Uses of Water Vapor for Solar Cell Fabrication

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1 Uses of Water Vapor for Solar Cell Fabrication Abstract Solar cell cost-per-watt is five to ten times higher than coal-fired electricity the baseline cost of today s commercial power. To be competitive, solar cell costs must be significantly reduced and solar energy efficiency increased. The photovoltaic (PV) industry is increasing efficiency and reducing cost through reduced silicon content and improved cell design. To make this happen, new materials and process steps need to be introduced to the manufacturing process. These processes require the use of water vapor during PV cell fabrication. New technologies are available that improve safety, increase growth rate and process uniformity, and remove particles and contaminants in solar cell manufacturing. Search Terms: Solar Cells, Passivation, Interlayer Oxide Films, TCO, Backside Thick Oxide, Water Vapor, RASIRC Steamer, RainMaker Humidification System, Photovoltaic Cells The purpose of this paper is to provide a quick review of how water vapor can improve the performance of Inorganic Photovoltaic Cells. Solar cell cost-per-watt is five to ten times higher than coal-fired electricity (1), which is the baseline cost for today s commercial power. To reduce solar cell per-watt cost, the PV industry is attempting four main tactics: 1. Move to large-scale manufacturing, playing off the already existing semiconductor and flat panel infrastructure. This can be seen by the recent entry of Applied Materials into the market for equipment supply. 2. Double the useful life of the PV cell from 15 to 30 years. The operating costs of PV cells are virtually nil, so by extending the useful life of the cell by 2X the cost per watt is reduced by 50%. 3. Reduce the material cost by limiting the amount of silicon needed in the PV cell. The silicon substrate is the main cost driver today for silicon based PV cells. The industry is moving rapidly to work with thinner substrates. The difficulties include handling and performance. Thinner structures also have higher parasitic losses and lower overall efficiencies. 4. Improve the design of the PV cell. These improvements in efficiencies need to address both thinner substrates and alternative design approaches. The rest of this paper will focus on how the use of water vapor during PV cell fabrication can enable improvements in efficiency. RASIRC, Inc. Page 1 of 6

2 Background Efficiency Improvements are being addressed at all parts of the Photovoltaic Structure. Reference: 2. Solar cells have several major layers that may include: the glass substrate, the transparent conductive oxide (TCO) layer, the front contact layer, the P, I, and N layers, TCO/Back Reflector, Back Metal contact, and a passivation layer. To improve efficiencies, new films and additional layers are being added to the PV structure. These films reduce losses of electrons due to the recombination of charge carriers within the cell. The overall efficiency of the cell is limited by the bulk recombination rate of charge carriers within the cell. Two types of defects promote recombination: localized impurities within each grain and extended defects such as occur at grain boundaries. Both of these act as recombination centers. Localized impurities include: source materials, dopant ions diffused from adjacent layers, metal contaminants from contacts and reflective coatings, and environmental contaminants. Extended defects occur at grain boundaries and dislocation in the grain structures. These act as recombination centers by generating holes or fractures within the structures. These sites allow for recombination of charge carriers within the defect, which short circuits the PV cell. These losses convert to waste heat and lower the overall efficiency for a given solar input. Passivation It is well understood that crystal defects reduce efficiencies. Atomic defects within the crystal propagate into the macrostructure. To repair these defects passivation is used. Hydrogen is used to form SiH at the grain boundaries. High temperature annealing can increase carrier lifetimes by injecting H into the Si/SiO 2 interface. Passivation of the interface limits hole/electron recombination. This process is referred to as thermal annealing. It reduces vacancies and dislocations at grain boundaries. RASIRC, Inc. Page 2 of 6

3 Hydrogen is sourced from H 2, H+ radicals from plasma, or H 2 0 water vapor. The three common techniques are referred to as hydrogen plasma anneal, forming gas anneal, or high temperature steam anneal. Traditional methods used Hydrogen Radical Annealing (HRA), which uses hydrogen gas and microwave energy. Forming Gas Annealing (FGA) uses H 2 at C. A new approach is the use of High Temperature Steam Annealing (HSA), which uses water vapor at C. Comparison of High Temperature Steam Annealing at 400 C to FGA and HRA. HSA generate the best carrier lifetimes with very short annealing cycle time. Reference: 3. In a test conducted by Abe, et.al, (3) the process recipe was 400 C for 5 minutes with a 13.4 slm H 2 0 flow rate. There are multiple advantages to the HSA approach. The steam readily inserts H and O atoms into the Si/SiO 2 interface. This process is much faster than FGA and comparable to the HRA process. Without the plasma, the device is subjected to less structural damages and obtained higher effective carrier lifetimes. The process is much simpler than a plasma process. The elimination of hydrogen reduces cost and is safer. Interlayer Oxide Films In addition to hydrogen passivation, techniques that are being adopted to address recombination losses include insertion of a thin SiO 2 layer between film interfaces to prevent ionic contamination from migrating (4). P-n and p-i-n efficiency can be improved by providing a passivation layer between the p-layer and i- or n- layers. By limiting boron diffusion and minimizing interface resistance, charge separation is improved, directly increasing cell efficiency. These passivating oxide layers can be performed by water vapor at low temperatures of 200 C to 450 C to prevent damage to the glass substrate. RASIRC, Inc. Page 3 of 6

