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

Two types of coadsorbents based on a carbazole unit containing a carboxylic acid acceptor linked by extended pi-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 pi-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 pi-conjugated linkers is too high for electron injection into the TiO2 CB, and has less preventing effect on the pi-pi 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 pi-conjugated linkers for efficient electron injection and a compact blocking layer on the TiO2 surface for preventing the pi-pi stacking of dye molecules, simultaneously.