Avoiding Resistance Limitations in High-Performance Transparent Supercapacitor Electrodes Based on Large-Area, High-Conductivity PEDOT:PSS Films

This work describes the potential of thin, spray-deposited, large-area poly­(3,4-ethylenedioxythiophene)/poly­(styrene-4-sulfonate) (PEDOT:PSS) conducting polymer films for use as transparent supercapacitor electrodes. To facilitate this, we provide a detailed explanation of the factors limiting the performance of such electrodes. These films have a very low optical conductivity of σ_op = 24 S/cm (at 550 nm), crucial for this application, and a reasonable volumetric capacitance of <i>C</i>_V = 41 F/cm³. Secondary doping with formic acid gives these films a DC conductivity of σ_DC = 936 S/cm, allowing them to perform both as a transparent conductor/current collector and transparent supercapacitor electrode. Small-area films (<i>A</i> ∼ 1 cm²) display measured areal capacitance as high as 1 mF/cm², even for reasonably transparent electrodes (<i>T</i> ∼ 80%). However, in real devices, the absolute capacitance will be maximized by increasing the device area. As such, here, we measure the electrode performance as a function of its length and width. We find that the measured areal capacitance falls dramatically with scan rate and sample length but is independent of width. We show that this is because the measured areal capacitance is limited by the electrical resistance of the electrode. We have derived an equation for the measured areal capacitance as a function of scan rate and electrode lateral dimensions that fits the data extremely well up to scan rates of ∼1000 mV/s (corresponding to charge/discharge times > 0.6 s). These results are self-consistent with independent analysis of the electrical and impedance properties of the electrodes. These results can be used to find limiting combinations of electrode length and scan rate, beyond which electrode performance falls dramatically. We use these insights to build large-area (∼100 cm²) supercapacitors using electrodes that are 95% transparent, providing a capacitance of ∼12 mF (at 50 mV/s), significantly higher than that of any previously reported transparent supercapacitor