REGENTS OF THE UNIVERSITY OF MICHIGAN

In Photovoltaic (PV) and thermoelectric (TE) energy conversion, the process by which incident light or heat energy is transformed into useful electricity is composed of a series of mechanisms that each have an intrinsic efficiency. Energy loss in these mechanisms, primarily due to coupling between charge carriers and thermal vibrations, determines the overall conversion efficiency of the process. However, fundamental study of PV and TE loss mechanisms has proven difficult due to the short time and length scales over which they occur. Emerging advances in ultrafast optical science hold promise for understanding these building blocks of energy conversion. Pump-probe techniques that can span femtosecond to nanosecond time scales and simultaneously achieve atomic-scale resolution have recently been developed, in particular by the ultrafast optics community at the University of Michigan. Furthermore, advances in the synthesis and fabrication of nanocomposites with precisely tuned physical and chemical structure have provided a materials basis on which to conduct fundamental measurements of electron, phonon, and photon transport. We propose to apply these techniques in establishing an Energy Frontier Research Center (EFRC) devoted to the study of fundamental loss mechanisms in PV and TE energy conversion. The integrated theoretical, experimental and computational framework of this Center will examine transport processes in organic, inorganic, and organic-inorganic hybrid materials, developing basic understanding to underpin approaches for radically increased conversion efficiency. We will examine the influence of atomic and electronic structure on the dynamics of transport by phonons and free or bound (exciton) charge carriers that govern the TE figure of merit and PV efficiency. In particular, we will utilize low-dimensional structures such as epitaxial quantum dot (QD) superlattices and colloidal nanocrystals to investigate the influence of nanostructure size, shape, band offset, arrangement, and surface polarity on electronic and thermal transport. Exciton generation, relaxation, diffusion, and dissociation in highly controlled organic and hybrid PV materials will be studied using ultrafast optical and scanning probe techniques. We will examine charge injection at organic-inorganic interfaces, as well as charge transport in ordered and disordered organic thin films. For TE material in particular, we will utilize low-dimensional structures to examine phonon scattering and electron-phonon coupling mechanisms, analyzing the effects of carrier confinement and modifications to the electronic density of states. The impact of this research will address two important questions: (1) How does the surface/interface of low-dimensional structures such as nanoparticle, nanowire, or thin film influence the structure’s energy level and coupling rates? (2) How does the arrangement of interacting, low-dimensional structures (in, for example, a QD superlattice) control the macroscale energy levels and coupling rates?

Clarification of Codes