Electronic Ratcheting in an Organic Photovoltaic Material

Ofer Kedem, Center for Bio-Inspired Energy Science, Northwestern University, Evanston, IL, USA
Mario Tagliazucchi, Department of Biomedical Engineering, Northwestern University, Evanston, IL, USA
Bryan T. G. Lau, Department of Chemistry, Northwestern University, Evanston, IL, USA
Emily A. Weiss, Department of Chemistry, Northwestern University, Evanston, IL, USA

Biological systems contain multiple examples of mechanisms for extracting directed motion from non-directed forces in highly-damped (viscous) environments, such as the unidirectional rotation of ATP synthase, and the linear motion of kinesin. These natural systems have inspired efforts to create synthetic analogs, commonly called “molecular motors”, to produce useful motion on the nano-scale, using thermal noise, unbiased ac fields, or chemical energy.^[1] One such class of systems are electronic ratchets, capable of directing a flow of electrons using unbiased, non-directed forces.^[2] Ratcheting requires: (i) breaking the spatial inversion symmetry of the system along the direction of transport, either by the physical structure of the system itself, or by the perturbation applied to it;^[3] and (ii) the external application of energy to push the system away from equilibrium, as directional transport is prohibited by the second law of thermodynamics for equilibrium systems.

Here we present a new system for ratcheting of charges in an organic photovoltaic (OPV) layer, using a time-oscillating (flashing), periodic, asymmetric electric potential (Fig. 1). Charge transport in OPV materials is an over-damped (non-inertial) process, where charges travel by a diffusive hopping mechanism, and where recombination rates are high. Electronic ratcheting can provide a local driving force for charge separation and extraction. We select the shapes of the potential surfaces based on one-dimensional quantum dynamics simulations, and apply the potential surface to the OPV film using an array of finger electrodes (FEs) with an asymmetric thickness profile (Fig. 1), designed using electrostatic simulations.

Figure 1. Schematic representation of a typical ratchet potential in the “ON” and “OFF” states (left) and an atomic force microscopy image of the asymmetric electrode array (right)

[1]  R. D. Astumian, Phys. Chem. Chem. Phys. 2007, 9, 5067.

[2]  E. M. Roeling, et al., Nature Mater. 2011, 10, 51.

[3]  P. Reimann, Phys. Rep. 2002, 361, 57.