Enhancing the Performance of Solution-Processed Bulk-Heterojunction Solar Cells Using Hydrogen-Bonding-Induced Self-Organization of Small Molecules

Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular connectivity of the molecules. In order to enhance the crystallinity for the solution-processed small molecule, it is possible to make use of carboxylic acid end-functional groups to drive hydrogen-bonding-induced π-π stacking of conjugated molecules. Herein, we report the synthesis and characterization of quarterthiophenes with carboxylic acid as end groups. The formation of hydrogen bonds between neighboring acid groups gives rise to a pseudo-polymeric structure in the molecules, which leads to substantial improvement in the organization and crystallinity of the active layers. This resulted in a four-fold increase in the hole mobility and a two-fold improvement in the performance of the solar cell device for the acid-functionalized molecule, compared to its ester analogue. More importantly, optimal device performance for the acid-functionalized molecule was achieved for the as-cast film, thereby reducing the reliance on thermal annealing and solvent additives.