Two-dimensional Conjugated Polymers Containing Isotruxene and Oligothiophene Moieties: Synthesis, Characterization and Application to solar cells

Polythiophene (PTs) is an important class of conducting polymers for polymer solar cells (PSCs) owing to its broad absorption range and high electronic conductivity. Recently, we introduced oligothiophene unit to the ortho-, para-conjugated isotruxene core to form two-dimensional conjugated oligomers and polymers. The ITTn (n=1-4) oligomers that contain an isotruxene core and oligothiophene arms. Then oligomers ITTn were converted to the two-dimensional polymers. Two different polymerization methods have been tested and compared. One method is the direct oxidative polymerization of ITTn using FeCl3 as oxidant to form PITTn. The other method is through Stille coupling reaction for ITTn and 3-hexylthiophene (HT) to form PITTnHT.
The results indicate that the degree of polymerization was controlled by the solubility and steric effect. Due to the similar solubility, the monomers can’t be removed from PITTn. PITTnHT have the lower band gap because of the higher degree of polymerization. When applied in PSCs, the PCE of PITTnHT/PCBM (1:2 % wt) is 1.00%, and that of the PITTn series reaches as PCE of 1.66%.
The comparison of PITT2HT and PTT2HT indicates that intramolecular charge transportation plays an important factor in determining the performance of PSCs. A material of excellent intramolecular charge transportation can significantly enhance the performance of PSCs.