An Insight into the Potential of Random Poly(heteroarylene-vinylene)s as Donor Materials in Bulk Heterojunction Solar Cells

The manuscript describes the design, preparation and characterization of two structurally isomeric random poly(arylene−vinylene)s, the properties of which have been optimized for their use as donor materials in BHJ solar cells. The structure of the polymers was aimed at broadening as much as possible their absorption profile. Poly[9,9-dioctylfluorene− vinylene-co-4,7-dithiophen-2-yl-benzo[1,2,5]thiadiazole−vinylene] (P1) and poly[2,7-dithiophen-2-yl-9,9-dioctylfluorene−vinylene- co-4,7-benzo[1,2,5]thiadiazole−vinylene] (P2) were prepared using the Suzuki−Heck polymerization. The polymers were characterized by elemental analysis, NMR, UV−vis absorption and photoluminescence, cyclic voltammetry, and GPC. The electrochemical characterization of P1 and P2 revealed similar HOMO/LUMO energy levels, although the UV−vis absorption profile of P2 is markedly broader than the one exhibited by P1. The more panchromatic absorption of P2 was explained by DFT and TDDFT calculations showing that the model systems, contributing together to the description of the random polymeric structure, exhibited different calculated excitation energies, that cover a broader portion of the absorption spectrum. In BHJ solar cells, the broadness of the absorption strongly influences the BHJ solar cell performances of P2 compared to P1 leading to higher short circuit currents and to a 3-fold higher power conversion efficiency. The PCE value (0.6%) obtained with P2 is in line with those obtained for other poly(heteroarylene−vinylene)s donors and is amenable to improvement by optimizing the device construction (PC61BM amount in the blend or use of annealing processes). These results demonstrate how combination of a suitable choice of the sequence of aryl units together with the potentialities offered by random polymers, can be useful tools in the design of new light-harvesting polymers in BHJ.