Climate change is widely recognized as one of the most critical challenges facing global society today. To address this issue, international agreements have been established to reduce greenhouse gas emissions, with many countries setting strategic goals for carbon peaking and carbon neutrality to promote sustainable development. Solar energy, as a clean and renewable resource, holds immense potential for growth. Among emerging photovoltaic technologies, organic-inorganic halide perovskite solar cells have shown rapid advancements due to their high efficiency and ease of fabrication. However, lead-based perovskite materials commonly used in these cells pose significant environmental and health risks due to the toxicity of lead, which can harm human kidneys and nervous systems. Studies indicate that lead halide perovskites have more severe effects on plants in nature compared to other lead compounds. Therefore, there is an urgent need to find efficient and less toxic elements to replace lead in perovskite solar cells.
In recent years, extensive research has focused on using environmentally friendly elements such as tin ions (Sn²⁺) from the same group as lead. Tin-based perovskits exhibit suitable band gaps and are abundant in nature, minimizing environmental impact. However, Sn²⁺ in these materials is prone to oxidation to Sn⁴⁺, leading to self-doping issues that degrade performance over time. Recent discoveries suggest that inorganic cesium halides are promising for enhancing the stability of lead-free perovskite solar cells. Among these, Cs₂AgBiBr₆ stands out as a viable structure, where silver (Ag⁺) and bismuth (Bi³⁺) cations replace lead, offering longer carrier lifetimes and improved stability compared to lead-based perovskites. Cs₂AgBiBr₆, as a lead-free perovskite material, demonstrates extended carrier diffusion lengths and high crystal quality. Nonetheless, the power conversion efficiency of Cs₂AgBiBr₆-based perovskite solar cells still lags behind that of lead-based counterparts, necessitating further optimization.
In solar cell architecture, energy level regulation engineering plays a crucial role in enhancing device performance by adjusting transport layer materials to align with the band structure of the perovskite layer. Proper band alignment between transport layers and the perovskite layer can optimize carrier transport pathways, enabling more efficient extraction of charge carriers to the external circuit and thus improving overall device efficiency. Various n-type semiconductor materials (e.g., C₆₀, CdS, SnS₂, IZGO, ZnO) and p-type semiconductor materials (e.g., CuI, Spiro-OMeTAD, SrCu₂O₂, Cu₂O, V₂O₅) have been explored as electron and hole transport layers in perovskite solar cells to boost efficiency and stability. However, current experimental studies on Cs₂AgBiBr₆ perovskite solar cells lack comprehensive theoretical insights into the selection of transport layer materials and their internal influencing factors. Therefore, simulation-based investigations and theoretical analyses of the optoelectronic properties and energy level matching mechanisms in Cs₂AgBiBr₆ perovskite solar cells are essential for advancing high-performance, lead-free perovskite solar cells and facilitating their commercial application.
In this work, we propose a novel lead-free perovskite solar cell with a micro-offset energy level structure based on Cs₂AgBiBr₆. We employ density functional theory (DFT) and one-dimensional solar cell simulation software SCAPS-1D to investigate its optoelectronic characteristics and energy level matching mechanisms. The initial SCAPS-1D model is validated against experimental designs, and we regulate the energy level structure of the perovskite solar cells using ten different transport layer materials. Additionally, we comprehensively analyze the effects of internal factors and operating temperature on device performance. Our results demonstrate that Cs₂AgBiBr₆ exhibits favorable optical and electronic properties. By achieving a conduction band offset of 0.03 eV and a valence band offset of -0.23 eV, the constructed IZGO/Cs₂AgBiBr₆/Cu₂O micro-offset energy level structure significantly enhances the photovoltaic performance of the device. This improvement is attributed to the optimized carrier transport mechanism facilitated by the micro-offset structure, which promotes the movement of photogenerated electron-hole pairs across layers. The optimized perovskite solar cells achieve a power conversion efficiency of 20.89%, a substantial increase from the initial 1.44%. These findings provide new insights for the high-performance development of lead-free perovskite solar cells.
