In recent years, perovskite solar cells have emerged as a promising technology for renewable energy due to their high power conversion efficiencies, ease of fabrication, and tunable optoelectronic properties. However, the presence of toxic lead in conventional perovskite materials, such as MAPbI3, poses significant environmental and health concerns, limiting their widespread adoption. Additionally, issues related to stability under operational conditions, such as moisture, heat, and UV exposure, further hinder their commercial viability. To address these challenges, researchers have been exploring lead-free perovskite alternatives, among which double perovskite materials with the general formula A2BX6 have gained considerable attention. These materials, particularly those with cesium as the A-site cation, offer enhanced stability and non-toxicity while maintaining favorable photovoltaic characteristics. In this study, we focus on Cs2PtI6, a lead-free double perovskite that exhibits a suitable bandgap, high absorption coefficient, and excellent stability, making it a compelling candidate for the photoactive layer in perovskite solar cells.
The performance of perovskite solar cells is not solely dependent on the photoactive material; it also relies heavily on the choice of charge transport layers and device architecture. Electron transport layers (ETLs) and hole transport layers (HTLs) play a critical role in facilitating the separation and collection of photogenerated charge carriers, thereby influencing key parameters such as open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and overall power conversion efficiency (PCE). Inorganic materials like SnO2 have been widely used as ETLs due to their high electron mobility and stability. Similarly, inorganic HTLs, such as quantum dot-based materials, offer advantages in terms of cost-effectiveness, tunable bandgaps, and improved charge extraction. In this work, we investigate two novel device structures: FTO/SnO2/Cs2PtI6/CuInS2-QD/Au and FTO/SnO2/Cs2PtI6/GaAs-QD/Au, where CuInS2 quantum dots (CuInS2-QD) and GaAs quantum dots (GaAs-QD) serve as the HTLs, respectively. These structures are designed to leverage the benefits of inorganic transport layers while minimizing the environmental impact associated with lead-based perovskites.
To comprehensively analyze the performance of these proposed perovskite solar cell designs, we employ numerical simulations using the SCAPS (Solar Cell Capacitance Simulator) software, version 3.3.10. SCAPS is a one-dimensional device simulation tool that solves the fundamental semiconductor equations, including the Poisson equation, continuity equations for electrons and holes, and the drift-diffusion model. This approach allows for a detailed investigation of the device physics under various operational conditions without the complexities and costs associated with experimental fabrication. The Poisson equation describes the electrostatic potential distribution within the device and is given by:
$$ \frac{d^2\psi}{dx^2} = \frac{e}{\epsilon_0 \epsilon_r} \left[ p(x) – n(x) + N_D – N_A + \rho_p – \rho_n \right] $$
where ψ is the electrostatic potential, e is the elementary charge, ε0 is the vacuum permittivity, εr is the relative permittivity, n and p are the electron and hole concentrations, ND and NA are the donor and acceptor doping densities, and ρp and ρn represent the hole and electron distributions, respectively. The continuity equations for electrons and holes account for the generation and recombination processes:
$$ \frac{dJ_p}{dx} = \frac{dJ_n}{dx} = G – R $$
where Jp and Jn are the hole and electron current densities, G is the carrier generation rate, and R is the recombination rate. The drift-diffusion equations model the current transport mechanisms:
$$ J_n = \mu_n n \frac{d\phi}{dx} + D_n \frac{dn}{dx} $$
$$ J_p = \mu_p p \frac{d\phi}{dx} + D_p \frac{dp}{dx} $$
Here, μn and μp are the electron and hole mobilities, Dn and Dp are the diffusion coefficients, and φ is the electric potential. These equations are solved numerically under standard test conditions (AM1.5G illumination, 100 mW/cm² incident power density) to obtain the current-voltage (J-V) characteristics, quantum efficiency, and other performance metrics of the perovskite solar cell.
The device structures under investigation are illustrated in the following schematic, which depicts the layered architecture with light incident from the FTO side. The use of quantum dots as HTLs is intended to enhance hole extraction and reduce recombination losses, thereby improving the overall efficiency of the perovskite solar cell.
