Spin dynamics of photoexcited polarons in MEH-PPV: optically detected magnetic resonance studies

We studied the \textit{full dynamics }of the photoluminescence detected magnetic resonance (PLDMR), photoinduced absorption (PA), and PA detected magnetic resonance (PADMR) in MEH-PPV films, as a function of microwave power, P, and modulation frequency, f$_{M}$. We found it critically important to measure both in-phase and quadrature components; otherwise key characteristics of the dynamics are not unraveled. For example, the PLDMR in-phase component \textit{changes sign }at f$_{M }=f_{0}$ of about 30 kHz before decaying at higher frequencies. In contrast the quadrature PLDMR component retains its sign within the same experimental frequency range. We account for these peculiar dynamics by a model in which the polaron recombination is spin dependent (SDR). Specifically by solving the SDR rate equations we found that it correctly explains the PLDMR frequency dependent phase, and reproduce the obtained increase of $f_{0}$ with $P$. Also the SDR model explains equally well the PADMR and PLDMR time resolved measurements.