New insights into energy loss open doors to promising solar technology – ScienceDaily


Organic solar cells are a promising emerging technology. Unlike the ubiquitous silicon solar panels, they have the potential to be lightweight, flexible, and offer a wide variety of colors, making them particularly attractive for urban or facade applications. However, continued progress on device performance has been slow as researchers work to understand the basic processes behind how organic solar cells work.

Now, engineers at Princeton University and the King Abdullah University of Science and Technology have described a new way to express energy loss in organic solar cells and have expanded that description to make recommendations for engineering the best devices. This breakthrough could reimagine the traditional approach to building organic solar cells. Their work was published on November 18 in Joule.

said Barry Rand, study co-author and associate professor of electrical and computer engineering and the Andlinger Center for Energy and the Environment.

Rand pointed out that the traditional way of describing energy loss does not account for a disturbance in an organic solar cell. One type of turbulence, dynamic turbulence, results from the non-uniform motion of molecules at the micro level, which leads to energy losses that are practically inevitable at most temperatures. The other type, structural or static disorder, is the product of the intrinsic structures of the various materials used in the organic solar cell, as well as their arrangement within the device.

Previous research on organic solar cells that did not account for perturbation in energy loss calculations yielded values ​​of approximately 0.6 eV, regardless of device materials. But when Rand and his team incorporated turbulence into their method of calculating energy loss and testing different devices, they found that the level of turbulence plays a significant role in determining the overall energy loss of an organic solar cell.

“As solar cell disruption increases, we see the non-radiative energy loss component — the component we control — grow rapidly,” Rand said. “Non-radiative energy loss grows with the square of the perturbation component.”

After demonstrating that increased disorder sharply increases energy losses in devices, the researchers were able to make recommendations on materials that reduce disorder and thus lead to more efficient devices. Because scientists can choose the materials they use as well as how they are arranged in an organic solar cell, they have some control over the level of structural disorder in a given device.

When engineering an organic solar cell, researchers can focus on creating a homogeneous mixture of materials, in which parts of the film are either crystalline or all of them amorphous, or a heterogeneous mixture, in which some parts of the film are crystalline and other parts are amorphous.

Through their work, the Rand team showed that when it comes to building organic solar cells, homogeneous mixtures prevail. For the best-performing organic solar cells, Rand said scientists should use highly crystalline or highly amorphous materials and avoid mixing the two inside a device.

“If you have anything in between, there’s some heterogeneity where parts of the movie are a little bit crystalline and some parts are amorphous, that’s when you lose the most energy,” said Rand.

This finding goes against convention, as researchers previously believed that some level of heterogeneity in solar cell mixtures was beneficial to overall performance. But because Rand’s team found that mixtures of heterojunctions had high levels of disorder and lost large amounts of energy, he said their discovery could provide a new focus for researchers as they pursue more efficient organic solar cells.

“Inhomogeneities were often the focal point of devices,” Rand said. “Some levels of crystallinity were thought to be beneficial. But it turns out that’s not what we saw.” He noted that many of today’s best-performing organic solar cells consist of highly amorphous films, and suggested that with current technologies, completely amorphous mixtures are more realistic than fully crystalline ones.

Although his team’s research primarily sought to understand the science behind organic solar cells, Rand hopes that others can use their work to build more efficient devices and eventually reach new performance standards for this promising solar technology.

“This discovery is another aspect of organic solar cells that we can add to what we already know, which will help us improve their efficiency in the future,” said Rand.



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