Numerical analysis of the MASnI₃/CZT(Se_1−xS_x) interface to boost the performance via band offset engineering

Abstract

This study investigates a tin-based perovskite solar cell (PSC) incorporating an inorganic hole transport layer, examined through simulations with the SCAPS simulator. The chosen CuZnSn(Se_1−xS_x) compound emerges as a promising candidate for the hole transport layer, allowing for a tunable band gap via adjustments to the S/(S + Se) ratio. The band gap varies from 0.95 eV for Cu₂ZnSnSe₄ to 1.5 eV for Cu₂ZnSnS₄, achieved through strategic valence band offset engineering at the MAPbI₃/CuZnSn(Se_1−xS_x) interface. However, achieving an optimal Valence Band Offset (VBO) at MASnI₃/CuZnSn(Se_1−xS_x) remains challenging yet crucial for realizing high-performance Perovskite Solar Cells. The device efficiency is systematically optimized by manipulating the S content, resulting in a noteworthy Power Conversion Efficiency of 18.29%. Furthermore, it is uncovered that a carefully selected VBO (0.22 eV) is achieved with the CZTSe_0.4S_0.6 hole transport layer, contributing significantly to the improved performance of the PSC. These findings underscore the importance of precise engineering in achieving optimal device properties for advanced solar energy conversion applications.

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1 Introduction

In recent years, there has been substantial progress in advancing perovskite solar cells (PSCs), a trend that began with the initial report in 2009.¹ A lot of work has gone into producing solar cells with greater efficiency, with perovskite materials emerging as one of the most promising alternatives for future developments. The power conversion efficiency (PCE) of hybrid halide perovskite has increased dramatically, from 3.8% to an astounding 25.2%, which justifies the material's increased attention on a global scale.^2,3 Metal halide perovskites, or PSCs, are represented by the formula ABX₃, where A stands for non-bonding monovalent cations, such as methylammonium (MA)⁺, formamidinium (FA)⁺, and cesium (Cs)⁺; B represents octahedral divalent ions, usually Pb²⁺; and X is a monoanionic ion, usually from the halide group (Cl⁻, Br⁻, I⁻, or a mixture).

Compared to more known technologies such as CdTe, CIGS, and silicon solar cells, this photovoltaic (PV) breakthrough has been achieved rather quickly. Characteristics such as an appropriate band gap, high absorption coefficient, ambipolar charge-carrier transport, small exciton binding energy, long diffusion length, defects tolerance, low fabrication cost, and low effective carrier masses are responsible for PSCs success.^4,5

Despite the remarkable accomplishments of PSCs, challenges remain for their broader adoption. Lead (Pb), a toxic element, risks human health and the natural environment.⁶ Additionally, MAPbI₃ experiences reduced performance due to poor stability when exposed to moisture and sunlight.⁷ To address these issues, various nontoxic metals such as copper, bismuth, germanium, antimony, and tin have been explored as substitutes for Pb in PSCs.^8,9 Sn has been identified as the most promising substitution for improving PSC performance.

MASnI₃, with its narrow band gap of 1.3 eV compared to MAPbI₃, exhibits a similar valency. The slight reduction in the radius of Sn²⁺ (1.35 Å) compared to Pb²⁺ (1.49 Å) allows for the replacement of Pb²⁺ with Sn²⁺ while maintaining the perovskite structure.¹⁰ The Goldschmidt tolerance factor and octahedral factor play a critical role in assessing the structural stability and feasibility of forming perovskite phases, thereby serving as essential criteria for the rational design and selection of high-performance perovskite materials in solar cell applications.¹¹ Sn²⁺ substitution adheres to coordination, causes minimal lattice constant perturbation, and maintains ionic size and charge balance.^12,13 Tin-based, environmentally friendly perovskite materials hold the potential to enhance device performance and stability. However, tin-based PSCs face challenges, including sensitivity to oxygen leading to rapid oxidation of Sn²⁺ into Sn⁴⁺, as well as the self-doping effect and rapid crystallization rate during solution preparation, resulting in current losses and reduced power conversion efficiency (PCE).^14,15

