Advancements in bandgap engineering: bromide-doped cesium lead perovskite thin films

Perovskite materials have emerged as promising candidates for next-generation photovoltaic devices due to their unique optoelectronic properties. In this study, we investigate the incorporation of bromine into cesium lead mixed iodide and bromide perovskites (CsPbI 3(1-x) Br 3x ) to enhance their performance. By depositing films with varying bromine concentrations ( x = 0, 0.25, 0.5, 0.75), we employ a combination of structural and optical characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible spectroscopy, and photoluminescence. Our analysis reveals that introducing bromine leads to structural modifications, influencing the perovskite films’ optical properties and energy gap. Specifically, we observe semiconductor behavior with a tunable energy gap controlled by the intercalation of bromine atoms into the CsPbI 3 lattice. Furthermore, heat treatment induces phase transitions in the perovskite films, affecting their optical responses and crystalline quality. SCAPS-1D simulations confirm the improved stability and efficiency of bromine-doped CsPbI 3 films compared to undoped counterparts. Our findings demonstrate that bromine incorporation facilitates the formation of highly crystalline perovskite films with reduced trap defects and enhanced carrier transport properties. These results underscore the potential of bromine-doped CsPbI 3 perovskites as promising materials for high-performance photovoltaic applications, paving the way for further optimization and device integration.


Introduction
In recent years, researchers have focused on hybrid organic-inorganic perovskite-based solar cells; these perovskite materials have seen upward efficiency from 3.8% to over 25.2% in a short period (Jeong et al. 2020;Kim et al. 2021).In general, perovskite materials are in the form of ABX 3 , A represents the organic cation, B is a metal, and the halogen is expressed by X (Bouich et al. 2021;Wang et al. 2022).Despite these perovskite solar cell hybrids exhibiting remarkable efficiency, these perovskite films are susceptible to degradation due to humidity and temperature conditions (Dunfield et al. 2020;Sutton et al. 2016;Liu et al. 2015).Significantly, the methylammonium lead iodide thin films thermally decompose to lead iodide at 100 °C (Fradi et al. 2022).Thermally, we find as results of Page 2 of 13 Fradi et al. J Mater. Sci: Mater Eng. (2024) 19:14 analyses in the literature that perovskites based on formamidinium are much less degradable than perovskites based on methylammonium, researchers tested a mixture of cations in the same perovskite film, it is an organic-inorganic mixture between formamidinium (organic cation) and cesium (inorganic cation), and they proved some thermal stability can exceed the value of 100 °C (McMeekin et al. 2016).From where comes The idea to enhance the stability of the thermal composition by thinking of partially changing the organic component with an inorganic cation, for example, the cesium element.Many studies have been carried out on the properties of lead cesium halides due to their excellent stability (Akkerman et al. 2018;Huang et al. 2020).Our work considers the two compounds, lead cesium bromide and cesium lead iodide, as a reference in our research.Both materials have slight stability up to the melting points.At room temperature, CsPbBr 3 reaches the crystallization phenomenon of the orthorhombic phase.It transitions to the tetragonal phase of perovskite at 87 °C and the cubic phase with orange color at 120 °C (Kulbak et al. 2015;Piotrowski et al. 2022).However, for CsPbI 3 , a quick transition can occur to the yellow phase (δ phase) at temperatures below 315 °C, which does not absorb much sunlight.This problem was addressed by doping CsPbI 3 with other compounds to gain higher stability and better performance (Mir et al. 2019;Guvenc et al. 2019;Zhao et al. 2023).Under various conditions, black α-CsPbI 3 perovskites cannot reach their phase stability; this phase quickly changes to thermodynamically stable δ-CsPbI 3 (yellow color) in a short time, which has a non-perovskite structure and high bandgap for photovoltaic devices.Our idea is to prevent this phase transformation from taking advantage of the good properties of CsPbI 3 .Various methods have been granted to inorganic perovskites to resolve this stability handicap; in this study, Br doping was used to increase the stability and improve the optical and structural properties of α-CsPbI 3 .This idea leads to the surface passivation induced by the network modification, which causes stable perovskite nanocrystals under normal conditions (Bera et al. 2019;Lu et al. 2018).Besides,doping is the most widely used technique in various materials synthesis systems for different applications.After further research, experience shows that the reduction of the degradation phenomenon in perovskite materials is made under the effect of the deformation of the cubic phase.There was a decrease in the density of the defects and an increase in the tolerance factor of Goldschmidt (Fu et al. 2019).Thin film deposition of CsPbI3(1-x)Br3x is shown in Fig. 1 Manufacture of cesium lead mixed halide Inorganic lead-based perovskites containing a bromineiodine mixture were developed using different available precursors.Mainly, cesium iodide compounds (CsI), lead iodide (PbI 2 ), and lead bromide (PbBr 2 ) were applied and dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF) as stabilizing solvents.CsPbI 3(1-x) Br 3x solution was spin-coated by centrifugation fluorine-doped tin oxide (FTO) substrate at 4500 rpm for the 30 s.The rapid vaporization of DMF/DMSO causes disturbances on the surfaces of the samples; to overcome this drawback, four to five drops of toluene are added, while the substrate is rotating on the spin coating; the addition of toluene produces optimal morphology and good crystallinity.Also, the injection of toluene must be rapid and not drop by drop because it causes poor morphology and crystallinity.For 5 min, the transparent thin films were heated at a low temperature (slow thermal heating), 70 °C, which transformed from yellow to orange depending on the bromine's addition concentration.After successful deposition, these thin films were subjected to surface morphology, optical bandgap, and structure characterization.But before conducting these studies, we reheated the same series of films on the heating plate at 180 °C for a structural and optical study for 10 min.During this reheating time, it is noticed that the films gradually take on a darker color by removing any remaining solvent.The appearance of a darker shade of the film proves the complete formation of the perovskite phase during annealing.

