Photoexcitation Spectroscopy of Conjugated Polymers

We attempted to understand the photoexcited state of the substituted poly-phenylene-vinylene family of conjugated polymers, materials which have proven particularly useful for use in light emitting diodes and lasers. We used photoinduced absorption spectroscopy, light induced electron spin resonance measurements, photoluminescence spectroscopy, and time resolved photoluminescence measurements to probe the excited states of poly(2-methoxy,5-(2 ^' -ethyl-hexyloxy)-p-phenylene -vinylene), MEH-PPV. This material is studied in several different forms; in solution, as films cast on substrates, in blends with ultrahigh molecular weight polyethylene (PE), and in stretch oriented blends with PE. As the quality and ordering of the material is improved, the long lived excitations shift from trapped charged excitations to metastable triplet states. This is observed in MEH-PPV on going from a disordered cast film to a stretch oriented PE blend. The decay mechanisms for the singlet excited states are studied by making time resolved photoluminescence measurements. In all forms of MEH-PPV, the luminescence decay is dominated by non-radiative decay channels with decay lifetimes on the order of several hundred picoseconds at room temperature. These lifetimes approximately double when the samples are cooled to 80 Kelvin. The quantum efficiency is highest in solution, at approximately 60-70%. The lowest quantum efficiency for emission is in the disordered cast film of the material. When mixed with electron accepting materials, the photoexcitations can be further manipulated by the removal of the photogenerated electron from the electron hole pair. This cuts off the route of formation of neutral excitations and allows us to directly photogenerate charged excitations only. This charge transfer is observed in polymer-C_{60} mixtures. The suppression of the back charge transfer process, possibly by the lattice relaxation stabilizing the charged excitations, allows for cw spectroscopy of the charge transfer states. The photoinduced charge transfer reaction is studied for several different polymer 'donors' and time resolved photoluminescence studies are done to determine the charge transfer rates. The results are discussed in terms of designing suitable donor -acceptor couples for photoinduced electron transfer using conducting polymers and C_{60} as donor and acceptor, respectively. (Abstract shortened by UMI.).