Solar power conversion: CuI hole transport layer and Ba₃NCl₃ absorber enable advanced solar cell technology boosting efficiency over 30%

Abstract

Researchers are becoming more interested in novel barium-nitride-chloride (Ba₃NCl₃) hybrid perovskite solar cells (HPSCs) due to their remarkable semiconductor properties. An electron transport layer (ETL) built from TiO₂ and a hole transport layer (HTL) made of CuI have been studied in Ba₃NCl₃-based single junction photovoltaic cells in a variety of variations. Through extensive numerical analysis using SCAPS-1D simulation software, we investigated elements such as layer thickness, defect density, doping concentration, interface defect density, carrier concentration, generation, recombination, temperature, series and shunt resistance, open circuit voltage (V_OC), short circuit current (J_SC), fill factor (FF), and power conversion efficiency (PCE). The study found that the HTL CuI design reached the highest PCE at 30.47% with a V_OC of 1.0649 V, a J_SC of 38.2609 mA cm⁻², and an FF of 74.78%. These findings offer useful data and a practical plan for producing inexpensive, Ba₃NCl₃-based thin-film solar cells.

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1. Introduction

The development of perovskite materials for potential incorporation into efficient solar cell technologies has resulted in significant advancements in photovoltaics.¹ In photovoltaics, perovskite materials have made substantial progress and are predicted to provide high-efficiency solar cell technology substitutes.^2,3 More specifically, the unique and intriguing structural, electrical, and optical properties of inorganic perovskites have garnered a lot of interest. Several A₃MX₃ (where a cation is indicated by A, M refers to a metal ion, and X is a halogen anion) group perovskites have recently surfaced as outstanding candidates among these materials.^4–8 While hybrid organic–inorganic perovskite solar cells (PSCs) have minimal trap presence, excellent light absorption, superior mobility of charge carriers, reduced binding energy for excitons, and an extended lifespan for charge carriers, these features significantly improve PV performance.^9–12 But the main problems with the extensive industrial use of organic cations are their thermal instability and volatility.^13,14 Certain materials, such as methylammonium lead iodide (MAPbI₃) and formamidinium lead iodide (FAPbI₃),¹⁵ have disadvantages such instability in ambient settings, lead toxicity, and difficulties with scaling in large-scale manufacture. These problems impair their long-term effectiveness in real-world applications as well as their commercial feasibility.

The exceptional properties of perovskites, such as their high mobility of electrons and ions, changeable bandgap, and efficient absorption of light, render them ideal for a variety of uses, such as energy storage, photovoltaic electronic devices, and catalysis.¹⁶ Scientists are interested in perovskite materials because they may be able to produce extremely efficient photovoltaic solar cells.¹⁷ Ba₃NCl₃ perovskite offers several advantages in solar cell performance. It exhibits better thermal and chemical stability compared to organic–inorganic perovskites, which enhances the lifespan of solar cells. It has a suitable bandgap for efficient light absorption, which can be optimized for improved solar energy conversion. Being lead-free, it addresses environmental and health concerns associated with lead-based perovskites. It has a better absorption coefficient compare to others perovskites. With an adjustable band gap (1.20 eV), exceptional compositional stability, and thermal stability, inorganic Ba₃NCl₃ perovskites emerged as a promising absorber for use in the creation of high-performance perovskite solar cells.¹⁸ Perovskite solar cells have garnered a lot of interest in the last few years because of their improved power conversion efficiency (PCE), which has surpassed 26%.^19,20 Thus, to stabilize Ba₃NCl₃ in the black phase, several techniques like doping, decreasing grain size, adjusting the value of tolerance factor, or modifying surface energy are applied. In addition, the doping method—which stabilizes the black phase in Ba₃NCl₃ by introducing the appropriate element ions—is frequently employed. It is possible to improve photovoltaic performance by adding beneficial element ions to the Ba₃NCl₃ host lattices and choosing the right hole transport layer (HTL) and electron transport layer (ETL).^21,22 The V_OC can be enhanced by interlayer insertion between Ba₃NCl₃ and the back contact by lowering the unanticipated level of Fermi trapping at the absorber–metal interaction that inhibits carriers having bends in the band.²³ By utilizing a highly doped p-type semiconductor as the hole transport layer (HTL) at the back side of the, this pinning effect can be avoided.²⁴ Therefore, by creating a p⁺–p junction with a high electric field, the absorber layer can limit the loss of carrier recombination at the HTL/absorber contact.²⁵ The transmission of holes can also be enhanced by a strongly doped HTL (p⁺ type) that has a little offset of valence band (VBO). Research is being done to develop an HTL between the back contact and the absorber.^26,27 The goal of the research is to create an HTL material, such as CuI, with improved electrical and optical properties.²⁸ Thus far, the only reports of improved PV performance have been of combined high-efficiency Ba₃NCl₃ absorber setups with Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni design.

