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

Abstract

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 (Es) 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 (Voc) of 0.90 V, a short-circuit current density (Jsc) 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 Voc and FF by the PFN layer, where it is possible that a built-in field potential exists between the active layer and cathode.