Effect of degree of crystallinity on charge transport in ferroelectric semicrystalline polymer film

Abstract

This paper describes 3D particle-in-cell (PIC) simulation of charge injection and transport through ferroelectric semicrystalline polymer (e.g. PVDF) film comprised of nanocrystallites in an amorphous matrix with varying degrees of crystallinity. The classical electrical double layer (EDL) model for a monopolar core is extended (eEDL) to represent the nanocrystallite by replacing it with a dipolar core. Charge injection at the electrodes assumes metal-polymer Schottky emission at low to moderate fields and Fowler-Nordheim tunneling at high fields. Injected particles propagate via field-dependent Poole-Frenkel mobility. In the eEDL model, the initial attachment of charge particles forms the bound Stern-Helmholtz layer leading to Maxwell-Wagner-Sillars polarization. Subsequent waves of charge particles are repelled by the attached charge resulting in the diffuse Gouy- Chapman transport layer. The simulation algorithm uses a boundary integral equation method (BIEM) for solution of the Poisson equation coupled with a second-order predictorcorrector scheme for robust time integration of the equations of motion. The stability criterion of the explicit algorithm conforms to the Courant-Friedrichs-Levy (CFL) limit assuring robust and rapid convergence. The model is capable of simulating a wide dynamic range spanning leakage current to pre-breakdown levels. Simulation results for semicrystalline film with varying degrees of crystallinity indicate that charge transport behavior depends on nanoparticle polarization with anti-parallel orientation showing the highest conduction and therefore lowest level of charge trapping in the interaction zone. Charge attachment to nanocrystallites increase with vol% loading or degree of crystallinity, and saturates at 40 vol% for the set of simulation parameters. The eEDL model predicts the intuitive meandering pathways of charge particle trajectories, which get progressively more inclined from normal incidence with increasing vol% loading.