A new device combination featuring Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni is utilized in this study. We obtained the highest PCE at 30.47%, with a V_OC of 1.0649 V, a J_SC of 38.2609 mA cm⁻², and an FF of 74.78% using the Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni structure. These results are superior than those of other reported Ba₃NCl₃ based devices, which is the novelty of this work. This suggested Ba₃NCl₃-based solar cell shows exceptional maintenance efficiency when compared to conventional thin-film solar cells, demonstrating Ba₃NCl₃'s appeal as a solar energy system material while removing potential toxicity issues.

In this study, a detailed analysis and investigation of the photovoltaic performance of Ba₃NCl₃ absorber-based double-heterojunction solar cells with and without HTL have been conducted. Analyzing the effects of many metals as front and rear contacts was first assumed to identify the least resistive junction at the metal–semiconductor interface. The work function of the materials used in the contacts is crucial because it determines the efficiency of charge extraction and transport. Proper alignment of the energy levels between different layers ensures minimal energy barriers for charge carriers, leading to efficient operation of the solar cell. Aluminum (Al) has been proposed as the optimal choice for the front contact, while nickel (Ni) has been recommended for the rear contact.²⁹ Subsequently, the photovoltaic (PV) acts of innovative Ba₃NCl₃ absorber-based cell structures for HTL of CuI with ETL of TiO₂ were computed for various working temperatures, thicknesses of layer, densities for doping, total defect as well as interface defect densities, series and shunt resistance, including rate of carrier generation and recombination, using SCAPS-1D simulator software to determine quantum efficiency (QE) and the current density–voltage (J–V).

2. Device construction and simulation techniques

The suggested Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni photovoltaic cells have been modelled using the one-dimensional Solar Cell Capacitance Simulator (SCAPS) tool. A one-dimensional solar cell modelling tool called SCAPS (a Solar Cell Capacitance Simulator) was created at the University of Gent, Belgium's Department of Electronics and Information Systems (ELIS). Basic equations were solved to predict and systematically assess both structures: Fig. 1a and b simultaneously display equations of continuity for both holes and electrons, alongside equations describing the electrostatic potential, all under conditions of steady-state without HTL and with HTL symmetrical structures. The layers that make up the n⁺–n–p–p⁺ structure are as follows: a 0.05 μm thick layer of FTO; a 0.05 μm thick layer of n-type TiO₂ ETL; a 1.0 μm thick layer of Ba₃NCl₃ absorber; and a 0.05 μm thick layer of p⁺-type HTL CuI with Ni and Al serving as the back and front metal electrodes. Fig. 1c and d show the energy band diagrams with and without HTL concurrently. The band alignment for all layers of Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni structure is shown in Fig. 1(e). The positions of quasi-Fermi states F_n and F_p under illumination confirm the production of electron–hole pairs in the gadget. The offset between absorber CB and VB confirms that electron–hole pairs have formed inside the device. The difference in work function (WF) between the absorber and transmission layers is responsible for the offset that is seen. Light generated at the absorber/ETL interface is separated with the help of integrated voltage and related electrical fields. The fundamental equations of one-dimensional semiconductors were solved with the SCAPS-1D simulator in order to get the PV parameters. Numerous input and output parameters exist in this work, including:

Fig. 1 The Device structures (a) without HTL, (b) with HTL, energy band diagram (c) without HTL, (d) with HTL CuI, and (e) band alignment of Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni.

2.1. Efficiency

The efficiency of solar cell materials is defined as the percentage of solar radiation that is transformed into electrical energy, indicating how well they can convert sunlight into electricity. Greater power output per unit area of a solar cell is indicated by high efficiency.

2.2. Short circuit current

In solar cell materials, the highest current produced when the cell is exposed to light and its terminals are shorted is known as the short circuit current (ISC). It is a crucial factor in determining the efficiency of the cell since it represents its capacity to produce charge carriers.

2.3. Open circuit voltage

The highest voltage that a solar cell can produce while there is no current flowing through the circuit is known as the open circuit voltage (V_OC) in sun cell materials. It symbolizes the potential difference between the cell's positive and negative terminals when it is not in circuit.

2.4. Fill factor

The ratio of the actual maximum attainable power to the theoretical power if both current and voltage were at their maximum is known as the fill factor (FF) in solar cell materials. It is a crucial factor in determining a solar cell's effectiveness since it indicates the calibre of the material and construction used in the cell.

