K. Hoffman

Amorphous Silicon Alloy Photovoltaic Technology - from R&D to Production

The key requirements for photovoltaic modules to be accepted for large-scale terrestrial applications are (i) low material cost, (ii) high efficiency with good stability, (iii) low manufacturing cost with good yield and (iv) environmental safety. Thin films of amorphous silicon alloy are inexpensive; the products are also environmentally benign. The challenge has been to improve the stable efficiency of these modules and transfer the R&D results into production. Using a Multijunction, Multi-bandgap approach to capture the solar spectrum more efficiently, we have developed one-square-foot modules with initial efficiency of 11.8%. After 1000 h of one-sun light soaking, a stable efficiency of 10.2% was obtained. Both the efficiency values were confirmed by National Renewable Energy Laboratory. The technology has been transferred to production using an automated roll-to-roll process in which different layers of the cell structure are deposited in a continuous manner onto stainless steel rolls, 14” wide and half a mile long. The rolls are next processed into modules of different sizes. This inexpensive manufacturing process produces high efficiency modules with subcell yields greater than 99%. The key features of the technology transfer and future scope for improvement are discussed.

Triple-junction amorphous silicon alloy PV manufacturing plant of 5 MW annual capacity

A spectral-splitting, triple-junction a-Si alloy solar cell processor has been designed, built and optimized. A roll-to-roll process has been used to deposit two layers of back reflector, a triple-cell structure with nine layers of a-Si and a-SiGe alloys and a single layer of antireflection coating consecutively on a half-a-mile roll of stainless steel. The coated web is next slabbed and processed to make a variety of products. The design of the machine and processes used incorporate several key features developed for improving cell efficiency. In order to reduce manufacturing cost, higher deposition rates and thinner cells than are used in R&D have been used. The back reflector also consists of Al/ZnO rather than Ag/ZnO. Large-scale production has begun, and products are being shipped for a wide range of applications.

Lightweight flexible rooftop PV module

Energy Conversion Devices, Inc. (ECD) and United Solar Systems Corp. (United Solar) are developing lightweight, flexible photovoltaic modules that can replace conventional roofing materials and be economically and aesthetically integrated into residential and commercial buildings. The modules will be fabricated from high-efficiency multi-junction a-Si alloy solar cells developed by ECD and United Solar. These cells are produced on thin, flexible, stainless steel substrates. Two types of products, 1 ft by 10 ft overlapping PV shingles and 1.3 ft by 20 ft PV roof panels are being developed by United Solar and ECD, respectively. United Solars shingle type design uses a roof mounting procedures similar to those used with conventional asphalt shingles, while ECDs PV panel uses mounting procedures conforming to metal roof systems. Thus, they can be installed on roof sheathings, replacing ordinary shingles or metal roofing panels, on a standard wood roof construction.

VHF Plasma Deposition of μc-Si p-Layer Materials

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.

Back reflector texture and stability issues in high efficiency multijunction amorphous silicon alloy solar cells

The role of the back reflector texture on the initial and stabilized efficiency of high efficiency triple-junction triple-bandgap amorphous silicon alloy based cells has been investigated. The devices have been deposited on Ag/ZnO back reflector possessing three different textures. The performance of the bottom single-junction a-SiGe alloy n-i-p cell on the three textures has also been analyzed. The value of the short-circuit current density of the n-i-p cell initially increases and then decreases with increasing texture. Light soaking results show that the higher textures exhibit superior device stability. Degradation as low as 8% has been obtained on the triple-junction cells.

Yield and performance of amorphous silicon based solar cells using roll-to-roll deposition

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.<<ETX>>

A new, inexpensive, thin film photovoltaic power module

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.<<ETX>>

Production of 20 A sec/sup -1/ a-Si alloys for use in solar cells

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.<<ETX>>

A PV module that emulates an asphalt shingle

The authors have developed a shingle roofing module designed to emulate the conventional asphalt shingle in form, structural function and installation. The PV shingle module consists of a series of interconnected, coated stainless steel tabs laminated together in EVA/Tefzel polymers. The PV shingle design allows the mechanical and electrical installation to be performed independently, thereby minimizing coordination between the roofing and electrical tradesman. The installation procedure is so similar to conventional asphalt shingles that an experienced roofing contractor, with minimal training, can install the modules. The authors show results of testing that demonstrate that the PV shingle serves the dual function of electrical generator as well as a roofing material. Finally, they demonstrate the feasibility of the PV shingle by describing a 1.8 kW AC system installed on the Southface Energy Institutes Energy and Environmental Resource Center House in Atlanta, Georgia, USA.

Accelerated environmental testing of amorphous silicon PV modules

A roll-to-roll deposition technology for amorphous silicon-based alloys used in photovoltaic (PV) materials is discussed. There is a need to establish design test procedures for flexible panels. All the test procedures were used in the evolution of the product lines from design inception to mass production. During the design development stage, concepts of product design were formulated, materials chosen, and sample modules were produced. In this case, most of the construction materials had been evaluated and used prior to this work. The tests to be used for product evaluation were formulated at this time. Results were fed back to pertinent design parameters and new products were fabricated, then retested until all performance specifications were successfully met.<<ETX>>