The causes of a hitherto mysterious process that could enhance the power of solar cells have been explained in a new study led by two St John’s researchers.
The underlying mechanism behind an enigmatic process called “singlet exciton fission”, which could enable the development of significantly more powerful solar cells, has been identified by scientists in a new study.
The process is only known to happen in certain materials, and occurs when they absorb light. As the light particles come into contact with electrons within the material, the electrons are excited by the light, and the resulting “excited state” splits into two.
If singlet exciton fission can be controlled and incorporated into solar cells, it has the potential to double the amount of electrical current produced from highly energetic blue and green light, capturing a great deal of energy that would normally be wasted as heat and significantly enhancing the efficiency of solar cells as a source of green energy. Until now, however, scientists have not really understood what causes the process, and this has limited their ability to integrate it into solar devices.
Writing in the journal Nature Physics, a team of researchers shows that there is an unexpected link between the splitting process and the vibration of the molecule that occurs when light comes into contact with the electrons. This vibration is thought to drive the production of two excited electrons, revealing for the first time how singlet exciton fission happens.
The study was carried out by researchers from the Cavendish Laboratory at the University of Cambridge, and the University of Oxford. As well as solving a hitherto mysterious problem of quantum physics, it potentially provides a basis on which new singlet fission materials could be developed for use in solar cells.
Dr Andrew Musser, a post-doctoral research associate and former PhD student at St John’s College, University of Cambridge, who co-authored the research paper, said: “We tend to characterise singlet exciton fission as a sort of two for the price of one deal on electrons, because you get twice as much electrical current. The problem is that if we want to implement this in a solar cell, the material needs to be engineered so that it is compatible with all the other components in the device. That means that we need to design a range of materials that could be used, and to do that, we need to understand more about why and how singlet exciton fission occurs in the first place.”
“We are fairly confident that this underlies all ultrafast singlet fission,” Dr Akshay Rao, a Research Associate at St John’s College, Cambridge, who led the Cambridge team, said. “The picture that emerges is that when they are excited by light, the intrinsic vibrations drive the development of a new electronic state.”
By understanding the fundamentals of singlet exciton fission, the study opens up the possibility of designing new singlet fission materials that would enable the process to be effectively integrated into a new generation of highly efficient solar cells. Future research is already being planned in which the group will examine the precise vibrational states that are required for singlet exciton fission to happen, which will further add to this knowledge.
The work at Cambridge forms part of a broader initiative to harness high tech knowledge in the physical sciences to tackle global challenges such as climate change and renewable energy. This initiative is backed by the UK Engineering and Physical Sciences Research Council (EPSRC) and the Winton Programme for the Physics of Sustainability.
Publication: Andrew J. Musser, Matz Liebel, Christoph Schnedermann, Torsten Wende, Tom B. Kehoe, Akshay Rao, Philipp Kukura. Evidence for conical intersection dynamics mediating ultrafast singlet exciton fission. Nature Physics, 2015
Story: Scientists move closer to “two for one deal” on solar cell efficiency | St John’s College, University of Cambridge — March 16, 2015