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Performance Optimization of Lead-Free Double Perovskite Solar Cells Employing CsPbI₃ and Cs₂TiBr₆ Absorbers a Numerical Study

This numerical study utilizing SCAPS-1D simulation demonstrates that optimizing device architecture, bandgap engineering, and transport layers can significantly enhance the power conversion efficiency of lead-free CsPbI₃ and Cs₂TiBr₆ perovskite solar cells, with the Cs₂TiBr₆-based configuration achieving a peak efficiency of 22.29%.

Original authors: Md Shawon

Published 2026-06-25

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Md Shawon

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the sun as a giant, endless battery charger, and solar cells as the plugs that try to catch that energy. For a long time, the best plugs have been made with a material called "perovskite." However, the most efficient versions of these plugs contain lead, which is toxic—like having a highly efficient battery that also leaks poison if it breaks.

This research paper is a virtual experiment (a computer simulation) that asks: "Can we build just as good solar plugs using safe, non-toxic materials instead?"

The authors, researchers from Bangladesh, used a digital tool called SCAPS-1D (think of it as a super-advanced video game simulator for solar cells) to test two specific "safe" materials: CsPbI₃ and Cs₂TiBr₆. They didn't build physical cells; they built them inside the computer to see how they would perform under different conditions.

Here is the breakdown of their findings using simple analogies:

1. The Two Contenders: The "Heavy" vs. The "Light"

The researchers tested two different materials as the main "sponge" (absorber layer) that soaks up sunlight:

CsPbI₃: Think of this as a thick, heavy sponge. It needs to be quite thick (about 800 nanometers, which is roughly the width of a human hair divided by 100) to catch all the light.
Cs₂TiBr₆: Think of this as a super-concentrated, lightweight sponge. It is so good at soaking up light that it only needs to be very thin (about 250 nanometers) to do the job.

2. The Setup: Building the Sandwich

A solar cell is like a sandwich. You have the bread (the metal contacts), the filling (the sponge/absorber), and the condiments (the layers that help move the electricity).

The researchers tried different "condiments" (called Electron Transport Layers and Hole Transport Layers) to see which combination made the electricity flow the smoothest.
They found that the best "sandwich" for the CsPbI₃ sponge used a specific mix of Zinc Oxide and Copper Antimony Sulfide.
For the Cs₂TiBr₆ sponge, the best mix involved Zinc Oxide and Copper Oxide (or Platinum in some cases).

3. The Results: Who Won the Race?

The goal was to get the highest "Power Conversion Efficiency" (PCE), which is basically the score on how much sunlight turns into electricity.

The CsPbI₃ (Lead-containing but safer than others) Result:
- With the right thickness and layers, it reached a score of 20.27%.
- The Catch: It needed to be thick, and if it got too hot, its performance dropped.
The Cs₂TiBr₆ (Lead-free and Titanium-based) Result:
- This was the surprise winner! It reached a score of 22.29%.
- The Advantage: It did this while being much thinner and using materials that are not toxic. It proved that you don't need lead to get top-tier performance.

4. The "Goldilocks" Factors

The study tested three main variables, like tuning a radio:

Thickness:
- Analogy: If the sponge is too thin, it misses the rain (sunlight). If it's too thick, the water gets stuck inside and rots (recombination).
- Finding: The "Goldilocks" thickness for the Titanium sponge was 250nm. For the other one, it was 800nm.
Bandgap (The "Color" of Light it Catches):
- Analogy: Imagine the solar cell is a net. If the holes in the net are too big, small fish (light particles) slip through. If they are too small, big fish can't get in.
- Finding: The researchers found the perfect "hole size" (bandgap) for these materials was around 1.8 to 1.85 eV. This allowed them to catch the most energy without losing voltage.
Temperature:
- Analogy: Solar cells are like athletes; they hate running in extreme heat.
- Finding: When the temperature went up (from a comfortable 300K to a hot 380K), the efficiency of all the cells dropped. They work best when they are cool.

5. The Bottom Line

The paper concludes that Cs₂TiBr₆ is a very promising "hero" material. It is:

Non-toxic: No lead, so it's safer for the environment.
Efficient: It beat the other materials in the simulation, reaching over 22% efficiency.
Thin: It works great even when made very thin, which could save money on materials.

Important Note: The authors emphasize that this is purely a computer simulation. They have not built these cells in a lab yet. The results show that these materials should work well, but real-world manufacturing and stability (like how they hold up in rain or humidity) still need to be tested by building physical devices.

In short: The computer says, "If you build a solar cell out of this Titanium material, it should be a champion that is both powerful and safe."

Technical Summary: Performance Optimization of Lead-Free Double Perovskite Solar Cells Employing CsPbI₃ and Cs₂TiBr₆ Absorbers

Problem Statement
While lead-based perovskite solar cells (PSCs) have achieved remarkable power conversion efficiencies (PCEs) approaching commercial silicon, their commercial viability is hindered by the inherent toxicity of lead and stability challenges. Consequently, there is an urgent need for environmentally benign, lead-free alternatives that maintain high photovoltaic performance. This study addresses the optimization of two specific lead-free inorganic perovskite absorbers: Cesium Lead Iodide (CsPbI₃) and Cesium Titanium Bromide (Cs₂TiBr₆). The research focuses on identifying optimal device architectures, absorber thicknesses, bandgaps, and operating temperatures to maximize efficiency while mitigating the trade-offs between light absorption and carrier recombination.

