Metal–organic frameworks: promising materials for improving the open circuit voltage of dye-sensitized solar cells

Abstract

Metal–organic frameworks (MOFs) were used for the first time to coat TiO2 to enhance the open circuit voltage (Voc) of dye-sensitized solar cells (DSC). ZIF-8 was selected as a representative MOF for a case study. After a growth time of 30 min, a very thin ZIF-8 film was coated on TiO2 with a thickness of about 2 nm. A linear relationship between the thickness of ZIF-8 coating layer and the Voc is found, and the value of Voc was increased by 55 and 66 mV for the dye N719 and D131, respectively. Mott–Schottky measurement demonstrated that the enhanced Voc was not caused by the changed conduction band edge of TiO2 due to its possible coordination with 2-methyl imidazole. The electrochemical impedance spectra and I–V characteristics of the cells measured under dark conditions indicated the increased Voc could be ascribed to the inhibited interfacial charge recombination due to the introduction of ZIF-8 shell material. Although the amount of adsorption of dyes for the TiO2/ZIF-8 electrode was increased significantly, the core/shell structure inhibited the injection of electrons from dyes into the conduction band edge of TiO2, which decreased the short circuit current.