Thickness-Dependent Charge Storage Dynamics in Polydithienothiophene Thin Films for Next-Generation Hybrid Supercapacitors

Conducting polymers are known for their outstanding pseudocapacitance, but their low power density often limits their efficiency as energy storage devices. In this work, we have introduced for the first time a metal- and binder-free working electrode based on polydithienothiophene (PDTT) thin films, a polymeric form of dithieno[3,2-b:2′,3′-d]thiophene (DTT). The PDTT thin films were prepared through electrochemical deposition on ITO glasses without the use of any nonconducting binding agents. Unlike other composite-based hybrid supercapacitors, our synthesized PDTT thin films can act as a stand-alone hybrid supercapacitor, thus offering a high energy density while maintaining a high power density. We have compared the thickness-dependent charge storage dynamics of the PDTT-4 and -6 thin films. PDTT-4 exhibits a balanced contribution of 51.7% of electric double-layer capacitance (EDLC) and 48.3% of pseudocapacitance to the total capacitance, which results in their superior overall capacitance (1010.10 F/g) as compared to PDTT-6 (961.53 F/g). Increased thickness of the polymer thin films causes a rise in diffusion-limited processes and internal resistance, which deteriorates their charge storage efficiency. PDTT-4 exhibited a high specific capacitance of 1482.6 F/g, which is more than twice the value of PDTT-6 (681.1 F/g) at 10 A/g. Moreover, PDTT-4 showcased a hike in energy density value (166.7 Wh/kg) compared to PDTT-6 (76.62 Wh/kg) while delivering close values of power densities (4497.3 and 4499.7 W/kg, respectively). PDTT-4 unveils comparatively better stability with a retention of 99% of specific capacitance after 10,000 cycles. A symmetric supercapacitor device was also fabricated, which revealed the superior supercapacitive behavior of PDTT-4 to that of PDTT-6. Interestingly, unlike the three-electrode system, the increased pseudocapacitance is responsible for enhanced specific capacitance (846.11 F/g), energy density (95.18 Wh/kg), and power density (2249.8 W/kg) of the PDTT-4-based device as compared to PDTT-6 (551.72 F/g, 62.06 Wh/kg and 2249.68 W/kg) at current density 2.5 A/g.