  We discovered a new metastable polaron-supporting phase in pristine films of
a soluble derivative of poly-p-phenylene vinylene (MEH-PPV) that is induced by
UV illumination. In the initial un-illuminated phase A, the films do not show
long-lived photogenerated polarons. However, prolonged UV illumination for
several hours induces a reversible, me-tastable phase B that shows abundant
long-lived photogenerated polarons. Phase B films transform back to the
original phase A within 1/2 hour in the dark at room temperature. We propose a
reversible mechanism in which UV illumination creates metastable deep defects
that substantially increase the photogenerated polaron lifetime.