Numerous techniques have emerged to enhance the performance of MASnI₃-based devices. For instance, adopting a n–i–p device configuration with mesoscopic TiO₂ and Spiro-OMeTAD as the hole transport material has achieved a PCE of 6.4%.¹⁶ In 2019, MASnI₃ achieved a PCE of 7.19% through a normal device by employing cation exchange approaches.¹⁷ The proper selection of electron transport materials (ETM) and hole transport materials (HTM) is crucial for improving the efficiency, reproducibility, and stability of solar cell. However, the use of organic materials such as Spiro-OMeTAD, PATT, and PEDOT:PSS poses challenges due to potential degradation and high costs, hindering the commercialization of perovskites.¹⁸

Inorganic copper-based HTLs, including CuSbS₂ and CuSCN, have been investigated for their suitable band gap and long-term stability.¹⁹ However, the performance of devices using these materials falls short compared to their organic counterparts. To further enhance the efficiency of PSCs with in-organic HTLs, it is crucial to identify other materials with appropriate energy band positions, inherent properties, excellent chemical stability, and high conductivity.²⁰

Copper-based materials possess chemical stability, high conductivity, and hole transport mobility, making them suitable candidates as HTLs in PSCs. Cu₂ZnSnS₄ is an environmentally friendly, nontoxic, and abundant material with a high absorption coefficient of 10⁵ cm⁻¹, an heightened energy band gap of 1.5 eV, and excellent stability, making it viable for use in low price devices. While the maximum achieved PCE of CZTS is 9.6%, the alloying of Cu₂ZnSn(Se_1−xS_x)₄ compound has demonstrated superior performance, reaching 12.6% efficiency with a tunable band gap.^21,22 Notably, CZTSe_1−xS_x has not only been explored as a potential light absorber in thin-film solar cells but has also shown promise as an HTL in PSCs, achieving an efficiency of 22.77%.²³ CZTSe_1−xS_x can be synthesized using a nanoparticle ink method, eliminating the need for high-temperature sulfuration and enabling its use at lower temperatures. A study in 2016 investigated the band level alignment and valence band position influence of CZTS and CZTSe nanoparticles ink as HTL on the performance of PSCs.²⁴

This manuscript introduces the novel concept of utilizing CZTSe_1−xS_x as an HTL in MASnI₃ PSCs through Solar Cell Capacitance Simulator (SCAPS), marking the first time this device modeling approach has been considered. The traditional structure of PSCs is examined and discussed, focusing on the valence band offset between the perovskite light absorber and CZTSe_1−xS_x, as well as the band gap of the HTL controlled through composition engineering of the CZTSe_1−xS_x compound. The effects of these factors on the IV characteristics and efficiency parameters are thoroughly investigated, leading to an optimized power conversion efficiency achieved with a specific S composition. At CZTSe_0.4S_0.6 composition, a notable efficiency of 17.29% is attained. Further optimization of carrier concentration, defect density, and diffusion length of tin-based perovskite in CZTSe_0.4S_0.6 results in an enhanced PCE of 17.34%.

2 Device configuration

The schematic representation of the simulated structure and the energy level alignment of the device structure are illustrated in Fig. 1. The cell configuration comprises a SLG substrate/FTO (used as TCO)/TiO₂ (serving as ETL)/MASnI₃ as the absorber/CZTSe_1−xS_x (employed as HTL)/Ni (functioning as the back contact), as depicted in Fig. 1(a). The energy levels and electron affinities of the main layers are presented in Fig. 1(b).

Fig. 1 (a) Architecture of modelling device (b) energy level alignment.

