Chain Length Dependence of the Photovoltaic Properties of Monodisperse Donor-Acceptor Oligomers as Model Compounds of Polydisperse Low Band Gap Polymers

Well-defined conjugated oligomers (Sn) containing from 1 to 8 units of a tricyclic building block involving a dioctyloxybenzothiadiazole unit with two thienyl side rings (S1) are synthesized by a bottom-up approach. UV-Vis absorption data of solutions show that chain extension produces a narrowing of the HOMO-LUMO gap (Delta E) to values slightly smaller than that of the parent polymer (P1). Plots of Delta E and of the band gap of films (E-g) versus the reciprocal chain length show that Delta E and E-g converge towards a limit corresponding to an effective conjugation length (ECL) of 7-8 S1 units. UV-Vis absorption and photoluminescence data of solutions and solid films show that chain extension enhances the propensity to inter-chain aggregation. This conclusion is confirmed by GIXD analyses which reveal that the edge-on orientation of short-chain systems evolves toward a face-on orientation as chain length increases while the pi-stacking distance decreases beyond 7 units. The results obtained on solution-processed BHJ solar cells show a progressive improvement of power conversion efficiency (PCE) with chain extension; however, the convergence limit of PCE remains inferior to that obtained with the polymer. These results are discussed with regard to the role of mono/polydispersity and chain aggregation.