Polyacetylene p–n Junctions with Varying Dopant Density by Polyelectrolyte-Mediated Electrochemistry

Abstract

Polyelectrolyte-mediated electrochemistry is demonstrated as a three-electrode technique to fabricate polyacetylene-based p–n junctions where the dopant density of the n-type layer is systematically varied. The approach utilizes a bilayer structure consisting of anionically and cationically functionalized polyacetylenes and a polyelectrolyte-based supporting electrolyte to control the ions available to support electrochemistry in the selective formation of n- and p-doped regions. Through control over the cationic functional group density and electrode potential, the dopant density of the n-type region is varied from 1019 to 1020 cm–3. Due in part to low carrier mobility and the presence of ionic functional groups, the properties of the junctions are distinct from classic inorganic p–n junctions in that they exhibit neither the voltage-dependent capacitance of a semiconductor depletion layer nor the classic Shockley, diffusion-based current–voltage behavior. Variation of the ionic functional group and dopant density enables the current–voltage curves to be tuned from symmetric to asymmetric with rectification ratios up to 1600 at 1.8 V and with equilibrium exchange current densities varying from 1 × 10–10 to 1 × 10–4 A cm–2. Under illumination, open-circuit voltages near 0.5 V are observed despite the absence of any substantial offset in frontier orbital positions of the constituent materials, as is the common motif in organic photovoltaics.