Triplet electron injection

In the light of the rapid excited-state relaxation dynamics and the observed <100 fs electron injection component, the fast injection component must occur from non-equilibrated excited states. Distinct singlet and triplet electron-injection pathways have been observed for Ru polypyridyl complexes on SnOa (Iwai et al, 2000) and TiOi (Benko et al, 2002 Kalloinen et al, 2002). In a detailed study of electron injection dynamics in N3/TiOi (Benko et al, 2002 Kalloinen et al, 2002), 55% of electron injection was shown to occur with 50 fs injection time from singlet MLCT... 

Figure 5.5 Triplet electron injection from RnN3 to Ti02 and Sn02 shows differences after red wing excitation of the visible absorption band of the dye. Low lying triplet states of RnN3 are situated below the conduction band edge of Ti02 and this leads to activated slow electron injection. For Sn02 the conduction band edge is sufficiently low for all triplets to inject efficiently.

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

Transient UV-vis absorption spectra showed that theTi02/Ru(II) films yield prompt electron injection upon photolysis ( >108s 1) These same films displayed photoluminescence decays with parallel first- and second-order components, the first-order component having a rate constant of about lxl06s-1. These two sets of results provide further support for the existence of at least two populations of adsorbed Ru(II), one of which injects electrons rapidly and another which does not inject electrons and is thus capable of luminescing on a longer time scale. The second-order component of the luminescence decay is attributed to bimolecular triplet-triplet annihilation of surface-bound Ru(II). (Note that the second-order rate constants reported for luminescence decay have units of s-1 because they are actually values for k2(Asi))... 

Photosensitized electron injection in colloidal TiOz has been reported by Moser and Gratzel as part of a scheme to photoreduce water [155], Moser et al. [156] and Rosetti and Brus [157] saw the formation of semioxidized Eosin by monitoring its absorption in the visible and by Raman spectroscopy. No oxidation of the triplet of Eosin adsorbed in Ti02 is observed, the lifetime... 

The /3-diketonate [Nd(dbm)3bath] (see figs. 41 and 117) has a photoluminescence quantum efficiency of 0.33% in dmso-7r, solution at a 1 mM concentration. It has been introduced as the active 20-nm thick layer into an OLED having an ITO electrode with a sheet resistance of 40 il cm-2, TPD as hole transporting layer with a thickness of 40 nm, and bathocuproine (BCP) (40 nm) as the electron injection and transporting layer (see fig. 117). The electroluminescence spectrum is identical to the photoluminescence emission the luminescence intensity at 1.07 pm versus current density curve deviates from linearity from approximately 10 mA cm-2 on, due to triplet-triplet annihilation. Near-IR electroluminescent efficiency <2el has been determined by comparison with [Eu(dbm)3bath] for which the total photoluminescence quantum yield in dmso-tig at a concentration of 1 mM is Dpi, = 6% upon ligand excitation, while its external electroluminescence efficiency is 0.14% (3.2 cdm-2 at 1 mAcm-2) ... 

More recently, a RuPc bearing a carboxylic moiety in its axial position was also tested for DSSC applications [281]. Transient absorption and steady-state (absorption and emission) measurements revealed that for such a system the electron injection occurs from the RuPc triplet state into the Ti02 conduction band. A detailed study on the variation of the recombination rate constants for a series of metal complexes and organic dyes, including some RuPcs, has been carried out in order to identify some important parameters that should be taken into account for the implementation of DSSCs [282],... 

Benko G., Kallioinen J., Myllyperkio P., Trif F., Korppi-Tommola J. E. I., Yartsev A. P. and Sundstrom V. (2004), Interligand electron transfer determines triplet excited state electron injection in RuN3-sensitized TiOi films , J. Phys. Chem. B 108, 2862-2867. 

Some of these carriers may recombine within the emissive layer yielding excited electron-hole pairs, termed excitons. These excitons may be produced in either the singlet or triplet states and may radiatively decay to the ground state by phosphorescence (PL) or fluorescence (FL) pathways (Fig. 1-2). An important figure of merit for electroluminescent materials is the number of photons emitted per electron injected and this is termed the internal quantum efficiency. It is clear, therefore, that the statistical maximum internal efficiency for an EL device is 25% as only one quarter of the excitons are produced in the singlet state. In practice, this maximum value is diminished further because not all of the light generated is visi-... 

The novel ETM B3PYMPM was reported to have high electron mobility and relatively low LUMO level. The combination of these properties led to effective electron injection and transport from the cathode into the ETL of B3PYMPM. TAPC used in EBL on the anode side of the EML is a well-known HTM with a high triplet energy of 2.89 eV and a hole mobility of at least 7 X10 cm /V/s.i Thus, extremely efficient charge-transporting materials... 

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