2.5. Electron affinity

The energy change that occurs when an electron is introduced to a neutral atom or molecule is known as electron affinity and is commonly expressed in electron volts (eV) in solar cell materials. It affects how well charges are separated and transferred within the solar cell.

2.6. Band gap

The energy differential between the bottom of the conduction (free electron flow) and the top of the valence (outer electron) band is referred to as the band gap.

2.7. Density of states

The amount of electronic states that are accessible in a material at each energy level is described by the density of states (DOS) in solar cell materials. It is essential for figuring out how well charge carrier recombination and generation work together, which affects the solar cell's overall performance.

2.8. Dielectric permittivity

The ability of a material to polarize in response to an electric field, which affects charge separation and transport, is measured by its dielectric permittivity in solar cell materials. Dielectric permittivity influences solar cell performance by enhancing internal electric fields, reducing charge recombination, and improving charge carrier mobility, leading to higher efficiency. It affects the optical properties, improving light absorption and reducing reflection losses. Proper dielectric permittivity aids in better impedance matching and capacitance management, ensuring efficient power transfer. It also affects band alignment and stability, optimizing energy levels and device longevity. In essence, optimizing dielectric permittivity is crucial for maximizing solar cell efficiency and stability.

2.9. Acceptor and donar density

The concentration of acceptor impurities, which produce holes (positive charge carriers), usually in p-type semiconductors, is referred to as acceptor density in solar cell materials. The concentration of donor impurities, which are typically found in n-type semiconductors, that supply free electrons (negative charge carriers) is referred to as donor density.

2.10. Electron and hole mobility

The ease with which electrons and holes—positive charge carriers—can travel through various materials used in solar cells is referred to as electron and hole mobility. Improved charge transport characteristics, which are essential for effective solar cell operation, are indicated by higher mobilities.

2.11. Interface defect density

The number of flaws per unit area at the junctions between different layers in solar cell materials is known as the interface defect density. These imperfections have the potential to trap charge carriers and lower the efficiency of the cell. It is a crucial factor affecting the stability and overall performance of the solar cell.

2.12. Defect density

The number of structural flaws per unit area or volume that can negatively impact the performance of the cell by obstructing charge carrier flow is referred to as defect density in solar cell materials. Higher efficiency and improved overall performance of the solar cells are usually the result of lower fault levels.

The set of equations includes Poisson's equation alongside the continuity equations for both electrons and holes, followed by expressions for drift and diffusion drift denoted by eqn (1)–(5).^30,31

The subsequent information has been gathered from available sources in addition to the experiment results and implemented as simulation variables: dielectric permittivity ε_r, thickness of layers (nm), energy gap of bands E_g (eV), the valence band density N_V (cm⁻³), affinity of electron χ (eV), density of the conduction band N_C (cm⁻³), mobility of hole μ_h (cm² V⁻¹ s⁻¹), donor density N_D (cm⁻³), mobility of electron μ_n (cm² V⁻¹ s⁻¹), recombination coefficient (cm³ s⁻¹), and acceptor density N_A (cm⁻³) were taken into account. An overview of all these simulating parameters is provided in Tables (1 as well as 2) for the anticipated heterostructures. For TiO₂ and CuI, the authors have chosen and verified the absorption coefficients from credible literature, ensuring future simulations beyond a reasonable doubt. By using SCAPS 1D software, we have added neutral type bulk defect densities in all layers to this study. These defects are distributed throughout the volume of the absorber material. They represent intrinsic defects within the bulk of the material that affect bulk recombination. Whenever an experimental version of the proposed device is produced, the limitations on design methodology, manufacturing, environmental impacts, and high precision measurement might lead to a notable discrepancy between simulation and experiment.

Table 1 Specifications for the absorber layer, FTO, ETL, and HTL layer

Parameters	FTO^32	TiO2 (ref. 33)	Ba3NCl3 (ref. 18)	CuI^28
Thickness (nm)	50	50	1000	50
Band gap, Eg (eV)	3.60	3.20	1.20	3.10
Electron affinity, χ (eV)	4.5	3.9	4.0	2.1
Dielectric permittivity (relative), εr	10	9	5.32	6.5
CB effective density of states, NC (cm^−3)	2 × 10^18	1 × 10^19	3.96 × 10^18	2.5 × 10^19
VB effective density of states, NV (cm^−3)	1.8 × 10^19	1 × 10^19	1.83 × 10^19	2.5 × 10^19
Electron mobility, μn (cm^2 V^−1 s^−1)	100	200	50	100
Hole mobility, μh (cm^2 V^−1 s^−1)	20	10	50	43.9
Shallow uniform acceptor density, NA (cm^−3)	0	0	1 × 10^18	1 × 10^18
Hallow uniform donor density, ND (cm^−3)	1 × 10^18	1 × 10^17	0	0
Defect density, Nt (cm^−3)	1 × 10^14	1 × 10^14	1 × 10^12	1 × 10^14

