An octyloxy substituted benzofurazan based poly(carbazole-alt-thiophene-benzofurazan) (PCzDTBF) was synthesized by Suzuki polycondensation. The absorption spectra of the polymer were peaked at 385 and 540 nm, and the HOMO and LUMO energy levels were −5.34 and −3.46 eV, respectively. The hole mobility was found to be 4.8 × 10⁻⁴ cm² V⁻¹ S⁻¹ by the space charge limited current (SCLC) method, and the surface energy (E_s) was calculated to be 35.8 mJ m⁻² from the contact-angle measurement. Polymer solar cells (PSCs) based on the blend of PCzDTBF and PCBM with a weight ratio of 1 : 2 were fabricated. The devices were found to have a power conversion efficiency (PCE) of 4.2% in the device configuration of ITO/PSS:PEDOT/PCzDTBF:PC₇₀BM/Al. By utilizing poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) as the cathode interfacial layer, the PCE was enhanced to 5.48%, with an open-circuit voltage (V_oc) of 0.90 V, a short-circuit current density (J_sc) of 7.73 mA cm⁻², and a fill factor (FF) of 67%. The high efficiencies could be ascribed to the good morphology, resulting from the matched surface energies of the PCzDTBF and PCBM components, and the distinct enhancement on V_oc and FF by the PFN layer, where it is possible that a built-in field potential exists between the active layer and cathode.