Potential Range of Photocurrent Generation

Prior to the j-V analysis, the conductivity type (p or n) of the semiconductor should be known. The experiment should be carried out under small forward bias potentials (i.e. at potentials between the flat-band potential and the open circuit potential) towards reverse bias potentials. It is important to keep in mind that 

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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As discussed in section Potential Range of Photocurrent Generation , the conductivity type of the semiconductor determines the direction of the potential scan (anodic of Eonset for n-type and cathodic of Eonset for p-type). The initial potential should be close to Eonset- In theory, Eonset should be close to which can be determined using an illuminated OCP measurement (see section Illuminated Open-Circuit Potential (OCP) ). In practice these values can differ by a few hundred mV. Consequently, the user may need to adjust the window of the sweep to collect the full j-V curve. 

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