Defect-Induced Modulation of Electronic and Optical Properties in Monolayer CsPb₂Br₅: Implications for Fiber-Optic Sensing Applications

Abstract

Two−dimensional halide perovskites have emerged as promising optoelectronic materials, yet the uncontrolled defect formation during synthesis remains a critical challenge for their practical applications. In this work, we systematically investigate the structural, electronic, and optical properties of monolayer CsPb₂Br₅ in two representative configurations: ds−CsPb₂Br₅ and ss−CsPb₂Br₅. By introducing four types of vacancy defects—V_Br−c, V_Br−b, V_Cs, and V_Pb, we analyze their structural distortions, formation energies, and their impact on band structure and optical response using first−principles calculations. Our results reveal that Br−related vacancies are energetically most favorable and induce shallow defect levels and absorption edge redshifts in the ds−CsPb₂Br₅ structure, while in the ss−CsPb₂Br₅ configuration, only V_Br−b forms a defect state. V_Pb and V_Cs lead to significant sub−bandgap absorption enhancement and dielectric response due to band−edge reorganization, despite not introducing in−gap states. Notably, V_Br−c exhibits distinct infrared absorption in the ss−CsPb₂Br₅ model without electronic trap formation. These findings underscore the critical influence of defect type and slab asymmetry on the optoelectronic behavior of CsPb₂Br₅, providing guidance for defect engineering in perovskite−based optoelectronic applications.

1. Introduction

Metal halide perovskites have attracted widespread attention in recent years due to their exceptional optoelectronic properties, tunable bandgaps, and low fabrication costs, making them promising candidates for next−generation optoelectronic and photovoltaic devices [1,2,3]. Among these, two−dimensional (2D) layered perovskites have emerged as particularly intriguing owing to their enhanced environmental stability, quantum confinement effects, and strong light–matter interactions [4,5,6]. In this context, CsPb₂Br₅, a lead−based layered halide perovskite, has gained increasing interest due to its unique quasi−two−dimensional structure, high photoluminescence quantum yield, and long carrier diffusion length [7,8,9,10].

However, as in many halide perovskites, intrinsic point defects, particularly halogen and metal vacancies, can significantly impact the optoelectronic properties of the material [11,12,13,14]. These defects may introduce localized electronic states, perturb the band edges, and alter optical absorption behavior, ultimately affecting carrier mobility, recombination rates, and device efficiency. Extensive studies have been conducted on the defect physics of 3D CsPbBr₃, revealing that Br vacancies typically introduce shallow donor states, while Pb vacancies often act as deep traps detrimental to device performance [15,16,17,18,19,20]. In 2D perovskites, recent works have demonstrated that dimensional confinement can either suppress or enhance defect tolerance depending on the specific lattice symmetry and surface termination [21,22,23,24]. Meanwhile, several theoretical studies on bulk CsPb₂Br₅ have shown that its unique layered structure leads to defect formation energetics and optical transitions that differ markedly from its 3D analogues [25,26,27,28]. By engineering the defect landscape and optimizing the slab configuration of monolayer CsPb₂Br₅, these defect−induced optical features—such as green emission and tunable bandgaps—can be exploited for integration into fiber−integrated optoelectronic systems [29,30]. The ability to control defect states via alloying or surface passivation enables the design of high−performance optoelectronic components with enhanced stability and emission properties, making them promising candidates for future photonic and optoelectronic technologies [31]. However, a systematic investigation of point defects in monolayer CsPb₂Br₅, especially under different slab configurations, remains lacking.

In this work, we perform a comprehensive first−principles study of monolayer CsPb₂Br₅, focusing on the formation and effects of four common vacancy defects: Br monovacancy (V_Br−c and V_Br−b), Cs vacancy (V_Cs), and Pb vacancy (V_Pb). By examining the structural relaxation, formation energies, electronic band structures, and optical responses, we reveal the distinct defect−induced modifications in each configuration. Our findings highlight the interplay between defect type and slab symmetry in governing the material’s functionality, offering insight into defect engineering strategies for 2D perovskite devices.

2. Computational Conditions

2.1. Modeling of Defect

The top and side views of the tetragonal CsPb₂Br₅ crystal structure are presented in Figure 1a,b. When constructing the monolayer CsPb₂Br₅ structure, different surface terminations of Cs atoms were considered. Two main configurations were examined: the model with Cs atom termination on both surfaces, denoted as ds−CsPb₂Br₅ (Figure 1c,d), and the model with Cs atom termination on only one surface, denoted as ss−CsPb₂Br₅, as shown in Figure 1e,f.

