An In‐Depth Investigation of the CombinedOptoelectronicand Photovoltaic Properties of Lead‐Free Cs2AgBiBr6DoublePerovskite Solar CellsUsing DFT and …

1 Introduction Perovskite solar cells (PSCs) have gained considerable attention in response to the rapid progress in solar technology, [1-8] as seen by the notable enhancement in efficiency from a modest 3.8% to 25.7% over the course of a decade. [9-13] This incredible enhancement was made likely by the halide perovskite materials' superior photo-electronic properties, which include their higher absorption coefficient, long-range charge diffusion lengths, low exciton binding energies, adequate bandgaps, and higher charge carrier mobility. [10, 11, 13-16] In general, lead-based PSCs yield better efficiency. [17-20] Lead-based cells, however, have several problems, like performance declines when exposed to moisture and light, and a major problem is lead toxicity which is thought to be a significant barrier to the commercialization of such cells.

[21-23] Other additives with the same group of lead (Pb) were thoroughly explored in order to address the existing deficit brought on by the presence of Pb in PSCs including tin (Sn) and germanium (Ge). [24-26] But Tin-based SCs have some disadvantages of being oxidized from Sn 2+ to Sn 4+ when kept open to the air for a long time. [27] Because of these insecurities, other PSCs needed to be considered that can replace Pb 2+ ion containing a monovalent and trivalent ion mixed, such as Cs 2 AgBiBr 6 , [28] Cs 2 AgBiCl 6 , [29] (MA) 2 AgBiBr 6 [30] which are known as double perovskite SCs. Among them, Cs 2 AgBiBr 6 is the most promising structure, in which lead is substituted by silver (Ag + ) and bismuth (Bi 3+ ) cations, has been described as having a good crystal structure, a long lifetime of charge carrier, and good stability when compared to lead-based perovskites.

[31] Some experiments have been conducted to improve the features of Cs 2 AgBiBr 6 in recent years. Greul et al. investigated Cs 2 AgBiBr 6 experimentally and got a PCE of 1.66%. [32] Another experimental investigation was done by Igbari et al.

and ended up getting 2.51% PCE. [33] Theoretical investigations were also done by some researchers on Cs 2 AgBiBr 6 . Zhang et al. and Alanazi et al.

combined SnO 2 and Spiro-MeOTAD with Cs 2 AgBiBr 6 and got a PCE of 6.37% [34] and 14.29% [35] respectively. Islam et al. combined TiO 2 and Cu 2 O with Cs 2 AgBiBr 6 and got a PCE of 7.25% [36] while Alkhammash et al. used ZnO and NiO to get a PCE of 21.88%.

[37] Chabri et al. and his team investigated Cs 2 AgBiBr 6 and got a PCE of 7.16%. [38] So, it is noteworthy that further research on Cs 2 AgBiBr 6 is needed to bring out the best possible combination and further improvement of photovoltaic (PV) properties for this PSC. Notably, every layer in PSC including fluorine doped tin oxide (FTO), hole transport layer (HTL), electron transport layer (ETL), perovskite absorber layer (PAL), and back metal contact (BMC) aids in working the device properly.

[15, 39, 40] The movement of charge carriers in PAL is greatly influenced by ETL and HTL in PSC. The most used ETL in PSCs is TiO 2 and HTL is Spiro-MeOTAD for its excellent bandgap, charge mobility, and band alignment. [33, 35, 41, 42] Due to the continuous improvement of the PSCs, there are some ETLs and HTLs that are not frequently used. Among them aluminum doped zinc oxide (AZnO), [43] cadmium zinc oxide (CdZnS), [44] La-doped BaSnO 3 (LBSO), [45] niobium pentoxide (Nb 2 O 5 ) [46] make good band alignments with Cs 2 AgBiBr 6 absorber.

In the case of HTLs, copper zinc tin selenide (CZTSe), [44] cadmium telluride (CdTe), [47] copper nickel tin sulfide (CNTS), [44] nitrogen-doped titanium dioxide (TiO 2 :N), [48] poly(triarylamine) (PTAA), [49] nickel cobaltite (NiCo 2 O 4 ), [50] antimony sulfide (Sb 2 S 3 ), [51] n-propyl bromide (nPB), [52] gallium arsenide (GaAs), [53] Zinc telluride (ZnTe) [54] makes good band alignment with Cs 2 AgBiBr 6 , although they are not used frequently. CNTS emerges as a promising choice for the hole transport layer in perovskite solar cells. Its high hole mobility, favorable energy levels, chemical stability, earth-abundant composition, reduced environmental impact (lead-free), facile synthesis, and tunable properties collectively position CNTS as a potential contributor to the enhanced performance and sustainability of perovskite solar cell technologies. [44] Interestingly, electronic assets like band structure, bandgap, DOS (density-of-states), electron density mapping, etc.

are very important parameters for the exploration of each element's contributions to electronic properties. [55] A significant number of researchers are performing theoretical calculations to understand the physical properties of the DFT (density function theory) method. According to these reports, [56, 57] a lot of halide perovskite exhibits promising physical properties such as electrical, structural, optical, etc. suit them for photovoltaics and optoelectronic applications.

McClure et al. [29] and Lei et al. [58] reported that Cs 2 AgBiBr 6 has almost similar electronic properties but is more stable and non-toxic compared to organic–inorganic lead halide perovskite absorbers. Some similar compounds such as Cs 2 AgBiCl 6 has a bandgap of 2.77 eV, [29] and Cs 2 AgInCl 6 has a bandgap of 3.23 eV [59] are less suitable for their relatively higher bandgap.

However, this is the first combined DFT and SCAPS-1D study of Cs 2 AgBiBr 6 double perovskite to evaluate optoelectronic properties for solar cell applications. In this study, 4 ETLs and 10 HTLs are combined together with Cs 2 AgBiBr 6 PAL to investigate and find the best four structures among 40 combinations. First, DFT is utilized to compute the bandgap of Cs 2 AgBiBr 6 (1.654 eV), and as well as optoelectrical properties are identified. Then SCAPS 1D simulation computer software [60-62] is equipped to simulate the structures for calculating performance parameters by employing the bandgap value of the DFT result.

After finding the best four combinations among 40 different structures, the effect of absorber and ETL layer thickness on the performance parameters is observed. Furthermore, the effect of series like resistance, shunt, in addition, temperature on PSCs are also investigated. Additionally, the current–voltage density ( J–V ) and quantum efficiency ( QE ) characteristics along with the generation and recombination rates of charge carriers for the PSCs are also shown. Finally, a detailed comparison of experimental and theoretical analysis was studied to validate the outcomes of this study.

From the investigation, it is anticipated that the tools will assist academics in looking into a more prevalent arrangement of solar cells based on Cs 2 AgBiBr 6 -based PAL. In this study, we have introduced and proposed novel ETLs and HTLs to enhance the novelty the our work. Given the extensive research on conventional transport layers like TiO 2 , SnO 2 , and Spiro-MeOTAD, among others, opting for these familiar choices would not align with our goal of advancing scientific understanding in this field. These choices were made to address specific performance criteria in our study.

The introduction of new transport layers is intended to contribute to the diversification and progression of materials used in perovskite solar cells. Moreover, our considered HTLs offer distinct advantages, CZTSe and CdTe provide cost-effective options, PTAA boasts high hole mobility, NiCo 2 O 4 demonstrates good stability, and Sb 2 S 3 is both cost-effective and earth-abundant.