Electroabsorption

The electroabsorption is defined as the normalized change in transmission arising from a DC electric field, 

Schematic plot of the third harmonic optical intensity (solid) and linear absorption (dashed). 

In principle, electroabsorption experiments enable excitons to be distinguished from interband transitions, as excitons are subject to the Stark effect, while interband transitions are subject to the Franz-Keldysh effect. We now describe these two effects. 

1 The Stark effect An electric field only affects the relative motion of an exciton and has no affect on the centre-of-mass motion. Thus, the total potential experienced by the electron-hole pair, Viot r), is 

If the exciton binding energy is less that FT, the exciton immediately dissociates under the influence of the electric field. If the exciton binding energy is greater 

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Table 11-2 shows the built-in potential in metal/MEH-PPV/metal structures measured by either electroabsorption [15] or photocurrenl techniques [37] for a variety of contact metals. The uncertainty in both the work function differences and the built-in potential measurements is about 0.1 eV. For all of the structures except the Pt-Ca and Al-Sm devices there is good agreement between the metal work function difference, AW, and the built-in potential, Vhi. This indicates that for a wide range of metal contacts the Schottky energy barrier between the metal and MEH-PPV is well approximated by the ideal Schottky model and that state chaiging, which pins the Schottky energy barrier, is not significant. A built-in potential smaller than the difference between the contact work functions implies that... 

A further clear establishment of the absoiption due to singlet excitons and the phonons coupled to them is the electroabsorption experiment reported in Ref. [18]. The main results are shown in Figure 9-14 the top panel shows the absorption spectrum of m-LPPP at 20 K. It becomes clear that the peaks at 2.7, 2.9, and 3.1 eV, representing A0, A i, and A2 (see Fig. 9-10) are not the only vibronic replicas. There are additional peaks between these dominant ones if the experiment is conducted at low temperature. The bottom panel in Figure 9-14 shows a so-called electroabsorption spectrum which is obtained as the modulation (or change) of the absorption under the application of an electric field. Below 3.2 eV the electroab-... 

Another way to measure the Vhi is by means of photovoltaic measurements [97, 113. The technique is based on the fact that, at near zero applied bias, the OLED acts as a photovoltaic cell, where photogencraled carriers drift under the influence of Vhi to produce a current in an external cireuit. In a way similar to electroabsorption, an external bias is applied in order to compensate the built-in potential and null the net pholocurrent (Fig. 13-6). However, it has been shown that the measurement produces accurate results only at low temperatures, where diffusive transport of charges that are phoiogcneraled at the interlaces is negligible [97]. 

Fischer R (1972) Absorption and electroabsorption of trigonal selenium near the fundamental absorption edge. Phys Rev B 5 3087-3094... 

Figure 6 compares the electroabsorption spectra of the three type of LB films in the wavelength range that corresponds to the absorption due to the transition moment in the direction of the long molecular axis. The applied field was 3.2 x 10s Vcm1 in each case. In the Y-type deposition film, a small Stark signal is observed nevertheless, the Y-type film is assumed to possess a symmetrical molecular orientation. The reason for this weak signal may be that the fluctuation of molecular orientation across the films induced a small asymmetry in the multilayer structure. 

The electroabsorption intensity of the Z-type deposition film is comparable to that of the Y-type film. The values of for the Z-type film and the Y-type film were estimated to he 0.10 and 0.04, respectively. The molecular orientation in the Z-type turn out to possess a small asymmetry, although the asymmetrical deposition methods was applied. We should, therefore, recognize that the structure of the Z-type film is close to that of the Y-type film a turnover of molecules was believed to occur. 

T.M. Brown, J.S. Kim, R.H. Friend, F. Cacialli, R. Daik, and W.J. Feast, Built-in electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene diox-ythiophene) hole injection layer, Appl. Phys. Lett., 75 1679-1681, 1999. 

GG Roberts, M McGinnity, WA Barlow, and PS Vincett, Electroluminescence, photoluminescence and electroabsorption of a lightly substituted anthracene Langmuir film, Solid State Commun., 32 683-686, 1979. 

B. O. Seraphin, Electroreflectance R L. Aggarwal, Modulated Interband Magnetooptics D. F. Blossey and Paul Handler, Electroabsorption B. Batz, Thermal and Wavelength Modulation Spectroscopy 7. Balslev, Piezopptical Effects... 

Electric fleld gradient, 22 214-218 Electroabsorption spectroscopy, 41 279 class II mixed-valence complexes, 41 289, 291, 294-297 [j(jl-pyz)]=+, 41 294, 296 Electrocatalytic reduction, nickel(n) macro-cyclic complexes, 44 119-121 Electrochemical interconversions, heteronuclear gold cluster compounds, 39 338-339 Electrochemical oxidation, of iron triazenide complexes, 30 21 Electrochemical properties fullerene adducts, 44 19-21, 33-34 nickeljll) macrocyclic complexes, 44 112-113... 

Blossey DF (1971) Wannier exciton in an electric field. 2. Electroabsorption in direct-band-... 

Table I consists of a compilation of r /4> ratios as a function of X. Our results and those presented for p-GaP and n-ZnO are in rough agreement with this simple model (8,9,30,31,32). Construction of a more refined model awaits incorporation of other data (nonexponential lifetimes, electroabsorption, carrier properties, intensity effects, quantitative evaluation of 4>nr by photothermal spectroscopy, e.g.) and examination of other systems.

Asymmetric induction during the reduction of 4-(48) was observed when a surface-modified carbon cathode was used.70 Optical yields were low but the effect of the chiral amino acid bound to the carbon surface was proved to be a true surface phenomenon. Induction of chirality by homogeneous rather than surface-bound agents has also been studied.71 All the isomeric acetylpyridines (48) were reduced in the presence of three different chiral alkaloids. Both carbinol products 2- and 4-(49) were shown to possess induced chirality, but the 3-carbinol (49) had none under any of the conditions tried. More rapid protonation of the intermediate was proposed to account for the lack of induced chirality. Optimization of optical yields was done.72 The pinacols (50) formed along with 49 were found to have no induced chirality. Optical yields have been as high as 50%.73 The role of electroabsorption was found to be important in the reduction of 2-(48).74 Product distributions were noted as a function of surfactant present in the electrolyte, carbinol 49 being favored... 

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