Quenching sites

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

In general, a two-layer device structure is more efficient than single-layer architectures. There are two key reasons for this. First, each layer can be separately optimized for the injection and transport of one carrier type. Second, exciton formation and radiative decay take place close to the HTL-ETL interface away from the quenching sites at the organic-metal contacts. 

The extra quenching sites have less influence on samples with low quantum efficiency. 

If we consider now transfer between two identical ions the same considerations can be used. If transfer between S ions occurs at a high rate, in a lattice of S ions there is no reason why the transfer should be restricted to one step. This can bring the excitation energy far from the site where the absorption took place. If in this way, the excitation energy reaches a site where it is lost nonradiatively (quenching site), the luminescence will be quenched. This phenomenon is called concentration quenching. 

Photoresponsive electron donor species such as Ru(bpy)3 + [151-159] and acceptor species such as MV + (MV = methylviologen) [153, 160] have been intercalated into montmorillonite/kaolin clays. Luminescence of Ru(bpy)3 + was found to be strongly quenched by lattice Fe- + [152, 154]. A random distribution of quenching sites in the lattice has been proposed [155]. Self-quenching due to segregation has also been reported [153]. The segregation of Ru(bpy)3"+ from co-intercalated... 

In order to arrive at meaningful (exponential) decay rates in such experiments, non-linear triplet quenching by diffusion, has to be avoided. For example, the initial accelerated decay in Fig. 7 is caused by bimolecular triplet-triplet annihilation dominating the decay of the triplets. Similarly, a faster decay is observed at higher temperature, where triplet exciton diffusion to quenching sites is faster than monomolecular decay. Nevertheless, by using low temperatures and low excitation doses exponential decay kinetics are observed yielding radiative decay rates as low as 1 s x, which sets an upper limit for the triplet excited state lifetime [28,34],... 

The term "disruptive quencher" is applied to the case in which all exciton contacts with the quencher results in completely effective quenchingIt is not at all clear that this property is applicable to Cu quenching. The case of a non-dlsruptive quencher is much harder to analyze and does not lead to a convenient expression for the excitation decay, analogous to eqn. (18). Based on classical diffusion equations it seems plausible that the disruptive quencher model is applicable so long as the quenching rate at the quenched site is of the same order of magnitude as the transfer rate from that lattice site to neighboring sites. ... 

IV.B.2). There is considerable debate concerning whether there is a specific quenching site residing in one type of LHCII it has been suggested that the minor complexes CP29 and CP26 may contain... 

Optimum Activator Concentration in Terms of Number of Emitting Sites and Number of Quenching Sites... 

The films and LEDs also exhibited PL-, EL-, and a-detected triplet exciton resonances. The primary PL-detected patterns were attributed to triplet excitons localized on the thiophene and phenylene rings, and only slightly larger. However, patterns due to other distinct triplets were observed in some films and LEDs. The nature of the triplet exciton ODMR was discussed in relation to the role of triplets as Si quenching sites, ground state recovery , and triplet-triplet fusion to S,. ... 

---