Silver-Doped CsPbI 2 Br Perovskite Semiconductor Thin Films

: All-inorganic perovskite semiconductors have received significant interest for their potential stability over heat and humidity. However, the typical CsPbI 3 displays phase instability despite its desirable bandgap of ~1.73 eV. Herein, we studied the mixed halide perovskite CsPbI 2 Br by varying the silver doping concentration. For this purpose, we examined its bandgap tunability as a function of the silver doping by using density functional theory. Then, we studied the effect of silver on the structural and optical properties of CsPbI 2 Br. Resultantly, we found that ‘silver doping’ allowed for partial bandgap tunability from 1.91 eV to 2.05 eV, increasing the photoluminescence (PL) lifetime from 0.990 ns to 1.187 ns, and, finally, contributing to the structural stability when examining the aging effect via X-ray diffraction. Then, through the analysis of the intermolecular interactions based on the solubility parameter, we explain the solvent engineering process in relation to the solvent trapping phenomena in CsPbI 2 Br thin films. However, silver doping may induce a defect morphology (e.g., a pinhole) during the formation of the thin films.

In the meantime, in 2014, the Snaith group compared three materials, CsPbI 3 , MAPbI 3 , and FAPbI 3 , which have bandgaps (E g ) of 1.73 eV, 1.57 eV, and 1.48 eV, respectively, and focused on the FAPbI y Br 3−y (y = 0 to 1) perovskite system, demonstrating the usefulness of the slightly larger FA cation as well as the tunability of the bandgap [6].Then, in 2015, CsPbI 2 Br using the density functional theory (DFT).Then, by adding silver bromide into the perovskite precursor solutions with a PbI 2 /AgBr = 1:0.01-0.03molar ratio, we studied the doping effects on the optical, structural, and morphological properties of semiconductor thin films.In addition, we explained the solvent-trapping phenomenon during the solvent engineering process [2,35], because this trapping was detected by the infrared (IR) spectroscopy.Herein, we employed the Hildebrand and Hansen solubility parameter [36,37] to explain the intermolecular interactions between the solvent and antisolvent.

Methods
The perovskite precursor solutions were prepared to synthesize the CsPbI 2 Br perovskite semiconductor without or with AgBr, for which the composition was controlled as follows: (CsBr) 1 (PbI 2 ) 1−x (AgBr) x with x = 0.00, 0.01, 0.02, and 0.03.Here, the control sample was composed of CsBr (1M) and PbI 2 (1M) in 1 mL DMSO.Then, after stirring at 500 rpm using a magnetic stirrer for 12 h at 70 • C, the solutions were filtered by a polytetrafluoroethylene (PTFE) syringe filter with a 0.22 µm pore size.Then, 70 µL of solution was dropped on top of the glass substrate (Microscope Slides; Grade-P.2;model number: ISO8037/IS3099 [38]; size: 76 mm × 26 mm × 1.35 mm; Rohem Instruments, Maharashtra, India) and spin-coated at 1500 rpm for 45 s using a spin coater (model: spinNXG-P2; Apex Instruments, Kolkata, India).During spinning, after ~25 s, 200 µL of CB was dispensed on top of the wet perovskite precursor film.After spin-coating, the sample was annealed thermally at 70 • C for 2 min and subsequently at 280 • C for 10 min.Then, all the characterizations were carried out under ambient conditions.

Thin Film Characterizations
The X-ray diffraction (XRD) patterns were obtained using the Drawell XRD-7000 diffractometer (Shanghai, China) with Cu Kα (3 KW) X-ray radiation (λ = 1.5406Å), having a source potential of 30 kV and source current of 25 mA (here, with a 2θ range of 10 • to 60 • ; a scan rate of 1 • per minute; a step size of 0.01 • ).Scanning electron microscope (SEM) images were obtained using the benchtop SEM (JCM-6000 Plus, JEOL, Tokyo, Japan) at 5 kV.On the other hand, the high-resolution transmission electron microscopy (HR-TEM) images were investigated by using the model JEM-2100 (Peabody, MA, USA) at 200 kV.The absorption spectra of the film were investigated by ultraviolet-visible (UV-Vis) spectroscopy (PerkinElmer Lambda 25, Kyoto, Japan) in the wavelength range of 350 nm to 750 nm at the scanning speed of 240 nm per minute and the step size of 1 nm.The photoluminescence (PL) lifetime curves were recorded by using the time-correlated singlephoton counting (TCSPC) (model: Fluor log 3 TCSPC, Horiba, and Houston, TX, USA) with an excitation wavelength of 570 nm.The Fourier transform infrared (FT-IR) spectroscopy data were obtained using the PerkinElmer spectrum two FT-IR spectrometer (Waltham, MA, USA).Here, the attenuated total reflection (ATR) was employed to record the transmittance data from 4000 to 400 cm −1 with a resolution of 0.5 cm −1 .All measurements were taken at room temperature in ambient conditions.

