Poly(3-hexylthiophene-co-benzothiadiazole) (THBT) as an electron-accepting polymer for normal and inverted type all-polymer solar cells

Abstract

A new n-type polymer, poly(3-hexylthiophene-co-benzothiadiazole) (THBT), which contains the same molar ratio of p-type (3-hexylthiophene) and n-type (benzothiadiazole) units in the main chain, was synthesized by Suzuki coupling reaction. Optical and photoelectron measurements showed that the THBT polymer features an optical band gap energy of 1.8 eV, a highest occupied molecular orbital energy of 5.7 eV and a lowest unoccupied molecular orbital energy of 3.9 eV. Synchrotron-radiation grazing incidence angle X-ray diffraction measurements disclosed a strong chain stacking effect in the THBT polymer film, which is similar to the poly(3-hexylthiophene) (P3HT) film case, but the chain stacking distance in the THBT film was different from that in the P3HT film. The zero-field electron mobility of the THBT film was measured as ca. 10⁻⁶ cm² V⁻¹ s⁻¹ when a space-charge limited current method was employed. Based on the characteristics of the THBT polymer, two types (normal and inverted types) of all-polymer solar cells were fabricated using the P3HT:THBT films as an active layer. The result showed that the maximum open circuit voltage reached ca. 0.86 V and the solar cell performance was found to be strongly dependent on the nanomorphology of the P3HT:THBT films upon changing the composition of co-solvents. The fill factor of the inverted solar cells was higher than that of the normal type solar cells, leading to a higher power conversion efficiency for the inverted type P3HT:THBT solar cells.