  In this paper we consider the essential electronic excited states in parallel
chains of semiconducting polymers that are currently being explored for
photovoltaic and light-emitting diode applications. In particular, we focus
upon various type II donor-acceptor heterojunctions and explore the relation
between the exciton binding energy to the band off-set in determining the
device characteristic of a particular type II heterojunction material. As a
general rule, when the exciton binding energy is greater than the band off-set
at the heterojunction, the exciton will remain the lowest energy excited state
and the junction will make an efficient light-emitting diode. On the other
hand, if the off-set is greater than the exciton binding energy, either the
electron or hole can be transferred from one chain to the other. Here we use a
two-band exciton to predict the vibronic absorption and emission spectra of
model polymer heterojunctions. Our results underscore the role of vibrational
relaxation and suggest that intersystem crossings may play some part in the
formation of charge-transfer states following photoexcitation in certain cases.