We utilized density functional theory (DFT) to study the electronic and optical properties of Cs₂AgBiBr₆. The calculations were performed using the Vienna Ab initio Simulation Package (VASP) with the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Structural optimization and convergence tests were conducted with a plane-wave cutoff energy of 450 eV and a 2×2×2 k-point grid for sampling the Brillouin zone. Atomic forces and energy changes were optimized until they converged below 2×10⁻⁸ eV/m and 1×10⁻⁵ eV, respectively. The crystal structure of Cs₂AgBiBr₆, as shown in the provided figure, has a space group of Fm-3m (No. 164) with lattice parameters a = b = c = 1.149 nm and angles α = β = γ = 90°.
For device simulation, we used the one-dimensional solar cell simulation software SCAPS-1D, which is widely employed in theoretical studies of thin-film solar cells, including perovskite solar cells. It effectively simulates and analyzes the optoelectronic performance of devices, with output results that align closely with experimental data. The software solves key equations describing the device physics, including the Poisson equation for electrostatic potential and charge density distribution, and the continuity equations for charge carriers. The Poisson equation is given by:
$$ \frac{d^2 \phi(x)}{dx^2} = \frac{q}{\epsilon \epsilon_0} \left[ p(x) – n(x) + N_D – N_A + \rho_p – \rho_n \right] $$
where \( \phi(x) \) is the electric potential, \( x \) is the spatial coordinate, \( q \) is the elementary charge, \( p(x) \) and \( n(x) \) are the hole and electron densities at position \( x \), \( N_D \) and \( N_A \) are the donor and acceptor doping concentrations, \( \epsilon_0 \) and \( \epsilon \) are the permittivity of free space and the semiconductor, respectively, and \( \rho_p \) and \( \rho_n \) are the charge densities for holes and electrons. The continuity equations for electrons and holes are:
$$ \frac{dn}{dt} = \frac{1}{q} \frac{\partial J_n}{\partial x} + (G_n – R_n) $$
$$ \frac{dp}{dt} = \frac{1}{q} \frac{\partial J_p}{\partial x} + (G_p – R_p) $$
where \( n \) and \( p \) are the electron and hole concentrations, \( J_n \) and \( J_p \) are the electron and hole current densities, \( G_n \) and \( G_p \) are the generation rates, and \( R_n \) and \( R_p \) are the recombination rates for electrons and holes, respectively.
The initial parameters for the perovskite solar cell structure are summarized in Table 1. The device configuration consists of indium tin oxide (ITO) as the front electrode, Cs₂AgBiBr₆ as the perovskite absorber layer, with electron transport layers (ETL) and hole transport layers (HTL) on either side. The operating temperature was maintained at 300 K unless otherwise specified, with an incident light intensity of 1000 W/m² under AM1.5G illumination.