In both the ds−CsPb₂Br₅ and ss−CsPb₂Br₅ models, we considered vacancy defects of three different atomic species: V_Br, V_Cs, and V_Pb. Among them, Br vacancies exist in two distinct forms: one located at the center of four neighboring Pb atoms, denoted as V_Br−b, and the other situated on the Pb−Br−Pb bridge bond, denoted as V_Br−c.

2.2. Computational Methods

All the calculations based on density functional theory (DFT) were performed using the Vienna ab initio Simulation Package (VASP) [32,33]. The exchange–correlation interaction was treated within the framework of the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) functional [34]. The projector augmented−wave (PAW) method was employed to describe the interaction between the core and valence electrons [35,36]. A plane−wave cutoff energy of 500 eV was used throughout all calculations to ensure good convergence.

In the simulation of monolayer CsPb₂Br₅, a 2 × 2 × 1 supercell with in−plane lattice constants a = b = 17 Å was adopted, a vacuum spacing of at least 15 Å was employed along the out−of−plane axis to avoid artificial interactions between periodic replicas. The Brillouin zone was sampled using a Γ−centered Monkhorst–Pack k−point mesh with a density of 0.04 Å⁻¹ for both structural optimization and electronic property calculations. The systems were considered converged when the forces acting on individual atoms were smaller than 0.01 eV/Å and the total energy difference between iterations was less than 10⁻⁷ eV.

The formation energy of each vacancy defect was calculated using the following equation [37]:

$$E_{form}=E_{defect}-E_{pristine}+{\mu }_i$$

(1)

where E_defect and E_pristine represent the total energies of the defective and pristine monolayer CsPb₂Br₅ structures, respectively, and μ_i denotes the chemical potential of the removed atom i (eg. Cs, Pb, or Br). The chemical potentials were chosen under thermodynamic equilibrium conditions.

Here, μ_Br, μ_Cs, and μ_Pb represent the energies per atom in crystalline Br₂, metallic Cs, and Pb, respectively. The calculated values of μ_Br, μ_Cs, and μ_Pb are −1.623, −1.890 and −2.823 eV, respectively.

The electronic band structures and density of states (DOS) were obtained after single−point self−consistent calculations based on the relaxed geometries. The complex dielectric function ε(ω) = ε₁(ω) + iε₂(ω) was calculated using density functional perturbation theory (DFPT). The optical properties were derived from the complex dielectric function [33].

3. Results and Discussion

3.1. Structures and Formation Energies of Vacancy Defects

To investigate the influence of intrinsic point defects on the structural stability of monolayer CsPb₂Br₅, four types of vacancy defects were introduced into both ds−CsPb₂Br₅ and ss−CsPb₂Br₅ configurations. The fully optimized atomic structures are shown in Figure 2 and Figure 3 for ds−CsPb₂Br₅ and ss−CsPb₂Br₅, respectively. In both models, the crystal lattice retains the basic layered framework after defect introduction, but notable local distortions are observed around the vacancy sites.

In the pristine monolayer CsPb₂Br₅, the Pb−Br octahedra are connected to form a layered framework, in which the Pb−Br bonds can be categorized into three distinct types according to their spatial orientations and coordination environments: Pb−Br₁, Pb−Br₂, and Pb−Br₃. As illustrated in Figure 4, these three types of bonds are represented by blue, green and red lines within the polyhedral layer, respectively.

To quantitatively evaluate the structural distortion induced by different vacancy defects, we extracted the variations in these three types of Pb−Br bond lengths surrounding the defect sites. The detailed bond length values for both ds−CsPb₂Br₅ and ss−CsPb₂Br₅ models are summarized in

Table 1. The Pb−Br bond lengths of different structures.

