Influence of alkoxy substituents on the exciton binding energy of conjugated polymers

Scanning tunneling microscope (STM) spectroscopy and optical absorption measurements were used to determine the exciton binding energy (E-b) of poly(p-phenylenevinylene) (PPV), We find E-b = 0.48 +/- 0.14 eV, which is significantly higher than the value of E-b reported previously for an alkoxy-PPV derivative. Furthermore, an analysis of photoluminescence (PL) and STM excited cathodoluminescence (CL) spectra, performed on PPV and on alkoxy-PPV derivatives, reveals that the energy separation between the dominant peaks in the vibronic progression is significantly smaller in the alkoxy-substituted compounds. The reduction of E-b appears to be related to the influence of the alkoxy side-chains, inducing a reduction of both the Coulomb interaction as well as the molecular structural relaxation energy of the exciton. (C) 2000 Elsevier Science S.A. All rights reserved.