Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014)
    Energy Harvesting & Storage Monday Sessions
       Session EH-MoM

Invited Paper EH-MoM8
Charge Transport in PbS Quantum Dot Arrays for Photovoltaic Applications

Monday, December 8, 2014, 11:00 am, Room Lehua

Session: Nano-based Approaches for Photovoltaics
Presenter: Miquel B. Salmeron, Lawrence Berkeley Lab, USA
Correspondent: Click to Email
New forms of matter can be designed by combining nanoscale building blocks to create artificial solids with novel functionalities for device applications, such as field effect transistors, solar cells, photodetectors, and light emitting diodes. Artificial atoms or quantum dots (QDs) are ideal elements for this purpose since their electronic properties can be tuned through control of their size, shape and composition, while charge transport through QD assemblies can be engineered by controlling the hopping barrier between neighboring dots. However, fabrication of the QDs comprised of hundreds to thousands of atoms is inevitably imperfect and defects and impurities are always present. The nature of the defect states is unknown and the microscopic mechanisms of charge transport are far from being understood. In this presentation we show, by means of nanoscale imaging, spectroscopy, and density functional theory (DFT), that charge transport takes place in PbS QD arrays following percolation pathways that are spatially different for electrons and holes, electrons via in-gap states (IGS) instead of conduction band states, and holes via valence band states. This novel and exotic transport phenomenon is explained by the measured electronic structure and energy level alignment of the individual QDs. In contrast with previous hypothesis that surface stoichiometry and ligands are responsible for IGS, we found that the IGS are induced by adsorbed oxygen molecules whose p* states hybridize with QD valence band states, thus enabling inter-particle coupling and electron transport. We demonstrate that chemical treatments can be used for surface impurity engineering to achieve tunable electronic structure and transport properties.