Low Energy Loss of 0.57 eV and High Efficiency of 8.80% in Porphyrin-Based BHJ Solar Cells

A new A-pi-D-pi-A small molecule, denoted as MV72, has been designed and synthesized. The molecule comprises a Zn-porphyrin central donor unit and dicyanovinylene-substituted rhodanine (RhCN) end-capping acceptor units linked by bisthienylenevinylene groups as pi-conjugated bridges. The optical and electrochemical properties of MV72 have been investigated as an electron donor for solution-processed organic solar cells (OSCs), and the results were compared to those of MV71, which consists of the same molecular backbone but with different rhodanine end-capping acceptor units. The effects of the end-capping terminal units on the absorption spectra, molecular crystallinity, frontier molecular energy levels, charge transport properties, morphology of active layers, and photovoltaic performance have been investigated. The OSCs based on optimized active layers of MV71/PC71BM and MV72/PC71BM (donor and acceptor weight ratios and solvent vapor annealing) deliver overall power conversion efficiencies (PCEs) of 7.61% and a remarkable 8.80%, respectively. MV72/PC71BM showed impressively low energy loss, with a value as low as 0.57 eV; accordingly, a high open-voltage (V-OC) of 0.88 V was achieved. The low energy loss in the MV72-based OSCs is related to the increased dipole moment change for MV72, which in turn results in low exciton binding energy.