Directed Self-Assembly of Block Copolymers for High Energy Density Polymer Film Capacitors

The emerging needs for high-power, lightweight and flexible electronics require the use of polymer film based solid-state capacitors with pulsed power. The fast charge/discharge rates of film capacitors (on the order of µs) afford a high power density as compared to slower charging conventional batteries and supercapacitors. However, their energy storage densities fall significantly short of rising demand in advanced applications. An improvement in the energy storage density of polymer film capacitors over current performance metrics are reliant on increasing the permittivity (εr) and/or breakdown strength (EBD) of the dielectric and necessitates the development of new materials and processing methods. Experimental and simulation studies have shown that the presence of barriers within the dielectric can significantly enhance the EBD by hindering electrical tree propagation during breakdown. In this regard, co-extruded multilayer composite films have shown much promise to improve the EBD over that of individual components. In this work, a new material design and processing strategy was developed for fabrication of multilayered, multicomponent, barrier dielectric films using self-assembling diblock copolymers (BCP). BCPs are versatile materials which self-assemble into highly ordered periodic nanostructures (5-100 nm), offering extraordinary tunability of dielectric contrast, orientation, layer thickness and interfacial width, from the intrinsic nanoscale molecular level to macroscopic thickness for device-level compatibility. Directed self-assembly (DSA) using a thermal-gradient based Cold Zone Annealing-Soft Shear (CZA-SS) method was applied to model lamellae-forming PS-b-PMMA BCP films to obtain highly oriented multilayered dielectric films, as confirmed by cross-sectional TEM, Neutron Reflectivity and X-ray scattering measurements. Dielectric breakdown measurements of the BCP films revealed a ~20-50% enhancement in EBD for the self-assembled multilayer BCP films as compared to unordered single-phase as-cast films, suggesting that the breakdown pathway is highly selective to the BCP nanostructure. The enhancement in EBD was attributed to the “barrier effect”, where the multiple interfaces between the component PS and PMMA lamellar blocks act as barriers to the dielectric breakdown through the film. As such, the effects of block molecular weight, lamellar thickness and number of layers on the breakdown strength of multilayered BCP films were also studied. Dielectric spectroscopy measurements on the BCP samples revealed no deleterious changes to the permittivity or dielectric loss with self-assembly. Thus, using the simple but robust self-assembly approach, the breakdown strength of films was significantly increased without compromising on other dielectric properties. Given that the energy density scales as the square of the breakdown strength (U~ EBD2), this novel concept of DSA aligned BCP dielectric films provides a new nanomaterial paradigm for designing high energy density polymer film capacitors.