Minority carrier transfer

A) minority carrier transfer catalysis and/or surface state passivation (B) electrostatic modification (C) catalysis of multi-electron photoprocesses (refer to text). 

This relation is identical to that derived for a pure solid state device which is determined by minority carrier transfer and recombination, such as a pn junction (see Section 2.3) or semiconductor-metal contact (see Section 2.2.3). The corresponding current-potential curves in the dark and under illumination are given by the solid lines in Fig. 7.16. Taking the complete Eq. (7.71), there may be a certain potential range where the recombination current determines the process until the current levels off to a constant jy. For very large jy values, the cathodic current can ultimately be diffusion-limited, which can be checked experimentally by using a rotating electrode. 

Fig. 7.58 a) Excitation of electron hole pairs and minority carrier transfer, b) Injection of minority carriers from the solution and recombination processes... 

A minority carrier transfer catalysis and or surface state passivation ... 

This difference was discussed to arise from a missing pathway of minority carrier transfer from the surface traps to the electrolyte and hence a negligible ks pointing at the presence of traps in the surface region which are not, however, accessible, even by adsorbed electrolyte species and were therefore referred to as near-surface defects . 

Fig. 9.3 Calculation of incident photon-to-cnrrent efficiency (IPCE) as a functitm of potentiai drop across the space charge layer and Ti/te, the ratio of the transit time to the recombination time. The rate of minority carrier transfer to the surface is set to equai the rate of transfer to the electrolyte (i.e., no surface recombinati

This relation is identical to that derived for a pure solid state device which is determined by minority carrier transfer and recombination, such as a p-n junction (see Section 2.3) or semiconductor-metal contact (see Section 2.2.3). The corresponding current-potential curves in the dark and under illumination are given... 

This means that the PMC signal will, apart from the generation rate of minority carriers and a proportionality constant, be determined by the interfacial charge transfer rate constant kr and the interfacial charge recombination rate sr... 

There is an additional simple relation between the surface concentration Aps of photogenerated minority carriers and the charge recombination and charge transfer rates sr and kr to be considered ... 

Let us now investigate the case of a semiconductor with a relatively slow interfacial charge transfer. In this case the surface concentration of minority carriers is high and we can neglect the second term (which does not contain Ps). For higher values of electrode potential, the term L Qxp(-AUqfkT) can also be neglected. 

Figure 13. Numerically calculated PMC potential curves from transport equations (14)—(17) without simplifications for different interfacial reaction rate constants for minority carriers (holes in n-type semiconductor) (a) PMC peak in depletion region. Bulk lifetime 10" s, combined interfacial rate constants (sr = sr + kr) inserted in drawing. Dark points, calculation from analytical formula (18). (b) PMC peak in accumulation region. Bulk lifetime 10 5s. The combined interfacial charge-transfer and recombination rate ranges from 10 (1), 100 (2), 103 (3), 3 x 103 (4), 104 (5), 3 x 104 (6) to 106 (7) cm s"1. The flatband potential is indicated.

Figure 14. PMC potential dependence, calculated from analytical formula (18) for different interfacial rate constants for minority carriers S = 1 cm, minority carrier flux toward interface I,- 1 cm-2s 1, a= 780enr1, L = 0.01 cm, 0=11.65 cmV, Ld = 2x 0"3cm), (a) sr = 0 and different charge-transfer rates (inserted in the figures in cm s 1), (b) Constant charge-transfer rate and different surface recombination rates (indicated in the figure).

As outlined at the beginning of this chapter, combined photocurrent and microwave conductivity measurements supply the information needed to determine three relevant potential-dependent quantities the surface concentration of excess minority carriers (Aps), the interfacial recombination rate (sr), and the interfacial charge-transfer rate ( r). By inserting the... 

The photovoltage is esentially determined by the ratio of the photo- and saturation current. Since io oomrs as a pre-exponential factor in Eq. 1 it determines also the dark current. Actually this is the main reason that it limits the photovoltage via Eq. 2, The value of io depends on the mechanism of charge transfer at the interface under forward bias and is normally different for a pn-junction and a metal-semiconductor contact. In the first case electrons are injected into the p-region and holes into the n-region. These minority carriers recombine somewhere in the bulk as illustrated in Fig. 1 c. In such a minority carrier device the forward current is essentially determined... 

The existence of two types of mobile charge carriers in semiconductors enables us to distinguish between a majority charge carrier transferred from the electrode into the electrolyte and a minority charge carrier injected from the electrolyte into the electrode. Minority carrier injection causes significant reverse currents, but may also contribute to the total current under forward conditions. 

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