Bandgap reduction and efficiency enhancement in Cs₂AgBiBr₆ double perovskite solar cells through gallium substitution

Abstract

Lead-free halide double perovskite (LFHDP) Cs₂AgBiBr₆ has emerged as a promising alternative to traditional lead-based perovskites (LBPs), offering notable advantages in terms of chemical stability and non-toxicity. However, the efficiency of Cs₂AgBiBr₆ solar cells faces challenges due to their wide bandgap (E_g). As a viable strategy to settle this problem, we consider optimization of the optical and photovoltaic properties of Cs₂AgBiBr₆ by Gallium (Ga) substitution. The synthesized Cs₂Ag_0.95Ga_0.05BiBr₆ is rigorously characterized by means of X-ray diffraction (XRD), UV-vis spectroscopy, and solar simulator measurements. XRD analysis reveals shifts in peak positions, indicating changes in the crystal lattice due to Ga substitution. The optical analysis demonstrates a reduction in the E_g, leading to improvement of the light absorption within the visible spectrum. Importantly, the Cs₂Ag_0.95Ga_0.05BiBr₆ solar cell exhibits enhanced performance, as evidenced by higher values of open circuit voltage (V_oc), short-circuit current (J_sc), and fill factor (FF), which are 0.94 V, 6.01 mA cm⁻², and 0.80, respectively: this results in an increased power conversion efficiency (PCE) from 3.51% to 4.52%. This research not only helps to overcome film formation challenges, but also enables stable Cs₂Ag_0.95Ga_0.05BiBr₆ to be established as a high-performance material for photovoltaic applications. Overall, our development contributes to the advancement of environmentally friendly solar technologies.

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1. Introduction

As a sustainable energy resource, solar energy emerges as a distinctive solution to cope with the global energy crisis and environmental pollution that accompanies the use of traditional energy. Consequently, the exploration and advancement of high-efficiency and economically viable solar cells underpin the technological progress for securing energy. Within this domain, conventional crystalline silicon-based solar cells have achieved successful commercialization notwithstanding certain constraints formerly. Additionally, dye-sensitized solar cells, organic photovoltaic devices, quantum dot solar cells, and perovskite solar cells (PSCs) have been extensively investigated and developed to enhance cell performance and efficiency.^1–4 LBPs have been elucidated as a highly promising alternative to silicon-based technology, thanks to their robust light absorption characteristics and superior ambipolar charge transport capabilities. They exhibit a lengthy carrier diffusion length (exceeding 1 mm), an extended charge carrier lifetime (surpassing 100 ns), and a low exciton binding energy (less than 25 meV). Recently, substantial research endeavors have been directed towards the advancements of PSCs, resulting in a notable enhancement of their PCE from a mere 3.8% to an impressive 22.7%.^5–8 While LBPs offer notable advantages of efficiency and cost, significant concerns persist regarding their toxic nature and susceptibility to instability, particularly in humid conditions. These issues demand careful attention and targeted strategies for effective mitigation of the problems.^9–11 Rigorous endeavors for addressing these challenges have been undertaken in the advancement of LBP solar cells.

The focus is to integrate the exceptional optoelectronic characteristics of LBPs with heightened stability and non-toxic attributes. Substitutes, including bivalent tin (Sn²⁺) and germanium (Ge²⁺), as well as trivalent bismuth (Bi³⁺) and antimony (Sb³⁺), have been examined as potential replacements for lead (Pb²⁺) in perovskite materials. The primary objective of exploring these alternatives is to uphold the exceptional performance of perovskite materials in optoelectronic devices and catalytic applications, while concurrently diminishing the environmental impact associated with the use of lead.^1212–14 Nevertheless, these alternative materials are deemed suboptimal due to inherent issues related to stability and subpar performance.^15–18 An emerging and promising strategy that we take notice is the substitution of one univalent and one trivalent metal cation for Pb within the crystal structure. This leads to the formation of double perovskites (DP), characterized by a formula of A₂B⁺B³⁺X₆. Notably, there is a growing interest in the development of LFHDP, where A = Cs⁺; B′ = Ag⁺, Na⁺, Cu⁺; B′′ = In³⁺, Bi³⁺, Sb³⁺; X = Cl⁻, Br⁻, or I⁻. The appeal of LFHDP lies in their small carrier effective mass, high stability, and non-toxic nature, rendering them promising alternatives to LBPs.^19–22

