Synthesis of perylene monoimide derivative and its use for quasi-solid-state dye-sensitized solar cells based on bare and modified nano-crystalline ZnO photoelectrodes

A novel perylene monoimide derivative, PCA, with a strongly electron-donating cyclohexylimide segment, bulky alkylphenoxy substituents at the 1,7-bay positions of the perylene core and an acid anhydride as the strong coupling group was synthesized and used as sensitizer for dye-sensitized solar cells (DSSCs). PCA was readily soluble in common organic solvents, decomposed above 390 degrees C and had glass transition temperature of 64 degrees C. The absorption curves had maximum at 505-512 nm, with thin film absorption onset at 643 nm corresponding to an optical band gap of 1.93 eV. We have fabricated the quasi-solid-state DSSCs with ZnO nano-crystalline films deposited from different values of pH. It is found that the power conversion efficiency (PCE) is high when the pH value of sol gel is about 9. The PCE of DSSC fabricated with the gold-coated ZnO photoanode is higher than that for bare ZnO photoanode. This enhancement is attributed to the formation of Schottky barrier at the Au/ZnO interface that blocks the electron transfer from ZnO to dye and electrolyte, increasing the electron density in the conduction band of ZnO. The incorporation of TiO2 nanoparticles in polymer gel electrolyte further increases the PCE of DSSC up to 3.0%. The enhancement is primarily explained by studying the dark reaction, diffusion coefficient of tri-iodide and exchange current density in the interface of electrolyte/Pt counter electrolyte, and lifetime of electrons in the anode film.