Dark Current and Photocurrent

1) A large program Energy Frontier Research Centers has been introduced in the USA with topical programs on solar fuels and photovoltaics. 

2) A new funding agency, Advanced Research Projeas Agency-Energy (ARPA-E) has been founded in the USA. 

The total current, including the thermally generated part becomes 

---

The IR, UV-VIS, and XPS, dark current and photocurrent have been compared for a number of the polymorphs of CuPc (Knudsen 1966 Enokida and Hirohashi 1991). The most important characteristic of a pigment is its colour the variation in spectral response five CuPc polymorphs, presented in Fig. 6.9, shows considerable... 

N. G. Tarr and D. L. Pulfrey, An investigation of dark current and photocurrent superposition in photovoltaic devices, Solid State Electron. 22 (1979) 265-270. 

A pH sensitive photoconductor based on PPV has been reported. The detection of local changes in pH by small-scale sensors is of particular interest for a variety of medical, biological, and environmental applications. In thin sheets of 25-30 nm of PPV, an increase in dark current and photocurrent is observed upon exposure to aqueous solutions. The change in photocurrent is a function of the pH. 

The values of and depend on the shape of the current-potential curve under illumination (i.e. on both the dark current and photocurrent-potential... 

N. G. Tarr and D. L. Pulfrey, An investigation of dark current and photocurrent superposition in photovoltaic devices. Solid State Electron, 22 (1979) 265-270. R. Tenne, N. Muller, Y. Mirovsky, and D. Lando, The relation between performance and stability of Cd-chalcogenide/polysulfide photoelectrochemical cells I. Model and the effect of photoetching, J, Electrochem, Soc, 130 (1983) 852-860. K. M. Van Vliet and A. H. Marshak, The Schottky-like equations for the carrier densities and the current flows in materials with a nonuniform composition. Solid State Electron, 23 (1980) 49-53. 

Fig. 40. (a) Dependence of the open-circuit potential E x on the illumination intensity J, and (b) depen-dence of the (1) dark current and (2) photocurrent density jpb on the electrode potential E. Potentials given vs. Ag,AgCl electrode [172], Reproduced by permission of The Electrochemical Society, Inc. 

The detection wavelength was 700 nm (PFB F8BT) and 580 nm (TFB F8BT), respectively. The current was not corrected for the dark current and hence is not the pure photocurrent. 

Although the chemical reactions at the sili-con/HF interface are the same for n- and p-type silicon, there is a basic asymmetry in their electronic properties. For the dissolution of silicon under an anodic bias , the p-type Si is forward-biased and the current is caused by thermally generated majority carriers. The n-type Si is reverse-biased and undergoes charge depletion. The current is characteristic of a minority carrier flow. For low-doped n-Si, anodic dissolution uses photogenerated minority carriers. The i-V curve for p-type Si has been discussed earher and comparison with the i-V curves for n-Si are presented here. For n-Si under reverse bias condition, a dark current and/or a photocurrent are observed, depending on the doping density and Ulumination level. 

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]. 

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)]. 

---