Influence of Dielectric Layers on Charge Transport through Diketopyrrolopyrrole-Containing Polymer Films: Dielectric Polarizability vs Capacitance

Field-effect mobility of a polymer semiconductor film is known to be enhanced when the gate dielectric interfacing with the film is weakly polarizable. Accordingly, gate dielectrics with lower dielectric constant (k) are preferred for attaining polymer field-effect transistors (PFETs) with larger mobilities. At the same time, it is also known that inducing more charge carriers into the polymer semiconductor films helps in enhancing their field-effect mobility, because the large number of traps presented in such a disorder system can be compensated substantially: In this sense, it may seem that employing higher k dielectrics is rather beneficial because capacitance is proportional to the dielectric constant. This, however, contradicts with the statement above. In this study, we compare the impact of the two, i.e., the polarizability and the capacitance of the gate dielectric, on the transport properties of poly[(diketopyrrolopyrrole)-alt-(2,2'-(1,4-phenylene)bisthiophene)] (PDPPTPT) semiconductor layers in an FET architecture. For the study, three different dielectric layers were employed: fluorinated organic CYTOP (k = similar to 2) poly(methyl methacrylate) (k = similar to 4) and relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethykne) (k = similar to 60). The beneficial influence of attaining more carriers in the PDPPTPT films on their charge transport properties was consistently observed from all three systems. However, the more dominant factor determining the large carrier mobility was the low polarizability of the gate dielectric rather than its large capacitance; field-effect mobilities of PDPPTPT films were always larger when lower k dielectric was employed than when higher k dielectric was used. The higher mobilities obtained when using lower k dielectrics could be attributed to the suppressed distribution of the density of localized states (DOS) near the transport level and to the resulting enhanced electronic coupling between the macromolecules.This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning of Korea (Code No. 2015R1C1A1A02037534) and partially supported by the Development of R&D Professionals on LED Convergence Lighting for Shipbuilding/Marine Plant and Marine Environments (Project No. N0001363) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). J.W.C. also is thankful for financial support from the Global Leading Technology Program of the Office of Strategic R&D Planning (OSP) funded by the Ministry of Commerce, Industry and Energy, Korea (No. 10042537