The material parameters used in the simulations are based on reliable literature sources to ensure the accuracy and relevance of our findings. These parameters include bandgap energy, electron affinity, dielectric constant, carrier mobilities, doping concentrations, and defect densities. The following table summarizes the key material properties for each layer in the perovskite solar cell structures:
| Parameter | SnO2 | Cs2PtI6 | CuInS2-QD | GaAs-QD |
|---|---|---|---|---|
| Thickness (nm) | 10 | 300 | 30 | 30 |
| Bandgap Eg (eV) | 3.50 | 1.37 | 1.60 | 1.43 |
| Electron Affinity χ (eV) | 4.00 | 4.30 | 4.10 | 4.07 |
| Relative Dielectric Constant ε | 9.00 | 4.80 | 9.76 | 12.90 |
| Effective Conduction Band Density Nc (cm⁻³) | 2.2 × 10¹⁷ | 3.0 × 10¹⁴ | 2.0 × 10¹⁹ | 4.7 × 10¹⁷ |
| Effective Valence Band Density Nv (cm⁻³) | 2.2 × 10¹⁶ | 1.0 × 10¹⁷ | 2.0 × 10¹⁸ | 9.0 × 10¹⁸ |
| Electron Mobility μn (cm²/V·s) | 20 | 62.6 | 100 | 8500 |
| Hole Mobility μp (cm²/V·s) | 10.0 | 62.6 | 25.0 | 400.0 |
| Donor Doping Concentration ND (cm⁻³) | 1 × 10¹⁸ | 1 × 10¹² | 0 | 0 |
| Acceptor Doping Concentration NA (cm⁻³) | 0 | 1 × 10¹⁵ | 2.0 × 10¹⁶ | 1.0 × 10¹⁶ |
| Defect Density Nt (cm⁻³) | 1 × 10¹⁵ | 1 × 10¹⁷ | 1 × 10¹⁴ | 1 × 10¹⁴ |
In our simulations, we assume that the electron and hole thermal velocities are set to 10⁷ cm/s for all materials, and the interface defects between layers are initially considered to be minimal unless specified otherwise. The primary goal is to optimize the device performance by systematically varying critical parameters, such as the back electrode material, thickness and defect density of the Cs2PtI6 layer, doping concentration of the HTL, interface defect density between the perovskite and HTL, and operating temperature. Each of these factors is explored in detail to understand their impact on the photovoltaic properties of the perovskite solar cell.
We begin by examining the influence of the back electrode material on the device performance. The work function of the back electrode plays a crucial role in determining the built-in electric field, which affects hole collection and overall charge carrier dynamics. We tested several common electrode materials with varying work functions: Ag (4.74 eV), Fe (4.81 eV), GO (4.90 eV), C (5.00 eV), and Au (5.10 eV). The simulation results indicate that as the work function increases, the open-circuit voltage (Voc) and power conversion efficiency (PCE) improve significantly. This enhancement can be attributed to the increased band bending at the interface, which strengthens the built-in electric field and facilitates more efficient hole extraction. The relationship between the work function and the built-in potential can be described by:
$$ V_{bi} = \frac{1}{e} \left( \phi_{\text{electrode}} – \phi_{\text{HTL}} \right) $$
where Vbi is the built-in potential, φelectrode is the work function of the back electrode, and φHTL is the work function of the hole transport layer. A higher Vbi reduces the recombination losses and improves Voc. For both device structures, Au yielded the best performance, with Voc values of 1.16 V and 1.14 V for the CuInS2-QD and GaAs-QD based perovskite solar cells, respectively. The following table summarizes the output parameters for different back electrode materials in the CuInS2-QD based device:
| Back Electrode | Work Function (eV) | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|---|
| Ag | 4.74 | 0.98 | 31.65 | 70.12 | 21.75 |
| Fe | 4.81 | 1.02 | 31.65 | 72.34 | 23.34 |
| GO | 4.90 | 1.07 | 31.66 | 74.56 | 25.22 |
| C | 5.00 | 1.12 | 31.66 | 76.45 | 27.11 |
| Au | 5.10 | 1.16 | 31.66 | 77.73 | 28.52 |
Similar trends were observed for the GaAs-QD based perovskite solar cell, confirming that Au is the optimal choice for the back electrode in these lead-free perovskite solar cell designs. The improvement in performance with higher work function electrodes is consistent with the principles of semiconductor device physics, where better band alignment minimizes contact resistance and enhances charge collection.
