Electrochemical Investigations of the N‑Type Semiconducting Polymer P(NDI2OD-T2) and Its Monomer: New Insights in the Reduction Behavior

This manuscript provides the first systematic characterization of the electrochemical properties of the high mobility n-type polymer poly­{[N,N′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis (dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P­(NDI2OD-T2)) and its corresponding monomer 2,6-bis­(2-bromothien-5-yl)­naphthalene-1,4,5,8-tetracarboxylic-N,N′-bis­(2-octyldodecyl) diimide (Br-NDI2OD-T2-Br) by cyclic voltammetry and in situ spectroelectrochemistry. Both monomer and polymer reveal a 2-fold reduction to the dianion via a radical anion species. The comparison between monomeric and polymeric species allows the explanation of the electrochemical behavior of P­(NDI2OD-T2) according to redox polymers with localization of charges on the naphthalene bisimide unit. Measurements with electrolyte gated transistors suggest electron hopping transport according to mixed valence conductivity. In the last section of this paper we discuss a significant first cycle effect upon electrochemical reduction which had not been reported for n-type polymers before. The effect is even more pronounced for samples with controlled morphology, that is, high amounts of aggregation in the films. In agreement with solution experiments we attribute the appearance of the signal at −1.04 V (E_1/2 = −1.00 V) to the radical anion form of the solvated species.