In 2025, we face both a climate and energy crisis, with the majority of our energy mix still reliant on fossil fuels. Looking ahead, we must halve emissions by 2030 to prevent catastrophic climate change and reach net-zero emissions by 2050. Since the 1960s, the transition to renewable energy has been steady, yet it remains off track to meet these critical targets. Despite numerous technological breakthroughs, significant challenges persist.
Energy Potential and the Role of Solar Power
The amount of solar energy that reaches the Earth’s surface in just an hour and a half is sufficient to meet global annual energy consumption. Given this, solar technology is a logical choice for renewable energy. Adjusted for inflation, the price per watt of photovoltaic panels has dropped by more than 99% since the 1970s. However, solar power still accounts for less than 6% of global energy production.
The global push for renewable energy has led to substantial advancements in solar technology, which is one of the safest and cleanest energy sources. However, it still faces key challenges. Notably, solar energy has the lowest capacity factor of any major energy source, meaning it is less reliable than fossil fuels or nuclear power. Additionally, solar power is space-inefficient, requiring large areas of land to generate the same amount of energy as other sources.
One of the primary reasons for this inefficiency is the current limitations of traditional silicon-based photovoltaic cells. These cells are constrained by relatively low efficiency and high production costs. This article explores the science and innovation behind perovskite and tandem solar cells - emerging as promising candidates for the next generation of efficient and cost-effective solar technology. We will examine their benefits, challenges, and future implications for the energy industry.
Background: Understanding Solar Cells
To appreciate the impact of perovskite materials and tandem cell design, it is essential to understand how a solar cell functions.
A solar cell generates electricity using sunlight through the photovoltaic effect, the process of converting light into electrical energy. Traditional solar cells are made from stacked layers of n-type and p-type silicon semiconductors. The n-type material is rich in electrons, giving it a negative charge, while the p-type material contains ‘holes’ - vacancies for electrons - which behave as positive charge carriers.
In a standard photovoltaic cell, the junction between the n-type and p-type materials forms a depletion region. When photons of light strike this region, they can generate electron-hole pairs (excitons). The electric field at the junction directs the electrons towards the n-type layer and the holes towards the p-type layer, generating an electric potential. When the cell is connected to an external circuit, electrons flow, creating electricity.
One of the critical factors in solar cell efficiency is ensuring that electrons travel correctly within the cell. Researchers at the University of Rochester found that incorporating a silver mirror layer increased efficiency by 250% by leveraging the Purcell effect, which enhances optical density states.
However, a major limitation of traditional solar cells is that they can only absorb light within a specific wavelength range. Silicon is commonly used because it absorbs light across a broad spectrum (400–800 nm), but any photon energy below this range is not absorbed, and excess energy is lost as heat. This results in significant efficiency losses, with the maximum theoretical efficiency, known as the Shockley-Queisser limit, capped at around 30% for silicon cells. The challenge is surpassing this limit—an area where perovskite and tandem solar cells offer promising solutions.
Overcoming the Shockley-Queisser Limit: Perovskite and Tandem Solar Cells
Perovskites are a class of materials that form a cubic lattice structure. Their composition can be fine-tuned to modify their physical properties, making them highly versatile. Originally developed for high-temperature superconductors, perovskites have been adapted for use in photovoltaics. Unlike silicon, perovskites can be engineered to absorb specific wavelengths of light more effectively, addressing one of silicon’s key limitations.
Despite these advantages, perovskite-only solar cells are still subject to the Shockley-Queisser limit. This is where tandem solar cell design becomes a game-changer.
Tandem solar cells, also known as multi-junction cells, involve stacking multiple layers of solar cells, each designed to absorb different parts of the light spectrum. This design dramatically increases efficiency: moving from a single-layer silicon cell to a two-layer tandem cell raises the theoretical efficiency from 27–29% to approximately 43%.
Perovskite materials are particularly well-suited for tandem solar cells due to their thinness. A perovskite layer is 200 times thinner than a silicon layer of equivalent efficiency, making layering more practical without significant efficiency loss. By carefully selecting materials that absorb different parts of the solar spectrum, tandem cells maximise energy capture, pushing beyond silicon’s efficiency constraints.
Perovskite can be engineered to absorb specific wavelengths of light, which, in theory, means an infinite number of layers could be stacked, each targeting a narrow frequency range to capture the entire light spectrum. However, physical and practical constraints make this unfeasible. In reality, targeting the entire spectrum is unnecessary - very few high-energy photons reach the Earth's surface, while low-energy photons carry minimal energy. As a result, the extreme ends of the spectrum are typically not the focus when designing solar cells.
Additionally, as more layers are added, less light reaches the lower layers, reducing their effectiveness and ultimately counteracting the intended benefit. This imposes a practical limit on the number of layers that can be used.
The most effective approach is to use a minimal number of layers to optimise absorption within the desired spectrum range. One successful design pairs a perovskite top layer with a silicon bottom layer. This combination leverages the mature and widely available silicon solar technology while enhancing its efficiency with a thin perovskite substrate. While even more efficient designs will likely emerge, this tandem structure has already increased commercial solar cell efficiency by 20%, marking a significant step forward in the evolution of solar energy technology.
Below is a graphic illustrating how a perovskite-silicon solar cell extends the coverage of the light spectrum.
By carefully tuning perovskite materials, researchers have developed perovskite-silicon tandem cells, significantly increasing commercial solar panel efficiency. Oxford PV, a leading innovator, has achieved 28% efficiency in laboratory conditions and 24% in commercial units, marking a major step toward practical adoption.
Advantages and Challenges of Perovskite Solar Cells
Advantages
Higher Efficiency: Tandem designs using perovskites surpass traditional silicon efficiency limits.
Lower Material Usage: Perovskite layers are incredibly thin, reducing material costs and enabling flexible solar applications.
Easier Manufacturing: Unlike silicon, which requires high temperatures and energy-intensive purification, perovskites can be processed using low-cost techniques like screen printing, spin coating, and sputtering.
Challenges
Despite their advantages, perovskite solar cells face serious challenges:
Instability: Perovskites degrade when exposed to heat, moisture, oxygen, and UV radiation.
Toxicity: Many perovskite structures contain lead, posing environmental and safety concerns.
High Production Costs: Although easier to manufacture than silicon, tandem solar cells are currently more expensive due to added protective layers needed for real-world durability.
Ongoing research is tackling these issues. Scientists are investigating lead-free perovskite alternatives and protective coatings to enhance long-term stability.
Overall, perovskite and tandem solar cells have immense potential to revolutionise renewable energy. By offering higher efficiency and lower production costs compared to traditional silicon-based cells, they directly address the key challenges of solar energy: efficiency limitations and land use constraints. While stability and environmental concerns remain hurdles, continued advancements in materials science and engineering are bringing perovskite solar cells closer to widespread commercial adoption. With further research and innovation, these next-generation solar technologies could play a crucial role in achieving a sustainable energy future and combating the global climate crisis.