High Energy Density and Increased Efficiency in Polymer Capacitors with Dilute Blended Electroactive Small Molecules

The energy storage density (<i>U</i>_e) of a capacitor is governed by its dielectric constant (ε_r) and breakdown strength (<i>E</i>_b). In this work, single-layer capacitors of the wide-bandgap-insulating polymers polystyrene (PS), polycarbonate (PC), and poly(methyl methacrylate) (PMMA) are fabricated on indium tin oxide (ITO)-coated borosilicate glass substrates with gold top-contacts. Dielectric layers approximately 2-5 μm thick are spin-coated and deposited on the ITO-coated substrates. Dilute concentrations of the small molecules dibenzotetrathiafulvalene (DBTTF), tetrakis(methylthio)tetrathiafulvalene (TMT-TTF), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), and 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) are solution-blended into each dielectric layer, and their effects on device performance are analyzed. When added in concentrations ranging from 0.01 to 1 wt % with respect to the concentration of polymer in the spinning solution, it is found in nearly all cases that these small molecules enhance the breakdown electric field strength of the polymer capacitors and as a result improve their maximum energy density by as much as 190% relative to control devices with no additives present. A maximum breakdown electrical field strength of 850 MV/m and a corresponding energy density of 16.2 J/cm³ are observed in PMMA with 0.1 wt % F4TCNQ capacitors, the best-performing devices in this study. The efficiency of the capacitors also improves at submaximal electric fields when small molecules are included. This work demonstrates the ability to use dilute blended electroactive additives in polymer capacitors to improve key performance metrics while helping to decrease the energy losses that hinder the applicability of capacitors comprising solution-processable engineering polymers such as PC and PMMA. The choice of readily soluble polymer dielectrics and complementary use of simple solution processing offer a scalable, cost-effective method for the production of high-performance polymer capacitors.