Synthesis and photovoltaic properties of two star-shaped molecules involving phenylquinoxaline as core and triphenylamine and thiophene units as arms

Two novel star-shaped D-A-type small molecules of TPA-PQ-T and TPA-PQ-2T were designed and synthesized, in which the hole-transporting moieties of triphenylamine (TPA) and thiophene (T) or double thiophene (2T) were used as arms and donor units, phenylquinoxaline (PQ) moiety was used as central core and acceptor unit, respectively. Their optical, thermal, electrochemical and photovoltaic properties were investigated. Both compounds films show a strong absorption peak in the range 250-600 nm, and a high thermal stability at more than 430 °C, respectively. The highest power conversion efficiency (PCE) of 1.28% with a short-circuit current density (J_sc) of 6.13 mA/cm², an open circuit voltage (V_oc) of 0.74 V and a fill factor (FF) of 32% was obtained in the solution-processed TPA-PQ-2T-based solar cells using [6,6]-phenyl-C-61-butyric-acid-methyl-ester (PC₆₁BM) as acceptor under the illumination of AM 1.5 G, 100 mW/cm². The PCE and J_sc values are 2.98 and 2.59 times higher than those of the TPA-PQ-T-based cells, respectively.