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Table 2 Data for interface parameters used in the Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni solar cell

Parameter	Ba3NCl3/TiO2	CuI/Ba3NCl3
Type of defect	Neutral	Neutral
σh (cm^2)	1 × 10^19	1 × 10^19
σe (cm^2)	1 × 10^19	1 × 10^19
Total density of defect	10^10	10^10
Er	0.6	0.6
Working temperature (K)	300	300
Energetic distribution	Single	Single

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3. Result and discussion

3.1. Thickness optimization of the absorber and ETL layers

Fig. 2(a) illustrates how varying the depth of the absorber layer between 300 and 3000 nm affects the recommended devices' capacity to maximize performance, while keeping other parameters as stated in Table 1. Significant increases in absorber layer thickness are seen in both carrier production and recombination rates. With an increase in absorber thickness, the V_OC of the without HTL group was nearly constant at 0.9764–0.9777 V, whereas the with HTL group saw a modest drop in V_OC, from 1.10561–1.0296 V. However, because of increased spectrum absorption,³⁴ especially at longer wavelengths, the J_SC increases with larger absorber layers. J_SC values have significantly increased both with and without HTL: with HTL, they have increased from 32.337 to 38.877 mA cm⁻², while without HTL, they have increased from 32.331 to 38.868 mA cm⁻². These differences between J_SC and V_OC indicate a tendency that is consistent with research from papers.^35–37 The FF is relatively stable at 74.78% for with HTL and 73.27% for without HTL. For both with HTL and without HTL, PCE grew as absorber thickness increased. When the length of charge carrier diffusion and thickness of absorber layer are equivalent, PSC performance is demonstrated to peak. It is evident from Fig. 2(a) that altering the thickness of the absorber layer has a major impact on the overall performance of PV systems. It is clear that we operate at our best when Ba₃NCl₃ is thicker than 1000 nm. Performance does not significantly increase or, in certain circumstances, decreases as thickness increases over ∼1000 nm. As a result, we have decided that the optimal thickness for the absorber to be 1000 nm. This led to the following results: V_OC of 0.9803 and 1.0697 V, J_SC of 38.2534 and 38.2609 mA cm⁻², FF of 73.27 and 74.78%, and PCE of 27.48 and 30.47% for both without and with HTL structures.

Fig. 2 The thickness variation effect of (a) absorber Ba₃NCl₃ and (b) ETL TiO₂ layers on PV parameters of V_OC, PCE, J_SC, and FF.

As seen in Fig. 2(b), the thickness of the ETL layer can significantly affect the performance of PV devices. Buffer layers help to move charge carriers from the absorber layer to the electrode more easily, which increases the stability and efficiency of PV devices. Several properties of PV devices, like as light absorption, charge carrier recombination, and interlayer contact resistance, can be influenced by the thickness of the ETL layer. Wider band gap smaller layers efficiently transport nearly all incident photons to the Ba₃NCl₃ absorber layer, improving light absorption in the cells. Despite the ETL layer being thicker, the V_OC remained nearly constant at 0.96 and 1.03 V, respectively, with and without HTL. However, there was a modest fall in the J_SC values for both the without HTL and with HTL groups, with declines of 38.2538 to 38.2285 mA cm⁻² and 38.2611 to 38.2358 mA cm⁻², respectively. The changing thickness of the ETL layer has minimal impact on J_SC as the layer remains within an optimal range as shown in Fig. 2(b), ensuring efficient electron extraction and minimal recombination. As the ETL is neither too thin to cause leakage nor too thick to hinder transport, J_SC remains relatively stable. Proper ETL design maintains effective charge collection across varying thicknesses. Additionally, there was some variation in the FF values between without HTL (73.25% to 75.26%, respectively). At 74.78%, the FF for with HTL was essentially stable. The PCE for the absence of HTL increased slightly, from 27.43% to 28.13. The PCE for with HTL remained relatively consistent at 30.47% as the thickness of the ETL layer grew. As a result of the ETL layer's thickness modifications, PCE, J_SC, FF, and V_OC have not significantly altered, as shown in Fig. 2(b). This suggests that there is no appreciable difference in the thickness of the ETL. In order to lower the cost of production, we have decided that 50 nm is the ideal thickness for the ETL layer. Because producing something that is practically thinner than 50 nm is nearly impossible. The output values of the suggested devices from this work are compared to those from earlier works in Table 3.

