Highly efficient emerging Ag2BaTiSe4 solar cells using a new class of alkaline earth metal-based chalcogenide buffers alternative to CdS

Cu2ZnSn(S,Se)4 is a non-toxic, earth-abundant photovoltaic absorber. However, its efficiency is limited by a large open circuit voltage (VOC) deficit occurring due to its antisite defects and improper band alignment with toxic CdS buffer. Therefore, finding an absorber and non-toxic buffers that reduce VOC deficit is crucial. Herein, for the first time, Ag2BaTiSe4 is proposed as an alternative absorber using SCAPS-1D wherein a new class of alkaline earth metal chalcogenide such as MgS, CaS, SrS, and BaS is applied as buffers, and their characteristics are compared with CdS to identify their potential and suitability. The buffer and absorber properties are elucidated by tuning their thickness, carrier concentration, and defect density. Interestingly, optimization of the buffer’s carrier concentration suppressed the barrier height and accumulation of charge carriers at the absorber/buffer interface, leading to efficiencies of 18.81%, 17.17%, 20.6%, 20.85%, 20.08% in MgS, CaS, SrS, BaS, and CdS-based solar cells respectively. Upon optimizing Ag2BaTiSe4, MoSe2, and interface defects maximum efficiency of > 28% is achieved with less VOC loss (~ 0.3 V) in all solar cells at absorber’s thickness, carrier concentration, and defect density of 1 µm, 1018 cm−3, 1015 cm−3 respectively, underscoring the promising nature of Ag2BaTiSe4 absorber and new alkaline earth metal chalcogenide buffers in photovoltaics.


Device structure and method
SCAPS-1D (version 3.3.10) is a solar cell simulation software used to extensively study the properties of each layer in the solar cell 40 .It was developed by Marc Burgelman in the Department of Electronics & Information Systems at the University of Gent, Belgium 41 .It is widely utilized to simulate various solar cells, including CdTe, CIGSe, CZTS, perovskites, etc. [42][43][44] .It assists the photovoltaic community by providing current density-voltage (J-V), capacitance-voltage (C-V), capacitance-frequency (C-F), quantum efficiency (QE), energy band diagram, carrier density, electric field, and recombination profiles of the modeled solar cells.In addition, other output parameters such as V OC , short circuit current density (J SC ), fill factor (FF), and PCE corresponding to each layer's parameters can be analyzed.It performs these operations by solving three fundamental equations: Poisson, continuity, and transport equations of charge carriers integrated in SCAPS-1D software 45 .Poisson (Eq.1), continuity (Eq. 2 and Eq.3), and charge transport (Eq. 4 and Eq. 5) equations of charge carriers are provided below: where q, ε, p, n, N + D , N − A , G n , G p , R n , R p , φ, D n , D p , J p , J n , μ n , and μ p are elemental charge, dielectric constant, hole concentration, electron concentration, donor-type doping concentration, acceptor-type doping concentration, generation rate of electrons, generation rate of holes, recombination rate of electrons, recombination rate of holes, (1) (5) J p = qD p ∂p ∂x − qµ p p ∂ϕ ∂x electric potential, coefficient of electron diffusion, coefficient of hole diffusion, hole current density, electron current density, electron mobility, and hole mobility respectively.In this work, we have investigated the performance of novel Ag 2 BaTiSe 4 solar cells with five buffers: MgS, CaS, SrS, BaS, and CdS, using SCAPS-1D.It is simulated in substrate device configuration of front contact/ Al:ZnO(AZO)/i:ZnO(IZO)/buffer/Ag 2 BaTiSe 4 /MoSe 2 /Mo/glass as shown in Fig. 1a.The initial parameters of each layer used for the simulation of the solar cell are listed in Table 1.These parameters are taken from the literature [45][46][47][48][49][50] .In Table 1, E g, χ, ε r, N C , N V , µ n , µ p, N D , N A , N t , SA, and SD represent bandgap, affinity, dielectric permittivity, effective density of states in the conduction band, effective density of states in the valence band, electron mobility, hole mobility, donor concentration, acceptor concentration, defect density, single acceptor, and single donor respectively.The thermal velocity of electrons and holes are fixed at 10 7 cm s -1 for all the layers, and flat band condition is applied to the front contact.The simulations are carried out at 300 K under AM 1.5G spectral irradiance, wherein shunt and series resistance are not considered.In addition, neutral defects are introduced at the Ag 2 BaTiSe 4 /MoSe 2 and buffer/Ag 2 BaTiSe 4 interfaces according to the parameters listed in Table 2 to simulate realistic conditions of solar cells.
Throughout the simulation, the parameters of AZO and IZO were fixed according to Table 1.The thickness of AZO in thin-film solar cells usually ranges from 0.1 to 0.5 µm [51][52][53][54][55][56] .However, experimental results have shown that a thickness of 0.2 µm is optimal in terms of low resistivity, high mobility, and carrier concentration 57 .Therefore, we have chosen a thickness of 0.2 µm for the AZO layer.Similarly, for IZO, the optimal thickness for high solar cell performance is between 0.04 and 0.1 µm, as reported by experiments 51,[53][54][55]58 . If he thickness is less than 0.04 µm, it increases the leakage current, while for a thickness greater than 0.1 µm, the series resistance increases and the built-in field reduces, leading to degraded performance [59][60][61] .Based on material usage and experimental range, we have selected a thickness of 0.05 µm for IZO.Likewise, the other material parameters of AZO and IZO were adapted from the experiments 57,61 .The important parameters of other layers such as buffers, Ag 2 BaTiSe 4 and MoSe 2 were tuned to discover their optimum range and understand their influence on the solar cell performance.
To begin with, initial solar cells are designed with the parameters listed in Table 1.After that, the performance of the solar cells is studied as a function of buffer thickness (0.05 to 0.2 µm), carrier concentration (10 12  Table 1.Input parameters of different layers of novel Ag 2 BaTiSe 4 solar cells used in the simulation [45][46][47][48][49][50]

Optimization of buffers
The buffer plays a vital role in thin-film solar cells.It develops a p-n junction with the absorber and reduces the pinhole effect and leakage current by terminating the interaction between the absorber and the window layers 62 .Thus, studying their properties to obtain a high PCE is crucial.Therefore, the critical parameters such as thickness, carrier concentration, and defect density of all buffers, namely MgS, CaS, SrS, BaS, and CdS, are varied from 0.05 to 0.2 µm, 10 12 to 10 20 cm −3 , and 10 12 to 10 20 cm −3 respectively, to broadly investigate their impact on the performance of solar cells.

