Optimization of side chains in alkylthiothiophene-substituted benzo[1,2-b:4,5-b′]dithiophene-based photovoltaic polymers

Alkyl side chains play critical roles in the molecular design of conjugated polymers for applications in bulk-heterojunction (BHJ) polymer solar cells (PSCs). Recently, the introduction of alkylthio substituents onto poly(benzo[1,2-b:4,5-b′]dithiophene-alt-thieno[3,4-b]thiophene) (PBDTTT)-based conjugated polymers has been proved to be an effective method to improve the photovoltaic properties of the polymers. In this contribution, three alkylthiothiophene-substituted benzodithiophene (BDT-TS) based polymers, named PBDT-TS1, PBDT-TS2 and PBDT-TS3, were synthesized and applied as donor materials in PSCs. In these three polymers, octyl, 2-ethylhexyl and 3,7-dimethyloctyl are used on their BDT units, respectively. The polymers were characterized in parallel by absorption spectroscopy, thermogravimetric analysis (TGA), electrochemical cyclic voltammetry (CV) and grazing-incidence wide-angle X-ray scattering (GI-WAXS), and also their photovoltaic properties in PSCs were studied and compared. The results reveal that the alkyls have little influence on absorption spectra and molecular energy levels of the polymers. The GI-WAXS results show that PBDT-TS1 has stronger and tighter π-π stacking than the other two polymers, implying that linear alkyls may reduce steric hindrance than branched alkyl chains in an aggregation state. As a consequence of the strong π-π inter-chain packing of PBDT-TS1, an increased short circuit current density (J_SC) and fill factor (FF) as well as a power conversion efficiency of over 9.5% are achieved in single-cell BHJ devices, which are obviously higher than those for devices based on the other two polymers. Overall, the results of this work suggest that alkyl side groups play an important role in affecting the π-π stacking of the conjugated polymers, i.e., the linear octyl has weaker steric hindrance for the inter-chain π-π stack than the branched 2-ethylhexyl and 3,7-dimethyloctyl, and for the highly efficient polymer based on the 2-alkylthiothiophene-substituted BDT, PBDT-TS1 has the optimal structure.