Hole controlled displacement behaviour of conducting polymer actuators

Abstract

The role of holes in the displacement behaviour of conducting polymer actuators is not emphasized much, hindering the design of actuators with a better response. Generally, it is assumed that the motion of ions limits the displacement of a conducting polymer because of their higher atomic mass compared to the effective mass of a hole or electron. Here, we report that the hole density of state (DOS) of a conducting polymer actuator could be another limiting factor for its displacement behaviour. Electrochemical techniques are used to estimate the DOS of a state-of-the-art conducting polymer, PEDOT:PSS. To capture the subsequent effect of changing the hole doping level and the kinetics of hole-ion transport, the electrochemical impedance of the PEDOT:PSS layer is measured while it is held at various constant voltages. To illustrate the effect of hole dynamics on the displacement of a conducting polymer actuator, the displacement of a PEDOT:PSS/bacterial cellulose actuator is recorded at various voltages and for different periods. The depletion and accumulation mode of operation is explained. The transients in the displacement of the actuator to the steady state are identified and explained, incorporating the electrochemical findings. The rate and magnitude of the displacement are found to be dependent on the hole doping level in a conducting polymer. The displacement of the actuator can be divided into three time scales; initial space charge (driven by drift current), filling up of high energy states (drift and diffusion), diffusion of ions (reflective or transmissive)