The interaction of aluminum with a urethane-substituted polythiophene with electroluminescence applications

The interaction of aluminum with spin-cast films of poly[2(3-thienyl)ethanol n-butoxycarbonylmethylurethane], a polythiophene having a hydrogen-bond-forming urethane side chain, has been studied by X-ray photoelectron spectroscopy (XPS). The aluminum was evaporated in situ under ultra-high vacuum conditions, and changes in the Al(2p), C(1s), O(1s), N(1s) and S(2p) electronic levels were monitored as a function of aluminum coverage. O(1s) and N(1s) shifts to lower binding energies indicate that initially deposited aluminum preferentially bonds with oxygen atoms of the urethane side-chain, transferring electrons to it. Interaction with the side-chain is so strong that it prevents electron donation into the thiophene ring, as evidenced by a lack of aluminum-induced S(2p) features. This is in contrast to reactive metal deposition on aliphatic-substituted polythiophenes, such as poly(3-hexylthiophene). Al(2p) spectra show that the metal in contact with the polymer electronically resembles that of aluminum oxide, but deposition beyond about 4×1015 Al/cm2 leads to metallic aluminum. Comparison of aluminum intensities on the polymer with aluminum on gold suggest limited diffusion of metal into the bulk.