Electronic Structures and Optical Properties of p-Type/n-Type Polymer Blends: Density Functional Theory Study

Abstract

A blend made of p-type and n-type polymers can act as bipolar/ambipolar material composites that transport both electrons and holes. Although several experimental efforts are currently devoted to p-/n-type blends of conducting polymers, theoretical studies of these systems are missing to a large extent. In the current paper, using the density functional theory (DFT) and the time-dependent DFT, we calculate electronic and optical properties of a p-type/n-type polymeric blend, where we have chosen the poly(3,4-ethylenedioxythiophene)/benzimidazo-benzophenanthroline ladder (PEDOT/BBL) as a model composite system. We demonstrate that in the blend, PEDOT acts as an electron donor and BBL acts as an electron acceptor under doped conditions. However, no charge transfer between the chains takes place for an undoped composite system. Due to a significant difference in the electron affinities and the ionization energies of PEDOT and BBL, the electronic properties of a negatively (positively) doped PEDOT/BBL blend are primarily governed by the chains where negative (positive) charges are localized, i.e., the BBL chains (the PEDOT chains). However, this is no longer valid for the optical absorption where the electronic transition occurs between the two chains and, therefore, the calculated UV–vis–near-infrared (NIR) absorption spectra of the negatively (positively) doped PEDOT/BBL blend are rather different compared to the corresponding spectra of the single BBL chains (PEDOT chains). The electronic coupling between the photoexcited state and the final charge-transfer state of the blend was calculated to be ∼0.08 eV. The results presented here are generic to a wide class of p-type/n-type combinations, which was further confirmed by calculations performed on the polythiophene (PT)/BBL blend