A-π-D-π-A type oligomer based on carbazole and benzothiadiazole for organic solar cells

Abstract

ABSTRACTAn acceptor-donor-acceptor (A-p-D-p-A) type oligomer based on benzothiadiazole and 2,7 carbazole, 2,7-Bis(5-{7–2[-(4-decyloxy-phenyl)-vinyl]-benzo[1,2,5]thiadiazol-4-yl}-thiophen-2-yl)-9-(2-octyl-dodecyl)-9 H-carbazole (CzBT-DP), is synthesized by the Suzuki Coupling reaction. In addition, to extend the effective of conjugated length, we introduce thiophene (T) ring as a p-bridge and stilbene in the end of the conjugated chain. Optical, electrical and thermal properties and photovoltaic performances of CzBT-DP is systematically investigated. UV-vis absorption spectra and cyclic voltammograms (CV) reveal that HOMO/LUMO level of CzBT-DP is −5.37 eV/−3.38eV with an optical band gap of 1.99 eV, respectively, due to the addition of conjugated chain using stilbene compound system. The HOMO energies of CzBT-DP were calculated from the measured onset potential of oxidation by assuming the energy level of ferrocene (Fc) as −4.8 eV by CV. Addition of thiophene affects the HOMO level which makes CzBT-DP has high-lying HOMO level. Due to higher LUMO energy values above 0.3 eV than PC71 BM, CzBT-DP could be applied to BHJ OSCs as the donor [1]. Inverted type organic solar cells (OSCs) with a configuration of ITO/ZnO/CzBT-DP:PC71BM/MoO3/Al are fabricated. Figure 1 shows the current density-voltage (J-V) characteristics of the optimized BHJ OSCs based on CzBT-DP:PC71BM blend ratios from 3:2 to 3:6 (w/w). Maximum power conversion efficiency (PCE) is 0.64% with a short-circuit current density (Jsc) of 1.78 mA/cm2, Voc of 0.94 V, and fill factor (FF) of 38.3%. The best blend ratio between CzBT-DP and PC71BM having the best PCE appeared at 3:3 (w/w).