Abstract
We report on the luminescence of a tetraphenylporphyrin, TPP-d, blended into poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV. We find significant energy transfer from MEH-PPV to the porphyrin, in spite of the low absorption of the porphyrin at the emission wavelength of MEH-PPV, reflected in a Forster transfer radius (2.5 nm) smaller than for materials with larger spectral overlap. The overall photoluminescence, PL, efficiency decreases monotonically with increasing porphyrin content, whereas the porphyrin contribution to the total efficiency, referred as an "apparent" PL efficiency, exhibits a maximum at 1.4% porphyrin content (by weight). We attribute this non-monotonic behaviour to the interplay of the exciton transfer probability and PL quenching, both of which increase with concentration. We also observed the energy transfer under electrical excitation, but noticed that, at low concentrations, the porphyrin contribution to the electroluminescence is higher than that observed in PL. This indicates significant emission from excitons formed directly at the porphyrin sites, which are likely to act as charge trapping sites. We also compare the luminescence properties of the blends with those of copolymers based on the same host-guest pair.