The simulation of the device is conducted using SCAPS 3.310, a software developed by the Department of Electronics and Information Systems (ELIS) at the University of Gent.²⁵ The simulation is performed under 1 Sun illumination with an incident power density of 1 kW m⁻² at a temperature of 300 K. SCAPS utilizes well-established equations from literature,²⁶ including the continuity equation, Poisson's equation, and electron/hole transport equations, to model various recombination mechanisms for solar cell simulation.

Table 1 presents fundamental parameters obtained from experimental and published data for the HTL, ETL, and FTO, as documented in sources. To represent the optical behavior of each layer accurately, the absorption coefficients for all active layers were incorporated. The absorption coefficient of MASnI₃ was modeled using the Tauc relation with a pre-factor A_α = 10⁵, while the absorption profiles of TiO₂ and CZT(Se_1−xS_x) were extracted from reported experimental literature.^27,30,31 The effective density states for valence and conduction bands is established at 1.8 × 10¹⁹ and 2.2 × 10¹⁸ cm⁻³, respectively, with the exception of the ETL.³² The thermal velocities of holes and electrons are set at 1 × 10⁷ cm s⁻¹. For accurate representation, MASnI₃ is assigned an electron carrier density of 2.0 × 10¹⁶ cm⁻³.

Table 1 The initial input parameters of the simulated device^27–29

Parameters	FTO	TiO2	MASnI3	Spiro-OMeTAD	CZTSe	CZTS
εr	9	10	8.2	3.0	10	10
χ (eV)	4	4	4.17	2.45	4.35	4.5
Eg (eV)	3.5	3.2	1.3	3.0	0.95	1.5
Nc (cm^−3)	2.2 × 10^18	2 × 10^17	1.0 × 10^18	2.2 × 10^18	2.2 × 10^18	2.2 × 10^18
Nv (cm^−3)	1.8 × 10^19	6 × 10^17	1.0 × 10^18	1.8 × 10^19	1.8 × 10^19	1.8 × 10^19
μn (cm^−2 V^−1 s^−1)	20	100	1.6	2 × 10^−4	60	100
μp (cm^−2 V^−1 s^−1)	10	25	1.6	2 × 10^−4	20	25
Na (cm^−3)	0	0	0	2 × 10^18	5 × 10^16	5 × 10^16
Nd (cm^−3)	2 × 10^19	2 × 10^19	2 × 10^16	10	10	10
ve (cm s^−1)	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7
vh (cm s^−1)	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7	1 × 10^7
d (nm)	50	50	450	200	200	200
Nt (cm^−3)	1 × 10^15	1 × 10^14	2.5 × 10^13	1 × 10^14	1 × 10^14	1 × 10^14

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To account for interface recombination, two Interface Defect Layers (IDLs) are introduced between TiO₂/perovskite and MASnI₃/HTL. These IDLs share identical parameters with MASnI₃ for consistency.

To achieve a carrier lifetime of 1 ns in the absorber layer, a defect density of 2.5 × 10¹³ cm⁻³ is presumed, aligning with the theoretical range of 1 ns to 4 ns. Table 2 compiles information on interface defects and absorber defects. Front optical filtration employs Transmission Solar Glass, and the back contact metal work function is set to 5.3 eV.³³

Table 2 The input parameters of defect in MASnI₃ and interface of layers

Parameters	MASnI3	TiO2/IDL	IDL2/HTL
Defect type	Neutral	Neutral	Neutral
Capture cross section for electrons/holes (/cm^2)	2 × 10^−14	1 × 10^−18	1 × 10^−18
	2 × 10^−14	1 × 10^−17	1 × 10^−19
Energetic distribution	Gaussian	Single	Single
Energy level w.r.t Ev (above Ev, eV)	0.65	0.07	0.32
Characteristics energy/eV	0.1	—	—
Total density/cm^−3	Variable	1 × 10^9	1 × 10^9

