How Tandem Solar Cells Improve Efficiency
- Spectrum Splitting Across Multiple Bandgaps
Tandem solar cells consist typically of two sub-cells stacked vertically, each optimized to absorb a different portion of the solar spectrum. For example, in perovskite-silicon tandem cells, the top perovskite cell absorbs high-energy (shorter wavelength) photons in the visible range, while the silicon bottom cell absorbs lower-energy (longer wavelength) near-infrared photons that pass through the top cell. This complementary absorption increases the total fraction of sunlight converted into electricity compared to single-junction cells, which can only optimally convert a narrow band of photon energies. - Surpassing the Shockley-Queisser Limit
Single-junction silicon solar cells face a theoretical maximum efficiency (the Shockley-Queisser limit) around 29%, limited by fundamental losses such as thermalization and transmission of photons not absorbed. By using multiple junctions with different bandgaps in tandem, these losses are reduced since each sub-cell converts its targeted wavelength range more efficiently. This allows tandem cells to exceed the single-junction efficiency limit, with recent perovskite-silicon tandems achieving certified efficiencies over 34.6% and theoretical limits up to 47% under standard solar illumination (AM1.5G). - Electrical and Optical Optimization
Different tandem architectures exist:
– Two-terminal tandems electrically connect the cells in series, requiring current matching between sub-cells to avoid recombination losses.
– Four-terminal tandems keep sub-cells electrically isolated, allowing independent optimization and avoiding current matching constraints. This configuration can achieve slightly higher theoretical efficiencies but is more complex and costly.
– Three-terminal tandems offer a middle ground but are less developed. - Advanced Material Engineering
Materials such as perovskite can be tuned chemically to absorb specific wavelength ranges, making them ideal top cells combined with silicon bottom cells. Perovskites are also thin, lightweight, and lower cost, facilitating their integration in tandems without significantly increasing manufacturing complexity or cost. - Reduced Thermal and Optical Losses
By converting photons closer to their optimal bandgap energies, tandem cells reduce energy lost as heat from photon thermalization and minimize transmission losses. This boosts the converted electrical power for the same incident sunlight.
Summary of Impact
| Feature | Single-Junction Cell | Tandem Cell |
|---|---|---|
| Spectrum Utilization | Narrow bandgap range | Broader, split by multiple bandgaps |
| Theoretical Efficiency Limit | ~29% (Silicon Shockley-Queisser limit) | Up to ~47% for two-junction under AM1.5G |
| Typical Record Efficiency (2024) | ~26-27% for advanced Si cells | 34.6% for perovskite-silicon tandem |
| Loss Reduction | Thermalization and transmission losses high | Losses significantly reduced via spectrum splitting |
| Design Complexity | Simple single layer | More complex layering and electrical design |
In essence, tandem solar cell designs improve overall efficiency by harnessing a wider range of the solar spectrum more effectively through multiple, bandgap-tailored sub-cells stacked together, enabling them to surpass the fundamental efficiency limits of single-material, single-junction solar cells. This makes them a promising technology for faster renewable energy adoption and cost reduction in solar power generation.
Original article by NenPower, If reposted, please credit the source: https://nenpower.com/blog/how-does-the-tandem-solar-cell-design-improve-overall-efficiency/
