An effect on the side chain position of D-π-A-type conjugated polymers with sp2-hybridized orbitals for organic photovoltaics

A D-pi-A-type poly[alkylidenefluorene-alt-di-2-thienyl-2,1,3-benzothiadiazole] (P1) was synthesized via Suzuki coupling reaction. Based on the positions of the spacer (p) and acceptor (A) in the polymers, a dodecyl chain (P2) and an octyloxy chain (P3), respectively, were introduced. Both chains were introduced to the spacer and acceptor of P4. The obtained polymers (P2-P4) were soluble in organic solvents such as chlorobenzene, THF and o-dichlorobenzene at room temperature. Also, the introduction of a dodecyl chain to the spacer reduced the energy of the highest occupied molecular orbital (HOMO) level (-5.5 to -5.56 eV) but increased the tilt angle (45.4-53.0 degrees), which had prevented the main chain from pi-pi stacking. The orientation of the obtained polymers in thin films was confirmed by XRD measurement. P3 with only an octyloxy chain showed a face-on-rich structure with a pi-pi stacking distance of 3.7 angstrom compared to other polymers. The polymer solar cells were fabricated through a solution process, and showed a power conversion efficiency (PCE) of 3.6%, with a short-circuit current density (J(sc)) of 8.9 mA cm(-2), an open-circuit voltage (V-oc) of 0.88 V, and a fill factor (FF) of 45.7%. With the introduction of poly[9,9-bis(6'-(diethanolamino)hexyl)fluorene] (PFN-OH), a PCE of 3.9% was confirmed.