MIT Department of Chemistry
Cambridge, MA 02139
mit.edu email: firstname.lastname@example.org
Year joined: 2008
|2008||BS in Chemistry and Biochemistry||University of California Santa Barbara||Advisor: Bernard Kirtman|
Improving Organic Photovoltaics Through Quantum Chemistry Modelling
I am currently working on projects involving organic photovoltaics. The four main steps to create electricity from sunlight in an organic photovoltaic are: (i) the sunlight is absorbed in the organic material to form what is called an exciton, (ii) the exciton diffuses to the interface of two different organic materials, (iii) at the organic/organic interface the exciton breaks apart into a charge transfer state, with the electron on the acceptor molecule and the hole on the donor molecule, and (iv) the electron and hole separate apart to the electrodes. The goal of my thesis work is to try and understand these processes and collaborate with experimental groups to create better design principles for organic photovoltaics.
To study the excited state we applied an old computational method to organic molecules used in electronic applications. The method we used is called ΔSCF. For this method we enforce a non-Aufbau occupation of the density during a normal self consistent field optimization in order to converge onto the excited state density (or wavefunction). We found that the ΔSCF method has an accuracy similar to the typically used linear-response time dependent density functional theory (TDDFT). The advantage of the ΔSCF method is that it is a ground state based method, so things like analytical gradients and hessians are already available. This allows us to more readily do dynamics on the electronic excited state. Recently we have been looking at expanding the ΔSCF method to yield more accurate excited states. To do this we take a set of ΔSCF wavefunctions and add MP2 corrections to them in order to create an active space where we can then diagonalize the Hamiltonian. The advantage of this procedure is that both the ground and excited states are all treated with the same level of theory.
Current bilayer organic photovoltaics cannot be made thick enough to absorb all incident solar radiation due to the short diffusion lengths (~10 nm) of singlet excitons. Thus, the diffusion length sets an upper bound on the efficiency of these devices. By contrast, triplet excitons can have very long diffusion lengths (as large as 10 microns) in organic solids, leading some to speculate that triplet excitonic solar cells could be more efficient than their singlet counterparts. We examined the nature of singlet and triplet exciton diffusion. We found that while there are fundamental physical upper bounds on the distance singlet excitons can travel by hopping, there are no corresponding limits on triplet diffusion lengths. This conclusion strongly supports the idea that triplet diffusion should be more controllable than singlet diffusion in organic photovoltaics. To validate our predictions, we modeled triplet diffusion by purely ab initio means in various crystals, achieving good agreement with experimental values. We further showed that in at least one example (tetracene) triplet diffusion is fairly robust to disorder in thin films, due to the formation of semi-crystalline domains and the high internal reorganization energy for triplet hopping. These results support the potential usefulness of triplet excitons in achieving maximum organic photovoltaic device efficiency.
The dissociation of the charge transfer state at the organic/organic interface is not well understood. The charge transfer state has a binding energy 30-40 times greater than the available thermal energy at room temperature, but despite this fact many organic photovoltaic systems display near unity charge separation efficiency at the organic/organic interface. In order to study this system we use a combined molecular mechanics/quantum mechanics (QM/MM) model. We applied the QM/MM model to a number of different organic/organic interfaces and found that the energy levels of the electrons and holes are different at the organic/organic interface. We studied environmental effects such as dielectric miss-match at the organic/organic interface, poor packing at the interface, and molecular multipole moments. All of these different effects can modify the energy of the electron and hole at the organic/organic interface in such a way that they can actually drive the charges away from the interface.
Other projects I have been involved in include collaborative efforts with the Baldo group here at MIT. Some of this work has been done to further model and understand the charge separation process at the organic/organic interface. Other collaborative work is being done on studying the process known as singlet fission. Singlet fission is an electronic process where the singlet excited state on one molecule splits into two triplet excited states on two molecules. This process offers a way of efficiently down-converting high energy photons to twice as many low energy photons, but the mechanism of the process is not understood. We have been working on a combined theoretical and experimental effort to determine the mechanism for singlet fission and understand how one could design new molecules and organic based devices around the process of singlet fission.