The complex permittivity of PEDOT:PSS

High permittivity materials are required for efficient organic photovoltaic devices, and the addition of the conjugated polymer composite poly(3,4-ethylenedioxythiophen) polystyrene sulfonate (PEDOT:PSS) to dielectric polymers has been shown to significantly heighten their permittivity. The permittivity of PEDOT:PSS at the optical and microwave frequencies has been investigated, but PEDOT:PSS layers are mainly used for low-frequency device applications, where accurate dielectric property measurements are hindered by their high electrical conductivity and the problems arising from the metal-polymer interfaces. Here, we determine the complex relative permittivity (E*(r) = E'(r) - jE'' (r) ) of PEDOT:PSS layers perpendicular to the layer plane in the 10(-2)-10(6) Hz range by combining data from the reactive energy estimations and electrochemical impedance spectroscopy, and discover that: E'(r) at <1 Hz is ultra-high (-106) decreasing with frequency to similar to 5 at 10(6) Hz; the experimental data fit the Cole-Cole dielectric relaxation model by considering multiple relaxation mechanisms; PEDOT:PSS polarizes nonlinearly and E'(r) increases with the intensity of the applied external field; low frequency E'(r) increases with both thickness and temperature of the layer, opposite trend of temperature-dependence prevails at >103 Hz; the dielectric properties of PEDOT:PSS are highly anisotropic and the in-plane E'(r) at 1.0 kHz is three orders of magnitude higher than the vertical E'(r); and that the E'' (r) decreases proportional to the reciprocal of frequency (1/f). The latter finding provides an explanation for the ubiquitous pink noise accompanying signals transmitted through organic conductor links. The described methodology can be adopted for investigations on other conjugated polymers.

Automated summary

High permittivity materials are important for organic photovoltaic devices. PEDOT:PSS, a conjugated polymer composite, has been shown to increase the permittivity of dielectric polymers. However, its high electrical conductivity and metal-polymer interfaces make accurate measurements difficult. In this study, the complex relative permittivity of PEDOT:PSS layers was determined using reactive energy estimations and electrochemical impedance spectroscopy. The results showed that PEDOT:PSS has highly anisotropic dielectric properties, with the in-plane permittivity three orders of magnitude higher than the vertical permittivity at 1.0 kHz.