Empirical Mappings of the Frequency Response of an Electron Ratchet to the Characteristics of the Polymer Transport Layer

Flashing electron ratchets oscillate a periodic asymmetric potential to rectify nondirectional forces and thereby produce directional transport of electrons with zero source-drain bias. The relationship between the oscillation frequency of the potential and the ratchet (short-circuit) current reflects microscopic mechanisms of charge transport within the device. This paper describes experimental mappings of the "optimal frequency(ies)" of the ratchet f_peak - the oscillation frequencies that produce the largest ratchet current - to the carrier concentration, n_h, and to the linear field effect transistor mobility, μ_h, for a poly(3-hexylthiophene-2,5-diyl) (P3HT) transport layer. Measurements on multiple devices, multiple P3HT films per device, and a range of annealing and photoexcitation conditions yield the empirical relationships f_peak ∝ n_h and f_peak ∝ μh2/3. Finite-element simulations suggest the sublinear relationship between mobility and peak frequency arises due to a combination of damped and inertial motion of the holes. This work also provides evidence that the frequency response of ratchets is sensitive to multiple length scales of asymmetry encoded within the periodic electrical potential. These multiple asymmetries cause changes in the polarity of the ratchet current at points within the frequency response, a long-mysterious characteristic of particle ratchets called "current inversion", by encouraging transport in opposite directions in different frequency regimes.