Emil J. W. List

Charged defects in highly emissive organic wide-band-gap semiconductors

A combined photoluminescence (PL) -detected magnetic-resonance (PLDMR) and thermally stimulated current (TSC) study of defects in wide-band-gap para-phenylene-type semiconductors 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 reported trap densities, which are 2×1016 for the polymer and 1×1014 cm−3, for the oligomer, are the lowest reported for para-phenylene-type materials.

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

Abstract The photoluminescence (PL)-detected magnetic resonance (PLDMR) of various π-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≥l.6×10 16 and l.4×10 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 (SEs) at polarons is stronger in PHP than in m-LPPP due to a higher diffusivity of SEs 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 SEs. The results also suggest that this quenching process is very significant in luminescent π-conjugated materials and organic light-emitting devices, and should be taken into account, especially at high excitation densities such as in lasing action.

Quantitative analysis of the singlet exciton-polaron interaction in para-phenylene-type ladder polymers

The steady state photoinduced absorption (PA), photoluminescence (PL), PL detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) NO dependence of the PA, PL, and PLDMR signals are analyzed by means of a new rate equation model. It describes the dynamics of singlet excitons (SEs) and polarons in all three experiments with the same set of parameters. This yields the first quantitative analysis of the interaction of SEs and polarons in conjugated polymers. The model is based on the observations that mobile SEs are quenched by trapped and free polarons and that the spin 1/2 magnetic resonance conditions reduce the total polaron and triplet exciton (TE) populations. Since the sublinear NO dependence of the positive (PL-enhancing) spin 1/2 PLDMR and the polaron PA band are essentially the same, we conclude that that PLDMR is due to reduced quenching of SEs by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. It also suggests that it is a very significant process in luminescent (pi) - conjugated materials and organic light-emitting devices, which needs to be taken into account especially at high excitation densities such as in lasing action.

Excitation energy migration in highly emissive semiconducting polymer blends probed by photoluminescence detected magnetic resonance

Abstract The excitation energy migration (EEM) in methyl-substituted ladder-type poly(para-phenylene) (m-LPPP) doped with small amounts of the red emitter poly(perylene-co-diethynylbenzene) (PPDB) was studied by photoluminescence (PL) detected magnetic resonance (PLDMR). It is suggested that the EEM process proceeds via two steps: (1) migration within the host and (2) transfer from the host to the guest. The contributions of the emissions from m-LPPP and PPDB to the PL-enhancing polaron PLDMR at g=2, which are due to a reduction in the density of polarons acting as singlet exciton (SE) quenching centers, evolve differently with temperature. This provides clear evidence for SE migration in m-LPPP. The triplet exciton (TE) PLDMR at g=4 shows a distinct peak for each polymer, with the intensity of the PPDB feature being proportional to its concentration. However, the spectral dependence recorded at the peak of each resonance is the same. This rules out the triplet–triplet-annihilation-mechanism in these blends for simple energetic reasons. Instead we propose that the resonance at g=4 is due to a SE-quenching mechanism similar to that for the polaron resonance at g=2. At the field-for-resonance the number of TE’s decreases, and hence the PL increases.

Frequency-resolved delayed fluorescence (DF) and photoluminescence detected magnetic resonance (PLDMR) studies of triplet and polaron dynamics in π-conjugated materials and devices

The DF and PLDMR of triplet exciton and polaron dynamics in (pi) -conjugated materials and devices is reviewed, and the significance of various processes involving these long-lived excitations is considered. These include the generation of singlet excitons by triplet-triplet annihilation, which leads to DF, and nonradiative quenching of such singlets by polarons or triplet excitons. The clear differences between the roles of these processes in (pi) -conjugated polymers vs small molecules and their implications for organic light emitting devices are discussed.