Onset driving voltage

Electrical measurements on devices with different layer thickness have shown that the diode current depends on the applied field rather than the drive voltage. This is similar to what has been observed with our alternating PPV copolymers [68]. It indicates that field-driven injection determines the electrical characteristics. From Figure 16-39 it is evident that U-OPV5 has the lowest onset for both current and emission. By means of Fowler-Nordhcini analysis of the /-V -charac-teristics and optical absorption measurements, wc estimated the injection barrier for holes and the HOMO-LUMO gap, respectively [119]. The results of... 

Fig. 4.7. Flexodomains (a), quiescent state (b) and conductive oblique EC rolls (c) in Phase 5. The initial director is horizontal, (d) Temporal evolution of the contrast C within a period T of the driving voltage above the onset of instabilities at / = 0.1 Hz, close to the transition frequency ft- Arrows indicate the times when the photographs in the corresponding subfigures were taken.

We may now consider on a more quantitative basis what influence the applied potential has. If the cell could behave as a thermodynamic cell and the reactants were under standard conditions, we might expect that the voltage needed would be that given by the net reaction above, net = 0.892 V. But this would be for an infinitesimal, reversible change and, similar to any other chemical reaction, an electrochemical reaction has an activation energy that must be overcome for the reaction to occur. In fact, an additional voltage termed the overpotential rj is required to drive the electrode reactions. About 2 V has to be applied to reach the current onset of the cell described, and this threshold is sometimes called the decomposition voltage. Thus the total potential that we must apply across the cell is... 

When illuminated with energy equal to or above the band gap hv > Eg) at these operating potentials, minority hole carriers in n-type electrodes drive the OER at the electrode-electrolyte interface while minority electron carriers in p-type electrodes drive the HER at this interface. The potential at which this phenomenon begins to occur is the photocurrent onset potential (Eonset). which is offset relative to the flat-band potential (Efb) by the required kinetic overpotentials for the reaction of interest. The difference between the photocurrent onset potential (Eonset) and the reversible redox potential of interest (E°) is the onset voltage (Eonset)- A band diagram of a n-type photoanode and its hypothetical j-V response is shown in Fig. 6.7. 

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