Exploring theOptoelectronicand Photovoltaic Characteristics of Lead‐Free Cs2TiBr6DoublePerovskite Solar Cells: A DFT and SCAPS‐1D Investigations

1 Introduction With the demand for excess power, energy resources are being diminished by issues such as environmental pollution, greenhouse effects, etc. Ample research has been carried out to replace the reliance on fossil fuels. [1-4] Perovskite solar cells (PSCs) that have more than 25% power conversion efficiency (PCE), show outstanding optoelectronic characteristics such as suitable bandgap, high absorption coefficient, low exciton binding energy, efficient charge transport characteristics and thus PSCs are becoming increasingly the subject of investigations in recent years. [2, 5, 6] However, lead causes toxicity concerns and Pb-based PSCs have organic cations like methylammonium and formamidinium which are responsible for making the device unsteady and intolerant of bad weather.

[1, 2, 7-11] Much research has been conducted to avoid the above issues and enhance the PCEs although they are not effective enough to mitigate the toxicity of lead halide PSCs. [1, 8, 10] The introduction of all inorganic lead-free halide perovskites [12-21] based on Ti (IV), A 2 +1 Ti +4 X 6 −1 , especially Cs 2 TiBr 6 , has emerged as a stable and promising compound for solar cell applications. [22-25] Cs₂TiBr₆, a lead-free double perovskite material, offers advantages in terms of reduced toxicity compared to traditional lead-based perovskites. However, like any material used on a large scale, there are potential environmental and health risks throughout its lifecycle.

[26] However, the synthesis of Cs₂TiBr₆ involves the use of organic solvents and generates chemical by-products that require proper handling and disposal to avoid environmental contamination. [27] Exposure to moisture, UV light, and oxygen can degrade Cs₂TiBr₆, potentially releasing bromine compounds [28] into the environment, which can be harmful. In outdoor applications, there is a risk of leaching of toxic elements, though this is significantly lower compared to lead-based perovskites. [29] Workers involved in the production and handling of Cs₂TiBr₆ may be exposed to bromine, which can be toxic if inhaled or ingested.

If Cs₂TiBr₆ ends up in landfills, there is a potential for leaching of bromine compounds into the soil and groundwater, posing environmental and health risks. Advancing recycling technologies to efficiently recover and reuse materials from spent Cs₂TiBr₆ devices can mitigate waste and reduce resource consumption. Addressing these through improved production methods, recycling technologies, and regulatory measures is essential for minimizing its overall impact. Since the earth-abundant Titanium is non-toxic, thermally stable, and biocompatible, Cs 2 TiBr 6 , a direct energy bandgap perovskite element with reasonable optical and photovoltaic (PV) properties, can undergo thermal stress under persistent irradiation and humid environments.

[5, 8, 10] Cs 2 TiBr 6 (cesium titanium bromide) perovskites have garnered attention in the field of solar cell research due to several potential advantages over other inorganic perovskite photovoltaic materials. Cs 2 TiBr 6 has shown enhanced thermal and moisture stability compared to other inorganic perovskites like lead-based perovskites (e.g., CsPbBr 3 ). This makes Cs 2 TiBr 6 more suitable for long-term applications in varying environmental conditions. Cs 2 TiBr 6 -based solar cells achieved a power conversion efficiency (PCE) of ≈24.82% which is better than earlier reports.

[30, 31] Unlike lead-based perovskites, Cs 2 TiBr 6 eliminates the toxicity concerns associated with lead-based perovskites. This makes Cs 2 TiBr 6 a more environmentally friendly and safer alternative for solar cell applications. Moreover, it is shown that Cs 2 TiBr 6 has a suitable bandgap (≈1.534 eV) for solar absorption, which allows it to effectively harness sunlight and convert it into electricity. The bandgap of Cs 2 TiBr 6 can be tuned by modifying its composition, which provides flexibility in optimizing the material for different applications.

Cs 2 TiBr 6 exhibits good charge carrier mobility, which is crucial for efficient charge transport within the solar cell. This property can lead to higher power conversion efficiencies as well. The absorber Cs 2 TiBr 6 has shown a higher tolerance to defects compared to other relevant perovskites. This means that imperfections in the crystal structure have a lesser impact on its performance, leading to more reliable and consistent solar cell operation.

The synthesis of Cs 2 TiBr 6 is relatively straightforward and can be achieved using solution-based methods, which are scalable and cost-effective for large-scale production. It can be deposited on flexible substrates, making it suitable for applications in flexible and wearable solar devices. Cs 2 TiBr 6 has a high absorption coefficient, meaning it can absorb a significant amount of sunlight with a thin layer of material. This can lead to lighter and potentially more cost-effective solar cells.

However, it's worth noting that while Cs 2 TiBr 6 has these advantages, the research is still ongoing to fully understand and optimize its properties for commercial solar cell applications. Continuous advancements in material science and engineering are necessary to further enhance its performance and integration into practical devices. The experiment done by Shivesh et al. showed a maximum PCE of 29.13% in Cs 2 TiBr 6 -based PSCs.

[9] While M. Mottakin et al. investigated the photoelectric performance of an environment-friendly device, Ti-based PSC, and his proposed structure showed excellent thermal stability having C T of −0.20881% K ─1 , that performs 19.3% PCE at optimum operational conditions. [8] Ahmed et al.

simulated the Cs 2 TiBr 6 solar cell and found ≈5.82% efficiency. [32] As there is not much literature regarding the performance of Cs 2 TiBr 6 -based solar cells (SCs), that's why Cs 2 TiBr 6 based PSCs could be a solution for nontoxic renewable energy resources in the near future due to their good optoelectronic properties. The absorber layer which is the prime part of PV, is sandwiched between the hole transport layer (HTL) and electron transport layer (ETL). The ETL has a wide bandgap and proper alignment that helps electrons reach the cathode and improves the flow of produced holes.

