Spin dynamics in organic optoelectronic devices

We investigate the spin-dependent opto-electronic pathways of organic semiconductors using pulsed electrically and optically detected magnetic resonance. This work is motivated by the need to provide unambiguous and independent verification of the underlying spin-dependent processes in organic devices, and to quantify their microscopic properties. This work is advanced through a series of studies.Electrical and optical detection are sensitive to the spin-dependent recombination of polaron pairs that occurs within the singlet-triplet basis. The change in observable from conventional inductively-detected electron spin resonance introduces several new effects. We show that additional time constants and echoes can occur in multi-pulse sequences, such as the Hahn echo and inversion recovery. We demonstrate that phase cycling removes the spin beating component and allows the true spin relaxation times to be recovered. Several exciton-mediated processes are believed to contribute to organic magnetoresistance. We develop a time domain theory of triplet-exciton polaron quenching for organic devices. Magnetic resonance drives spins between the doublet and quartet manifolds, producing macroscopic changes in the sample conductivity. Using this formalism we are able to place bounds on the spin-spin interactions present in the exciton-polaron complexes present in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) devices and to theoretically describe the transition between a weakly-coupled exciton-polaron complex and a strongly-bound trion state. Triplet-triplet annihilation allows two low energy photons to be upconverted into a single high energy photon, substantially increasing the efficiency of solar cells. We develop a theoretical description of triplet-triplet annihilation, and show that the signal is strongly dependent on both the spin-selection rules and the energetics of the system. Since the underlying Rabi frequencies are sensitive to the dipolar and exchange coupling between the two excitons, they can be used to accurately characterize the spin Hamiltonian