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

ABSTRACT

An 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).

KEYWORDS:

• Benzothiadiazole
• organic solar cells
• phenotiazine
• small-molecule

Funding

This research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (20153010140030) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (201603930154).