Semiconductors conduction band processes

The sensitization of semiconductors is a special example of electron transfer quenching and may prove to be very important. A photoexcited electron may, for example, be injected with high quantum yield into the semiconductor conduction band, to produce a photovoltaic device. The hole that is left behind may then perform some useful oxidation process. 

Electron-phonon interaction in a semiconductor is the main factor for relaxation of a transferred electron. There are two different relaxation processes that decrease the efficiency of light conversion in a solar system (1) relaxation of an electron from a semiconductor conduction band to a valence band and (2) a backward electron transfer reaction. The forward and backward electron transfer processes have been already included in the tunneling interaction, HSm-qd, described by Eq. (108). However, the effect of SM e-ph interaction is important for the correct description of electron transfer in the SM-QD solar cell system. In the previous section, we have gradually considered different types of interactions in the quantum dot and obtained the exact expression for the photocurrent (128) where the exact nonequilibrium QD Green s functions determined from Eq. (127) have been used. However, in... 

This condition must also be fulfilled if both bands are involved in one redox reaction at a semiconductor electrode. In addition we have s = Zs nd = p% In the case of a conduction band process one obtains then, by using Eqs. (7.52) and (7.54), and with... 

Figure 9.6 Charge transfer between a semiconductor electrode and a redox system. (A) Conduction band processes and (B) valence band processes.

Thermodynamic factors are expected to be different for the various stepwise mono-oxidation processes showed in eqs. 8-11. Equation 12 is another poisoning reaction, which can involve any species (with n = 1—3) and could compete with eq. 4, but it can be minimized by keeping low the concentration of C. It is worth noting that in the above scheme, it is implicitly assumed that SA is a chemical species also a semiconductor electrode can take this role. In the latter case, the optimization of the reactions involved in eq. 4 vs. eq. 5 can be obtained by modifying the level of the semiconductor conduction band by an applied bias. The irreversibility of the overall process is given, rather than by eq. 7, by removal of the negative charge from SA via an external circuit. 

High efficiency of the electron injection step and a low yield of electron recombination between electrons in the semiconductor conduction band and oxidized dye are essential for an efficient material. The electron injection efficiency can be optimized by ensuring very fast injection, much faster than all competing processes deactivating the excited state of the sensitizer. The fraction of injected electrons that recombine with the oxidized dye may be influenced in several different ways. One possibility is to choose the sensitizer and redox couple in such a way that their electrochemical properties maximize the ratio of rates for oxidized dye reduction by the redox couple and electron recombination from the semiconductor. Another possibility is to decrease the rate of back-electron transfer from the semiconductor to the oxidized dye, by increasing the distance between the semiconductor and sensitizer (see e.g. Burfeindt et al ). 

In an intrinsic semiconductor, tlie conductivity is limited by tlie tlieniial excitation of electrons from a filled valence band (VB) into an empty conduction band (CB), across a forbidden energy gap of widtli E. The process... 

Semiconductors may also be made from a maferial which is normally an insulator by infroducing an impurify, a process known as doping. Figure 9.9 shows fwo ways in which an impurify may promote semiconducting properties. In Figure 9.9(a) fhe dopanf has one more valence election per atom fhan fhe hosf and confribufes a band of filled impurify levels 1 close to fhe conduction band of fhe hosf. This characterizes an n-fype semiconductor. An example is silicon (KL3s 3p ) doped wifh phosphoms (KL3s 3p ), which reduces fhe band gap to abouf 0.05 eY Since kT af room femperafure is abouf 0.025 eY the phosphoms... 

It is because these exU insic elecU ons can so readily be activated thermally to the conduction band, tlrat great care must be taken in producing the elemental semiconductors to a high state of purity, by such processes as zone refining. 

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