Device Tosses Out Unusable PV Wafers

Manufacturers need better, less expensive ways to make photovoltaics, and now a solar furnace that kicks out unusable silicon wafers before they become solar cells could potentially save the industry billions of dollars annually.

Five to 10 percent of solar cells are destined to fail during manufacture due to weak silicon wafers that end up on the assembly line. The wafers take a bruising when going through assembly lines, where they are oxidized, annealed, purified, diffused, etched and layered. While the global photovoltaics (PV) industry generated revenue of about $82 billion in 2010, a 10 percent failure rate means the loss reached into the billions.

Now scientists at the US Department of Energy’s National Renewable Energy Laboratory (NREL) have developed an instrument that tests silicon wafers for their strength before they reach the assembly line.

NREL scientist Bhushan Sopori stands beside the Silicon Photovoltaic Wafer Screening System, which puts thermal pressure on wafers to determine if they will survive the rigors of being turned into solar cells. Images courtesy of Dennis Schroeder.

The device, dubbed the Silicon Photovoltaic Wafer Screening System, or SPWSS, is a cube-shaped furnace about 15 in. on each side, which puts thermal pressure on the wafers to determine whether they will survive the rigors of the manufacturing process. Similar to the toasting belt used to heat submarine sandwiches, each wafer passes through a 15-mm narrow, high-intensity illumination zone, where they are exposed to thermal stress in the form of precisely calibrated high temperatures.

“We create a very high temperature peak,” said NREL scientist Bhushan Sopori, principal investigator for the SPWSS. “The idea is to create a thermal stress, like putting very hot water in a glass.”

To create the SPWSS, Sopori built on the success of his optical cavity furnace, which uses optics to heat and purify solar cells at unmatched precision while sharply boosting the cells’ efficiency. (See: Optical Cavity Furnace Could Cut Solar Costs) The device’s trapezoidal prism shape enables it to narrow the focus of the light and increase its intensity. The furnace’s ceramic sides reflect the light to the intensity zone and ensure that almost no energy is wasted.

A PV wafer emerges from the Silicon Photovoltaic Wafer Screening System. This tool tests a pre-selection of wafers for high fracture strength, improving the yield of silicon solar cells by preventing breakage during cell fabrication.

“The most efficient way to generate light for the Silicon Photovoltaic Wafer Screening System is to use it in conjunction with the optical cavity furnace,” Sopori said.

It’s the rapid increase in thermal energy — made possible by the geometry of the furnace and its highly reflective surfaces — that causes the stress to travel through the wafer. While one 15-mm strip of the wafer feels 500 °C of stress, the strip adjacent to it feels much cooler. The hot strip wants to expand, but the cool strip doesn’t want any part of that, the team said. It’s these competing forces that cause the stress.

The microcracks, or breaks, that occasionally develop from the thermal stress mirror the stress that will happen to weak wafers as they go through the assembly process. The difference is that the thermal testing happens first, before the expensive wafer coatings and layers are added.

The loss in revenue due to broken wafers — which increases dramatically as the wafers move closer to completion — is an important barrier to solar energy’s cost competitiveness with other energy technologies. In recent years, the US has lost a large portion of its global market share of PV production, from 42 percent in 1997 to merely 4 percent in 2011.

NREL postdoctoral scientist Rene Rivero readies a wafer for the Silicon Photovoltaic Wafer Screening System.

Because of this trend, it will take dramatic changes in solar cell manufacturing to make the US a competitive PV manufacturing source in the industry again, analysts say.

The manual version of SPWSS can screen 1200 wafers an hour and costs $60,000. That’s quick enough for most manufacturers to screen every wafer without slowing down their conveyors. An upgraded version of the device, which costs $100,000, can automatically separate broken wafers; the failed wafers are swept out and melted down to be processed again into boules of solar-grade silicon.

The system can be retrofitted to any assembly line and could save PV manufacturers in the US billions of dollars annually.

For more information, visit: www.nrel.gov