Organic solar cells are a promising new technology. Unlike the ubiquitous silicon solar panels, they can be lightweight, flexible and come in many colors, making them particularly attractive for urban or facade applications. However, continued improvement in device performance has been sluggish as researchers work to understand the fundamental processes behind how organic solar cells work.
Now, engineers from Princeton University and King Abdullah University of Science and Technology have described a new way of expressing energy loss in organic solar cells and have extended that description to provide guidance for designing the best devices. This breakthrough could redefine the traditional approach to building organic solar cells. Their work was published on November 18 in Joule.
“There was a way in which energy losses in organic solar cells were traditionally described and quantified. And it turns out that description wasn’t quite right,” said Barry Rand, co-author of the study and associate professor in the Department of Electrical and Computer Engineering and the Andlinger Center for Energy and the Environment.
Rand noted that the traditional method of describing energy loss does not take into account the presence of disorder in an organic solar cell. One type of disorder, dynamic disorder, is caused by the disordered motion of molecules at the microscopic level, resulting in a loss of energy that is virtually unavoidable at most temperatures. Another type, structural or static disorder, is a product of the internal structures of the various materials used in an organic solar cell and their arrangement within the device.
Past studies of organic solar cells that did not account for disorder in energy loss calculations yielded values of about 0.6 electron volts, regardless of device materials. But when Rand and his team incorporated disorder into the way they calculated energy loss and tested different devices, they found that the level of disorder plays an important role in determining the overall energy loss of an organic solar cell.
“As the solar mess increases, we’re seeing our non-radiative component of energy loss — the component we control — grow rapidly,” Rand said. “Nonradiative energy loss increases with the square of the disorder component.”
By demonstrating that increasing disorder causes a dramatic increase in energy loss in devices, the researchers were able to provide recommendations for materials that minimize disorder and thus lead to more efficient devices. Because scientists can choose the materials they use and how they are arranged in an organic solar cell, they have some control over the level of structural disorder in a given device.
In developing an organic solar cell, researchers can focus on creating a homogeneous mixture of materials in which parts of the film are either all crystalline or all amorphous, or a heterogeneous mixture in which some parts of the film are crystalline and other parts are amorphous.
Through their work, Rand’s team demonstrated that when it comes to making organic solar cells, homogeneous mixtures reign supreme. Rand said that to improve the performance of organic solar cells, scientists must use either highly crystalline or highly amorphous materials and avoid mixing them in the device.
“When you have something in between, some inhomogeneity where parts of the film are a little bit crystalline and some parts are amorphous, that’s when you lose the most energy,” Rand said.
This finding breaks convention, as researchers previously believed that some level of heterogeneity in the solar cell mix contributed to overall performance. But because Rand’s team found that heterogeneous device mixtures have a high level of disorder and lose significant amounts of energy, he said their discovery could provide new focus for researchers pursuing more efficient organic solar cells.
“Heterogeneity has often been the focal point of devices. A certain level of crystallinity was considered beneficial. But it turns out that’s not what we saw,” Rand said. He noted that many of the most efficient organic solar cells today are composed of highly amorphous films, and suggested that with existing technologies, fully amorphous compounds are more pragmatic than fully crystalline ones.
Although his team’s research has primarily sought to understand the science behind organic solar cells, Rand hopes others can use their work to create more efficient devices and ultimately achieve new performance figures 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,” Rand said.