Table 3 A comparison between this work's PV parameters versus those from earlier studies

Structures	VOC (V)	JSC (mA cm^−2)	FF (%)	PCE (%)	Ref.
FTO/TiO2/Ba3NCl3	0.980	38.2534	73.27	27.48	This work
FTO/TiO2/Ba3NCl3/CuI	1.064	38.2609	74.78	30.47	This work
FTO/SnS2/Sr3SbI3	0.93	34.67	87.31	28.05	38
FTO/SnS2/Sr3PI3	0.92	34.65	34.65	28.15	39
FTO/SnS2/Ba3NCl3	0.94	38.26	79.91	28.81	18
Al/FTO/WS2/RbPbBr3/Au	0.8967	34.5466	79.23	24.56	40
FTO/SnS2/Sr3NCl3	1.24	16.79	86.44	18.11	18
CH3NH3PbI3/CuSCN	1.0	23.12	83.56	19.33	41
CIGS/CH3NH3PbI3	0.84	27.2	84	19.29	42
n-TiO2/i-CH3NH3SnBr3/p-NiO	0.80	31.88	84.89	21.66	43

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3.2. Influence of Ba₃NCl₃ absorber's bulk defect and acceptor density variation on PV performance

Fig. 3 shows how variations in defect density (N_t) and acceptor density (N_A), which range from 10¹⁰ to 10¹⁷ cm⁻³ and 10¹² to 10²⁰ cm⁻³, respectively, affect the performance of solar cells using CuI as HTL. When the N_t increases over 10¹² cm⁻³, solar cell metrics exhibit a sharp decline.⁴⁴ Specifically, for the Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni structure, the PCE, FF, J_SC, and V_OC fluctuate from 8.04 to 37.98%, 44.93 to 79.09%, 21.72 to 38.27 mA cm⁻², and 0.5024 to 1.2546 V, respectively, when the N_t and N_A change from 10¹⁰ to 10¹⁷ cm⁻³ and 10¹² to 10²⁰ cm⁻³. As Fig. 3(a) illustrates, a N_A greater than 10¹⁷ cm⁻³ and a N_t less than 10¹² cm⁻³ are needed to achieve the maximum V_OC of 1.2546 V. In contrast, the V_OC dramatically decreases to 0.502 V when the N_t is greater than 10¹⁶ cm⁻³ and N_A is lesser than 10¹⁵ cm⁻³. According to Fig. 3(b), the N_t needs to be less than 10¹⁴ cm⁻³ and the N_A needs to be more than 10¹⁸ cm⁻³ in order to get the maximum J_SC value of 38.272 mA cm⁻². As Fig. 3(c) illustrates, when the absorber layer's N_A is at least over 10¹⁸ cm⁻³ and the N_t is at or below 10¹² cm⁻³, the FF peaks at 79.09%. Nevertheless, when this N_t is exceeded from 10¹⁶ cm⁻³, FF lowers dramatically to 44.93%. Meanwhile, the combination that produces the maximum PCE of more than 37.998% falls at 10¹⁹ cm⁻³ in N_A and a N_t of less than 10¹² cm⁻³, as shown in Fig. 3(d). High-defect states accelerate carrier recombination, which lowers cell performance when they are put into the absorber layer.^45,46 With HTL CuI, the ideal parameters to achieve the highest PCE of 30.47% are a fixed Ba₃NCl₃ absorber layer N_A of 10¹⁸ cm⁻³ and a N_t of 10¹² cm⁻³. The solar cell furthermore matched the following specifications: V_OC of 1.0649 V, J_SC of 38.2609 mA cm⁻², and FF of 74.78%.

Fig. 3 Effects of Ba₃NCl₃ absorber acceptor and bulk defect density variations with HTL CuI on photovoltaic parameters; (a) V_OC (b) J_SC (c) FF and (d) PCE.

3.3. Impact of HTL CuI layer's bulk defect and acceptor density variation on PV performance

The doping level (N_A) of hole transport layers (HTL) can significantly affect the efficiency of photovoltaic devices. A common method for improving conductivity and band alignment between the electrode and absorber layer is doping HTL. Among other things, bandgap, conductivity, and carrier concentration are all impacted by N_A in PV devices. Fig. 4 displays the impact of acceptor density N_A and bulk defect N_t in HTL CuI on the PV characteristics at 10¹⁰ to 10¹⁷ cm⁻³ and 10¹² to 10²⁰ cm⁻³. The PV characteristics with HTL CuI range from 1.0647 to 1.0655 V, 38.260913 to 38.260939 mA cm⁻², 74.7104 to 74.7880%, and 30.4357 to 30.4895%, respectively, with the variation of N_t and N_A. However, CuI HTL produced the maximum FF of 74.78, J_SC of 38.397 mA cm⁻², V_OC of 1.0655 V, and PCE of 30.48%. To achieve cost-effective manufacturing and high efficiency in practical applications, a N_t of 10¹² and a N_A level of 10¹⁸ cm⁻³ were selected for more research with HTL CuI. Variations in the defect density (N_t) in PV devices' HTL layer can cause recombination, obstruct charge carrier passage, and jeopardize the stability of the device. Non-uniform fault distribution throughout the HTL can affect optical properties that affect photon absorption and lead to spatial variations in device performance. Ensuring uniformity in the HTL layer and decreasing fault density are the primary ways to optimize PV efficiency and dependability.

