EPVs Glass PV Panels Mean A Clear Future For Solar
by Lee H. Goldberg & Dave Bell with Photography by Dave Bell

A short SUV ride from the mall-infested lanes of New Jersey's infamous Route 1, there's a small factory where the future is being built at the rate of 2 kW/hr per hour. Huddled on one of the few remaining rural roads in Lawrenceville, the modest building that houses Energy Photovoltaics Inc. (EPV) looks much like any other industrial building that dots the New Jersey landscape in these parts -- until you see the racks of solar panels. Hung on wood and metal racks next to the plant's parking lot like so much laundry in the breeze, an assortment of window-sized sandwiches of ordinary plate glass turn as much as 8% of the afternoon sun into electricity.

While one or two of the larger racks actually offset the building's electrical demand from the grid, most of the glass panels are undergoing life testing to better understand how their output changes under constant exposure to "real world" conditions over their anticipated 30+ year lifespan. Dolores Philips, Director of Government Relations at EPV, explains that making sure that these low-cost panels age as gracefully as their traditional silicon-based cousins will be a key to gaining market share for the thin-film technology that could cut the cost of solar-generated electricity by a third or more. The low cost, simple construction, and versatile form factors offered by these thin-film glass plates make them equally at home driving multi-acre solar farms or doing double duty as windows and skylights that produce electricity for the buildings they shelter.

After nearly 20 years of labor (some of which was done when the company was known as Chronar and APS), EPV's amorphous thin-film-on-glass technology seems to have matured to the point where it is ready to challenge the more efficient, but more expensive silicon wafer technologies that currently dominate the industry in several key applications. While conventional solar panels are made by attaching individual solar cells from silicon wafers onto panels and wiring them together, the thin-film process creates entire arrays of cells with a single series of steps on sheets of low-cost substrate, such as glass.

EPV's thin film process uses standard vapor deposition techniques to build up photodiodes from several micron-thin layers of amorphous silicon. Just like their wafer-based cousins, each PV junction produces around 1.7 V, with the current output depending on its area. The junctions are built up on transparent film of tin oxide on the glass plate that both acts as a conductor to tie front side of the individual PV cells together and boosts the pane's light absorption in the red spectrum, adding 7% to its overall efficiency.

A tour of the compact Lawrenceville pilot plant reveals that most of the production steps are straightforward and can be done in the relatively "dirty" environment of a normal manufacturing facility rather than the clean-room conditions needed to make the silicon wafers used in conventional panels. The factory uses a relatively simple 10-step process for turning ordinary 2 ft x 4 ft plate glass panes into electron pumps. The recipe is as follows:

• Glass Preparation: A sheet of tin oxide-coated plate glass and a sheet of untreated glass (the cover glass) are seamed and washed, then a hole is drilled in the cover glass for the electrical leads.
• Tin Oxide Patterning: A laser scribes the tin oxide backside interconnect layer.
• a-Si Deposition: Six layers of thin-film amorphous silicon are vapor-deposited on the plate glass to form the PV junctions.
• a-Si Patterning: A laser is used to scribe the silicon layers.
• Aluminum Deposition: The aluminum rear electrode is deposited using a continuous vacuum sputtering process.
• Aluminum Patterning: A laser scribes the aluminum layer to form the cells' other interconnect traces.
• Plate Testing: Panels are sampled and light-tested for quality control.
• Heat Aging: Panels are sampled and heat-aged for quality control.
• Encapsulation: The active thin-film on the edge of the plate is removed, aluminum foil strips are bonded to the aluminum layer, a layer of ethyl vinyl acetate (EVA) and the cover glass are added to the plate glass for protection, and electrical connections and mounting brackets are installed.
• Module Testing: The finished modules' outputs and spectral response are checked with a flash illumination tester.
  
  This facility is a relatively small operation, intended as much to be a test bed for refining the processes it licenses to larger factories as it is an actual manufacturing center.

Consequently, manual labor is currently used instead of automated handling equipment to move batches of 40 panels at a time through the production line. The good news is that most operations, other than the actual vapor deposition (a multi-hour batch process), can be done in a continuous flow, and easily scaled up as necessary: something that's been done in plants in China and several other countries where the system has been licensed.