Structures	Bond Lengths (Å)
	Pb−Br1	Pb−Br2	Pb−Br3
ds−CsPb2Br5	3.180	2.949	3.408
ds−CsPb2Br5−VBr−c	3.153–3.244	2.951	3.291
ds−CsPb2Br5−VBr−b	3.054–3.277	2.858–2.893	3.088–3.244
ds−CsPb2Br5−VCs	3.125	2.899	3.236
ds−CsPb2Br5−VPb	2.994–3.255	2.892–3.099	3.107–3.479
ss−CsPb2Br5	3.184	2.868	3.226
ss−CsPb2Br5−VBr−c	3.081–3.230	2.875–2.884	3.101–3.251
ss−CsPb2Br5−VBr−b	3.157–3.277	2.858–2.893	3.088–3.244
ss−CsPb2Br5−VCs	3.109	2.884	3.237
ss−CsPb2Br5−VPb	2.966–3.212	2.832–2.875	2.934–3.426

. The results reveal that defect introduction leads to asymmetric contraction or elongation of Pb−Br bonds, with the extent of distortion depending on the type and location of the vacancy. In the ds−CsPb₂Br₅ model, the most prominent structural distortions occur in the V_Br−b and V_Pb configurations. For instance, in ds−CsPb₂Br₅−V_Pb, the Pb−Br₃ bond length varies substantially from 3.107 to 3.479 Å, compared to 3.408 Å in the pristine structure. These large fluctuations highlight the significant local lattice relaxation induced by the removal of terminal Br atoms or the central Pb atom, which disturbs the octahedral connectivity and symmetry of the Pb−Br framework. Similarly, the ss−CsPb₂Br₅ configurations show significant bond distortions upon defect formation, particularly for V_Br−b and V_Pb. In addition, compared to the ds−CsPb₂Br₅ structure, the ss−CsPb₂Br₅ model lacks one surface Cs layer, which results in a more asymmetric coordination environment and reduced electrostatic screening. This structural asymmetry allows for greater local lattice relaxation around the vacancy sites, leading to slightly larger variations in Pb−Br bond lengths and defect−induced distortions.

We also calculated the formation energies of V_Br−c, V_Br−b, V_Cs, and V_Pb in both ds−CsPb₂Br₅ and ss−CsPb₂Br₅ models, as shown in Figure 5. These results reveal that Br vacancies possess the lowest formation energies among all considered defects in both configurations. In contrast, V_Pb exhibits the highest formation energy, indicating it is energetically the least favorable to form. This trend in formation energies correlates well with the structural responses observed in Figure 2 and Figure 3 and detailed in Table 1. The high formation energy of the V_Pb defect is consistent with the significant structural distortions it induces, particularly the large variation in Pb−Br₃ bond lengths exceeding 0.37 Å in the ds−CsPb₂Br₅ model. This reflects the need to break multiple strong Pb−Br bonds and rearrange the local coordination environment, which requires substantial energy input and explains the unfavorable formation of Pb vacancies. In contrast, although the V_Br−b defect also causes notable local distortions, its formation energy remains the lowest among all the considered defects. This can be attributed to the nature of Br atoms, which form more ionic and weaker bonds with Pb atoms. Additionally, Br ion possesses a relatively small ionic radius and is located at the periphery of the PbBr6 octahedra, making its removal energetically more favorable and structurally more tolerable for the lattice. Therefore, the lattice can accommodate multiple Br vacancies with moderate relaxation, supporting their high likelihood of occurrence under Br−deficient conditions.

The formation energy of the V_Cs is also relatively low in both ds−CsPb₂Br₅ and ss−CsPb₂Br₅ models, following that of the Br−related vacancies. This can be explained by the weak interaction between Cs ions and the surrounding lattice. Cs atoms reside in the interlayer spaces and primarily serve as charge−balancing species rather than forming strong directional bonds with neighboring atoms. As a result, their removal does not require significant bond breaking or structural reconstruction, which keeps the formation energy modest. Moreover, the large ionic radius of Cs ions and its peripheral position in the lattice make its removal energetically more favorable than Pb, which is deeply embedded within the Pb−Br octahedral framework. This structural role difference contributes to the contrast in formation energies: V_Cs induces only slight local relaxation, as reflected in the minimal changes in Pb−Br bond lengths listed in Table 1, while V_Pb leads to severe lattice distortion.

These distinct structural responses are expected to have a profound impact on the electronic band structure and optical absorption behavior, as discussed in the following sections.

3.2. Electronic Properties of Vacancy Defects

As illustrated in Figure 6a, bulk CsPb₂Br₅ exhibits an indirect band gap of approximately 3.05 eV, which is consistent with previous report [38,39]. with the valence band maximum (VBM) located at the N point and the conduction band minimum (CBM) at the Γ point. When reduced to the monolayer limit, the band gap shows a clear quantum confinement effect [25]. The ds−CsPb₂Br₅ and ss−CsPb₂Br₅ model (Figure 6b,c) retains an indirect band gap character but increases to 3.16 eV. Both monolayer structures preserve the indirect nature of the band gap, suggesting that the separation of electron−hole pairs may be less efficient in pristine 2D CsPb₂Br₅.

