Optical- and photocurrent-detected magnetic resonance studies on conjugated polymer/fullerene composites

X-band photocurrent- and photoluminescence-detected magnetic resonance spectra of films of MDMO-PPV [${\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{y}(2-\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y},5-(3\mathrm{}}^{\prime }$ ${,7\mathrm{}}^{\prime }$-dimethyloctyloxy)-1,4-phenylenevinylene] with different concentrations of PCBM $[1-(3-\mathrm{methoxycarbonyl})-\mathrm{propyl}-1-\mathrm{phenyl}-(6,6\left)\mathrm{}{\mathrm{C}}_{61}\right]$ are described and discussed. Magnetic resonance at $g\approx 2$ is found to reduce the photocurrent and increase the photoluminescence. Upon increasing the fullerene concentration, the intensity of the photocurrent-detected magnetic resonance (PCDMR) spectrum decreases. Compared to the pure polymer sample, the resonance position shifts to a smaller g value and the line shape changes from one Lorentzian line to a line with a pronounced shoulder at smaller g values. This is attributed to different free-charge-carrier generation processes in the polymer and polymer-fullerene mixtures. In the pristine material at the magnetic resonance condition the recombination of polymer anion-cation pairs (polaron pairs) increases, while in the mixture the polymer cation-fullerene anion pair recombination is enhanced. Both species are precursors for free charge carriers. At high PCBM concentrations free charge carriers are no longer created via a precursor, but free charges are formed immediately after photoexcited charge transfer from polymer onto fullerenes. Adding small amounts of PCBM to MDMO-PPV, both the narrow (polaron) resonance at $g\approx 2$ and the triplet resonances are enhanced in the photoluminescence-detected magnetic resonance spectra. Similar to the PCDMR experiment, the position of the polaron resonance shifts to smaller g values and the line shape changes from one Lorentzian line to a line with a shoulder at lower g values. Increasing the fullerene concentration in the polymer matrix, the triplet resonance is quenched, while the polaron resonance amplitude is almost constant up to the highest PCBM concentrations investigated. The observed polaron resonance line shape suggests that the observed radicals result from photoinduced charge transfer.