| Parameter | ITO | SnO₂ | C₆₀ | CdS | SnS₂ | IZGO | ZnO | Cs₂AgBiBr₆ | P3HT |
|---|---|---|---|---|---|---|---|---|---|
| Thickness (nm) | 200 | 50 | 50 | 50 | 50 | 50 | 50 | 400 | 100 |
| Bandgap (eV) | 3.65 | 4.04 | 1.70 | 2.40 | 1.85 | 3.05 | 3.30 | 1.64 | 2.00 |
| Electron Affinity (eV) | 4.80 | 4.09 | 4.50 | 4.50 | 4.26 | 4.16 | 4.10 | 4.19 | 3.20 |
| Relative Permittivity | 8.9 | 9.0 | 10.0 | 10.0 | 17.7 | 10.0 | 9.0 | 5.8 | 3.0 |
| Conduction Band Density (cm⁻³) | 5.2×10¹⁸ | 2.2×10¹⁸ | 2.2×10¹⁸ | 2.2×10¹⁸ | 7.3×10¹⁸ | 5.0×10¹⁸ | 4.0×10¹⁸ | 1.0×10¹⁶ | 2.5×10¹⁸ |
| Valence Band Density (cm⁻³) | 1.0×10¹⁸ | 1.8×10¹⁹ | 1.8×10¹⁹ | 1.8×10¹⁹ | 1.0×10¹⁹ | 5.0×10¹⁸ | 1.0×10¹⁹ | 1.0×10¹⁶ | 1.8×10¹⁹ |
| Electron Mobility (cm²/V·s) | 10 | 240 | 0.1 | 100 | 50 | 15 | 100 | 11.81 | 1×10⁻⁴ |
| Hole Mobility (cm²/V·s) | 10 | 25 | 0.10 | 25 | 25 | 0.1 | 25 | 0.49 | 1×10⁻⁴ |
| Donor Density (cm⁻³) | 1×10²⁰ | 2×10¹⁹ | 1×10¹⁸ | 1×10¹⁷ | 1×10¹⁸ | 1×10¹⁷ | 1×10¹⁸ | 1×10¹⁹ | 0 |
| Acceptor Density (cm⁻³) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1×10¹⁹ | 2×10¹⁸ |
| Defect Density (cm⁻³) | 1×10¹⁵ | 1×10¹⁵ | 1×10¹⁵ | 1×10¹⁶ | 1×10¹⁵ | 1×10¹⁵ | 1×10¹⁵ | 1×10¹⁵ | 1×10¹⁴ |
To validate our initial model, we simulated the structure ITO/SnO₂/Cs₂AgBiBr₆/P3HT/Au based on experimental reports and compared the results. Accounting for interface defects and resistances, the simulated current-density voltage curves showed good agreement with experimental data, confirming the reliability of our approach. The initial power conversion efficiency was 1.44%, matching the experimental value. We then extended the study to include various ETL and HTL materials to investigate their impact on the energy level structure and performance of perovskite solar cells.
Using DFT, we explored the electronic and optical properties of Cs₂AgBiBr₆. The band structure calculation along the high-symmetry points (Γ-L-W-X-Γ) revealed that Cs₂AgBiBr₆ is an indirect bandgap semiconductor with a bandgap of 1.34 eV, where the conduction band minimum is at the L point and the valence band maximum is at the Γ point. This aligns with previous computational studies. The density of states (DOS) and partial density of states (PDOS) analysis, as shown in Figure 3(b), indicated that the valence band edge is primarily composed of Br 4p and Ag 4d orbitals, while the conduction band edge is dominated by Bi 6p orbitals with contributions from Br 4p orbitals. The Cs atoms showed minimal contribution to the total DOS, suggesting that the bandgap is controlled mainly by Br, Bi, and Ag atoms.
The optical properties were evaluated through the dielectric function and absorption coefficient. The dielectric function \( \epsilon(\omega) \) is expressed as:
$$ \epsilon(\omega) = \epsilon_1(\omega) + i\epsilon_2(\omega) $$
where \( \epsilon_1(\omega) \) and \( \epsilon_2(\omega) \) are the real and imaginary parts, respectively. The absorption coefficient \( \alpha(\omega) \) is related to the dielectric function by:
$$ \alpha(\omega) = \sqrt{2} \omega \sqrt{ -\epsilon_1(\omega) + \sqrt{\epsilon_1^2(\omega) + \epsilon_2^2(\omega)} } $$
The real part of the dielectric function, \( \epsilon_1(\omega) \), reached a static value of 5.17, indicating a high dielectric constant at low frequencies, with a peak of 10.29 at 2.96 eV. The imaginary part, \( \epsilon_2(\omega) \), associated with interband transitions, began to rise rapidly around 1.5 eV, corresponding to electron transitions from the Br 4p and Ag 4d orbitals in the valence band to the Bi 6p and Br 4p orbitals in the conduction band, consistent with the DOS results. The absorption coefficient exhibited significant peaks in the near-UV and visible light ranges (approximately 300–600 nm), demonstrating strong light response capabilities in these regions. These findings confirm that Cs₂AgBiBr₆ possesses excellent optoelectronic properties, making it a promising material for perovskite solar cells.
