Sb2Se3 as an HTL for Mo/Sb2Se3/Cs2TiF6/TiO2 solar structure: performance evaluation with SCAPS-1D.

Perovskite-based solar cells (PSCs) have recently gained much attention due to their distinctive optical and electrical properties. Cesium titanium fluoride (Cs2TiF6) is an example of lead-free perovskite absorber material with a bandgap of 1.9 eV, making it suitable for a solar device. However, the high cost of the hole transport material (HTM) and other considerations prevent their commercial production. Antimony selenide (Sb2Se3) is well suited for HTM as it is low-cost material with a tunable bandgap. The work presents the TiO2/Cs2TiF6/Sb2Se3-based solar cell performance using SCAPS-1D simulation software. The effect of all the active layer thicknesses, defect density, hole-electron mobility, and temperature on the device is also simulated. I-V, C-V, and QE curves and energy band diagrams show the photovoltaic device's excellent performance. The outputs are competent enough with an efficiency of 22.10 % when Sb2Se3 is used as a hole transport layer (HTL) in the device architecture. The results suggest that the lead-free solar cell is a promising future option for the solar cell community regarding environmental friendliness and high efficiency.

Introduction

Perovskite solar cells have received much attention in recent years because of their simple structure [1], low production cost [2], tunable bandgap [3], and flexibility. Current research on PSCs has delivered excellent performance; however, these perovskite materials used to absorb the sunlight contain the toxic element (lead) that is not good for the environment. So, there is an urgent need to produce lead-free perovskite solar cells for large-scale production. The Cesium Titanium (IV) Halide materials are excellent for solar applications because they have a tuneable band gap in the range of 1.4–1.9 eV. Additionally, it is known that these compounds' electron and hole diffusion lengths are balanced. The current work is an optimization of the Cs2TiF6 perovskite material's device parameters. Cs2TiF6 absorber material [4] has shown potential light-harvesting properties, suitable bandgap, high absorption coefficient, and better stability. Cs2TiF6 is used as an absorber in this study as it displays higher heat, moisture, and light effects stability than methylammonium lead iodide (MAPbI3). In 2018, Ju et al. produced cesium titanium mixed halide-based solar cells with a 1.0–1.8 eV bandgap [5]. Kunal et al. proposed a CuSCN/Cs2TiF6/CdS device structure. They achieved a maximum efficiency of 21 % [4]. Zhang et al. have reported a bilateral interface modification to perovskites by doping room-temperature synthesized CsPbBr3 nanocrystals (CN) and achieved an average efficiency exceeding 20% [6]. Chandan et al. reported 28.82 % and 29.48 % efficiency for n-type doped perovskite with a p-type based crystalline silicon solar structure and p-type doped perovskite with n-type based crystalline silicon solar structure [7]. The key challenges with perovskite photovoltaics are long-term stability and upscaling. Perovskite researchers have looked into various techniques, including deposition processes, perovskite composition, stability, and the development a hole-transport layer (HTL). Riyad et al. reported on performance enhancement in Ni/Sb2S3/CZTS/WS2/FTO/Al-based solar cell structure, where Sb2S3 is used as a hole transport layer (HTL) and achieved an efficiency of 30.63 % [8]. Ahmad et al. reported the performance evaluation of Au/p-CdTe/Cs2TiI6/n-TiO2/ITO-based solar cell, where CdTe is used as an HTL and TiO2 is used as an ETL and achieved an efficiency of 15.06 % [9]. Atul et al. reported the computational analysis of chalcogenides as an inorganic hole transport layer in perovskite solar cells [10].

TiO2 film deposition generally needs a high-temperature procedure. Still, recently, the method of colloid-spray coating has been reported to enable 100 °C deposition of the TiO2 layer. TiO2 is highly stable, non-toxic, and considered a very safe material. Sb2Se3 is also a thermally stable material. The stability of Sb2Se3 material can be enhanced by selenization. In this work, Sb2Se3 is used as an HTL layer as this material has an optimal bandgap of 1.2 eV [11], high absorption coefficient [12], highly chemically stable, and excellent optoelectronic properties. Sb2Se3 can be easily deposited using atomic layer deposition (ALD), spray pyrolysis, thermal vapor deposition, and sputtering. An HTL layer plays a vital role in the better performance of the device as it extracts and transfers holes to the electrode and serves as an energy barrier to restrict the electron's transfer to the anode. It is also expected that the Sb2Se3 HTL and perovskite absorber will have an effective band alignment, which will be beneficial for carrier (hole) transportation and prevent minority carriers (electrons) from the absorber. This work presents simulation results for a solar cell configuration consisting of Cs2TiF6 (absorber), Sb2Se3 (HTL), and TiO2 [13] [14] (ETL) active layers. The ETL and HTL, respectively, will absorb high-energy and low-energy photons. The aim is to achieve a highly efficient perovskite solar cell. It is done with the help of the Sb2Se3-based HTL layer. The HTL and absorber should have almost similar valence band maximum energy. The valence band offset (VBO) at the perovskite/Sb2Se3 interface is suitable (VBO = -1.12 eV) in the current PV device, confirming smooth transportation of photo-generated holes from the perovskite absorber to the back contact metal through the Sb2Se3 HTL.