Computational Method
The electronic band structures of the compounds (CsPbI 2 Br and Ag-doped CsPbI 2 Br) were calculated based on the DFT using the Vienna Ab initio Simulation Package (VASP) software (version 4.3.3) in the supercomputing resources provided by the Indian Institute of Science (Bengaluru, India).

Results and Discussion
The projector-augmented-wave (PAW) method, which is included in the VASP code, was used for all DFT calculations [39].The Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional was used for all structural relaxations [40].After optimizing the CsPbI 2 Br structure based on the tetragonal β-CsPbI 3 crystal (space group P4/mbm), the lattice parameters were determined to be a = b = 6.395Å and c = 5.988 Å.These values were very close to Chen et al.'s reports of a = b = 6.40 Å and c = 5.97 Å for the pseudo-cubic α-phase CsPbI 2 Br [41].Next, the 2 × 2 × 1 supercells (Figure 1) were built using the optimized structure as a basis to study the electronic structures of Cs 4 Pb 4−x Ag x I 8 Br 4 (x = 0, 1, 2, 3, and 4) perovskites.Here, when x = 0, the structure is equivalent to four times of the pure CsPbI 2 Br perovskite, as shown in Figure 1a (supercell), whereas, when x = 1, 2, 3, or 4, the structures become the silver-doped perovskite, as shown in Figure 1b.Here, note that the radius of Br − is smaller than that of I − , which allows the CsPbI 2 Br to form a stable crystal structure compared to CsPbI 3 .In this study, Pb-Br and Pb-I have bond lengths of 2.9940 Å and 3.1975 Å, respectively.According to Shannon [42], the effective ionic radius at the relevant coordination number (CN) is as follows: Cs + (CN = XII) (1.88 Å), Pb 2+ (VI) (1.19 Å), I − (VI) (2.20 Å), and Br − (VI) (1.96 Å).Hence, if we calculate Goldschmidt's tolerance factor, , t is 0.851 for CsPbI3, 0.855 for CsPbI2Br, and 0.862 for CsPbBr3, respectively.Here, A R , B R , and X R are the radii of A, B, and X, respectively.On the other hand, if we calculate the effective octahedral factor, , μ is 0.541 for CsPbI3, 0.561 for CsPbI2Br, and 0.607 for CsPbBr3, respectively.Note that, for CsPbI2Br, we used the average ( )  [17,18].The ionic radius of cesium is dependent on the CN.For example, if cesium has a CN of VI, VIII, IX, X, XI, and XII, the ionic radius is 1.67 Å, 1.74 Å, 1.78 Å, 1.81 Å, 1.85 Å, and 1.88 Å, respectively.Hence, if some studies in the literature adopt CN = VI instead , t is 0.851 for CsPbI 3 , 0.855 for CsPbI 2 Br, and 0.862 for CsPbBr 3 , respectively.Here, R A , R B , and R X are the radii of A, B, and X, respectively.On the other hand, if we calculate the effective octahedral factor, µ = R B /R X , µ is 0.541 for CsPbI 3 , 0.561 for CsPbI 2 Br, and 0.607 for CsPbBr 3 , respectively.Note that, for CsPbI 2 Br, we used the average 12 Å for the radius of the mixed halogen.Moreover, to be a cubic phase, the perovskite should have 0.81 < t < 1.11 and 0.44 < µ < 0.90 [17,18].
The ionic radius of cesium is dependent on the CN.For example, if cesium has a CN of VI, VIII, IX, X, XI, and XII, the ionic radius is 1.67 Å, 1.74 Å, 1.78 Å, 1.81 Å, 1.85 Å, and 1.88 Å, respectively.Hence, if some studies in the literature adopt CN = VI instead of XII for the cesium ion, the ionic radius will be 1.67 Å.Therefore, t = 0.807 could be reported for CsPbI 3 and t = 0.815 for CsPbBr 3 , respectively [26].However, the Cs + cation has a 12-fold coordination site (the A-site has CN = 12 for all the perovskites), not a 6-fold one [43], indicating that Cs + (CN = XII) (1.88 Å) should be correct.Importantly, the material should have a tolerance factor of 0.9-1.0 to form an ideal cubic structure [44], indicating the aforementioned t values are somewhat away from this ideal range.In other words, they can easily undergo structural deformation for reducing the Gibbs free energy.Furthermore, thermodynamically, the most stable structure of the CsPbI 3 is non-perovskite orthorhombic yellow delta-phase, with E g = ~2.82eV at the RT, whereas that of the CsPbBr 3 is the orthorhombic gamma-phase, with E g = ~2.3eV (here, it is notable that CsPbBr 3 has no yellow phase) [45][46][47].Therefore, it should be reasonable to study CsPbI 2 Br for improving the structural stability of CsPbI 3 via composition engineering (e.g., the B-and X-site modification in the ABX 3 structure) for PV applications.
