation of Cs + into the perovskite improves humidity stability and decreases the phase transition temperature, which is attributed to the enhanced interaction between formamidinium HC(NH2) 2+ and iodide due to contraction of volume of the cubic-octahedral.The ionic radius of Cs + being smaller than that of the FA + cation helps reduce the Goldschmidt tolerance factor of 1.04 to the ideal range and, therefore, induces phase stability at moderately lower temperatures.It could be said that due to the low transition temperature of powders it can be used in single source vacuum evaporation processes [16].The UV-Vis absorbance spectra of both powder samples were measured, and the results are presented in Figure 4.The absorption edge changed from 610 nm (for -CsFAPbI3) to 880 nm ( -CsFAPbI3), respectively.The optical absorption of -CsFAPbI3 covers the entire visible region as well as the near-IR region.The optical bandgap was obtained from Tauc-plots approximation, which showed the values of 1.45 eV and 2.09 eV for -CsFAPbI3 and -CsFAPbI3 crystal powders, respectively, which agrees with previous reports [17].Figure 5 shows the curves to obtain the Urbach energy according to equation: ( where is the absorption coefficient, and are constants, and is the photon energy.We calculate the exponential relation between the absorption coefficient and the photon energy and the from the slope of the exponential decay of the absorption onset.-CsFAPbI3 registered the lowest values of 41.31 meV, while -CsFAPbI3 exhibited a higher value of 27.60 meV.The significantly low value for -CsFAPbI3 is related to a better crystallinity as observed in the diffraction patterns of Figure 3, where the -CsFAPbI3 phase does not contain the formation of PbI2, and the intensity of the peaks is greater with respect to the -CsFAPbI3 phase.It has been reported that this decrease in is also associated with a low density of defects and/or impurities [18].The thermogravimetric analysis (TGA) values were measured for -CsFAPbI3 powders and it's shown in Figure 6.At 330 °C, FAI evaporated before the inorganic components of CsFAPbI3 powders.Hightemperature stability and the low phase-transition temperature of pre-synthesized perovskite powder suggest that could be used in single-source evaporation method.This opened the pathway of replacing the complicated multisource evaporation.To test this easy thermal evaporation method, we deposited a CsFAPbI3 film using the presynthesized crystal powders.The structural properties of the deposited CsFAPbI3 thin film were analyzed by XRD measurements.As the diffractogram shown in Figure 8, the obtained CsFAPbI3 film presents a mixedphase perovskite, according to the diffraction peaks at 11.79° and 26.29° which correspond to (100) and (211) diffraction planes of the hexagonal -CsFAPbI3 phase and the diffraction peaks at 13.89° and 26.20°, which correspond to (022) and (211) diffraction planes of the photoactive cubic α-CsFAPbI3 phase.In this method, it is difficult to control the vapor reaction between different molecules of the precursors simultaneously, which easily leads to the presence of phase mixing.As shown in the diffractogram, phase-pure -CsFAPbI3 can be only achieved if the films are annealed at 150 °C for 10 minutes.Since the crystalline size affect the electronic properties of the film, an estimation of this parameter was calculated by Debye-Scherrer equation.The crystallite size of -CsFAPbI3 thin film was calculated in the range of 69.24 nm with lattice parameters a= 6.3522 consist with the cubic -CsFAPbI3 phase.For -CsFAPbI3 thin film the crystallite size was around 27.64 nm with lattice parameters a=6.6468 , b=5.9475 and c= 11.2162 corresponding with hexagonal -CsFAPbI3 phase, similar results was presented by Lee et al [19].The annealed film exhibited higher value of crystallite size due to a lower density of structural defects.The optical properties of the thin films were analyzed by UV-vis absorption and PL photoluminescence spectrum as shown in Figure 9.The absorption edge is observed at 810 nm.The energy bandgap of the -CsFAPbI3 perovskite thin film was calculated from Tauc-plot approximation, which has a value of 1.53 eV.As seen in Figure 7b, the maximum photoluminescence peak of the -CsFAPbI3 perovskite thin film is located at a wavelength value of 809 nm, which corresponds to a bandgap energy value of 1.53 eV.This means that this emission is due to edge transactions and/or defects close to the valence band, associated with vacancies or interstices of FAI.
Here the energy bandgap value is calculated using the equation: (3) where is the bandgap energy and is the wavelength (in nm) corresponding to the maximum value of PL intensity.This bandgap energy value is consistent with the value obtained from the UV-Vis absorption spectrum of the same sample.Finally, we obtained the Urbach energy of the -CsFAPbI3 thin film (Figure 10) which was 24.85 meV.Biwas et al., report a value of 27 meV [20], they mention that this value is associated to films with low density of deep levels and high crystallinity.In our case, high crystallinity is observed according to the XRD diffractogram as shown in Figure 8.The -CsFAPbI3 films without thermal treatment have a greater number of defects such as FA vacancies which can increase the density of deep levels in the bandgap that act as recombination centers, and therefore, reducing the intensity of photoluminescence emission [21].When the films are annealed, these defects are reduced and according to the PL spectrum, the emission comes from states located at the band edge of CsFAPbI3 perovskite.The asymmetry of PL spectrum can be ascribed to the radiative recombination of photocarriers localized at the band tails near the band-edge of the perovskite.

Conclusions
This work highlights the promising application of the single-source flash evaporation method to deposited CsFAPbI3 perovskite thin films using crystal powders.The deposited films were uniform and showed both phases, including the desirable -phase and the undesirable -phase.However, postannealing treatment improves the crystalline phase of the thin films.Our results suggest that controlling the growth conditions during evaporation of the films and post-annealing improves the intrinsic stability of the films since the formation of non-radiative recombination centers is decreased.Optimizing the deposition parameters in this method will improve the evaporation and condensation dynamics of formamidinium and its degradation products.

Figure 8 .
Figure 8. XRD patterns of As- deposited and annealed CsFAPbI3 thin films.