Steady-State Photocurrents

In general, both the drift and diffusion components contribute additively to the photocurrent. The steady-state efficiency of the photogeneration process can be found by solving the drift and diffusion equations and adding the currents. When assuming that the built-in field is located primarily in either the n-type or p-type side of the junction, the Gartner equation for monochromatic photocurrent generation applies, which reads 

it is assumed that the applied field fully drops over the n-type side of the junction. Eq. (21) is only valid if electron-hole pairs do not recombine in the space-charge region, or at the surface. For practical devices this assumption usually does not hold, but to avoid complications we shall not consider these types of electron-hole pair recombination here. 

In a steady-state photocurrent experiment, a monochromatic light source is used to excite the semiconductor and the photocurrent and photovoltage are measured (Green [1992]). If the load resistance is zero, the short-circuit photocurrent is mea-smed and the photovoltage is zero. If the load is infinite, the photocurrent is zero and the open-circuit photovoltage is measured. 

In this chapter, the effect of preexcitation with the light of band-gap energy on trapping and thermal generation is examined in selenium and selenium-rich As-Se alloy films by several techniques. Results suggest that excess carrier trapping and dark-carrier generation are controlled by deep defect centers whose population can temporarily be altered by photoexcitation. 

we consider the associated changes in the photoelectronic properties of the samples. The spectral characteristics of photoconductivity of the samples display a red shift after irradiation. Such behavior of the photoconductivity is not surprising, because it is in full agreement with the shift in the absorption edge. Additionally, the photoconductivity decreases after photodarkening [2]. The decrease may be attributed to the creation of new defect states or altering the existing localized states. 

Trap Level Spectroscopy in Amorphous Semiconductors. DOI 10.1016/B978-0-12-384715-7.00006-1 

Copyright 2010 by Elsevier Inc, All rights of reproduction in any form reserved 

Lux-ampere characteristics for annealed films are characterized by a sublinear dependence /pj, in a wide light intensity range, n being approximately equal to 0.6 for As2Se3. After darkening, the index n increases. 

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The basic idea of most diffusion length measurement techniques is to generate a certain number of minority carriers inside the bulk Si, for example by illumination, and to measure the fraction of these carriers that diffuse to a collecting interface. This fraction can be determined capacitively [Bo6], as well as by measurements of the steady-state photocurrent [Dr2, Lei 1], The parameter obtained by these measurements is the minority carrier diffusion length ID of electrons in... 

Figure 6.1 Steady-state photocurrent in an a-As2Se3 sample at 700 nm illumination with approximate intensity of lO photons/cm /s as a function of inverse temperature (curve 1). Also shown is the effect induced hy laser irradiation (A = 633 nm) at 7 = 100 K (curve 2).

Light is switched off at +0.5 V vs. SCE for the cathodic sweep. In (a) there is no added reductant (b), (c), and (d) contain 0.5mM ferrocene, 1, l -dimethylferrocene, and acetyl-ferrocene, respectively. Acetylferrocene does not attenuate the surface ferricenium surface ferrocene wave since it is not a sufficiently powerful reductant. Ferrocene and 1, l -dimelhylferrocene both attenuate the surface ferricenium - surface ferrocene wave. But l,l -dimethylferrocene is more effective under identical conditions despite the fact that the same, mass transport-limited, steady-state photocurrent is found for these two reductants. These data suggest that after the light is switched off the reduction of surface ferricenium is controlled partially by mass transport and partly by the electron transfer rate (see text). 

The acetylferrocene does not consume the (FeCp2+)surf., Figure Ad, because the reaction is not thermodynamically spontaneous. The conflict in our data is that steady-state photocurrents for the fast reductants is the same, but the (FeCp2+)surf. can be consumed at different rates in the dark for the various fast reductants. 

PET across a phospholipid bilayer was also shown to occur when triad molecules of the type pictured in Fig. 21a, were brought into BLM [167,185], A steady-state photocurrent across the membrane was observed in these experiments. 

In the presence of hydroquinone, the quasi-steady-state photocurrent sensitized by partially aggregated Dye II will be governed by the following relations ... 

Analysis of the ratio of the instantaneous to steady-state photocurrent. J. Phys. Chem., 89, 3863-3869. 

Sodergren et al. obtained the steady state solution of equation (8.52) by assuming that D and r are constant and 17 = 1. Under these conditions, the photocurrent due to photoinjected electrons is independent of voltage, and the steady state photocurrent is given by... 

Figure 47 Fluorescence quenching efficiency (a) and steady-state photocurrent (b) as a function of electric field at two different excitation wavelengths. From Ref. 306. Copyright 2002 American Institute of Physics.

We saw that the observed steady-state photocurrent is given by... 

Steady-state photocurrent-voltage behavior quantifies the bottom line efficiency of operational solar cells. Unfortunately, it provides little molecular information, particularly when the photocurrent yields are low. A variety of transient, time and frequency domain electrochemical techniques have been employed to provide further insights into the molecular origin of loss mechanisms. Space restrictions do not allow these electrochemical techniques to be discussed here [34-37]. 