Effect of buffer thickness
The thickness of the buffer greatly influences the transportation of electrons from the absorber to the front contact.Therefore, it is crucial to determine the optimum thickness of each buffer to attain maximum solar cell performance.In this regard, we varied the thickness of MgS, CaS, SrS, BaS, and CdS from 0.05 to 0.2 µm, and the respective changes in V OC , J SC , FF, and PCE are displayed in Fig. 2. The corresponding J-V graphs are given in Fig. S1.It can be observed from Fig. 2a that the V OC of SrS decreases from 1.1 to 0.67 V when the thickness is enhanced from 0.05 to 0.09 µm.The drastic decline may occur due to the reduced quasi-fermi level splitting in SrS-based solar cells with increasing thickness 63 .Nevertheless, it saturates beyond 0.09 µm.The V OC of other buffers remains unaffected, indicating no change in the positions of energy bands and fermi levels in these solar cells corresponding to the buffer thickness 63 .On the other hand, the J SC value increases from 39.27 to 39.38 mA cm −2 with an increase in BaS thickness.This is unusual because J SC generally decreases with an increase in buffer  thickness.It is suspected that this may be due to the characteristics of BaS, and further experimental study is required.In other cases, a small decrease in J SC was observed, which could be attributed to the minute light absorption in buffers and the resulting reduction in charge carrier generation in the absorber 43 .Other researchers have noticed a minor change in J SC due to an increase in buffer thickness, which is considered a constraint in numerical simulations [64][65][66][67] .Figure 2c shows that FF significantly increases from 35.47 to 58.64% till 0.09 µm in SrS-based solar cells and improves slightly to 61.8% on further increase to 0.2 µm.Similarly, it also enhances from 79.77 to 80.05%, 60.75 to 61.27%, 57.35 to 68.57%, and 68.71 to 68.93% in MgS, CaS, BaS, and CdS-based solar cells, respectively for the thickness range 0.050 to 0.2 µm.This increment originates from the reduction in the R S of the solar cells at enhanced buffer thickness 68 .Consequently, PCE also increases in all solar cells (Fig. 2d).However, the improvement is minute, demonstrating that the impact of buffer thickness on the overall performance of all the solar cells is negligible.When making solar cells, the buffer thickness is a critical factor.If it's too thin (below 0.05 µm), it may not cover the entire substrate, causing a high leakage current and a poor p-n junction at the buffer/absorber interface.This, in turn, negatively impacts the spectral response of the solar cell 53,62,69,70 .On the other hand, if the buffer is too thick (over 0.1 µm), it leads to parasitic absorption, reducing the amount of charge carriers that reach the absorber, and reducing the built-in potential at the interface, which makes it harder to generate and collect charge carriers 62,70,71 .To achieve high PCE, it's recommended to use a buffer thickness in the range of 0.05 to 0.1 µm for I 2 -II-IV-VI 4 solar cells as found in the literature 16,51,55,58,71 .
After considering all these factors, an optimal thickness of 0.080 µm is selected for the alkaline earth metal-based chalcogenide buffers, which falls within the suggested range.

Effect of buffer's carrier concentration and defect density
The buffer's carrier concentration strongly influences the interface properties between the absorber and buffer in solar cells 72 .Thus, to study its influence on solar cell parameters, each buffer's carrier concentration (N D buffer ) is varied from 10 12 to 10 20 cm −3 while fixing the absorber's carrier concentration (N A absorber ) at 10 15 cm −3 as seen in Fig. 3.The respective J-V graphs are provided in Fig. S1.It can be observed that the V OC of all solar cells stays unaltered till 10 16 cm −3 and slightly increases for CaS and BaS-based solar cells while decreasing for SrS, MgS, and BaS-based solar cells beyond the mentioned value.It is well known that the V OC of the solar cells increases with the splitting of electron and hole quasi-fermi levels, which is produced by the electrochemical potential difference of electrons and holes in each layer of solar cell 73 .In this view, the observed increase or decrease in V OC can be attributed to the increment or reduction in the quasi-fermi level splitting in the respective solar cells with the increasing N D buffer .On the other hand, J SC and FF values of all solar cells significantly improve above 10 15 cm −3 , leading to improved PCE.When N D buffer > N A absorber , the concentration of electrons at the interface region rises, lowering the barrier height at the absorber/buffer interface.This subsequently elevates the built-in potential and conductivity of the solar cells, resulting in enhanced solar cell performance 72 .However, when the N D buffer is increased beyond the optimum value, large electron-electron scattering occurs in solar cells, which hinders the carrier transportation, resulting in a slight decrease of J SC and PCE for concentrations above 10 19 cm −3 in MgS and BaS-based solar cells 74 .Thus, an optimum N D buffer of 10 20 cm −3 is chosen for CaS, SrS, and CdS buffers, while 10 19 cm −3 is selected for MgS and BaS buffers to obtain the maximum solar cell performance.