Characterization techniques
The X-ray diffraction is used with the Cu ka radiation (k = 1.5418A°) of Rigaku Ultima IV to characterize the CsPbI 3(1-x) Br 3x thin films.The morphology of the films was characterized using scanning electron microscopy (Quanta 200-FEI) with 1.5-kV capture conditions at different magnifications.The measurements of absorption and transmission were taken by an Ocean Optics HR4000 spectrophotometer (Si-CCD) in the range of 350-nm and 850-nm wavelengths.We did the photoluminescence measurements using the He-Cd laser source emitting at 325 nm, and we detected the photoluminescence emissions by a back-thinned Si-CCD detector (Hamamatsu).

X-ray diffraction analysis
The structural characteristics of CsPbI 3 and CsPbI 3(1-x) Br 3x films by X-ray diffractograms are exhibited in Fig. 2. It was identified that for the α-CsPbI 3 phase, many XRD peaks are assigned to the (110) planes, (110), (200), and (211), corresponding to angles at 14.6°, 20.7°, 31.1°, and 33.28° (JCPDS, 01-075-0412).A slight shift at the peaks towards the lower angles after the intercalation of Br in thin films of CsPbI 3 is remarkable; the large size of bromine relative to the iodine halide is the cause of this transformation.The peak located at 12.8° is the peak that indicates the appearance of the undesirable δ-CsPbI 3 phase (Fig. 2).Furthermore, the integration of Br substance from (0-100%) the secondary phaseδ-CsPbI 3 gradually disappears and changes to the desired α-CsPbI 3(1-x) Br 3x perovskite structure.For CsPbI 3(1-x) Br 3x (x = 0, 0.5) perovskites, a peak close to 13° appears which explains the non-perovskite phase of CsPbI 3 , whereas for CsPbI 3(1-x) Br 3x (x = 0.25), this peak no longer exists.We conclude that the film doped with 25% bromine shows some compositional stability under the same conditions.The shift of XRD peaks to higher angles is noticed during the doping procedure.This change can prove that the doping process is taking place accurately in the perovskite structure.Because of the smaller ionic radius of the brome halide (115 pm) than the iodide halide (140 pm), peaks shift to higher angles.Another fundamental remark in the XRD model that has been noticed is the division of the peak of ( 200) for the sample CsPbI 3(1-x) Br 3x (x = 0.25); the division can be done by scaled-down symmetry in the cubic structure perovskite.A prolongation of stability may occur in the α phase due to lattice distortion.The ionic radius of bromine is smaller than the ionic radius of iodine.Concerning the Goldschmidt tolerance factor (Eq. 1) (Bouich et al. 2022), it is improved under the effect of the intercalation of bromine, which has a size of the ionic radius smaller than the ionic radius of iodine.
In the perovskite structure, the radius of the site A cation is presented by R A , R B expresses the radius of the site B cation, and R X marks the radius of the anion (Bouich et al. 2022).