Figure 2 shows (a-e) the electronic structures of the pseudo-cubic α-phase Cs 4 Pb 4−x Ag x I 8 Br 4 and (f) the resulting bandgap as a function of the silver doping level.Here, we assumed that the silver atom may stay with CsPbI 2 Br as a substitutional dopant according to Chen et al.'s study [32].First, except for Cs 4 Pb 4−x Ag x I 8 Br 4 (x = 3), all the others display the promising 'direct bandgap' characteristics.Second, although the bandgap of CsPbI 2 Br is known to be ~1.8-1.9 eV [48][49][50], the DFT results exhibit the small value, 1.361 eV, indicating the typical underestimation of the bandgap in the PBE-based DFT calculation [51,52].Hence, we need to focus on the trend of the bandgap instead of the exact value itself.Third, when Cs 4 Pb 4−x Ag x I 8 Br 4 (x = 2), the bandgap is the largest, 1.966 eV.Fourth, when Cs 4 Pb 4−x Ag x I 8 Br 4 (x = 4), i.e., 4[CsAgI 2 Br], the bandgap is the smallest, 1.060 eV.Finally, this trend of the bandgap is summarized in Figure 2f, providing the insightful silver-doping effect on the electronic structure of CsPbI 2 Br qualitatively.
In our experimental study, we introduced Ag atoms into the CsPbI 2 Br crystals by dissolving AgBr into the perovskite precursor solutions, indicating that there should be combined effects from both Ag + cations and Br − anions because our samples are mixedhalide perovskites.Here, in the perovskite precursor solutions, Br − anions can serve as a processing additive/dopant because the bromine anions have a high Gutmann's donor number (D N = 33.7,Lewis basicity) [33,34] affecting the crystallization of the perovskites via the modified interactions between DMSO (D N = 29.8)and the perovskite precursors in the solution state.Figure 3 shows each optical bandgap at the onset of the absorption, for which the Tauc plot was employed for clarity (Figure 4).Here, the film thickness (l) was estimated to be ~148 nm (0% AgBr and 1% AgBr) and ~332 nm (2% AgBr and 3% AgBr), respectively.For this purpose, the following absorption coefficient (α = ~5 × 10 4 cm −1 at 600 nm [53]) was employed: l = 2.302 × Abs/α, where 'Abs' denotes the absorbance.First of all, for the CsPbI 2 Br perovskite without AgBr, the bandgap is 1.84 eV, which falls in the general bandgap (~1.8-1.9 eV) of the CsPbI 2 Br perovskite [48][49][50].Here, it is notable that CsPbI 3 and CsPbBr 3 have the bandgaps of ~1.73 eV and ~2.3 eV, respectively.However, when AgBr was employed into the perovskite precursor solutions, the resulting bandgap increased slightly from 1.87 eV (at 1% AgBr) to 1.95 eV (at 2% AgBr) and 1.96 eV (at 3% AgBr).Figure 3b shows the summary of the results, i.e., the bandgap as a function of the AgBr doping concentrations.It is worthy to remind that the bandgap is a key factor in the 'stability and cost' for practical solar cells, determining the theoretical PCE based on the Shockley-Queisser limit [54].Importantly, Ravi et al. pointed out that, in CsPbX 3 perovskite, the conduction band minimum (CBM) is dominantly affected by Pb 6p orbitals, whereas the valence band maximum (VBM) is mainly determined by anti-bonding hybridization Pb 6s and X np orbitals, specifically, the major effect from X np [55].Therefore, the bandgap shift in Figure 4 could be explained as follows.Ag-doping may affect the CBM shift, whereas Br codoping may contribute to the VBM move.However, these effects will be very small because of the slight AgBr doping concentration of ~1-3%.Then, we measured the PL lifetime for the CsPbI 2 Br when AgBr was 0, 1, 2, and 3%.Accordingly, as shown in Figure 5, the PL lifetime was enhanced from 0.990 ns (at 0% AgBr) to 1.187 ns (at 3% AgBr) with increasing AgBr amounts, suggesting that the AgBr doping helps minimize the nonradiative transition according to the literature reports [22][23][24][25][26][27][28][29][30]32].In our experimental study, we introduced Ag atoms into the CsPbI2Br crystals by dissolving AgBr into the perovskite precursor solutions, indicating that there should be combined effects from both Ag + cations and Br − anions because our samples are VBM move.However, these effects will be very small because of the slight AgBr doping concentration of ~1-3%.Then, we measured the PL lifetime for the CsPbI2Br when AgBr was 0, 1, 2, and 3%.Accordingly, as shown in Figure 5, the PL lifetime was enhanced from 0.990 ns (at 0% AgBr) to 1.187 ns (at 3% AgBr) with increasing AgBr amounts, suggesting that the AgBr doping helps minimize the nonradiative transition according to the literature reports [22][23][24][25][26][27][28][29][30]32].The