CBES Investigators

Emily Weiss

Ofer Kedem

Postdoctoral Fellow
Chemistry

Bryan Lau

PhD Candidate
Chemistry

James Schwabacher

PhD Candidate
Chemistry

Nathaniel Swenson

PhD Candidate
Chemistry

Daniel Kwaskieski

Graduate Student
Chemistry

Research Overview

CBES research in the Weiss group focuses on movement of charge and energy over long distances and electronic ratchets (Thrust 3). The performance of third-generation “excitonic” solar energy conversion devices comprising solution-processable materials such as molecules, polymers, or nanoparticles is limited by random diffusion of electrons or weakly biased drift currents and an associated poor yield of charge collection. In these third-generation solar cells, energy from thermalization of electrons from above-band-edge excitations is lost as heat. We aim to transform random diffusive motion of electrons, powered by light and thermal energy, into vectorial charge transport through a phenomenon called “ratcheting”. A ratchet is a non-equilibrium construct that induces unidirectional movement in otherwise randomly moving particles (classical or quantum) by continuously switching between two states of the potential surface within which the particle travels: (i) a state that causes an asymmetric relaxation of the particle, and (ii) a state that allows random, uniform diffusion of the particle. The periodic modulation of the potential surface serves to drive the system away from equilibrium. Additionally, a ratcheting potential surface must not be symmetric under inversion in the direction of transport.

Atomic Force Microscope top-down image and line-scan of an asymmetric, double-sawtooth ratchet potential, etched into Silicon by Focused Ion Beam (FIB) milling. We use such shaped potentials to create gate electrodes for devices in which we apply oscillating asymmetric gate potentials to transform random drift currents into directional current without application of a static source-drain bias.

Classical Brownian ratchets are theoretically well-studied systems that are prominent in biology, where they convert chemical to mechanical energy. For example, linear motor proteins are ratchets responsible for muscle contraction, and ATP synthase is a rotary motor that enables ATP synthesis and hydrolysis. We use these motors as inspiration to design quantum ratchets, where the switching of the potential surface produces a net electrical current despite a zero source-drain bias and exerting a zero time-averaged force on the charge carriers. We are carrying out the computational design and experimental realization of quantum ratchets for directional transport of electrons through nominally insulating materials, such as organic photovoltaic active materials.