ICFO boosts organic solar cell efficiency by limiting emission angles

Researchers at Barcelona-based photonics research center ICFO have increased the power conversion efficiency of organic solar cells by restricting the angles at which they emit light. The strategy, described in Energy Environmental Science, offers a "promising path to surpass the standard theoretical efficiency limit for single-junction solar cells," says the research team. 

Conventionally, the process of absorption of sunlight by solar cells, is not perfect, and a significant 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. From Earth, the Sun appears very small in the sky, so 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. 

Reducing losses 

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. The team used an inverted organic solar cell based on the PM6:Y6 blend (the donor-acceptor pair of polymer PM6 and the small molecule acceptor Y6 is a material system for high-efficiency single-junction organic solar cells) 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,” said 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, commented, “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.” 

Key is metal

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,” said 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.”