In general, N D buffer primarily impacts the bending of energy bands at the absorber/buffer interface, facilitating the separation of charge carriers in solar cells 45 .Thus, to attain significant insights into its impact on band bending, the energy band diagram is extracted from SCAPS-1D as a function of N D buffer, as shown in Fig. 4a-e, where E C is the conduction band minimum, and E V is the valence band maximum.It can be seen that when the N D buffer is less than or equal to the N A absorber , no change is observed in the band alignment, attributing to the unaltered solar cell performance till 10 15 cm −3 .In addition, the barrier for electrons at the absorber/buffer and buffer/IZO interface is large.On the contrary, as the N D buffer increases above the N A absorber , the E C and E V of the buffer move downwards, reducing the barrier at both interfaces and boosting the transport efficiency of charge carriers.This occurs because, at N D buffer < N A absorber , the holes primarily occupy the absorber/buffer interface energy states, which act as a recombination center for the photogenerated electron, impeding its flow towards respective contact 72 .Moreover, as the electron concentration at the interface is very low, a large spike is formed at the absorber/buffer interface 75 .Nevertheless, for N D buffer > N A absorber , the electrons predominantly occupy the lower density of states in the buffer's conduction band and near interface states of absorber/buffer junction.This eventually improves the interaction and exchange potential between the charge carriers, resulting in a downward shift of energy bands.As a result, the barriers at the absorber/buffer and buffer/IZO interface shrink, enhancing the conductivity of the solar cells 45,76 .Furthermore, it could be noticed that the bending of buffer's E C and E V is large in MgS, SrS, and BaS-based solar cells while it is less in CaS and CdS-based solar cells.This reveals that the influence of N D buffer on the band bending of CaS and CdS-based solar cells is comparatively smaller than MgS, SrS, and BaS-based solar cells.
Impedance spectroscopy is a vital characterization technique to investigate the accumulation of charge carriers at the interface in solar cells 77 .Therefore, C-V measurements are performed between 10 12 cm −3 and the optimum N D buffer of each solar cell to analyze the accumulation of charge carriers at the absorber/buffer interface (Fig. 4f-j).All measurements are executed at 1 MHz frequency to address the repercussions of deep-level defects.From the C-V plots of all solar cells, it can be observed that the capacitance remains constant and has less values at lower voltages, corresponding to the depletion capacitance (C DEP ), which originates from the depletion region at the absorber/buffer junction.Conversely, it is comparatively large at higher voltages.This behavior occurs because the depletion region shrinks as the voltage increases, resulting in a large accumulation of charge carriers at the interface.As a consequence, capacitance increases, which is termed as accumulation capacitance (C ACC ) 78 .It can be noticed in Fig. 4f-j that the C ACC of all solar cells at 10 12 cm −3 is very high compared to the optimized N D buffer , denoting that the accumulation of charge carriers at the interfaces is intense at 10 12 cm −3 , which boosts the recombination in the bulk region.This is also evident in Fig. 4a-e, where the barrier for electrons at the Ag 2 BaTiSe 4 /buffer interface is large at 10 12 cm −3 .Thus, the photogenerated electrons in the absorber require more energy to cross the barriers and get collected at the front contact.As a result, they accumulate in the absorber and eventually recombine with the holes.Whereas, at an optimized N D buffer , the possibility for charge carrier accumulation decreases due to the reduction in the barrier, resulting in a fast collection of charge carriers without recombination.This has led to improved solar cell performance at an optimized N D buffer .Furthermore, in the C-V plot, we could notice that the voltage at which the capacitance begins to rise, shifts to a higher voltage for optimized N D buffer in all solar cells.In other words, it can be said that C DEP extends to higher voltage when the N D buffer is increased, revealing that the depletion width at the p-n junction has been improved at a higher N D buffer .To witness it, the Mott Schottky (1/C 2 ) graph is plotted from the C-V, and the built-in potential (V b ) is determined from the intercept of the plot.It is clear from Fig. 4k-o that V b has been drastically improved from 0.56 to 0.84 V, 0.42 to 0.63 V, 0.49 to 0.88 V, 0.56 to 0.91 V, 0.46 to 0.79 V for MgS, CaS, SrS, BaS, and CdS-based solar cells respectively, after optimizing the N D buffer .The corresponding depletion width (W) of each solar cell was calculated using the following equation 79 : where ε 0 is the dielectric permittivity of free space, ε S is the dielectric constant, and q is the elementary charge.At 10 12 cm −3 , W is deduced to be 0.61 µm, 0.51 µm, 0.57 µm, 0.60 µm and 0.55 µm for MgS, CaS, SrS, BaS, and CdS-based solar cells respectively and improved to 0.74 µm, 0.64 µm, 0.76 µm, 0.77 µm and 0.72 µm at optimized N D buffer .It is essential to highlight that the W will extend towards the absorber as N D buffer > N A absorber .Since most of the light absorption and charge carrier generation occurs in the depletion region along the absorber side, the increase in W boosts the amount of carrier generation in solar cells.Moreover, the elevation in V b would hasten the charge carrier separation and improve their collection at the respective contacts 62 .Thus, final PCE of 18.81%, 17.17%, 20.6%, 20.85%, and 20.08% are obtained for MgS, CaS, SrS, BaS, and CdS-based solar cells, respectively, at the optimized N D buffer .The discussed research findings reveal N D buffer's dominant role in enhancing solar cell performance.
After that, the effect of defect density of each buffer is investigated by tuning it from 10 12 to 10 20 cm −3 (Fig. 5).The corresponding J-V graph is given in Fig. S1.All the photovoltaic parameters remain almost constant up to a specific range in all solar cells and slightly decrease to lower values with further increase in defects.In particular, PCE reduced from 18.86 to 18.3%, 17.17 to 15.61%, 20.63 to 19.87%, 20.9 to 20.49%, 18.69 to 17.72% for MgS, CaS, SrS, BaS, and CdS-based solar cells respectively, when the defect density is increased from 10 12 to 10 20 cm −3 .The reduction in solar cell performance occurs because the rise in the defect states acts as traps for the charge carriers, which boosts the recombination rate in solar cells 80 .However, it can be noticed that the level of decrement is small, i.e., 0.56%, 1.56%, 0.76%, 0.41%, and 0.97% for MgS, CaS, SrS, BaS, and CdS-based solar cells.This indicates that the influence of the buffer's defect density on the solar cell performance is negligible.Based on the results, an optimum defect density of 10 16 cm −3 for MgS, 10 15 cm −3 for BaS and SrS, and 10 17 cm -3 for CdS and CaS are selected for further simulations.www.nature.com/scientificreports/