Fig. 4 Effects of HTL CuI shallow acceptor N_A and bulk defect density variations on photovoltaic parameters; (a) V_OC (b) J_SC (c) FF and (d) PCE.

3.4. Analysis of generation-recombination rate and carrier concentration

Fig. 5(a) and (b) display the carrier production and recombination rates for the two scenarios with HTL and without HTL for different locations. An electron creates electron–hole pairs with a hole remaining in the valence band (VB) when it moves from the VB to the conduction band (CB) during carrier creation. This leads to the release of electrons and holes, which increases carrier synthesis in PSCs. Surprisingly, the generation rate with HTL is the lowest at 0.1 μm, whereas the rate without HTL is around 1.03 μm. This difference can be explained by the fact that certain places absorb photons at higher rates than others. Eqn (6) makes it easier to find G(x), and the N_phot (λ, x) SCAPS-1D model uses the incoming photon flux to measure the creation of electron–hole pairs.

G(λ, x) = α(λ, x)·N_phot(λ, x) (6)

Fig. 5 (a) The rate of total generation; (b) the rate of recombination; (c) the concentration of hole; and (d) the concentration of electron carrier with and without HTL.

Recombination occurs in photovoltaic systems when electrons and holes mix in the conduction band. Recombination rates in precisely designed and optimized PSCs are highly dependent on the density and lifespan of these charge carriers. Moreover, the defect state of every layer affects the recombination process in PSCs. In particular, more electrons from the conduction band pass over the energy barrier and pair with holes in the valence band.

As such, the energy levels related to this transition have an impact on the kinetics of electron–hole recombination. The differences in the thickness of the Ba₃NCl₃ absorber layer with and without HTL, as well as how they impact the concentrations of electron and hole carriers, are depicted in Fig. 5(c) and (d). The varying absorber concentrations result in different densities of states (DOS) inside the valence bands, which in turn affect the hole concentration in the Ba₃NCl₃ absorber. It's interesting to note that the former is more prevalent when comparing the concentrations of electrons and holes.^47,48 This study shows that employing CuI as HTL in combination with Ba₃NCl₃ based on barium nitride chloride decreases electron–hole recombination and enhances carrier generation, hence increasing the absorber materials' effectiveness. These advancements may lead to the creation of high-performance PSCs.

3.5. Analysis of interface defect density on PV performance

The influence of defect interface states on the output parameters is comparable to that of bulk defects. Fig. 6 illustrates how interfacial imperfections at with HTL CuI/Ba₃NCl₃ and without HTL Ba₃NCl₃/TiO₂ impact the PV characteristics of a newly designed Ba₃NCl₃-absorber based PSC. Reduced interface defects are essential for optimizing solar cell performance through careful material engineering and interface passivation. Lattice mismatch, thermal expansion differences, chemical reactions, and grain boundaries are common sources of interface defect densities in an Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni solar cell structure, which impact its efficiency as recombination centers for charge carriers. The analysis and measurement of these defects is done using techniques such as capacitance.

Fig. 6 Defect impact on PV parameters at the Ba₃NCl₃/TiO₂ and CuI/Ba₃NCl₃ interfaces.

The concentration of interface faults was varied in this simulated investigation from 10⁹ to 10¹⁶ cm⁻² while maintaining the same values for all other parameters. With increasing defect density starting at 10⁹ cm⁻², PV metrics including PCE, FF, J_SC, and V_OC are shown in Fig. 6 drastically decline at the with HTL CuI/Ba₃NCl₃ and without HTL Ba₃NCl₃/TiO₂ interface, going from 30.48 to 18.70% and 27.48 to 15.64%, 74.78 to 63.71% and 73.27 to 56.40%, 38.26 to 35.47 mA cm⁻² and 38.25 to 34.24 mA cm⁻², and 1.0654 to 0.8278 V and 0.9804 to 0.809 V, respectively. When interface faults exist, series resistance can increase significantly.^49,50 However, this resistance has very little effect on J_SC, which leads to nearly constant J_SC at all three interfaces with varying defect concentrations. The decline in performance is caused by the increased interface carrier recombination rate when defect density rises.^51–53 Therefore, it is suggested that interface defects at the Ba₃NCl₃/TiO₂ and CuI/Ba₃NCl₃ layers have a major role in determining the performance characteristics of PV devices.^54,55