Next, we investigate the effect of the Cs2PtI6 photoactive layer thickness on the perovskite solar cell performance. The thickness of the perovskite layer directly influences light absorption and carrier collection. A thicker layer can absorb more photons, leading to higher photocurrent, but it may also increase recombination losses if the thickness exceeds the carrier diffusion length. We varied the Cs2PtI6 thickness from 100 nm to 1000 nm and monitored the changes in Jsc, Voc, FF, and PCE. The results show that Jsc and Voc increase with thickness up to 800 nm, beyond which they saturate. This saturation occurs because the absorption depth is optimized at around 800 nm, and further increases in thickness do not significantly enhance photon absorption but instead promote recombination due to longer carrier travel paths. The quantum efficiency (QE) spectra confirm this behavior, with higher QE values observed in the visible and near-infrared regions for thicker layers. The fill factor decreases gradually with thickness due to increased series resistance, which can be modeled as:
$$ R_s = \rho \frac{L}{A} $$
where Rs is the series resistance, ρ is the resistivity of the perovskite layer, L is the thickness, and A is the cross-sectional area. The following table illustrates the performance parameters for different Cs2PtI6 thicknesses in the CuInS2-QD based perovskite solar cell:
| Thickness (nm) | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 100 | 0.95 | 28.45 | 80.12 | 21.65 |
| 300 | 1.05 | 30.12 | 78.34 | 24.78 |
| 500 | 1.12 | 31.23 | 77.89 | 27.21 |
| 800 | 1.16 | 31.66 | 77.73 | 28.52 |
| 1000 | 1.16 | 31.67 | 77.12 | 28.48 |
Based on these results, we select 800 nm as the optimal thickness for the Cs2PtI6 layer in both perovskite solar cell structures, as it provides a balance between high absorption and minimal recombination losses.
The defect density in the Cs2PtI6 layer is another critical parameter that affects the performance of perovskite solar cells. Defects act as recombination centers, reducing carrier lifetime and diffusion length. We simulated the devices with defect densities ranging from 10¹³ cm⁻³ to 10¹⁹ cm⁻³. The results indicate that for defect densities below 10¹⁷ cm⁻³, the performance remains relatively stable, but beyond this threshold, Voc, FF, and PCE degrade rapidly. At a defect density of 10¹⁹ cm⁻³, the J-V curves exhibit significant S-shaped characteristics, indicating severe recombination. The Shockley-Read-Hall (SRH) recombination model describes this behavior:
$$ R_{SRH} = \frac{np – n_i^2}{\tau_p (n + n_i) + \tau_n (p + p_i)} $$
where RSRH is the SRH recombination rate, ni is the intrinsic carrier concentration, and τn and τp are the electron and hole lifetimes, respectively. The carrier lifetime is inversely related to the defect density:
$$ \tau = \frac{1}{\sigma v_{th} N_t} $$
where σ is the capture cross-section, vth is the thermal velocity, and Nt is the defect density. As Nt increases, τ decreases, leading to higher recombination and lower efficiency. The following table shows the impact of defect density on the performance of the GaAs-QD based perovskite solar cell:
| Defect Density (cm⁻³) | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 10¹³ | 1.14 | 31.66 | 81.36 | 29.43 |
| 10¹⁵ | 1.14 | 31.66 | 81.35 | 29.42 |
| 10¹⁷ | 1.13 | 31.65 | 80.12 | 28.65 |
| 10¹⁹ | 0.95 | 30.45 | 65.34 | 18.45 |
To achieve high efficiency, it is essential to maintain the defect density in the Cs2PtI6 layer below 10¹⁶ cm⁻³, which can be realized through careful material synthesis and processing techniques.
