It is shown that Coulomb attraction between electron and hole in coupled quantum
wells leads to localization of states in the direction of growth, i.e. in one of the
wells, even when the wells are symmetric and unbiased. As a result, oscillator
strength of the lowest excitonic transition can increase by more than 100%, while
the oscillator strengths of higher energy transitions can decrease. These results,
obtained using novel model, are important for understanding of the Wannier-Stark
localization effect in quantum wells and superlattices.