We investigated the influence of ETL energy levels on the performance of perovskite solar cells. The conduction band offset, defined as the difference between the ETL conduction band and the perovskite layer conduction band, is a key parameter for assessing energy level matching. We tested five ETL materials: C₆₀, CdS, SnS₂, IZGO, and ZnO, in the structure ITO/ETL/Cs₂AgBiBr₆/P3HT/Au. The conduction band offset values were -0.31 eV for C₆₀ and CdS, -0.07 eV for SnS₂, 0.03 eV for IZGO, and 0.09 eV for ZnO. The positive and negative signs indicate the direction of the offset. IZGO, with a conduction band offset of 0.03 eV, showed the best alignment with the Cs₂AgBiBr₆ perovskite layer.
| ETL Material | Voltage (V) | Current Density (A/m²) | Fill Factor (%) | Power Conversion Efficiency (%) |
|---|---|---|---|---|
| C₆₀ | 0.96 | 103.6 | 69.10 | 6.88 |
| CdS | 0.97 | 125.3 | 68.15 | 8.31 |
| SnS₂ | 1.07 | 102.8 | 75.99 | 8.38 |
| IZGO | 1.08 | 128.1 | 69.91 | 9.68 |
| ZnO | 1.08 | 127.2 | 68.28 | 9.39 |
As shown in Table 2, the device with C₆₀ as the ETL exhibited the poorest performance, while IZGO yielded the highest power conversion efficiency of 9.68%. This is attributed to the better energy level matching, which reduces carrier recombination and enhances electron extraction. The current-density voltage curves in Figure 5(a) shifted rightward with decreasing conduction band offset, and the IZGO-based device achieved a maximum voltage of 1.08 V and current density of 128.1 A/m². The external quantum efficiency (EQE) curve for IZGO in Figure 5(b) followed a similar trend.
To understand the underlying mechanisms, we examined the carrier transport and recombination behaviors. The conduction band offset influences the carrier transport mechanism: negative offsets create cliff structures at the interface, while positive offsets form spike structures. For IZGO/Cs₂AgBiBr₆, the minimal offset of 0.03 eV resulted in a micro-offset structure that facilitated carrier transport by reducing interface recombination and backflow, thereby improving the photovoltaic performance of the perovskite solar cell.
Similarly, we studied the effect of HTL energy levels on perovskite solar cell performance. The valence band offset, defined as the difference between the perovskite layer valence band and the HTL valence band, determines the hole transport efficiency. We evaluated five HTL materials: CuI, Spiro-OMeTAD, SrCu₂O₂, Cu₂O, and V₂O₅, in the structure ITO/IZGO/Cs₂AgBiBr₆/HTL/Au. The valence band offset values were -0.63 eV for CuI and Spiro-OMeTAD, -0.33 eV for SrCu₂O₂, -0.23 eV for Cu₂O, and 0.37 eV for V₂O₅.
