Heterojunction interfaces

Enhancement in the performance of OLEDs can be achieved by balanced charge injection and charge transport. The charge transport is related to the drift mobility of charge carriers. Liu et al. [166] reported blue emission from OLED based on mixed host structure. A mixed host structure consists of two different hosts NPB and 9,10-bis(2 -naphthyl)anthracene (BNA) and one dopant 4,4 -bis(2,2-diphenylvinyl)-l,l -biphenyl (ethylhexyloxy)-l,4-phenylene vinylene (DPVBi) material. They reported significant improvement in device lifetime compared to single host OLEDs. The improvement in the lifetime was attributed to the elimination of heterojunction interface and prevention to formation of fluorescence quenchers. Luminance of 80,370 cd/m2 at 10 V and luminous efficiency of 1.8 cd/A were reported. 

The formation of junctions in carbon nanotubes is important for use in electronic application. The SWNTs have been shown to react with silicon and transition metals and form metal carbide nanorods and nanoparticles at high temperature under high vacuum. A heterojunction interface with metal carbide at the tips of SWNTs is possible. Silicon and metal substrates such as Ti and Nb have been used with SWNTs to form long SiC, TiC, and NbC nanorods respectively. A small number of SWNTs with partial reaction are found to have coimected to SiC nanorods (Figure 15). The formation of such a carbide heterojunctions in carbon nanotubes with... 

A model where the resistance Rs of the heterojunction interface PSi/ Si-single crystal is connected in series with the base resistance of the PSi layer was applied for the study of CVCs. As in the field of small direct bias, the voltage drop is insufficient and displacement on the base resistance may be ignored the initial part of the direct branch is approximated by the exponential dependence of current J on voltage U with a large factor of ideality m ... 

At the polymer/acceptor interface, the arrangement of the polymer and acceptor relative to each other can have a profound effect on charge dissociation [197,198,209]. The heterojunction interface complexity effectively limits the characterization techniques to those capable of providing angstrom-level resolution. As mentioned before, solid-state NMR is one of the few techniques that fit this requirement. 

All of these morphologies showed the expected characteristic peak in photocurrent when the domain size was approximately equal to the exciton diffusion length. However, what was more pronounced was the influence of the interfacial morphology. A morphology with domains of only 4 nm and sharp heterojunction interfaces was found to out-perform a morphology with optimal sized domains of 8 nm with diffuse interfaces. This suggests that the environment around the... 

It is anticipated therefore that a thin insulating layer could form in a heterojunction interface because of the interfacial bond nature alteration and the charge... 

As expected from the observation of fast photo-induced electron transfer from MEH-PPV to Cgo [53,119], the major increase in both forward and reverse bias current results from photoinduced charge separation at the heterojunction interface. The strong increase in photocurrent upon illumination at —1 V (reverse) bias demonstrates that the heterojunction serves as a relatively sensitive photodiode. 

The rapidly rising component of the charge carrier dynamics in Figure 8.3 is known to arise from ultrafast electron transfer over distances of 10-20 nm between the electron donor and the fullerene acceptor. The slower rising dynamics occurs after the collapse of the initially delocalized coherent state. As a result, the remaining excitons diffuse to a heterojunction interface... 

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