We show that one-electron band theory fails to describe optical absorp
tion in PPV and the absorption spectrum can be described only within a
Coulomb-correlated model. The lowest optical state is an exciton, who
se binding energy is estimated theoretically to be 0.90+/-0.15 eV. Mea
surements of the photoconductivity quantum efficiency in poly[2-methox
y,5-(2'-ethyl-hexyloxy)-1,4 phenylenevinylene] reveal a binding energy
in good agreement with the theoretical value.