Absorber optimization
The absorber is the most critical layer in solar cells, as most charge carriers are generated here.Thus, the quality of the absorber is crucial to attain maximum solar cell performance.Therefore, the material parameters of Ag 2 BaTiSe 4, such as electron affinity, thickness, carrier concentration, and defect density, are varied from 4.1 to 4.7 eV, 0.1 to 2 µm, 10 12 to 10 18 cm −3 and 10 12 to 10 20 cm −3 respectively to understand their influence on the solar cell performance.The results of the Ag 2 BaTiSe 4 optimization are elucidated in the following sections.

Effect of absorber's electron affinity
Adjusting the energy band offsets at the absorber/buffer and MoSe 2 /absorber interface is essential to alleviate the formation of energy barriers and enhance the collection of charge carriers in solar cells 81 .This can be directly achieved by tuning the electron affinity of the absorber as it governs the energy band offsets at both interfaces.Herein, the electron affinity of Ag 2 BaTiSe 4 is varied from 4.1 to 4.7 eV to identify the optimum energy band offset values in all solar cells.The variation in photovoltaic parameters as a function of electron affinity is displayed in Fig. 6a-d, and the corresponding J-V graphs are given in Fig. S2.The V OC of all solar cells increases till 4.6 eV and reduces at 4.7 eV, while J SC remains constant throughout the affinity range in all the solar cells except for SrS-based solar cells, where it drastically decreases after 4.6 eV.On the other hand, FF and PCE follow the same trend in all solar cells, and their maximum values are demonstrated at 4.4 eV.The affinity values above or below 4.4 eV degrade the solar cell performance.The obtained results can be clearly understood in light of variations in energy band offsets concerning the absorber's electron affinity.High conduction band offset (CBO) and low valence band offset (VBO) are generally required at the MoSe 2 /absorber interface to restrict electrons and efficiently transport the photogenerated holes.Low CBO and high VBO are essential at the absorber/buffer interface to effectively collect electrons at the front contact 82 .Hence, CBO and VBO at both interfaces corresponding to each affinity value of absorber is calculated using the following formula 83 : Absorber/buffer interface MoSe 2 /absorber interface where χ MoSe 2 , χ buffer , χ absorber and E G MoSe 2 , E G buffer , E G absorber are the affinity and bandgap of MoSe 2 , buffer, and absorber respectively.The calculated CBO and VBO of all solar cells at both interfaces are explicitly provided in Table 4. On analyzing the MoSe 2 /absorber interface, VBO holds positive and negative values with the absorber's electron affinity variation.At positive VBO, a spike is created at the MoSe 2 /absorber interface, which hinders the diffusion of holes from the absorber to MoSe 2 .In the case of negative VBO, the photogenerated holes in the absorber have a cliff-like barrier that boosts the collection of holes at the back contact.However, a large cliff elevates the accumulation of holes in MoSe 2 , which enhances the recombination at the back contact, deteriorating the solar cell performance 84 .Thus, the optimum VBO at the MoSe 2 /absorber interface is identified to be − 0.06 eV to demonstrate maximum PCE.Similarly, two conditions are observed at the absorber/buffer interface 84 : (1) When the affinity of the absorber is less than the buffer, negative CBO is obtained, representing cliff formation.
(2) Positive CBO is attained when it is higher than the buffer, leading to spike formation at the interface.
It has been commonly believed for a long time that a cliff at the absorber/buffer interface is beneficial for solar cells, as the separation and extraction of the charge carriers are not constrained while it is restricted by spikes due to the formation of barrier at the interface 85 .Later, the development of simulation tools and experimental results proved that a moderate spike-like barrier is also advantageous for solar cells as it creates strong V b at the interface, enhancing the carrier collection at the contacts 86,87 .Moreover, in some cases, a large cliff-like barrier has also been observed, which leads to charge carrier accumulation at the interface due to the weak potential barrier, improving the interfacial recombination and thereby affecting the solar cell performance 88 .Thus, it is important to mention that the ideal type of barrier (either spike or cliff) required at the absorber/buffer interface and optimum values of the barrier height in solar cells are scattered in the literature, revealing that it primarily depends on the adopted material system 75,87,89 .The results show that CaS, CdS-based solar cells require a cliff-like barrier with a height of − 0.2 eV and − 0.1 eV to attain maximum solar cell performance.Whereas the spikes of 0.1 eV, 0.4 eV, and 0.25 eV are essential in MgS, SrS, and BaS-based solar cells.The positive or negative CBO above or below the mentioned values would degrade the solar cell performance due to the unfavorable band alignment at the absorber/buffer interface.Furthermore, the obtained results exhibit that the difference in the type of barrier and their respective barrier height mainly stems from the material characteristics of the buffer, as the other layers in these solar cells are similar, revealing the dominant role of buffer properties for the efficient transportation of charge carriers.It has also been reported elsewhere that buffer's parameters, such as carrier concentration, the effective density of states, interface defect states, etc., determine the barrier height at the interface, thus confirming the demonstrated results 75 .Moreover, the optimum VBO and CBO calculated at the absorber/buffer and MoSe 2 /absorber interface, respectively, are also ideal for restricting the holes and electrons in the respective interfaces.Hence, the electron affinity of 4.4 eV is required in Ag 2 BaTiSe 4 to achieve high PCE.
In addition, to gain a deep understanding of the variations in the transportation and recombination of charge carriers corresponding to the absorber's electron affinity, Nyquist plots are plotted from C-F measurements in all solar cells, as shown in Fig. 6e-i.Generally, the Nyquist plot of solar cells consists of two semi-circles at separate frequency regions.The semi-circle at the low-frequency range denotes the recombination resistance (R rec ) at the absorber/buffer interface, while the high-frequency semicircle signifies the hole transfer resistance at the MoSe 2 / absorber interface 90 .Interestingly, a single semi-circle is observed in all these solar cells in the whole frequency range.In addition, the semicircle obtained at the optimum electron affinity, i.e., 4.4 eV, is larger than the other affinity values in all solar cells.This indicates that the obtained semicircle represents the R rec .The absence of high-frequency semicircles reveals no hole transfer resistance in these solar cells.As discussed above, the large semicircle at 4.4 eV displays that the solar cells have high R rec at the optimum electron affinity, occurring due to the proper CBO and VBO at the interfaces of the absorber and transporting layers.The shrinkage in the semicircle for the affinity values above or below 4.4 eV happens due to the surging recombination rate of charge carriers resulting from improper interface barriers.From the above discussions, it is apparent that proper CBO and VBO at the absorber/buffer and MoSe 2 /absorber interface are essential to effectively transport the photogenerated charge carriers to the contacts without recombination.

Effect of absorber's thickness
The thickness of the absorber plays a vital role in understanding the performance of solar cells.Thus, the thickness of Ag 2 BaTiSe 4 is varied from 0.1 to 2 µm to identify the optimum value.Figure 7a-d  V OC , J SC , FF, and PCE as a function of absorber thickness.The respective J-V graphs are given in Fig. S2.It can be noticed that the V OC of all solar cells improves till ~ 0.3 µm and slightly decreases after the mentioned value.
The initial improvement is attributed to enhanced quasi-fermi level splitting with the large generation of charge carriers whereas its reduction originates from the elevating dark saturation current and recombination rate with the thickness 91 .In addition, FF rises till a certain thickness range and saturates in all the solar cells, occurring due to the enhanced R S in a solar cell for thicker absorber 91 .On the other hand, when the thickness is increased from 0.1 to 2 µm, J SC drastically improves from ~ 21 to ~ 40 mA cm −2 in all solar cells.This has subsequently elevated PCE from 10.05 to 20.74%, 11.27 to 22.75%, 10.13 to 22.75%, 10.13 to 22.5%, 10.15 to 22.79%, and 10.54 to 22.79% in MgS, CaS, SrS, BaS and CdS-based solar cells respectively.This happens because when the absorber is very thin, photons of longer wavelengths from the sun are not absorbed, and most of the light is transmitted.This results in the poor generation of charge carriers in solar cells due to low performance.As the thickness increases, the photon absorption in solar cells is enhanced, elevating the generation rate of charge carriers 92 .This has led to a steep rise in solar cell performance.However, we could see that J SC significantly improves till 1 µm and saturates beyond the mentioned value.A similar trend has also been observed in PCE.In particular, when the thickness is enhanced from 0.1 to 1 µm, the increment in PCE is about 10.57%, 11.23%, 12.12%, 12.38%, and 12% for MgS, CaS, SrS, BaS, and CdS-based solar cells respectively.While it increased by just ~ 0.25% in all the solar cells when the thickness was extended 1-2 µm.When the absorber is too thick, the generated charge carriers must travel long distances to reach the respective contacts.Thus, the majority of them tend to recombine due to shorter diffusion lengths than the absorber thickness, causing saturated solar cell performance 93 .This is also evident in QE measurements (Fig. 7e-i), where the absorption increases by ~ 34% in all the solar cells for the thickness range of 0.1 to 1 µm.Whereas it improves by ~ 1.5% on further increment to 2 µm.In I 2 -II-IV-VI 4 solar cells, experiments have shown that an absorber thickness of approximately 1 µm is ideal for majority photon absorption due to its high absorption coefficient (~ 10 5 cm −1 ) 54,[94][95][96] .For example, when the thickness of CZTS was adjusted from 0.5 to 2 µm, the PCE gradually increased up to 1 µm and then stabilized beyond that.QE measurements revealed that the collection depth of minority carriers ranged from 0.75 to 1 µm, indicating that recombination was high for thicknesses greater than 1 µm.Conversely, for absorber thicknesses less than 0.75 µm, there were high collection losses of charge carriers in the solar cells.Therefore, the optimal thickness www.nature.com/scientificreports/for the CZTS absorber was determined to be around 1 µm for achieving high PCE 97 .Our simulation results were compared with the experiments, and we concluded that a thickness of 1 µm for Ag 2 BaTiSe 4 would be sufficient to achieve high solar cell performance.

