Weak Donor, Strong Acceptor Thienopyrazine-Based Polymers for Fine Tuning of LUMO Levels—Suitable Materials for Energy and Storage Solutions

Abstract

One approach creating and to tuning the energy levels of small band gap polymers or molecules is alternating donor-like (electron donating) and acceptor-like (electron withdrawing) monomers. Polymer for solar cells need to maintain a low highest occupied molecular orbital (HOMO) energy level in order to maximize the open circuit voltage (Voc) and have a small band gap in order to maximize the short-circuit current (Jsh). Alternating a “weak donor” and a “strong acceptor” will maintain a low HOMO energy level and reduce the band gap via intramolecular charge transfer (ICT). Here we present the synthesis and characterization of a series of polymers based on thienopyrazine (acceptor) and carbazole (donor). We compared the optoelectronic properties of the classic polymer poly-carbazole-dithiophene-benzothiadiazole (PCDTBT) with one where benzothiadiazole was substituted with thienopyrazine and one where we randomly alternated thienopyrazine and benzothiadiazole as the acceptor unit while maintaining the same donor unit. The approach of alternating weak donor and strong acceptor moieties allowed us to obtain fine LUMO level modulation of the materials, opening the way to the design of new donor materials.

Keywords

• low band-gap polymers
• OPVs
• semiconducting polymers
• thienopyrazine