Influence of Covalent and Noncovalent Backbone Rigidification Strategies on the Aggregation Structures of a Wide-Band-Gap Polymer for Photovoltaic Cells

With the purpose of improving the backbone planarity and thus the charge transport property of the wide-band-gap ester-modified polymer, the covalent and noncovalent (F...S conformational lock) backbone rigidification strategies are, respectively, employed to design two new benzodithiophene and ester-modified oligothiophene-based copolymers (PBDE-TT and PBDE-DFDT). Although thieno[3,2-b]thiophene (TT) and difluorinated 2,2'-bithiophene (DFDT) possess planar conformations with similar conjugation length, polymer PBDE-DFDT shows much stronger aggregation effects in diluted solution and more compact and ordered pi-pi stacking in thin film compared to the polymer PBDE-TT, which are associated with the existence of several nontraditional hydrogen-bonding interactions for the DFDT-based oligothiophene unit. Thus, the hole mobility of polymer PBDE-DFDT is over two times higher than that of its counterpart. The photovoltaic device based on PBDE-DFDT:IT-4F blend achieves a distinctly higher power-conversion efficiency of 14.16%, which is mainly attributed to the higher ordering of aggregates and more symmetric charge transport in the blend film. The results reveal that optimizing the aggregation structures via the modulation of intermolecular interactions is of great importance for enhancing the charge mobility and photovoltaic performance of materials, which shall be considered for further designing high-performance donor polymers with simple chemical structures.