Quality: Approximately 92% of all PV cells produced are the crystalline silicon (c-Si) type. Many manufacturers in the silicon PV industry utilize the gravitational deposition of silicon, be it either as molten silicon, particulate silicon in a solvent, or decomposition of silane on backing plates. In most of these PV cell fabrication processes, recrystallization at high temperature is done. However, there are still quality problems.
There are several technical reports concerning silicon layer uniformity and performance by the industry's current method. Hsu, et.al., have indicated uniformity problems (1). Oxygen precipitates (SiO2) which interrupt the uniformity of silicon layers is discussed by Wang, et. al (2). P-type and n-type doping via ion implantation results in induced lattice damage and non-uniformity (3). Plasma enhanced chemical vapor deposition processes have resulted in chronic non-uniformity of deposited layers (4). Graf, et.al., has shown improved silicon wafer integrity after annealing at 1200 degrees C in argon and hydrogen atmospheres. This shows some reduction of crystal defects with some improvement in uniformity (5). Shimizu, et.al. has used the Bridgman method for phosphorus deposition that resulted in crystal defects (6).
Conversion Efficiency: Solar conversion efficiency of the PV cells is important from several aspects. The efficiency appears several times in the cost factor equation as shown later.
A number of reported attempts have been made in improving silicon's direct conversion efficiency by improving high temperature annealing or sintering. K. Drain has announced a silane laser decomposition method for deposition of silicon on monolithic large glass substrate. Recrystallization was conducted at high temperatures (8). Konarka has shown a 12% efficiency with its thin film hybrid silicon (a-Si) PV cells (9). Although, no aging studies have been done. They are doing further research with IMEC (Belgium) to improve that to >20%. Signet Solar has started manufacturing thin-film silicon modules that achieves 6.2% efficiency (9). NREL tested Global Solar Energy's (GSE) new prototype CIGS cell at 15.45% but GSE's production cells are still 11.7% (10). In addition, CdTe production solar cells as made by First Solar achieves 9% efficiency (8). EMPA, the Swiss Federal Labs have improved the efficiency of flexible CdTe cells to 12.4% (11). Both CdTe and CIGS are susceptible to efficiency degradation by moisture.
A recent contract signed by Evergreen Solar (c-Si) requires PV panels of 19% efficiency (9). The "Q-Cells" Company has achieved 15% efficiency for their crystalline Si cells (10). However, in all cases, the efficiency for field installed solar PV cells should achieve 20% or higher for good return on investment.
Solar Tracking: There are two types of solar tracking equipment for mounting the PV modules. These are 2-axis and single-axis tracking units. The current 2-axis tracking units do not gain sufficient increase in wattage to be economical during the lifetime of the PV modules. For the single-axis tracking units, they would be a marginal investment if their installed price is below $0.50/watt.
Cost Factors: There are three major cost factors that can be developed for the cost of the installed PV modules. The solar PV efficiency is an important factor in two of the three major cost factors. The three major factors are:
Total Cost = Land area cost + PV module cost + Installation cost
For a good return on the investment, all three factors should be minimized. The land area required for a given PV solar farm, e.g. 20 MWe, is dependent inversely on the solar cell efficiency:
Land area required: 20 x 103 kW/(1 kW/m2 x solar PV efficiency)
For the given solar farm size of 20 MWe, the PV module cost can be expressed as:
PV module cost = (cost/ m2) (20 x 103kW/PV efficiency)(m2/1 kW)
(The factor 1 kW/m2 is the solar insolation factor at noon.)
The third factor, installation cost, is composed of a number of items including module support structure, labor, land preparation, etc.
In summary, for a given "farm" size and when the solar conversion efficiencies approach 22% vs. current 9-14%, then fewer acres are required and, hence, the land cost is lower. In addition, fewer PV panels are required and, therefore, installation costs are less.
Cost Reduction: For the past three years, ICT Corporation, a little known company in upper Michigan, has experimented and developed a unique and innovative method for producing crystalline PV cells. This new development produces the Solar Industry's lowest cost PV cells based on their 40 years experience in high temperature furnaces.
ICT's innovative high temperature process directs the silicon material to the proper surface area without any loss of expensive silicon material. This process overcomes the surface energy of molten silicon using a new unique vacuum/purge high temperature centrifuge process designated as the "FW"process. This method spreads the molten silicon uniformly to any chosen thickness. The extreme thinness of the silicon layer (0.0017") on the backing material enables a very significant lowering of manufacturing energy use and material cost.
