Conjugated polymers based on benzodithiophene and fluorinated quinoxaline for bulk heterojunction solar cells: thiophene versus thieno[3,2-b]-thiophene as pi-conjugated spacers

Two conjugated donor-acceptor copolymers based on a benzodithiophene donor unit and a fluorinated quinoxaline acceptor unit, spaced with either thiophene or thieno[3,2-b] thiophene pi-bridges, were designed and synthesized. The effect of different pi-bridges and of the processing conditions on optical, electrical, morphological and photovoltaic properties of the polymer: fullerene blend films were investigated. The polymer containing the thieno[3,2-b] thiophene pi-bridge (PBDTFQ-TT) showed a red-shifted absorption and an enhanced charge carrier mobility, as compared to its analogue with the thiophene pi-bridge (PBDTFQ-T), due to its narrower optical gap (by similar to 0.1 eV) and stronger inter-chain interactions, favored by the structural planarity and increased linearity of the polymer backbone, as also supported by DFT calculations. The blend of PBDTFQ-TT and PC61BM ([6,6]-phenyl-C-61-butyric acid methyl ester), compared to the PBDTFQ-T: PC61BM one processed under the same conditions (by blade-coating technique), showed greatly enhanced photovoltaic performance, with more than doubled power conversion efficiency (PCE up to 5.60% for the best device) due to the increased short-circuit current density and fill factor. However, similar PCEs were also achieved for PBDTFQ-T:PC61BM-based devices by optimizing the processing conditions through the addition of 1,8-diiodooctane (DIO) as the solvent additive. Through morphological and electrical analysis of the films, produced with and without additive, it was observed that the addition of DIO greatly enhances the self-organization, and consequently the charge mobility, of the thiophene pi-bridge-based polymer, while it was detrimental for the nanoscale morphology and photovoltaic performances of the thieno[3,2-b] thiophene pi-bridge-based polymer in the corresponding blend.