Push-pull architecture eliminates chain length effects on exciton dissociation

Recent development of small molecule non-fullerene acceptors has led to remarkable performance when incorporated in organic photovoltaic devices. These non-fullerene acceptors typically consist of about 10 aromatic rings arranged in an alternating electron-rich/electron-deficient architecture reminiscent of push-pull polymers, making them "push-pull oligomers". Without the extended conjugation length of a polymer, it is perhaps surprising that devices incorporating oligomeric non-fullerene acceptors perform so well. To investigate exciton dissociation as a function of chain length, we synthesized a series of donor-acceptor block copolymers consisting of a conjugated homopolymer electron donor, poly(3-hexylthiophene-2,5-diyl) (P3HT), covalently linked to a push-pull polymer electron acceptor, poly-((2,5-dihexylphenylene)-1,4-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (PPT6BT). By adjusting synthetic parameters, the chain length of each block is selectively tuned. The block copolymers are dissolved as isolated chains in dilute solutions and intramolecular charge transfer is quantified. When the P3HT block is very short (<3 nm), charge transfer is inhibited. Nevertheless, efficient charge transfer is observed for PPT6BT block lengths ranging from essentially a single repeat unit to 16 nm. This indicates that the polarized nature and charge transfer character of excited states generated along push-pull polymers facilitate exciton dissociation.