Physicists at ICFO boost solar cell efficiency by using a silver film to reflect lost light, potentially surpassing a long-standing theoretical limit.
New material boosts solar cell efficiency
At the Institute of Photonic Sciences in Barcelona (ICFO), a team of physicists replaced the transparent electrode of an organic solar cell with a six-nanometer-thick silver film. This partial mirror reflects a portion of the light usually lost by the cell back inside, increasing its efficiency.Light loss and theoretical limits
Under the sun, a solar cell absorbs light, but a small part of this energy is re-emitted as a weak infrared glow. From Earth, the sun occupies a very small portion of the sky, and its rays therefore strike the cell within a very narrow cone. The cell's glow, on the other hand, is emitted in almost all directions of the hemisphere above it.Boltzmann loss and efficiency ceiling
Physicists refer to this imbalance as the Boltzmann loss, which is incorporated into the calculation of the Shockley-Queisser limit. For a single-junction cell, this theoretical efficiency ceiling is set at 33.16%. By narrowing the emission cone of their prototype, the researchers at ICFO have reduced this loss, long considered unavoidable, with the hope of surpassing this limit.Alternative to stacked solar cells
To surpass the Shockley-Queisser limit, manufacturers generally stack two materials, and each captures a different portion of the solar spectrum. The Chinese company Longi raised the record for its silicon-perovskite tandem cells to 35.5% in July. This result was certified by the solar testing laboratory of the Joint Research Centre of the European Commission.Simpler design for higher efficiency
The team led by Jordi Martorell, a professor at ICFO and the Polytechnic University of Catalonia, kept a single active layer, a mixed organic PM6:Y6. On the front of the cell, their researchers replaced the classic indium tin oxide electrode with a six-nanometer-thick silver film, covered with tungsten oxide. Between this film and the back electrode, also made of silver, the active layer is enclosed in an optical cavity.Experimental results and future potential
In their article, the authors also present their method as an alternative to multi-junction cells. The argument is also industrial. "The simplicity of design of the cavity cell introduces no additional manufacturing complexity, unlike many approaches currently explored to surpass the Shockley-Queisser limit," notes Francisco Bernal Texca, first author of the study.Research background and improvements
Francisco Bernal Texca had dedicated a chapter of his thesis, defended at ICFO in November 2024, to increasing the voltage of a cell by restricting its photon emission. In the measurements published on September 3 in the journal Energy & Environmental Science, the efficiency of the best cavity cell is 19.08%, compared to 18.23% for the best reference cell with an indium tin oxide electrode.Theoretical projections and practical gains
The 43% efficiency highlighted by ICFO is a theoretical calculation. According to the researchers, a single-junction cell could slightly exceed this threshold if its emission cone were as narrow as that of solar light, for a material with a band gap of 1.16 electron volts. TechRadar notes that this is an estimate of the method's potential and that the gains observed in the lab are small.Broader applications and conditions
Jordi Martorell nevertheless defends the race for efficiency points. "It is essential to achieve higher efficiencies, as this directly translates into a lower cost per watt and more efficient use of land and materials for the deployment of solar energy," explains the researcher. The physicist adds that "the underlying principle is not specific to organic materials and, in principle, is applicable to all photovoltaic technologies." The study authors, however, set a condition for this extension. The fluorescence responsible for the voltage gain must occur near the interface between the active layer and the charge transport layers.
