X-BAND OPTICALLY DETECTED MAGNETIC-RESONANCE (ODMR) STUDY OF C-60-DOPED PI-CONJUGATED POLYMERS

The X-band ODMR of 0.1 - 10 mol.% C-60-doped 2,5-dihexoxy poly(p-phenylenevinylene) (DHOPPV) and 2,5-dibutoxy poly(p-phenyleneacetylene) (DBOPPA) is described. As invariably observed in undoped pi-conjugated polymers, it includes a narrow PL-enhancing polaron resonance at g approximate to 2.002, and full- and half-field triplet exciton powder patterns. The polaron resonance is surprisingly weak in undoped DHOPPV, but its intensity increases twentyfold at 0.1 mol.% C-60, and then decreases one hundredfold at 10 mol.%. In undoped DBOPPA the narrow resonance is intense and initially decreases with C-60 content. C-60 doping generally drastically weakens the full- and half-field triplet exciton resonances. The results are discussed in relation to the role of positive (p(+)) and negative (p(-)) polarons and bipolarons as nonradiative singlet and triplet quenching centers; the polaron resonance is then attributed to their removal by spin-dependent p(+)-p(-) recombination, which may also generate radiative singlet excitons.