Synthesis and characterization of novel conjugated copolymers for application in third generation photovoltaic solar cells

An organic photovoltaic cell (OPV) is a subcategory of the third generation of photovoltaic cells. Advantages of OPV include lightness and ease of production in industrial scale. This work proposes the synthesis and characterization of two novel random and block copolymers for OPV containing fluorene (F), thiophene (T) and benzothidiazole (B) as structural units. Polymerizations were performed via the Suzuki-Miyaura mechanism. The yields obtained for both copolymers were around 65% with corresponding molar masses measured by GPC of 3394 and 4598 g/mol. From TGA, the onset degradation temperature was approximately 415 °C, while the maximum degradation temperature was around 440 °C, for both copolymers. FTIR and NMR analyses confirmed the ratio of aliphatic and aromatic carbons in the structure of both copolymers. Difference of chaining between both copolymers was verified by 13CNMR and XRD. The energy values of HOMO were -5.76 eV for random and -5.74 eV for block copolymer. Through UV-Vis spectroscopy, optical bandgaps of 2.39 and 2.34 eV were obtained for random and block polymers, respectively. Finally, LUMO energy levels of -3.37 eV (random) and -3.40 eV (block) were calculated by subtracting the optical band gap from the HOMO energy. Mobility measurements by JV method were performed in pure polymer films, rendering 1.91 × 10−6 and 1.35 × 10-8 cm2 /V.s for random and block copolymers, respectively. Copolymers were applied in conventional and inverted devices, presenting diode behavior. Energy efficiency obtained was higher for the block copolymer, which was consistent with the obtained interplanar distances estimated by XRD.