How a New Kind of Solar Cell Can Boost the Efficiency and Lower the Cost of Solar Power Generation
Prof. Aécio D’Silva, Ph.D
AquaUniversity
Perovskite-Silicon Tandem Solar Cells – Solar energy is one of the most abundant and clean sources of renewable energy, but it still faces some challenges, such as low efficiency, high cost, and intermittency. To overcome these challenges, researchers and companies are developing a new kind of solar cell that combines two different materials: perovskite and silicon. These perovskite-silicon tandem solar cells have the potential to deliver more power, use less space, and reduce the environmental impact of solar power generation.
What are perovskite-silicon tandem solar cells?
Perovskite-silicon tandem solar cells are composed of two layers of photovoltaic materials: a perovskite layer on top of a silicon layer. Perovskites are a class of materials that have a crystal structure similar to that of calcium titanium oxide (CaTiO3), and can be made from cheap and abundant elements, such as lead, iodine, and carbon. Silicon is the most widely used material for solar cells, as it has high stability, durability, and compatibility with existing infrastructure.
The advantage of combining perovskite and silicon is that they can absorb different wavelengths of sunlight more efficiently than either material alone. Perovskites have a wider band gap, which means they can absorb more visible light, while silicon has a narrower band gap, which means it can absorb more infrared light. By stacking them together, the tandem solar cell can harvest more energy from the solar spectrum, and thus increase the power conversion efficiency (PCE), which is the ratio of electrical power output to solar power input.
According to the Shockley-Queisser limit, the theoretical maximum PCE of a single-junction solar cell is 33.7%, but the tandem solar cell can exceed this limit by using two or more junctions. The current record PCE for a perovskite-silicon tandem solar cell is 33.9%, achieved by Oxford PV, a UK-based company that is commercializing this technology. (1). Some researchers project that the PCE of perovskite-silicon tandem solar cells could reach up to 45% in the future. (2).
The Shockley-Queisser limit is the maximum theoretical efficiency of a conventional solar cell using a single p-n junction, where the only loss mechanism is radiative recombination in the solar cell. It was first calculated by William Shockley and Hans-Joachim Queisser in 1961, giving a maximum efficiency of 30% at 1.1 eV. However, more recent calculations give a maximum efficiency of 33.7% at 1.34 eV, using a more realistic solar spectrum and a back surface mirror. The limit is one of the most fundamental and important concepts in solar energy research and development. (3)
What are the benefits of perovskite-silicon tandem solar cells?
Perovskite-silicon tandem solar cells have several benefits over conventional silicon solar cells, such as:
- Higher power density: Since the tandem solar cell can produce more power from the same area, it can reduce the land use and installation cost of solar power generation, especially in crowded urban areas or industrial sites where space is limited.
- Lower levelized cost of electricity (LCOE): LCOE is the average cost of producing one unit of electricity over the lifetime of a power plant, considering the capital, operation, maintenance, and fuel costs. The LCOE of solar power depends on the PCE, the initial investment, and the degradation rate of the solar cell. By increasing the PCE and reducing the degradation rate, the tandem solar cell can lower the LCOE of solar power, and make it more competitive with other energy sources.
- Reduced environmental impact: The tandem solar cell can reduce the environmental impact of solar power generation by using less raw materials, energy, and water, and by emitting fewer greenhouse gases and toxic wastes. For example, perovskites can be synthesized at low temperatures and deposited by solution-based methods, which can save energy and water compared to the high-temperature and vacuum-based methods used for silicon. Moreover, perovskites can be recycled and reused, which can reduce the waste generation and disposal. (4)
What are the challenges of perovskite-silicon tandem solar cells?
Despite the promising potential of perovskite-silicon tandem solar cells, they also face some challenges that need to be addressed before they can be widely adopted, such as:
- Stability: Perovskites are sensitive to moisture, heat, and light, which can cause degradation of their structure, performance, and stability. To prevent this, perovskites need to be protected by encapsulation and passivation layers, which can increase the complexity and cost of the tandem solar cell. Moreover, the mismatch of thermal expansion coefficients between perovskite and silicon can cause mechanical stress and cracks in the tandem solar cell, especially under varying temperature conditions.
- Scalability: Perovskite-silicon tandem solar cells require precise control of the thickness, composition, and alignment of the perovskite and silicon layers, which can be challenging to achieve at large scales and high speeds. Moreover, the integration of the tandem solar cell with the existing silicon solar cell manufacturing processes and equipment can pose technical and economic barriers, such as compatibility, reliability, and standardization issues.
- Toxicity: Perovskites contain lead, which is a toxic and hazardous element that can pose health and environmental risks if it leaks or spills during the production, use, or disposal of the tandem solar cell. To mitigate this, researchers and companies are developing lead-free or low-lead perovskite materials, such as tin-based, germanium-based, or organic-inorganic hybrid perovskites, which can reduce the toxicity and improve the stability of the tandem solar cell. (5)
What are the prospects of perovskite-silicon tandem solar cells?
Perovskite-silicon tandem solar cells are an emerging technology that can boost efficiency and lower the cost of solar power generation, thus contributing to the transition to a sustainable and clean energy future. However, they must also overcome technical and commercial challenges, such as stability, scalability, and toxicity, before they can be widely deployed and accepted. To achieve this, further research and development are needed to improve the understanding of the physics and chemistry of the tandem solar cell, as well as to optimize the design and fabrication of the tandem solar cell. Moreover, collaboration and communication among researchers, industry, policymakers, and consumers are needed to address the regulatory, ethical, and social aspects of the tandem solar cell. (6,)
Perovskite-silicon tandem solar cells are not a miracle material that can change the world overnight, but they are a promising technology that can make a difference in the long run.
References:
(1) https://www.nature.com/articles/d41586-023-03714-y
(2) https://link.springer.com/article/10.1007/s43630-023-00500-7
(3) https://solaredition.com/shockley-queisser-limit-theoretical-maximum-solar-cell-efficiency
(4)https://www.ise.fraunhofer.de/en/press-media/press-releases/2022/advancing-perovskite-silicon-tandem-solar-cell-and-module-technology.html
(5)https://www.ise.fraunhofer.de/en/business-areas/photovoltaics/perovskite-and-organic-photovoltaics/perovskite-silicon-tandem-photovoltaics.html
(6) https://www.sciencedaily.com/releases/2023/08/230822111648.htm