In the ds−CsPb₂Br₅ system (Figure 7a,b), the introduction of Br vacancies, including both V_Br−c and V_Br−b, leads to a remarkable transformation of the electronic band structure. Specifically, the original indirect band gap in the pristine system is converted into a direct band gap at the Γ point. This transition may be attributed to the local symmetry breaking and the modified electrostatic environment around the defect sites, which alter the energy alignment of the conduction and valence band edges and enhance orbital overlap at the Γ point. In addition to the band gap type transition, shallow defect levels emerge within the band gap. For V_Br−c, a defect state appears approximately 0.26 eV below the CBM, while V_Br−b introduces a slightly deeper level at ~0.56 eV below the CBM. These states originate from the undercoordinated Pb atoms adjacent to the Br vacancies, where unpaired Pb 6p orbitals from weakly hybridized localized states. These shallow donor−like levels may facilitate sub−bandgap absorption and act as intermediate states for carrier excitation, although they may also contribute to nonradiative recombination depending on their population and density [16]. Nevertheless, the conduction and valence band edges remain well−defined, indicating that Br vacancies, despite introducing shallow levels, do not compromise the overall electronic integrity of the material.

In contrast, the introduction of V_Cs and V_Pb defects does not lead to the formation of in−gap states (Figure 7c,d). Instead, both defects slightly increase the band gap relative to the pristine ds−CsPb₂Br₅: to 3.18 eV for VCs and 3.19 eV for VPb. These modest shifts are attributed to structural relaxation around the vacancy sites, which slightly modifies the Pb−Br orbital interactions at the band edges. This modification may be partially attributed to local coordination deficiency and structural asymmetry experienced by neighboring atoms, particularly Pb and Br, which alters their bonding environment and electronic coupling. Notably, the V_Cs defect transforms the original indirect band gap into a direct band gap at the Γ point. This transition may be related to the breaking of out−of−plane symmetry caused by the removal of Cs⁺ ions, which weakens interlayer electrostatic coupling and redistributes the orbital contributions near the conduction band minimum. Since Cs plays a minimal role in electronic band formation, its removal leads to relatively minor structural distortions (as also seen in Table 1), but enough to slightly lift degeneracies and alter the k−space alignment of the band edges. Conversely, V_Pb maintains the indirect band gap, as the system accommodates the missing Pb atom via local rearrangements without introducing defect states. The small band gap increase may be due to the reduced Pb 6p contributions at the CBM and partial stabilization of Br−derived valence states. The absence of Pb likely enhances the structural asymmetry and under−coordination of surrounding Br atoms, further modulating the dispersion of the band edges and contributing to the observed band gap widening. This contrast highlights that Pb plays a more central role in maintaining the band structure topology, and its removal, while impactful on the band gap magnitude, does not significantly shift the momentum−space alignment of the band extrema. As a result, the system retains its indirect character through local structural adaptations, without requiring a fundamental reconfiguration of the band edge positions in k−space.

In the ss−CsPb₂Br₅ system (Figure 8a,b), Br vacancies also convert the original indirect band gap into a direct one, consistent with the increased asymmetry and reduced dielectric screening of the single−slab configuration. However, their impact on the band structure is somewhat different from that in the ds−CsPb₂Br₅ model. Specifically, V_Br−c does not introduce any defect levels, and the band edges remain clean and well−structured. This suggests that the lattice effectively accommodates the single Br vacancy in the central octahedron without significant disruption. In contrast, V_Br−b introduces a shallow defect level ~0.84 eV below the CBM, arising from the localized 6p orbitals of undercoordinated Pb atoms near the octahedral edge, which are less screened in the single−slab configuration.

As shown in Figure 8c, the V_Cs defect in ss−CsPb₂Br₅ causes a slight band gap reduction from 3.16 eV (pristine) to 3.15 eV, accompanied by a transition from an indirect to a direct band gap. This differs from the ds−CsPb₂Br₅ model, where V_Cs caused a band gap increase. The difference in band gap variation between the two models can be attributed to their structural asymmetry: the ss−CsPb₂Br₅ structure, lacking the upper Cs⁺ layer, is already less electrostatically stabilized and more susceptible to symmetry−breaking perturbations. The further removal of the remaining Cs ion in ss−CsPb₂Br₅ enhances this asymmetry, slightly distorting the Pb−Br framework and promoting Γ−point localization of the band edges. Although Cs itself does not contribute directly to the frontier orbitals, its absence alters the local potential environment and orbital overlap conditions, resulting in a reduced and direct band gap.

