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

Metal-organic frameworks (MOFs) were used for the first time to coat TiO(2) 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 TiO(2) 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 TiO(2) 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 TiO(2)/ZIF-8 electrode was increased significantly, the core/shell structure inhibited the injection of electrons from dyes into the conduction band edge of TiO(2), which decreased the short circuit current.