Large built-in piezoelectric fields are observed in strained CdTe quantum wells grown along a polar axis with CdZnTe or CdMnTe barriers. Photoluminescence and absorption spectra give direct evidence of the progressive spatial separation of electrons and holes when the electric field increases (i.e., when the misfit strain increases) and when the quantum well thickness increases: the lines rapidly shift to the red, the e₁–h₁ oscillator strength decreases while the e₁–h₂ one increases. The observed redshift allows one to measure the built-in field for different compositions of the barrier (CdMnTe or CdZnTe) and the quantum well (CdTe or CdZnTe). Significant nonlinearity is found. Such structures are expected to have interesting applications in electro-optics or non-linear optics. However, their electronic properties result from a balance between the confinement due to band offsets, which tends to enhance excitonic effects, and the built-in field, which tends to separate the electron-hole pair leading to reduced oscillator strength. By suitably choosing the parameters of the heterostructure, and with a good control of growth, multiple quantum wells are realized which exhibit both intense narrow absorption lines and a sizable redshift.