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In order to improve further the economies of scale which are inherent in continuous roll-to-roll amorphous silicon alloy solar cell manufacturing processes, the authors have developed a concept for a serpentine web plasma CVD deposition process to maximize throughput while keeping the size of the deposition chambers small. When this technique is incorporated into a continuous roll-to-roll PV manufacturing process, it will maximize the throughput for a high volume production plant, reduce the machine cost, improve gas utilization, reduce power consumption, and improve the solar cell stability. To demonstrate the serpentine web deposition concept, the authors have constructed a single loop serpentine deposition chamber to deposit an a-Si for n-i-p structure solar cells. During the initial process of optimization, the authors have produced single-junction a-Si solar cells with an 8.6% efficiency, and triple-junction a-Si solar cells with a 9.5% initial efficiency, where the top cell intrinsic layer was deposited in the serpentine deposition chamber.

The engineering principles used to make >20-A-s/sup -1/ intrinsic layers of good-photovoltaic-quality amorphous silicon over large areas are addressed. Deposition equipment was designed and installed in a plasma-assisted CVD (chemical vapor deposition) roll-to-roll processor. Films were made in this machine with gas mixtures containing silane. Same-bandgap tandem solar cells using this material had 8.2% (average) to 8.6% (best cell) conversion efficiencies. >

Hydrogen $({\mathrm{H}}_{2})$ dilution of the source gas is known to be a key factor in producing hydrogenated amorphous silicon films that demonstrate a high degree of optoelectronic stability. In this work, we investigate, using Raman spectroscopy and high-resolution transmission electron microscopy (TEM), whether microstructural differences exist between such films and those made with no ${\mathrm{H}}_{2}$ dilution (i.e., that have greater instabilities). The key variable is the ${\mathrm{H}}_{2}$ dilution, which ranges from none to very high levels, producing amorphous and microcrystalline silicon films. The TEM results show that embedded within the amorphous matrix are chainlike objects (CLO's) having \ensuremath{\sim}3 nm widths, \ensuremath{\sim}30 nm lengths, and showing a high degree of order along their length. Such order implies vanishing levels of bond-angle distortion (BAD). These CLO's are present in all samples investigated, but their density increases with the level of ${\mathrm{H}}_{2}$ dilution. The Raman spectra show a TO band centered at $490 {\mathrm{cm}}^{\ensuremath{-}1}$ $(37\ifmmode\pm\else\textpm\fi{}3{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ full width). Quantitative analysis shows this band to exist in all samples investigated, but increases in magnitude with increasing ${\mathrm{H}}_{2}$ dilution. In the highest dilutions when microcrystallites are observed, the band is distinctly evident. Its position and width are also consistent with very low (crystallinelike) levels of BAD \ensuremath{\sim}0\ifmmode^\circ\else\textdegree\fi{}. It is thus likely the $490 {\mathrm{cm}}^{\ensuremath{-}1}$ Raman band is a signature of the intermediate ordered CLO's.

Under the PVMat 2A Program, Energy Conversion Devices, Inc. (ECD) has performed manufacturing technology development work utilizing its proprietary continuous roll-to-roll triple-junction a-Si alloy solar cell production line. Among the accomplishments achieved under this program, ECD demonstrated the production of the world’s first 4 ft2 PV modules utilizing triple-junction two-bandgap solar cells manufactured in a commercial, continuous roll-to-roll production line. These 4 ft2 modules had 9.5% initial efficiency and 8% stable module efficiency. ECD has recently designed and constructed a 5 MW continuous roll-to-roll a-Si solar cell processor for its U.S. joint venture, United Solar Systems Corp. (United Solar). The state-of-the-art processor incorporates major advances in solar cell design and manufacturing processes achieved by United Solar and ECD, with support from DOE/NREL. The advanced continuous roll-to-roll triple-junction a-Si module production technology will reduce module production costs, in...

Microcrystalline silicon (μc-Si) p-layers have been widely used in amorphous silicon (a-Si) solar cell research and manufacturing to achieve record high solar cell efficiency. In order to further improve the solar cell performance and achieve wider parameter windows for the process conditions, we studied the deposition of high quality μc-Si p-layer material using a very high frequency (VHF) plasma enhanced CVD process. A design of experiment (DOE) approach was used for the exploration and optimization of deposition parameters. The usage of DOE leads to a quick optimization of the deposition process within a short time frame. In addition, by using a modified VHF deposition process, we have improved the solar cell blue response which leads to a 6–10% improvement in the solar cell efficiency. Such an improvement is likely due to an improved microcrystalline formation in the p-layer.

