Researchers identify and remove barrier to efficiency of organic solar cells

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Researchers have identified a key mechanism responsible for lowering the efficiency of organic solar cells and have shown a way to overcome this obstacle.

The international group of researchers, led by the University of Cambridge, have identified a loss pathway in organic solar cells that makes them less efficient than silicon-based cells at converting sunlight into electricity. Additionally, they identified a way to suppress this pathway by manipulating molecules inside the solar cell to prevent loss of electrical current through an unwanted state, known as the triplet exciton.

their results, reported in the newspaper Nature, suggest that it might be possible for organic solar cells to compete more closely with silicon-based cells for efficiency.

Organic solar cells, which are flexible, semi-transparent and inexpensive, can greatly expand the range of applications of solar technology. They could be wrapped around the exterior of buildings and can be used for efficient recycling of energy used for interior lighting, which is not possible with conventional silicon panels. They are also much more environmentally friendly to produce.

“Organic solar cells can do a lot of things that inorganic solar cells cannot, but their commercial development has leveled off in recent years, in part because of their lower efficiency,” said the lab’s Dr Alexander Gillett. Cavendish of Cambridge, first author of the article. . “A typical silicon-based solar cell can achieve efficiencies of up to 20-25%, while organic solar cells can achieve efficiencies of around 19% under laboratory conditions and actual efficiencies of around 10%. at 12 %.

Organic solar cells generate electricity by loosely mimicking the natural process of photosynthesis in plants, except that they ultimately use energy from the sun to create electricity rather than converting carbon dioxide and water to glucose. When a light particle, or photon, hits a solar cell, an electron is excited by the light and leaves behind a “hole” in the electronic structure of the material. The combination of this excited electron and this hole is known as the exciton. If the mutual attraction between the negatively charged electron and the positively charged hole in the exciton, similar to the attraction between the positive and negative poles of a magnet, can be overcome, it is possible to harvest these electrons and holes in the form of electric current.

However, electrons in solar cells can be lost through a process called recombination, where electrons lose their energy – or excited state – and fall back into the empty hole state. Since there is a stronger attraction between the electron and the hole in carbon-based materials than in silicon, organic solar cells are more prone to recombination, which in turn affects their efficiency. This requires the use of two components to prevent the electron and the hole from recombining quickly: an electron “donor” material and an electron “acceptor” material.

By combining spectroscopy and computer modeling, the researchers were able to follow the mechanisms at work in organic solar cells, from the absorption of photons to recombination. They discovered that a key loss mechanism in organic solar cells is caused by recombination to a particular type of exciton, known as the triplet exciton.

In organic solar cells, triplet excitons present a difficult problem to overcome, as it is energetically favorable for them to form from electrons and holes. Researchers have found that by creating strong molecular interactions between electron donor and electron acceptor materials, it is possible to keep the electron and hole further apart, thus preventing recombination. in tripled excitons.

Computer modeling suggests that by adjusting the components of organic solar cells in this way, the timescales of recombination to these triple exciton states could be reduced by an order of magnitude, allowing more efficient operation of solar cells.

“The fact that we could use the interactions between components of a solar cell to turn off the triplet exciton loss pathway was really surprising,” said Gillett. “Our method shows how you can manipulate molecules to prevent recombination from occurring. “

“Now, synthetic chemists can design the next generation of donor and acceptor materials with strong molecular interactions to suppress this loss pathway,” said co-author Dr. Thuc-Quyen Nguyen of the University of California at Santa Barbara. “The work shows the way forward to achieve increased device efficiency. “

The researchers say their method provides a clear strategy to obtain organic solar cells with yields of 20% or more by stopping recombination in tripled exciton states. As part of their study, the authors were also able to provide design rules for electron donor and acceptor materials to achieve this goal. They believe the guidelines will allow chemistry groups to design new materials that block recombination into triplet excitons, allowing organic solar cells to be made with efficiencies closer to silicon.

– This press release was originally posted on the University of Cambridge website


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