Scanning tunneling microscope (STM) spectroscopy and optical absorption mea
surements 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 signifi
cantly higher than the value of E-b reported previously for an alkoxy-PPV d
erivative. Furthermore, an analysis of photoluminescence (PL) and STM excit
ed cathodoluminescence (CL) spectra, performed on PPV and on alkoxy-PPV der
ivatives, 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 interactio
n as well as the molecular structural relaxation energy of the exciton. (C)
2000 Elsevier Science S.A. All rights reserved.