Significant progress has been made recently at ECD (Energy Conversion Devices, Inc.), in developing new manufacturing technologies for a‐Si (amorphous silicon) photovoltaics. In a 2 MW continuous roll‐to‐roll a‐Si PV (photovoltaics) manufacturing plant, designed and constructed by ECD, we have advanced ECD’s PV technology and demonstrated the manufacturing of 4 ft2 PV modules with 9.5% initial and 8% stable efficiencies. In addition, high subcell yield up to 99.7% has been demonstrated in 600 m long (about 15 kWp) production runs with high uniformity and consistency. ECD has recently designed and constructed an additional pilot manufacturing machine to develop new technologies including high throughput serpentine web continuous roll‐to‐roll deposition of a‐Si alloy materials. When it is incorporated into a large‐scale production line, serpentine technology can maximize the throughput for a high volume production plant, reduce the machine cost, improve the gas utilization, reduce power consumption, and improve the material stability. Using a single loop serpentine chamber to deposit the top i layer, we have demonstrated triple‐junction triple‐bandgap solar cells with 95% initial efficiency.

Energy Conversion Devices, Inc. (ECD) has designed and constructed a 2 Megawatt (mW) manufacturing line that produces triple‐junction spectrum‐splitting a‐Si alloy solar cells in a continuous roll‐to‐roll process. This manufacturing line has reliably and consistently produced high efficiency solar cells. We have demonstrated the production of 4ft 2 triple‐junction two band‐gap a‐Si alloy PV production modules with 8% stable aperture area efficiency. The production line has successfully incorporated: 1) a band‐gap profiled a‐Si‐Ge narrow band‐gap solar cell deposited in a continuous roll‐to‐roll process using a proprietary gas distribution manifold and cathode configuration; and 2) a textured Ag/ZnO back‐reflector deposited in a continuous roll‐to‐roll sputtering machine with production subcell yields greater than 99%.

We report on evidence that fluorine, properly incorporated into a-Si, replaces weakly bonded hydrogen and improves the material stability under light soaking. Our fluorinated amorphous silicon (a-Si:H:F) is made by if glow discharge at high deposition temperatures up to 430 C from a gas mixture of SiH4 or Si2H6 and F2. These a-Si:H:F films show much lower density of states in the light soaking saturated state than device quality a-Si:H prepared in the same deposition system. It is evident from our results that fluorine incorporated into the network at such high deposition temperature makes for a new configuration which minimizes dangling bonds and other defects.

The US Department of Energy is probably the best known body to have claimed that photovoltaic conversion devices cannot compete with fossil tuels or uranium until at the earliest 1986, or more likely 2000. Such statements are based on the known current technological problems of three types of silicon—single crystal, polycrystalline, and amorphous. With amorphous silicon the chief difficulty has been the high density of states (as explained by Taylor on p 672) which restricts the necessary current flow.

Photovoltaic devices based on amorphous alloys of silicon with H and/or F have recently generated substantial interest in the photovoltaic community and the larger solid state physics and electronics communities as well. These developments have been reviewed recently by D. Carlson of RCA Labs(1). It was Carlson’s early collaboration with C. Wronski(2) which served to rekindle scientific and technological interest in a-Si:H produced by the glow discharge induced decomposition of SiH4 gas and the deposition of Si-rich fragments of this gas onto substrates held at 200–300°C(3–5).

There is a need for alternative energy sources. Photovoltaics are an attractive possibility, but their application has been limited by economic considerations in single-crystal materials, and for physical reasons such as grain boundaries in poly-crystalline materials. Amorphous semiconductors are especially attractive in this regard because they are basically much less expensive than their crystalline counterparts and because they possess a direct band gap with a high value for the optical absorption coefficient. We report here the development of a new alloy that eliminates the physical problems associated with the silicon-hydrogen alloys.

Amorphous Si:F:H with desirable properties for photovoltaic applications can be fabricated by the glow discharge of SiF4 and H2. The preparation conditions influence the properties of the resultant alloy. For instance, altering the ratio of SiF4 to H2 from 80 to 5 can alter the localized state density from 1019cm−3eV−1 to ≃ 1016cm−3eV-1, respectively. The conduction mechanisms are altered and there are vast changes in the photoconductivity as the density of recombination centers is decreased. The lower density of states achieved in the a-Si:F:H alloy reflects in the ease of doping. In addition, the lower density of states in a-Si:F:H alloy should result in a wider depletion region than reported for the a-Si:H alloy when fabricated within the device configuration. Results of C-V measurements using Au Schottky barrier devices confirm this.

