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

The photoluminescence (PL)-detected magnetic resonance (PLDMR) of various p i -conjugated materials, such as methyl-substituted ladder-type poly(p-phen ylene), para-hexaphenyl (PHP) films. and ladder-type oligophenylenes are de scribed. The optical measurements are compared to a thermally stimulated cu rrent (TSC) study of defects in m-LPPP and PHP. As TSC probes the density o f mobile charge carriers after detrapping and PLDMR reveals the influence o f trapped charges on the PL, their combination yields the concentration of traps, their energetic position, and their contribution to PL quenching. Th e 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 compari son of the PLDMR and TSC results one finds that the interaction and hence t he nonradiative quenching of singlet excitons (SE's) at polarons is stronge r 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 mo del 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 tha t this quenching process is very significant in luminescent pi -conjugated materials and organic light-emitting devices, and should be taken into acco unt, especially at high excitation densities such as in lasing action. (C) 2001 Elsevier Science B.V. All rights reserved.