Tolerance factor calculation
Taking our case of doping, the calculation of the influential tolerance factor is done using the formulas in Eq. 2.
For ordered double perovskites of the AB(X'X) 3 type with two different halogens in the X sites, since the tolerance factor relationship is applicable to both single and double perovskites, the Goldschmidt factor can be calculated by replacing the bond length B-X bond length in the case of the single perovskite by the average of the bond lengths bonds < B-X, B-X′ > to give the following (Bouich et al. 2022):

Influence of bromine doping on film stability
The influential tolerance factor also increased when bromine was introduced into the CsPbI 3 perovskite (1) structure.The increase in this factor translates into a more stable perovskite structure.In our work, we have guaranteed the stability of CsPbI 3(1-x) Br 3x compound since the further the Goldschmidt factor deviates from 1.1, the more deformed the lattice becomes.It has usually been believed that a suitable range for perovskite is 0.8-1.10(Table 1) (Bouich et al. 2022).

Scanning electron microscopy (SEM) analysis
However, the structural and morphological response of films based on CsPbI 3 and CsPbBr 3 is unsatisfactory due to the poor quality of perovskite thin films.In this situation, by combining both substrates with preheat treatment CsPbI 3(1-x) Br 3x and the additive precursor of bromine, we composed the high-quality CsPbI 3(1-x) Br 3x perovskite film, giving full coverage, excellent crystallinity, and high crystal grain size, which improves the light collection and weakens non-radiative charge recombination.The SEM images picked up from thin films of perovskite with intercalation of the halide Br and the pure compound CsPbI 3 are shown in Fig. 3.The perovskite film obtained reveals poor morphology with nonhomogeneous coverage and low crystallinity without adding bromine.However, the CsPbI 3(1-x) Br 3x film shows full substrate coverage without voids and crystal grains of large sizes.It offers that the combination of bromine and iodide significantly affects the construction of the perovskite film morphology, in good agreement with the literature (Guo et al. 2019;Wang et al. 2020).
SEM images are shown in Fig. 3; we note that for pure CsPbI 3 substrates, several holes appear on the surface, indicating a specific magnification at the level of the surface morphology.The 25% Br sample shows excellent morphology, which agrees with the results of the XRD,  and PL-confirmed stability against the pure CsPbI 3 thin film.Besides, substituting the halide Br content stabilizes the yellow phase of CsPbI 3 with enhanced XRD, suppressing the undesirable perovskite phase transition.

Optical properties analysis
Adding the amount of Br camouflages the diffuse absorption of the pure CsPbI 3 .Also, the absorption edge observed a remarkable enhancement at the highest energy wavelength.Furthermore, one sharp peak was observed in the 500-600 nm range in Fig. 4, possibly because of absorbent materials' electronic transition, rotational, or vibrational energy levels.This information is consistent with the increase in crystallinity in the results of the XRD.The bandgap is measured by the Tauc method, illustrated in Fig. 5.

Photoluminescence spectra analysis
The present study reports the optical characteristics of cesium perovskites based on lead and halides.CsPbI 3(1-x) Br 3x (X = 0, 0.25, 0.5, 0.75, 1).The computed optical structure profile proves that these materials expose semiconductor behavior with a photoluminescence value of CsPbI 3 regulated by the intercalation of an atom of bromine by an atom of iodine in CsPbI 3 .Bromine belongs to the same column as iodine in the periodic table, so they have properties in common.The elements to which they belong have the same number of electrons on their outer layer, which gives them similar properties, and it is this outer layer that is responsible for their similar reactivity.
Our work is to understand the ionic transport mechanism by introducing suitable halogen substitution with insignificant impact on perovskite photoluminescence value to hinder ion diffusion and thereby to seek a method to improve the stability.The research results show that a bromine substitution not only prevents ion migration in perovskite but also promotes the intensity of photoluminescence; this result is very clear in the example.In Fig. 4, the strong absorption at 660 nm corresponds to the perovskite CsPbI 3(1-x) Br 3x (x = 0.25).Furthermore, smaller atomic substitution for the halogen atoms may be essential for increasing the diffusion barrier.Bromine doping can also introduce defects into the perovskite crystal structure.These defects can act as recombination sites for charge carriers, which reduces the efficiency of photoluminescence; this interpretation explains the increase in the values of the widths at half maximum, at the level of the perovskites doped with bromine, and also at the level of the peak which corresponds to the value 660 nm where we note that the intercalation of bromine can also form complexes with the perovskite, which can modify the photoluminescence properties of the material.We suggest that partial bromine substitution is beneficial for the CsPbI 3 perovskite to have the most intense peak of photoluminescence, although none of the substitutions has better optical proprieties than lead iodine perovskite with 25% of bromine does currently.

