Howard Branz, front row, center, principal investigator for the team that won an R&D 100 Award for a new “black silicon” process to improve the efficiency of solar cells, poses with his National Renewable Energy Laboratory teammates at the PDIL facility. On Branz’s right is Anna Duda; on his left is Vern Yost. In the back row, left to right, are Matt Page, Scott Ward and Hao-chih Yuan.
Natcore Technology Inc. has been granted a patent license agreement from the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) to develop and commercialize a line of black silicon products—including equipment, chemicals, and solar cells—based on NREL patents. The license grants Natcore exclusivity in the field of diffused emitters with liquid phase passivation.
Natcore and NREL have also agreed to enter into a Cooperative Research and Development Agreement to develop commercial prototypes that embody NREL's black silicon inventions.
"Black silicon" refers to the apparent color of the surface of a silicon wafer after it has been etched with nano-scale pores; the black color results from the absence of reflected light from the porous wafer surface. R&D Magazine awarded the black silicon technology an R&D 100 Award in 2010, identifying it as one of the top 100 technological innovations of the year.
A panel made from black silicon solar cells will produce a significantly greater amount of energy (KwHrs) on a daily basis than will a panel made from cells using the industry standard thin film coating, not only because the reflectance is lower but also because the angular dependence of the reflectance from black silicon is much lower as well. The latter fact means a black silicon panel will perform better during the morning and afternoon hours when the sun hits at an angle and will also outperform standard cell panels on cloudy days. The combination of lower cost and higher energy output per kilowatt of installed array peak power should quickly make black silicon the antireflection control technology of choice in the industry.
For solar cells, minimum reflectivity is desirable because sunlight that is reflected, rather than absorbed, is "wasted." The reflectivity of a polished silicon wafer surface approaches 40%, giving the wafer its shiny appearance. Adding the industry's typical antireflective coating reduces the average reflectivity to approximately 6% and gives the cells their distinctive dark blue color. The black silicon process has been shown by Natcore scientists and NREL researchers to reduce average reflectivity to less than 1.5%.
Black silicon solar cells have been studied since the 1980s because of their potential for significantly improved performance compared to standard production cells. But a key obstacle to turning their increased light absorption into increased power output is a significantly increased area of exposed silicon on the sidewalls of the pores and on the small mesas that remain at the top surface of the wafer itself. This increased area must be passivated, or treated to keep it from trapping the light-generated electric charges as they migrate toward the contacts of the solar cell, a process that robs the cell of output power.
"Natcore has the ability to passivate black silicon cells using their liquid phase deposition (LPD) technology. That has been the missing piece. It's what will enable black silicon to reach its potential," says Dr. Dennis Flood, Natcore's Chief Technology Officer."Natcore has the ability to passivate black silicon cells using their liquid phase deposition (LPD) technology. That has been the missing piece. It's what will enable black silicon to reach its potential," says Dr. Dennis Flood, Natcore's Chief Technology Officer.
"Before Natcore's passivation technology, it was necessary to put coated cells into a 1,000 deg. C. furnace to create a thermal oxide," continues Flood. "Natcore's LPD silica coating achieves passivation without requiring an extra thermal process."
Prior to today's announcement, NREL sent black silicon wafers with junctions—unfinished cells—to Natcore. Natcore coated them with SiO2 and passivated them. NREL then applied contacts and tested the completed cells in their labs in Golden, CO. According to Flood, the result persuaded NREL to grant Natcore a license to develop and commercialize products based on the NREL black silicon technology.
"Double the output, halve the cost," says Natcore President and CEO Chuck Provini. "That's our mantra. To make solar cells cost-competitive, we must reduce their cost and increase their output. The combined NREL-Natcore technologies will reduce cost by eliminating the need for thermal oxidation. And they'll increase output by enabling cells to be more productive throughout all daylight hours."
"We're combining NREL's black silicon technology with our LPD and passivation technologies, Provini adds. "We'll optimize the combined processes and incorporate them into our AR-Box. AR-Box enables use of an all-liquid phase process for creating ultralow reflectivity, high-performance silicon solar cells at high volume production rates." AR-Box is Natcore's device that uses the company's liquid phase deposition (LPD) process to grow antireflective (AR) coatings on silicon wafers.
The NREL license contains a development and commercialization plan that establishes technical and market milestones for Natcore, along with a royalty structure. These are subject to confidentiality provisions set by the parties. The technical milestones include solar cell efficiency goals, some of which are to be met by August of 2012. The market milestones include commercial sale dates and dollar targets. The agreement is dated December 12, 2011, and is effective for as long as the NREL patents are enforceable.
NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for DOE by the Alliance for Sustainable Energy LLC.