Multi-Band Filling and Non-Equilibrium Charge Transport in Conjugated Polymers at Charge Densities on The Order of 10²¹ cm^-3

Organic electrochemical transistors (OECTs) provide us a powerful tool for studying charge transport in conjugated polymers over a wide range of charge densities. It has been reported that in both n-type and p-type OECTs, charge densities on the order of one charge per monomer (~10²¹ cm^-3) can be achieved. At such high charge densities, Coulomb interactions among electrons and between electrons and counter-ions are expected to play a role in charge transport. However, charge transport in this regime, as well as the many-body correlated physics, is still not as well understood as charge transport in devices with much lower charge densities (e.g., organic field-effect transistors).<br/><br/>In this work, we show that in OECTs based on a class of p-type donor-acceptor polymers, it is possible to completely empty the HOMO and eventually access the HOMO-1 orbitals without any degradation, which is supported by a combination of electrical, thermoelectric, and photoemission spectroscopic measurements. More interestingly, under such extreme band filling conditions, by adding a second field-effect back-gate to the OECT, we observe unusual field-effect response when the ionic motions are frozen. Both the shape of the back-gate transfer curve (graphene-like) and the magnitude of drain current modulation (up to 300%) are substantially different from what one would expect from a conventional field-effect device based on an ultra-heavily doped polymer. Temperature-dependent measurements suggest that the unusual back-gate field-effect response is a non-equilibrium phenomenon, which provides unique insight into the formation of a frozen, soft gap in the density of states driven by Coulomb interactions.