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 (V_oc) 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 V_oc is found, and the value of V_oc was increased by 55 and 66 mV for the dye N719 and D131, respectively. Mott–Schottky measurement demonstrated that the enhanced V_oc 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 V_oc 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.