3.6. Optimized J–V and QE characteristics

There is a lot of promise for Ba₃NCl₃ as an absorber layer in solar cells. The quantum efficiency (Q-E) and current–voltage (J–V) properties of the solar cell can be significantly impacted by the thickness of the Ba₃NCl₃ absorber layer. Increased J_SC is frequently the result of increased photocurrent and light absorption from a bigger Ba₃NCl₃ absorber layer. But a bigger absorber layer might also result in higher recombination losses, which would lower the FF and V_OC and eventually lower efficiency. Recent research indicates that the doping level, processing technique, and device design all affect the ideal thickness of the Ba₃NCl₃ absorber for the fabrication of high-efficiency solar cells.^56,57 The J–V characteristics and QE properties are critical indicators of solar cell performance.⁵⁸ The current density (J) when the voltage (V) across the solar cell is zero is called short-circuit current density (J_SC). A higher J_SC indicates efficient light absorption and charge carrier collection. V_OC is the voltage (V) when the current (J) is zero. V_OC is influenced by the material's bandgap and the recombination of charge carriers. QE is the ratio of the number of charge carriers collected by the solar cell to the number of photons incident on the cell at a given wavelength. A higher QE indicates better photon-to-electron conversion efficiency. In some cases, a lighter Ba₃NCl₃ absorber layer might increase charge carrier collection and decrease recombination losses to reach better efficiency. Because of its impact on photocurrent output and photon absorption efficiency, the thickness of the Ba₃NCl₃ absorption layer can affect Q-E. Longer wavelength Q-E is greater because more photons can be absorbed by the thicker Ba₃NCl₃ absorber layer.

On the other hand, if the absorber layer is too thick, light may be absorbed too deeply, which would reduce Q-E and charge carrier collecting efficiency. Fig. 9a and b display the J–V and Q-E characteristics variation with CuI HTL for the variation of absorber thickness. The optimised configurations' PCE reached its highest point at 30.47% CuI HTL, with a V_OC 1.0649 V, a FF of 74.78%, and J_SC of 38.2609 mA cm⁻². The optimised structure's current density zeroes out at 1.06 V with CuI HTL, which is remarkable. It is significant to remember that earlier research has demonstrated a consistent drop in current density as voltage increases, as Fig. 7a and c shows.^59,60 The wavelength range covered by the QE profiles from 300–1100 nm. For with HTL, the QE begins at around 100% and gradually decreases to 0% as the wavelength approaches 1040 nm. This drop in QE is consistent with the J–V characteristics results.

Fig. 7 Impact of the absorber layer's (a and b) thickness and (c and d) doping concentration on the J–V and QE properties.

The effect of the Ba₃NCl₃ absorber layer doping concentration on the J–V and QE spectra of the corresponding solar cells is seen in Fig. 7c and d, respectively. When the hole doping level surpasses 10¹⁸ cm⁻³, the response of QE significantly decreases in both absorber designs, further revealing the recombination of the photogenerated carrier as predicted by a rise in the recombination of free carrier charges inside the bulk. In the absorber layer, the lower energy (longer wavelength) photons are absorbed significantly. As a result, it was shown that the doping concentration had a significant effect on the total conversion efficiency.⁶¹

3.7. Effect of temperature

A comprehensive numerical study that looks at the effects of changing the temperature from 275 to 475 K on PV properties is presented in Fig. 8. Critical metrics such as PCE, V_OC, J_SC, and FF have consistently been demonstrated to have higher values in produced PSCs with Hole Transport Layer (HTL) than in PSCs without HTL over the complete temperature range. The production and recombination of carriers exhibit a constant rate over the temperature range of 275 to 475 K in this graph. The estimated PCE for the baseline structure without HTL spans from 27.55 to 22.14% for the variation of temperature from 275 K to 475 K, respectively, and ranges from 31.31% to 28.17% with CuI HTL. Temperature can accelerate electron recombination between the valence and conduction bands in some PSC configurations, which can result in an increase in reverse saturation current and a decrease in PCE and V_OC.⁶²

Fig. 8 Effect of temperature change on PV solar cell.

