Singlet exciton quenching by polarons in π-conjugated wide bandgap semiconductors:: a combined optical and charge transport study

The photoluminescence (PL)-detected magnetic resonance (PLDMR) of various pi -conjugated materials, such as methyl-substituted ladder-type poly(p-phenylene), para-hexaphenyl (PHP) films. and ladder-type oligophenylenes are described. The optical measurements are compared to a thermally stimulated current (TSC) study of defects in m-LPPP and PHP. As TSC probes the density of mobile charge carriers after detrapping and PLDMR reveals the influence of trapped charges on the PL, their combination yields the concentration of traps, their energetic position, and their contribution to PL quenching. The TSC measurements reveal trap densities greater than or equal to 1.6 x 10(16) and 1.4 x 10(14) cm(-3) in m-LPPP and PHP, respectively. From a comparison of the PLDMR and TSC results one finds that the interaction and hence the nonradiative quenching of singlet excitons (SE's) at polarons is stronger in PHP than in m-LPPP due to a higher diffusivity of SE's in PHP. All of the results are in excellent quantitative agreement with a rate-equation model in which the positive (PL-enhancing) spin 1/2 PLDMR is due to the role of polarons in nonradiative quenching of SE's. The results also suggest that this quenching process is very significant in luminescent pi -conjugated materials and organic light-emitting devices, and should be taken into account, especially at high excitation densities such as in lasing action. (C) 2001 Elsevier Science B.V. All rights reserved.