Effect of absorber's carrier concentration
The N A absorber modifies its electrical conductivity and determines the charge separation efficiency of the solar cells 98 .According to the Mott criterion, the maximum carrier concentration limit of CZTS is 10 18 cm −3 .Beyond this, it degenerates and loses its semiconducting property, adversely affecting the J SC of solar cells 99 .Thus, taking insights from the parent material, the carrier concentration of Ag 2 BaTiSe 4 is varied from 10 12 to 10 18 cm −3 in all solar cells.The changes in the solar cell parameters concerning the N A absorber are displayed in Fig. 8.The J-V graphs are provided in Fig. S3.It can be seen that the V OC and FF of all solar cells are unchanged till 10 14 cm −3 , and then it drastically increases with the N A absorber .
In general, the improvement in the N A absorber amplifies the V b at the interface, promoting the separation efficiency of photogenerated charge carriers 100 .Mott-Schottky plots for 10 12 and 10 18 cm −3 and their corresponding V b are shown in Fig. 9a-e.When N A absorber is 10 12 cm −3 , V b of MgS, CaS, SrS, BaS, and CdS-based solar cells are identified to be 0.54 V, 0.61 V, 0.6 V, 0.59 V and 0.64 V respectively which are then increased to 0.9 V, 0.95 V, 1.01 V, 1.03 V and 1.18 V at 10 18 cm −3 .Thus, the attained increment in V b assists in efficiently separating and collecting the generated charge carriers at the respective electrodes without recombination, leading to a drastic rise in V OC and FF.In addition, the increase in N A absorber modifies the energy band alignment and enhances the quasi-fermi level splitting, boosting the V OC and overall performance of solar cells.Therefore, to witness it, energy band diagrams for 10 12 and 10 18 cm −3 are extracted from SCAPS-1D in all solar cells (Fig. 9f-j).When N A is increased from 10 12 to 10 18 cm −3 , the energy bands of all the layers in the solar cell shift upwards, such that the E V of the absorber moves closer to the hole quasi-fermi level (F P ).This strongly elevates the conductivity of solar cells, leading to improved solar cell performance.Notably, no change is observed in the position of F P along Ag 2 BaTiSe 4 and MoSe 2 , whereas the electron quasi-fermi level (F N ) shifts upward along with the energy bands with increasing N A absorber , disclosing the increment in the splitting of quasi-fermi levels, which directly improves the V OC of solar cells.Contrastingly, J SC remains constant till 10 15 cm −3 and decreases with a further increase in the N A absorber .This happens because as the N A absorber increases, the width of the depletion region decreases along the absorber region while improving towards the buffer, which reduces the light absorption in solar cells 45 .This can also be evidenced in Fig. 9k-o, where QE is performed between 10 12 and 10 18 cm −3 in all solar cells.The absorption slightly decreases by 1.18%, 1.14%, 1.2%, 1.17%, and 1.16% in MgS, CaS, SrS, BaS, and CdS-based solar cells, causing a reduction in J SC .In addition, the possibility of charge carrier generation in the quasi-neutral region of the absorber increases with a decrement of W in the absorber, and the charge carriers generated far from the junction have to diffuse through W to reach the contacts.Some of them with insufficient diffusion length and lifetime may recombine, diminishing the J SC of solar cells 100 .However, when the N A absorber is increased from 10 12 to 10 18 cm −3 , PCE drastically rises from 19.49 to 26.62%, 21.37 to 28.47%, 21.18 to 28.22%, 21.46 to 28.54% and 21.45 to 28.55% for MgS, CaS, SrS, BaS, and CdS-based solar cells respectively which mainly comes from the combined improvement of V OC and FF values while the demise in J SC does not affect the overall performance of the solar cells.

Effect of absorber's defect density
The optoelectronic properties of solar cells are adversely affected by defects in the material system 81 .Therefore, the influence of the absorber's defect density on the photovoltaic parameters is examined by tuning it from 10 12 to 10 20 cm −3 in all solar cells.Figure 10a-d shows the changes in V OC , J SC , FF, and PCE as a function of absorber defect density, and the respective J-V is displayed in Fig. S3.All the solar cell parameters are almost unaltered till 10 15 cm −3 and dramatically falls on further increase in defect density.To be specific, when defect density is improved from 10 12 to 10 20 cm −3 , PCE sharply declines from 27.3 to 1.68%, 28.9 to 2.64%, 28.8 to 2.22%, 29.13 to 1.6% and 29.13 to 2.33% for MgS, CaS, SrS, BaS, and CdS-based solar cells respectively.The massive decline occurs due to the increase in the recombination sites at the path of photogenerated charge carriers with increasing defects, which shortens their diffusion length and lifetime 101 .Generally, the minority carrier lifetime ( τ ) is calculated using Eq. ( 11) 102 Here σ is the capture cross-section of the charge carrier, v th is the thermal velocity, and N t is the defect density.The diffusion length of charge carriers is given as where L is the minority carrier diffusion length and D is the diffusion coefficient.Herein, the L and τ are acquired from SCAPS-1D for the varying defect density of the absorber with a thickness of 1 µm (Fig. 10e).When the defect density is raised from 10 12 to 10 20 cm −3 , the τ considerably decreased from 10 5 to 10 -3 ns and their L drastically reduced from 160 µm to 0.016 µm.On comparing the obtained solar cell parameters with Fig. 10e, it can be observed that the PCE almost remains unaffected till L of 5.1 µm, slightly decreases for L = 1.6 µm while firmly falls for L ≥ 0.5 µm, confirming that L less than the thickness of the absorber adversely affects the solar cell performance.Moreover, the observed decrement in L and τ elevates the recombination rate of charge carriers, leading to poor solar cell performance.
The recombination rate concerning the increasing defect density in all solar cells is shown in Fig. 11a-e.As the defect density increases, the recombination rate shoots up in the absorber region near the absorber/buffer junction of all solar cells, giving rise to deterioration in solar cell performance.Interestingly, the recombination at the absorber/MoSe 2 interface decreases with defect density.This occurs because, as the recombination in the absorber region increases, the density of photogenerated carriers drastically reduces in the entire bulk region of the absorber and the near interface regions.This subsequently declines the number of charge carriers reaching the electrodes, reducing the recombination rate at the absorber/MoSe 2 interface.This is also evident from Fig. 11f-j where the variation in density of minority carriers (electrons 'n') in the absorber region for the defect density 10 12 and 10 20 cm −3 of all solar cells are explicitly displayed.It can be seen that the density of electrons in the absorber region becomes negligible at 10 20 cm −3 , resulting from the hiking recombination rate in the absorber.In addition, electrons at the absorber/buffer interface of MgS, SrS, and BaS-based solar cells are diminished at 10 20 cm −3 , indicating that there is sharp recombination at the absorber/buffer interface of these solar cells as witnessed in Fig. 11a, c, d.Furthermore, the electric field at the interface of the absorber and buffer is minimized with increasing defects, as shown in Fig. 11k-o.Thus, the separation and collection of charge carriers are considerably affected, worsening the performance of solar cells.Overall, the results show that the defect density rise greatly affects the L and τ of charge carriers, increasing the recombination rate in solar cells.Consequently, the density of minority carriers and built-in electric field at the p-n junction declined, drastically reducing the solar In addition, it is important to study the influence of absorber's shallow and deep level defects on the performance of solar cells.Hence, we investigated the impact of varying the defect location and density in Ag 2 BaTiSe 4 absorbers on the photovoltaic parameters of solar cells.The observed variation in V OC and J SC are displayed in Fig. 12 while the changes in FF and PCE are provided in Fig. 13.We varied the defect location from − 0.1 to 1.3 eV, corresponding to the valence band and defect density from 10 12 to 10 20 cm −3 .We found that the photovoltaic parameters were independent of the defect's position up to 10 15 cm −3 , after which they declined with increasing defects.Shallow-level defects, located near/inside the valence and conduction band, were observed in the regions between − 0.1 and 0.1 eV and 1.1 to 1.3 eV, while deep-level defects were present in the region between 0.1 and 1.1 eV due to their presence away from the energy bands 103 .V OC and FF were almost unchanged at shallow levels near the valence band when the defect density was larger than 10 15 cm −3 .However, they slightly decreased when the defects were near the conduction band and drastically fell at the deep levels in all solar cells.Similarly, J SC nearly retained its maximum value at shallow levels when the defect density was increased from 10 15 to 10 18 cm −3 , while it decreased for deep levels.For defects > 10 18 cm −3 , it showed a decline, even at shallow levels, where the decrement was more pronounced at the deep levels.PCE was largely dependent on J SC rather than V OC and FF.Shallow-level defects aided in the formation of proper band alignment at the interface of transport layers, enriching charge carrier transportation, and had little effect on PCE until 10 18 cm −3 , beyond which they degraded PCE 45 .On the other hand, deep-level defects caused a dramatic reduction in PCE, with the lowest PCEs of 1.71%, 2.67%, 2.24%, 0.45%, and 2.35% in MgS, CaS, SrS, BaS, and CdS-based solar cells respectively, obtained at ~ 0.6 eV for 10 20 cm −3 .This occurs because a large number of photogenerated electrons and holes are trapped due to the presence of deep-level defects in the mid-bandgap regions, leading to high recombination and limiting the separation of charge carriers to the transport layers, degrading the overall performance of solar cells 45,104 .Therefore, to achieve the highest PCE, the deep-level and shallow-level defects in Ag 2 BaTiSe 4 absorbers must be less than 10 15 cm −3 and 10 18 cm −3 , respectively.

