| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014) | |
| Energy Harvesting & Storage | Wednesday Sessions |
| Session EH-WeE |
| Session: | Characterization of Materials for Energy Applications II |
| Presenter: | Alexander Weber-Bargioni, Lawrence Berkeley Lab, USA |
| Authors: | A. Weber-Bargioni, Lawrence Berkeley Lab, USA S.Y. Leblebici, Lawrence Berkeley Lab, USA J. Lee, Lawrence Berkeley Lab, USA M. Melli, Lawrence Berkeley Lab, USA W. Bao, Lawrence Berkeley Lab, USA K. Munechika, Lawrence Berkeley Lab, USA S. Barja, Lawrence Berkeley Lab, USA S. Aloni, Lawrence Berkeley Lab, USA F. Intonti, European Laboratory for Non Linear Spectroscopy D.F. Ogletree, Lawrence Berkeley Lab, USA |
| Correspondent: | Click to Email |
Here we present unprecedented insight into the local exciton transport through organic and inorganic semiconducting nano building block assemblies using state of the art near field optics, hyperspectral mapping, and Field Effect Transistors to control the exciton transport electronically.
Controlling individual excitons and their deliberate movement through a material will provide the access to a new parameter space for the development of next generation light harvesting materials. Nano materials have in principle the potential to realize this vision due to their tuneability. However, the lack of spatial resolution has so far prevented the insight needed to control the transport of optically excited electronic states at their native length scale.
To study the local exciton transport we use optical antennae to locally excite our sample optically and map spatially independent the energy flow by detecting either the local photo luminescence or the local photo current. We use this approach to study exciton transport through three model systems: Inorganic nano wires, 2-D assemblies of inorganic nano crystals, and through organic PV materials.
In InP nanowire system we demonstrate that the transport id mediated by locally enhanced exciton recombination velocity due to charge puddles on nanowire surfaces. CdSe Quantum Dot assemblies are another excellent absorber material system for light harvesting purposes. We determined exciton transport length through well ordered 2-D films of CdSe Nano Crystals of 80 nm and 120 nm for the 1-D case, mediated by Foerster Resonance Energy Transfer (FRET). To develop a better understanding of FRET between quantum dots (which is still not really understood) we used a graphene Field Effect Transistor to study FRET between individual quantum dots and graphene. In this device we can systematically tune with high precision the distance between graphene and quantum dot and the electronic structure of the exciton adsorber (graphene), while building the currently smallest light switch in the world.
Exciton diffusion is also a key hurdle for the systematic development of Organic Photo Voltaic. We used our techniques to directly measure the exciton diffusion length in polymer (P3HT) and small molecule (rubrene) organic photo voltaic materials and show a crystallinity dependent exciton diffusion length that correlates to the OPV dedvice power conversion efficiency. Furthermore we have evidence that local electric field gradient can modify the exciton diffusion length in organic semiconductors, where the exciton binding energy is large (1 eV) and the transport is mainly mediated by tunneling processes.