Additionally, the adequate potential barrier created at the perovskite/Sb2Se3 interface with a proper conduction band offset (CBO = +0.31 eV) can impede the photo-generated minority electrons. The impact of the device's absorber layer, HTL and ETL thicknesses, total defect density, and temperature have been reported. Open circuit voltage (Voc) produced in a solar cell at zero current is the maximum voltage, and it is influenced by temperature, and fill factor (FF) shows the maximum power generated by a solar after evaluating the square of the I–V curve, as shown in Eq. (1) and Eq. (2) respectively

Here, IL is the light-generated current, and I0 is the dark saturation current. A brief examination of Eq. (1) could lead one to believe that VOC increases linearly with temperature. It is not the case, though, as I0 rapidly rises with temperature due to variations in the intrinsic carrier concentration ni. The impact of temperature varies with cell technology and is complex. I0 depends on recombination in the solar cell. The device's open-circuit voltage then serves as a measure for the amount of recombination there.

PMP and JSC are the maximum power and short-circuit current, respectively.

Solar device modeling and simulation are the best things to analyze their physical mechanism without the fabrication of the device. Here, the simulation is done by the SCAPS-1D software [15, 16, 17, 18]. The software helps optimize the device's material properties. With the help of various input parameters of different layers, the performance of the solar device has been studied. The software works on two basic semiconductor equations; Poisson and Continuity equations of electrons and holes [19], as represented by Eqs. (3), (4), and (5), respectively. The Poisson and the continuity equations are considered the basic semiconductor equations. They are utilized to find solutions for the electrical performance of the electronic systems upon applying stimuli to them. These stimuli can be in the form of voltages, photons, or thermal energy. It means that we can compute the electrical performance of the devices and develop I–V relations for them using these equations. They, therefore, serve as the theoretical foundation for evaluating and deciphering the measured performance of such devices. They also allow the performance of new device structures to be predicted without the need for very costly experimental work, resulting in significant effort and cost savings. By making the mathematical analysis easier, they can be used to determine the ideal device structure parameters. We can obtain the potential, electron, and hole distribution by solving these equations in a predefined macroscopic device region with the appropriate boundary conditions and obtain the electron current and the hole current at any appropriate cross-section.

and are donor and acceptor concentrations. ψ is the electrostatic potential, and are the trapped holes and electrons, GL and R are the generations and recombination rates, and ε is the dielectric constant. SCAPS performs a numerical evaluation of the system's steady-state and small-signal behavior. The differential Eqs. (3), (4), and (5) are discretized into the set of algebraic equations by SCAPS, which separates a photovoltaic cell into slabs and main grid points. Parameters apart from I–V, such as C–V characteristics, and energy band can be derived by evaluating the above equations.

Solar cell structure and simulation parameters

Solar cell structure

The solar device consists of a structure of Mo/Sb2Se3/Cs2TiF6/TiO2 where the perovskite absorbing material, Cs2TiF6 is sandwiched between the two transport layers, ETL (TiO2) and HTL (Sb2Se3), respectively, with a transparent electrode at the front side (ITO) and a metal electrode at the backside (Mo). These electrodes are responsible for collecting the segregated electron-hole pairs. The front contact collects the electrons. The back contact collects the holes. Back contact is corrosion-resistant, provides an ohmic connection, and has a lower recombination rate for the minority carriers. We used a continuous light of 1000 W/m2 at AM_1.5G and 300 K temperature for illumination. The proposed device structure and its energy band diagram are shown in Figure 1(a-b).

Figure 1

(a) Schematic and (b) energy band diagram of TiO2/Cs2TiF6/Sb2Se3 based solar cell.