Optimization of MoSe 2
In I 2 -II-IV-VI 4 thin film solar cells, the formation of MoSe 2 is inevitable at the absorber/Mo interface due to the requisite selenization process 105 .MoSe 2 belongs to the transition metal dichalcogenide semiconductor family which possess excellent physical and optoelectronic properties owing to its 2D structure 106 .It displays both n and p-type characteristics.In practice, it has been found that p-MoSe 2 is helpful for improving solar cell performance as it forms ohmic contact at the junction between the absorber and Mo.Conversely  www.nature.com/scientificreports/back contact and suppresses interfacial recombination at the absorber/Mo junction 108 .Thus, to understand the role of p-MoSe 2 properties on the performance of novel Ag 2 BaTiSe 4 solar cells, its thickness and carrier concentration are varied from 0.050 to 0.2 µm and 10 12 to 10 20 cm −3 respectively.

Effect of MoSe 2 's thickness and carrier concentration
Optimizing the thickness of MoSe 2 is critical for improving the performance of solar cells.When the MoSe 2 layer is very thin, it increases recombination near the back contact.On the other hand, if the MoSe 2 layer is too thick, it reduces the thickness of the Mo layer, degrading the electrical contact of the absorber to Mo and increasing the R S in the solar cells 105,109 .Thus, in this study, the MoSe 2 thickness was varied from 0.050 to 0.2 µm in all solar cells to determine the optimal value (Fig. S5).However, it was found that the variations in the V OC , J SC , FF, and PCE with respect to MoSe 2 's thickness were insignificant.According to literature reports, the thickness of MoSe 2 in I 2 -II-IV-VI 4 solar cells is typically in the range of a few 0.1 µm to ~ 1 µm at the interface of absorber/ Mo 96,105,109,110 .However, high PCEs were reported for thicknesses in the range of ~ 0.1 to 0.2 µm, indicating that the R S and back contact recombination could be minimized in this range 7,52 .Therefore, considering the material cost and overall performance, 0.1 µm is considered the optimal value for further simulations.Thereafter, the influence of MoSe 2 's carrier concentration on the characteristics of the solar cells is analyzed by differing it from 10 12 to 10 20 cm −3 .Figure 14 demonstrates the changes in solar cell parameters as a function of MoSe 2 's carrier concentration.The corresponding J-V is given in Fig. S6.It can be observed that all solar cells' V OC , J SC , and PCE almost remain constant till 10 16 cm −3 and then improve to higher values.Conversely, FF drops after 10 17 cm −3 in all solar cells.This observance can be elucidated by the changes in the band alignment corresponding to MoSe 2 's carrier concentration (Fig. 15a-e).No change in energy bands is noticed for the concentrations 10 12 -10 16 cm −3 , referring to the constant solar cell performance till the mentioned range.This also reveals that MoSe 2 's carrier concentration must be greater than 10 16 cm −3 to contribute to the solar cell performance significantly.Interestingly, both E C and E V of MoSe 2 shift upwards for concentrations > 10 16 cm −3 , in consequence of which the barrier for holes is gradually diminished while the electron's barrier is boosted at the Ag 2 BaTiSe 4 /MoSe 2 interface.This facilitates the transportation of holes from Ag 2 BaTiSe 4 to MoSe 2 while restricting the flow of electrons.As a result, the probability of interface recombination declines, which is also witnessed in Fig. 15f-j, where the recombination rate of charge carriers at the Ag 2 BaTiSe 4 /MoSe 2 interface as a function of MoSe 2 's carrier concentration is depicted.The figure shows that as the carrier concentration increases, the recombination rate at the Ag 2 4 / MoSe 2 interface drastically falls in all solar cells due to the attainment of appropriate barriers for electrons and holes.This has led to an improvement in V OC .In addition, the strong built-in electric field will also be generated at the Ag 2 BaTiSe 4 /MoSe 2 interface with increasing MoSe 2 's carrier concentration, which promotes the collection of holes at the back contact without recombination, causing an enhancement in J SC 111,112 .However, the R S may  increase at higher concentrations, resulting in FF decrement 113

