Air-Exposure-Induced Gas-Molecule Incorporation into Spiro-MeOTAD Films

Combined photoemission and charge-transport property studies of the organic hole transport material 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) under air exposure and controlled environments of O-2, H2O + N-2, and N-2 (1 atm and under dark conditions) reveal the incorporation of gas molecules causing a decrease in charge mobility. Ultraviolet photoelectron spectroscopy shows the Fermi level shifts toward the highest occupied molecular orbital of spiro-MeOTAD when exposed to air, O-2, and H2O resembling p-type doping. However, no traces of oxidized spiro-MeOTAD(+) are observed by X-ray photoelectron spectroscopy (XPS) and UV-visible spectroscopy. The charge-transport properties were investigated by fabricating organic field-effect transistors with the 10 nm active layer at the semiconductor-insulator interface exposed to different gases. The hole mobility decreases substantially upon exposure to air, O-2, and H2O. In the case of N-2, XPS reveals the incorporation of N-2 molecules into the film, but the decrease in the hole mobility is much smaller.