FT-IR spectra were characterized for the CsPbI2Br thin film as a function of the AgBr doping in the perovskite precursor solutions.As shown in Figure 6a, we can find only the FT-IR peaks from the solvents remaining inside of the CsPbI2Br thin film.This observation indicates that the trace amounts of the solvent molecules may survive in the trapped state inside of the perovskite film, although the annealing temperature (>250 °C) could also be trapped into the crystal structure of CsPbI2Br, although CB and CsPbI2Br have two different polarities, i.e., CB is slightly polar (polarity index = 2.7) but CsPbI2Br and DMSO (polarity index = 7.2) are highly polar [63].Hence, CB's trapping could be understood based on the physical trap instead of the chemical affinity between the CB and the CsPbI2Br perovskite.On the other hand, DMSO can be trapped for two reasons, i.e., affinity and physical confinement.Accordingly, even after thermal annealing at 280° for 10 min, trace amounts of the solvents could be trapped, as demonstrated in the FT-IR spectra in Figure 6a.The FT-IR spectra were characterized for the CsPbI 2 Br thin film as a function of the AgBr doping in the perovskite precursor solutions.As shown in Figure 6a, we can find only the FT-IR peaks from the solvents remaining inside the CsPbI 2 Br thin film.This observation indicates that the trace amounts of the solvent molecules may survive in the trapped state inside of the perovskite film, although the annealing temperature (>250 • C) was higher than the boiling points of each solvent molecules (DMSO: 189 • C and CB: 132 • C; their structures are given in Figure 6b).Here, the detailed peak assignment is as follows [56]: First, in the high frequency regions, 3025-2849 cm −1 , =C-H and -C-H vibrations were observed from the DMSO and CB molecules inside of the CsPbI 2 Br crystals.At 1676 cm −1 , a -C=C vibration from the aromatic ring of CB was detected.On the other hand, at 1387 cm −1 , 1092 cm −1 , and 738 cm −1 , -CH 3 , S=O, and -C-Cl vibrations were displayed, respectively.Finally, the small peak at 463 cm −1 is ascribed to the molecular vibration of the antisolvent CB.
Importantly, Figure 6c shows the solvent engineering process [35] to explain the solvent trapping phenomena detected via the FT-IR.Here, the solvent engineering procedure is as follows.The antisolvent (chlorobenzene) dripping on top of the wet perovskite precursor film (solvent: DMSO) during spinning brings forth the fast crystallization and deposition of a perovskite film.Here, to understand the intermolecular interactions, the solubility parameter (δ) data [36] of the solvent, antisolvent, and perovskite are required.First, DMSO and CB have δ = 14.5 (cal/cm 3 ) 2 and δ = 9.5 (cal/cm 3 ) 2 , respectively [57].In the case of CsPbI 2 Br, we may estimate it from the water contact angle (θ c = 32.76• ) data reported by Chen and coworkers [58].Li and Neumann [59] suggested the relation between the contact angle and surface energy, cos , where γ lv , γ sv , and γ sl are the surface energies for liquid-vapor, solid-vapor, and solid-liquid, respectively.The constant β is 0.000115 m 4 /mJ 2 and γ lv is 72.8 mJ/m 2 for water, respectively.Then, by inputting θ c = 32.76• into the aforementioned Li-Neumann's equation, we may estimate γ sv = 63.05 mJ/m 2 .Then, from the relation of δ cal/cm 3 2 = 1.829058 √ γ sv [60-62], we obtained δ = 14.5 (cal/cm 3 ) 2 or δ ′ (SI unit) = δ × 2.0455 = 29.7 MPa 1/2 , respectively (Table 1).Hence, because CsPbI 2 Br and DMSO have (almost) the same solubility parameter, there is high probability that DMSO may be trapped in the CsPbI 2 Br.However, for the case of CB, the 'solvent-antisolvent' (DMSO and CB) molecules are miscible because of the entropy-driven mixing, affording CB to wash and remove the DMSO molecules during its dripping process [35].However, when CB was dropped on top of the wet perovskite precursor film, CB could also be trapped into the crystal structure of CsPbI 2 Br, although CB and CsPbI 2 Br have two different polarities, i.e., CB is slightly polar (polarity index = 2.7) but CsPbI 2 Br and DMSO (polarity index = 7.2) are highly polar [63].Hence, CB's trapping could be understood based on the physical trap instead of the chemical affinity between the CB and the CsPbI 2 Br perovskite.On the other hand, DMSO can be trapped for two reasons, i.e., affinity and physical confinement.Accordingly, even after thermal annealing at 280 • for 10 min, trace amounts of the solvents could be trapped, as demonstrated in the FT-IR spectra in Figure 6a.a This is estimated based on the lattice parameters, a = b = 0.640 nm and c = 0.597 nm.
