The indirect sensing of alkali metal counterions using P3HT-based organic electrochemical transistors working in the accumulation mode

Abstract

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 Xray 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.