Energetic characterization of thin-film p-type copper dicyanoanthraquinone diimine electrodes

Thin-film p-type copper dicyanoanthraquinone diimine electrodes, (DCNAQI)Cu, were fabricated and characterized. Powder X-ray diffractograms and scanning electron micrographs show these organic semiconductors possess tetragonal or distorted tetragonal structure and uniformly deposit at the copper substrate after reaction of DCNAQI with copper. Mott-Schottky plots and electrochemical probling with methyl viologen consistently show these thin-film electrodes are of p-type semiconductors with flatband potentials located at -0.3 ± 0.2 V vs SCE. Admittance studies suggest two major surface states are distributed, respectively, around -0.4 and -0.8 V vs SCE, at most (DCNAQI)Cu electrodes. The former (surface state 1, density ca. 10¹² cm^-2) is relatively more equally populated at most (DCNAQI)Cu electrodes than the latter (surface state 2). Surface state 1 is postulated to manipulate charge transfer for DCNAQI^-/2-, theoretically forbidden, at (DCNAQI)Cu electrodes; however, surface state 2 is likely to induce a strong specific oxygen adsorption at the (DCNAQI)Cu electrode surface and counteract the effect of surface state 1. With these competitive interactions, a suppression relationship between DCNAQI^-2/- and O₂ was observed at (β-Cl-DCNAQI)Cu electrodes because of a heavier density of surface state 2, but current enhancement (EC′ process) was recorded at (β-Me-DCNAQI)Cu electrodes, relatively free of surface state 2, under similar conditions.