Electron in the dark

Cao et al. and other authors [191] have observed that the photocurrent risetime decreases as the light intensity is increased. The risetime appears to follow a power law of the form t /2 x/o" with n = 0.5-0.6. This effect may arise from intensity dependent occupancy of electron traps. Cao et al. therefore assumed as a first approximation that the diffusion coefficient of electrons is a linear function of electron concentration. Numerical solution of Eq. 88 then yields transients that depend on light intensity. In principle this approach allows the incorporation of any arbitrary dependence of D on but a more satisfactory approach would be to relate the diffusion coefficient directly to the trap occupancy or to separate diffusion and trapping. Cao et al. estimate an upper limit of D = 10 cm s for the diffusion of electrons in the dark, whereas the D value for intensities corresponding to solar illumination levels are two orders of magnitude higher. 

It is these quasi-Fermi levels that are important in defining the thermodynamic criteria for light-driven water splitting. In the case of an n-type semiconductor such as rutile, the concentration of electrons in the dark is high (typically >10 cm ), whereas the concentration of holes is vanishingly small since the law of mass action applies. 

Band scheme for CU/CU2O with the hydrogen and vacuum scale, the [Co(NH3)6Cl] redox system, the subband and the path for transfer of photoelectrons via surface states and of electrons in the dark via the subband. (From Strehblow, Fl.-Fl. et al.. International Symposium on Control of Copper and Copper Alloys Oxidation, Rouen 1992, Edition de la Revue de Metallurgie, Paris, France, p. 33,1993.)... 

Color Centers. Characteristics of a color center (1,3,7) include production by irradiation and destmction by heating. Exposure to light or even merely time in the dark may be sufficient to destroy these centers. Color arises from light absorption either from an electron missing from a normally occupied position, ie, a hole color center, or from an extra electron, ie, an electron color center. If the electron is a valence electron of a transition element, the term color center is not usually used. 

Fig. 2. (a) A schematic diagram of a n—p junction, including the charge distribution around the junction, where 0 represents the donor ion 0, acceptor ion , electron °, hole, (b) A simplified electron energy band diagram for a n—p junction cell in the dark and in thermal equilibrium under short-circuit... 

The first realization of a conjugated polymer/fullerene diode [89] was achieved only recently after the detection of the ultrafasl phoioinduced electron transfer for an lTO/MEH-PPV/CW)/Au system. The device is shown in Figure 15-18. Figure 15-19 shows the current-voltage characteristics of such a bilayer in the dark at room temperature. The devices discussed in the following section typically had a thickness of 100 nm for the MEH-PPV as well as the fullerene layer. Positive bias is defined as positive voltage applied to the 1TO contact. The exponential current tum-on at 0.5 V in forward bias is clearly observable. The rectification ratio at 2 V is approximately l()4. 

Since under normal depletion conditions, conductivity changes are dominated by majority carriers, and interfacial electron transfer can be neglected in the dark, the carrier profile can be found by solving Poisson s equation ... 

Dark current comes from the thermal excitation of electrons in the detector material - thermally generated electrons can not be distinguished from photoelectrons. 

When exposed to daylight, the sulfide and selenide halides HgsY2X2 are blackened within a few minutes. This black color reversibly disappears when the sample is heated to 90 to 120°C, or stored in the dark for several days 204, 375-377). The nature of this phototropic behavior has now been widely investigated by analytical, spectroscopic, structural, magnetic, EPR, and radiotracer investigations 205, 233, 375-377, 379, 380, 382). During irradiation of the compounds, electrons belonging to or I ions are excited to upper states. The result-... 

The color hues produced in the reaction do not appear to be affected by differing substituents at the amine nitrogen however electron-attracting substituents at the a-C atom appear to reduce the detection sensitivity of the reaction [1]. The colors produced remain stable for months in the dark. In the light the zones produced by primary amines fade more rapidly than those from secondary amines [1]. 

RCII may subsequently have been transformed into RCI by formation of the Fx cluster and eventually the capturing of a soluble 2[4Fe-4S] protein as an RC-associated subunit. These additions would have allowed electrons to leave the space of the membrane and serve for reductive processes in the dark reactions of photosynthesis. Our present knowledge concerning distribution of HiPIPs among species indicate that this electron carrier would have been invented only lately within the branch of the proteobacteria. Tbe evolutionary driving... 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

Fig. 5.2 The n-Cd(Se,Te)/aqueous Cs2Sx/SnS solar cell. P, S, and L indicate the direction of electron flow through the photoelectrode, tin electrode, and external load, respectively (a) in an illuminated cell and (b) in the dark. For electrolytes, m represents molal. Electron transfer is driven both through an external load and also into electrochemical storage by reduction of SnS to metaUic tin. In the dark, the potential drop below that of tin sulfide reduction induces spontaneous oxidation of tin and electron flow through the external load. Independent of illumination conditions, electrons are driven through the load in the same direction, ensuring continuous power output. (Reproduced with permission from Macmillan Publishers Ltd [Nature] [60], Copyright 2009)...

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