Defect-assisted photoluminescence intensity enhancement in poly(p-phenylene vinylene) films probed by time-resolved photoluminescence

We report on picosecond time resolved photoluminescence measurements in poly(p-phenylene vinylene) (PPV) films of thickness which exceeds the penetration depth of the exciting laser, irradiated by laser in air. The photoluminescence (PL) intensity increases during the irradiation process despite the increasing density of photo-oxidation induced carbonyl groups, usually acting as quenching centers. We study the PL enhancement and the dynamics of the PL decay for different irradiation conditions and different sample thicknesses. The PL decay surprisingly shows a reduced importance of defects trapping as the defect concentration increases, thus explaining the PL enhancement. We explain our results in terms of a chain shortening due to carbonyl incorporation and formation of an energy profile that extends and migrates into the film, enabling efficient spectral diffusion of excited carriers into nondegraded PPV segments by Forster energy transfer.