interface
Interface defects are inevitable in solar cells, which are formed during fabrication due to structural imperfections.These defects boost the interface recombination of charge carriers that are detrimental to solar cell performance 67 .Thus, it is crucial to analyze their influence on these solar cells and obtain the optimum value to fabricate them practically.So far, all the simulations were performed with the neutral interface defect density of 10 12 cm −3 at Ag 2 BaTiSe 4 /buffer and Ag 2 BaTiSe 4 /MoSe 2 interface where the defect levels are fixed at 0.6 eV above E V in all solar cells.Herein, the impact of interface defect density on the photovoltaic parameters is estimated by varying it from 10 12 to 10 20 cm −3 at both interfaces.The respective variations in J-V characteristics are depicted in Fig. S7. Figure 16a-d demonstrates the outcomes of all solar cells regarding the Ag 2 BaTiSe 4 /buffer interface defects.V OC and FF are approximately the same till 10 15 cm −3 , 10 17 cm −3 , 10 16 cm −3 , and 10 14 cm −3 for MgS, SrS, BaS, and CdS-based solar cells, respectively, and decline afterward.In the case of CaS-based solar cells, they drastically reduce beyond 10 12 cm −3 .In contrast, J SC remains stable till 10 18 cm −3 in all solar cells and then decreases.However, the reduction in J SC is less significant compared to V OC and FF.Moreover, the PCE of all solar cells follows a similar trend as V OC and FF, indicating that the overall performance of all solar cells is determined mainly by the changes in V OC and FF while less influenced by J SC at Ag 2 BaTiSe 4 /buffer interface.The level of degradation in PCE is observed to be 8.48%, 18.03%, 4.55%, 6.22%, and 13.86% in MgS, CaS, SrS, BaS, and CdS-based solar cells, respectively, for the defect density range 10 12 -10 20 cm −3 .This clearly reveals that CaS and CdS-based solar cells are more sensitive to the Ag 2 BaTiSe 4 /buffer interface defects than the other solar cells.In contrast, SrS-based solar cell is comparatively stable with the defects.The overall degradation in solar cell performance with the interface defects occurs due to the increasing trap-assisted recombination of photogenerated electrons at the buffer/Ag 2 BaTiSe 4 interface, which restricts their flow towards the front contact 114 .Thus, the optimum defect density of 10 15 cm −3 , 10 12 cm −3 , 10 17 cm −3 , 10 16 cm −3 , and 10 14 cm −3 are selected for MgS, CaS, SrS, BaS, and CdS-based solar cells, respectively at Ag 2 BaTiSe 4 /buffer interface to attain maximum solar cell performance.These values are higher than the defect densities reported in the experiments at the CZTS/buffer interface, revealing the superiority of novel Ag 2 BaTiSe 4 solar cells over their predecessors 115,116 .Similarly, the defect density at the Ag 2 BaTiSe 4 /MoSe 2 interface is shifted from 10 12 to 10 20 cm −3 (Fig. 16e-h).Here, all the solar cell parameters start to reduce for defect density above 10 15 cm -3 and begin to saturate at 10 18 cm −3 .It can be noticed that J SC is adversely affected than V OC and FF, i.e., it decreases from ~ 39 to ~ 34 mA cm −2 when defect density is raised from 10 12 to 10 18 cm −3 .This has led to a decline in PCE by ~ 6.5% in all solar cells.The drop in the solar cell performance may happen because holes travelling from Ag 2 BaTiSe 4 to MoSe 2 have a high chance of being trapped or recombined when the defects at the Ag 2 BaTiSe 4 /MoSe 2 interface increases 114 .The results show that defect density < 10 15 cm −3 is required at the Ag 2 BaTiSe 4 /MoSe 2 interface for exceptional solar cell performance.Overall, the investigation of interface defects suggests that it has a massive impact on solar cell performance.These defects are generally produced due to the structural defects between different layers and metal cation diffusion through the absorber during fabrication 67 .Thus, effective techniques must be employed for the deposition of layers, and methods such as etching, post-heat treatment, and inserting a passivation layer can be used to minimize the interface defects in solar cells [117][118][119] .Table 5 displays the final solar cell performance of the novel Ag 2 BaTiSe 4 solar cells with diverse buffers after optimization, and the corresponding J-V is displayed in Fig. 17.As discussed in the introduction, the main problem that limits the PCE of I 2 -II-IV-VI 4 solar cells is their large V OC deficit.In addition, CdS is used as a buffer in most solar cells containing the toxic element Cd, creating severe problems when dumped into the environment.Therefore, developing efficient absorbers and eco-friendly buffers is always highly interesting to the photovoltaic community.In this regard, for the first time, we have reported Ag 2 BaTiSe 4 of group I 2 -II-IV-VI 4 as a potential absorber and a new class of alkaline earth metal chalcogenides, namely MgS, CaS, SrS, and BaS as alternative buffers using SCAPS-1D.Herein, we have accomplished high PCEs of 28.00%, 30.02%, 29.87%, 30.23%, and 29.68% for MgS, CaS, SrS, BaS, and CdS-based Ag 2 BaTiSe 4 solar cells, respectively.Surprisingly, the PCEs achieved in Ag 2 BaTiSe 4 solar cells with alkaline earth metal chalcogenides buffers are comparable with CdS, proving their potential and suitability to be applied as alternative, non-toxic buffers in thin-film solar cells.Moreover, the V OC loss in these solar cells is less, specifically 0.3 V, 0.29 V, 0.34 V, 0.28 V, and 0.29 V for MgS, CaS, SrS, BaS, and CdS-based Ag 2 BaTiSe 4 solar cells respectively, displaying the superior properties of Ag 2 BaTiSe 4 as an alternative absorber.The low V OC deficit in alkaline earth metal chalcogenides buffer-based Ag 2 BaTiSe 4 solar cells is highly possible in experiments due to the proper band alignment at absorber/buffer interface and low antisite defects in Ag 2 BaTiSe 4 absorber because of the large atomic size difference between the constituent elements.Thus, this work would kindle the photovoltaic community's interest in fabricating novel efficient Ag 2 BaTiSe 4 solar cells with new alkaline earth metal chalcogenides buffers and achieve high PCE.In addition, based on our research outcomes, we propose that these new alkaline earth metal chalcogenides buffers have immense potential to be utilized in other conventional solar cells such as CdTe, CIGSSe, perovskites, etc.
Overall, after the optimization of buffer, Ag 2 BaTiSe 4 , MoSe 2 , and interface properties, the PCE incredibly improved from 18.72%, 11.65%, 15.93%, 15.47% and 14.99% to 28.00%, 30.02%, 29.87%, 30.23% and 29.68% for MgS, CaS, SrS, BaS and CdS-based Ag 2 BaTiSe 4 solar cells respectively.Further, the final solar cells where the high PCEs have been demonstrated are selected for the upcoming studies to investigate the impact of R S , R Sh , and working temperature on their performance.Effect of series shunt resistances R S and R Sh display significant impact on the performance of solar cells.R S is the sum of resistance at the front and back contacts and between various layers of solar cells.On the other R Sh originates from the reverse saturation current in solar cells that is produced by the manufacturing defects 120 .Here, the influence of R S and R Sh on the solar cell parameters is investigated using SCAPS-1D.Figure 18a-d displays V OC , J SC , FF, and PCE as a function of R S , where it is varied from 0.5 to 6 Ω cm 2 for all solar cells.The respective variations in J-V characteristics are depicted in Fig. S8.It can be observed that V OC and J SC remain unaffected throughout the R S range.Whereas, FF drastically decreases from 82.12 to 59.27%, 83.34 to 61.27%, 82.72 to 61.35%, 83.05 to 61.49%, and 82.99 to 60.96% in MgS, CaS, SrS, BaS and CdS-based solar cells respectively.The massive reduction in FF is attributed to the colossal power loss in the solar cells with increasing R S , which adversely affects their performance 121 .Thus, when R S is improved from 0.5 to 6 Ω cm 2 , PCE dramatically declined by ~ 7.5% in all the solar cells.Similarly, R Sh is tuned from 1000 to 100,000 Ω cm 2 in all solar cells, as shown in Fig. 18e-h.The respective changes in J-V are depicted in Fig. S8.In this case, V OC and J SC are almost the same for all R Sh values.On the contrary, the FF and PCE values enhanced with an increment in the R Sh till 2000 Ω cm 2 and saturate on further increase in R Sh .Nevertheless, the rise in PCE is about 0.53%, 0.61%, 0.62%, 0.7%, and 0.6% in MgS, CaS, SrS, BaS, and CdS-based solar cells, respectively.This exhibits that the effect of R Sh on the performance of these solar cells is negligible compared to R S .Therefore, a low R S of 0.5 Ω cm 2 is highly recommended for the efficient functioning of these novel Ag 2 BaTiSe 4 solar cells.

