Near-IR Absorbing D-A-D Zn-Porphyrin-Based Small-Molecule Donors for Organic Solar Cells with Low-Voltage Loss.

Two D-A-D small molecules with a DPP acceptor core and Zn-porphyrin donor with different electron-donating substituents, namely, 2,6-bis(dodecyloxy)phenyl and 5-hexylthieno[3,2- b]thiophen-2-yl at mesopositions, VC4 and VC5, were synthesized, and their optical and electrochemical properties were investigated. The results reveal that both molecules are suitable as donors for organic solar cells (OSCs) in which PCBM is employed as the acceptor. Overall power conversion efficiencies of 8.05% ( J = 13.83 mA/cm, V = 0.91 V, and FF = 0.64) and 8.89% ( J = 16.98 mA/cm, V = 0.79 V, and FF = 0.663) were obtained, respectively. The high V value for the VC4-based OSC correlates with the deeper HOMO, whereas the high J value for VC5 may be attributed to the extended absorption spectrum toward the longer wavelength region. Moreover, the relatively high FF value for VC5-based OSCs as compared to the VC4 counterparts may be related to the more balanced charge transport in the active layer, reduced charge recombination, and efficient charge collection. The energy loss for VC5 is smaller (0.52 eV) than that for VC4 (0.56 eV).