Mechanism of one- and two-photon absorption induced photoluminescence in PPV type, electroluminescent polymer

We report the luminescence properties in a poly(phenylenevinylene) derivative with the carbazole pendent and alkoxy group (CzEH-PPV). The two-photon excitation spectrum showed that the threshold energy (2homega = 2.64 eV) of two-photon absorption (TPA) is larger than that (2.34 eV) expected by one-photon excitation spectra. This implies that the two- and one-photon absorptions satisfy different selection rules. Meanwhile, the two- and one-photon absorption (OPA) induced photoluminescences (PLs) were found to occur from the same exciton band that has a double minimum adiabatic potential. The lifetimes of the upper and lower exciton states are measured to be 280 and 370 ps, respectively. By comparing the PL spectrum of CzEH-PPV film to its electroluminescence spectrum of single-layer CzEH-PPV light-emitting device (ITO/CzEH-PPV/Al), the species generated by OPA or TPA are found to be the charged carriers. For OPA-PL, the excitations having the pulse-energy larger than 2.3 muJ at 2.96 eV produce a spectrally narrowed emission band with its maximum located at 2.14 eV with the spectral width of 23 meV. This is ascribed to the amplified spontaneous emission enhanced by the optical wave guiding in the polymer film. (C) 2002 Elsevier Science B.V. All rights reserved.