The Indirect Sensing of Alkali Metal Counterions Using P3HT-Based Organic Electrochemical Transistors Working in the Accumulation Mode

Here, we report the alkali metal counterion effect and its corresponding indirect sensing using poly(3-hexylthiophene)-based organic electrochemical transistor (OECT) devices working in the accumulation mode with bis(trifluoromethanesulfonyl)imidate salt aqueous solution as electrolyte. Our in-operando microRaman imaging studies confirmed that the charge transport and transfer processes are associated with the accumulation mode configuration, while our in-operando X-ray scattering studies indicate that the injection of dopants reached not only the swollen amorphous but also the crystalline regions of poly(3-hexylthiophene) (P3HT). Interestingly, we have observed characteristic features depending on the alkali metal counterion in both the output and transfer characteristics for these OECT devices. The channel current/transconductance exhibited a progressive increase, while the threshold voltage exhibited a progressive decrease in the OECT response with increasing the alkali metal ionic size. For instance, for OECT with W/L = 500/20 geometries, the maximum transconductance values were gmax = 11.9, 14.1, and 22.3 mS, the threshold voltage values were -Vth = 0.31, 0.27, and 0.20 V, and the figure of merit values were μC* = 3.51, 4.81, and 7.31 F·cm–1·V–1·s–1, for LiTFSI, NaTFSI, and KTFSI 0.1 M, respectively. In addition, the OECT response yielded excellent sensitivity for alkali metal counterions using secondary electrolytes such as LiCl, NaCl, and KCl in a broad range of concentrations from 10 to 1000 mM. We evidence that these P3HT-based OECT devices can be extremely useful to determinate Li+, Na+, and K+ working in the accumulation mode exhibiting sensitivities ranging from Δ(−Ids) = −0.3 to −1.3 mA, −0.6 to −2.2 mA, and −2.3 to −4.5 mA in the case of Li+, Na+, and K+, respectively. This indicates that the cation expulsion mechanism can be used for the indirect sensing of alkali metal ions in P3HT-based OECT devices working in the accumulation mode, yielding excellent response compared with those typically observed for PEDOT:PSS-based OECT devices working in the depletion mode.