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 (R_c) 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 R_c 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). R_c 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 R_c. 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 R_c.