Organic Mixed Ion-Electron Conductors for THz Metadevices

In the last decades, materials that can efficiently transport ionic and electronic charges become key in advancements in the fields of bioelectronic devices, energy storing or harvesting, and healthcare. Organic mixed ionic-electronic conductors (OMIECs) are organic materials that can transport both ionic and electronic charge¹. The large variety of organic molecules that can be synthesized and the number of ways in which they can be arranged provide a great number of degrees of freedom that influence the OMIECs’ properties and device functionalities. Recent progress in the design and synthesis of organic mixed ion-electron conductors is actively contributing to developing exciting new technologies based on organic electrochemical transistors (OECTs). Among recent applications, OECT-based resonant devices have been proposed in the microwave spectral region showing excellent amplitude and frequency tuning performances in the sub-5GHz range². In these devices, the 3-dimensional charge modulation capabilities of OMIECs are key in achieving excellent tunability of both individual and arrays of microwave resonators, hence leading to a novel class of microwave photonic devices based on organic materials. The next step is to explore the applicability of this approach toward the THz frequencies, as well as the investigation of the OMIEC charge transport properties in this spectral range.<br/>In this work, we study the far-infrared conductivity of a state-of-the-art OMIEC, namely p(g2T-TT), with field-assisted THz Time Domain Spectroscopy (THz-TDS). Through this technique, we non-destructively investigate the dielectric and electronic properties of the OMIEC during operation, thus gaining crucial insights for the physical modeling of charge transport at high frequencies. By varying the applied voltage bias from -800 to 600 mV, we modulate the dielectric response of the polymer and obtain information about the charge injection and its effect on the polymer conductivity. We observe an increased conductivity by tuning the bias from 600 to -800 mV indicating an increase of the injected charge. More in detail, we report a positive real accompanied by a negative imaginary part of the conductivity, as already shown in the literature³. This behavior is well described by the Drude-Smith model that considers charge localization effects due to the finite polymer chain size and the energetic disorder. From the fit, we extract the charge density injected in the OMIEC as a function of the applied voltage bias and its carrier mobility.<br/>To demonstrate the effectiveness of the charge carrier screening effects in p(g2T-TT) at THz frequencies, we investigate the amplitude modulation performances of a THz metadevice based on this OMIEC. The prototypical device is based on metallic Split Ring Resonators with a resonance at 0.7 THz, which are electrostatically tuned with the p(g2T-TT). We show that despite the relatively low mobilities of the charge carriers, their large density modulation leads to excellent modulation depth – exceeding 60% - with polarization voltages below 1 V.<br/>Our results highlight the potential of organic THz devices based on OMIECs, for instance as optical modulators for telecom applications. Further research efforts in this direction could lead to a new generation of reconfigurable photonic technologies that take benefit of the many desirable properties of organic semiconductors, such as the ease of processability on large-area flexible substrates.<br/><br/>References:<br/>1. B. D. Paulsen, K. Tybrandt, E. Stavrinidou, and J. Rivnay. Organic mixed ionic–electronic conductors. Nat. Mater. 19, 13–26 (2020).<br/>2. G. E. Bonacchini, & F. G. Omenetto. Reconfigurable microwave metadevices based on organic electrochemical transistors. <i>Nat. Electron. 2021 46</i> <b>4</b>, 424–428 (2021).<br/>3.D. Tsokkou, P. Cavassin, G. Rebeteza & N. Banerji. Bipolarons rule the short-range terahertz conductivity in electrochemically doped P3HT. Mater. Horiz. 9, 482-491 (2022).