Charge transfer transitions in solid tetracene and pentacene studied by electroabsorption

Abstract

Electric field modulated absorption spectra of vapor deposited layers of tetracene and pentacene indicate existence of charge transfer transitions. In tetracene the transitions from a molecule located at (0, 0, 0) to molecules located at ( 1 2 , 1 2 , 0), (0, 1, 0), (1, 0, 0), and (1, 1, 0) or 1 2 , 3 2 , 0) occur at 2.71 eV, 2.779 eV, 2.895 eV and 3.063 eV, respectively. The dependence of the electron—hole binding energy on the pair separation is coulombic and yields a bandgap E g= 3.4 = 0.05 eV. The oscillator strengths of the transition between nearest neighbors is about 0.03. In pentacene the transition between molecules located at (0, 0, 0) and ( 1 2 , 1 2 , 0), (0, 1, 0), (1, 0, 0) could be resolved. They appear at 2.120 eV, 2.270 eV and 2.345 eV, respectively, and are the dominant features in the absorption spectrum. The electron—hole pair binding energy decreases faster than r CT −1 with increasing separation. This effect explains the high carrier generation efficiency in pentacene. The results are in accord with recent work of Bounds and Siebrand suggesting that CT excitons are generated by direct transitions rather than by autoionization of a localized excited molecular state followed by thermalization. The influence of the external field on the singlet exciton transition is explained in terms of a quadratic Stark effect.