Scientists at the Center for Computational Study of Excited-State Phenomena in Energy Materials (C2SEPEM) at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) in California (US), have discovered a new mechanism that could improve solar cell efficiency through singlet fission.
Singlet fission is the process in which one high-energy exciton quickly splits into two lower-energy excitons, doubling the number of charge carriers and decreasing the energy loss to heat. And exploiting singlet fission is one way to circumvent the Shockley-Queisser limit of 33% solar cell efficiency, which is why researchers are very much interested in optimizing this process in light harvesting materials.
“In our research, we discovered a new pathway for a very fast singlet-fission transition in the pentacene molecular crystal, a well-studied system for this process,” says Steven Louie, director of C2SEPEM. His team uncovered this mechanism using a new predictive theoretical framework they had developed. “Our findings show that there is a strong relation between fission efficiency and crystal symmetry and present a new paradigm to control singlet fission via crystal design and structural manipulations.”
A new singlet fission paradigm
The C2SEPEM team explains that singlet fission has been traditionally studied using methods from quantum chemistry. While these approaches can accurately take into account electron correlation effects, they are typically limited to studying dimers of molecules or small clusters. “On the other hand, our approach is formulated such that it can directly deal with an infinitely extended crystal, revealing several constraints on the singlet-fission mechanism which were previously unknown,” says project co-leader Jeff Neaton. “Indeed, using our new approach, we find that singlet-fission efficiency can be dramatically increased if the sample displays a particular crystal symmetry that is very common in the materials used in the field.”
Another substantial advantage of the C2SEPEM team’s new approach is that it is predictive. Until now, singlet fission has most commonly been investigated using approaches that rely on a series of fitting parameters. “Our approach does not require external parameters and is fully ab initio, meaning it does not depend on fitting parameters or other input from experiments,” points out postdoctoral fellow Sivan Refaely-Abramson, one of the paper’s co-lead authors. “Starting from a set of atomic positions that make up a crystal, we can predict the singlet-fission decay rate for this material through the channel discovered.”
The researchers note that being free from experimental parameters is “of fundamental importance” in predicting new materials with high light-harvesting efficiency.
Designing new light-harvesting materials with efficient singlet-fission rates
“Our ultimate goal is to enable rational design of materials with efficient singlet-fission rates,” says postdoctoral fellow Felipe da Jornada, the paper’s other co-lead author. To achieve this, they believe it is critical to understand the underlying mechanism behind singlet fission well, and that the predictive modeling does not have to rely on many parameters from experiments. “Additionally, by generalizing the framework we recently developed, we believe that not only singlet fission, but also exciton dissociation and carrier extraction will be eventually accessible, so that one may understand the whole picture of light harvesting in complex materials,” Jornada adds.
Project leader Louie interjects: “Apart from these accurate calculations, our work also elucidates the role of crystal symmetry on singlet-fission rates, and provides some simple yet general rules to be kept in mind when developing new materials with high light-harvesting efficiency.”
The predictive computational method developed by the C2SEPEM could help to create more efficient light-harvesting materials in multiple ways, including directly applying the new approach to predict singlet-fission rates for a variety of materials, though the team points out that this is a computationally challenging task, at this point.
“We actually believe that the most impactful and direct outcome of our work is to help guide future experiments by changing the paradigm of what is a good material for singlet fission, which is done by putting emphasis on the crystal symmetry,” says Louie. “We believe that our theory will not only bring a predictive way to compute singlet fission in solids, but will also give a fresh perspective when designing future experiments, for instance, by considering inversion symmetry and number of symmetry-inequivalent molecules in the crystal as relevant degrees of freedom, instead of putting emphasis on traditional parameters such as the angle or overlap between neighboring molecules.”
Overcoming computational challenges
The team points out that although it is well known that the crystalline environment is an integral part of excited state phenomena in energy materials, an ab initio method to compute complex exciton-exciton interactions such as singlet fission in crystals did not exist. “Since these processes are of extremely high experimental interest, we wanted to take the first step in developing new approaches to better understand the involved mechanisms,” says Louie. “So, our first big challenge was to come up with the theoretical methodology.”
However, even after they developed the new formalism and coded their new approach in the BerkeleyGW software package, Louie says the computational cost to apply such robust methods on real materials was still very large. The team overcame this computational hurdle by using the NERSC supercomputers at Berkeley Lab.
Clear selection rules in crystals — the biggest surprise
“Even though we obtained singlet-fission rates without any fitting parameters and which matched remarkably well with experiment, what surprised us most were the clear selection rules we found for this process in crystals,” reveals Louie. “These selection rules were not recognized in previous theories, and hence open the door to a completely new understanding of the reaction mechanism, suggesting many paths for new and exciting research directions.”
In continuing their research, the team’s next steps are going to be two-fold: First, they plan to apply their new formalism and code to a variety of other materials and further make predictions and validations of the approach. Second, they want to generalize their formalism to deal with situations the predictive modeling method cannot currently handle: “For instance, we only considered electron-electron interactions in our theory, which is appropriate when dealing with systems with fast singlet-fission rates,” Neaton says. “However, we did not consider an additional, perhaps slower, decay channel that arises due to the interaction of electrons with lattice vibrations. We also did not consider the case where the electron-electron interaction is so strong that the biexciton binding energy may be large — i.e., when the energy needed to break a composite two-exciton particle is large.”
The research is described in detail in the paper “Origins of Singlet Fission in Solid Pentacene from an ab initio Green's Function Approach,” published in Physical Review Letters.
Written by Sandra Henderson, Research Editor, Solar Novus Today