Versatile vapor phase deposition approach to cesium tin bromide materials CsSnBr3, CsSn2Br5 and Cs2SnBr6

We report on the successful application of RF-magnetron sputtering to deposit, by using a single type of target, three different materials in the form of thin films within the Cs–Sn–Br compositional range, namely, CsSnBr3, CsSn2Br5 and Cs2SnBr6. It is shown that, by playing with the deposition parameters and post-deposition treatments, it is possible to stabilize these three perovskites or perovskite related compounds by exploiting the versatility of vapor phase deposition. Full characterization in terms of crystal structure, optical properties and morphology is reported. The power of vapor phase methods in growing all-inorganic materials of interest for photovoltaic and optoelectronic applications is demonstrated here, indicating the advantageous use of sputtering for these complex materials.


Introduction
In recent times, there has been a growing interest towards allinorganic perovskite materials for their application in perovskite solar cells (PSCs) and optoelectronics. Among others, it is possible to mention the use of CsPbBr 3 and CsPbI 3 , and their solid solutions, in the fabrication of PCSs, as well as their possible use in optical devices due to the superior emission properties when dealing with nanosized materials. [1][2][3][4][5] Besides the well-established Pb-based all inorganic 3D perovskites, there is an intense and continuous interest in developing leadfree phases together with the search of more stable compositions by reducing the dimensionality of 3D materials, thus exploring 2D, 1D, 0D, and perovskite related phases. 6-10 3D leadfree all-inorganic materials are now currently employed in the fabrication of photovoltaic devices, as can be seen by the recent use of CsSnI 3 or Bi-based double perovkites. 11 On the other hand, the exploration of lower-dimensional lead-free perovskite and perovskite-related phases is still a challenge. One of the main reason can be found in the difficulty in achieving thin lms of all-inorganic materials, where the common wetchemistry protocols used for hybrid organic-inorganic (HOIP) phases do not reliably assure good results. For example, CsBr has a poor solubility in some apolar solvents such as dimethyl fomamide, DMF, dimethyl sulfoxide, DMSO, etc. thus limiting the use of one-step depositions methods. 12 Even with modied one-step depositions, the methods do not provide uniform lms and in other cases annealing at elevated temperatures is required, which is impractical and leads to a reduction of the overall device performance. 13 Two-step deposition methods provide better results in terms of uniformity but are more timeconsuming and require the control of several parameters. 12 One possible method to overcome such limitation is the use of vapor phase deposition methods, but also in this case the current methods of target evaporation by heating (commonly used for HOIPs) cannot be easily used for inorganic materials showing low volatility. In many cases, aer vacuum deposition methods, a thermal annealing at temperature up to 320 C is required. 14 In addition to this, when dealing with tin-based systems, one of the most exploited choice to substitute for lead, the stabilization of Sn 2+ oxidation state during usual solution-based synthetic procedures can be a challenge, thus requiring the use of additives or complex synthetic approaches. 7 In this paper, we are going to focus on a series of phases, within the Cs-Sn-Br compositional phase diagram, by showing a vapor phase deposition approach based on RF-magnetron sputtering which allows, by tuning the deposition parameters or the post-synthetic treatments, to access at least three singlephase compounds, namely: CsSnBr 3 , CsSn 2 Br 5 and Cs 2 SnBr 6 , by using the same starting target material. RF-magnetron sputtering has been already shown, by our group, to be a suitable technique for metal halide perovskites because of its benets in terms of reliability, simplicity, and scalability, among others, In addition, in most of the cases, a fully crystalline and uniform lm is obtained without the requirement of any thermal annealing. Quite surprisingly, however, this method has not yet been fully explored in the photovoltaic eld. 15

Post-deposition heating
Aer the deposition selected lms have been heated and cooled in vacuum by using a BÜCHI glass drying oven. Others were instead heated and cooled in air by means of an oven.