Simulation parameters

The parameters used to simulate the solar cell are listed in Table 1. It consists of the parameters for Cs2TiF6, TiO2, and Sb2Se3 to be varied, and these parameters are obtained from the literature [4, 20, 21]. The thickness of the absorber layer, ETL, and HTL are varied to check the device's performance. The thicknesses, defect density, hole-electron mobility, and device temperature were varied. The back contact (Mo) work function is taken as 5.0 eV. Defect density values inside the absorber layer, HTL, and ETL are listed in Table 2.

Table 1

Different electrical properties of proposed solar cell active layers.

Parameters	TiO2	Sb2Se3	Cs2TiF6
Thickness (μm)	Varied	Varied	Varied
Band Gap (eV)	3.20	1.2	1.90
Electron Affinity (eV)	4.0	4.04	3.70
Dielectric Permittivity	10	18	18
Conduction-band density of states (cm−3)	1.0 × 1021	2.2 × 1018	1.0 × 1019
Valence-band density of states (cm−3)	2.0 × 1020	1.8 × 1019	1.0 × 1019
Electron Mobility (cm2/Vs)	20	15	4.4
Hole Mobility (cm2/Vs)	10	5.0	2.5
Shallow uniform donor density ND (cm−3)	1 × 1019	0	1.9 × 1019
Shallow uniform acceptor density NA (cm−3)	0	1 × 1018	1 × 1019

Table 2

Defect density parameters and values for absorber, HTL, and ETL.

Parameters	Cs2TiF6	Sb2Se3	TiO2
Defect's Type	Neutral	Neutral	Neutral
Capture cross-section for electrons and holes (cm−2)	1 × 10−15	1 × 10−15	1 × 10−15
Energetic Distribution	Single	Single	Single
Energy level with respect to Ev	0.6	0.6	0.6
Characteristic Energy (eV)	0	0	0
Trap Density, Nt (cm−3)	1 × 1013 (Varied)	1 × 1013 (Varied)	1 × 1014

Results and discussion

The paper is aimed at how a Sb2Se3-based HTL can affect the efficiency of a Cs2TiF6 perovskite solar cell when the absorber layer, HTL, and ETL thickness, total defect density, and hole-electron mobility is varied. The temperature of the device also affects the device's performance. After the optimization of all these parameters, I–V, C–V, QE, and energy band structure represented the overall performance of the solar cell.

Effect of the absorber, HTL, and ETL thickness

The absorber layer's thickness must be assessed to evaluate solar cell efficiency since it is one of the essential characteristics of photovoltaic cell optimization. The thickness must be carefully taken to enhance the current density. Solar cells with a thinner absorber layer will reduce the current density and efficiency due to inadequate light absorption. A thicker absorber layer is ineffective as it produces a longer path for photogenerated charge carriers to travel, increasing recombination. For perovskite solar cells, the device's spectral response strongly depends on the absorption layer thickness, which significantly impacts the device's efficiency. The absorber layer thickness is varied from 0.25 to 2.5 μm, as shown in Figure (2). With the increasing thickness, Jsc (Figure 2(b)) and efficiency (Figure 2(d)) decrease, and the FF (Figure 2(c)) increases while Voc (2(a)) remains the same. The PCE will theoretically drop while the FF will increase when the thickness of the perovskite layer is increased beyond the diffusion length due to the active layer's sheet resistance decrease. The highest efficiency is achieved at 0.5 μm thickness of the absorber layer.

Figure 2

Impact of Cs2TiF6 layer on the (a) Voc, (b) Jsc, (c) FF and (d) efficiency.

Device stability and performance are both impacted by the HTL's thickness. Thinning the HTL should decrease the distance that holes must travel to reach the CE and lower the probability that they would experience a recombination event; nevertheless, it may also result in a reduction in coverage uniformity, mainly if the perovskite surface is made up of massive crystallites. HTL is introduced to improve the flow of holes in the cell and prevent electrons from entering the hole majority area once the active layer has been optimized. Sb2Se3 is used as an HTL and is low-cost and less-toxic material. We varied the thickness from 0.25 -2.5 um to achieve the maximum efficiency, as shown in Figure (3(d)). Since a thin HTL (less than 250 nm) does not completely cover the perovskite layer, the chance of recombination is higher. The longer path length for charge carriers to reach the back electrode increases series resistance. As a result of better coverage, the fill factor (Figure 3(c)) increases as the HTL thickness grows. The optimized thickness of 2 μm is adequate to achieve high performance. Jsc (Figure 3(b)) and Voc (Figure 3(a)) increase from 21.62 mA/cm2 to 30.26 mA/cm2 and 0.79 V–0.86 V, respectively. The absorber layer thickness was optimized at 0.5 μm. The dependence of solar cell parameters on ETL thickness is also studied in the range of 0.03–0.30 μm, as shown in Figure (4). With the increasing ETL (TiO2) thickness values, Voc (Figure 4(a)) remained the same, there is a decrease in Jsc (Figure 4(b)), η (Figure 4(d)), and a slight increment in FF (Figure 4(c)). It justifies the fundamental concept of decreased ETL thickness. A too-thick electron transport layer can reduce recombination in the cell; high resistance can impede electron flow. At 0.03 μm thickness of ETL, maximum efficiency of 22.10 % is achieved.

