Polaron spin-lattice relaxation time in π-conjugated polymers from optically detected magnetic resonance

We describe a method for obtaining the polaron spin-lattice relaxation time T SL in π-conjugated polymers by measuring the optically detected magnetic resonance (ODMR) dynamics as a function of microwave power and laser intensity. The peculiar ODMR dynamics is well described by a spin dependent recombination model where both recombination and spin relaxation rates determine together the response dynamics. We apply this method to the spin 1/2 ODMR in films of pristine 2-methoxy-5-(2'-ethylhexyloxy) phenylene vinylene [MEH-PPV] polymer, as well as MEH-PPV doped with various concentrations of radical impurities. We obtained T SL ∼ 30 μs in pristine MEH-PPV, but substantially shorter when the magnetic impurities are added.