Rather, the atoms form a continuous random network. A wide variety of gas mixtures has been explored to improve material quality, and dilution of silane with hydrogen has been the most successful. a-Si differs from monocrystalline or polycrystalline silicon in that instead of forming a four-bonded tetrahedral structure, the atoms form a continuous random network with numerous unconnected dangling bonds. 6). a-SiGe alloys have met with more success and are used in both the middle and the bottom cells. a-Si:H is a very intriguing material. We should mention that high-quality material showing improved order has also been obtained using deposition conditions that form silicon clusters in the plasma (Roca i Cabarrocas 1998). The use of stacked multijunction cells based on a-Si:H and a-SiGe:H has led to record amorphous silicon-based cells. Recent reports suggest production of a-Si:H PV will increase to 5000 MW in 2012 from the current level of about 200 MW in 2007 (PV News, August 2008). Courtesy of E. Fortunato et al. Amorphous silicon-based TFTs are by far the most common, due to their lower production cost, whereas polycrystalline silicon TFTs are more costly and much more difficult to produce. However, a-Si passivated by hydrogen, where hydrogen atoms bond with the dangling bonds to produce hydrogenated, Flexible photovoltaic cells embedded into textile structures, Amorphous and Nanocrystalline Silicon Solar Cells and Modules, Materials Science of Thin Films (Second Edition), Advances in Plasma-Grown Hydrogenated Films, Encyclopedia of Materials: Science and Technology, Photovoltaics: Advanced Inorganic Materials, Encyclopedia of Physical Science and Technology (Third Edition), Handbook of Silicon Based MEMS Materials and Technologies (Second Edition). So far, we have discussed the optimization methods for a-Si alloys only. Order also improves as the thickness increases and the quality of the material becomes more inhomogeneous in the direction of growth. Hydrogenated amorphous silicon, a-Si:H, was first fabricated in 1969 using a silane gas (SiH4) precursor (Chittick et al., 1969). 1998) where, for a given dilution, the material changes from amorphous to microcrystalline at a certain thickness. hydrogenated amorphous Si ~c-Si and a-Si!11–13 are also shown for comparison. In amorphous silicon (a-Si) almost every Si atom is tetrahedrally bonded to four nearest neighbor Si atoms—just as in crystalline Si (c-Si). Recombination centers reduce the carrier lifetimes, while in the presence of an electric field, the generation centers produce an excess of electron–hole pairs that contribute to leakage current and to noise. Because of the short-range order, material properties of amorphous semiconductors are similar to their crystalline counterparts. The CRN may contain defects, but the crystalline concepts of interstitials or vacancies are not valid here. Silicon is a fourfold coordinated atom that is normally tetrahedrally bonded to four neighboring silicon atoms. Material properties that affect solar cell performance are briefly discussed; for detailed description of the physics of the materials, the readers are referred to other treatises (Street, 1991; Fritzsche, 1988). In the ideal CRN model for amorphous silicon, each atom is fourfold coordinated, with bond lengths similar (within 1% [50]) to that in the crystal. This substitutional doping mechanism was described by Street [52], thereby resolving the apparent discrepancy with the so-called 8 – N rule, with N the number of valence electrons, as originally proposed by Mott [53]. The dangling bonds in a-Si cause anomalous electrical behavior, poor photoconductivity, and prevents doping, which is critical to producing semiconductor properties (Collins et al., 2003). The disorder inherent in the material allows it to absorb light efficiently. In amorphous silicon this long range order is not present. Amorphous silicon has temperature coefficient of resistances (TCRs) from 1.8%/K to 5.5%/K, depending on the growth techniques. However, a-Si passivated by hydrogen, where hydrogen atoms bond with the dangling bonds to produce hydrogenated amorphous silicon (a-Si:H), has better performance when used in PV applications. Furthermore, these materials contain several percent of hydrogen, which assists in neutralizing or passivating grain boundaries. Examples include small high-resolution displays such as those found in projectors or viewfinders. The μc-Si:H layer consists of an amorphous tissue with coherent regions of crystalline grains having sizes from nanometer scale to micrometer scale. Amorphous silicon is often viewed as a continuous random network (CRN) [48, 49]. The average bond-angle variation ∆Θ reflects the degree of structural disorder in the random network. Below 4 nm, the bandgap increases monotonically to greater than 2 eV. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780128035818103558, URL: https://www.sciencedirect.com/science/article/pii/B9780081005743000187, URL: https://www.sciencedirect.com/science/article/pii/B9780128035818008353, URL: https://www.sciencedirect.com/science/article/pii/B9780125249751500094, URL: https://www.sciencedirect.com/science/article/pii/S1079405002800047, URL: https://www.sciencedirect.com/science/article/pii/B0080431526000565, URL: https://www.sciencedirect.com/science/article/pii/B9780080431529021965, URL: https://www.sciencedirect.com/science/article/pii/B012227410500689X, URL: https://www.sciencedirect.com/science/article/pii/B9780323299657000063, Handbook of Infra-red Detection Technologies, 2002, Inorganic Thin Film Materials for Solar Cell Applications, Reference Module in Materials Science and Materials Engineering. Physically, these dangling bonds represent defects in the continuous random network and may cause anomalous electrical behavior. Despite these limitations, single- and multijunction solar cells and large-area modules based on a-Si:H and related alloys, e.g., a-Si1–xGex:H, are now well established in the market. The design of the PECVD system has great impact on the production cost of such panel, therefore most equipment suppliers put their focus on the design of PECVD for higher throughput, that leads to lower manufacturing cost[7] particularly when the silane is recycled.[8]. Apart from the triple-bandgap a-Si/a-SiGe/μc-Si:H approach, the a-Si/μc-Si/μc-Si structure is also under investigation. The material has an improved order, as confirmed by Raman, transmission electron microscopy, and x-ray diffraction studies. The plasma contains many different ions and neutral radicals generated by electron–molecule collisions which, in turn, undergo various secondary reactions as they move to the substrate. Hydrogen is an etchant, and may be responsible for elimination of weak bonds at the growing surface. One can represent the disorder by the atom pair distribution function, which is the probability of finding an atom at a distance r from another atom. Exposure to light increases the defect density in the material and decreases the cell efficiency.