| HTL Material | Voltage (V) | Current Density (A/m²) | Fill Factor (%) | Power Conversion Efficiency (%) |
|---|---|---|---|---|
| CuI | 1.03 | 128.0 | 68.05 | 8.97 |
| Spiro-OMeTAD | 1.07 | 128.0 | 68.94 | 9.51 |
| SrCu₂O₂ | 1.16 | 128.0 | 76.99 | 11.45 |
| Cu₂O | 1.41 | 128.1 | 80.24 | 14.46 |
| V₂O₅ | 1.01 | 132.8 | 70.34 | 9.46 |
As summarized in Table 3, Cu₂O as the HTL achieved the highest power conversion efficiency of 14.46%, with a voltage of 1.41 V and a fill factor of 80.24%. Although V₂O₅ yielded a higher current density of 132.8 A/m², its voltage was lower at 1.01 V, resulting in a lower efficiency. The superior performance with Cu₂O is due to its minimal valence band offset of -0.23 eV, which forms a micro-offset structure that optimizes hole transport and minimizes recombination. The current-density voltage curves in Figure 6(a) showed significant variations with different HTLs, while the EQE curves in Figure 6(b) were less affected. The valence band structures in Figure 6(c) and (d) illustrate that cliff structures (for CuI, Spiro-OMeTAD, SrCu₂O₂, and Cu₂O) primarily influence voltage, whereas spike structures (for V₂O₅) affect current density. Excessive spikes can increase recombination, explaining the lower fill factor for V₂O₅. Thus, the micro-offset structure with Cu₂O enhances the overall performance of the perovskite solar cell.
We also analyzed the impact of absorber layer thickness and operating temperature on the perovskite solar cell performance. The thickness of the Cs₂AgBiBr₆ layer affects light absorption and carrier generation. We varied the thickness from 200 nm to 1200 nm and observed the changes in photovoltaic parameters. The voltage remained relatively constant at around 1.4 V, while the current density increased significantly from 79.3 A/m² at 200 nm to 195.1 A/m² at 1000 nm, indicating improved light absorption and carrier generation. However, further increases in thickness beyond 1000 nm led to a decrease in fill factor due to extended carrier transport paths and reduced built-in electric field, slowing the growth in power conversion efficiency. Therefore, we recommend keeping the Cs₂AgBiBr₆ thickness below 1000 nm for optimal performance.
The operating temperature is another critical factor for the practical application of perovskite solar cells. We evaluated the device performance at temperatures ranging from 300 K to 400 K under standard illumination conditions. The voltage decreased with increasing temperature, as described by the equations:
$$ V_{OC} = \frac{E_g}{q} – \frac{n k_B T}{q} \ln \left( \frac{I_{SC}}{I_0} \right) $$
$$ E_g = E_{g,0} – \frac{A T^2}{B + T} $$
where \( V_{OC} \) is the open-circuit voltage, \( E_g \) is the bandgap, \( k_B \) is Boltzmann’s constant, \( I_{SC} \) is the short-circuit current, \( I_0 \) is the reverse saturation current, \( E_{g,0} \) is the bandgap at 0 K, and A and B are constants. The bandgap decreases with temperature, leading to a reduction in voltage. Despite this, the power conversion efficiency decreased by only 1% from 300 K to 400 K, demonstrating excellent thermal stability. This is attributed to the improved carrier transport in the micro-offset energy level structure, which maintains high photoconversion efficiency even at elevated temperatures.
In conclusion, Cs₂AgBiBr₆ is a promising lead-free perovskite material for solar cells, with an indirect bandgap of 1.34 eV and strong absorption in the near-UV and visible regions. By optimizing the energy level structure with IZGO as the ETL (conduction band offset of 0.03 eV) and Cu₂O as the HTL (valence band offset of -0.23 eV), we constructed a micro-offset energy level structure that significantly enhances the performance of perovskite solar cells. This structure improves carrier transport mechanisms, reducing recombination and facilitating the movement of photogenerated electron-hole pairs. The optimized perovskite solar cell achieved a power conversion efficiency of 20.89%, a substantial improvement from the initial 1.44%. Additionally, the device exhibited robust thermal stability, with minimal efficiency loss over a temperature range of 300 K to 400 K. These results provide valuable insights for the development of high-performance, lead-free perovskite solar cells, paving the way for their commercial adoption in sustainable energy solutions.
Future work could focus on experimental validation of these simulations, exploring additional transport layer materials, and investigating the long-term stability of Cs₂AgBiBr₆-based perovskite solar cells under various environmental conditions. The integration of advanced computational methods with experimental approaches will further accelerate the progress in lead-free perovskite solar cell technology.