Recombination of electron-hole pairs

The emission of a range of photons from UV to infrared, called cathodolumines-cence, is mainly caused by recombination of electron-hole pairs in the sample. 

The most important feature in Fig. 8.4 is the existence of states in the Au-doped Si within the band gap of pure Si. These new states make the recombination of electron-hole pairs far more rapid in the doped material than in pure Si. That is, the existence of Au impurities in crystalline Si has serious negative effects on the properties of the material as a solar cell. This problem does not only occur with Au a large number of metal impurities are known to cause similar problems. More information about this topic is available from the sources in the Further Reading section at the end of the chapter. 

Recall that the concept of Fermi quasilevels, suggested by Shockley (1950), can be introduced as follows. Under steady state photogeneration of charge carriers, a dynamic equilibrium arises in a semiconductor between generation and recombination of electron-hole pairs. As a result, certain steady state (but not equilibrium ) concentration values nj and p are established. The quasiequilibrium concentrations ng and pg are defined by the relations ng = n0 + A and Po = Po + Ap> and since photogeneration of carriers occurs in pairs, we have An = Ap = A. Let the following inequalities be satisfied ... 

Photocatalytic reactions at the semiconductor surface can be described by the following six steps as shown in Fig. 5.3. (D Absorption of a unit of light associated with the formation of a conduction band electron and a valence band hole in the semiconductor. (2) Transfer of an electron and a hole to the surface. (D Recombination of electron-hole pairs during the reaction processes. Stabilization of an electron and a hole at the surface to form a trapped electron and a trapped hole, respectively. (D Reduction and oxidation of molecules at the surface. (6) Exchange of a product at the surface with a reactant at a medium. Among these reaction steps, the absorption of light in the bulk (step CD) and... 

As other semiconductors, QDs are characterized by a certain band gap between their valence and conduction electron bands.20 When a photon having an excitation energy exceeding the semiconductor band gap is absorbed by a QD, electron-hole pairs are generated (electrons are excited from the valence to the conduction band) and the recombination of electron-hole pairs (the relaxation of the excited state) results in the emission of the measured fluorescence light. 

These equations mean that a linear dependence of the reaction rate with light intensity (LVRPA) is observed when intensities are small, while square root dependence is observed when intensity is high. This latter dependence occurs for high intensities because the recombination of electron-hole pairs starts to limit the efficient use of the available photons (Alfano et al., 1997). The intensity at which the crossover between these two types of behavior occurs depends on the value of the lumped kinetic parameter 7, which in turn depends on the specific reaction under consideration. 

Figure 4.4. Schematic of recombination of electron-hole pairs generating either a photon of energy or heat.

Figure 4.45. Multilayered structure of OLEDs/PLEDs. Also shown are the relative energy levels for individual layers hght is emitted as a result of the radiative recombination of electron-hole pairs.

According to this definition = 1 in the range of saturated photocurrent. The current decrease observed at lower potentials, is caused by recombination of electron-hole pairs. The current is then given by... 

The physical mechanism by which semiconductor light-emitting diodes (LEDs) emit light is spontaneous recombination of electron-hole pairs and simultaneous emission of photons. The spontaneous emission process is fundamentally different from the stimulated emission process occurring in semiconductor lasers and superluminescent LEDs. The characteristics of spontaneous emission that determine the optical properties of LEDs will be discussed in this section. 

Figure 2 In highly-excited quantum-confined structures, photoexcited carriers form electron and hole Fermi seas in conduction and valence bands, respectively Uth is the energy distance between the corresponding Fermi levels. Recombination of electron-hole pairs, belonging to space-quantization energy levels, gives rise to discrete lines in the luminescence spectrum.

The recombination of electron/hole pairs can take place either between energy bands or on the surface. As a result the photocatalytic efficiency is reduced. To impede the recombination process, conducting materials such as noble metals can be incorporated into the semiconductor to facilitate the electron transfer and prolong the lifetime of the electron/hole separation process (10). Although, there has been considerable efforts in using photocatalysis for complete oxidation of organic compounds in air and water streams, incomplete or partial oxidation has been reported (//, J2). 

The conclusion that can be drawn from the experiments just discussed is that, except for the very small particles, the photoluminescence of our Si nanocrystals, which are produced by CO2 laser-assisted pyrolysis of silane and which are gently oxidized in air under normal conditions, can be perfectly explained on the basis of the quantum confinement model, that is, by the radiative recombination of electron-hole pairs confined in the nanocrystals [15]. In order to obtain high quantum yields, the nanoparticles must be defect-free in particular, they must be perfectly monocrystalline and all dangling bonds must be passivated, for example by a silicon oxide layer. Indeed, high-resolution electron microscopy (HREM) studies have shown that our Si nanoparticles are composed of a perfect diamond-phase crystalline core and a surrounding layer of SiO [19]. 

The generation, separation, and recombination of electron-hole pairs, along with electronic transport can be studied in great detail with time-resolved photocurrent generation techniques. The examples given above are by no means limiting and do not cover aU possible fields of applications. Yet, they demonstrate the enormous potential that these techniqnes offer. In the smdy of dye-sensitized solar cells the use of IMPS have proven invalnable and has lead to fnndamental knowledge about... 

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