Illuminated currents

The results of Figure 6 exhibit the same general behavior as our experimental scans reported above the magnitude of the overshoot current relative to the difference between the steady state dark and illuminated current levels decreases with increasing bias (or decreasing photocathode potential). 

Fig. 5.18. Variation of dark and illuminated current with voltage for ITO/PEDOT/MEH-PPV PCBM/Al blends for different concentrations of PCBM. The contacts were ITO and A1 [43].

The dark and illuminated characteristics of the PCBM PPV solar cells for different concentrations of PCBM are shown in Fig. 5.18. It can be seen that the dark characteristics are not described by Shockley type equation. Forward illuminated I(V) curves are strongly dependent on the applied voltage. Under reverse bias the illuminated current is not so sensitive to the voltage. 

Fig. 5.21. Top figure The experimental plots of dark and illuminated J—V characteristics of a typical MDMO-PPV/PCBM solar cell for different concentrations of PCBM. The currents are plotted on the log scale. Curves 1 are for dark currents and curves 2 are for illuminated currents. The open circles show the dark currents needed to make the output current zero at the open circuit voltage. Bottom figure The current densities are plotted on the linear scale for two PCBM concentrations. Curves 1 are for dark currents and curves 2 are for illuminated currents.

Brabec and coworkers have made extensive measurements of the dark and illuminated currents of MDMO-PPV/PCBM BHSCs [124,141], The experimental results for the BH-SCs from their paper [124] are shown in Fig. 5.22. There are points of inflexion in the dark current of the organic solar cell confirming that the dark current is the space charge limited currents. These features, particularly the points of inflexion, are absent in the illuminated characteristics. At larger voltages the forward dark current intersects the illuminated current and the illuminated current becomes larger than the dark current. 

In 1995 Yu et al. reported J-V characteristics of a BHSC (layer structure Ca/MEH-PPV PCBM/ITO) device in their classical papers [119]. Dark forward current had the usual shape with a point of inflexion characteristic of the space charge limited currents. However the dark forward current was smaller than the illuminated curve at all voltages. The result is not acceptable, for if this were true the observed illuminated current should have flown in the opposite direction, i.e. it should have been negative like the short circuit current. In their paper in Science [119] Yu et al. observed similar discrepancy in the single layer organic solar cells also. 

The brominated specimens may be studied by a SEM-EDXA or a TEM-EDXA system. Since the intensity of the X-ray emission is directly proportional to the illuminating current, it is essential, for quantitative assay, to ascertain the current during the analysis. A Faraday cup (Howitt et al. 1976) in conjunction with an electrometer (e.g., Keithley 616 Digital Electrometer) may be used. The illuminating current should preferably be in the range of 10-9-10-11) amperes. 

Figure 2. Dark vs. illuminated current/voltage curves for two types of phthalocyanine-modified SnOg electrodes.

Figure 3. Dark vs. illuminated current/voltage curves for thin-layer voltammetry on plain and GaPc-Cl-modified Au-MPOTE s. Conditions as for Figure 2, except the potential scan was carried out at 2 mV/s.

Figure 5. Photosensitization vs. photoelectrocatalytic reactions. At top, the illuminated current/voltage curve is placed negative of the current/voltage response on bare Pt in the dark. At bottom the current/voltage curve indicates an increase in reaction rate, but no change in the apparent emf of the reaction.

Figure 5.27 Illuminated current-voltage characteristics of the n-CdS/p-CdTe heterojunction devices (a) for CdS Im deposited without STA and (b) for CdS Im deposited with STA.

Some of the PVK films have been redissolved in chlorobenzene and then redried either in absence or in presence of an electric field. Then, the illuminated current-voltage characteristics were measured. 

Fourth in a series dating from 1977, this volume illuminates current trends and likely future developments in the field of chemical signaling in vertebrates. Going back even farther, the first chapter, a personal account of the past quarter century by Dr. Mykytowycz recalls the most important milestones, such as symposia, or the founding of societies and journals. He also credits those investigators who stand out by their seminal studies. 

The equivalent circuit diagram used to model solar cell current-voltage characteristics is shown at the top of Figure 1.1. The schematic energy level diagram of a DSSC at the bottom of Figure 1.1 shows the various charge transfer processes that occur in photoelectrochemical cells and relates these processes to current pathways via components of the model circuit. An illumination current density /l is induced upon photoexcitation of the... 

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