Structural effect of carbazole-based coadsorbents on the photovoltaic performance of organic dye-sensitized solar cells

Abstract

Two types of coadsorbents based on a carbazole unit containing a carboxylic acid acceptor linked by extended π-conjugated linkers, e.g., biphenyl (HC-A′) and tetramethylbiphenyl (HC-A′′), were synthesized. They were used as coadsorbents in dye-sensitized solar cells (DSSCs) based on the porphyrin dye (2Flu–ZnP–CN–COOH:FP). For comparison, the π-conjugated phenyl linker (HC-A1) previously developed by our group was used as a coadsorbent. As a result, the DSSCs based on HC-A′ and HC-A′′ displayed power conversion efficiencies (PCEs) of 6.47 and 5.85%, respectively, while the HC-A1-based DSSC achieved a PCE of 7.46%. The HC-A′′-based DSSC exhibited lower short-circuit current (Jsc) and Voc compared to the HC-A′-based DSSC, due to the fact that the dihedral angle of the π-conjugated linkers is too high for electron injection into the TiO2 CB, and has less preventing effect on the π–π stacking of dye molecules due to its lower adsorbed amount, resulting in lower Jsc and Voc values. Therefore, it is important for coadsorbents to have a smaller dihedral angle of the π-conjugated linkers for efficient electron injection and a compact blocking layer on the TiO2 surface for preventing the π–π stacking of dye molecules, simultaneously.