Understanding the Microscopic Origin of the Contact Resistance at the Polymer-Electrode Interface

Contact resistance (R_C) in organic devices originates from a mismatch in energy levels between injecting electrodes and organic semiconductors (OSCs). However, the microscopic effects governing charge transfer between electrodes and the OSCs have not been analyzed in detail. We fabricated transistors with different OSCs (PTB7, PCDTBT, and PTB7-Th) and electrodes (MoO₃, Au, and Ag) and measured their contact resistance. Regardless of the electrodes, devices with PTB7-Th exhibit the lowest values of R_C. To explain the trends observed, first-principles computations were performed on contact interfaces based on the projector operator diabatization method. Our results revealed that differences in energy levels and the electronic couplings between OSCs' highest occupied molecular orbitals and vacant states on the electrodes influence device R_C. Further, based on values obtained from the first-principles, the rate of charge transfer between OSCs and electrodes is calculated and found to correlate strongly with trends in R_C for devices with different OSCs. We thus show that device R_C is governed by the feasibility of charge transfer at the contact interface and hence determined by energy levels and electronic coupling among orbitals and states located on OSCs and electrodes.