Bandgap variation with bromine content
The bandgap energies have decreased to the lower point with Br from 100 to 0%.In our case, incorporating 25% Br content reinforced the lattice parameter changes proposed in the previous section.
Table 2 explains the values of the optical bandgap (Eg) of CsPbI 3(1-x) Br 3x , removed from PL and UV measurements.When adding the bromine amount assay, the absorption limit decreased significantly from 2.24 to a value of 1.68 eV.The overall difference in PL emission between the CsPbI 3 and CsPbI 3(1-x) Br 3x samples is from 2.34 to 1.7 eV.The bandgap measurements of CsPbI 3(1-x) Br 3x substrates are characterized in a typical environment after 1 week called aged thin films.The recovered samples' band gap value was fixed ~ 2 times less than the sample characterized a week later.In the air, the improvement in absorbance can be attributed to the creation of vacancies under the effect of the intercalation of Br in the CsPbI 3 network.Charge recombination is decreased by lowering surface trap states, leading to increased photoelectric efficiency.
Figure 4a and b confirms the growth of the spectra of emission, which correspond to CsPbI 3 varying the percentage of Br (x = 0%, 25%, 50%, 75%, and 100%).An apparent change was noted in the normalized photoluminescence spectrum at the lowest wavelengths.This wavelength deviation proves the assembly of I 3 /Br 3 in the perovskite latticework.The number of non-radiative emissions decreases, which results in a reduction in the level of the density of states of the traps; this result is explained in the amplified emission spectrum of PL.The transformation of pure CsPbI 3 to the mixed material CsPbI 3(1-x) Br 3x was noticed in the photoluminescence spectra; this result is in good agreement with the results of the X-ray and absorption spectra.In summary, Table 2 summarizes the responses of the photoluminescence spectra and visible UV spectroscopy of CsPbI 3 for different Br 3 contents.In Table 2, we can conclude that for 25% of Br 3 , the value of the gap energy of CsPbI 3(0.75)Br 3(0.25) is smaller compared to the value of the bandgap of CsPbBr 3 , which the intercalation of Br can explain, and then the creation of one more energy space between the valence and the conduction band, and consequently to the reduction of the gap.
It can be noted that CsPbI 3 has a higher absorbance value than CsPbBr 3 , science explains the variation of the value of the bandgap by the phenomenon of absorption in the visible range, and it is an internal characteristic for each type of material, which causes a weakening of the energy of the bandgap due to the quantities adding bromine content.
The halide is the most crucial element of hybrid perovskites, the atomic size increases by descending in group VIIA from the chloride halide to the iodine, the changed absorption spectrum translates to longer wavelengths, and a decrease in energy occurs as the absorption spectrum moves toward the highest wavelengths, i.e., a redshift is created (Jeong et al. 2021); the reduction can explain this in electronegativity to have such compatibility with lead, increasing the covalency and effectively minimizing the ionic character.Bromide was most profitably used to adjust the bandgap hybrid perovskites (Lu et al. 2021).Iodide is the closest lead on the periodic table, so it shares a similar covalent character, which leads to a similar covalent character to form a stable structure.When Br is included in CsPbI 3 , compression stress between Pb-I bonds is introduced due to the size of the lattice parameters, leading to a structural distortion that causes an increase in the bandgap value (Appadurai et al. 2021).For our case, Fig. 5 confirms that the bandgap value of CsPbI 3(1-x) Br 3x was limited from 1.6 (CsPbI 3 ) to 2.1 eV (CsPbBr 3 ).An adequate amount of 25% of bromine intercalated in CsPbI 3 was confirmed by the results with a slow annealing treatment guide to successfully synthesize perovskite films with large absorbance and bandgap equal to 1.65 eV.

