BODIPY derivatives with near infra-red absorption as small molecule donors for bulk heterojunction solar cells

The study of small donor molecules as the active component of organic solar cells continues to attract considerable attention due to the range of advantages these molecules have over their polymeric counterparts. Here we report the synthesis and solar cell fabrication of three BODIPY small molecule donors. Two of the dyes feature triphenylamine and phenothiazine as donor units attached to the meso and -positions of the BODIPY core (TPA-PTZ-DBP and PTZ-TPA-BDP). Additionally, we have synthesised a push-pull derivative featuring phenothiazine moieties in the -positions and a nitrobenzene in the meso-position (N-TPA-BDP) in order to investigate what effect this type of functionalisation has on the photovoltaic properties compared to the other dyes. The optoelectronic properties were investigated and the dyes showed broad absorption in the near-infrared with high extinction coefficients. Electrochemical measurements indicated good reversibility for the dyes redox processes. In contrast with the all-donor functionalised systems, N-TPA-BDP demonstrated extensive HOMO-LUMO overlap by DFT. The dyes were investigated as donor molecules in bulk heterojunction solar cells along with PC71BM, and under optimal donor to acceptor ratio PTZ-TPA-BDP showed the highest PCE of 1.62%. N-PTZ-BDP:PC71BM was the only blend to further improve upon thermal annealing reaching the highest conversion efficiency among the dyes of 1.71%. A morphology comprised of finely mixed donor and acceptor components is observed for BHJ blends of each of the three donors at their optimum fullerene content. Upon thermal annealing, these morphological features remain mostly the same for PTZ-TPA-BDP:PC71BM and TPA-PTZ-DBP:PC71BM blends whereas for N-PTZ-BDP:PC71BM the domains show a larger size. These dyes show that phenothiazine functionalisation of BODIPY is useful for solar cells because it gives strong and broad absorption extending to the near infra-red and materials with reversible redox properties - both of which are desirable for organic solar cells.