Spectroscopy of conducting and insulating ladder-type poly(para-phenylene)device structures

We have applied the electroabsorption (EA) and charge-induced absorption (C IA) spectroscopic techniques to methyl substituted laddertype poly(para-phe nylene) (m-LPPP) sandwich device structures. The device structures have bee n either of conducting (ITO/m-LPPP/Al) or insulating (ITO/SiO/m-LPPP/Al) ty pe. For both devices we have observed a CIA band with its 0-0 transition at 1.95 eV. From a comparison to doping induced and photoinduced absorption s pectra we assign this band to polarons. In addition, in the insulating devi ce we observed a derivative-like band with zero crossing at 1.15 eV. In the conducting device two derivative bands are observed with zero crossing at approximately 0.8 and 1.2 eV, respectively. Their intensity increases linea rly with the modulus of the applied electric field and the features are ind ependent of the modulation frequency. We tentatively assign these derivativ e-like bands to a Stark-effect related to charge transfer transitions at th e ITO and Al electrodes. At high currents in the conducting device we also observed a relatively broad, structureless CIA band peaking at 1.3 eV, that we assign to biplarons. (C) 2001 Elsevier Science B.V. All rights reserved .