Within the realm of metal halide perovskites, Cs₂AgBiBr₆ double perovskite has surfaced as a compelling substitute for toxic and unstable lead halide perovskites in diverse optoelectronic applications including solar cells. This is attributed to its exceptional chemical stability, non-toxic nature, and remarkable optoelectronic properties, including prolonged carrier lifetimes.^23–26 However, the Cs₂AgBiBr₆ solar cell encounters challenges related to efficiency, primarily attributed to its wide E_g.^27–29 The process of substitution with impurity ions has been acknowledged as a potent approach for augmenting the optical characteristics of halide perovskites. Indeed, among the array of strategies, substitution stands out for its simplicity and ease of application. Additionally, it offers the advantage of modifying the properties of LFHDP without perturbing their crystal structure.^30–33 The incorporation of Ga ions has been substantiated as a promising dopant, exhibiting noteworthy enhancements in both open circuit voltage (V_oc) and fill factor (FF) through the mitigation of recombination, consequently resulting in increased efficiency. Ga substitution has demonstrated the potential to ameliorate charge transport by diminishing surface traps.^34–37 Boudoir et al. have doped Ga in Mg_xZn_1−xO for photovoltaic devices with its concentration of 0.05 (5%), and they showed that this concentration for Ga is optimal.³⁸ This specific concentration enhances device performance improving the efficiency of capturing charge carriers. Regarding this consequence, 0.05 Ga concentration has been used in this research.

This study presents a novel exploration focused on the successful incorporation of Ga into Cs₂AgBiBr₆, a promising LFHDP material. The synthesized Cs₂Ag_0.95Ga_0.05BiBr₆ underwent comprehensive examinations utilizing XRD, UV-vis spectroscopy, and solar simulator measurements. The crystalline structure and phase purity of the material have been elucidated via XRD analysis, leading to insights into the influence of Ga substitution on the perovskite lattice. UV-vis spectroscopy delved into the optical properties, revealing shifts in absorption spectra indicative of modifications in the electronic structure. Additionally, solar simulator measurements assessed the photoconversion efficiency and performance of Cs₂Ag_0.95Ga_0.05BiBr₆, positioning it as a compelling candidate for photovoltaic applications. The synergistic application of these characterization techniques provides a holistic understanding of the structural, optical, and photovoltaic attributes of the pioneering Cs₂Ag_0.95Ga_0.05BiBr₆. This contribution offers invaluable insights to the evolving realm of LFHDPs for sustainable energy technologies. Importantly, this research provides the first comprehensive explanation of the enhanced solar-cell properties caused by Cs₂Ag_0.95Ga_0.05BiBr₆.

2. Experimentation

213 mg of CsBr (with a purity of 99.9% from Sigma-Aldrich), 225 mg of BiBr₃ (with a purity of 99.99% from Sigma-Aldrich), and 94 mg of AgBr (with a purity of 99.9% from Alfa Aesar) were introduced into a 25 mL solution of 47% hydrobromic acid (HBr, Alfa Aesar). The resultant mixture underwent stirring at a temperature of 120 °C until the solvent evaporated, yielding red crystals. Stirring was then terminated, and the mixture underwent controlled cooling at a rate of 5 °C per hour. Subsequently, the mixture was allowed to stand undisturbed overnight. The red crystals that precipitated were subsequently subjected to filtration and collection. A subsequent ethanol wash was administered to the collected crystals. The resultant solid product underwent drying under vacuum conditions at 60 °C, culminating in the formation of Cs₂AgBiBr₆ crystals. The identical procedure was replicated for Ga substitution, where 5 wt% Ga was incorporated along with all the chemical components.

The phase structure and particle size of both pristine and Cs₂Ag_0.95Ga_0.05BiBr₆ films were investigated using XRD. XRD analysis was performed employing a PANalytical X'Pert PRO MRD X-ray diffractometer equipped with a Ni-filtered Cu Kα source (wavelength = 1.5418 Å). This instrument facilitated high-resolution and lower-resolution measurements on diverse thin film and powder samples. The X-ray diffractometer operated under conditions of 40.0 kV and 40.0 mA. For the optical characterization of the films, a Shimadzu UV-2101 spectrometer was employed.

