In a groundbreaking study published in Advanced Materials, Chan and his colleagues—including graduate students Kushal Rijal, Neno Fuller, and Fatimah Rudayni from the University of Kansas’ Department of Physics & Astronomy—along with KU chemistry professor Cindy Berrie have uncovered a microscopic mechanism that partially explains the remarkable performance attained by an NFA.
Kushal Rijal (right) and Neno Fuller (left) performed the TR-TPPE measurement using the ultra-high vacuum photoemission spectroscopy system shown in the picture. Image Credit: The University of Kansas
Solar energy is essential for a sustainable energy future. Traditionally, silicon—the same semiconductor material found in common electrical devices—has been utilized to gather solar energy. However, silicon solar panels have shortcomings. For example, they are costly and challenging to install on curved surfaces.
To address these issues, scientists have created alternative substances for solar energy harvesting. The most promising of them are carbon-based semiconductors, or "organic" semiconductors; they are less expensive, more readily available from Earth, and less harmful to the environment.
They can potentially lower the production cost for solar panels because these materials can be coated on arbitrary surfaces using solution-based methods — just like how we paint a wall. These organic materials can be tuned to absorb light at selected wavelengths, which can be used to create transparent solar panels or panels with different colors. These characteristics make organic solar panels particularly suitable for use in next-generation green and sustainable buildings.
Wai-Lun Chan, Associate Professor, University of Kansas
Organic semiconductors are not yet commonly utilized in commercial solar panels despite being used in the display panels of consumer gadgets like TVs, mobile phones, and virtual reality headsets. A drawback of organic solar cells has been their poor efficiency of light-to-electric conversion, which is just 12%, as opposed to the 25% efficiency of single-crystalline silicon solar cells.
In organic semiconductors, electrons usually bond to their positive counterparts, called "holes," according to Chan. In this sense, electrically neutral quasiparticles known as "excitons" are frequently produced when organic semiconductors absorb light.
However, this paradigm was altered by the recent discovery of a brand-new family of organic semiconductors called non-fullerene acceptors (NFAs). NFA-based organic solar cells have an efficiency that is closer to 20%.
The scientific community is still unsure of the reason for this new class of NFAs' exceptional performance relative to existing organic semiconductors.
Lead author Rijal's data utilizing an experimental method known as "time-resolved two photon photoemission spectroscopy," or TR-TPPE, were crucial to this finding. With the use of this technique, the researchers were able to monitor excited electron energy at a temporal resolution of less than a trillionth of a second, or sub-picosecond.
Chan added, “In these measurements, Kushal (Rijal) observed that some of the optically excited electrons in the NFA can gain energy from the environment instead of losing energy to the environment. This observation is counterintuitive because excited electrons typically lose their energy to the environment like a cup of hot coffee losing its heat to the surrounding.”
The group, which received funding for their research from the Office of Basic Energy Sciences of the US Department of Energy, thinks that this peculiar process takes place on a microscopic level as a result of electrons' quantum nature, which permits one excited electron to appear on several molecules at once. The second rule of thermodynamics, which states that all physical processes result in a rise in total entropy, or what is commonly referred to as "disorder," interacts with this quantum strangeness to generate the odd energy gain process.
In most cases, a hot object transfers heat to its cold surroundings because the heat transfer leads to an increase in the total entropy. But we found for organic molecules arranged in a specific nanoscale structure, the typical direction of the heat flow is reversed for the total entropy to increase. This reversed heat flow allows neutral excitons to gain heat from the environment and dissociates into a pair of positive and negative charges. These free charges can in turn produce electrical current.
Kushal Rijal, Graduate Student, University of Kansas
The research suggests that organic solar cells built using NFAs can reach substantially higher efficiency due to an entropy-driven charge separation process based on their experimental findings.
Rijal concluded, “Understanding the underlying charge separation mechanism will allow researchers to design new nanostructures to take advantage of entropy to direct heat, or energy, flow on the nanoscale. Despite entropy being a well-known concept in physics and chemistry, it’s rarely been actively utilized to improve the performance of energy conversion devices.”
Furthermore, the KU team members think the mechanism they have uncovered in their study can be applied to the development of more effective solar cells, but it can also aid in the design of more effective photocatalysts for the photochemical conversion of carbon dioxide into organic fuels, a process known as solar fuel production.
Journal Reference:
Rijal, K., et. al. (2024) Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction. Advanced Materials. doi:10.1002/adma.202400578