Enhancing the photovoltaic performance of triphenylamine based star-shaped molecules by tuning the moiety sequence of their arms in organic solar cells

Isomeric star-shaped molecules of TPA(T-BT-3Cz)₃ and TPA(BT-T-3Cz)₃ with different moiety sequences in their arms based on the triphenylamine (TPA) core were designed and synthesized in order to study the influence of inserting thiophene (T) units in their arms and the effects of their positions on the photo-physical and photovoltaic properties of their molecules. For comparison, parent molecules of TPA(BT-3Cz)₃ were also prepared with three arms containing 4,7-benzothiadiazole (BT) and 3-carbazolyl (3Cz) units. After the thiophene unit was inserted, the molecular planarity, absorption spectra and hole mobility improved significantly in the resulting TPA(BT-T-3Cz)₃ and TPA(T-BT-T-3Cz)₃. However, only TPA(BT-T-3Cz)₃ exhibited improved photovoltaic properties in contrast to TPA(BT-3Cz)₃ in the solution-processable solar cells with a device structure of ITO/PEDOT:PSS/SMs:PC₇₁BM/Ca/Al. A maximum power conversion efficiency (PCE) of up to 5.07% with a short-circuit current density (J_sc) of 10.08 mA cm^-2, an open circuit voltage (V_oc) of 0.90 V and a fill factor (FF) of 54.05% was obtained in the TPA(BT-T-3Cz)₃-based solar cells. To the best of our knowledge, this PCE is one of the highest PCE values for star shaped molecules with the TPA core in organic solar cells. Our work further indicates that the PCE and J_sc values of organic solar cells with TPA-based star-shaped molecules can be increased by inserting T units and tuning the moiety sequence of their arms.