Synthesis and Photovoltaic Properties of Solution Processable Small Molecules Containing 2-Pyran-4-ylidenemalononitrile and Oligothiophene Moieties

A series of solution processable small molecules (4TPM, 6TPM, and 8TPM) were synthesized with 2-pyran-4-ylidenemalononitrile (PM) as the electron-accepting unit and oligothiophene with different numbers as the electron-donating unit. Differential scanning calorimetry (DSC) measurement indicated that melting point and crystal temperature of molecules increased with the increase of thiophene number. UV-vis absorption demonstrated that the combination of PM with oligothiophene resulted in an enhanced intramolecular charge transfer (ICT) transition, which led to an extension of the absorption of the molecules. Cyclic voltammetry investigation displayed that the highest occupied molecular orbital (HOMO) energy levels of the three molecules were relatively low, which promised good air stability and high open circuit voltage (V-oc) for photovoltaic application. Theoretical calculations revealed that the variation laws of HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels are well consistent with cyclic voltammetry measurement. The bulk heterojunction (BHJ) photovoltaic devices with the structure of ITO/PEDOT-PSS/small molecules-PCBM/LiF/Al were fabricated, with the three molecules as donor and (6,6)-phenyl-C-61-butyric acid methyl ester (PCBM) as the acceptor. The device based on 6TPM/PCBM (30:70 w/w) successfully achieved a maximum power conversion efficiency (PCE) of 1.15% under the illumination of AM 1.5, 100 mW/cm(2).