An organic solar cell from the Institut de Ciències Fotòniques (ICFO) increased its efficiency by around 0.85% by reducing the light it emits at wide angles. The experimental result is modest; the projection proposed by the team, however, suggests that this strategy could theoretically bring the efficiency of single-junction cells to 43%, above the Shockley-Queisser limit of 33.16%.
The idea is not to capture more light through a new combination of materials, but to better control the light the cell sends back outside. To do this, the researchers replaced the usual front electrode made of indium tin oxide with a thin layer of silver and configured the device as an optical cavity. The structure lets solar radiation in while restricting some of the emission at wide angles.
The work was published in Energy & Environmental Science. The difference between the small measured gain and the theoretical potential is central: the study experimentally demonstrates a way to reduce a specific loss, but it does not present a commercial panel with an efficiency close to 43%.
A loss distinct from heat
In a photovoltaic cell, not all the absorbed energy is converted into electricity. Some is dissipated as heat, and some is lost when the material emits light again. The ICFO team focused on this second pathway, known as Boltzmann loss, which limits the voltage and efficiency the device can achieve.
The problem is related to the difference between the angles at which sunlight arrives and those at which a cell can emit photons. Incoming radiation occupies a relatively narrow angular range; emission covers a much wider one. According to pv magazine Global, this disparity increases the entropy associated with emission and reduces the maximum possible performance.
The ICFO research sought to narrow the emission cone to bring it closer to the absorption cone. The team does not aim to prevent the cell from absorbing sunlight: the goal is to maintain that incoming light while also reducing the number of photons that escape at wide angles. This approach differs from more common attempts to improve efficiency, which focus on thermal losses or materials capable of absorbing different parts of the spectrum.
Silver and an optical cavity in an organic cell
The device studied was an inverted organic solar cell based on a PM6 blend, according to pv magazine Global. Instead of the front electrode made of indium tin oxide, the researchers incorporated a thin layer of silver. Combined with another silver electrode at the back and intermediate layers, it forms an optical cavity.
The front silver serves two functions: it conducts electricity and strongly reflects light trying to escape at wide angles. At the same time, the configuration maintains high transmission in the direction of the incoming sunlight. The design, therefore, is not simply a mirror that blocks radiation: it aims to distinguish between incoming light and light emitted by the device.
Engadget describes the result as a cell that still lets sunlight in but emits less light at wide angles. Efficiency increased by approximately 0.85%. This figure corresponds to the reported experiment and should not be confused with the 43% figure, which is an estimate of what could be achieved if the emission cone were narrowed further.
Francisco Bernal Texca, the study's first author, explained in an ICFO statement: “If the emission cone can be narrowed to match more closely the absorption cone of incoming sunlight, which is exactly what our work demonstrates experimentally, solar cells could, in principle, approach efficiencies close to 43%.”
If the emission cone can be narrowed to match more closely the absorption cone of incoming sunlight, which is exactly what our work demonstrates experimentally, solar cells could, in principle, approach efficiencies close to 43%.
— Dr. Francisco Bernal Texca, first author of the study
From the theoretical limit to today's panels
The 33.16% figure is the Shockley-Queisser theoretical limit cited for a single-junction solar cell. It is not the efficiency of panels installed today: Engadget puts the typical performance of consumer panels at between 20% and 23%. The ICFO projection suggests that controlling emission could open a path to exceeding that conventional theoretical ceiling; it does not mean that residential panels will immediately reach that figure.
The potential scope is broader than the organic cell used in the experiment. Engadget notes that the principle should apply to different types of photovoltaic cells, and pv magazine Global considers that the cavity could be integrated into solar devices with a relatively simple structure, without significantly increasing manufacturing complexity. Extending it to other technologies, however, is a possibility proposed by the researchers, not a result demonstrated for every type of cell.
If a technology of this kind were transferred to commercial products, generating more electricity per unit of area could reduce the amount of material and land needed for a given installed capacity, according to pv magazine Global. Engadget also points to potential benefits for consumer solar panels, including plug-in balcony systems whose adoption is growing in the United States. None of these effects was measured in the ICFO experiment.
A different path from efficiency records
The new approach comes as other teams work on different sources of losses. In July, Chinese company LONGi presented, according to Engadget, a solar cell with an efficiency of 35.5% and said the photovoltaic cell used could theoretically reach 43%. TechRadar describes that result as a perovskite cell stacked on top of a conventional silicon one. These figures relate to different devices and methods, not comparable measurements of the same configuration.
LONGi's strategy is related to tandem cells: stacking materials with different bandgaps makes it possible to capture different energy ranges of light. The ICFO work, by contrast, aims to limit photon emission and reduce Boltzmann losses. TechRadar also mentions singlet fission, an approach that can split high-energy photons; the result cited there is a quantum yield of 130%, not a solar conversion efficiency of 130%.
These distinctions matter because efficiency figures depend on the technology and material used. In the case of ICFO, the observed result was an improvement of around 0.85% in an organic cell; 43% describes the idea's theoretical potential. The next step toward turning that promise into a useful improvement for panels would be to transfer and develop the method in other configurations, something the sources describe as a possibility rather than an available product.
For now, the advance consists of proof that emission can be experimentally narrowed using an optical cavity based on silver electrodes. Commercial panels, which typically deliver between 20% and 23%, remain far from the 43% projection.
Sources: Engadget, ICFO, pv magazine Global, TechRadar
