Metastable polaron supporting phase in poly-p-phenylene films induced by   UV illumination
  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. 
T. Drori, E. Gershman, C.X. Sheng, Y.Eichen, Z.V. Vardeny, E.   Ehrenfreund