For the V_Pb defect in ss−CsPb₂Br₅ (Figure 8d), the band gap increases to 3.24 eV, consistent with the trend observed in the ds model (3.19 eV). However, unlike the ds−CsPb₂Br₅ configuration which maintains an indirect band gap, the VPb defect in the ss−CsPb₂Br₅ structure induces a transition to a direct band gap. This qualitative change can be attributed to the combination of Pb removal and enhanced structural asymmetry in the ss−CsPb₂Br₅ model. The absence of the central Pb atom suppresses the Pb 6p contribution at the conduction band minimum, while the surrounding Br atoms and residual Pb−Br bonds reconfigure in such a way that conduction and valence band extrema shift to the Γ point. The reduced dielectric screening and quantum confinement in the monolayer further enhance this effect, stabilizing the direct transition character.

Shallow defect levels are also present in CsPbBr₃ and MAPbBr₃. In CsPbBr₃, these levels are predominantly located near the edges of valence and conduction bands. The presence of such shallow defect states contributes to an enhanced defect tolerance in these materials. As a result, the optoelectronic properties of these materials remain largely unaffected by the presence of defects [19]. In MAPbBr₃, the formation energy of intrinsic defects depends on Fermi level position, indicating that shallow defect states are more likely to form under certain chemical potentials [40].

In summary, vacancy defects in monolayer CsPb₂Br₅ induce a range of electronic responses, from shallow defect level formation to band gap transition type changes. While Br vacancies introduce shallow donor−like states and convert the band gap to direct, Cs and Pb vacancies affect the band gap magnitude and symmetry without introducing trap states. The electronic effects of these defects are strongly influenced by the dimensionality and symmetry of the system, with ss−CsPb₂Br₅ configurations showing enhanced sensitivity due to reduced screening and structural asymmetry. These insights underscore the need for precise structural control in tailoring the defect landscape for optoelectronic applications.

3.3. Optical Properties of Vacancy Defects

To further evaluate the effects of vacancy defects on the optoelectronic performance of monolayer CsPb₂Br₅, optical properties including absorption, reflectivity, transmission, and the imaginary part of the dielectric function Im(ε) were calculated for both ds−CsPb₂Br₅ and ss−CsPb₂Br₅ structures (Figure 9 and Figure 10). The simulated spectra cover the infrared (0–1.7 eV), visible (1.7–3.1 eV), and ultraviolet (3.1–5 eV) regions.

3.3.1. Optical Response of ds−CsPb₂Br₅

As shown in Figure 9a, the pristine ds−CsPb₂Br₅ displays a sharp absorption onset at ~3.2 eV, consistent with its electronic band gap. Upon defect introduction, absorption is markedly enhanced in the sub−bandgap region. Among all configurations, V_Pb exhibits the strongest absorption enhancement, followed by V_Cs, both showing significant increases across 0.5–3.0 eV. This is consistent with their pronounced structural relaxation or band−edge reorganization, which introduce additional allowed optical transitions at lower photon energies. V_Br−c and V_Br−b also broaden the absorption edge, inducing a clear redshift. These redshifts are attributed to shallow defect states near the CBM (0.26 eV for V_Br−c and 0.56 eV for V_Br−b), facilitating sub−bandgap transitions. However, their absorption intensity increase remains moderate due to their more localized structural impact.

The reflectivity spectra (Figure 9b) support the absorption behavior. Both V_Cs and V_Pb show a dual response: increased reflectivity below ~1.7 eV and decreased reflectivity above, indicating enhanced infrared light retention and stronger visible absorption. V_Br−c and V_Br−b, in contrast, follow the pristine reflectivity trend closely, with V_Br−c displaying a distinct peak at 4.79 eV in the ultraviolet, likely due to local resonance or defect−induced polarization effects.

As expected, transmission (Figure 9c) decreases where absorption increases. V_Pb and V_Cs cause the most notable transmission suppression between 1.0–3.0 eV. V_Br−b shows moderate transmission loss due to its redshifted absorption tail, while V_Br−c maintains relatively high transparency in the visible region.

