Spin injection efficiency

As a result, nearly perfect interfaces between the ferromagnetic material and the semiconductor are not a prerequisite for efficient spin injection. It is for example possible to insert a non-magnetic seed layer between the ferromagnetic base layer and the semiconductor collector. Since hot electrons retain their spin moment while traversing the thin non-magnetic layer this will not drastically reduce the spin polarization of the injected current. Finally, since electron injection is ballistic in SVT and MTT devices the spin injection efficiency is not fundamentally limited by a substantial conductivity mismatch between metals and semiconductors [161, 162], The latter is the case in diffusive ferromagnetic metal/semiconductor contacts [163],... 

Pending further work on these new magnetic semiconductors, metallic ferromagnets are in principle, the most convenient spin polarized sources for spin device work. The obvious configuration of direct Ohmic contact between metal and semiconductor proved to have fundamental shortcomings. The conductivity mismatch between the two materials implies very indifferent spin injection efficiency [174, 175], However it transpires that this difficulty is surmountable [176] by placing a tunnel barrier between the... 

Fert A, Jaffres H (2001) Conditions for efficient spin injection from a ferromagnetic metal into a semiconductor. Phys Rev B 64 184420... 

The value of y is related to the injection process and depends on electrode materials. InP-LJBD, a calcium electrode is used to increase the electron injection efficiency due to its low work function(12,13). The value of 7 e.h related to not only materials but also device structure. Multi-layer structure is commonly used to increase the value of 7, generating the hole and/or electron accumulation near a light-emitting layer surface(l,14). The maximum value of 7 1 has been suggested to be 25% because of the spin statistics of singlet exciton formation (11). Toin rove g, we have to use highly efficient materials or dye-doped materials(15). 

A number of difficulties have to be resolved to create successful devices. These include efficient spin injection into semiconductors and heterostructures, and a search for new spin-polarized materials. Other effects potentially important for spintronic devices include optical and electrical manipulation of ferromagnetism, current-induced switching and precessing of magnetization, and the possibility of a long coherence time for optically excited spins in semiconductors. For a good overview of the issues in spin electronics, including the prospects for spintronic quantum devices, see [3.118]. 

To measure the spin coherence time and to estimate the spin-polarized carrier injection efficiency from the electroluminescence data, the selection rules and the valence band structure in ZnO must be understood. The valence band in wurtzite materials is split into three bands (A, B, and C) due to crystal field and spin-orbit coupling as discussed before in Chapter 3. The spin degeneracy of these three bands and the conduction band is lifted in magnetic field resulting in small symmetric Zeeman splittings as shown in Figure 5.6 near the F point [48]. The allowed transitions following the selection rules AI = 1 (for 0 polarization) are indicated... 

In electroluminescent applications, electrons and holes are injected from opposite electrodes into the conjugated polymers to form excitons. Due to the spin symmetry, only the antisymmetric excitons known as singlets could induce fluorescent emission. The spin-symmetric excitons known as triplets could not decay radiatively to the ground state in most organic molecules [65], Spin statistics predicts that the maximum internal quantum efficiency for EL cannot exceed 25% of the PL efficiency, since the ratio of triplets to singlets is 3 1. This was confirmed by the performance data obtained from OLEDs made with fluorescent organic... 

Gratzel and co-workers reported an N3-dye-sensitized nanocrystalline Ti02 solar cell using a hole-transport material such as 2,2, 7,7 -tetrakis (N, N-di-p-methoxyphenyl-amine) 9,9 -spirobifluorene (OMeTAD) as shown in Fig. 17, as a solid electrolyte [143]. OMeTAD was spin-coated on the surface of the N3 dye/Ti02 electrode and then Au was deposited by vacuum evaporation as the counterelectrode. The cell efficiency was 0.7% under 9.4 mW/cm2 irradiation, and 3.18 mA/cm2 of Jsc was obtained under AM 1.5 (100 mW/cm2) [143]. The maximum IPCE was 33% at 520 nm. The rate for electron injection from OMeTAD into cations of N3 dyes has been estimated as 3 psec, which is faster than that of the I ion case [144]. 

The operation of the SPICE design is closely analogous to that of a conventional bipolar transistor comprising distinct emitter, base, and collector stages but with some differences that are forced by materials considerations. The central base layer is a doped semiconductor and in it is established a non-equilibrium minority carrier population by injection from the emitter. Additionally these injected carriers are partly spin polarized and when they are harvested by the spin-dependent collector, the collection efficiency is dependent on the relative orientations of the emitter and collector orientations. The feasibility of these devices depend on several issues,... 

Indeed, spin statistics mandate that if the rates of reactions (1) and (2) are the same, then the nongeminate polaron pairs generated by carrier injection in OLEDs would yield 3 TEs for every SE. This SE/TE branching ratio is one of the most important factors suppressing the efficiency of OLEDs based on the fluorescent decay of SEs. However, recent studies suggest that in luminescent -conjugated polymers the rate of reaction (1) is higher than that of (2), so the yield of SEs is... 

Meerholz and coworkers incorporated the HTM (triphenyldiamine derivative) into the backbone of SPF to elevate the device efficiency through promoting hole injection/transport properties [24]. The cross-linkable oxetane-functionalized SPF derivatives 10-12 were also synthesized to realize full color display via spin-coating processes. The resulting EL devices (ITO/PEDOT PSS/10, 11, or 12/Ca/Ag) showed maximum efficiencies of 2.9, 7.0, and 1.0 cd A-1 for blue, green, and red emissions, respectively. 

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