Defect characterization of highly emissive para-phenylene-type molecular films by photoluminescence-detected magnetic resonance and thermally stimulated charge transport

A new combined photoluminescence-detected magnetic resonance (PLDMR) and thermally stimulated current (TSC) study of defects in wide bandgap amorphous methyl-substituted ladder-type poly(para-phenylene) (m-LPPP) and polycrystalline para-hexaphenyl (PHP) films, both with PL quantum yields of ∼30%, is described. 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≥1.6×1016 and 1.4×1014 cm−3 for 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 at polarons is stronger in PHP than in m-LPPP due to a higher diffusivity of singlet excitons in PHP. Furthermore we show that PLDMR results are in agreement with photoinduced absorption results of pristine and photo-oxidized m-LPPP.