Ohmic injection

Second, the ratio /di//o should be 2.72. However, in the real case, the transient current consists of a dark-injection current and a RC response of the circuit. The latter comes from the inevitable fact that the sample itself acts as a capacitor and its needs a finite time to charge up. Thus Jg is overestimated due to the effect of RC response, and therefore the ratio Joi/Jo is always smaller than the ideal value. Third, another ratio /di//scl should be 1.21 [41,51]. Departure from the ideal value of 1.21 may indicate that the injecting contact deviates from ohmic injection. Alternatively, if the charge injection remains ohmic, deviation of this ratio could also be a consequence of nonnegligible charge diffusion or field-dependent mobilities. A critical examination of this ratio under the condition of ohmic injection has been reported elsewhere [52]. 

In a light-emitting MSM structure the two metal electrodes selected such that the work functions of the electrodes are near the edge of the valence band (VB) and the conducting band (CB) of the semiconductor, respectively, so that oppositely charge carriers are injected from the opposite electrodes. An ohmic and a rectifying contact is therefore formed in the MSM structure (see Fig. 9-22). 

In bilayer LEDs the field distribution within the device can be modified and the transport of the carriers can be controlled so that, in principle, higher efficiencies can be achieved. On considering the influence of the field modification, one has to bear in mind that the overall field drop over the whole device is given by the effective voltage divided by the device thickness. If therefore a hole-blocking layer (electron transporting layer) is introduced to a hole-dominated device, then the electron injection and hence the efficiency of the device can be improved due to the electric field enhancement at the interface to the electron-injection contact, but only at expense of the field drop at the interface to the hole injection contact This disadvantage can be partly overcome, if three layer- instead of two layer devices are used, so that ohmic contacts are formed at the interfaces [112]. 

Another issue that can be clarified with the aid of numerical simulations is that of the recombination profile. Mailiaras and Scott [145] have found that recombination takes place closer to the contact that injects the less mobile carrier, regardless of the injection characteristics. In Figure 13-12, the calculated recombination profiles arc shown for an OLED with an ohmic anode and an injection-limited cathode. When the two carriers have equal mobilities, despite the fact that the hole density is substantially larger than the electron density, electrons make it all the way to the anode and the recombination profile is uniform throughout the sample. 

Molecular wire behavior The bridge states are energetically comparable to the donor level the electron may be thermally injected into the bridge, whereupon, it moves from donor to acceptor incoherently, as a defect such as a polaron. The distance dependence behavior is Ohmic (varies inversely with distance). 

D. Poplavsky, J. Nelson, and D.D.C. Bradley, Ohmic hole injection in poly(9,9-dioctylfluorene) polymer light-emitting diodes, Appl. Phys. Lett., 83 707-709, 2003. 

An example of the difficulties encountered when trying to fabricate an ohmic electrode, able to sustain a space-charge-limited current, is the recent work of the Neher group [179]. The authors deposited barium as an electron injection cathode on top of an electron transporting polymer based on a naphthalene diimide core whose LUMO is as low as 4 eV below vacuum level. Although the Fermi level of barium should be above the LUMO of the polymer, the electron current is. 

Koo Y-M, Choi S-J, Chu T-Y, Song O-K, Shin W-J, Lee J-Y, Kim JC, Yoon T-H (2008) Ohmic contact probed by dark injection space-charge-limited current measurements. J Appl Phys 104 123707... 

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