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The band gap (E_g) and electron affinity (χ) of the CZTSe_1−xS_x HTL are adjusted according to the sulfur content (x). Previous studies indicated that the E_g values for CZTS (x = 1) and CZTSe (x = 0) were 1.5 and 0.95 eV, respectively. The determination of E_g and χ for CZTSe_1−xS_x at various sulfur concentrations (x) is accomplished using equations from the literature.²⁸

3 Results and discussion

The simulation primarily focuses on the tin-based perovskite structure with Spiro-OMeTAD as the Hole Transport Layer (HTL), aiming to compare CZTSe and CZTS, respectively. Fig. 2 illustrates the numerical simulated J–V curve of the reference solar cell. The reference cell, utilizing Spiro-OMeTAD, exhibits a Power Conversion Efficiency of 23.36%, V_oc of 0.93 V, J_sc of 31.60 mA cm⁻², and a FF of 79.99%. These values strongly correlate with reported experimental and theoretical data for high-efficiency PSCs with performance exceeding 20%.^34,35

Fig. 2 The numerical simulated J–V properties of different HTL.

In addition to the reference cell, the study explores CZTSe and CZTS as HTLs, keeping all other parameters constant except for those related to the HTL. The simulated J–V properties with CZTSe HTL show a V_oc of 0.52 V, J_sc of 39.88 mA cm⁻², FF of 66.28%, and an efficiency of 13.95%. Notably, all efficiency parameters in this simulation are lower than those with Spiro-OMeTAD HTL in tin-based perovskite solar cells, yet these results align with previously reported experimental outcomes. The simulations are conducted with a thickness of 200 nm and a total defect density of 5 × 10¹⁶ cm⁻³ for both CZTSe and CZTS.

Our simulation reveals that CZTSe, as an HTL, demonstrates a power conversion efficiency comparable to Spiro-OMeTAD in tin-based PSCs. This similarity is attributed to the approximately aligned band structures of MASnI₃/Spiro-OMeTAD and MASnI₃/CZTSe interfaces.

The CZTSe alloys possess the capability of a tunable energy band gap. Therefore, employing a configuration engineering method allows for the investigation of the CZTSe_1−xS_x compound and the optimization of band alignment at interfaces, consequently influencing PCE in tin-based devices. The changes in performance of the IV parameters with different sulfur concentrations in CZTSe_1−xS_x as the Hole Transport Layer (HTL) is depicted in Fig. 3.

Fig. 3 The variation in the performance of the IV parameters with the different S concentration in CZTSe_1−xS_x as HTL in structure.

Tin-based PSCs with CZTSe as the HTL exhibit relatively lower performance IV parameters. With the addition of a small fraction of S to CZTSe, J_sc experiences a slight decrease, followed by a more substantial decrease. As the S concentration increases up to approximately 0.6, V_oc, J_sc, and η show an increase, while FF initially decreases before starting to increase again, as shown in Fig. 3(c). Beyond x = 0.7, a significant decrease in all parameters is observed. Therefore, optimized performance is observed at x = 0.6.

At the adjusted of the sulfur content, the tin-based PSCs demonstrate a V_oc of 0.79 V, J_sc of 28.20 mA cm⁻², FF of 76.71%, and PCE of 17.29%. This efficiency is in correspondence with Spiro-OMeTAD-based lead-free PSCs.

The S/(S + Se) ratio serves to control the band gap (E_g) and electron affinity (χ) of the CZTSe_1−xS_x Hole Transport Layer (HTL). Through composition engineering, the Conduction Band Offset (CBO) and Valence Band Offset (VBO) at the interface of MASnI₃/CZTSe_1−xS_x can be practically manipulated. The band diagram of MASnI₃ with CZTSe_1−xS_x HTL is presented in Fig. 4(a). To facilitate the hole charge transportation from the tin-based absorber layer to the HTL, the valence energy band level of the HTL is usually slightly lower than the conduction band level of MASnI₃, leading to a reduction in the Built-in Potential (V_bi). It is related that a small negative VBO can result in a low V_oc. CZTSe shows a −0.17 eV VBO and the lowest V_bi (−0.35 V), which corresponds to the smallest V_oc in Fig. 3(a).

