High energy density, and low loss polymer dielectrics for energy storage capacitors and organic electronics

Open Access
Author:
Wu, Shan
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 27, 2014
Committee Members:
  • Qiming Zhang, Dissertation Advisor
  • Noel Christopher Giebink, Committee Member
  • Shizhuo Yin, Committee Member
  • Christopher Rahn, Committee Member
Keywords:
  • energy storage
  • capacitor
  • polymer
  • dielectric
  • energy density
  • loss
  • dielectric breakdown
Abstract:
Electrical energy storage devices are among the most important components for a broad range of applications in modern electronics and electrical power systems such as hybrid electric vehicles (HEV), medical defibrillators, filters, and switched-mode power supplies. Due to these applications, electrical energy storage devices have been growing rapidly in recent years. Desired properties of the dielectrics for energy storage include high electric energy density, high charge-discharge efficiency, high electric breakdown, and high operation temperature. Compared with ceramic capacitors, polymer thin film capacitors are inexpensive, possess high dielectric strength, high energy density and low dielectric loss, and fail gracefully. The continuous miniaturization and increased functionality in modern electronics and electric power systems demand further increases in energy and power density of dielectric materials since these capacitors contribute significant (>30%) volume and weight to systems. One major challenge in developing dielectric polymers is realizing high energy density while maintaining low dielectric loss, even when high electric fields are applied. The traditional dielectric polymers have a relatively low dielectric constant around 2-3, and the energy density is limited to below 5 J/cm3. Recently, PVDF (polyvinylidene fluoride) based dielectric polymers such as P(VDF-CTFE) (CTFE: chlorotrifluoroethylene) and P(VDF-HFP) (HFP: hexafluoropropylene) have been studied and demonstrated to achieve very high energy densities (>25 J/cm3). Unfortunately, it is still a challenge to reduce the ferroelectric loss in PVDF based polymers by the strongly coupled dipoles and the high electric field conduction loss. Two approaches are introduced in this dissertation on how to develop the next generation polymer dielectrics with high energy density, low loss, high breakdown strength, and high temperature stability. The first approach is modification of high K polymer dielectrics to reduce the ferroelectric loss and conduction loss. The second approach is start from intrinsically low loss materials, then enhance the dielectric properties by increasing the dipole moment and dipole density. A polar-fluoropolymer blend consisting of a high energy density P(VDF-CTFE) and a low dielectric loss poly(ethylene-chlorotrifluoroethylene) (ECTFE) was developed. Both the blend and crosslinked blend films exhibit a dielectric constant of 7 and low loss (1%), as expected from the classical composite theory. Moreover, introducing crosslinking can lead to a marked reduction of losses in blend films at high electric fields while maintaining a high energy density. At 250 MV/m, a loss of 3% can be achieved in the crosslinked blend compared with 7% loss in pure blend, which is already much below that of pure P(VDF-CTFE) (35%). Furthermore, uniaxially stretch can improve the dielectric breakdown strength and mechanical properties. The promise of aromatic, amorphous, and polar polymers containing high dipolar moments with very low defect levels is demonstrated for future dielectric materials with ultrahigh electric-energy density, low loss at high applied fields, and ultrahigh breakdown strengths. Specifically, an amorphous, polar, and glass-phase dielectric polymer aromatic polythiourea (ArPTU) features extremely high dielectric breakdown strength (>1.1 GV/m), low loss at high electric fields (10% at 1.1 GV/m), and a high maximum electrical energy density (>24 J/cm3). This dissertation presents a study of the structure-property relationships and electrical properties study in ArPTU, and offers a phenomenological explanation for the experimentally observed high-field loss characteristics which facilitate the excellent energy storage properties. Besides the aromatic polythiourea, meta-aromatic polyurea (meta-PU) was developed and investigated for energy storage capacitors. Modifications to the molecular structure can tune the dipolar density and dipole moment in the polyurea systems to improve the dielectric properties. The meta-PU has an enhanced dielectric constant from the higher volume dipolar density, higher energy density, and a high electrical breakdown. A high storage electrical energy density of 13 J/cm3 with energy storage efficiency of 91% can be achieved at 670 MV/m electric field. Other polyureas, polythioureas based dielectrics with tunable dielectric properties are also summarized. Polymer dielectrics possessing high dielectric constant, low loss are not only of great importance for energy storage capacitors, but also attractive as gate dielectrics in organic thin film field effect transistors (OTFTs). In this work, solution processable PVDF based polymers, with tunable dielectric constant from 7 to more than 50 as well as ferroelectricity, were used as the gate insulator in bottom gated OTFTs with a pentacene semiconductor layer. Due to the high dielectric constant of P(VDF-TrFE-CFE), a large capacitive coupling between the gate and channel can be achieved which causes a high charge concentration at the interface of the semiconductor and dielectric layers. In devices with the P(VDF-TrFE-CFE) dielectric layer, high performances and a low minimum operation gate voltage (5-10 V) were attained. Also, the ferroelectric thin film transistor with the P(VDF-TrFE) dielectric has a high remnant polarization, which is desired for memory applications.