Researchers at the Institute of Photonic Sciences (ICFO, Institut de Ciències Fotòniques) in Barcelona have found a way to cut a largely overlooked source of efficiency loss in solar cells, known as Boltzmann losses. They have published their results in the journal Energy & Environmental Science.
What are Boltzmann losses?
Solar cells absorb light and convert it into electricity, but in the process they inevitably emit some light too. The trouble is in the geometry. Seen from Earth, the sun is a small point in the sky, so its light reaches the cell from a narrow range of angles.
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The cell, by contrast, emits light in almost every direction above it, across a cone that covers nearly the entire hemisphere. The gap between the narrow angle at which light arrives and the wide angle at which it leaves costs the cell a portion of its efficiency. These are known as Boltzmann losses, after the Austrian physicist Ludwig Boltzmann, who first linked entropy to the behaviour of particles.
Silver on both sides
To narrow the gap between incoming and outgoing light, the team led by Professor Jordi Martorell (ICFO and the Polytechnic University of Catalonia) took an organic solar cell based on the PM6:Y6 blend and turned it into an optical cavity. A silver electrode on the front of the cell reflects light across a wide range of angles.
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Together with a second silver electrode on the rear, it forms an optical cavity that suppresses light emission at wide angles and in this way narrows the emission cone, while still letting through sunlight arriving from the front. Co-author Francisco Bernal-Texca says: “Our study is a significant step forward in the field of solar energy conversion, as it opens up a completely unexplored route to overcoming the Shockley-Queisser limit.” That limit, namely the theoretical maximum efficiency for a single-junction solar cell, stands at 33.16 percent.
In principle, the gains could be substantial. “If the emission cone can be narrowed enough to better match the absorption cone of the incoming sunlight, which is exactly what our work demonstrates experimentally, solar cells will in principle be able to reach efficiencies close to 43 percent,” Bernal-Texca adds.
Not just for organic cells
Martorell expects the principle to travel well beyond his immediate field of expertise: “Although the route to achieving such gains is demonstrated here on an organic solar cell, the underlying principle is not specific to organic materials and is in principle applicable to all photovoltaic technologies.” Bernal-Texca also points out that “the simplicity of the cavity cell design introduces no additional manufacturing complexity, unlike many of the approaches currently being explored to overcome the Shockley-Queisser limit.”
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For Martorell, the case for higher efficiency is best framed in practical terms: “Achieving higher efficiencies is essential because they translate directly into lower costs per watt and a more efficient use of area and materials in the deployment of solar energy.” (nhp)