This very thin layer is the underlying reason for many advantages of this approach. Primarily, minimizing the loss of expensive silicon, i.e. approximately 1/15th of the amount of the sawn methods. Secondarily, the method only heats about 1/10th the amount of silicon resulting in an obvious energy savings. Finally, the method crystallizes the silicon much faster due to the small volume. A crudely-fabricated first version of the high temperature "FW" unit has been used to make small prototype PV cells. Chemical analysis has indicated no harmful defects and the silicon is crystalline. ICT has received one U.S. Patent with six more U.S. Patent applications made and international patent applications being developed.
Independent external review of prototype PV cell has been completed at MIT, Arthur D. Little and Miami Research Labs with highly positive reports. M.I.T. has stated: "these were made of silicon on graphite cells provided by ICT... of the curves, the one in (a) is the one closest to ideal and is the best of all cells tested, including single crystal cells... control of the degree of penetration of the graphite by the silicon layer is unique to the ICT process -- in no other process known at this time is there a way of controlling substrate contact area.
Independent cost analysis has shown a $0.76/watt production cost for up to 40 MWe. This is 25% lower than First Solar's recently announced "lowest production cost" of $0.98/watt (March 2009). First Solar's FOB factory selling price is $2.50/watt (8) while ICT's price is projected at $1.75 (FOB, Michigan). Their current goal is fabrication of PV cells with 20% efficiency. This will make inexpensive solar energy available to homes and businesses.
The innovative "FW" method is broadly applicable such that it can be applied to the improvement in efficiency and cost for the electrolysis electrodes. By applying ICT's "FW" method to the development of nickel electrodes, this removes the expensive platinum electrodes as well as increasing the electrolysis efficiency and, hence, reduces the size and cost of the electrolysis unit. This can also be done using the nickel electrodes for fuel cells. Hence, this would bring down the cost of fuel cells and make them available for auto makers to produce fuel cell autos. ICT's innovative development has far-reaching applications.
Summary: Rapid strides are being made to improve the quality and efficiency of the crystalline silicon solar cells. Silicon solar cells have the added advantage of having a much higher efficiency and lower cost than either the current CdTe or CIGS PV cells.
(1) Hsu, J-C, Lee, C-C, Kuo, C-C, Chen, S-H, Wu, J-Y, Chen, H-L and Wei, C-Y, Applied Optics 44(20) 4402-07, 2005
(2) Wang, Q., Daggubati, M., Yu, R., and Zhang, X-F., Appl. Phys. Lett. 88, 242112, 2008
(3) Sundaresan, S., Doctoral Diss. George Mason Univ., Oct. 2007
(4) Zhao, A., Bulgar, J.M., Green, L.P., Harris, W.C., Hunt, T.J. Marchesseault, W.R., Shuman, R.F., French, M.C. and Lisy, H.C. Adv. Semiconductor Manufacturing Conf. IEEE/SEMI issue 11-12, 143-148, 2005. See also: Xiang, B. et. al.,Nano lett., 2007, 7(2) 323-328.
(5) D. Graf, U. Lambert, M. Brohl, A. Ehlert, R. Wahlich and P. Wagner, J. Electrochem. Soc. 142 (9) 3189-92, 1995.
(6) A. Shimizu, J-I Nishizawa, Y. Oyawa, K. Suto. J. Of Cryst. Growth, Techn. Conf. #1 Sendai, Japan, 2001 229, p119-123.
(7) K. Drain's NanoGram Corp. development as reported by D. Vogler, Sr. Tech. Ed., PV World 21 May 2009.
(8) Doty, "Solar Photovoltaic (PV)" www.dotyenergy.com/markets. See also: Plenary Lecture: "The Solar-Hydrogen Economy - An Analysis" by W.D. Reynolds, presented August, 2007 at SPIE's International Solar Energy Conference, San Diego, CA.
(9) See www.q-cells.com /en/products
(10) See "Global Solar Energy's CIGS Production Material Achieves 15.45% Efficiency" Photovoltaic Wire News: www.electroiq.com 9/18/09
(11) See Thin Film Today Newsletter, 3 September 2009.
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