3.8. The effect of the resistances on the PV performance of the proposed solar cell

Using SCAPS-1D software, the impact of the series (R_s) and shunt resistances (R_sh) on the solar device is examined. The circuit terminal resistance, the bulk resistance, and the front and rear metallic contact resistance combine to produce the solar cell's R_s.⁶³ The main result of the series resistance is to reduce the fill factor, which means that a 100% FF is unachievable. To improve efficiency, lower series and greater shunt resistances must be reached. The resistance values have an impact on J_SC and V_OC as well. The value of R_s is changed using SCAPS-1D to see how it affects the performance of the suggested and conventional solar cells. While the shunt resistance (R_sh) is set at 10⁵ Ω cm², the value of R_s can vary from 1 to 10 Ω cm².

As widely noted in the previous literature,⁶⁴ Fig. 9(a) shows the impact of varying R_s on the proposed cell, including the without HTL and with CuI HTL layer. It is evident from this that raising R_s considerably reduces the solar cell's efficiency. The efficiency decreased from 26.12 to 15.16% and from 29.10 to 18.87% when R_s were changed within the specified range, according to these data. The value of R_sh is now changed from 10² to 10¹⁰ Ω cm² after the series resistance, R_s, is fixed at 0.5 Ω cm². The results of varying the cell's R_sh value as previously described are shown in Fig. 9(b), which also demonstrates that the efficiency of the suggested solar cell rises as R_sh grows. The efficiency increases with raising the R_sh value of the proposed solar cell, which includes the without HTL and with CuI HTL layer, from 21.55 to 26.80% to 23.35 to 29.79%.

Fig. 9 The impact of varying series and shunt (a and b) without HTL and with HTL CuI layers on PV solar cell.

4. Conclusion

Using SCAPS-1D, the performance of a novel heterostructure including Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni in solar devices was examined and assessed. Computational simulations were used to evaluate key parameters of Ba₃NCl₃-based solar cells with TiO₂ serving as the electron transport layer (ETL) and with or without CuI-based hole transport layer (HTL). When comparing this new design to the conventional structure, V_OC and PCE showed significant improvements. It was looked at how different gadget features affected solar cell performance. The optimal configurations were determined to be the absorber thickness of 1000 nm and the doping concentration of 10¹⁸ cm⁻³, the HTL CuI thickness of 50 nm, and the TiO₂ ETL thickness of 50 nm. There are four different PV values for the proposed Ba₃NCl₃-based heterojunction with CuI HTL: 30.47% for PCE, 1.0649 V for V_OC, 38.2609 mA cm⁻² for J_SC, and 74.78% for FF. These findings show that the Al/FTO/TiO₂/Ba₃NCl₃/CuI/Ni design has the potential to enable the production of high-efficiency solar cells by the solar cell industry. On the other hand, Ba₃NCl₃-based photovoltaic cells are limited by assumptions derived from simulations that do not fully capture the complexities of real-world systems. Processing challenges and potential stability and efficiency issues make it challenging to implement recommended solar cell configurations. Obtaining consistent and dependable device performance is also a significant barrier to scaling up production. Ultimately, overcoming these limitations necessitates comprehensive understanding as well as innovative solutions in the domains of material science and device engineering.

Abbreviations

HTL	Hole transport layer
ETL	Electron transport layer
PSC	Perovskite solar cell
VOC	Open voltage current
FF	Fill factor
EA	Electron affinity
J–V	Current voltage density
WF	Work function
JSC	Short circuit current density
Nt	Defect density
μh	Hole mobility
χ	Electron affinity
Rs	Series resistances
SCAPS	Solar cell capacitance simulator
SRH	Shockley–Read–Hall
EBD	Energy band diagram
PV	Photovoltaic
FTO	Fluorine-doped tin oxide
εr	Dielectric permittivity (relative)
PCE	Power conversion efficiency
BMC	Back metal contact
Q-E	Quantum efficiency
ND	Shallow uniform donor density
NA	Shallow uniform acceptor density
μn	Electron mobility
Rsh	Shunt resistances

Ethical statement

All the authors declare that the manuscript does not have studies on human subjects, human data or tissue, or animals.

Data availability

Data will be made available on reasonable request.

Author contributions

Avijit Ghosh: conceptualization, methodology, software, validation, formal analysis, visualization, investigation, data curation, supervision, writing—original draft, and review and editing. Abdullah Al Hossain Newaz, Abdullah AL Baki, Nasser S. Awwad, and Hala A. Ibrahium: methodology, software, validation, formal analysis, data curation, writing—original draft, and review and editing. Mohammad Shakhawat Hossain, and Md Muminur Rahman Sonic, Md Saiful Islam, and Md Khaledur Rahman: software, validation, formal analysis, writing—original draft, and review and editing.

Conflicts of interest

The authors have no conflicts of interest.

Acknowledgements

The authors extend their appreciation to the Deanship of Research and Graduate Studies at King Khalid University for funding this work under grant number RGP2/148/45.

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