But even with the modest scale and limited automation that's been implemented in the Lawrenceville plant, the economics of thin-film manufacturing are apparent. The fabrication process uses less expensive materials and less energy than crystalline silicon processes, and the large monolithic arrays require much less labor to assemble into solar modules. This, along with recent improvements in the percentage of sunlight that EPV's amorphous cells turn into electricity, has made them a low-cost alternative to traditional solar panels for many applications.

Boosting the efficiency of amorphous cells while maintaining low production costs has been a big part of EPV's long development effort. Over the years, they've seen the conversion rates of their amorphous cells climb from 3% to between 6% - 8% in their current products. While this does not seem very impressive in comparison to the 12% - 20% typical for conventional cells, it's important to note that amorphous cells do a much better job of converting the lower light levels prevalent on overcast days and in the early mornings and late afternoons when the sun is low in the sky. This allows the EPV's panels to be generating at reduced power levels during many hours per month that conventional units would not be running at all. EPV's Dolores Philips explained, "Amorphous silicon modules generate more electricity per unit of installed capacity than do crystalline silicon modules, leading to lower electricity generating costs and superior economics for many applications."

Even with their lower efficiencies, the amorphous panel's lower materials costs and simpler assembly process yields a much lower cost-per-raw watt than competing products. With most conventional solar panel makers struggling to approach a $3.50/W wholesale price point, EPV can comfortably sell its products at under $2/W.

Of course, nothing this good comes without a downside, and these low cost panels do pay the price of being 2 - 3x the size of a crystalline panel with equivalent output. This can be a distinct disadvantage in space-constrained applications, such as a building rooftop. Requiring more panels per kWh also partially offsets the panel's cost advantages in large "solar farm" generating facilities because of the additional installation labor and mounting hardware required. But despite these hidden costs, amorphous solar panels usually still manage to deliver a clear cost advantage over other commercially-available solutions.

Thin-film products also enjoy several other less obvious advantages, including better temperature vs output coefficients that allow them to provide a greater percentage of their rated power production during the summer months when crystalline cells outputs tend to sag. In addition, their one-piece construction avoids most of the potential for failures caused by inter-cell contact failure in conventional panels. EPV also argues that their thin-film modules don't suffer from many of the feedstock availability issues that the recent spike in worldwide solar panel demand is causing.

While EPV's a-Si modules are not the ideal solution for all PV applications, their ability to act as a structural element in a building allows them to find their way into new and exciting places that would be unheard of for a traditional solar panel. Their dark color makes them especially appealing for integrated building panels. Since they are frameless, and don't have to be individually grounded, the panels can be assembled into large awnings, canopies, and other architectural features. The panel's photolithographic manufacturing process also allows a percentage of the glass to be left bare to let in 20% - 60% of the incoming sun. These partially-clear panels can do double duty in office windows, skylights, and atriums, generating power while providing controlled light to large open spaces.

As revolutionary as EPV's technology is, the little factory's production capacity of a few MW a year will not seriously impact the world energy market by itself. That's why much of their efforts have gone into making their process portable, and easy to license. The company can put together a complete integrated manufacturing system consisting of its proprietary manufacturing technology, specially designed equipment, installation, commissioning, and training. With EPV's system and about 26,000 square feet of space, you can start producing modules. Producing the panels close to where they're being used also dramatically reduces transportation costs, both for the raw materials, and finished products.

Thanks to their flexible business model, EPV's technology is spreading like wildfire across the planet. The company has already been involved in designing and building 14 thin-film PV manufacturing facilities around the world, including an independently-owned factory in Tianjin, PRC. EPV's joint venture company is building a manufacturing facility in northern Greece that will produce 5 MW per year of a-Si modules and also have the capability of converting up to 1.25 MW per year into building-integrated PV panels.

Many factors seem to favor amorphous technologies winning a substantial niche in the photovoltaic market. Because the process couples a relatively simple manufacturing process to the mature manufacturing infrastructure of the plate glass industry, starting an a-Si PV plant requires a fraction of the capital required for a conventional solar cell fab. Rising energy prices should also favor a-Si technology, since the panels take only 1 - 2 years to pay back the energy used to manufacture them (including the glass substrate), vs 3 - 4 years for traditional silicon cells. Developments in the works could eventually boost amorphous panels' efficiency to 10%, or more, making it increasingly likely that the shiny black panels will someday be a common sight on the roofs of homes across the world.

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