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

We have applied the electroabsorption (EA) and charge-induced absorption (CIA) spectroscopic techniques to methyl substituted ladder-type poly(para-phenylene) (m-LPPP) sandwich device structures. The device structures have been either of conducting (ITO/m-LPPP/Al) or insulating (ITO/SiO/m-LPPP/Al) type. 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 spectra we assign this band to polarons. In addition, in the insulating device 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 linearly with the modulus of the applied electric field and the features are independent of the modulation frequency. We tentatively assign these derivative-like bands to a Stark-effect related to charge transfer transitions at the 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.