Molecular modification on dye-sensitized solar cells by phosphonate self-assembled monolayers

Molecular modification by phosphonic acids forms phosphonate dipole layers on TiO2 and influences photoelectrochemical characteristics of DSSCs. V-oc is dipole-related and eta is dominated by V-oc. A phosphonic acid with a negative dipole moment shifts the TiO2 E-CB closer to the vacuum level and causes a larger V-oc. A higher E-CB increases the E-F gradient and results in a larger J(sc). APPA has the largest negative dipole moment, so the APPA-modified DSSC shows the largest V-oc and J(sc), and the highest eta. The dark currents of modified DSSCs are not just dipole-related. They are dominated mainly by the electron tunneling effect. A shorter phosphonate dipole layer offers a smaller tunneling barrier and makes reverse electron injection easier. So the CEPA- and APPA-modified DSSCs show smaller charge transfer impedances at the TiO2/dye/electrolyte interfaces and larger dark currents than the other four modified DSSCs. Although the TFBPA dipole layer is not the longest, the TFBPA-modified DSSC exhibits the largest impedance and smallest dark current because the charge recombinations at the interface and reverse injection of electrons are suppressed by the strong electron-withdrawing property.