The first two terms represent the electron flux and the change in electron concentration with time, respectively. The third term represents the recombination rate and is assumed to be first order with electron density. The fourth term is the generation term that assumes the dye concentration is uniform throughout the film. Eq. 33 has been solved analytically. Sddergren and co-workers [155] have shown that the steady-state photocurrent is consistent with this model and the assumption that electron transport occurs by diffusion. 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

Despite the extensive photoconductivity data, the nature of photoexcitations in poly(phenylenevinylene), PPV, and its soluble derivatives has remained controversial. In part, the controversy arises from the conflict between the results obtained with fast time-resolved photoconductivity and those obtained by the more familiar steady-state photoconductivity the latter indicate a strong T-dependence for the np product. Recent experiments have resolved the apparent conflict [203]. The idea is rather simple If the sample is sufficiently thin that the photocarriers can be swept out before a significant fraction fall into traps, the steady-state photocurrent will provide information similar to that obtained at short times by transient photoconductivity (in the latter, sample thickness is not important since pre-trapping and trap dominated transport are separated in the time domain). 

The T-dependences of the steady state photocurrent (J j) at E= 1.33 x 10 V/cm in four samples with thicknesses from 120 nm to 5700 nm (a range of nearly fifty) are compared in Fig. VD-2. The thinnest sample (120 nm) exhibits the weakest T-dependence initially decreases but remains nearly constant below about 80 K, behavior which is similar to that obtained from transient experiments in the sub-ns regime. When thicker films are used, the dependence of on T increases. By fitting the low temperature data to a thermally activated form, exp( - A/kgT), the activation energy can be obtained for samples with various thicknesses. As shown in Fig. VD-2, A increases with the thickness of the semiconducting polymer film. 

The similarity of the temperature dependence of steady-state photocurrent in thin films to that of the transient photoconductivity in the sub-ns regime implies that, in thin films, carrier sweep-out occurs prior to deep trapping. The weak residual T-dependence above 80 K in the thinnest sample is again similar to that observed in sub-ns time-resolved experiments. In the time domain, this corresponds to the temperature dependent tail characteristic of the transient photocurrent [199]. This weak T-dependence arises from the effect of shallow traps with multiple release and retrapping during carrier sweep-out. Note that the... 

Figure VD-2 Steady state photocurrent in spin-cast MEH-PPV vs. 10 /T for samples of various thicknesses at E= 1.33x10 V/cm under illumination by 10 W/cm solid circles, 120 nm solid squares, 500 nm up triangles, 4100 nm down triangles, 5700 nm.

More detailed studies are required in order to check whether the above correlation, established on the basis of relatively rapid potential sweep measurements, holds also for the steady-state photocurrents, i.e., in the situation when the Ti02 surface becomes covered with the peroxo-titanate species. These should also include water photocleavage experiments onto titanium dioxide powders loaded with some of the catalysts investigated by Contractor and Bockris. The difficulty, associated with the fact that most of... 

FIGURE 6,2. Steady-state photocurrent-voltage curves for naked p-type Si and for platinized p-Si. Aqueous solution buffered to pH 6.5. Illumination is at 632.8 nm, 2.5 mW/cm. The platinized p-type Si was prepared by photoelectrochemical redution from 1 x 10 MK2PtCl6in0.1 MNaCI04/H20 at-0.3 until 1.1 x lO -C/cm had passed. After Dominey et... 

Fig. 8 Steady-state photocurrent-voltage curves for naked p-type Si and for platinized p-Si. Aqueous solution buffered to pH 6.6 illumination is at 632.8 nm,...

Figure 2. Photocurrent-voltage curves (10 mV/s) for a p-type Si/[(PQ 2C )J Pd(0)]mrt photocathode where Pd(0) is deposited only on the outer surface of the redox polymer. The illumination source is a He-Ne laser, 632.8 nm, at 10 mW/ cm2, and the exposed electrode area is 0.1 cmi2. The inset shows the power conversion efficiency peaking at pH = 4. Steady-state photocurrent corresponds to H, evolution. Data are from Ref. 35.

In the absence of bimolecular recombination, experimentally verified via the intensity dependence of the steady state photocurrent, and of defect-catalyzed recombination ("Shockley-Read recombination") ( ), K must con-... 

The photovoltaic effect was first observed in the phthalocyanines in 1948 by Putseiko (291). Since then the photoconductivity of polycrystalline samples and of sublimed films has been extensively investigated (32, 33, 66, 69,104, H5,184, %18,292,294,848,858,860-864). The majority carriers are assumed to be holes (174)- In vacuo the steady state photocurrent is related to the activation energy AE for photoconductivity by ... 

It is important to recognise that a sub-band gap optical transition leads to a delocalised carrier of one type and a localised carrier of opposite type. Steady-state photocurrent flow requires that the localised carrier is excited subsequently to the valence or conduction band, either by absorption of a second photon (process (b) in Fig. 3) or by thermal excitation (processes (c, d)). Bandgap states localised at the semiconductor surface may be of special importance for sub-band gap photocurrent flow. In process (e), an electron (majority carrier) is optically excited into the conduction band, and the resulting empty surface state is refilled by an interfacial electron transfer process. The latter process is similar to the process of dye sensitised electron injection in the nanocrystalline Ti02 solar cell [20-26, 129). 

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