Fig. 4 (a) Energy band diagram of tin based PSCs with CZTSe_1−xS_x HTL (b) MASnI₃/HTL valence band offset.

The VBO of CZTSe_1−xS_x consistently shifts to deeper energy levels, resulting in values of VBO and V_bi transitioning from negative to positive with the addition of sulfur concentration in CZTSe, as indicated in Fig. 4b and 5. A positive (+ive) VBO is most favorable for reducing charge carrier recombination.³⁶ The complete valence energy band alignment at the MASnI₃/CZTSe_1−xS_x interface is illustrated in Fig. 4(a and b) with a distinctly vertical transfer. The fluctuation in the valence energy band position of CZTSe_1−xS_x alters the V_bi in the absorber layer.

Fig. 5 The change of VBO and V_bi with different S content in CZTSe_1−xS_x HTL.

The V_bi is defined as and the VBO = χ(CZTSe_1−xS_x) + E_g(CZTSe_1−xS_x) − χ(MASnI₃) − E_g(MASnI₃). The values of VBO and V_bi are attained and depicted in Fig. 5. CZTS shows the maximum VBO (0.53 eV) and V_bi (0.89 V) as the VBO becomes deeper with increasing sulfur content, leading to an increase in V_bi. In our study, tin-based PSCs with CZTSe_0.6S_0.4 or CZTSe_0.4S_0.6 HTLs show a +ive VBO of 0.08 eV and 0.22 eV, respectively. From Fig. 5, it can be inferred that tin-based PSC with CZTSe_0.4S_0.6 HTL demonstrates sufficient V_bi, which significantly participate to the larger V_oc examined in Fig. 3(a).

The overall rate of generation and recombination in tin-based PSCs with CZTSe_1−xS_x is depicted in Fig. 6. With a thickness of 450 nm for MASnI₃, the majority of incident light is absorbed, allowing only a small portion to be transmitted, given its large range of the absorption co-efficient (10⁴–10⁵ cm⁻³). As seen in Fig. 6(a), carrier generation primarily appears in the MASnI₃ layer. The suitable band gap of CZTSe_1−xS_x in the range of 0.95–1.5 eV impacts the generation rate and recombination rate of the charge carriers for the transmitted light.

Fig. 6 The recombination rate and charge carrier generation of the PSCs with CZTSe_1−xS_x HTL.

When CZTSe_1−xS_x is employed as the Hole Transport Layer (HTL), maximum charge carrier generation was examined in CZTSe. The charge carrier generation rate decreases as the sulfur content increases, attributed to the widening band gap. The interface of MASnI₃/CZTSe_1−xS_x, a potential barrier hinders the motion of electrons from the HTL to MASnI₃ due to the Conduction Band Offset (CBO). Simultaneously, the V_bi in the perovskite layer propels photon-generated majority charge carriers toward the CZTSe_1−xS_x, facilitating easy recombination with already available minority carriers in the CZTSe_1−xS_x.³⁷

Hence, the total recombination rate in CZTSe_1−xS_x is noted to be less than Spiro-OMeTAD. For certain concentrations, the recombination rate is particularly small due to the reduced accessibility of the recombined electrons, driven by the high sulfur concentration and an appropriate Valence Band Offset (VBO). Among the CZTSe_1−xS_x compositions, CZTSe_0.4S _0.6 exhibits a slightly lower total recombination rate.