Organic charge transport materials expose reduced stability in the presence of oxygen, light, and moisture. For these disadvantages, inorganic charge transport materials are highly used because they are transparent in UVs, have thermal and chemical stability, and have a simple preparation process. [7] In this present work, four ETLs, aluminum (Al) doped ZnO (AZnO) and zinc stannate (Zn 2 SnO 4 ), have been to find the best possible layout for the device. Because of its flexible electronic properties, Aluminum (Al) doped ZnO has received significant attraction as an ETL.

Besides, ZnO ETL doped with Al which plays a crucial role in mitigating recombination at the interface. [33, 34] It is reported that Zn 2 SnO 4 ETL is developed by a scalable chemical bath deposition (CBD) technique that symbolizes significant PV performances such as enhanced grain size low hysteresis index, and high device reproducibility, etc. [35] Similarly, the HTL has the ability to let holes flow to the anode but impede electrons. For the HTL material, 2,20,7,70-tetrakis (N, N-di-p-methoxy phenylamine)−90,9-spirobifluorene, also known as spiro-OMeTAD was the primitive choice.

However, their instability in chemical and thermal conditions and high cost have worked as a barrier to the development of PSCs. We have examined some highly potential materials for hole transport layers such as Solution processed Zinc Phosphide (Zn 3 P 2 ), [36] Molybdenum disulfide (MoS 2 ), [37-39] Gallium arsenide (GaAs), [40-42] Carbon nanotubes (CNTS), [43, 44] Zinc telluride (ZnTe), [45, 46] Poly[2-methoxy-5-(2′-ethylhexyloxy)−1,4-phenylene vinylene] (MEH-PPV), [47] Copper Aluminum Oxide (CuAlO 2 ), [48] Molybdenum trioxide (MoO 3 ). [49, 50] Recent research supports the use of CNTS-based materials as promising candidates for the HTL in perovskite solar cells due to their stability and adaptability under various environmental conditions. The incorporation of materials like CsNbTiS into the HTL has been shown to contribute positively to the device's overall performance, ensuring better protection against environmental factors while maintaining high conductivity and efficient hole extraction.

[51, 52] These enhancements in HTL technology underline the potential of CNTS-based materials in achieving durable and efficient solar cells suitable for practical applications. Furthermore, researchers have already demonstrated the vapor-based synthesis of Cs2TiBr6 thin films. The authors achieved good stability with this method, with the films maintaining over 80% of their initial PCE under ambient conditions. [53] However, we need to thoroughly and precisely look into the electronic and optical characteristics of a specific material system in a way to assess whether it's appropriate for solar energy usage.

To do this, we must use DFT approaches, [17] which have become widely utilized in physical, chemical, and biological applications. The Pb-free cubic Cs 2 TiBr 6 perovskite is computationally modeled in this research utilizing the renowned CASTEP program for its structural, electrical, and optical characteristics. It can be an excellent choice for materials screening because of the isotropic ion property of Cs. Based on the DFT calculations, we investigate the band dispersion, bonding, and optical behavior of the subject material.

[54-56] In addition, several halide perovskites show interesting physical characteristics, including those that are optical, structural, electrical, etc., making them suitable for solar and optoelectronic devices. For this material, a measurement bandgap of 1.8 eV was discovered. To support and comprehend the experimental findings, theoretical calculations utilizing density functional theory (DFT) were carried out. The authors demonstrate the phase-pure Cs 2 TiBr 6 powder production using an XRD pattern that is in line with our DFT simulations and exhibits a lattice value of 10.64 Å.

Additionally, researchers observed a faint photoluminescence response and a measurement absorbance onset of roughly 2.0 eV, which are both in agreement with our DFT estimates [57] and shed light on how the indirect and parity-forbidden transitions contributed to the restricted emission of light. First-principal studies on the electrical and optical characteristics of this absorber material and its doping were conducted where the band gap varies from 1.4 to 1.8 eV, which is also within the range of our calculations. [9] This makes it suitable for solar device design. Therefore, it is very efficient to use a hybrid strategy that combines both DFT and SCAPS-1D simulators to investigate an optimized design of Cs 2 TiB r6 double perovskite solar cell.

The Ti-based PSC is investigated in this study using DFT to identify the optoelectronics features like structural, band architecture, charge density mapping, and optical attributes. Then SCAPS-1D simulation software [58] was used to determine the optimal performance of FTO/Zn 2 SnO 4 /Cs 2 TiBr 6 /HTL/Au structure based on several HTLs like MoO 3 , CuAlO 2 , MEH-PPV, ZnTe, CNTS, GaAs, MoS 2 , PTAA, Cu 2 Te, and Zn 3 P 2 . After that, with the best optimized HTL evaluate and compare the structure's several features with four ETLs. Finally, the outcomes of the study were validated through comparison with previously studied experimental and theoretical investigations.

For functional lead-free PSCs, this mathematical study on the basis for selecting suitable hole and electron transport materials compatible with Cs 2 TiBr 6 would provide a new avenue in manufacturing industrial PSC. Therefore, our report is mainly a combination of the SCAPS-1D simulation package and DFT method which are used to calculate various solar cell parameters, power conversion efficiency, structural, as well as optoelectronic properties to investigate potent/specific applications of the material as a solar absorber in the photovoltaic design for the first time. In addition, to our knowledge, we have not found such a type of combined study on this compound for solar absorber application in the earlier studies. Notably, we summarized our obtained results to compare with previous reports of similar solar absorber materials where our achieved data are superior and noteworthy than earlier reports.