Synthesis and characterization of fluorene-based low-band gap copolymers containing propylenedioxythiophene and benzothiadiazole derivatives for bulk heterojunction photovoltaic cell applications

The following noble series of soluble π-conjugated statistical copolymers was synthesized by palladium catalyzed Suzuki polymerization: poly[(9,9-dioctylfluorene)-alt-(4,7-bis(3',3'-dihepyl-3,4-propylenedioxythienyl)-2,1,3-benzothiadiazole)] (PFO-PTBT) derived from poly(9,9-dioctylfluorene) (PFO) and poly[(4,7-bis(3',3'-dihepyl-3,4-propylenedioxythienyl)-2,1,3-benzothiadiazole)] poly(heptyl₄-PTBT). The structure and properties of these polymers were characterized using ¹H-, ¹³C-NMR, UV-visible spectroscopy, elemental analysis, GPC, DSC, TGA, photoluminescence (PL), and cyclic voltammetry (CV). The statistical copolymers, PFO-PTBT (9:1, 8.4:1.6, 6.5:3.5), were soluble in common organic solvents and easily spin coated onto indium-tin oxide (ITO) coated glass substrates. The weight-average molecular weight (Mw) and polydispersity of the PFO-PTBT ranged from (1.0-4.2) x 10⁴ and 1.5-2.3, respectively. Bulk heterojunction photovoltaic cells with an ITO/PEDOT/PFO-PTBT:PCBM/LiF/Al configuration were fabricated, and the devices using PFOPTBT (6.5:3.5) showed the best performance compared with those using PFO-PTBT (9:1, 8.4:1.6). A maximum power conversion efficiency (PCE) of 0.50% (Voc = 0.66 V, FF = 0.29) was achieved with PFO-PTBT (6.5:3.5).