NERSC Initiative for Scientific Exploration (NISE) 2009 Awards

Understanding organic photovoltaics using ab initio many-body perturbation theory

Peter Doak, Berkeley Lab

Sponsoring NERSC Project: Theory of nanostructured materials (m387), Principal Investigator: Jeffrey Neaton, Berkeley Lab

A fundamental global challenge is to develop a scalable technology, with nontoxic abundant components, for efficiently and inexpensively harvesting solar photon energy and then converting it into convenient forms for storage and transportation. Nanostructure-based organic solar cells are now being explored expressly for this purpose. Fundamentally different from more costly silicon solar cells, these devices rely on a high density of nanoscale interfaces to separate and transport electrons and holes. Despite their considerable promise, there is little microscopic intuition or theory to guide material and device design for such systems. A primary reason is the absence of a quantitative picture of the fundamental non-equilibrium electronic structure underlying key processes in solar energy conversion---absorption, charge separation, charge transport, and charge collection.

In this work, we will use accurate ab initio many-body perturbation theory to predict optical absorption and examine charge separation in a “model” organic solar cell system, donor-acceptor “pn” molecules, to understand the mechanisms of third-generation molecular solar cells. These calculations are part of an effort supported by the Helios Solar Energy Research Center at LBNL, where nanoscale organic components are being explored for artificial photosynthesis.

Using many-body perturbation theory (MBPT) to extend DFT calculations of a donor-acceptor molecule, we have recently been able to calculate all relevant single electron addition and removal energies as well as optical excitations. This has been done using the GW approximation and a Bethe-Salpeter equation approach, respectively. We seek to extend this work beyond a proof of concept to a set of prototypical molecules that, on publishing will provide a useful guide to design for the community. Additionally, we propose to calculate the effects of static electric fields on the dissociation and evolution of the excitonic states in such systems. Using these results, we can address the relationship between donor and acceptor level alignment, exciton binding, and efficient charge separation. These are fundamental questions in organic solar cells and other excitonic photovoltaic systems that have not previously been addressed from first principles. This is a unique application of large-scale DFT and MBPT theory, only possible using parallelization across thousands of cores.

Previously we have been successful in scaling the PWSCF NSCF portion of the calculation to thousands of cores. Recent improvements in the BerkeleyGW MBPT code allow better distribution of wave functions across cores allowing greatly enhanced parallelization. It is now possible to run MBPT calculations on larger systems across thousands of cores. This will allow us to take full advantage of massively parallel computing throughout all stages of the calculation. Good scaling may be possible over greater than 10,000 cores, greatly reducing the wall-time cost of the calculations.

We will be able to explore the important effects of binding groups on the free donor-acceptor molecules. We will be able to examine theoretically for the first time the effect of a static electric field on the dissociation of excitons in a D/A molecule. We will be able to demonstrate extreme scaling of MBPT calculations. This will require significant new calculations to be run on larger numbers of cores than we anticipated would be possible at the beginning of our allocation.