Photocurrents pulses

For positive lit electrodes one can register the drift of holes, and for negative ones- the drift of the electrons. The photosensitizer (for example Se) may be used for carrier photoinjection in the polymer materials if the polymer has poor photosensitivity itself. The analysis of the electrical pulse shape permits direct measurement of the effective drift mobility and photogeneration efficiency. The transit time is defined when the carriers reach the opposite electrode and the photocurrent becomes zero. The condition RC < tlr and tr > t,r should be obeyed for correct transit time measurement. Here R - the load resistance, Tr -dielectric relaxation time. Usually ttras 0, 1-100 ms, RC < 0.1 ms and rr > 1 s. Effective drift mobility may be calculated from Eq. (4). The quantum yield (photogenerated charge carriers per absorbed photon) may be obtained from the photocurrent pulse shape analysis. 

In claiming (i) that v E and (ii) that photocurrent relaxation sets in on a 10 ns time scale we also have to cope with the experiment of Baumann et al. (17) who report on photocurrent pulses excited by 25 ps laser flashes falling off with an instrument limited decay time ( 100 ps) and increasing linearly with field. A numerical estimate which will be presented elsewhere indicates that, contrary to earlier reasoning (18), these photocurrent pulses can be associated with the primary generation of geminate e...h pairs whose dipole moments will preferentially align parallel to the applied field as E increases (19 20). 

Photovoltaic effects in polymers have continued to attract interest. Polyethylene films sandwiched between parallel transparent electrodes exhibit photocurrent pulses on illumination after being subjected to d.c. fields of up to 105 V mm-1 the effect is attributed to a hopping motion of detrapped electrons in the field of an injected space-charge.156 Similar studies have been carried out on polyethylene terephthalate),157 other organic polymers,168 and polymer- 158 ... 

Photocurrent pulses observed in a PTS crystal and polyacetylene, respectively after excitation with 25 ps laser pulses (from ref.(43)). 

Fig. 5. Schematic diagram of the photocurrent pulses from a photomultiplier tube as a function of time. The autocorrelation function is obtained by measuring the photocurrent at time i(t) and i(r + r) for times f = rj,. .. etc. which are selected along the graph and then calculating the average of the product. If sufficient points are taken the average will converge to a single value C (t) which will only be a function of the value of z chosen...

The rise time of the anode voltage under pulsed illumination can be made short because of the small stray capacitance of the anode. In a photocell of biplanar geometry (Fig.4.95) where the semitransparent anode has a distance from the cathode of only a few millimeters, an electric field of about 1 kV/mm allows large photocurrent pulses up to several amperes without distortion by space charges. With a coaxial design, matched to a 50 n cable, rise times of 100 ps can be achieved for pulses with peak heights of some volts, which can be directly viewed on a fast sampling... 

In dc amplification, the signal pulses from the phototube anode are converted into photocurrent and the voltage drop produced is read and the output displayed on a recorder. 

Figure 23. Influence of light pulsing frequency on peak height and peak position of n-WSe2 in contact with 50 mMFe2+/3+ (5 mM H2SO4). Pulsing frequencies between 11 and 110 cps are compared forthe PMC and photocurrent curves (hght intensity, 50 mW cm-2).

The ability to create and observe coherent dynamics in heterostructures offers the intriguing possibility to control the dynamics of the charge carriers. Recent experiments have shown that control in such systems is indeed possible. For example, phase-locked laser pulses can be used to coherently amplify or suppress THz radiation in a coupled quantum well [5]. The direction of a photocurrent can be controlled by exciting a structure with a laser field and its second harmonic, and then varying the phase difference between the two fields [8,9]. Phase-locked pulses tuned to excitonic resonances allow population control and coherent destruction of heavy hole wave packets [10]. Complex filters can be designed to enhance specific characteristics of the THz emission [11,12]. These experiments are impressive demonstrations of the ability to control the microscopic and macroscopic dynamics of solid-state systems. 

Affer drying fhe film completely in a vacuum for several days, a photocurrent was observed using the same electrode setup shown in Fig. 15, with pulsed irradiation of light above 380 nm using a 150 W xenon lamp (Hamamatsu Photonics). As shown in Fig. 15, a large ohmic phofocurrenf was observed when DNA strands were aligned perpendicular, but not paral-... 

Fig. 19. Photocurrent during light pulse (Ar-Laser, 514 nm, 1 W) at moderately doped Z11O2 electrode (Nj) pa 1017 cm-3) with rhodamine B, in absence of hydrochinon (1), in presence of 10-3 M hydrochinon (2)...

Fig. 21. Photocurrent during light pulse as in Fig. 19 at highly doped ZnO electrode (jVd cm-3) (1) no hydrochinon (2) 10 3 M hydrochinon...

Stroboscopic method. In this method the photomultiplier is also gated or pulsed having an on-time of nanosecond or subnanosecond duration during which it operates at very high gain. The flash lamp and the detector systems are synchronized such that a suitable delay can be introduced between the two. The gated photomultiplier samples the photocurrent each time the lamp fires and the photocurrent is thus proportional to the... 

The photocurrent which is a series of discrete pulses gives n(t) and n(t t) directly and can be used to calculate C (t) digitally. 

The resemblance of the photocurrent to the optical adsorption spectrum has suggested the involvement of molecular excited states in the creation of charge carriers. While this resemblance is by no means universally observed, the concept of carrier creation via exciton interactions at or very near the illuminated electrode has become increasingly favored. Many of the data leading to these conclusions have been obtained by the use of pulsed light techniques (6, 7,3). These methods are virtually independent of electrode effects and the subsequent analysis of the transient current has led to considerable advances in the theory of charge transfer in molecular crystals. 

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