Small molecules incorporating regioregular oligothiophenes and fluorinated benzothiadiazole groups for solution-processed organic solar cells

Small molecules incorporating regioregular oligothiophenes and fluorinated benzothiadiazole groups were synthesized for solution-processed organic solar cells. The length of oligothiophene units showed profound influences on light absorption, energy levels, crystalline behaviors and active layer morphologies. The photovoltaic performances were tested using mixed-solvent processing methods. When blended with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the active layer, the shorter molecule, 4TA4T, achieved a power conversion efficiency (PCE) of 3.81%, with an open circuit voltage (V_oc) of 0.83 V, a short current density (J_sc) of 8.02 mA cm^-2 and a Fill Factor (FF) of 0.57. The longer molecule, 6TA6T, showed a lower PCE of 3.21% due to a lower V_oc of 0.74 V. This study provides detailed insights for the designing of small molecules by subtle changes in donor lengths.