XRD diffraction
The structural properties of the deposited thin lms were characterized by X-ray diffraction (XRD) by means of a Bruker D8 Advance instrument (Cu radiation) in a Bragg-Brentano setup. EDX analysis provided an agreement within 5% between nominal and experimental compositions. Microstructural characterization of the samples was made using a highresolution scanning electron microscope (SEM, TESCAN Mira 3) operated at 25 kV.

Optical properties measurement
Absorptance (A) spectra were collected by using a UV-vis spectrophotometer Jasco750 with an integration sphere.

AFM
Atomic Force Microscopy (AFM) images (256 Â 256 pixels) were obtained with an AutoProbe CP microscope (ThermoMicroscopes-VEECO), operating in contact mode (C-AFM), by means of sharpened silicon tips onto V-shaped cantilevers (resonance frequency: 15 kHz; force constant: 0.03 N m À1 ). For each analyzed lm, scans of 10 mm Â 10 mm and 4.0 mm Â 4.0 mm have been carried out with a scan rate ranging from 1.0 to 1.5 Hz. A standard second-order atten processing of the images has been performed to correct the scanner nonlinearity.

Results and discussion
CsSnBr 3 is a typical 3D perovskite with a cubic unit cell and a band-gap around 1.72 eV, and was object of few studies addressing the growth of lms by using spin-coating and reactive thermal deposition to achieve epitaxial materials. [16][17][18] CsSn 2 Br 5 has never been reported in the form of lm (except as an impurity phase in ref. 17), and the few data available refer to crystal structure investigation in single crystals. 19 CsSn 2 Br 5 has a 2D tetragonal crystal structure belonging to the I4/mcm space group, and is composed by two adjacent Sn 2 Br 5 layers separated by a Cs layer along the c-axis. Signicant work has been carried out on the Pb-based counterpart, namely CsPb 2 Br 5 , which is considered a promising candidate for optoelectronic applications. [20][21][22] Finally, Cs 2 SnBr 6 is a Sn(IV) containing phase of great actual interest for both solar cells and optoelectronic applications, and is a vacancy ordered double perovskite with cubic symmetry (space group Fm 3m), and a reported band-gap around 3.2 eV. [23][24][25] For this phase no reports on thin lm preparation have been reported. A sketch of the crystal structures of the three compounds is reported in Fig. 1.
The samples have been prepared in form of lm on fused silica substrates by RF-magnetron sputtering starting from a target made of CsBr and SnBr 2 (see details in the ESI †). In this paper, we are reporting representative data for the three singlephase compounds obtained, which are the results of several replica of lm depositions. As mentioned above, the three compositions have been prepared using a single target by varying the sputtering conditions as shown in Table 1.
Essentially, the lm growth conditions were quite similar for the three phases, with tuning of sputtering power and thermal treatments as key parameters to modulate the phase composition. By heating the substrate during lm depositions to 200 C, CsSnBr 3 was prepared, while a post-deposition annealing to 200 C allowed forming Cs 2 SnBr 6 . Without in situ or postdeposition heating, CsSn 2 Br 5 is the stable phase formed under selected conditions. Fig. 2a-c shows the X-ray diffraction patterns of three lms of about 1 mm thickness representing CsSnBr 3 , CsSn 2 Br 5 and Cs 2 SnBr 6 , together with the reference patterns for each of them (as vertical bars).
As can be seen from Fig. 2, the three lms are single-phase with very good crystallinity, also for the sample prepared without any thermal treatment (i.e. CsSn 2 Br 5 ). No signs of peculiar preferential orientation effects are found in the patterns. Hump around 22 in Fig. 2 is due to the amorphous nature of the substrate (fused silica). Chemical composition of the prepared lms was checked by EDX (energy dispersive X-ray analysis) and was found in very good agreement with nominal stoichiometries. The results reported above clearly indicate the versatility of sputtering approach in modulating the deposited phases by simply changing the deposition and/or heat treatment parameters. Being the sputtering a quite complex process, oen far from equilibrium, it is not simple, and goes beyond the scope of the present paper, to understand the specic conditions leading to the stability of the different phases, probably related to the different sputtering efficiencies of CsBr and SnBr 2 and to surface reactions. Notwithstanding, the method is extremely reliable and, by keeping the same sputtering conditions/thermal treatments, the three phases are always obtained. The lattice parameters determined from the renement of the patterns reported in Fig. 2  On the representative lms shown in Fig. 2, we performed UV-vis absorption spectroscopy measurements, which are shown in Fig. 3, below.
