Photocurrent dark current

The latter mainly results from the thermal emission current. The dark current is apparent mainly in the long-wavelength range of the spectrum when the photocurrent is appropriately small [53, 54, 131]. It is relatively small for alloy cathodes (e.g. Sb-Cs cathodes), but not small enough to be negligible. 

Photocells The basic construction of a photocell is illustrated in Figure 17. A photocurrent flows when the photocathode is illuminated, this is proportional to the intensity of illumination if the supply potential has been chosen to be higher than the saturation potential. A minimal potential is required between the photocathode and the anode in order to be able to collect the electrons that are emitted. The sensitivity is independent of frequency up to 10 Hz. The temperature sensitivity of evacuated photocells is very small. The dark current (see below) is ca. 10 " A[l]. 

In conclusion it should be mentioned that the same type of effects are possible for p-type electrodes. In this case an anodic dark current occurs whereas the photocurrent corresponds to an electron transfer via the conduction band (cathodic plEiotocurrent). 

FIG. 14 On-off photocurrent responses (a) associated with the reaction in Eq. (41) at Ao0 = —0.225 V. In this figure, positive currents correspond to the transfer of a negative charge from water to DCE. The potential dependence of the photocurrent (b) was obtained under chopped illumination and lock-in detection. The maximum in the photocurrent-potential curve contrasts with the small changes in the dark current shown in (c). These responses are developed within the polarizable window described in (d). (From Ref. 49. Reproduced by permission of The Royal Society of Chemistry.)... 

Halmann reported in 1978 the first example of the reduction of carbon dioxide at a p-GaP electrode in an aqueous solution (0.05 M phosphate buffer, pH 6.8).95 At -1.0 V versus SCE, the initial photocurrent under C02 was 6 mA/ cm2, decreasing to 1 mA/cm2 after 24 h, while the dark current was 0.1 mA/cm2. In contrast to the electrochemical reduction of C02 on metal electrodes, formic acid, which is a main product at metal electrodes, was further reduced to formaldehyde and methanol at an illuminated p-GaP. Analysis of the solution after photoassisted electrolysis for 18 and 90 h showed that the products were 1.2 x 10-2 and 5 x 10 2 M formic acid, 3.2 x 10 4 and 2.8 x 10-4 M formaldehyde, and 1.1 x 10-4 and 8.1xlO 4M methanol, respectively. The maximum optical conversion efficiency calculated from Eq. (23) for production of formaldehyde and methanol (assuming 100% current efficiency) was 5.6 and 3.6%, respectively, where the bias voltage against a carbon anode was -0.8 to -0.9 V and 365-nm monochromatic light was used. In a later publication,4 these values were given as ca. 1% or less, where actual current efficiencies were taken into account [Eq. (24)]. 

Fig. 5.43 Photocurrent density versus applied potential in 1 M KOH solution under UV (320 nm to 400 nm) illumination (96 mW/cm ). Anodic samples prepared as (a Titanium foil anodized at 20 V for 70 h in DSMO and ethanol mixture solution (1 1) containing 4% HF. (b) H2O-HF electrolyte at 20 V for 1 h. Both samples were annealed at 550°C 6 h in oxygen atmosphere prior to testing. Dark current for each sample is shown in (c).

Most of the available inorganic semiconductors with wide enough band gap to prevent energy transfer have -type character. The conditions for studying electron injection from excited dye molecules are therefore most favourable with these materials since a depletion layer (compare Fig. 11) can easily be formed by anodic polarisation. This barrier layer prevents electronic currents in the absence of illumination (or keeps the dark current at least very small) and makes the system most sensitive for photocurrents. 

In derivatives of 9-dicyanomethylene-2-nitrofluorene 7 the ratios of photocurrent to dark current range between 100 and 20.000 depending on the position of the introduced nitro groups 82). 

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

This analysis is valid for all solar cells that consist of interpenetrating chemical phases—of which there are an increasing number [51]. For those without mobile ions, the distributed resistor model alone leads to the conclusion that dark currents cannot be quantitatively compared to photocurrents for those with mobile electrolyte, the effect is quantitatively reinforced by the field-induced motion of the electrolyte ions. 

The photocurrent-voltage curve of a cell made with the I /I2 redox couple (Fig. 8) shows behavior typical of the standard DSSC. The substantial photovoltaic effect is expected from the fact that the dark current (Fig. 4) is negligible positive of about -0.5 V. On the other hand, a cell made with the FcCp2 70 redox couple shows no measurable photoeffect Its current under illumination (Fig. 8) is essentially equal to its dark current (Fig. 4). The photovoltaic effect is negligible because practically all photogenerated charge carriers recombine before they can be collected in the external circuit. In general, fast rates of reactions (4) and (5) tend to eliminate the photovoltaic effect in DSSCs. 

Photocurrent voltage curves have been studied with molybdenum selenide crystals of different orientation and different pretreatment. Figure 5 represents results for three typical surfaces of n-type MoSe (JJ+). An electrode with a very smooth surface cleaved parallel to the van der Waals-plane shows a very low dark current in contact with the KI containing electrolyte since iodide cannot directly inject electrons into the conduction band and can only be oxidized by holes. At a bias positive from the flat band potential U where a depletion layer is formed a photocurrent can be observed as shown in this Figure. This photocurrent reaches a saturation at a potential about 300 mV more positive than when surface recombination becomes negligible. 

A drastically different behavior was found if the electrode was prepared by a mechanical cut normal to the van der Waals-sur-face. The dark current increases steeply above a critical anodic potential as is seen in curve 3 of Figure 5- Only a small photocurrent can be observed which quickly becomes indistinguishable from the dark current, if the electrode potential is further increased anodically. This shows that the recombination rate is much higher at this kind of surface and that electron injection into the conduction band is now catalyzed by surface states generated by the formation of steps and other surface imperfections (16). 

Mechanisms 1 and 2 are included in the model that is used here for comparison with experimental data. Interface recombination and dark current effects are not included however, the experimental data have been adjusted to exclude the effects of dark current. To include the additional bulk and depletion layer recombination losses, the diffusion equation for minority carriers is solved using boundary conditions relevant to the S-E junction (i.e., the photocurrent is linearly related to the concentration of minority carriers at the interface). Using this boundary condition and assuming quasi-equilibrium conditions (flat quasi-Fermi levels) ( 4 ) in the depletion region, the following current-voltage relationship is obtained. 

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