Solar cells gain efficiency by emitting light in fewer directions

Solar cells absorb sunlight and convert it into electricity. This process, however, is not perfect, and a large part of the solar energy is simply lost along the way. Some of it is lost as heat, while photons with insufficient energy pass through the cell without generating electricity. These are the loss sources that most scientists in the field try to minimize, but there is another important one known as Boltzmann loss.

Boltzmann losses arise because materials that absorb light can also emit light, and in a solar cell this emission is an unavoidable part of the energy-conversion process. The problem with solar cells lies in how this occurs. Because the Sun appears very small in the sky, sunlight reaches the cell within a narrow cone of directions. The solar cell, by contrast, emits in all directions, over a broad cone that practically covers the full hemisphere above it. This mismatch between the narrow cone of incoming light and the broad cone of emitted light translates into a reduction in the useful work that can be extracted from the device.

ICFO researchers, Dr. Francisco Bernal Texca, Chiara Cortese, Dr. Mariia Kramarenko, and Dr. Quan Liu, led by ICFO and UPC Professor Jordi Martorell, have now found a way to reduce Boltzmann losses experimentally. In the study, recently published in Energy & Environmental Science, the team used an inverted organic solar cell based on the PM6:Y6 blend and, by converting the solar cell into an optical cavity, they inhibited photon emission at large angles, thereby narrowing the emission cone.

“Our study is a significant step forward in the solar energy conversion field as it opens a completely unexplored route to surpass the Shockley–Queisser limit,” says Dr. Francisco Bernal Texca, first author of the article. This efficiency limit, which accounts for the intrinsic loss mechanisms considered in the Shockley–Queisser model, is theoretically set at 33.16%. “If the emission cone can be narrowed to more closely match the absorption cone of incoming sunlight, which is exactly what our work demonstrates experimentally, solar cells will, in principle, be able to approach efficiencies close to 43%.” Professor Jordi Martorell, lead researcher of the study, adds that “although the path towards achieving such gains is demonstrated here in an organic solar cell, the underlying principle is not specific to organic materials and is, in principle, applicable across photovoltaic technologies.”

The key to narrowing the emission cone is the use of a metal as the front electrode, enabling high reflectivity over a wide angular range for the emitted light. A layered structure surrounds this silver-based electrode to form, together with the silver electrode at the back of the cell, an optical cavity. This makes it possible to combine broad-angle reflectivity, which effectively narrows the emission cone, with high transmission in the direction of the incoming sunlight. Moreover, “the simplicity of the cavity cell design does not introduce any additional fabrication complexity, unlike many of the approaches currently being explored to surpass the Shockley–Queisser limit,” explains Bernal Texca.

Overall, the framework opens a general route for improving photovoltaic efficiencies, pushing them closer to their theoretical limits. According to Prof. Martorell: “Achieving higher efficiencies is essential, because they directly translate into a lower cost per watt and more efficient use of land and materials for solar energy deployment.”

Reference:

Francisco Bernal-Texca, Chiara Cortese, Mariia Kramarenko, Quan Liu, Jordi Martorell; Inhibited photon emission to overcome Boltzmann losses in an organic solar cell with a cavity configuration. Energy Environ. Sci. 2026.

DOI: https://doi.org/10.1039/d6ee03772k

Acknowledgements:

The authors acknowledge the financial support by the European Commission through the SOREC2 project (101084326). Additionally, the authors acknowledge the financial support from the Ministerio de Ciencia, Innovacion y Universidades through the CROPS project (grant no. PID2024-161397OB-I00). Also, the work was partially funded by Ministerio de Ciencia e Innovación (grants nos. CEX2019-000910-S and PID2020-112650RB-I00), Fundació Cellex, Fundació Mir-Puig, and Generalitat de Catalunya through Centres de Recerca de Catalunya.