The spectra of CsSnBr 3 well matches with the data reported for the bulk phase, as well as for the few thin lms available, with a very sharp absorbance around 700 nm, and a band-gap of $1.73 eV. [16][17][18]26 For this material, having and absorption in a range of interest for photovoltaic applications, also the photoluminescence (PL) spectra has been determined, and it is shown in the inset of Fig. 3a, indicating a maximum of emission around 710 nm. This result is also in agreement with previous data. 26 The quality of the absorption spectra of CsSnBr 3 lm, prepared by sputtering, is signicantly higher with respect to the data reported for lms prepared by wet-chemistry route, showing edges extending from 400 to 700 nm. 11 The spectra of CsSn 2 Br 5 shows broader features, possibly related to the lack of any thermal treatment, with a rst edge around 390 nm and a band-gap of about 3.2 eV, in fair agreement with the only available report on this phase, which is however for a very thin lm used as a barrier layer. 17 Finally, Fig. 3c reports the absorption spectra of Cs 2 SnBr 6 showing again a very sharp edge and an estimated band-gap of about 2.85 eV, in good agreement with previous reports on bulk materials, being this the rst time Cs 2 SnBr 6 is prepared in form of lm. 23,24   Finally, the morphology of the three lms reported above has been determined by Atomic Force Microscopy (AFM) and some representative images (4 mm Â 4 mm area) are shown below (Fig. 4).
There is a markedly different morphology in the three lms deposited. CsSnBr 3 (a) shows well-dened spherical grains of average dimension around 100-150 nm and a surface roughness (R rms ) around 30 nm; CsSn 2 Br 5 (b) is composed of grains of about 150 nm agglomerated into relatively big spherical objects (500-700 nm) which can be the result of the absence of any thermal treatment; and nally Cs 2 SnBr 6 lms (c) is characterized by polygonalshaped grains of a size around 200 nm. The peculiar shape of these last grains are in some way reminiscent of the hexagonal form found in nanosized samples of Cs 2 SnBr 6 particles. 24 Substrate coverage as well resulted to be quite good for these sputtered lms as can be inferred by 10 mm Â 10 mm images reported in the ESI. † Unfortunately, there are not direct AFM images collected on analogous lm to perform any relevant comparison. It is interesting to note, however, that any specic composition leads to a peculiar morphology (for analogous lm thicknesses) (Fig. 4).

Conclusions
This paper reports the successful deposition of three distinct single-phase materials in form of thin lms of the Cs-Sn-Br system by RF-magnetron sputtering, namely CsSnBr 3 , CsSn 2 Br 5 and Cs 2 SnBr 6 . The deposition approach used in this work allowed using the same starting target material and tuning the preparation of the desired phase by changing the sputtering parameters or applying mild post-deposition heat treatments. Structural, optical and morphology measurements conrm the quality of the prepared lms. The accessibility of complex allinorganic phases, which may be difficult to deposit in form of lms by means of traditional wet-chemistry routes, is demonstrated through a simple, reliable and scalable vapor-phase method such as sputtering.

Conflicts of interest
There are no conicts to declare.

Acknowledgements
GB thanks Fondazione di Sardegna -Convenzione triennale tra la Fondazione di Sardegna e gli Atenei Sardi, Regione Sardegna, annualità 2018 -L. R. 7/2007 for funding the project "Lead-free halide perovskites for high efficiency solar cells". The authors gratefully acknowledge the project PERSEO-"PERovskite-based Solar cells: towards high Efficiency and lOng-term stability"