A total of 532 mg of Cs₂AgBiBr₆ and Cs₂Ag_0.95Ga_0.05BiBr₆ powders were dissolved in dimethyl sulfoxide (1 mL, DMSO, 99.9%, Sigma-Aldrich) to prepare 0.5 M precursor solutions for Cs₂AgBiBr₆ and Cs₂Ag_0.95Ga_0.05BiBr₆. Subsequently, 100 μL of these solutions underwent spin-coating on the surface of a glass/FTO/TiO₂ layer at 3000 revolutions per minute (rpm) for 1 minute. The resulting two films, Cs₂AgBiBr₆ and Cs₂Ag_0.95Ga_0.05BiBr₆, were annealed at 280 °C for 5 minutes to ensure the complete formation of a double perovskite phase. The TiO₂ film was synthesized following the procedures outlined in our previously published paper.³⁹

To fabricate the hole transport material (HTM), solution (A) was generated by blending 36 mL of tributylphosphine (TBP) with 22 mL of a stock solution containing 520 mg mL⁻¹ of lithium bis-trifluoromethyl sulfonyl-imide. Within solution (A), 72 mg of spiro-OMeTAD was introduced and stirred, resulting in the formation of a solution denoted as solution (B). Subsequently, 1 mL of chlorobenzene was incorporated into solution (B) to create the final solution. For enhanced connectivity of the device with an external source, an 80 nm-thick gold layer was deposited onto the electrode. These solar cells possessed an active area of 0.16 cm² and followed the configuration: glass/FTO/TiO₂/Cs₂AgBiBr₆/spiro-OMeTAD/Au.⁴⁰

For the evaluation of solar cells, a solar simulator was employed, featuring calibrated air mass (AM) 1.5 G and an intensity of 100 mW cm⁻². The testing apparatus was equipped with a computer-controlled source meter, specifically the Keithley 2400 (manufactured by Keithley Instruments, Inc., Cleveland, OH, USA). This setup was utilized to generate the J–V curve, and the voltage sweep was carried out in the reverse bias direction, ranging from 1.2 V to 0 V. The device underwent assessment under standard 1 sun illumination at an air mass (AM) of 1.5 G, and the voltage sweep was executed at a scan rate of 0.2 V s⁻¹ (Fig. 1).⁴¹

Fig. 1 Geometry of solar cell.

3. Results and discussion

3.1. Structure analysis

For characterization, the Cs₂AgBiBr₆ film was initially synthesized using the sol–gel method. The crystalline structure was determined through XRD.^42,43 The diffraction peaks for pure Cs₂AgBiBr₆ were identified on the (200), (220), (222), (400), (420), and (440) planes, corresponding to 2θ values of 16.56, 22.40, 27.50, 32.30, 36.95, and 38.62, degrees, respectively. The obtained results closely align with the standard JCPDS data for Cs₂AgBiBr₆, as indicated by the corresponding file number (01-084-8699). Remarkably, with a 5% Ga substitution, XRD peaks exhibited a shift toward higher 2θ angles, notably evident in the (400) diffraction peak as depicted in Fig. 2(a). Specifically, the peak observed at (400) for the undoped sample at 32.30° shifted to 32.59° with Ga substitution. Upon Ga substitution, no additional peaks were detected, affirming that Ga substitution did not introduce secondary phases or impurities alongside pristine Cs₂AgBiBr₆. This confirms Ga substitution in Cs₂AgBiBr₆. Also, there was an observed augmentation in peak intensity, indicative of the higher crystallinity by the substitution of Ga ions. The observed peak shifting toward higher angles corresponds to in-plane compressive stress in the film. The introduction of smaller ions into the host lattice results in a peak shifting, attributed to the replacement of Ag⁺ by Ga³⁺ due to the disparity in atomic radii between Ag⁺ (1.15 Å) and Ga³⁺ (0.62 Å).^44–47 The Scherer equation given below can be utilized to calculate the grain size of the material:^48,49

Fig. 2 (a) XRD pattern of pure and Cs₂Ag_0.95Ga_0.05BiBr₆ halide double perovskite solar cell. (b) Crystallite Size and Dislocation Line Density of Cs₂Ag_0.95Ga_0.05BiBr₆. (c) D-spacing, lattice parameter of pure and Cs₂Ag_0.95Ga_0.05BiBr₆. (d) Shows the volume for pure and substitution.

