Synthesis of conjugated polymers with directly coupled 2-butyloctyloxybenzodithiophene and benzothiadazole units for application as active layers in organic solar cells

Abstract

One of the fastest growing areas in polymer science is the photovoltaic organic devices, which conjugated polymers are an important part of, since they constitute the core of bulk heterojunction active layer. In this work two benzodithiophene based polymers containing fluorinated and non-fluorinated benzothiadiazole units were successfully synthesized. It was observed that chain growth depends upon the nature of the electron deficient monomer, being fluorinated monomer able to produce higher chain size and distribution. Low band gap was obtained (1.71 and 1.72 eV for fluorinated and non-fluorinated polymers, respectively) as well as values of −3.78 and −3.66 eV for LUMO orbitals energy were estimated. The synthesized polymers presented degradation temperatures above 280 °C, which makes them able to be processed through roll-to-roll deposition techniques. Charge carriers mobilities are 4.6 × 10−6 and 1.0 × 10−5 cm2 V−1.s−1 for non-fluorinated and fluorinated polymers, respectively: this last higher charge carrier mobility was attributed to the interaction between the fluorine atoms and the adjacent aromatic sulfur, which according to other authors promotes planarity and consequently, it favors the charge carriers mobility. AFM images showed a higher roughness correlated with phase separation for fluorinated polymer; conduction paths with dimensions less than 25 nm were detected which is positive for future improvement of solar devices.