All About the Interface: Do Residual Contaminants at A High‐Quality h‐BN Monolayer Perylene Diimide Interface Cause Charge Trapping?

Intrinsic charge transport in molecularly thin organic semiconducting crystals is critically sensitive to the quality of the interfaces required to perform the electrical measurements. Most prominent are the dielectric–semiconductor and semiconductor–metal interface. While impacts from the latter on charge transport can be extracted by four-terminal measurements, the impact of the dielectric interface can only be minimized, typically by utilizing inert dielectrics. Here, it is shown that charge transport in organic field-effect transistors based on the n-type small molecule N, N′-di((S)-1-methylpentyl)-1,7(6)-dicyano-perylene-3,4:9,10-bis(dicarboximide) (PDI1MPCN2) can be improved up to one order of magnitude by using hexagonal boron nitride (h-BN) as dielectric, compared to a standard SiO2 substrate. Using temperature-dependent electrical measurements, the charge-transport properties of devices are systematically analyzed, and high four-terminal mobilities of up to 5.0 cm2 V−1 s−1 are obtained. The high mobility likely stems from decreased charge-carrier trapping at the semiconductor-dielectric interface due to the smooth surface of the inert h-BN. Nevertheless, the temperature dependencies of the mobility, threshold voltage, and interface-state trap density suggest that charge-carrier trapping at the dielectric-semiconductor interface still exists. By comparing the data to transport studies performed on thin air-gapped organic films, it is concluded that an interfacial layer (likely water or solvent residues) between h-BN and the monolayer PDI1MPCN2 causes charge trapping.