Effect of working temperature
Long-term stability in the environment is an essential requirement for the application of solar cells 122 .Thus, the deterioration process of solar cells under ambient air conditions must be investigated.In the present study, the working temperature is varied from 300 to 400 K for analyzing its influence on all solar cells.Figure 18i-l demonstrates the response of solar cells concerning working temperature.The corresponding variations in J-V graphs are depicted in Fig. S8.It can be seen that V OC , FF, and PCE diminish with increasing temperature while J SC slightly increases in all solar cells.The temperature rise reduces the bandgap of the absorber, which enhances the charge carrier generation in solar cells, leading to enhancement in J SC 122 .However, the thermally generated electrons begin to vibrate at high temperatures, become unstable, and recombine with the holes before being collected at the contacts, reducing V OC .Moreover, the increasing temperature affects the transport efficiency of charge carriers, such as carrier concentration and mobility of charge carriers, thereby decreasing the FF of all solar cells 123 .Overall, the combined decrement in V OC and FF has led to a lowering of PCE from 28.0% to 21.64%, 30.02 to 24.46%, 29.87 to 24.23%, 30.23 to 24.39%, 29.68 to 24.17% for MgS, CaS, SrS, BaS, and CdSbased solar cells respectively.

Conclusion
In the present work, a comprehensive numerical study on the performance of novel Ag 2 BaTiSe 4 solar cells with new alkaline earth metal chalcogenides as alternative buffers to CdS was performed using SCAPS-1D.Upon optimizing buffer properties, maximum PCE of 18.84%, 17.17%, 20.65%, 20.87%, and 18.66% for MgS, CaS, SrS, BaS, and CdS-based respective solar cells were obtained, which mainly originated from the variation in N D buffer .The energy bands shifted downwards for N D buffer > N A absorber , diminishing the barrier height at the absorber/buffer interface, eventually suppressing the recombination rate and enhancing the V b and W of solar cells.Thereafter, at the optimized absorber's electron affinity (4.4 eV) and thickness (1 µm), PCE improved by 5.33% for CaSbased solar cells and ~ 2% for other solar cells due to the increment in R rec and light absorption of solar cells, as evidenced by Nyquist plots and QE measurements.Notably, the absorber's carrier concentration of 10 18 cm −3 drastically escalated the performance of all solar cells, while its defect density > 10 15 cm −3 declined the PCE owing to the dramatic decrease in the τ and L of charge carriers.Further, on tuning MoSe 2 parameters and interfacial properties, the best PCE of 28%, 30.02%, 29.87%, 30.23%, and 29.68% was achieved with minimal V OC loss

Figure 2 .
Figure 2. Effect of buffer's thickness on (a) V OC (b) J SC (c) FF (d) PCE of novel Ag 2 BaTiSe 4 based solar cells with diverse buffers.

Figure 3 .
Figure 3.Effect of buffer's carrier concentration on (a) V OC (b) J SC (c) FF (d) PCE of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.

Figure 4 .
Figure 4. (a-e) Variation in energy band diagram of novel Ag 2 BaTiSe 4 solar cells with diverse buffers as a function of N D buffer .(f-j) C-V and (k-o) Mott-Schottky plots with corresponding V b at 10 12 cm −3 and optimized N D buffer .

Figure 6 .
Figure 6.Effect of absorber's electron affinity on (a) V OC (b) J SC (c) FF (d) PCE of novel Ag 2 BaTiSe 4 solar cells with diverse buffers, (e-i) Nyquist plots as a function of absorber's electron affinity.

Figure 7 .
Figure 7. Effect of absorber's thickness on (a) V OC (b) J SC (c) FF (d) PCE (e-i) QE of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.

Figure 8 .
Figure 8.Effect of absorber's carrier concentration on (a) V OC (b) J SC (c) FF (d) PCE of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.

Figure 9 .
Figure 9. (a-e) Mott-Schottky plots with respective V b , (f-j) Energy band diagram and (k-o) Changes in QE (magnified image) at absorber's carrier concentration of 10 12 and 10 18 cm −3 in novel Ag 2 BaTiSe 4 solar cells with diverse buffers.The simplified illustration of QE is provided in Fig. S4.

Figure 10 .
Figure 10.Effect of absorber's defect density on (a) V OC (b) J SC (c) FF (d) PCE and (e) diffusion length and lifetime of charge carriers of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.

Figure 11 .
Figure 11.(a-e) Recombination rate as a function of defect density of absorber.(f-j) Change in the solar cells' density of electrons 'n' with the increasing defect density.The inset displays the variation in 'n' along the absorber region.(k-o) Electric field corresponding to defect density of absorber.

Figure 12 .
Figure 12.Effect of absorber's defect energy level and density on (a-e) V OC and (f-j) J SC of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.
, n-MoSe 2 creates a Schottky barrier that enhances back contact recombination 107 .Y. Song et al. have improved the p-type conductivity of MoSe 2 by doping Nb and have shown that p-MoSe 2 reduces the barrier for hole collection at the

Figure 13 .
Figure 13.Effect of absorber's defect energy level and density on (a-e) FF and (f-j) PCE of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.

Figure 14 .
Figure 14.Effect of MoSe 2 's carrier concentration on (a) V OC (b) J SC (c) FF (d) PCE of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.

Figure 15 .
Figure 15.Effect of MoSe 2 's carrier concentration (a-e) Energy band alignment and (f-j) Recombination rate at Ag 2 BaTiSe 4 /MoSe 2 interface in novel Ag 2 BaTiSe 4 solar cells with diverse buffers.

Figure 17 .
Figure 17.J-V graph of novel Ag 2 BaTiSe 4 solar cells with diverse buffers after optimization.

Figure 18 .
Figure 18.Effect of series resistance, shunt resistance, and working temperature on the performance of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.

Table 2 .
Simulation parameters at the interfaces.

Table 3 .
Initial solar cell parameters of novel Ag 2 BaTiSe 4 solar cells.

Table 4 .
displays the variations in Electron affinity of absorber with the corresponding CBO and VBO at the interfaces of novel Ag 2 BaTiSe 4 solar cells with diverse buffers.Optimum values are in [bold].

of defects at Ag 2 BaTiSe 4 /buffer and MoSe 2 /Ag 2 BaTiSe 4
. Nevertheless, the elevation in V OC and J SC has boosted the PCE from 26.35 to 28.07%, 28.18 to 30.04%, 27.93 to 29.96%, 28.25 to 30.31%, and 28.26 to 30.31% in MgS, CaS, SrS, BaS and CdS-based solar cells respectively.Thus, MoSe 2 's carrier concentration of 10 20 cm −3 is the optimum value for achieving high PCE in all solar cells.

Table 5 .
Solar cell parameters of novel Ag 2 BaTiSe 4 solar cells with diverse buffers after optimization.