Bipolar host

Typical host materials can be summarized as electron transport hosts, hole transport hosts, bipolar hosts, large band-gap hosts for blue or white emission, or polymer hosts. 

The properties of the host material generally determine device performance. Depending on the charge-transporting properties of the host, recombination can occur at the interface with the blocking layer or in the bulk of the EML. A bipolar host can promote bulk recombination, but can also lead to a situation where the recombination occurs at both the EML HTL and EML IETL interfaces, in particular, when the electric field and the mobility of charge carriers in the EML are high. ... 

Figure 21 Bipolar host materials for phosphorescent organic light-emitting diode.

Chien C-H, Hsu F-M, Shu C-F, Chi Y (2009) Efficient red electrophosphorescence from a fluorene-based bipolar host material. Org Electron 10 871-876... 

Fig. 5 OLEDs fabricated with iridium complexes bearing 6-fluoroquinoline-based ligands as emitters and TICCBI and TICNBI as donor-acceptor bipolar hosts...

We reported the preparation of sophisticated bipolar three-dye photonic antenna materials for light harvesting and transport [22]. The principle is illustrated in Figure 1.12. Zeolite L microcrystals of cylinder morphology are used as host for organizing several thousand dyes as monomers into well-defined zones. 

Adachi, C., Baldo, M.A., and Forrest, S.R. (2000) Electroluminescence mechanisms in organic light emitting devices employing a europium chelate doped in a wide energy gap bipolar conducting host. Journal of Applied Physics, 87, 8049-8055. 

Carbazoles are the most often used neat-host materials for Ir(ppy)3. Triplet energy and transport properties of this class of materials can be tuned by derivatization of the phenyl rings or the carbazole group. Green phosphorescent OLEDs comprising the CBP host doped with Ir(ppy)3 have been extensively CBP is reported to have bipolar transport-... 

Among the components of the fuel cell stack, the bipolar plate is considered to be one of the most costly and problematic. In addition to meeting cost constraints, bipolar plates must possess a host of other properties. The seareh for suitable, low-cost bipolar plate materials is a key concern in PEMFC stack development [34]. 

The whole progress can be approximated by a set of piecewise linear functions by a threshold bipolar (TB) protocol, which is very simple in its implementation and design philosophy. For a host, when the current Vp< a threshold, the urgent task is to download the most wanted chunks, while if Vp > the threshold, the job is switched to help spread the latest or rarest chunks over the network. We have ever observed some other peers startup procedures in our trace, and all of them can be interpreted easily by the TB protocol. 

In photoconductive polymers the intersite hopping distance is not variable it is determined by the structure and morphology of the polymer. In molecularly doped polymers the average hopping distance can be varied at will, simply by changing the concentration of the transport-active species in the host polymer. In the PVK/TNF bipolar system there is evidence that holes move via uncomplexed carbazole groups, while electrons move via both complexed and uncomplexed TNF molecules [14]. The hole mobility decreases and the electron mobility increases as the TNF concentration increases [14] (Fig. 8.10). 

---