Degradation study
Making the balance between the high-power conversion efficiency and the stability of hybrid perovskite solar cells is challenging to commercialize solar cells.For this reason, the value of the bandgap is the secret to the physical conversion process in the photovoltaic field; the optical response of organic-inorganic hybrid perovskite would change by its properties and the environmental situations, like temperature and pressure, which affected directly the stability of perovskite solar cells (Sun et al. 2017).This part will cover the stability under a thermal condition, like in our case, the relation of the optimization of the bandgap with the temperature to give a complete overview of the phenomenon of the degradation of perovskite by exposing it to the air for a week.However, research has explored CsPbX 3 compounds as photovoltaic materials.Still, α-CsPbI 3 thin films undergo an abrupt orthorhombic phase change.Oxygen and humidity are the most direct factors affecting photovoltaic stability.We aim to study the degradation phenomenon of CsPbI 3(1-x) Br 3x films deposited using heat treatment.Perovskite thin films were protected in the dark at room temperature for 1 week.After 1 week, to examine the degradation of samples, we reheated the perovskite films on the heating plate at 180 °C for 10 min.We studied the optical response of the samples under reheating.Molecules of H 2 O present in the air lead to the degradation of the films.They probably cause the breaking of the bonds between the synthesized mixed perovskite molecules and end with a change of the film color and appearance of a non-perovskite phase.In addition, in Fig. 6d, after a week in which the CsPbI 3 film is exposed to air and humidity, the curve shows that the energy of the CsPbI 3 material obtained is equal to 3.3 eV; this result indicates that the phase is degraded.It does not correspond to a perovskite phase.
On the other hand, we notice the appearance of the two phases by analyzing Fig. 6c, where the film is reheated at 180 °C. Figure 6a and b shows the UV-visible absorption spectra of the aged and recovered perovskite films to quantify the stability of films and the effect of temperature on the absorption and the bandgap of different concentrations added of bromine.It can be seen that the absorption is greater with increasing iodide contents.At the same time, similar absorption bandgaps were exhibited, suggesting that the addition of iodide and the idea of synthesizing a mixed compound can enhance the light absorption properties since the pure CsPbBr 3 compound has the lowest absorption band.For aged films, after 1 week, we noticed that the absorption range was lower than in recovered films; this result convinced us to admit that optical treatment has a nice effect on the absorbent surface of perovskite.By comparing the value of the bandgap from estimating the optical band gap via fitting the experimentally determined absorption coefficient from Tauc equation, we note that for the aged CsPbI 3(1-x) Br 3x (x = 0.25), Eg = 1.73 eV, while for the recovered CsPbI 3(1-x) Br 3x (x = 0.25), Eg = 1.65 eV.
Encouragingly, the photoluminescence spectrum response and the optical characterization of the pure material containing Br significantly improve film performance and stability.The 25% Br is critical in slowing down the poor quality of CsPbI 3 thin films.Certain optical characterization techniques can indeed accelerate the degradation of perovskite materials.This happens due to several factors like light exposure, and perovskites can be sensitive to light, particularly highenergy photons like UV light.Techniques like photoluminescence (PL) spectroscopy involve excitation by light, which can induce photodegradation processes.Additionally, some optical methods, like those using focused laser beams, can generate localized heating.This heat can accelerate chemical reactions within the perovskite, leading to degradation.The degradation can be also explained by ambient factors, and exposure during characterization can introduce the perovskite to ambient air or moisture, which can also contribute to degradation.Researchers minimize these degradation effects by using low-powered light sources, pulsed lasers to minimize heat generation, and performing measurements in inert atmospheres.
As a result, the study of absorbance spectroscopy has improved the degradation of the perovskite layers.The apparent color of the film is related to the growth stages of the perovskite layer, which begins to darken at the beginning and then changes to brown a week later, as visible in the photo (Fig. 7).