Methodology
The study employs a numerical simulation approach using SCAPS-1D (Solar Cell Capacitance Simulator), a one-dimensional device simulation tool. The methodology involves:

Device Modeling: Solving Poisson's equation and continuity equations for electrons and holes to simulate charge transport, recombination (Shockley-Read-Hall, radiative, and Auger), and generation under standard illumination.
Architectural Variations: Three distinct device structures were modeled for each absorber material, varying the Electron Transport Layers (ETLs) and Hole Transport Layers (HTLs):
- CsPbI₃ Structures: TCO/ZnO/CsPbI₃/Cu₂O/Ag (Str.1), TCO/ZnO/CsPbI₃/CuSbS₂/Ag (Str.2), and TCO/ZnO/CsPbI₃/CuSbS₂/Se (Str.3).
- Cs₂TiBr₆ Structures: TCO/TiO₂/Cs₂TiBr₆/CdTe/Ag (Str.1), TCO/ZnO/Cs₂TiBr₆/Cu₂O/Ag (Str.2), and TCO/ZnO/Cs₂TiBr₆/CuSbS₂/Pt (Str.3).
Parameter Optimization: Systematic variation of absorber thickness (ranging from 50 nm to 1000 nm depending on the material), bandgap (1.4 eV to 1.95 eV), and operating temperature (300 K to 380 K) to evaluate their impact on Open-Circuit Voltage ( $V_{OC}$ ), Short-Circuit Current Density ( $J_{SC}$ ), Fill Factor (FF), and PCE.

Key Contributions and Results

CsPbI₃ Performance Optimization:
- Structure 3 (TCO/ZnO/CsPbI₃/CuSbS₂/Se) demonstrated the highest performance among CsPbI₃ configurations.
- Optimal Thickness: An absorber thickness of 800 nm yielded the maximum PCE of 20.27% (noted as 20.35% in specific result sections) with a bandgap of 1.7 eV.
- Key Metrics: At this optimum, the device achieved a $V_{OC}$ of 1.1357 V, $J_{SC}$ of 21.15 mA/cm², and an FF of 84.71%.
- Temperature Sensitivity: Efficiency degraded significantly as temperature increased from 300 K to 380 K, primarily due to a reduction in $V_{OC}$ .
Cs₂TiBr₆ Performance Optimization:
- Structure 3 (TCO/ZnO/Cs₂TiBr₆/CuSbS₂/Pt) achieved the highest overall efficiency in the study.
- Optimal Thickness: Unlike CsPbI₃, the Cs₂TiBr₆ device performed best at a much thinner absorber layer of 250 nm, achieving a PCE of 22.29%.
- Bandgap Tuning: The highest efficiency for Cs₂TiBr₆ was observed at a bandgap of 1.85 eV, where the device reached a $V_{OC}$ of 1.2465 V, $J_{SC}$ of 19.27 mA/cm², and an FF of 88.61%.
- Comparison: The lead-free Cs₂TiBr₆ device outperformed the CsPbI₃ device, suggesting superior charge extraction capabilities in ultrathin layers due to a high absorption coefficient and reduced recombination sensitivity.
General Trends:
- Thickness Trade-off: Increasing absorber thickness generally increased $J_{SC}$ due to enhanced light absorption but decreased $V_{OC}$ and FF due to increased bulk recombination and shunt resistance.
- Bandgap Trade-off: A clear inverse relationship was observed between bandgap and $J_{SC}$ ; higher bandgaps reduced photon absorption (lowering current) but increased $V_{OC}$ .
- Thermal Stability: All configurations showed significant performance degradation at elevated temperatures (up to 380 K), highlighting the thermal sensitivity of these inorganic perovskite devices.

Significance and Claims
The paper claims that the numerical optimization of CsPbI₃ and Cs₂TiBr₆ absorbers provides a viable pathway toward sustainable, cost-effective, and environmentally friendly photovoltaic technologies. The study concludes that:

Bandgap Engineering is critical, with optimal efficiencies for both materials aligning closely with the Shockley-Queisser limit (near 1.8–1.85 eV).
Thickness Optimization is material-specific; while CsPbI₃ requires a thicker layer (~800 nm) for optimal absorption, Cs₂TiBr₆ achieves superior efficiency in ultrathin layers (250 nm), indicating its potential for reduced material usage and cost.
Lead-Free Viability: The results confirm that lead-free double perovskites, particularly Cs₂TiBr₆, can achieve high efficiencies (exceeding 22%) comparable to or surpassing some lead-based counterparts, offering a promising alternative for next-generation solar cells without the toxicity concerns of lead.

The authors emphasize that these findings serve as design guidelines for experimentalists, suggesting that precise control over transport layers, bandgaps, and absorber thickness is essential for realizing high-performance, lead-free perovskite solar cells.

1. The Two Contenders: The "Heavy" vs. The "Light"

2. The Setup: Building the Sandwich

3. The Results: Who Won the Race?

4. The "Goldilocks" Factors

5. The Bottom Line

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