The imaginary part of the dielectric function (Figure 9d) further confirms these observations. V_Pb and V_Cs show increased Im(ε) near ~1.7 eV, corresponding to enhanced absorption and modified polarization response. Beyond this energy, Im(ε) decreases slightly compared to the pristine, consistent with reduced interband transition strength. V_Br−b remains similar to the pristine case, and V_Br−c again exhibits a local maximum around 4.79 eV, reinforcing its unique optical signature.

3.3.2. Optical Response of ss−CsPb₂Br₅

The trends in the ss−CsPb₂Br₅ structure (Figure 10) are broadly consistent with the ds−CsPb₂Br₅ model, but amplified by enhanced asymmetry and reduced dielectric screening in the monolayer geometry.

As shown in Figure 10a, V_Pb again exhibits the strongest absorption enhancement, followed by V_Cs and V_Br−b. Notably, V_Br−b is the only defect in the ss−CsPb₂Br₅ system that introduces a shallow defect level, which explains its redshifted absorption edge. Interestingly, V_Br−c introduces a distinct infrared absorption peak, despite the absence of in−gap states, suggesting localized symmetry breaking and orbital rearrangement unique to the ss−CsPb₂Br₅ model.

The reflectivity spectra (Figure 10b) show a general reduction in reflectivity for all defective configurations compared to the pristine structure. V_Pb exhibits the lowest reflectivity, consistent with its strong absorption and dielectric polarization. In contrast to the ds−CsPb₂Br₅ case, the reflectivity variation here is more gradual and less localized, likely due to the more uniform light−matter interaction in the single−slab configuration.

The transmission spectra (Figure 10c) follows a trend similar to the ds−CsPb₂Br₅. The pristine system maintains high transmission up to ~3.4 eV. V_Pb and V_Cs cause strong suppression in the 1.0–3.0 eV region, while V_Br−b shows moderate suppression. Despite its new absorption peak, V_Br−c maintains high transparency in the visible region, indicating that its influence is spectrally localized.

The Im(ε) spectra (Figure 10d) closely follow those of the ds−CsPb₂Br₅ system. Both V_Cs and V_Pb display marked increases around ~1.7 eV, consistent with enhanced low−energy absorption. At higher energies, their Im(ε) values fall below those of the pristine structure. V_Br−c and V_Br−b match the pristine curve with minor deviation.

The vacancy defects significantly influence the optical properties of monolayer CsPb₂Br₅, with effects strongly dependent on defect type and slab configuration. V_Pb and V_Cs consistently enhance sub−bandgap absorption and dielectric response due to band−edge restructuring. V_Br−b induces absorption redshifts via shallow defect levels in both ds−CsPb₂Br₅ and ss−CsPb₂Br₅ structures. In contrast, V_Br−c introduces a shallow in−gap state only in the ds−CsPb₂Br₅ model, while in the ss−CsPb₂Br₅ model it yields infrared absorption without forming defect states. These findings highlight the critical role of structural dimensionality in modulating defect−related optical behavior and offer insights for defect engineering in perovskite optoelectronics.

4. Conclusions

In this study, we systematically investigated the effects of four types of vacancy defects on the structural, electronic, and optical properties of monolayer CsPb₂Br₅ using two representative models: ds−CsPb₂Br₅ and ss−CsPb₂Br₅ structures. Among these, Br−related vacancies, particularly V_Br−b, exhibit the lowest formation energies and induce moderate local distortion, whereas Pb vacancies are energetically less favorable and cause pronounced lattice relaxation. Electronic structure calculations reveal that Br vacancies convert the pristine indirect band gap into a direct one in both structures. Specifically, V_Br−c introduces a shallow in−gap state in the ds−CsPb₂Br₅ model but not in the ss−CsPb₂Br₅ model, while V_Br−b generates shallow defect levels in both. Although V_Cs and V_Pb do not introduce in−gap states, they modify the band−edge alignment, with V_Cs facilitating an indirect−to−direct band gap transition. From an optical perspective, V_Pb and V_Cs consistently enhance sub−bandgap absorption and increase the dielectric response, primarily due to band−edge reorganization. V_Br−b causes a redshift in the absorption onset associated with its defect−induced states, whereas V_Br−c produces a distinct infrared absorption peak in the ss−CsPb₂Br₅ structure without introducing electronic trap states. These findings highlight the critical role of structural dimensionality and defect positioning in governing the defect physics of CsPb₂Br₅, offering valuable guidance for tailoring its optoelectronic performance through defect engineering.