Charge transport in mix-conducting hetero-ionic junctions of polyacetylene ionomers
Lin, Fuding, 1975-
Experimental studies on mix-conducting hetero-ionic junctions of anionically (PA A ) and cationically (PA C ) functionalized polyacetylene ionomers, as well as each individual ionomer, in thin-film sandwich configurations are reported for the purpose of better understanding the interaction between ionic and electronic charge transports in mixed ionic-electronic conductor (MIEC) systems. The transport of ions in both individual ionomers as well as their hetero-ionic junction was investigated via small-amplitude AC impedance spectroscopy in the absence of significant interference from the electronic charge transport. Modeling of the impedance results reveal important information about the materials such as: ion conductivity, activation energy of ion conduction, ion hopping frequency, dielectric constant, interfacial capacitance, and estimates of effective ion density. Electrochemical injection of electronic charge carriers into PA A and PA C from gold electrodes was monitored to determine the applied potentials needed to drive hole and electron injection into each ionomer. It is found that for both ionomers, the onset voltages for unipolar and bipolar charge injection are similar, and holes can be injected at close to zero bias. The responses of the complete Au|PA A |PA C |Au hetero-ionic junction, as well as each constituent ionomer layer in Au|Ionomer|Au configuration, to various stepping biases were investigated through current-voltage and impedance measurements to study the origin of the asymmetric current-voltage response observed in the hetero-ionic junction. Analysis of the results reveal a working mechanism of a mix-conducting junction that is fundamentally different from that of a purely electronic pn junction. When illuminated with light, the Au|PA A |PA C |Au junction exhibits unidirectional photovoltage and photocurrent with the PA A side at higher potential, while the Au|PA A |Au and Au|PA C |Au samples exhibit symmetric photoresponses. The efficiency of photocurrent generation in the Au|PA A |PA C |Au junction was found to be strongly dependent on the direction of illumination and on the sample thickness. These observations can be explained by the difference in the mobility of holes and electrons and the existence of a built-in ionic space charge region at the PA A |PA C interface. A mechanism of photoresponse unique to MIEC junctions was proposed, and the magnitude of built-in potential was estimated.