Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.

Photoinduced effects and metastability in amorphous semiconductors and insulators

Density of the gap states in undoped and doped glow discharge a-Si:H

Optical characterization methods were applied to a series of microcrystalline silicon thin films and solar cells deposited by the very high frequency glow discharge technique. Bulk and surface light scattering effects were analyzed. A detailed theory for evaluation of the optical absorption coefficient α from transmittance, reflectance and absorptance (with the help of constant photocurrent method) measurements in a broad spectral region is presented for the case of surface and bulk light scattering. The spectral dependence of α is interpreted in terms of defect density, disorder, crystalline/amorphous fraction and material morphology. The enhanced light absorption in microcrystalline silicon films and solar cells is mainly due to a longer optical path as the result of an efficient diffuse light scattering at the textured film surface. This light scattering effect is a key characteristic of efficient thin-film-silicon solar cells.

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Optical absorption and light scattering in microcrystalline silicon thin films and solar cells

This chapter covers the current use and challenges of thin-film silicon solar cells, including conductivities and doping, the properties of microcrystalline silicon (the role of the internal electric field, shunts, series resistance problems, light trapping), tandem and multijunction solar cells (a-Si:H/a-Si:H tandems, triple-junction amorphous cells, microcrystalline–amorphous or “micromorph” tandems), module production (deposition of thin-film layers, substrate materials, laser scribing, cell interconnection, and module encapsulation), and module performance.

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Thin-Film Silicon Solar Cells

Defect states in a representative amorphous oxide semiconductor, a-InGaZnO4, were studied by optical analyses and first-principle calculations. The optical analyses suggested that the as-deposited a-IGZO film have weak subgap absorptions around 0.6 and 2 eV. Local density approximation calculations showed that an oxygen defect works as an electron trap as well as a shallow donor depending on its local structure. It suggests that a large vacancy space remained in an oxygen deficient structure forms a deep levels in the band gap and traps electrons, while if such a large space is annihilated e.g. by a film growth process and post thermal annealing, oxygen deficiency may form a shallow donor level. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure of oxygen deficient amorphous oxide semiconductor a-InGaZnO4–x : Optical analyses and first-principle calculations

Direct measurement of the gap states and band tail absorption by constant photocurrent method in amorphous silicon

The density of states in amorphous hydrogenated silicon is obtained by deconvolution of the optical absorption coefficient a in the full sub band-gap region. The spectral dependence of the optical absorption coefficient is measured by a constant photocurrent method (CPM) on forward-biased Schottky diodes. The spectral dependence of reflection on the semitransparent Pt electrode is determined. All assumptions used in the deconvolution of a are discussed. The difference in measured value of a (by CPM) in the region below approximately 0.9 eV compared to the measurement of α by photothermal deflection spectroscopy (PDS) is explained by an absorption process seen in PDS and not seen in CPM. This absorption is supposed to be a transition from a deep localized state to a deep localized gap state (two different charge states of the same defect).



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Gap states density in a-Si:H deduced from subgap optical absorption measurement on Schottky solar cells

Measurement of the spectral dependence of photoconductivity and quantum efficiency at liquid helium temperature is presented. The mobility gap value was found to be quite close to the Tauc optical gap value for undoped good quality material.

Determination of the mobility gap in amorphous silicon from a low temperature photoconductivity measurement

Influence of applied stress, impurities and doping on Staebler-Wronski effect (SWE) has been investigated, compared with the results of other authors and possible explanation of discrepancies is presented. Our model points to the importance of complexes of Si dangling bonds with impurities or dopant atoms in addition to breaking of weak SiSi bonds by light soaking.

J. Stuchlík

Papers

Density of the gap states in undoped and doped glow discharge a-Si:H

Direct measurement of the gap states and band tail absorption by constant photocurrent method in amorphous silicon

Gap states density in a-Si:H deduced from subgap optical absorption measurement on Schottky solar cells

Determination of the mobility gap in amorphous silicon from a low temperature photoconductivity measurement

Deep states in a-Si:H- changes with light soaking and applied stress