We also explore the influence of the HTL doping concentration and the interface defect density between the Cs2PtI6 layer and the HTL. The doping concentration in the HTL affects its conductivity and band alignment, while interface defects can lead to increased recombination. We varied the HTL doping concentration from 10¹⁵ cm⁻³ to 10¹⁸ cm⁻³ and the interface defect density from 10¹² cm⁻³ to 10¹⁸ cm⁻³. The results demonstrate that higher doping concentrations in the HTL improve Voc and PCE by enhancing conductivity and reducing series resistance. Conversely, higher interface defect densities degrade performance by promoting non-radiative recombination. The relationship between interface defects and recombination can be expressed as:
$$ R_{\text{interface}} = \frac{1}{2} \sigma v_{th} N_{\text{interface}} (n p – n_i^2) $$
where Rinterface is the interface recombination rate, and Ninterface is the interface defect density. The following table summarizes the PCE values for different HTL doping concentrations and interface defect densities in the CuInS2-QD based perovskite solar cell:
| HTL Doping (cm⁻³) | Interface Defect Density (cm⁻³) | PCE (%) |
|---|---|---|
| 10¹⁵ | 10¹² | 26.45 |
| 10¹⁶ | 10¹² | 27.89 |
| 10¹⁷ | 10¹² | 28.34 |
| 10¹⁸ | 10¹² | 28.52 |
| 10¹⁸ | 10¹⁵ | 25.67 |
| 10¹⁸ | 10¹⁸ | 20.12 |
These findings highlight the importance of optimizing both the HTL doping and interface quality to achieve high-performance perovskite solar cells. We recommend a doping concentration of 10¹⁸ cm⁻³ and an interface defect density of 10¹² cm⁻³ for the best results.
Finally, we examine the effect of operating temperature on the perovskite solar cell performance. Temperature influences carrier mobility, recombination rates, and bandgap energy. We simulated the devices at temperatures ranging from 300 K to 500 K. The results show that Voc and FF decrease with increasing temperature, while Jsc remains relatively stable. The decrease in Voc can be explained by the temperature dependence of the diode equation:
$$ V_{oc} = \frac{n k T}{e} \ln \left( \frac{I_L}{I_0} + 1 \right) $$
where n is the ideality factor, k is Boltzmann’s constant, T is the temperature, IL is the light-generated current, and I0 is the reverse saturation current. As T increases, I0 increases exponentially, leading to a reduction in Voc. Additionally, higher temperatures enhance carrier scattering and recombination, further degrading performance. The following table presents the performance parameters at different temperatures for the GaAs-QD based perovskite solar cell:
| Temperature (K) | Voc (V) | Jsc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| 300 | 1.14 | 31.66 | 81.36 | 29.43 |
| 350 | 1.10 | 31.65 | 79.45 | 27.56 |
| 400 | 1.05 | 31.64 | 76.34 | 25.34 |
| 450 | 0.98 | 31.63 | 72.12 | 22.34 |
| 500 | 0.90 | 31.62 | 68.45 | 19.56 |
These results underscore the need to operate perovskite solar cells at moderate temperatures to maintain high efficiency, which has implications for real-world applications and thermal management strategies.
In conclusion, our numerical study demonstrates that the proposed lead-free double perovskite solar cells based on Cs2PtI6 can achieve high power conversion efficiencies exceeding 28% under optimized conditions. The optimal parameters include a back electrode of Au, a Cs2PtI6 thickness of 800 nm, a defect density in the Cs2PtI6 layer below 10¹⁶ cm⁻³, an HTL doping concentration of 10¹⁸ cm⁻³, an interface defect density of 10¹² cm⁻³, and an operating temperature of 300 K. The FTO/SnO2/Cs2PtI6/CuInS2-QD/Au structure yields a PCE of 28.52%, while the FTO/SnO2/Cs2PtI6/GaAs-QD/Au structure achieves a PCE of 29.43%. These results surpass those of previously reported lead-free perovskite solar cells and highlight the potential of Cs2PtI6 as a viable alternative to lead-based perovskites. Future work should focus on experimental validation and further optimization of interface engineering and material properties to enhance the stability and scalability of these devices. This study provides valuable insights for the development of environmentally friendly and high-performance perovskite solar cells.