Method For Passivating Crystal Silicon Surfaces

Technology Marketing SummaryThe photovoltaic market remains dominated by silicon wafer-based solar cells. Therefore, there is a need for improvements in the manufacturing processes of first generation solar cells that can achieve higher conversion efficiency without exorbitant increases in production cost. Improvements to the minority charge carrier’s lifetime through new manufacturing methods offer the desired increase in overall efficiency without extreme capital costs. There is a need in the art of preparing crystal silicon surfaces to provide good surface passivation since the direct growth of hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) does not generally produce good surface passivation. NREL scientists have provided a method for producing good surface passivation when depositing a-Si:H on crystal silicon.DescriptionThe minority – carrier lifetime (t) is a quantitative measure of a silicon cell’s generated carrier’s recombination rate with defects, impurities and surface damage. The apparent minority-carrier lifetime value is a function of surface states. When surface states are not passivated silicon atoms on the surfaces have unsatisfied dangling bonds which act as very effective recombination centers for the charge carriers. This reduces the apparent minority-carrier lifetime to a value much lower than its true bulk lifetime. This invention improves the minority carrier’s life-time of c-Si based solar cells by providing a surface treatment prior to deposition of a thin a-Si: H, by mixing NH3 and H2 gases in a chemical vapor deposition (CVD) or hot wire chemical vapor deposition (HWCVD) process chamber at a low substrate temperature for a short time. An increase in the lifetime of the c-Si wafer of at least three-fold or to about 20µs compared to a lifetime of about 6 µs for a comparable c-Si wafer prepared without this method, for example, using NO or H2 has been seen in the lab.Benefits

• The passing of NH3 + H2 before the HWCVD deposition of thin film silicon leads to an immediate and true a-Si:H layer on the c-Si surface, resulting in an increase in the minority charge carriers lifetime
• The a-Si:H layer on a c-Si substrate has superior passivation properties of c-Si dangling bonds
• This process can be employed in a-Si/c-Si heterojunction solar cells by significantly reducing junction interface recombination and back surface recombination
• This process can be used in a c-Si homojunction solar cells by passivating either or both the front and back surface to improve its energy conversion efficiency

Applications and Industries

• This application is applicable to monosilicon, polysilicon, and microsilicon PV manufacturers

Patents and Patent Applications

ID Number	Title and Abstract	Primary Lab	Date
Patent 7,629,236	Method for passivating crystal silicon surfaces In a method of making a c-Si-based cell or a .mu.c-Si-based cell, the improvement of increasing the minority charge carrier's lifetime, comprising: a) placing a c-Si or polysilicon wafer into CVD reaction chamber under a low vacuum condition and subjecting the substrate of the wafer to heating; and b) passing mixing gases comprising NH.sub.3/H.sub.2 through the reaction chamber at a low vacuum pressure for a sufficient time and at a sufficient flow rate to enable growth of an a-Si:H layer sufficient to increase the lifetime of the c-Si or polysilicon cell beyond that of the growth of an a-Si:H layer without treatment of the wafer with NH.sub.3/H.sub.2.	National Renewable Energy Laboratory	12/08/2009 Issued

Technology Status

Technology ID	Development Stage	Availability	Published	Last Updated
US 7,629,236 B2	Development	Available	07/07/2010	07/28/2010