Effects of selenium incorporation on the performance of polythiophene based organic electrochemical transistors

Selenium incorporation is a widely used strategy for achieving high mobility semiconductors for a variety of organic optoelectronic devices, but it has received less attention in organic electrochemical transistors (OECTs). Herein, we developed two polythiophene derivatives (Pg2T-S and Pg2T-SVS) containing different selenophene loadings, as well as a selenophene-free polythiophene (Pg2T-T) as the control. The effects of selenium substitution/incorporation on the optical, electrical, electrochemical, and OECT performance of polythiophene-based organic mixed ionic-electronic conductors are systematically investigated. We discovered that introduction of selenium increases quinoidal character and crystallinity, which is beneficial for charge transport but makes ion penetration more challenging. In contrast, increasing selenium content increases the dipole moment and ion-polymer interaction, which is helpful for ion doping and volumetric capacitance but detrimental to charge transport. Therefore, appropriate selenium content should be evaluated to balance the trade-off between charge transfer and volumetric capacitance. As a result, Pg2T-S with moderate selenium loading achieves a high mu C* of 332.7 F cm-1 V-1 s-1 with increased mu of 1.31 cm2 V-1 s-1 and maintains the C* value of 254 F cm-3. It also has a quick tau on/tau off of 10.76 ms/5.90 ms and outstanding operational stability. Further introducing selenium boosts the dipole moment and ion-polymer interaction and therefore increased C*, but suppressed mu, resulting in inefficient mu C*. We indicated that systematic balancing of the effects of selenium incorporation is necessary for the development of mixed ionic-electronic conductors for high-performance OECTs. This study highlights that an appropriate selenium content should be evaluated to balance the trade-off between charge transfer and volumetric capacitance, paving the way for the optimal product of electronic mobility and volumetric charge storage.