The Scherer constant, denoted as K, is 0.9, and the wavelength (λ) is 1.54 Å. Here, β represents the Full Width at Half Maximum of the peak, and θ denotes the Bragg angle. Therefore, the grain size calculated using eqn (1) is 29 nm for pristine Cs₂AgBiBr₆, and is 35 nm for the substitution sample.

The dislocation line density is a metric that quantifies the density of dislocations within a defined volume of a crystalline material. It is defined as δ = D⁻² which denotes the length of dislocation lines per unit volume of the crystal and is conventionally expressed in meters per cubic meter (m m⁻³).⁴⁸ The dislocation line density for pristine Cs₂AgBiBr₆ was measured at 1.18 × 10⁻³ m m⁻³, whereas for the Ga substitution sample, it decreased to 8.16 × 10⁻⁴ m m⁻³. The reduction in dislocation line density has the potential to mitigate the recombination rate, leading to an increase in carrier lifetime. As a result, this enhancement can positively impact the V_oc, J_sc, and overall PCE of the solar cell.^50–53 Interplanar d-spacing (d) is a vital parameter in X-ray diffraction analysis, utilized for discerning the crystal structure of a material. The d-spacing signifies the perpendicular distance between two successive crystallographic planes of atoms within a crystal lattice. The value of d significantly influences the optical and electronic properties of the perovskite, thereby impacting the overall performance of the solar cell.⁵⁴ The d can be determined using a formula called Bragg's law:⁵⁵

d = λ/2 sin θ (2)

where λ is the wavelength and θ is the angle of incidence. The d for the pristine sample is 2.76 Å, and for the substitution sample, it is 2.74 Å. The lattice parameter (a) is a measurement characterizing the length of the edges of the unit cell, which serves as the smallest repeating structural building block within the crystal. The value of ‘a’, especially for the most prominent peak reflection, can be determined through a specific mathematical expression. The volume of the unit cell is directly proportional to the cube of this ‘a’, as defined by the following mathematical expression.⁵⁶

a = d(h² + k² + l²)^1/2 (3)

In this context, d represents the interplanar spacing, and h, k, and l are the Miller indices. The lattice parameter for the pure sample is 11.07 Å and decreases to 10.98 Å after substitution. The volume a³ can be easily computed using the relationship in eqn (3). For the pure sample, the volume is 2.22 × 10⁻⁸ m³, and for the substitution sample, it is 2.20 × 10⁻⁸ m³. This reveals discernible differences in the structural properties between the pristine and 5% substitution of Ga into Cs₂AgBiBr₆. Considering effective ionic radii, the Ga³⁺ ion (0.62 Å) exhibits a smaller size than the Ag⁺ ion (1.15 Å). As per Vegard's law, the introduction of Ga through substitution induces structural shrinkage, leading to a reduction in volume.⁵⁷

3.2. Optical analysis

To determine the optical E_g, UV-visible spectroscopy was conducted. The response of the material to light irradiation is significantly influenced by its E_g and the energy conveyed by the incident photons. When photons interact with the material, they impart energy (hν) to the electrons, elevating electrons' energy so that they occupy higher energy levels beyond the material's E_g. In this process, the excess energy are absorbed by the electrons through interband transitions. This phenomenon is crucial in scrutinizing the absorption edges of a material. The material's E_g can be ascertained by applying Tauc's formula to these absorption edges.^48,58,59

(αhν)² = B(hν − E_g) (4)

In this context, α represents the absorption coefficient, B is a constant, hν signifies the energy of the incident photon, and E_g denotes the material's bandgap. The value of E_g was determined from Fig. 3(a) by extrapolating it to the zero-ordinate. The optical E_g for pure Cs₂AgBiBr₆ was found to be 1.91 eV, and for Ga substitution, the E_g decreased to 1.86 eV. The observed reduction in E_g in the Cs₂Ag_0.95Ga_0.05BiBr₆ film occurred because the Ag 4d-derived valence band (VB) shifted to a higher energy level. This shift was primarily attributed to the overlap between the Ga 4p orbitals and the Ag 4d orbitals.⁶⁰ The reduction in E_g could be attributed to an increase in grain size and a decrease in the number of grain boundaries.⁶¹ This is advantageous because a decreased E_g can lead to improved light absorption within the visible spectrum. The bending and propagation of light rays when they transit from one medium to another can be explained by the refractive index (n), which is of the form.⁶²

where A and B are constants with values of 13.6 eV and 3.4 eV, the value of n for pristine Cs₂AgBiBr₆ is 2.74, and for Ga substitution, it is 2.77. Researchers have indicated that n and E_g are fundamental characteristics of optical materials due to their close connection with the electronic properties of the material. Eqn (5) demonstrates a strong correlation between the n and E_g. Specifically, a reduction in E_g corresponds to an elevation in the n. The increase in n signifies a higher concentration of charge carriers, leading to a reduction in E_g. This reduction is attributed to an increase in the number of energy states between the conduction and valence bands. Moreover, this increase in ‘n’ serves as proof of an enlarged presence of polarizable molecules, resulting in a modification of the speed of light within the material.⁶³

