Perovskite Tandem Solar Cells

Perovskite tandem solar cells are a hot topic for solar researchers due to their potential for achieving high efficiencies at lower costs. These cells have garnered significant attention, especially after LONGi Solar set a record efficiency of 33.9%. This achievement has popularized perovskite tandem solar cells and motivated ongoing research.

Perovskite tandem solar cells are a type of tandem solar cell, which uses perovskite materials as one, or both, of the active layers. The bandgap of a perovskite can be easily tuned by changing the perovskite composition, meaning that it can be paired with other solar cells, such as silicon, CIGS, or organic photovoltaics, to make hybrid tandem solar cells. Alternatively, two perovskites with complementary bandgaps can be used in tandem to absorb light from a wider range of the solar spectrum.

What are Tandem Solar Cells?

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Tandem solar cells stack solar cells of different bandgaps to capture and convert a wider range of solar spectrum into electricity. Tandem solar cells surpass the efficiency limit that exists for a single junction solar cell. The efficiency limit of a single junction solar cell exists due to transmission losses, thermalization losses, recombination losses, and parasitic resistance losses. The transmission and thermalization losses are dependent on the bandgap of the material and accounts for the major efficiency losses. These losses can be minimised to certain extent by the tandem and multijunction solar cells.

The tandem solar cells consist of two junctions (2J):

• a wider bandgap junction placed on top which absorbs high energy photons and transmits the lower energy photons.
• a narrower bandgap junction underneath, which absorbs lower energy photons transmitted by the wider bandgap junction.

Multijunction solar cells are triple junction (3J: wide, intermediate, and narrow bandgap) or more. The maximum efficiencies that different multijunction solar cells can attain are listed in the table below .

Solar Cell Type	Theoretical Max. Efficiency
Tandem (2J) Solar Cells	42%
Triple Junction (3J) Solar Cells	49%
Infinite Junction Solar Cells	68%

Such high potential maximum efficiencies are the main motivation behind the tandem and multijunction PV research.

Why Use Perovskites in Tandem Solar Cells

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Perovskites are typically attractive for tandem and multijunction PV for a range of reasons.

• Tuneable Bandgaps

  Silicon boasts a consistent 1.12 eV bandgap. The bandgap of III-V semiconductors are adjustable but this involves intricate alloying or compositional adjustments, which can be costly and complex.

  However, perovskites offer a simpler solution. With perovskites, bandgap tuning, spanning from 1.22 eV to 2.3 eV can be achieved through compositional adjustments of the A cation (Cs, methylammonium, formamidinium, or their mixtures), B metal (Pb, Sn, or their mixtures), and X halide (I, Br, or their mixtures) within the ABX₃ crystal structure.

• Ultrathin:

  Perovskites are direct bandgap materials with high absorption coefficients and hence can be made as thin as 0.3 – 0.5 µm.

• High Specific Power (Lightweight):

  Perovskites solar cells have 5-10x higher power-to-weight ratio than silicon and III-V semiconductors based solar cells. This feature is beneficial for space applications where form factor and weight are critical considerations.

• Potential for low-cost manufacturing:

  Perovskites-based PV devices are made using solution-processable methods.

  Hence the large-scale perovskite solar cells could be cost-effective compared to the dominant silicon PV technology. This also means perovskite materials can be coated onto many surfaces, including other photovoltaic cells.

• Compatibility with different semiconductors:

  Perovskites are versatile in terms of their compatibility with different semiconductors. They can make tandem solar cells with a silicon, organic or CIGS-based bottom sub-cell.

• Compatibility with flexible substrates:

  Perovskites are flexible unlike rigid silicon and III-V semiconductors. They make flexible solar cells possible with their compatibility with flexible substrates like PET (polyethylene terephthalate) and other plastics.

