Bulk heterojunction photovoltaic cells based on room temperature liquid crystalline tetrathiafulvalene derivatives

We synthesised a tetrathiafulvalene (TTF) derivative having an asymmetric structure of a TTF mesogenic core and long alkyl chains which shows a highly ordered columnar liquid crystalline phase at room temperature. A bulk heterojunction photovoltaic cell was fabricated by using the TTF liquid crystal compound (a-6TTF12) as an electron donor and [6,6]-phenyl C ₆₁-butyric acid methyl ester (PC ₆₀BM) as an electron acceptor. The photoconversion efficiency (PCE) of the cell was very low (0.063%) as prepared, which increased to 0.120% after thermal annealing at 80°C for 20 min. The annealing effect was investigated by means of UV spectroscopy, X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) and energy-dispersive X-ray spectroscopy (EDS). The morphological observations of the annealed a-6TTF12: PC ₆₀BM composite film indicate that the higher PCE was attributed to the enhanced ordering of TTF molecules by the thermal annealing process to form a highly ordered columnar structure, which results in better charge-carrier mobility.