Band photocurrent transients

The photocurrent responses of nanocrystalline electrodes to stepped or pulsed illumination exhibit features on rather slow timescales. This is illustrated, for example, by Fig. 8.26, which is a set of photocurrent transients reported by Solbrand et al. [78] for band-band excitation at 308 nm of nanocrystalline Ti02 films of differing thicknesses permeated by 0.7 mol dm-3 LiC104 in ethanol. The 30 ns excimer laser pulse was incident from the solution side, and since the penetration depth of the light was much smaller than the film thickness, electron-hole pairs were effectively... 

Fig. 79. Photocurrent transients with white light for n-MnTi03 in 1 MNaOH. Flat-band potential is ca. - 1.0 V/SSE and the dark-current density is shown. It can be seen that the transients become essentially square for V - VjJ 1 V.

Fe(phen)3] to the solution, the standard potential of which is located very close to the valence band of WSc2, the energy bands remain pinned to their dark values [78], i.e. holes created by light excitation are efficiently transferred from the valence band to the hole acceptor in the solution. In the absence of a redox system, typical photocurrent transients have been observed in the range Ufb(dark) and Ufb(hv). Details of the charge transfer mechanism will be given in Sect. 4.3. 

Bare and sensitised nanoporous TiOi electrodes with thicknesses between 0.1 and 20 pm have been studied extensively by several research groups, both by measuring photocurrent transients and IMPS responses. Although the excitation mechanisms differ (band-band excitation as opposed to dye sensitised electron injection), the... 

Redox Processes at Semiconductors-Gerischer Modei and Beyond, Fig, 5 Peak value filled square) and stationary plateau value filled circle) of the photocurrent transient in response to a rectangular illumination pulse of 1 ms duration in dependence on the voltage bias. The inbuilt depletion layer voltage is decreased with the bias shifting in negative direction. The flat-band potential is at - 0.95 V. The reduced redox ions that can discharge the holes at the surface are 0.005 M 1,1 -dimethylferrocene (Reproduced from upper part of Fig. 4, Ref. [16])... 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

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).

The net photocurrent and the quantum yield are a function of a number of competing processes " as shown in Fig. 1.22. For an n-type semiconductor, the externally measurable current i is the difference between the photocurrent and the forward current of electrons. The electron current is decreased to zero under certain anodic bias. While the flux of holes to the surface is exclusively controlled by the solid-state properties, all the other reaction steps depend on the surface properties of the semiconductor. The holes arriving at the surface can either (i) transfer to an electron donor in the solution, (ii) be trapped at the surface states, or (iii) recombine with electrons in the conduction band in the depletion region or at the surface. Process (iii) does not generate current in the external circuit, whereas process (ii) produces only transient current charging up the surface states. Only process (i) produces steady photocurrent. The measured photocurrent /ph can therefore be different from the flux of holes to the surface due to these processes. 

Illumination with light having a wavelength larger than the band gap of silicon generates a photocurrent under an anodic potential on an n-Si electrode but has essm-tially no effect on p-Si, as would be expected from the basic theories of semiconductor electrochemistry. However, the photocurrent may not be sustained because of the formation of an oxide film, which passivates the silicon surface to various degrees depending on the electrolyte composition. In solutions without fluoride species, the photocurrent is only a transient phenomenon before the formation of the oxide film. In fluoride solutions, in which oxide film is dissolved, a sustained photocurrent can be obtained. 

It is apparent that trap states influence conduction in several ways through dispersive photocurrent and dark current transients strongly space charge limited transport [59] and sub-band gap photo-conductive and photovoltaic effects. 

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