Weak donor- strong acceptor thienopyrazine based polymers for fine tune of LUMO levels in optoelectronic applications

In the last decades researchers focused on the synthesis and design of new materials for solar cells in order to boost the power conversion efficiency (PCE). One way it is to use NIR sensitizer since more than 50% of solar radiation falls in the NIR spectral region.

One approach in order to tune the energy levels and achieve small band gap polymers or molecules is alternating donor-like (electron donating) and acceptor-like (electron withdrawing) monomer. 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 small band gap in order to maximize the short circuit current (J_sh). Alternatig “weak donor” and “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 polymer based on thienopyrazine acceptor and carbazole donor. We have compared the optoelectronic properties of classic PCDTBT (poly-carbazole-dithiophene-benzothiadiazole) polymer with one where benzothiadiazole has been substituted with thienopyrazine and one where we have randomly alternate thienopyrazine and benzothiadiazole as acceptor unit, maintaining the same donor unit. The approach to alternate weak donor- strong acceptor moieties allow to obtain fine LUMO levels modulation of the materials opening the way to the design of new donor materials.