Field-modulation spectroscopy of pentacene thin films using field-effect devices: Reconsideration of the excitonic structure

We report pure electric-field effects on the excitonic absorbance of pentacene thin films as measured by unipolar field-effect devices that allowed us to separate the charge accumulation effects. The field-modulated spectra between 1.8 and 2.6 eV can be well fitted with the first derivative curve of Frenkel exciton absorption and its vibronic progression, and at higher energy a field-induced feature appears at around 2.95 eV. The results are in sharp contrast to the electroabsorption spectra reported by Sebastian et al. in previous studies [Chem. Phys. 61, 125 (1981)], and leads us to reconsider the excitonic structure including the location of charge-transfer excitons. Nonlinear pi-electronic response is discussed based on second-order electro-optic (Kerr) spectra.