Previously we have shown that amorphous silicon-based alloys containing F and H (1–7) possess desirable properties for photovoltaic application. Amorphous tetrahedral semiconductors ordinarily possess a very large density of states which act as traps leading to low values for drift mobility and low recombination lifetimes of free carriers. However, Spear and his group (8–10) reported that a-Si decomposed from SiH4 gas by r.f. glow discharge and deposited on a heated substrate produces a film which has a comparatively low density of states. However, materials prepared in this way possess a large concentration of H.(11) Because of the relatively low density of states, these types of films can be doped n type, albeit using relatively large concentration of the dopant. However, p-type doping using B2H6 is generally accompanied with the decrease in the band gap, indicating alloying action rather than conventional doping.(9) Furthermore, the material has apparently a sufficiently large carrier lifetime that efficiencies of photovoltaic devices exceeding 5% have been reported.(12)

THE AMORPHOUS-CRYSTALLINE transition in a-Si and a-Ge has been studied for many years [1,2]. However, unlike electrical and optical properties, no substantial modifications on the RDF (radial distribution function) for the amorphous phases have been reported for different methods of preparation or annealing conditions short of crystallization. More recently, Barna et al. [3] have found a small shift of the second peak in the RDF towards smaller r, and a more pronounced peak at 5 A for the a-Si: H prepared by glow discharge technique. These authors concluded that a higher degree of local order is present in the glow discharge a-Si: H although they stated that there exist only minor differences in amorphous silicon samples with different preparations. Earlier Raman measurements [4] have not been shown to be sensitive to preparation conditions, however, recent results indicated that Raman measurements can indeed differentiate various types of amorphous silicon samples. Tsu et al.

A roll-to-roll plasma deposition machine for depositing multilayered amorphous alloys has been developed. The plasma deposition machine has multiple deposition areas and processes a stainless steel substrate 16 in wide continuously. Amorphous photovoltaic thin films (less than 1 μm thick) with a six-layer structure (p-i-n-p-i-n) are deposited continuously in a single pass onto a roll of stainless steel substrate 16 in wide and 1000 ft long. Mass production of low cost tandem solar cells utilizing roll-to-roll processes is now possible. A commercial plant utilizing this plasma deposition machine for manufacturing tandem amorphous silicon alloy solar cells is now in operation.

As glow discharge amorphous silicon alloys become more important for commercial production of photovoltaic, xerographic, optical sensing, and microelectronic devices, economic considerations have motivated considerable research aimed at increasing deposition rate and gas utilization efficiency. Recently, a low pressure microwave glow discharge process was reported (1) which produces good quality a-Si:F:H films, with essentially 100% gas utilization at deposition rates > 100A/sec.

We have developed an amorphous silicon alloy based solar cell with a novel structure in which the optical gap of the intrinsic layer changes in a substantial portion of the bulk. Computer simulation studies show that for a given short circuit current, it is possible with this structure to obtain higher open circuit voltage and fill factor than in a conventional cell design. Experimental cell structures have been made and confirm the theoretical prediction. The new cell design shows a considerable improvement in efficiency. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination.

A process is described which provides a simple, automatic means for quickly reducing the deleterious effects of shunts in completed thin‐film solar cells. The method has been used to passivate current shunting defects in large area, mass produced solar cells made from amorphous silicon alloys. In the passivation process, photovoltaic devices are exposed to an electrochemical bath which selectively converts the top surface conducting oxide to a lesser conducting state at the vicinity of shunts. The process and its utility in repairing defects will be described.

The development of a photovoltaic power generating module is presented. The module is a tandem cell solar generator made using roll-to-roll amorphous silicon alloy thin-film deposition technology. This product is a large-area (1 ft*4 ft) single solar cell that eliminates the distinction between a cell and a module. Device series connections occur externally at the module level, resulting in substantial reductions in fabrication costs. >

We have used molecular-beam deposition (MBD) with a base pressure of 10–10 torr to prepare a-Si and a-Ge at relatively low substrate temperatures. The films contain voids which can adsorb gases such as H2, F2, SiF4, GeF4, O2, etc. Due to the high reactivity of the dangling bonds present, these molecules are broken down to their atomic constituents, thereby passivating the dangling bonds. Further annealing is required to form proper chemisorption and drive the passivating agents into smaller or isolated voids. Typically a decrease of dark conductivity by as much as 8 orders of magnitude is observed accompanied by the appearance of few decades of photocurrent above the dark current under (Air Mass 1) AM1. For a-Ge, using a similar procedure, fluorine is more effective as a passivating agent than hydrogen.

