Current/voltage characteristics

For an electrical current to flow through the 2-level system, the holes have to move in the direction of current flow and the electrons, due to their negative charge, in the opposite direction. We consider some sort of valve or semi-permeable membrane which allows the electrons to enter or leave the 2-level system at an energy of 2 only on one side and the holes at an energy of 

Integrating over the thickness l of the 2-level system, we find the total current jQ, since the hole current is zero at the membrane for the electrons (x = 0) and equals the full current at the hole membrane (x = /), where the electron current is zero  

By virtue of the membranes, the Fermi energy of the upper level translates into the Fermi energy at the electron membrane, whilst the Fermi energy of the lower level becomes the Fermi energy at the hole membrane. The difference between the Fermi energies of the two membranes is related to the voltage V between the contacts to the membranes by 

Equation (4.23) is the familiar current voltage characteristic of a diode. For large negative voltages and in the dark, the small reverse current is given by 

With these expressions, the current-voltage characteristic becomes 

Under reverse bias, the band bending and its associated electric field are increased by the applied voltage, and charge transport across the junction is blocked. Conversely, under forward bias, the external voltage tends to decrease the band bending, thus facilitating charge injection across the interface. Eq. 

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Fig. 5. Dark(current—voltage) characteristics ofp—n ( ) andp—i—n (A) junctions at 295 K.

ZnO blocks have extremely non-linear, current-voltage characteristics, typically represented by... 

Fig. 2. Current-voltage characteristics taken at points (1), (2), and (3) in Fig. 3. The top insert shows the conductance versus voltage plot, for the data taken at point (3) (adapted from Oik el al. I]).

This figure demonstrates that also under potentiometric conditions (- no external current flow) electrochemical net reactions occur. The EMF of the zinc-amalgam in a given Zn2 -ion solution depends on the current-voltage characteristic of other ions (in this example, Cu2 and Pb2 are interfering ions with respect to the Zn2 equilibrium potential) at the amalgam electrode. EMF drifts are thus explainable. 

Recently, Mailiaras et al. [ 1511 have shown that for the analysis of the current-voltage characteristics of single layer OLEDs, it is of fundamental importance to properly account for the built-in potential. The electrical characteristics of MEH-... 

To calculate the current-voltage characteristic of a MESFET, we assume that the conducting channel is at thermal equilibrium, and the space-chaige layer completely depleted. Eq. (14.28) becomes... 

However, although it allowed a correct description of the current-voltage characteristics, this model presents several inconsistencies. The main one concerns the mechanism of trap-free transport. As noted by Wu and Conwell [1191, the MTR model assumes a transport in delocalized levels, which is at variance with the low trap-free mobility found in 6T and DH6T (0.04 cm2 V-1 s l). Next, the estimated concentrations of traps are rather high as compared to the total density of molecules in the materials (see Table 14-4). Finally, recent measurements on single ciystals [15, 80, 81] show that the trap-free mobility of 6T could be at least ten times higher than that given in Table 14-4. 

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. 

The power density / (Wkg-1) of the element results if the power is related to the battery weight. Figure 7 shows the current-voltage characteristic of a Leclanche element. 

Figure 7. Current-voltage characteristic of a Le-clanche element.

An important experimentally available feature is the current-voltage characteristic, from which the terminal voltage ([/v ) supplied by the electrochemical cell at the corresponding discharge current may be determined. The product of current / and the accompanying terminal voltage is the electric power P delivered by the battery system at a given time. 

FIG. 10 Current-voltage characteristics of polyanihiies LS films of 40 monolayers deposited on interdigitated electrodes (1) PANI, (2) POT, (3) POAS, (4) PEOA. 

On the other hand, some work on the topic considers the presence of negative differential resistance in the current-voltage characteristics and the possibility of a coulomb... 

On the other hand, even in particle systems the coulomb blockade (Van Bentum et al. 1988a) and the coulomb staircase (Van Bentum et al. 1988b) were observed, some nonlinear effects were observed in the current-voltage characteristics (Wilkins et al. 1989), and behavior related to the quantized energy levels inside the particles was described (Crom-mie et al. 1993, Dubois et al. 1996). 

Figure 12. A ligand-protected Pt309 nanocluster between an STM tip and an Au (111) surface to determine the current-voltage characteristics.

Ohmic losses AEohmic originate from (i) membrane resistance, (ii) resistance of CLs and diffusion layers, and (iii) contact resistance between the flow field plates. Although it is common practice to split current-voltage characteristics of an MEA into three regions— kinetic (low currents), ohmic (intermediate currents), and mass transport (high currents) [Winter and Brodd, 2004]—implicit separation of vt Afiohmic is not always straightforward, and thus studies of size and... 

FIG. 60. Current-voltage characteristics of a solar cell made at 65 MHz and 42 mW/cm-, The dashed line indicates the maximum-power point. 

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