[Paper] p-Doping of Squaraine with F4-TCNQ by Solution Processing

Abstract

We report p-type doping of 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) by solution processing. We confirm the presence of F4-TCNQ in the doped films and observe the Fermi level shift towards the highest occupied molecular orbital (HOMO) as the dopant concentration increases. Furthermore we note that the doped film conductivities, as well as the threshold voltages and ON/OFF ratios of transistors incorporating those films, show trends as a function of F4-TCNQ concentration consistent with p-type doping. The number of charge carriers in the film increases with doping, but no dependence on the doping level is observed for the extracted field-effect mobility. The latter is likely due to a lack of control over the morphology formation during the film crystallization and not to fundamental factors governing charge transport. The present findings may serve as a basis for the development of higher performance organic solar cells using SQ as a donor material.