How Bismuth Is Replacing Lead in Perovskite Tech
Imagine a solar cell so efficient it competes with traditional silicon, yet made from a material as non-toxic as a common pharmaceutical ingredient.
This isn't science fiction—it's the promise of CH₃NH₃BiI₃ (methylammonium bismuth iodide), a lead-free perovskite material that is quietly revolutionizing solar energy research. Scientists are using sophisticated computer simulations to design and perfect this eco-friendly alternative, pushing its efficiency to levels once thought impossible without toxic lead. This is the story of how innovation and sustainability are converging to create the next generation of solar technology.
For over a decade, perovskite solar cells have been the rising stars of photovoltaics. Their efficiency at converting sunlight to electricity has skyrocketed from 3.8% to over 25% in just a few years, rivaling traditional silicon cells 1 . However, a dark cloud hangs over this success: the best-performing perovskites contain lead, a cumulative environmental toxin that poses serious risks to ecosystems and human health 3 .
As one research group noted, the "presence of lead (Pb) and its disadvantages" has prompted a global search for safer alternatives 1 . The ideal replacement needed to mimic lead's favorable electronic properties while being abundant, stable, and environmentally benign.
Bismuth, a element with low toxicity commonly used in medicines and cosmetics, sits near lead on the periodic table, sharing similar chemical characteristics 1 9 . This proximity suggested bismuth could potentially replicate lead's photovoltaic magic without the dangerous side effects.
Early theoretical work confirmed that replacing lead with bismuth in the perovskite structure creates a material with a narrower bandgap, enabling it to absorb not just visible light but also infrared wavelengths 9 . This broader absorption spectrum means CH₃NH₃BiI₃ could potentially harvest more energy from the same amount of sunlight.
Research into novel materials like CH₃NH₃BiI3 no longer happens solely in wet labs surrounded by beakers and chemicals. Today, much of the pioneering work occurs in digital laboratories where researchers use advanced simulation software to model and optimize solar cell designs before ever creating a physical prototype.
The most crucial tool in this digital arsenal is SCAPS-1D (Solar Cell Capacitance Simulator One Dimension), a software program that numerically solves fundamental semiconductor equations 1 . By inputting parameters for each layer of a solar cell, researchers can predict how different configurations will perform, saving tremendous time and resources in the development process 1 .
These simulations work by solving three critical equations that govern solar cell behavior:
This mathematical framework allows scientists to virtually test thousands of material combinations and structures, identifying the most promising candidates for real-world fabrication.
Recent research demonstrates the power of this simulation-driven approach. A 2025 study published in Applied Optics set out to design and optimize a high-performance CH₃NH₃BiI₃ solar cell through systematic simulation 2 . The research team employed SCAPS-1D to model a specific device structure and methodically analyze how each component affected overall performance.
The researchers proposed a structured approach to constructing their virtual solar cell:
This carefully orchestrated arrangement of materials created what scientists call a "band alignment" favorable for separating and extracting light-generated charges.
With the basic structure in place, the researchers began a systematic optimization process, tweaking key parameters to maximize performance:
They tested how varying the thickness of the CH₃NH₃BiI₃ layer between 0.1 and 2.0 micrometers affected light absorption and charge collection.
They adjusted the density of charge carriers in the absorber layer to find the optimal balance.
They analyzed performance across a temperature range from 240 to 420 Kelvin (-33°C to 147°C) to understand real-world operational stability 2 .
Each adjustment provided insights into the complex physics governing the solar cell's operation, moving the researchers closer to an optimally designed device.
After meticulous optimization, the simulated CH₃NH₃BiI₃ solar cell achieved extraordinary performance metrics that challenge the dominance of lead-based perovskites 2 .
Power Conversion Efficiency (PCE)
Open-Circuit Voltage (VOC)
Short-Circuit Current Density (JSC)
Fill Factor (FF)
These numbers become even more impressive when compared with other lead-free alternatives. Another study focusing on CH₃NH₃SnI₃ (tin-based) perovskite achieved only 12.37% efficiency despite extensive optimization 4 , while a different configuration of CH₃NH₃BiI₃ with MoS₂ as the hole transport layer reached 26.60% efficiency 1 . The 28.4% result thus represents a significant advancement in lead-free photovoltaic technology.
The researchers discovered that the thickness of the light-absorbing perovskite layer dramatically impacted performance 2 .
| Absorber Thickness (μm) | Efficiency (%) | Observation |
|---|---|---|
| 0.1 | 25.1% | Too thin - insufficient light absorption |
| 0.5 | 27.8% | 接近Optimal - good balance |
| 1.0 | 28.4% | Optimal - maximum efficiency |
| 1.5 | 28.1% | Slight decline - increased recombination |
| 2.0 | 27.5% | Too thick - charge collection difficulties |
This thickness optimization proved critical—too thin a layer couldn't absorb all available sunlight, while too thick a layer caused electrons and holes to recombine before they could be collected as electricity.
The study also provided crucial insights into how these solar cells would perform under different environmental conditions 2 .
| Temperature (K) | Temperature (°C) | Efficiency (%) | Stability Assessment |
|---|---|---|---|
| 240 | -33°C | 29.2% | Best performance but impractical conditions |
| 300 | 27°C | 28.4% | Excellent room-temperature performance |
| 350 | 77°C | 26.1% | Moderate decline - usable range |
| 400 | 127°C | 22.8% | Significant degradation |
| 420 | 147°C | 20.5% | Severe efficiency loss |
This temperature analysis confirmed that while performance peaks at low temperatures, the cell maintains reasonable efficiency up to about 350K (77°C), suggesting practical operational stability under normal conditions.
Creating high-performance perovskite solar cells requires carefully selected materials, each serving a specific function in the energy conversion process.
Function: Light Absorber
Captures photons, generates electrons and holes
Function: Electron Transport Layer
Extracts electrons from perovskite
Function: Hole Transport Layer
Extracts holes from perovskite
Function: Transparent Conductor
Front electrode allowing light entry
Function: Electrodes
Collect charges for external circuit
Each component plays a critical role in the sophisticated choreography of photon absorption and charge extraction that defines an efficient solar cell.
The impressive 28.4% efficiency achieved in simulation represents a milestone, but the journey continues. Researchers are now working to translate these digital achievements into physical devices that can be manufactured at scale. The stability studies showing maintained performance up to 350K (77°C) are particularly encouraging for real-world applications 2 .
Perhaps most exciting is the broader potential of bismuth-based perovskites. As one early theoretical paper suggested, CH₃NH₃BiI₃ "may be one of even more promising alternatives to CH₃NH₃PbI₃ for spectrum-broad and highly-efficient solar cells" 9 . This prediction, made in 2016, is now being validated through sophisticated numerical modeling.
The combination of advanced simulation tools like SCAPS-1D with innovative materials science is creating a virtuous cycle of discovery. Each digital experiment informs the next, accelerating progress toward commercially viable, eco-friendly photovoltaics.
As this research continues, we move closer to a future where clean energy comes not just from the sun, but from solar cells that are truly clean throughout their lifecycle—from manufacturing to disposal.
The rise of bismuth-based perovskites represents more than just a technical achievement; it signals a greener, more sustainable path for solar technology.