Perovskite Tandem Solar Cell Applications

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• Utility-scale Solar Farms: Perovskite-based tandem and multijunction PV systems offer the promise of generating greater power output at reduced costs, all while being lightweight. This quality makes them an exciting option for replacing traditional silicon solar panels in utility-scale solar energy production.
• BIPV (Building Integrated Photovoltaics): Similarly, perovskites possess appealing attributes such as high efficiency, cost-effectiveness, lightweight construction, and compatibility with various substrates, making them well-suited for integration into building structures for power generation. Oxford PV stands out as a company dedicated to realizing this potential.
• Electric Vehicles (EVs): With the surge in popularity of electric vehicles, perovskite-based tandem and multijunction PV systems emerge as promising renewable energy solutions. Their high efficiency, cost-effectiveness, and adaptability position them as viable options for powering EVs.
• Portable Electronics: Perovskite-based tandem and multijunction solar cells offer significant advantages for portable electronics, including enhanced efficiency, space optimization, versatile integration, and sustainability. These benefits make them a promising technology for the next generation of portable electronic devices. This could lead to advancements in the battery life and sustainability of such devices.
• Space Applications: Currently, III-V semiconductor-based triple junction PV systems dominate the space sector despite their rigidity, heavy weight, manufacturing complexity and expense. Perovskite-based tandem and multijunction PV systems offer a compelling alternative, boasting high specific power, moderate radiation stability, and flexibility at lower costs compared to III-V technologies.

Perovskite Tandem Solar Cell Efficiencies

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The efficiencies mentioned below are mostly based on the Solar cell efficiency tables (Version 63) .

Technology Type	Max Certified Efficiency
Silicon (single junction)	26.7%
Perovskite (single junction)	26.1%
Perovskite-Silicon	33.9%
Perovskite-CIGS	24.2%
Perovskite-Perovskite	29.1%
Perovskite-OPV	23.4%
Perovskite-Perovskite-Silicon	27.1%
Perovskite-Perovskite-Perovskite	25.1%

Some inferences to draw from the above table:

• For their wider acceptance, the 2J and 3J perovskites-based PV devices go beyond the single-junction silicon and perovskite cell efficiencies.
• Perovskite-on-silicon is currently the most successful 2J solar cell technology, surpassing III-V 2J technology and nearing to that of III-V triple junction technologies.
• Perovskite-on-CIGS 2J technology has lagged because of lattice mismatch, parasitic absorption, and other optoelectronic compatibility issues.
• Perovskite-on-perovskite 2J technology are attractive it uses only perovskites. And hence it can be made cost-effective and highly efficient.
• With the success of perovskite-silicon 2J technology, advancements have been made in the perovskite-perovskite-silicon 3J technology.
• All-perovskite triple junction PV is attractive because of its potential to produce cheapest electricity compared to other PV technologies. However, this technology has struggled with the performance.
• Through their study involving optical and electrical simulations, Hörantner et al., in 2017, have suggested realistically feasible PCEs of 33.8%, 36.6%, and 38.8% for all-perovskite tandem, all-perovskite triple junction, and perovskite-silicon tandem cells, respectively. This implies that there is still a long way to fully achieve their full potential in terms of efficiencies.

Perovskite Tandem Solar Cell Issues

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The major problem with the commercialization of perovskite-based tandem and multijunction photovoltaics is their operational stability. The maximum stable operational hours recorded for perovskites is about 10,000 hours (a little more than a year). This is mostly due to their instability when exposed to oxygen, moisture, temperature, and light.

Furthermore, light instability is a major factor in the poor performance of mixed-halide wide bandgap perovskites, which are used as the top sub-cell absorbers. These perovskites (like APbI_1-xBr_x) are typically preferred for their low-cost manufacturing. However, they undergo phase segregation, forming two phases (one iodide-rich and one bromide-rich). This effect is more detrimental in organic-inorganic hybrid perovskites (like MAPbI_1-xBr_x).

Inorganic perovskites (like CsPbI_1-xBr_x) last a bit longer than the hybrid ones but still no longer than an hour. This makes it more challenging to commercialize perovskite tandem and multijunction photovoltaics.

Currently, researchers are trying to minimize this halide segregation using various techniques like compositional engineering. One such attempt by Wang et al., in 2023, has improved the operational stability of CsPbI_1-xBr_x from like an hour to 420 hours by introducing rubidium doping. Further research is being done to improvise the performance and stability issues.

Outlook for Perovskite Tandem Solar Cells

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Perovskites, with their attractive features, are potentially ideal absorber materials for use in tandem and multijunction solar cells. They have the potential to replace silicon solar cells for terrestrial applications and III-V multijunction solar cells for space applications, offering a very low-cost solution for powering both terrestrial and space systems. However, their operational stability remains a significant challenge. Current research is focused on improving this stability to fully harness their vast potential.

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Further Reading and References

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1. The Physics of Solar Cells, Nelson J., J. (2003)
2. Detailed balance limit of efficiency of p-n junction solar cells., Shockley, W., & Queisser, H. J., Journal of Applied Physics (1961)
3. Methylammonium-free wide-bandgap metal halide perovskites for tandem photovoltaics., Ramadan, A. J., Nature Reviews Materials (2023)

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