A roll-to-roll plasma deposition machine for depositing multi-layered amorphous alloys has been developed. The plasma deposition machine (approximately 35 ft. long) has multiple deposition areas and processes 16-inch wide stainless steel substrate continuously. Amorphous photovoltaic thin films (less than 1pm) having a six layered structure (PINPIN) are deposited on a roll of 16-inch wide 1000 ft. long stainless steel substrate, continu-ously, in a single pass. Mass production of low-cost tandem amorphous solar cells utilizing roll-to-roll processes is now possible. A commercial plant utilizing this plasma deposition machine for manufacturing tandem amorphous silicon alloy solar cells is now in operation. At Energy Conversion Devices, Inc. (ECD), one of the major tasks of the photovoltaic group has been the scale-up of the plasma deposition process for the production of amorphous silicon alloy solar cells. Our object has been to develop the most cost effective way of producing amorphous silicon alloy solar cells having the highest efficiency. The amorphous silicon alloy solar cell which we produce has the following layer structure: 1. Thin steel substrate. 2. Multi-layered photovoltaic amorphous silicon alloy layers (approximately 1pm thick; tandem cells have six layers). 3. ITO. 4. Grid pattern. 5. Encapsulant. The deposition of the amorphous layer is technologically the key process. It was clear to us from the beginning of this scale-up program that amorphous silicon alloy solar cells produced in wide width, continuous roll-to-roll production process would be ultimate lowest cost solar cells according to the following reasons. First of all, the material cost of our solar cells is low because: (1) the total thickness of active material is less than 1pm, and the material usage is very small; (2) silicon, fluorine, hydrogen, and other materials used in the device are abundant and low cost; (3) thin, low-cost substrate is used; and (4) product yield is high. In addition, the development of high efficiency cells in future time will further reduce the material cost. Secondly, the labor cost associated with the production of our solar cells is low because our process utilizes simple, high production rate, highly automated processing for the complete fabrication of photovoltaic modules. Specifically, six layers of tandem amorphous silicon alloy solar cell are plasma-deposited on a roll of wide stainless steel substrate, continuously in a single pass. Over one order of magnitude increase in the line speed is straightforward from an engineering point of view. Other downstream process steps for the fabrication of photovoltaic modules also utilize simple, high production rate, highly automated machineries.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A manufacturing plant for producing thin-film amorphous silicon alloy based solar cells using roll-to-toll deposition is being operated by Sovonics Solar Systems in Troy, Michigan, USA. The authors detail the performance of solar cells produced at this plant. Roll-to-roll deposition on stainless steel has been shown to be a viable method of producing high-quality thin-film solar cell materials. Various thin-film depositions are made on 1000-foot-long rolls of 14-inch-wide, thin stainless steel in the deposition facility. The rolls are then cut into appropriate sizes for module fabrication. Quality analysis, in situ deposition monitoring, and post deposition measurements have enabled a detailed analysis of production run data, including such parameters as layer thickness, adhesion, conversion efficiency, yield, etc. These and other properties of the photovoltaic materials and devices have been monitored and studied in order to optimize the roll-to-roll deposition process. >

The authors have developed an amorphous silicon alloy-based solar cell with a novel structure. Computer simulation studies show that for a given short-circuit current, it is possible to obtain a higher open-circuit voltage and fill factor than in a conventional cell design. For a nominal 1.5 eV a-SiGe alloy, the fill factor under red illumination can be improved from 0.55 to 0.64 for the same short-circuit current. Experimental cell structures confirm the theoretical prediction. The novel cell design shows a considerable improvement in efficiency. Dynamic internal collection efficiency measurements show reduced recombination in these cells, which gives rise to the observed higher fill factors. Incorporation of this structure in the bottom cell of a triple device has resulted in the achievement of 13.7% efficiency under global AM1.5 illumination. >

The solid-state revolution, which began in 1947 with the invention of the transistor, was made possible by the ability to make crystalline materials (at that time germanium) sufficiently free of defects that substitutional dopants could overcome the background noise of other defects and control the electronic transport properties of the semiconductor. Since the early 1930’s Bloch, Wilson, and others had laid a sufficient theoretical groundwork in semiconductors so that transistor action could be predicted, demonstrated, and understood even though the point contact transistor had many mysteries associated with it. Figure 1 shows that historical lever that moved the world, the first transistor.