CPM Seminar

Shifting landscapes: energy level alignment and the role of polarization and Hubbard U at organic heterojunctions

Sarah A. Burke

Department of Physics & Astronomy, Department of Chemistry, and Stewart Blusson Quantum Matter Institute
University of British Columbia

Organic semiconductors offer numerous advantages for photovoltaic devices, yet their entry into the clean energy market has been limited to niche applications, largely hampered by efficiencies that have languished behind advances in competing technologies. As excitonic solar cells, the generation of free charges occurs only when the optically generated exciton is separated into free charges, usually at an interface between an acceptor and donor material. Thus, this energetic landscape at interfaces that drives exciton separation and gives rise to competing loss pathways is fundamental to device function: the interface is the device.

In an effort to gain a molecular-scale picture of these energetic landscapes relevant to charge separation at organic heterojunctions, we have applied low-temperature scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and atomic force microscopy (AFM) to determine the local structure and energy level alignment at organic interfaces. By examining a variety of interface types and morphologies, we have found that the single-particle excitation energies relevant for charge separation and transport are dramatically influenced by the local molecular structure and surroundings in most cases. I will describe our work on these different types of junctions and the influence of polarization and on-site interactions on these energetic landscapes.