Organic electronic ratchet devices : exploring the electronic ratchet and investigating a solar ratchet

Abstract

This presentation reports on the investigation of two related topics, the (in)-organic electronic ratchet and the organic solar ratchet. These are two implementations of the electronic ratchet device that will first be introduced. The electronic ratchet generates electrical current and voltage by rectifying an external perturbation, in the present case an oscillating voltage. The oscillation is rectified by a periodic, asymmetric potential in which electronic charges are placed. The external perturbation drives the system away from equilibrium, causing detailed balance to be broken; consequently the periodic asymmetry of the potential leads to a net motion of the charges, comparable to the net motion of marbles on a shaking washboard. Note that under equilibrium conditions no (spontaneous) current will flow in any potential. In this presentation we show that the generated current as a function of the frequency of the driving signal can be scaled onto a universal profile, covering many orders of magnitude in frequency. Simulations show that this profile is predominantly dependent on drift (as opposed to diffusion) currents in the ratchet. Experimentally this is verified by combining measurements on three different disordered semiconducting materials. IGZO (Indium Gallium Zinc Oxide) as an inorganic amorphous electron semiconductor and Pentacene and P3HT/PCBM as organic hole semiconductors. The combined scaled current profile shows a consistent behavior for the three materials over at least seven orders of magnitude of (scaled) frequency. The solar ratchet is a lateral solar cell which can in lowest order be seen as multiple solar cells in series, which is comparable to a multi-junction tandem solar cell. The goal is to create a large open circuit voltage with the intention to create a more efficient solar cell. Simulation of this device turned out to be troublesome due to numerical problems related to the calculation of diffusion currents and of bimolecular recombination. However the first version of the simulation tool, in which drift and diffusion currents are calculated sequentially, shows the possible behavior of the solar ratchet. The second version, in which the drift and diffusion currents are simultaneously calculated using the Boltzmann transport equation, predicts the solar ratchet to be non-functional due to zero recombination. However, the depletion length of the recombination zones can be calculated. On basis of this an analogy can be made with the recombination zone in the pn-junction in a conventional tandem solar cell. As the tandem solar cell is known to work the solar ratchet is predicted to work as well. Experimental proofs of principle of the ratchet solar cell were tried to be made but unfortunately no suitable solar ratchet was produced due to various problems that will be discussed.

Date of Award	31 Aug 2013
Original language	English
Supervisor	M. Kemerink (Supervisor 1)