Polythiophene derivative comprising carbazoles as pendant groups for polymer solar cell applications

We have prepared a polythiophene copolymer (CzPh-PT) containing 9-position substituted carbazoles as bulky pendants and investigated its application in polymer solar cell (PSC). Electrochemical study presented that the onset oxidation potential positively shifted 0.47 eV for CzPh-PT in comparison with that of poly(3-hexyl thiophene). By incorporating the bulky substituent of CzPh-PT, the effective conjugation length can be curtailed, and consequently a lowered HOMO energy level (-5.38 eV) was acquired. This promises better air stability and a high open circuit voltage (Voc) for the PSC. Moreover, the optical properties of CzPh-PT reveal that the presence of π-π interaction between carbazole units can enhance light harvesting ability in visible region, leading to a wide absorbance. PSC was fabricated based on an interpenetrating network of CzPh-PT as electron-sufficient polymer and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) as electron-deficient material. The PSC device based on the blend of CzPh-PT/PC61BM (w/w = 1:1) gave the best preliminary result with a Voc of 0.85 V, a short-circuit current of 1.57 mA/cm2, and a fill factor of 0.27, offering an overall power conversion efficiency of 0.36%. Our finding suggests that the polythiophene derivative comprising bulky steric groups as side chains is promising for PSC applications.