Highly efficient polymer solar cells based on poly(carbazole-alt-thiophene-benzofurazan)

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 x 10(-4) cm(2) 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.