Photovoltages and Photocurrents

In principle the same process occurs in minority carrier devices. In all cases the photocurrent is proportional to the light intensity and is independent of the applied potential. Accordingly, the photocurrent occurs in the diode equation (Eqs. (2.18) or (2.31) or (2.37)) as an additive term, so that we have 

We can distinguish between two cases first the short circuit condition and secondly the open circuit condition. The first is defined as a condition where the system is short circuited, i.e. 7 = 0. According to Eq. (2.41) the total current is then only determined 

The second case, the open circuit condition, means that the total current is zero (j = 0). Inserting this value into Eq. (2.41), one obtains the so-called photovoltage tZpi, as given by 

The discussion of photoeffects in terms of quasi-Fermi levels may seem to be rather pointless because there is no way of determing the quasi-Fermi levels at solid-solid junctions experimentally. It has been introduced here, however, because it is possible to obtain experimental information on quasi-Fermi levels in the case of semiconductor-liquid junctions, and it will be shown in Chapter 7 that the same principles can be applied for semiconductor-liquid and solid-solid junctions. 

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Differences in behavior between polycrystalline and single-crystal CdSe electrodes in polysulfide PEC involving the short- and long-term changes in photovoltage and photocurrents have been discussed by Cahen et al. [88], on the basis of XPS studies, which verified the occurrence of S/Se substitution in these electrodes when immersed in polysulfide solution, especially under illumination. The presence of a thin (several nanometers) layer of CdS on top of the CdSe was shown to influence... 

In solution, photovoltage and photocurrent between electrodes are controlled by the different redox couples present in solution such as foreign ions, but also dissolved oxygen and water. 

Enhanced photovoltage and photocurrent signals were observed by the authors of Refs. [183,184] with linked porphyrin-quinone molecules in planar bilayer lipid membranes (BLM) as compared with preparations containing the non-Iinked components. They interposed BLM between two aqueous compartments containing a secondary electron donor on one side and a secondary acceptor on the other side. The efficiency of PET increased when the P-L-Q molecules were oriented in the membrane. 

Kruger J., Plass R., Gratzel M., Cameron P. J. and Peter L. M. (2003), Charge transport and back reaction in solid-state dye-sensitized solar cells a study using intensity-modulated photovoltage and photocurrent spectroscopy , J. Phys. Chem. B 107, 7536-7539. 

The experiment of White, Abruna and Bard (46a) sheds some light on how polymers may enhance the PEC response of semiconductor electrodes. n-WSe2 and n-MoSe2 showed quite variable oxidation waves when Immersed in aqueous solutions of Fe(CN) / and 13 / couples. In general, those electrodes with numerous surface imperfections (edges, steps) showed the poorest photovoltages and photocurrents. Surface Imperfections historically have been considered to be centers for the recombination of (e /h ) pairs, and they may act as catalysts for dark "back... 

Schwarzburg K, Willig F (1999) Origin of photovoltage and photocurrent in the nanoporous dye-sensitized electrochemical solar cell. J Phys Chem B 28 5743-5746... 

ZnO is obviously one of the best candidates among semiconductors for DSSC applications as it can be synthesized easily and inexpensively into different shapes and sizes using various methods, and is environment friendly and stable indefinitely. Photovoltage and photocurrent measurements using modulated illumination on dye-sensitized ZnO solar cells have shown tens to hundreds of times faster electron transport in nanorod array electrodes compared to nanocrystalline particulate electrodes assembled from colloidal nanoparticles, with the electron lifetimes being only slightly smaller [237]. 

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