Fig. 3 (a) E_g of pure and Cs₂Ag_0.95Ga_0.05BiBr₆ films. (b) Refractive index, electronegativity, and extinction co-efficient of pure and Cs₂Ag_0.95Ga_0.05BiBr₆.

Electronegativity is defined as the atom's capacity within a molecule to attract electrons to itself. Optical electronegativity provides valuable insights into nature of chemical bonding.^64,65 Optical electronegativity not only offers profound insights into the essence of chemical bonding, but shedding light on the intricate connections that govern molecular structures as well.⁶⁶ On the Pauling scale, when the electronegativity difference is below 0.5, it commonly leads to a non-polar covalent bond. The connection between the E_g and optical electronegativity (χ*) in different binary systems is articulated as follows:^67–69

E_g = 3.72(Δχ*) (6)

where Δχ* is the difference of the electronegativity between anion and cation. In the case of pristine Cs₂AgBiBr₆, the Δχ* value is 0.51, while for Cs₂Ag_0.95Ga_0.05BiBr₆, it reduces to 0.5. This indicates that as Δχ* decreases, the E_g also tends to decrease. With an electronegativity difference of only 0.01, this observation implies the existence of a nonpolar covalent bond. The extinction coefficient (k) plays a crucial role in elucidating the dielectric properties of a material. It directly influences emissivity, absorption coefficient, and the total power radiated from a film. This relationship is expressed as k = n/(Δχ002a)^γ.⁵⁵ In this context, the value of γ is −0.32 (a constant). The calculated value of k is 2.220 for pristine Cs₂AgBiBr₆ and 2.221 for Cs₂Ag_0.95Ga_0.05BiBr₆. When electromagnetic radiation, such as a light wave, interacts with a material, the complex dielectric function provides valuable insights into the optical behavior of the material.^55,70 The dielectric constant (ε), closely linked to a solid material's polarizability, serves as an indicator of a medium's polarity. In this context, ε is typically represented by its real (ε_r) and imaginary (ε_i) parts as:⁷¹

ε = ε_r + iε_i (7)

ε_r and ε_i which appeared here can be represented as follows:

ε_r = n² − k² (8)

ε_i = 2nk (9)

ε_r portrays the degree of materials' polarization when subjected to incident light or an electromagnetic field.³⁹ ε_i signifies the light absorption in the material.⁷² The values of ε_r and ε_i for the undoped material are 2.62 and 12.21, respectively, whereas ε_r is 2.75 and ε_i is 12.31 for the substitution material, as shown in Fig. 4. From this, we see that both ε_r and ε_i increase with substitution. The increase of ε_i indicates enhanced light absorption due to substitution, and this enhancement leads to a greater generation of electron–hole pairs. Consequently, this results in a higher current flow through the circuit, boosting the current density of the solar cell. This, in turn, can contribute to an overall improvement in the efficiency of the solar cell, making it more effective at converting sunlight into electricity.

Fig. 4 (a) Shows the real part of ε. (b) Shows the imaginary part of ε for pristine and Cs₂Ag_0.95Ga_0.05BiBr₆.

3.3. J–V curve

Fig. 5 illustrates the J–V curves for the device fabricated with Ga substitution, and the essential photovoltaic parameters are detailed in Table 1. The Cs₂AgBiBr₆ solar cell exhibited a PCE of 3.51%, accompanied by a V_oc of 0.92 V, J_sc of 5.13 mA cm⁻², and FF of 0.73. Substitution with Ga in the LFHDP displayed significantly improved performance. Specifically, the PCE of the cell increased to 4.52%, with elevated values for V_oc, J_sc, and FF at 0.94 V, 6.01 mA cm⁻², and 0.80, respectively.

Fig. 5 J–V curve of pure and Ga substitution films.