Figure 7a shows the XRD patterns of CsPbI2Br as a function of the AgBr doping concentration.First, in the absence of AgBr, the CsPbI2Br perovskite thin film displays the typical (100) and ( 200) peaks [64].However, by increasing the AgBr concentration (see ~2-3% AgBr), the other peaks such as (211), (300), and (222) are intensified, indicating that the crystallographic ordering decreases with the increasing AgBr concentrations.a This is estimated based on the lattice parameters, a = b = 0.640 nm and c = 0.597 nm.
Figure 7a shows the XRD patterns of CsPbI 2 Br as a function of the AgBr doping concentration.First, in the absence of AgBr, the CsPbI 2 Br perovskite thin film displays the typical (100) and ( 200) peaks [64].However, by increasing the AgBr concentration (see ~2-3% AgBr), the other peaks such as (211), (300), and (222) are intensified, indicating that the crystallographic ordering decreases with the increasing AgBr concentrations.This observation implies that the crystallization kinetics were changed when the AgBr was introduced into the perovskite precursor solutions.Second, we estimated the crystallite size (D) by using Scherrer's relation of D = 0.9λ/(B • cos θ), where λ (=0.154 nm) is the wavelength of the X-ray, whereas B is the full width at half-maximum (FWHM) at the diffraction angle of θ.The results are summarized in Figure 7b and Table 2.As shown in Figure 7b, there was no clear linear trend when AgBr was introduced into the perovskite precursor, implying that, although the morphology might be changed through different crystallization kinetics, the crystallite size (i.e., the average single crystalline domains in the polycrystalline structure) were not much changed, but rather similar to all the conditions, whether doped or not.
Electron.Mater.2024, 5, FOR PEER REVIEW 12 This observation implies that the crystallization kinetics were changed when the AgBr was introduced into the perovskite precursor solutions.Second, we estimated the crystallite size (D) by using Scherrer's relation of ( ) , where λ (=0.154 nm) is the wavelength of the X-ray, whereas B is the full width at half-maximum (FWHM) at the diffraction angle of θ.The results are summarized in Figure 7b and Table 2.As shown in Figure 7b, there was no clear linear trend when AgBr was introduced into the perovskite precursor, implying that, although the morphology might be changed through different crystallization kinetics, the crystallite size (i.e., the average single crystalline domains in the polycrystalline structure) were not much changed, but rather similar to all the conditions, whether doped or not.8a,b show the stability test of the perovskite thin film: (a) CsPbI2Br without AgBr and (b) CsPbI2Br with 1% AgBr doping.These two samples were selected for this test because the surface morphologies were relatively uniform compared to the others (~2-3% AgBr-doped perovskite samples).Interestingly, both samples show the growth of the minor peaks at the (211) and (222) crystallographic planes with time, indicating that the orientational ordering decreases with time.Here, it is notable that, except for single crystalline perovskite thin films, all the polycrystalline films are thermodynamically metastable because the defect area (including polycrystalline nature) makes the surface energy increase.Hence, for the lowering of the Gibbs free energy, the sample can undergo phase transition.In this case, by decreasing the orientational order (i.e., increasing the (211)-(222) XRD peaks), the film may reduce its free energy.When we see the CsPbI2Br sample without the AgBr doping in Figure 8a, compared to the same sample (but different batch) in Figure 7 (black solid line), the additional strong peak at the   Br with 1% AgBr doping.These two samples were selected for this test because the surface morphologies were relatively uniform compared to the others (~2-3% AgBr-doped perovskite samples).Interestingly, both samples show the growth of the minor peaks at the (211) and (222) crystallographic planes with time, indicating that the orientational ordering decreases with time.Here, it is notable that, except for single crystalline perovskite thin films, all the polycrystalline films are