Figure 3

Impact of Sb2Se3 layer on the (a) Voc, (b) Jsc, (c) FF and (d) efficiency.

Figure 4

Effect of TiO2 layer on the (a) Voc, (b) Jsc, (c) FF, and (d) efficiency.

Impact of total defect density of absorber and HTL layer

Another critical factor that might greatly impact the device's performance is the active layer's total defect density. The quality of interfaces and the formation of defects such as point defects, stacking faults, dislocations, and grain boundaries affect the performance of thin-film solar cells. Higher pinhole creation and recombination due to defects in the absorber layer also result in faster film deterioration, less stability, and poorer device performance overall. Increased defect density will increase carrier recombination, directly impacting a device's efficiency. Total defect density for Cs2TiF6 (absorber) and Sb2Se3 (HTL) layers is varied from 1012-1020 cm−3 is depicted in Figures 5(a) and 5(b), respectively. By increasing defect density, the efficiency of the device decreases. Maximum efficiency of 22.10 % is achieved at ∼1013 cm−3 for both layers. When the defect density increased, the carrier recombination rate also increased and reduced the carrier's lifetime and diffusion length. Thus, the overall performance of the device decreased [22]. Thus, it can be observed from the simulation results that the total defect density for the absorber and HTL layers should be ∼1013 cm−3 for device simulation.

Figure 5

Impact of total defect density of (a) Cs2TiF6 and (b) Sb2Se3 on the efficiency of the device.

Impact of electron and hole mobilities of the absorber layer and HTL

The mobility of the material is the drift velocity achieved by carriers when an electric field is applied [23]. The mobility of the charge carrier in a semiconductor is among the essential parameters in electronic devices. It defines the capacity of charge carriers to go to and fro in the material as it is exposed to an external electric field. The mobility of an electron is more significant than a hole as the effective mass of an electron is generally smaller than the effective mass of a hole. Figs. (6 (a-b)) and (7 (a-b)) show the relation between the electron and hole mobilities of the absorber layer and HTL versus efficiency. For an absorber layer, the efficiency increases slightly with the increase in hole mobility, but it remains stable with increased electron mobility. The optimized hole and electron mobilities were found at 2.5 cm2/Vs and 4.4 cm2/Vs, respectively, with maximum efficiency of 22.10 %

Figure 6

Impact of Cs2TiF6 (a) electron and (b) hole mobility on the efficiency of the device.

Figure 7

Impact of Sb2Se3 (a) electron and (b) hole mobility on the efficiency of the device.

For an HTL layer, the electron and hole mobility varies from 15-95 cm2/Vs and 5–45 cm2/Vs, respectively. The efficiency decreases with increased electron mobility but remains the same for hole mobility variation. The hole and electron mobility for efficiency of 22.10 % are 5 cm2/Vs and 15 cm2/Vs.

Effect of temperature on the solar cell performance

Solar cells are used in various geographical regions and under different climatic conditions throughout the year, causing the operating temperature of the solar cell to fluctuate, and the solar cell's performance is highly influenced by its operating temperature. The simulation in this research has been performed at 300 K, the optimum operating temperature. For a solar cell in the temperature range of 280–400K under the constant light (1000W/m2), the temperature dependence of parameters for the Cs2TiF6/Sb2Se3-based cell is investigated as shown in Figure (8). It has been found that the efficiency (Figure 8(d)) and open-circuit voltage (Figure 8(a)) decrease by increasing the temperature. Jsc (Figure 8(b)) and FF (Figure 8(c)) changed very little with an increase in temperature. At higher temperatures, the parameters such as density, electron and hole mobility, bandgap, electron affinity, carrier concentration, etc., get highly affected and result in poor efficiency [24]. Maximum efficiency of 22.63 % is achieved at 280 K. It may be concluded that the perovskite device performs better when a solar cell works at lower temperatures.

Figure 8

Effect of temperature on (a)Voc (b) Jsc (c) FF and (d) efficiency of the device.

