The key component of an organic electrochemical transistor (OECT) is the channel material which is made of an organic mixed ionic-electronic conductor (OMIEC). Its role is to conduct both electronic and ionic charges within the entire volume of the channel and to modulate its redox states and conductivity through interactions with ions and solvent molecules of the electrolytes. Conjugated polymers quickly appear as an ideal platform for mixed conductance. The electronic charge transport is occurring along the conjugated backbone and the ionic transport is allowed through the bulk.

While many efforts have been made to understand how to design materials with high charge mobilities, such structure-property relationships are still lacking when these polymers are interacting with ions and solvent molecules. By using a combined classical/quantum approach, we have simulated the charge and ionic transports in different OMiEC candidates to get a better understanding of this mixed conductance and to propose design criteria for new materials.

We focused our attention on the IDTBT polymer, a state-of-the-art p-type polymer, for which we studied the impact of the incorporation of glycol side chains on both the charge transport properties and ion diffusion. While our simulations suggest that glycolated side chains do not alter the number and quality of interachain contacts for both dry and swollen films, experimental evidence points to disappointing OECT performances [1]. The latter could be rationalized theoretically by larger electrostatic energetic disorder due to the presence of polar atoms close to the conjugated backbones. Our calculations therefore suggest designing polar side chains by incorporating the hydrophilic segment between two apolar regions and using semicrystalline materials instead of near-amorphous materials.

pgBTTT and p(g2T-TT) are two interesting materials which demonstrate that subtle changes of the chemical structure of the glycolated PBTTT, i.e., regioisomers, can strongly influence their properties [2]. Interestingly, it has been shown that pgBTTT is more efficient in OECT than p(g2T-TT). By simulating the ion insertion process in crystalline stacks of both polymers, our calculations pointed out differences in the planarity of the conjugated backbones and, more importantly, the localization of the ions in the swollen films; rationalizing their different behaviors.

Finally, our latest results on the insertion of ions within BBL polymers, a state-of-the-art n-type polymer for OECT, will be presented.