SCAPS simulation
The results of the degradation study effect after Br doping on CsPbI 3 were further investigated by simulating a device structure in SCAPS that incorporates band gap variation along with a change in absorption profile for the device.There are various ways to analyze these results as there is no limitation to the structure of the device because CsPbI 3 can be utilized as an absorber layer or CsPbBr 3 due to its wide bandgap can be used as a hole transport layer (HTL) (Chen et al. 2020;Yadav et al. 2023).But however, in this work, we used the fabricated layer properties as an intermediate layer between the absorber layer and HTL.The device structure that was simulated in SCAPS-1D is given as TiO 2 /MAPbI 3 / CsPb(I,Br) 3 /Spiro-MeOTAD.The physical parameters for the device are given in Table 3.
First, the device was simulated for as-deposited samples of CsPbI 3 doped with different concentrations of Br by incorporating as-deposited band gap values and absorption profiles for the samples.The results for asdeposited samples are shown in Fig. 8. From Fig. 8, it is evident that the device that is doped 25% with Br shows the highest power conversion efficiency (PCE%).The reason for this is that it has a good band alignment with absorber and HTL layer, as discussed in detail in our previous work (Bouich et al. 2022).But if we look at the inset table of the figure, there is no big difference in PCE for all Br-doped intermediate layers on the performance of solar cells.
However, after simulating samples, it was observed in SCAPS-1D that there is a variation in the performance of solar cells.The results of the degradation of the device are shown in Fig. 9. From Fig. 9, it is evident that we are not able to get the results for the pure CsPbI 3 sample, and the main reason for this is its bandgap got much wider, thus creating a large band offset in between the absorber and HTL layer, which SCAPS-1D is not able to simulate.In contrast, there is an overall reduction in PCE of the device.
After applying the profiles recovery stage on Brdoped CsPbI 3 samples, results for bandgap and absorption profile were again incorporated in the SCAPS-1D environment to analyze how the recovery of samples plays a role in device performance.After incorporating these results, we found out that devices doped with Br showed a slight increase in device performance, and this is shown in Fig. 10, whereas a comparison was also drawn in Table 4. From Table 4, one can deduce that undoped CsPbI 3 faced degradation at a higher rate than that of doped samples.This work aimed to propose and study the potential of substituting a quantity of bromine in CsPbI 3 perovskites which will be ready for use in solar cells with a well-optimized gap value.The work focused mainly on the composition of alloys of the CsPbI 3(1-x) Br 3x type via changing the molar ratio of the precursors.After comparing our results with numerical and theoretical findings in existing studies, we have successfully created halogenated hybrid perovskites.These materials have been combined with other layers in solar cells.This breakthrough suggests a coming revolution in solar technology that could greatly enhance the efficiency of traditional solar cells.

Conclusion
In summary, we report a strategy induced by the institution of bromine to prepare CsPbI 3(1-x) Br 3x inorganic perovskite films.By combining the two compounds CsBr and CsI as the main precursors of synthesis, highly crystalline CsPbI 3(1-x) Br 3x (x = 0, 0.25, 0.5, 0.75) films are obtained via crystal rearrangement after annealing.In our article, the optimization of the amount of bromine addition in perovskite-based solar devices CsPbI 3 may well increase yield since it provides self-passivation at the grain boundaries and uniform grain sizes, which weakens the potential barrier between the crystal and the grain   (Bouich et al. 2022(Bouich et al. , 2023;;Sonmezoglu and Akin 2020;Akman et al. 2023Akman et al. , 2021;;Baig et al. 2020;Motta et al. 2015;Khattak et al. 2022Khattak et al. , 2021Khattak et al. , 2020Khattak et al. , 2023;;Marí Soucase et al. 2022) w thickness, E g bandgap, χ electron affinity, ε r dielectric permittivity, N c , N v density of states, µ e , µ p carrier mobility, n, p

Fig. 6
Fig. 6 Variation of absorbance in aged vs. recovered CsPbI 3(1-x) Br 3x films across bromine concentrations Table5below summarizes the different values of the optical bandgap (Eg) of CsPbI 3(1-x) Br 3x by comparing our work with the results found in the literature.A comparative analysis between the results of our experiment and those of other research allows us to conclude on the validity of our hypotheses.The knowledge constructed through experimentation is supplemented by the confirmation of results similar to our values obtained.

Table 1
Effect of bromine additive on tolerance factor in CsPbI 3 perovskites

Table 3
Physical parameters for device structure carrier concentrations

Table 5
Comparative table of results