I–V, C–V, and QE plots for the overall performance of a device

Figure 9 (a-c) shows the I–V, C–V, and QE curves for the device's overall performance. Electron–hole (e-h) pairs are produced after the illumination, and the holes and electrons move towards HTL and ETL because of the junction field. The cathode and anode collect the electrons and holes and produce the voltage [25]. The Voc = 0.86 V, Jsc = 29.88 mA/cm2, FF = 85.55 % and efficiency = 22.10 % is achieved. The device's current density is determined by material parameters such as bandgap, thickness, and mobility. Photocurrent will be high if the absorption coefficient is large [26, 27, 28, 29]. The other parameters are thickness and mobility, which are crucial in achieving a high current density as the Jsc is linearly proportional to the mobility. The capacitance-voltage (C–V) characteristic of Cs2TiF6/Sb2Se3-based solar cells helps identify their fundamental features. The capacitance quickly shows a difference in performance for an applied voltage up to 0.5 V. QE curve also shows that a solar cell with an HTL enhances the device's performance.

Figure 9

(a) I–V, (b) C–V, and (c) QE curve for the overall performance of the solar device for optimized parameters.

Energy band diagram

The energy band diagram depicts the energy levels and their respective positions. Sb2Se3/Cs2TiF6/TiO2's band structure and their respective band gaps are shown in Figure (10(a)). When the photogenerated carrier separates the absorber and ETL layers at the interface, they are accelerated by the high electric field and quickly pulled away from the junction. Back contact traps the holes when they pass through the absorber and HTL layer while the electrons move into the ETL layer. When the band offsets are suitable, the charge carrier flows to the metal contact (avoiding recombination) [30]. The energy band structure is for Sb2Se3 (HTL), Cs2TiF6 (absorber), and TiO2 (ETL) with 2 μm, 0.5 μm, and 0.03 μm, respectively. The use of various HTLs with various perovskite absorber layers has been compared in Table 3.

Figure 10

Energy band diagram of (a) with Sb2Se3 as an HTL (b) without HTL-based photovoltaic cell.

Table 3

Comparison of various HTLs for perovskite-based solar cells.

Structure	Efficiency	Reference
Au/p-CdTe/Cs2TiI6/n-TiO2/ITO	15.06 %	[9]
ITO/PEDOT:PSS/CH3NH3PbI3/PCBM	13.28 %	[31]
FTO/CuSCN/CH3NH3PbI3/TiO2	28.9 %	[32]
Au/Cu2O/CH3NH3PbI3/ZnO/ITO	21.5 %	[33]
Mo/Sb2Se3/Cs2TiF6/TiO2	22.10 %	This Work

Conclusion

This work proposes a novel perovskite solar cell structure of Mo/Sb2Se3/Cs2TiF6/TiO2 where Cs2TiF6, Sb2Se3, and TiO2 are absorber, HTL, and ETL, respectively. We used Sb2Se3 as an HTL as it is less toxic, low in price, and can achieve a highly efficient solar cell. After optimizing the various parameters, such as the thickness of all active layers, acceptor density, and mobility, the maximum solar cell efficiency was 22.10 %, with Voc = 0.86 V, Jsc = 29.88 mA/cm2, and FF = 85.55 %. All these output parameters are within the Shockley-Queisser limits. The study points to the possibility of producing highly-efficient solar cells. The proposed solar device's electrical and physical properties were studied using the SCAPS-1D numerical modeling software, which optimized the solar cell design by adjusting the thickness, defect density, hole, electron mobilities, and temperature. The efficiency was maximum when the thickness of the absorber layer, HTL, and ETL was taken as 0.5 μm, 2.0 μm, and 0.03 μm, respectively. The device's performance was enhanced by increasing Cs2TiF6 hole mobility and decreased by increasing the Sb2Se3 electron mobility, and the highest efficiency was obtained when NA was taken as 1018 cm−3. When the defect density exceeds 1013 cm−3, the efficiency decreases for both the absorber and HTL because when the defect density increases, the recombination rate also increases, reducing efficiency. The device's performance was excellent when operated in the temperature range of 280–300K. It may be concluded that the perovskite device performs better when a photovoltaic cell works at lower temperatures. These results demonstrated the need to strengthen and improve the research on sustainable energy.

Declarations

Author contribution statement

Mamta: Conceived and designed the experiment; Analyzed and interpreted the data; Wrote the paper.

K.K. Maurya, V.N. Singh: Analyzed and interpreted the data; Wrote the paper.

Funding statement

This research received no specific grant from public, commercial, or not-for-profit funding agencies.

Data availability statement

Data will be made available on request.

Declaration of interests statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.