Self-Assembly of Electroactive Small Molecule Crystallites in Polystyrene-Based Gate Dielectrics for Increased Threshold Voltage Shifts and Memristor Activity

Charging of organic-field effect transistors (OFETs) provides insight into the charge trapping behavior of polymer dielectrics via shifts in the threshold voltage (V_th) that occur after charging and can confirm memristor activity in these devices. Top-contact bottom-gate pentacene OFETs were fabricated with single layer dielectrics comprising either polystyrene (PS), poly(4-methylstyrene) (P4MS), or poly(4-tert-butylstyrene) (P4TBS). The polystyrenes were blended with varying concentrations of two different small molecules known as dibenzotetrathiafulvalene (DBTTF) and 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) to form self-assembled, small, separated crystallites contained throughout the polymer dielectric layer. The output and transfer characteristics of these devices were investigated and the shift in the threshold voltage of each device was measured after applying static charging of -70 V to source and drain electrodes for 5 minutes. Two-terminal measurements were conducted using multiple different gate biases in the range of -50 V to +50 V to investigate memristor behavior in the devices. The devices were subsequently analyzed via laser optical microscopy, x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopy (EDS) to confirm the presence and persistence of the small molecule crystallites while further characterizing their structure and morphology. Threshold voltage shift data obtained from the transfer characteristics of each device show that inclusion of DBTTF crystallites in the dielectric layer substantially increases the threshold voltage shift observed in the two substituted polystyrene matrices. Threshold voltage shift increases as large as 330% are observed in OFETs with P4MS dielectric layers when DBTTF is included, while a maximum average threshold voltage shift of 49.1±2.2 V is observed in the OFET with P4TBS and 10wt% DBTTF. OFETs comprising PS, P4MS, and P4TBS with at least 7.5wt% of DBTTF added showed memristor activity with gate voltage range set to -50 V and +50 V, where shifts in current are reversible and reproducible while all other devices break down under similar conditions. This work demonstrates that including small, separated crystallites in polymer dielectrics enhances their charge storage ability and could be promising for creating nonbinary memory devices for data processing, such as memristors used in neuromorphic computing.