Elevating Thermoelectric Performance by Compositing Dibromo-Substituted Thienoacene with SWCNTs

Composites of organic small molecules (OSMs) and single-walled carbon nanotubes (SWCNTs) have drawn great attention as flexible thermoelectric (TE) materials in recent years. Here, we synthesized thieno[2',3':4,5]thieno[3,2-b]thieno[2,3-d]thiophene (TTA) and 2,6-dibromothieno[2',3':4,5]thieno[3,2-b]thieno[2,3-d]thiophene (TTA-2Br) and compounded them with SWCNTs, obtaining thermoelectric TTA/SWCNT and TTA-2Br/SWCNT composites. The introduction of the electron-withdrawing Br group was found to decrease the highest molecular orbital energy level and bandgap (E_g) of TTA-2Br. As a result, the Seebeck coefficient (S) and power factor (PF) of the OSM/SWCNT composite films were significantly improved. Moreover, suitable energy barrier between TTA-2Br and SWCNTs facilitates the energy filtering effect, which further enhances thermoelectric properties of the 40 wt % TTA-2Br/SWCNT composite film with optimum thermoelectric properties (PF = 242.59 ± 9.42 μW m^-1 K^-2 at room temperature), good thermal stability, and mechanical flexibility. In addition, the thermoelectric generator (TEG) prepared using 40 wt % TTA-2Br/SWCNT composite films and n-type SWCNT films can generate an output power of 102.8 ± 7.4 nW at a temperature difference of 20 °C. This work provides new insights into the preparation of OSM/SWCNT composites with significantly enhanced thermoelectric properties.