Comparative study on the photovoltaic characteristics of A-D-A and D-A-D molecules based on Zn-porphyrin; a D-A-D molecule with over 8.0% efficiency

The synthesis of two symmetrical molecules with A-D-A (1) and D-A-D (2) structures is reported. The compounds have Zn-porphyrin as the donor component and 3,6-di(thiophen-2-yl) pyrrolo[3,4-c] pyrrole-1,4- dione (DT-DPP) as the acceptor unit, linked through ethynyl p-bridges. The effect of scaffold modification on the electrochemical, optical and photovoltaic characteristics has been investigated and favourable properties are found for the D-A-D system compared to the A-D-A. These two Zn-porphyrin-based small molecules are used as donors along with PC71BM as the acceptor for the fabrication of solution processed bulk heterojunction organic solar cells in order to obtain information about the effect of donor and acceptor positions on the photovoltaic response of the devices. A strategy for optimizing the morphology of the active layer using a binary solvent additive has been used. After the optimization of the weight ratios between 1 or 2 and PC71BM and the solvent additives, the devices based on 1 : PC71BM and 2 : PC71BM active layers show remarkable PCE values of 7.04% and 8.03%, respectively. The higher PCE obtained for 2 : PC71BM compared to 1 : PC71BM can be related to the increase in J(sc) and FF and attributed to lower energy loss, strong pi-pi intermolecular stacking and more balanced charge transport.