Table 1 Solar cell parameters of pure and Cs₂Ag_0.95Ga_0.05BiBr₆

Samples	Jsc (mA cm^−2)	Voc (V)	FF	Efficiency η (%)
Cs2AgBiBr6	5.13	0.92	0.73	3.48
Cs2Ag0.95Ga0.05BiBr6	6.01	0.94	0.80	4.52

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The J_sc signifies the electric current coursing through the external circuit when the solar cell's electrodes are directly linked. The J_sc value of a solar cell is affected by the photon flux produced by the incident light and is dictated by the spectral composition of the light. J_sc is a frequently employed term to characterize the maximum current output achievable from a solar cell.⁷³ The J_sc of Cs₂AgBiBr₆ for pristine material is 5.13 mA cm⁻², and following substitution with Ga, its value increases to 6.01 mA cm⁻². An increase in the J_sc in a solar cell contributes to improving power output, efficiency, and overall performance of the solar cell.⁷⁴ The V_oc is the electric potential difference between two terminals of a device when no external load is applied. It represents the maximum voltage achievable from a solar cell and corresponds to the forward bias voltage. Also, it relies on the photo-generated current density. The V_oc value for the undoped material is 0.92 V, and after substitution, it increases to 0.94 V.^75,76 It might be possible that Ga substitution in Cs₂AgBiBr₆ could increase the carrier concentration and induce an upward shift of the Fermi level of Cs₂AgBiBr₆, facilitating the injection and transfer of electrons to the conduction band of perovskite. Simultaneously, by increasing the carrier concentration, the electrons could effectively fill the interface traps, reducing interface trap density. This is beneficial for decreasing electron capture and preventing carrier recombination, eventually improving electron transport efficiency in perovskite. These enhancements could positively impact the photovoltaic properties of the device, such as J_sc and V_oc.^77–79 The FF is a parameter that characterizes the shape of the current–voltage (J–V) curve of a solar cell. It is the ratio of the maximum power (P_max) to the product of the V_oc and J_sc. Hence, the FF is calculated by the relation.⁸⁰

The FF value for the undoped sample is 0.73, and after substitution, its value increases to 0.80. The increase in FF value indicates that the solar cell is more efficient in converting sunlight into electricity.

The PCE of a solar cell is a measure of how well it converts sunlight into electricity. It is defined as the ratio of the maximum power output (P_max) to the incident solar power. Accordingly, the mathematical expression for PCE is of the form⁸¹

The PCE of the LFHDP is 3.48% for the pristine specimen, and with substitution, the value escalates to 4.52%. This increase in PCE is ascribed to the elevation of the values J_sc, V_oc, and FF, resulting from the substitution. The enhanced J–V parameters observed in cells with an optimal content of Cs₂Ag_0.95Ga_0.05BiBr₆ can be attributed to higher electrical conductivity and an upward shift of the Fermi level. Therefore, the improved photovoltaic results suggest a reduction in charge carrier recombination at Cs₂AgBiBr₆.

4. Conclusion

In conclusion, this research demonstrates the strong points of Cs₂Ag_0.95Ga_0.05BiBr₆ in relation with enhancing the performance of the device. Cs₂AgBiBr₆ films were successfully synthesized using the sol–gel method, as confirmed by XRD analysis. The Cs₂Ag_0.95Ga_0.05BiBr₆ revealed peak shifts towards higher angles, indicative of achieving Ga substitution. Then, we focused on exploring the impact of the presence of Ga on the structural, optical, and dielectric properties of Cs₂AgBiBr₆. The introduction of Ga led to larger grain sizes in the Cs₂AgBiBr₆ films. Optical properties, including E_g, electronegativity, k, and ε, were determined using UV-Vis spectroscopy. With 5% Ga substitution, the solar cell exhibited improved parameters: J_sc = 6.01 mA cm⁻², V_oc = 0.94 V, FF = 0.80, and PCE = 4.52%. This research not only successfully addresses challenges in film formation but also provides constructive insights into the potential of Cs₂Ag_0.95Ga_0.05BiBr₆ as a high-performance and stable material for photovoltaic applications. The findings contribute to the technology of developing environmentally friendly and efficient solar cells, paving the way for the future adoption of lead-free perovskites in the solar energy landscape.

Conflicts of interest

The authors have no conflict of interest.

Acknowledgements

This research was supported by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R327), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2021R1F1A1062849).

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