Electrical and Optical Properties of Diketopyrrolopyrrole-Based Copolymer Interfaces in Thin Film Devices

Two donor−acceptor diketopyrrolopyrrole (DPP)-based copolymers (PDPP-BBT and TDPP-BBT) have been synthesized for their application in organic devices such as metal−insulator semiconductor (MIS) diodes and field-effect transistors (FETs). The semiconductor-dielectric interface was characterized by capacitance−voltage and conductance−voltage methods. These measurements yield an interface trap density of 4.2 × 1012 eV−1cm−2 in TDPP-BBT and 3.5 × 1012 eV−1cm−2 in PDPP-BBT at the flat-band voltage. The FETs based on these spincoated DPP copolymers display p-channel behavior with hole mobilities of the order 10−3 cm2/(V s). Light scattering studies from PDPP-BBT FETs show almost no change in the Raman spectrum after the devices are allowed to operate at a gate voltage, indicating that the FETs suffer minimal damage due to the metal−polymer contact or the application of an electric field. As a comparison Raman intensity profile from the channel-Au contact layer in pentacene FETs are presented, which show a distinct change before and after biasing.