thermodynamically metastable because the defect area (including polycrystalline nature) makes the surface energy increase.Hence, for the lowering of the Gibbs free energy, the sample can undergo phase transition.In this case, by decreasing the orientational order (i.e., increasing the (211)-(222) XRD peaks), the film may reduce its free energy.When we see the CsPbI 2 Br sample without the AgBr doping in Figure 8a, compared to the same sample (but different batch) in Figure 7 (black solid line), the additional strong peak at the (300) crystallographic plane was observed [i.e., a more orientational order because the (100), (200), and (300) planes are equivalent], indicating the batch-to-batch partial uncertainty depending on the drying process in the laboratory under ambient conditions.Importantly, the 1% AgBr-doped CsPbI 2 Br shows the structural stability (i.e., the XRD peak position is the same with aging time), but the 0% AgBr sample clearly shows the major peak's shift to the left direction (i.e., a partial expansion of crystal; see the dotted red line box in Figure 8a).This aging effect data proves that the AgBr doping should contribute to the structural stability of the CsPbI 2 Br perovskite films.The results are reasonable because the AgBr addition increases the stability (a wider bandgap and improved tolerance to the environments).
Electron.Mater.2024, 5, FOR PEER REVIEW 13 (300) crystallographic plane was observed [i.e., a more orientational order because the (100), (200), and (300) planes are equivalent], indicating the batch-to-batch partial uncertainty depending on the drying process in the laboratory under ambient conditions.Importantly, the 1% AgBr-doped CsPbI2Br shows the structural stability (i.e., the XRD peak position is the same with aging time), but the 0% AgBr sample clearly shows the major peak's shift to the left direction (i.e., a partial expansion of crystal; see the dotted red line box in Figure 8a).This aging effect data proves that the AgBr doping should contribute to the structural stability of the CsPbI2Br perovskite films.The results are reasonable because the AgBr addition increases the stability (a wider bandgap and improved tolerance to the environments).Figure 9 shows the SEM images displaying the microstructural morphologies of the CsPbI2Br sample as a function of the AgBr doping concentration.First, the CsPbI2Br thin films (a) without the AgBr and (b) with the 1% AgBr are relatively uniform, whereas the other films with the ~2-3% AgBr are nonuniform, displaying the crystal domains and defect sites clearly.Probably, the samples (c and d) were grown very fast in the presence of high doping (~2-3% AgBr).However, it is worth reminding that, according to the XRD data in Figure 7, the crystallite size (the average single-crystalline domains) is not much different from sample-to-sample.The average crystallite size is 41.5 ± 3.4 nm and 39.3 ± 7.2 nm at the (100) and (200) crystallographic planes, respectively.However, as shown in Figure 9, the film processing condition should be optimized further for photonic devices, which will be included in our future work.Finally, we note that, without annealing (>250 °C), the CsPbI2Br sample shows phase impurities at room temperature due to polymorphism (see Table S1 and Figure S1 in Supplementary Materials). Figure 9 shows the SEM images displaying the microstructural morphologies of the CsPbI 2 Br sample as a function of the AgBr doping concentration.First, the CsPbI 2 Br thin films (a) without the AgBr and (b) with the 1% AgBr are relatively uniform, whereas the other films with the ~2-3% AgBr are nonuniform, displaying the crystal domains and defect sites clearly.Probably, the samples (c and d) were grown very fast in the presence of high doping (~2-3% AgBr).However, it is worth reminding that, according to the XRD data in Figure 7, the crystallite size (the average single-crystalline domains) is not much different from sample-to-sample.The average crystallite size is 41.5 ± 3.4 nm and 39.3 ± 7.2 nm at the (100) and (200) crystallographic planes, respectively.However, as shown in Figure 9, the film processing condition should be optimized further for photonic devices, which will be included in our future work.Finally, we note that, without annealing (>250 • C), the CsPbI 2 Br sample shows phase impurities at room temperature due to polymorphism (see Table S1 and Figure S1 in Supplementary Materials).

