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Amorphous-crystalline silicon (a-Si:H/c-Si) heterojunctions have recently drawn much attention owing to their low-temperature fabrication and high-efficiency photovoltaics. a-Si:H/c-Si heterojunctions were studied for the first time using the constant photocurrent method (CPM). The doping concentration in the p-type a-Si:H was varied. CPM derived absorption for energies greater than 1.4 eV is observed to increase with decreasing dopant concentration in the p- layer. This is attributed to a decrease in the density of defect states in the amorphous layer and the interface. A model is proposed wherein the amorphous layer and the interface constitute one absorbing layer while the crystalline substrate forms the other absorbing layer. A combined defect density in the amorphous layer and interface of 2.8x1018 cm-3 eV-1 at 0.4 eV from the valence band edge was measured for our best device. By comparing the combined defect density with that of a single amorphous layer the defect density at the interface is inferred to be 5x1012 cm -2.
Solar cells based on monocrystalline silicon (c-Si) can potentially achieve high sunlight energy conversion efficiencies and thus could reach grid parity despite the high cost of c-Si. The efficiency of standard c-Si solar cells featuring diffused emitters and aluminum back surface fields (BSF) is limited by interface recombination. Alternatively the growth of intrinsic/doped amorphous silicon (a- Si: H) layer stacks on c-Si effectively passivates the c-Si surface and simultaneously forms the emitter and BSF. Such Si heterojunction (HJ) solar cells can use thin c-Si wafers, benefit from low production cost of a-Si: H layers and enable the highest efficiencies. The focus of this work is the study of interfaces in a-Si: H/c-Si heterostructures, particularly the electronic quality of the a-Si: H/c-Si heterointerface and its effect on the subsequent a- Si: H/c-Si HJ solar cell fabrication. Interface recombination modeling by considering the amphoteric nature of Si dangling bonds is in excellent agreement with measurements, and provides insight into the microscopic passivation mechan
The main focus of the present work is related to the optimization of heterojunction solar cells. The key roles in obtaining high efficient heterojunction solar cells are mainly the plasma enhanced chemical vapor deposition of very low defect layers, and the sufficient surface passivation of all interfaces. In heterojunction solar cells, the a-Si: H/c-Si hetero-interface is of significant importance, since the hetero-interface characteristics directly affect the junction properties and thus solar cell efficiency. In this work, the deposition and film properties of various hydrogenated amorphous silicon alloys, such as a-SiC: H, a-SiO_x: H, and muc-Si: H (standard a-Si: H is used as reference), are employed. Special attention is paid to (i) the front and back surface passivation of the bulk material by high-quality wide-gap amorphous silicon suboxides (a-SiO_x: H), and (ii) the influence of wide-gap high-quality a-Si- and muc-Si-based alloys for use as emitter and back-surface-
This book gives the first systematic and complete survey of technology and application of amorphous silicon, a material with a huge potential in electronic applications. The book features contributions by world-wide leading researchers in this field.
The world of today must face up to two contradictory energy problems: on the one hand, there is the sharply growing consumer demand in countries such as China and India. On the other hand, natural resources are dwindling. Moreover, many of those countries which still possess substantial gas and oil supplies are politically unstable. As a result, renewable natural energy sources have received great attention. Among these, solar-cell technology is one of the most promising candidates. However, there still remains the problem of the manufacturing costs of such cells. Many attempts have been made to reduce the production costs of “conventional” solar cells (manufactured from monocrystalline silicon using diffusion methods) by instead using cheaper grades of silicon, and simpler pn-junction fabrication. That is the ‘hero’ of this book; the heterojunction solar cell.
Amorphous Silicon Kazunobu Tanaka National Institute for Advanced Interdisciplinary Research, Ibaraki, Japan Eiichi Maruyama Hitachi Ltd, Ibaraki, Japan Toshikazu Shimada Hitachi Ltd, Ibaraki, Japan Hiroaki Okamoto Osaka University, Osaka, Japan Translated by Takeshi Sato, National Institute for Advanced Interdisciplinary Research, Ibaraki, Japan Amorphous silicon has substantially different properties as compared to crystalline silicon. It has therefore become recognized as a fascinating and important material in its own right, with many interesting facets that lead to a range of novel and still developing applications. Amorphous Silicon introduces the reader to this field by first discussing what is meant by the amorphous state. It details the way in which amorphous silicon is prepared, and the growth mechanism. The main structural, optical and electronic properties are then covered in detail, and there is a full chapter on the structural stability of the material, including photoinduced effects. Finally, a number of the most exciting applications of amorphous silicon are presented, including its use in solar cells, photo-sensors and liquid crystal displays. Amorphous Silicon will be of great interest to all those working in solid state physics or chemistry, materials science and electronic engineering, from postgraduate students to more experienced workers in these fields.
The amorphous - crystalline silicon heterojunction (SHJ) represents a new paradigm in crystalline silicon (c-Si) photovoltaics (PV). To achieve the 27% efficiency target for SHJ PV, defects in the silicon heterointerface must be minimized by growing high-quality hydrogenated amorphous silicon (a-Si:H) onto the c-Si surfaces without deposition-related damage. Typically, a-Si:H is deposited using radio-frequency (RF) plasma enhanced chemical vapour deposition (PECVD), which in its conventional configuration directly exposes the c-Si growth surface to the ignited plasma. In this thesis, silicon heterostructures prepared by the grid-based triode RF PECVD method is investigated for the first time. The triode method allows for high-quality a-Si:H growth with the c-Si surfaces shielded from any potential plasma damage. Using a custom-built configurable PECVD facility, a systematic study was conducted and it was demonstrated that the triode method affords the preparation of a-Si:H with excellent bulk film quality and state-of-the-art passivation for c-Si surfaces. Using the triode method, an effective minority carrier lifetime (
Coverage: preparation and material quality; structural and vibrational properties; electronic structure; electronic transport recombination of excess carriers; Schottky junctions; multilayers; optical constants; mechanical and thermal properties; TFTs; solar cells; photodetectors; large area displays; LEDs; xerographic applications.