In the case of metal-semiconductor junctions, the barrier heights were found to be nearly independent of the type of metal, which was interpreted as strong Fermi level pinning. These semiconductors are of special interest here because many electrochemical experiments have been performed with these materials. The contact between metals and III-V semiconductors has been studied in detail.
In order to obtain a better characterization of the experimental data, they defined an index of the interface behavior S which they introduced into Eq. that the sensitivity of a barrier height to different metals increases with the ionicity of the semiconductor. However, in many cases the slope, d j,/ d, of a corresponding plot was much smaller than unity. Many researchers have also measured the barrier height as a function of the work function of the metal, and have mostly obtained a straight line, as expected from Eq. Many experimental values for barrier heights at semiconductor-metal junctions have been obtained. These methods are not described here some of them are discussed in Chapters 4 and 5 (see also ). The first-mentioned technique seems to be the most accurate. There are various experimental techniques for measuring barrier heights, such as photoelectric and capacity measurements and current-voltage investigations. Again assuming an ideal contact, the energy barrier is given by Ī large number of semiconductor metal junctions have been studied.
It should be mentioned that for p- type materials, usually, a negative space charge is formed because the work function of the semiconductor is below that of the metal. have described a high sensitivity variation of this technique where the time integrated light output is measured as a fimction of the duration of a voltage pulse and the transit time is extracted by extrapolation to ġ3 The Chemistry, Physics and Engineering of Organic Light-Emitting Diodes Ībsolute photoluminescence efficiency measurements in thin solid films are quite difficult, since light-trapping, waveguiding effects and, possibly, distributions in tlie emission dipole moments of individual chromophores modify the angular distribution of the emission. Therefore, the time onset for emission corresponds to the transit time of the most mobile carrier. The technique is based on the fact that for recombination to take place, the most mobile of the two carriers needs to transverse the sample and meet with the less mobile one. A way to measure charge carrier mobilities directly on an OFED geometry is by time-resolved electroluminescence measurements. The above technique requires organic layers that are of the order of a few microns thick, considerably thicker than those used in OFEDs. The experiment is performed at various voltages and the barrier height values are corrected for the image charge potential by extrapolating to zero voltage.
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