5 Figure 1 .
Figure 1.(a) Unit cell and supercell of CsPbI2Br with lattice parameters, a = b = 6.395Å and c = 5.988 Å.(b) B-site substitutional doping by silver for the CsPbI2Br supercell with a size of 2 × 2 × 1.

Figure 3 .
Figure 3. (a) UV-Vis spectra of CsPbI2Br as a function of the AgBr concentration.(b) Bandgap as a function of the AgBr doping concentration (%).Optical bandgap determination for CsPbI2Br as a function of the AgBr concentration by the Tauc plot.

Figure 3 .Figure 4 .
Figure 3. (a) UV-Vis spectra of CsPbI 2 Br as a function of the AgBr concentration.(b) Bandgap as a function of the AgBr doping concentration (%).Optical bandgap determination for CsPbI 2 Br as a function of the AgBr concentration by the Tauc plot.Electron.Mater.2024, 5, FOR PEER REVIEW 9

Figure 4 .
Figure 4. Optical bandgap determination for CsPbI2Br as a function of the AgBr concentration (a) 0%, (b) 1%, (c) 2% and (d) 3% by the Tauc plot.Here, each arrow indicates the tangent line for determining the optical bandgap.

Figure 5 .
Figure 5. PL lifetime of the CsPbI2Br thin film as a function of the AgBr concentration.

Figure 5 .
Figure 5. PL lifetime of the CsPbI 2 Br thin film as a function of the AgBr concentration.

11 Figure 6 .
Figure 6.(a) FT-IR spectra of CsPbI2Br with or without AgBr when processed with dimethyl sulfoxide (DMSO) and chlorobenzene (CB).(b) Chemical structures of DMSO and CB.(c) Solvent engineering process: when CB is dripping on top of the wet perovskite (precursor) film, DMSO can be washed away.During this process, some solvent molecules could be trapped in the perovskite thin film.

Figure 6 .
Figure 6.(a) FT-IR spectra of CsPbI 2 Br with or without AgBr when processed with dimethyl sulfoxide (DMSO) and chlorobenzene (CB).(b) Chemical structures of DMSO and CB.(c) Solvent engineering process: when CB is dripping on top of the wet perovskite (precursor) film, DMSO can be washed away.During this process, some solvent molecules could be trapped in the perovskite thin film.

Figure 8a ,
Figure 8a,b show the stability test of the perovskite thin film: (a) CsPbI 2 Br without AgBr and (b) CsPbI2 Br with 1% AgBr doping.These two samples were selected for this test because the surface morphologies were relatively uniform compared to the others (~2-3% AgBr-doped perovskite samples).Interestingly, both samples show the growth of the minor peaks at the (211) and (222) crystallographic planes with time, indicating that the orientational ordering decreases with time.Here, it is notable that, except for single crystalline perovskite thin films, all the polycrystalline films are thermodynamically metastable because the defect area (including polycrystalline nature) makes the surface

Table 2 .
Crystallite size in CsPbI2Br as a function of the AgBr doping concentration.

Table 2 .
Crystallite size in CsPbI 2 Br as a function of the AgBr doping concentration.