Electronic Transfer in Organic Photovoltaic Materials

Seminar series
Physical Chemistry Seminar
Thu, Feb 6 12:00pm
2033 Young Hall
Speaker Christopher Arntsen
University of California, Los Angeles
Dept. of Chemistry and Biochemistry

Abstract: Organic photovoltaic devices (OPVs) pose an inexpensive alternative to traditional inorganic solar cells. OPVs, however, are not yet efficient enough to be economically viable. There are several factors limiting the power conversion efficiency (PCE) of OPVs, though one main bottleneck is the extraction of free electrons. I present a computational study of the electronic coupling of fullerene dimers in order to improve understanding of which types of fullerene molecules make the most efficient OPV devices. One difficulty with performing electronic coupling calculations for isolated dimers is that for asymmetric systems, each fragment in a dimer will see a different chemical environment. This can result in the frontier orbitals involved in electron transfer being misplaced; for example, all frontier orbitals can be localized on the same molecule. The methods presented therefore attempt to mimic the bulk chemical environment. I will present a method which, when calculating the coupling between two fullerenes in a dimer system, applies an electric field in order to delocalize the frontier orbitals across the dimer. This method can be relatively computationally expensive for large systems, as many calculations are required to properly align the energy levels in the dimer. This approach is then modified by applying a potential directly to the Fock matrix. Self-consistent calculation of the direct potential reproduces the results of applying an electric field, but only requires one simulation, greatly improving computational efficiency.  It is found that fullerenes with highly delocalized LUMOs, and particularly LUMOs with spherical symmetry, have the largest intermolecular coupling. The results are compared with experimental mobilities and are found to be consistent.