LPPPs

EL experiments showed that the yellow-emitting LEDs prepared from LPPP 12 exhibit quite remarkable characteristics (single layer construction ITO/LPPP 12/Ca quantum efficiency ca. 1.0%, applied voltage 4-6 V 135]). These figures are in the range of the best values described hitherto for polymeric emitters in a single layer arrangement, for example, poly(pcira-phenylenevinylene) PPV and PPV derivatives. 

Figure 8-2. (a) Normalized PL spectra of m-LPPP films al 7=77 K for exeila ion ai 3.2 eV (390 am) with a circular spot al Iwo different flu-ences 0.3 mJ/eni2 (solid line) and 2.4 mJ/cm2 (dashed line). The inset shows ihe room lempcralure absorption spectrum of nt-LPPP. (b) Normalized PL spectrum for excitation with a rectangular spot at a fluence of 0.055 mJ/cm2 (from Ref. 125J with permission). 

Figure 8-5. Transmission difference spectra of m-LPPP films at 7=77 K excited at 3.2 eV for various pump-probe delays. The inset zooms out the low energy region for 0 ps (solid line) and 400 ps (dashed line) delay. Doping induced absorption (D1A) data are also shown for comparison (from Ref. (251 with permission).

Figure 8-7. SE decay al 2.53 cV in m-LPPP films il T=77 K versus pump-probe delay at lhrce different excitation fluences (from Ref. 125] with permission).

We have studied the temporal dynamics of CPG in m-LPPP by performing field-assisted pump-probe experiments on LED structures, as described in Section 8.3.2. The narrow line-width PA assigned to polarons (see Section 8.5.2) is a fingerprint of charge generation in m-LPPP. Monitoring the dynamics of these PA band enables us, for the first time, to directly observe the CPG dynamics in a conjugated polymer with sub-picosecond time resolution [40],... 

Figure 8-16. A picture of the pholoexcilalion scenario in m-LPPP, see text for a discussion. Pv is a positively charged chain (polaron), while X- can be either a negatively charged chain or an electron acceptor, such as oxygen.

Figure 9-1. Materials overview a few sclcclcd conjugated polymers and Ihcir properties have been compiled and ihe following abbreviations arc used DO-PPP...Poly(2-decyloxy-l,4-phcnylcnc), EHO-PPP...Poly(2-(2 -elhylliexyloxy)-l,4-phcnylenc), CN-PPP... Poly(2-(6 -cyano-6 -incthyl-licplyloxy)-l,4-phcnylene), m-LPPP... methyl-substituted ladder-type Poly( 1,4-phenylcne), and PLQY=phololuinincs-ecncc quanluni yield.

Figure 9-15. Triplet absorption, singlet emission, and absorption (from left to right) of (from top to bottom) ni-LPPP [I44 and hexaphcnyl films 1145] at 90 K, and /n/ra-quater- and iciplienyl at 77 K in tetrahydro-2-methylfijran [146].

In summary, all the transitions expected for the neutral states of a model system for conjugated polymers, the m-LPPP, were observed and described and all of these transitions also show clearly resolved vibronic replicas due to coupling to vibronic modes of the backbone. 

Figure 9-17. Photoinduced absorption in s-LPPP al 0.26 eV versus temperature (filled squares). Full lines represent the model results (lower curve for two activation energies, higher curve for one). The doited lines represent the decay rates for the 0.12eV (a) and 0.37 eV (b) activated processes the dash-dolled horizontal line represents the temperature-independent part E.

The electrical current of a coplanar interdigilal gold/LPPP/gold device is space charge limited due to p-type charge earner traps localized in the bandgap [28]. This can be inferred from the field dependence of the dark current at room temperature. The thermally stimulated current spectrum exhibits two peaks, corresponding to two distinct trap levels ,1 and ,", which can be calculated from the rise in current, /, below the peak temperature ... 

The photoinduced absorption and the electrical characteristics of the conjugated LPPP show that the optoelectrical properties are strongly dependent on charge carrier traps in the bandgap. From aromatic molecular crystals it is known that impurities and structural imperfections form localized states [34]. LPPP forms homogeneous and dense films with a mean interchain distance of about 20 A and ncgligi-... 

Concerning the nature of electronic traps for this class of ladder polymers, we would like to recall the experimental facts. On comparing the results of LPPP to those of poly(para-phenylene vinylene) (PPV) [38] it must be noted that the appearance of the maximum current at 167 K, for heating rates between 0.06 K/s and 0.25 K/s, can be attributed to monomolecular kinetics with non-retrapping traps [26]. In PPV the density of trap states is evaluated on the basis of a multiple trapping model [38], leading to a trap density which is comparable to the density of monomer units and very low mobilities of 10-8 cm2 V-1 s l. These values for PPV have to be compared to trap densities of 0.0002 and 0.00003 traps per monomer unit in the LPPP. As a consequence of the low trap densities, high mobility values of 0.1 cm2 V-1 s-1 for the LPPPs are obtained [39]. 

The final remark of this section concerns the polaronic transition of m-LPPP around 1.9 eV, where we can observe P2 with its vibronic replica P3 at 2.1 eV. In Figure 9-20 we show this polaronic absorption in m-LPPP as detected by photoin-duced absorption (a), chaige-induced absorption in conventional light-emitting devices (b), and chemical redox-reaction (c). Only under pholoexcilation, which creates both neutral and charged species, the triplet signal at 1.3 eV is also observed. 

Figure 10-10. (a) Semilogarillnnic plol of ihc stimulated emission transients for various excitation pulse energies measured for LPPP on glass. The excitation pulses have a duration of 150 fs and are centered at 400 nm. The probe pulse were spectrally filtered (Ao=500nin, Aa=l0nm). (b) Emission spectra recorded for the same excitation conditions. The spectra are normalized at the purely electronic emission baud (according lo Ref. [181). 

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