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 (J_sc) and V_oc 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 J_sc and V_oc 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.