Oliver L. A. Monti

Honors

• Swiss National Science Foundation Postdoctoral Fellow, 2001-2003
• Greendale Senior Scholar, Merton College, Oxford, 1998-2001
• 3M Non-Tenured Faculty Awardee, 2005-2007

Education and Appointments

• Dipl. Chem. ETH 1997, ETH
  
• D. Phil. Oxon 2001, Oxford
  
• Postdoctoral Fellow 2001-2004, JILA (National Institute of Standards and Technology and University of Colorado, Boulder)
  

Research Interests

• Physical
  
• Instrumentation
  
• Surface Science
  
• Materials Synthesis and Characterization
  
• Polymers
  

Research Summary

Organic Electronics, Nanoscience, Photoelectron Spectroscopy, Single Molecules, Scanning Probe Microscopy, Ultrafast Spectroscopy, Interfacial Processes, Surface Science

Research in my group is focused on obtaining a detailed understanding of interfacial processes in organic electronic devices such as organic photovoltaic cells. Their function is largely controlled by interfacial processes such as exciton dissociation, polaron formation, geminate recombination and carrier transport. Interfaces are intrinsically complex environments, with typically high defect densities and complex structure. This is compounded by the bulk heterojunction architecture, currently the most efficient architecture for organic solar cells, with structure on the micron to nanometer length-scale. Such heterogeneity makes it difficult to extract the underlying physics of charge generation and transport. As a consequence, the development of solar cells with significantly higher power conversion efficiencies presents a major challenge.

Our research seeks to elucidate the chemistry and physics of carriers in organic semiconductors at interfaces on the short length- and time-scales present in organic photovoltaic cells. We use novel forms of optical microscopy in combination with scanning probe methods such as scanning tunneling and atomic force microscopy to study the electronic structure and dynamics in solar cells. Because organic solar cell materials are typically poor fluorophores, we have developed a novel form of microscopy, scanning photoionization microscopy (SPIM). It relies on ionization induced by tunable, ultrashort laser pulses and is a spatially resolved, chemically selective form of photoelectron spectroscopy.

Using this approach, we are pursuing research in several complementary directions:

1. Interface formation and charge transport at organic/metal and organic/organic boundaries for organic solar cells, organic light emitting diodes and thin film transistors. We use spatially resolved photoelectron spectroscopy in conjunction with scanning probe and fluorescence microscopy to study charge generation and transport at such interfaces. The position of frontier orbitals (HOMO and LUMO) in bulk heterojunctions displays signficant disorder, thereby controlling charge generation and transport in solar cells. These effects must be understood in order to generate more efficient device architectures.

2. Exciton dissociation at efficient heterojunctions occurs on fs to ps timescales. We attempt to obtain an understanding of the dynamics of this process at a magnetically aligned bulk heterojunction by following the exciton survival with chemically selective, spatially resolved ultrafast pump-probe spectroscopy. This will allow us to develop a more efficient bulk heterojunction with fast carrier generation without compromising efficient carrier transport.

3. In order to obtain a detailed picture of the exciton dissociation process, we reduce the complex bulk heterojunction typically used in organic solar cells to a model system, where charge transfer occurs from a single molecule via a heteroepitaxially grown single crystalline insulator barrier layer onto a wide bandgap semiconductor under highly defined conditions. Ensemble measurements often obscure some of the most intriguing aspects of such small-scale systems. This requires us to look at individual molecules in order to capture the complete picture. This allows us to investigate effects of surface state densities, defect densities, distance etc. on exciton dissociation efficiencies. These experimental results may also be modelled by high level ab inito calculations.

More generally, we are interested in obtaining a detailed experimental understanding of nanoscale systems. These physical structures fall between the size range of individual atoms and the fully developed condensed phase and are currently the subject of intense interest in chemistry and physics. Nanometer-sized objects display a surprising range of unusual properties that are quite different from the isolated atom or the bulk phase.

Selected Publications

• O. L. A. Monti, L.K. Schirra, M.L. Blumenfeld, B. S. Tackett, "Interfacial Structure and Dynamics in molecular solar cells", Proc. SPIE, 6643, 6643C

• O. L. A. Monti, T.A. Baker, D. J. Nesbitt, "Imaging Nanostructures with Scanning Photoionization Microscopy", J. Chem. Phys. and Virtual J. Nanoscale Science and Technology, 125 (14), 154709

• O. L. A. Monti, J. T. Fourkas, D. J. Nesbitt, "Diffraction-Limited Photogeneration and Characterization of Silver Nanoparticles ", J. Phys. Chem. B, 108 (5), 1604-1612.

• O. L. A. Monti, H. A. Cruse, T. P. Softley, S. R. Mackenzie, "Spatial Discrimination of Rydberg-Tagged Molecular Photofragments in an Inhomogeneous Electric Field", J. Chem. Phys., 115 (17), 7924-7934.