Sub-band gap

Figure 1 Schematic representation of a Gratzel solar cell. Sub-band-gap light absorption leads to the formation of the sensitizer excited state, followed by electron injection into the conduction band of the high-area nanocrystalline semiconductor. The electrons can be drawn into a circuit to do useful work and returned to the system through the redox mediator, the I/Ij" couple, at the counterelectrode.

Adsorption of substrates may also be altered by photoexcitation or by the presence of other adsorbates. That adsorption and photoadsorption are influenced by thermal removal of water and surface hydroxyl groups has been shown in detailed studies On tin oxide, preadsorption of other reagents can produce a substantial increase in oxygen photoadsorption under sub-band gap irradiation and can shift the photosensitivity threshold to longer wavelengths... 

Optical transitions in semiconductors can also involve localized states in the band-gap. These become particularly important for semiconductors in nanocrystalline form (see below). Sub-band-gap transitions can be probed with photons of energy below the threshold defined by Eg. 

Surface state densities of the order of 10 cm are commonplace for semiconductor electrodes of the sort considered in previous sections of this chapter. These translate to equivalent volume densities of 10 cm for nanocrystalline films. Such high densities enhance light absorption by trapped electrons in surface states, giving rise to photochromic and electrochromic efiects [297-299] (see below). Unusually high photocurrent quantum yields are also observed with sub-band-gap light with these photoelectrode materials. Corresponding sub-band-gap phenomena are rather weak and difficult to detect with single crystal counterparts. 

Photocurrent on n-Si can also be caused by surface generation processes with sub-band-gap light illumination. Figure 5.16 shows that the quantum yield from sub-... 

Fig. 3 summarises the possible sub-band gap transitions between localised and delocalised states. 

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). 

Butler and Ginley have explored the properties of p-GaP in some detail. The stability of the material in acid and base solutions is poor, and the photoresponse is seriously reduced by the presence of surface states, which act as efficient recombination centres. Additional evidence for these surface states comes from measurements of the sub-band-gap response of GaP photoelectrodes, and Butler and Ginley argue convincingly that the same bulk state controls the electroluminescence efficiency of GaP. 

Somewhat larger siuface defect densities of iVj = 10 cm were found by Knights et al. (1977) and by Jackson et al. (1983a), who measured the thickness dependence of the spin density and of the sub-band-gap absorption hv <1.5 eV). It is not clear whether the defects measured in these experiments are near the substrate or at the free surface. The measured value is probably the sum of both contributions, so that we are back at defect concentrations near 5 X 10 cm. ... 

In previous studies of colloidal SQDs [5], a dominant feature of the PL was found to be due to the recombination from surface states in the gap. To examine such states, PL spectra were taken using a 514.53 nm Ar+ laser source to ensure the sub-band gap excitation. As a result the strong PL feature was not present due to the recombination of the electron-hole pairs at quantum confined states. 

Probably the first reported instance of observation of an APE was in 1977 for a n-Ti02-Na0H electrolyte interface [158]. The APE was observed in the saturation region of cathodic current flow and was induced by sub-band gap irradiation of the photoanode. A peak in the spectrum of the photoresponse at 800 nm (the corresponding photon energy being lower than the 3.0 eV band gap of Ti02) was used by the authors to invoke a surface... 

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. 

UC of sub-band gap photons for a conventional single-junction bifacial solar cell was presented by Trupke et al. in 2002 [44]. The upconverter, consisting of Er " -doped NaYp4, was located on the rear side of a bifacial cell, thus leading to a response of the cell when it was excited at 1500 nm. From then on, significant improvement, including both theoretical analysis and experimental achievements, has been reported in the field of solar UC. According to the literatures, UC is predicted to enhance the efficiency of solar cells when mounted on the rear of the solar cell [45, 46]. 

The Ho " ion has a relatively wide absorption band in the 1150-1225-nm spectral range due to Ig —> % transition. The irradiation power density of sunlight in this spectral range is approximately 40 W/m, which is approximately twice more intense than that in the 1480-1580-nm range. Lahoz in 2008 reported the use of Ho singly doped oxyfluoride glass ceramics as promising upconverters for efficiency enhancement in c-Si solar cells [50]. Under sub-band gap excitation at 1170 nm, upconversion emissions in the visible (approximately 650 nm, Ig... 

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