Natcore Technology Inc.and the National Renewable Energy Laboratory (NREL) have been selected to present a technical paper to the 38th IEEE Photovoltaic Specialists Conference, the pre-eminent global technical conference in photovoltaics. It is sponsored by the world’s largest professional association for the advancement of technology, the Institute of Electrical and Electronic Engineers. Titled “Antireflection and SiO2 Surface Passivation by Liquid-Phase Chemistry for Efficient Black Silicon Solar Cells,” the paper was co-authored by Hao-Chih Yuan, Jihun Oh and Howard M. Branz of NREL, and by Yuanchang Zhang, Oleg A. Kuznetsov and Dennis J. Flood of Natcore. The oral presentation was made by Dr. Yuan in Austin, TX, on June 4.
The paper concludes that the integration of black silicon solar cells with low-cost liquid phase deposition (LPD) silicon dioxide demonstrates that LPD solar cells (1) retain low reflectivity of the black silicon surface, and (2) provide better surface passivation quality compared with the thermally grown silicon dioxide. Further optimization of the key processing steps could promise an extremely low-cost and high-efficiency black silicon solar cell, work that is being carried out by Natcore and NREL. “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. Natcore has been granted an exclusive patent license from NREL in the field of diffused emitters with liquid phase passivation to develop and commercialize a line of black silicon products based on NREL patents. 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.
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. 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.
Although black silicon solar cells have been studied since the 1980s, 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 cost effectively using their LPD technology. That has been the missing piece. It’s what will enable black silicon to reach its potential,” says Dr. Dennis Flood, a Natcore founder and the company’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. 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.” “NREL asked Natcore if we would like to submit a joint paper for presentation at the conference,” says Dr. Flood. “I agreed, and the paper was accepted by the program committee for an oral presentation. Most of the papers are presented as posters, so getting selected for an oral presentation means the committee felt the paper had significant technical merit. About one in ten papers are selected for oral presentation.” Dr. Flood had been on the organizing committee for the conference since his days at NASA. He held several positions, from publication chair to program chair to general chair and international committee chair. He has been affiliated with the conference for over 32 years.
The IEEE is the world’s largest technical society and is the official conference sponsor. It will be attended by about 2000 persons in academic institutions and company laboratories from more than 40 countries worldwide. It is considered the premier technical conference on photovoltaic energy conversion and is the longest running conference on the topic.