Illuminated current/voltage response

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.

Of course, electrochemical systems can be perturbed in a large number of ways, e.g. pressure and temperature variation and the application of a time-dependent illumination leading to responses that are of an electrical nature (current or voltage). The discussions to follow will be restricted to electrical perturbations leading to responses that are also electrical. 

Figure 2.27 DSC solar cell characteristics. 1.2 nm thick nanowire ( ), 1.2 nm thick gyroid (A), and 1.4 jmi thick nanoparticle ( ) Ti02 arrays, (a) Absorption spectra after sensitization with N719 (inset), (b) Spectral response of liquid electrotyte dye-sensitized solar cells, (c) Current-voltage curves under simulated AM 1.5100 mW cm solar illumination. Reproduced with permission from Ref. [56].

Electrical transport models predict the device current-voltage characteristics and response to impulse, step and frequency modulated illumination. The power conversion efficiency of the cell q is determined by the open circuit voltage V c, the short circuit current J c, and the fill factor FF. FF is calculated from the maximum obtainable power max (JV) as shown in Fig. 3 ... 

The SC is characterized by its current-voltage dependence measured under standard solar simulator (CEI/IEC 904-3) as global air mass 1.5 (AM 1.5G) with the power density of 1000 W/m corresponding to spectral intensity distribution for wavelength 250-2500 nm and at an incident angle of 48.2° (Fig. 3.8). The intensity of the illumination should be verified before the measurements using a calibrated silicon diode with known spectral response. 

The simplest and most widely used model to explain the response of organic photovoltaic devices under illumination is a metal-insulaior-metal (MIM) tunnel diode [55] with asymmetrical work-function metal electrodes (see Fig. 15-10). In forward bias, holes from the high work-function metal and electrons from the low work-function metal are injected into the organic semiconductor thin film. Because of the asymmetry of the work-functions for the two different metals, forward bias currents are orders of magnitude larger than reverse bias currents at low voltages. The expansion of the current transport model described above to a carrier generation term was not taken into account until now. 

In-situ SPV measurements seem possible with minor modifications (1) the tip potential (versus the reference) is set at a value close to the rest potential of the semiconductor in darkness (this must be compatible with the electrochemical response of the tip), and (2) the tip current is quenched by adjusting the sample voltage (versus the reference) with the second feedback system. With p-type materials the method seems more obvious than with n-type specimens, since illumination promotes surface electrons. At n-type materials SPV measurements will induce corrosion since holes are driven to the interface. If absolute measurements of the SPV seem difficult, because they depend on the adjustment of the tip potential, differential measurements appear accessible to experiment. 

Blends of poly(ferrocenylmethylphenyisilane) (PFMPS) 48 and fullerenes as the active layers gave short circuit currents on the order of nanoamperes under white light illumination of ca. 160 mW cm" 2, with an open-circuit voltage from ca. 0.3 to 0.45 V [95]. Similar responses were obtained when the fullerenes were covalently attached to the polysilane backbone [96]. 

Quantitative measurements are best carried out at much lower light intensities than those responsible for the large effects illustrated in Fig. 5. It is desirable to avoid, as far as possible, the photoelectrochemical oxidation or reduction of the film on the time scale of the measurements and this generally restricts incident power densities to less than 10-4 Wcm"2. Since the photocurrents generated by such low levels of illumination are too small to be measured directly, it is necessary to use a lock-in amplifier in conjunction with a mechanical chopper in the experimental arrangement shown in Fig. 6. The sensitivity of the photocurrent spectrometer is usually determined by the noise current arising from the electrode capacitance and the noise voltage in the system. Under favourable conditions, it is possible to measure photocurrents as small as 10 10 A. For typical illumination inten-... 

Fig. 49. Experimental IMPS responses for dye sensitised cells (illumination from the substrate side, X = 514nm) measured at different voltages 90], Dc current 6.14mA, Note that the modulated photocurrent is 3 orders of magnitude lower than the dc photocurrent.

Figure 3.6 Response of PmPV polymer-coated CVD-grown SWNT-FET device to UV light (A- = 365 nm). (A) The source-drain current (/sd) versus the gate voltage (Kg) of the device in air (Ksd = 1 V) at UV-off (blue curves) and UV-on (red curves) conditions. The reversible hysteresis (forward 7sd -reverse 7sd) in the device measured in the range of 20 V (-10 V to +10 V) at the sweep rate of 4 Hz. The inset shows the polymer-coated CVD-grown SWNT-FET device geometry. (B) Current (7sd) versus time response to UV illumination of PmPV-coated SWNT-FET device in air at room temperature (Fq = 4 V, Fsd = 1 V). The inset shows no apparent recovery in the device conductance after 16 h at fixed Fg conditions. Shaded and unshaded regions mark the UV-on and -off periods, respectively. Reprinted (adapted) with permission from Star, A. et al. Nanotube Optoelectronic Memory Devices. Nano Letters, 2004. 4(9) pp. 1587-1591. Copyright (2004) American Chemical Society.

Voltage-clamp currents were recorded from lipid bilayers in which a small amount of photopigment was previously incorporated (see Methods). In these measurements the membrane was formed in red light and successively illuminated either with a continuous white light or with 1-4 flashes of light of a Xenon lamp (1 msec duration, 1 W/mm ). In 80% of the measurements light elicited a "response , after a lag time varying between some seconds and 150 seconds with a mean value of 70 seconds. Two types of responses were observed ... 

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