Study of Thickness Optimization, Temperature and Work Function for Cs2AgBi0.75Sb0.25Br6 Based Perovskite Solar Cell

INTRODUCTION
Improvements in technology, increased commercial manufacturing, and the identification of affordable materials have all contributed to solar photovoltaic (PV) technology’s remarkable progress in the last several decades [1-3]. Improving solar cells’ efficiency is key for commercializing this technology. This discrepancy between photon and bandgap energies causes energy loss in single-junction PV devices, which causes certain limitations. As is widely known, the bandgap energy must be equivalent to the photon energy for its efficient extraction as electric power. For photons with energies below the bandgap, absorption occurs automatically; for those with higher energies, carrier thermalization causes to lose energy, rendering them ineffective in the conduction process [4]. Although perovskite solar cells produce more light when their density is increased, the photocurrent efficiency (PCE) stays the same because of restrictions on electron length [5-7]. Perovskites made of tin halides, such as CsSnI3, MASnI3, and FASnI3, have been studied. There is an inherent lack of stability in the latter two perovskites [8, 9]. All-inorganic lead-free CsSnI3 perovskite is presently the most promising contender to Pb-based light harvesting materials. Recent research have demonstrated that using cesium (Cs) instead of organic cations in perovskite structures can greatly improve thermal stability and performance in outdoor and ambient devices [10, 11]. With a PCE of 10.1%, CsSnI3 was the most efficient lead-free all-inorganic PSC [12, 13]. As a potential alternative to lead halide perovskites, halide double perovskites with the formula A2B’B”X6 (where A = Cs, MA; B’ = Bi, Sb; B” = Cu, Ag; and X = Cl, Br, I) have been studied [14]. The stability issues with perovskite solar cells have been successfully addressed, and this new family of materials has proven to be quite stable when subjected to various weather conditions. Although Cs2AgBiBr6 deteriorated over many weeks when exposed to both ambient air and light, McClure et al. [15] discovered that Cs2AgBiCl6 and Cs2AgBiBr6 perovskites were stable in air. There are currently no Pb-free solar cells on the market that can compete with the efficiency of Pb-containing perovskites [16]. Perovskite absorber material (Cs2AgBi0.75Sb0.25Br6) is suggested by Min Chen et al. [17], in terms of performance and air stability. A PCE of 7% is demonstrated by the device based on Cs2AgBi0.75Sb0.25Br6. This model was obtained using fundamental literature data provided by the corresponding simulation studies [18, 19]. The present investigation relies on a perovskite Cs2AgBi0.75Sb0.25Br6. This material was selected for its 1.8 eV bandgap since it has several benefits over halogenated perovskites. It is linked to a ZnO-based electron transport layer and a Cu2O-based HTL. Since Cu2O is inexpensive, abundant in solar energy materials, and shows interest as an inorganic HTL for PSC applications, it was chosen to serve as the HTL [20]. In standalone settings, we predicted the implications on the photovoltaic characteristics of diverse thickness of the absorbent, electron transport, and hole transit layers. We identified the optimal cell functional parameters based on the results. We also investigated the impact of the working temperature and work function on the parameters and efficiency of the solar cell [21].

MATERIALS AND METHODS
Simulating devices and characterization
One of the most important tools in this field is simulation, which may help us understand proposed physical explanation, and the effect of initial parameters on the performance of cell systems (Fig. 1 and Table 1) [2, 22-27]. Current research provides use of SCAPS for modelling perovskite solar cells based on Cu2O/Cs2AgBi0.75Sb0.25Br6/ZnO (Fig. 2 and Table 2).
Fig. 3a shows the SEM image of ZnO nanoplate synthesized via the hydrothermal method, which is used as ETM in perovskits solar cells. This morphology indicates a high surface area-to-volume ratio, which is beneficial for electron transfer and interface contact. The ZnO plates appear well-aligned and densely packed, which can improve charge mobility and reduce recombination losses in devices. The SEM image in Fig. 3b reveals a dense and uniform distribution of Cu2O nanoparticles with roughly cubic or slightly rounded morphology. The particles appear to be nanometer-sized, in the range of 20–100 nm. Cu2O nanoparticles are used as a hole transport layer in perovskite solar cells. This topography promotes efficient hole extraction and transport, while suppress electron recombination and ensure good interfacial contact with the perovskite absorber layer.

RESULTS AND DISCUSSION
Impact of Cu2O layer thickness on solar cells
To maximize photon absorption and electron-hole pair generation, the absorber layer’s thickness must be fine-tuned. In the past, active layer dimension has varied between 0.1 and 0.7 μm. Light with a longer wavelength induces a good rate of generation of electron-hole pairs in absorber layer. Increasing the thickness of the absorber layer brings the depletion layer closer to the back contact, allowing the back contact to catch more electrons for recombination [25]. The relationship between absorber layer thickness and PV parameter variation is shown in Fig. 4. Filling factor, current density, voltage density, and efficiency all raise with increasing thickness. At a thickness of 0.55 µm, the efficiency hits 9.02, allowing it to achieve peak performance. The statistics from the drawings are displayed in Table 3.

