Charge Injection and Transport in an Isoindigo-Based Polymer Transistor

Polymer semiconductors hold great potential as active materials in (opto)electronic, thermoelectric, and biomedical devices. Their charge transport performance has seen tremendous progress, with mobilities exceeding 1 cm2 V−1 s−1 for a variety of donor-acceptor copolymers. Nevertheless, charge injection at the metal/polymer interface is still rather ineffective and poorly understood. In a field-effect transistor, this process is manifested by the contact resistance (Rc) which, for polymers, is several orders of magnitude higher than for their inorganic counterparts. Therefore, an in-depth investigation of the charge injection in metal/donor-acceptor polymer systems is sought-after. Here, the low-temperature dependent Rc and charge transport of a model isoindigo donor-acceptor copolymer-based transistor are studied. The metal/polymer interface is tuned by functionalizing the electrodes with different thiolated self-assembled monolayers (SAMs). Rc in devices with SAM-functionalized electrodes is generally lower and exhibited a weak temperature dependence. Counterintuitively, electrodes functionalized with SAMs expected to lead to an apparently unfavorable energy level alignment displayed the lowest Rc. The Fermi level is found to be pinned at all the encompassed interfaces. An energy-level alignment modeling is employed to understand this behavior. The findings reveal that simply looking at the energy levels alignment of metal/polymer interface does not necessarily lead to reduced Rc.