A structural study of p-type A-D-A oligothiophenes: effects of regioregular alkyl sidechains on annealing processes and photovoltaic performances

In this study we report on a series of 12 small molecular p-type penta- and hexathiophene semiconductors end-capped with either malononitrile (MN), hexyl rhodanine (HR) or dicyano hexyl rhodanine (CHR) acceptors and functionalized with alkyl sidechains of two different lengths, orientated away from the central core in a regioregular manner. Organic photovoltaic devices were fabricated, and annealed using conditions optimized for each material to produce a wide range of power-conversion efficiencies (PCEs) from 1-8%. We find that achieving high PCEs is critically dependent on matching the conjugation length to specific end-group acceptors, and also find a slight correlation between improved performances and longer sidechains. To understand this behavior we performed an in-depth analysis of the structure-property relationships of these materials on intermolecular packing through grazing-incidence wide-angle X-ray scattering (GIWAXS) studies, which yielded several key relationships between molecular structure, bulk crystallinity and optoelectronic properties.