Impact of Cs2AgBi0.75Sb0.25Br6 layer thickness change on solar cells
This research looks at a PV design that uses a perovskite absorber material that is lead free and analyzes it using computer modeling tools. The energy bandgap of this novel structure is 1.80 eV, and it is made of a cesium-based double perovskite material [20-23]. The desirable characteristics displayed by the Cs2AgBi0.75Sb0.25Br6 are well-aligned bandgaps and increased stability under room temperature. A solar cell’s efficiency is highly dependent on the thickness of its absorber layer. In terms of absorber layer thickness, Cs2AgBi0.75Sb0.25Br6 has varied between 0.1 μm and 1 μm. Fig. 5 shows how the photovoltaic properties of perovskite change with thickness. At 0.5 μm and 1 μm, the efficiency is 8.99 %. At 0.4 μm, efficiency rises to 9.02%, and at 0.3 μm, it remains constant almost. The optimal thickness for a solar cell is 0.1 μm, where the efficiency is 16.2%. Reduction in absorber layer thickness result in an enhancement in the number of electrons caught for recombination, making the depletion layer compatible with the back contact (Table 4). Observing the graphs of F.F/thickness, Voc/thickness, and Jsc/thickness, it is possible to observe that the FF value increases at a thickness of 0.1 μm, while Voc remains constant. Lastly, the value of Jsc grows as the thickness decreases.

Impact of ZnO layer thickness change on solar cells
Fig. 6 exhibits a findings of the research in which we projected the effect of increasing thickness of ZnO from 0.01 μm to 0.7 μm. A reduction in productivity can be observed as the dimension of the ZnO layer is increased; specifically, the efficiency value reaches its peak at 0.08 μm, which is 16.64%. This is due to the fact that a larger amount of radiation is absorbed, leading to the generation of a large number of excitons (Table 5). The value of the energy gap also plays a role in this equation [15, 17]. For example, when the material’s energy gap is minimal and its wavelength is close to that of red light, the number of excitons increases and the efficiency rises. While Voc drops from 1.3668 V to 1.3657 V as the thickness increases from 0.01 μm to 0.7 μm. The Jsc drops from 24.916751 to 23.966513 mA/cm2 as the thickness increases from 0.01 μm to 0.09 μm. The fill factor (FF) gets from 41.04% to 50.24% as the thickness increases from 0.01 μm to 0.7 μm.

Effect of annealing Temperatures for Cs2AgBi0.75Sb0.25Br6  
We find that a temperature of 300 K, Jsc = 23.794833 (mA/cm2), FF = 50.66%, and Voc = 1.4250 as model’s optimal operating conditions yield an efficiency value of 17.18%. An important factor influencing output is the ambient temperature. As demonstrated in Fig. 7, the PCE, FF, Voc, and Jsc all climb from 100 K to 250 K as a result of an enhancement on formation of hole-electron in the perovskite materials. However, when the temperature increases to 300 K, Jsc (mA/cm2) and FF decrease. The appropriateness temperature for using perovskites solar cells with Cs2AgBi0.75Sb0.25Br6 as a PVSC is 300 k, as we can see in Table 6. The device performance can be changed with temperature changes due to the ability for controlling the recombination, creating charge carriers, and so on.

Impact of various back contact materials
Two major obstacles to the widespread usage of perovskite solar cells are their expensive price and the fact that their back contact is thermally unstable. Based on Table 7, we used a many value for (Back) work function from 4.26 to 5.6 eV to get a back connection with higher specs. Fig. 8 shows that as the work function material increases, the values of Voc (1.2185 V to 1.4250 V), Jsc (19.546135 mA/cm2 to 23.794833 mA/cm2), Filling factor (47.64% for 50.66%), and η (11.35% to 17.18%) also rise. The parameters increaces as the work function (back contact) rises, which is related to the reduced Schottky barrier at the Cu2O-contact interface.

CONCLUSION
This study comprehensively examines a revolutionary lead-free perovskite solar cell based on Cs2AgBi0.75Sb0.25Br6. A standard configuration of FTO/Cu2O/ Cs2AgBi0.75Sb0.25Br6/ZnO/Pt was computed and examined utilizing SCAPS‐1D simulation software. The influence of absorber layer on device performance was analyzed to achieve optimal efficiency. Result has shown that a perovskite solar cell utilizing Cs2AgBi0.75Sb0.25Br6 exhibits superior performance due to its optimal band alignment with the electron transport layer and hole transport layer. Additionally, the photovoltaic efficiency of the cell has been enhanced by adjusting three key parameters: temperature, work function, and absorber layer thickness. Our investigation elucidated the substantial impact of these three factors on the electrical characteristics of the PSC. The findings indicated that the ideal thickness of the light absorber was 1.5 μm and the optimal temperature was 300 K. Minimizing the absorber layer thickness markedly improves the PCE, achieving a result of about 16.23%. Our innovative findings may offer a feasible approach to produce economical, highly efficient, and stable Cs2AgBi0.75Sb0.25Br6-based perovskites.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.