The defect density (N_t) of MASnI₃ is adjusted to 2.5 × 10¹³ cm⁻³, aligning with the carrier diffusion length of 0.9 μm based on earlier numerical simulated investigations of prime structure. To investigate the impact of N_t further, we varied N_t from 10¹²–10¹⁷ cm⁻³ and illustrated the change of I–V curves with N_t in Fig. 7. The cell demonstrates a substantial performance improvement with a decrease in N_t in tin-based perovskite, consistent with simulations of lead-based perovskite.³⁸ A lower N_t results in better photovoltaic performance, achieving V_oc of 0.80 V, J_sc of 29.13 mA cm⁻², FF of 78.31%, and PCE of 18.29%.

Fig. 7 Variation in I–V curves with increasing the values of N_t in CZTSe_0.4S_0.6.

Experimental literature indicates that Sn-based PSCs exhibits favorable charge-transport characteristics. To delve deeper into the effect of N_t on the device, we examine the impact of N_t on the charge carrier diffusion length (L) in Fig. 8, based on and , where μ represents carrier mobility and is used to determine the diffusion length of carriers. Lower N_t values correspond to longer diffusion lengths (L), which contributes to the enhancement of cell performance.

Fig. 8 Variation in device efficiency, with rising diffusion length in MASnI₃/CZTSe_0.4S_0.6.

Considering the influence of N_t and l_n(l_p), the devices performance parameters are optimum when N_t is as low as 2.491 × 10¹³ cm⁻³ (resulting in a l_n(l_p) of 0.9 μm), and the absorber layer thickness is 450 nm. This substantial performance improvement is attributed to the increased l_n(l_p) associated with the reduction in N_t. Table 3 provides a comparative summary of MASnI₃-based perovskite solar cells reported in the literature along with the current simulated device. In this study, a PCE of 18.29% was achieved for the MASnI₃/CuZnSn(Se_1−xS_x) heterostructure, demonstrating the role of optimized absorber thickness, defect passivation, and favorable band alignment through interface engineering. Notably, an experimental absorption model was employed in our simulation, resulting in more realistic and experimentally relevant efficiency values. In contrast, many reported studies rely on idealized absorption assumptions, often overestimating performance. Although some literature devices show higher PCEs (e.g., 25.05% with CuI HTL, 24.28% using BaTiO₃/CuO interfaces), they typically incorporate more complex or less stable components. Our proposed structure offers a cost-effective, stable, and environmentally friendly architecture, providing a promising foundation for experimental fabrication and further optimization of high-efficiency, lead-free perovskite solar cells.

Table 3 Comparative performance of MASnI₃-based perovskite solar cells reported in the literature and the material and interface engineering

Absorber material	Device structure	PCE (%)	References
MASnI3	MASnI3/CuZnSn(Se1−xSx)	18.29	This work
MASnI3	PCBM/MASnI3/CuI	25.05	39
MASnI3	MASnI3/CuSCN	20.17	40
MASnI3	p-P3HT/p-MASnI3	22.46	41
MASnI3	BaTiO3/MASnI3/CuO	24.28	42
MASnI3	BaTiO3/MASnI3/MASnBr3	24.09	42

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4 Conclusion

This study explores a Sn-based PSCs with an inorganic HTL, utilizing the SCAPS simulator. The compound Cu₂ZnSn(Se_1−xS_x)₄ emerges as a promising candidate for the HTL in tin-based PSCs, offering a tunable band gap through changing the S/(S + Se) ratio. This range spans from 0.95 eV for Cu₂ZnSnSe₄ to 1.5 eV for Cu₂ZnSnS₄, achieved through appropriate VBO engineering at the MAPbI₃/CuZnSn(Se_1−xS_x) interface. Achieving a proper VBO at MASnI₃/CuZnSn(Se_1−xS_x) is challenging but crucial for obtaining high-performance PSCs. Optimization of solar cell performance is carried out by adjusting the S concentration, resulting in PCE of 18.29%. Additionally, it is revealed that an appropriate VBO (0.22 eV) is achieved with the CZTSe_0.4S_0.6 hole transport layer, contributing to enhanced PSC performance.

Data availability

Data available on suitable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2025R316), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia for funding this research article.

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