4 Water vapor is commonly used as the source of oxide and can be cheaper and easier to control than other sources of oxygen. Boron migration into the transparent conductive oxide layer will lead to decreased efficiency due to Free Carrier Absorption (FCA) which reduces transmittance in the near infrared region(5). Passivation of the TCO before p-layer deposition can limit boron intrusion. Transparent Conductive Oxides TCOs are a necessary part of PV cells. They enable the cell to collect solar energy efficiently by reducing backscatter as well as allowing for conduction through the film to the front electrode. TCOs can be composed of ZnO, SnO, SiO, or TiO. These films need to be applied at low temperatures due to glass substrates. While commonly reacted with oxygen and plasma, the deposition rate can be increased two to five times by adding small amounts of water, which speeds the decomposition of the organometallic precursor. Too much water can lead to premature decomposition which leads to a high particulate rate. This can be caused by condensation of the water vapor in the delivery line or microdroplets from the water vapor delivery systems. Condensation is frequently caused by improper heating of the delivery line. Causes for microdroplets are more complicated. Due to high flow rates and low operating process temperatures, pyrolytic torches are not used. Instead, either a water bubbler or direct liquid injection system in conjunction with a vaporizer is used. Bubblers generate erratic flow rates and suffer from inaccuracies due to water temperature and level variations. In addition, at higher flow rates, bubble energy volatizes water microdroplets that are entrained in the carrier gas and lead to localized water vapor concentrations on the substrate and in the gas stream that react with the source chemistry and generate particulate. Alternatively, vaporizers atomize the water and then try to combust the small droplets into molecular water. The heat of vaporization is very high and the ability to the get the energy to the water molecule is limited by the heater transfer rate through the vaporizer plate and the carrier gas. In addition, the water is aggressive and can corrode the vaporizer s internal components, which leads to long term stability and reliability issues. For TCO films to work properly, the film thickness and uniformity are critical. The RASIRC Steamer and the RainMaker Humidification Systems are two new innovative water vapor delivery systems from RASIRC. These new products solve many of the challenges for direct delivery of water vapor by completely changing the way water molecules are converted from liquid to gas phase. Where bubblers and vaporizers depend on water molecules overcoming the surface tension and water molecule binding energies, the RASIRC products are based on a hydrophilic membrane that uses the ion charge of the membrane to separate each water droplet into its molecular components. The energy required to enter the membrane is equal to the heat of vaporization. Transfer across the membrane is restricted to single and small channel transfer rates. Once molecules cross the wall of the membrane, they are energized and ready to enter RASIRC, Inc. Page 4 of 6

5 the gas phase based solely on the vapor pressure curve that relates to the temperature of the water. By using the membrane as the phase separator, water droplets cannot permeate the membrane and very smooth consistent flow can be delivered. Backside Thick Oxide Another design change to improve overall efficiency is a move to eliminate the front electrical contact and relocate it to the backside. Traces on the front side of the PV cell block available light from entering the cell. By moving these to the backside, the entire front surface is available. Moving both contacts to the back side requires a good insulating layer to prevent current recombination. A thick SiO 2 layer is commonly used. In addition SiO 2 can be used as a rear TCO. The oxide layer provides good separation between the n-layer and metal reflector or metal connector. The oxide layer also provides good transmission as well as a barrier to metallic migration into the n-layer. The table below compares thick silicon oxide against silicon nitride and thin silicon oxide. The thick oxide film generated efficiencies greater than 21%. (6).The table below shows that highest efficiency was generated through the use of only thick oxide. All other combinations produced between 0.6% and 1.5% lower performance. The thick oxide provides the best insulator. When used with dual rear contacts, this benefit becomes more important. The RASIRC Steamer and RainMaker Humidification System can play a significant role in generating uniform thick oxides. Reference: 6. RASIRC, Inc. Page 5 of 6

6 Summary In conclusion, PV cell manufacturers are moving to improve cell efficiencies by adding water vapor to grow new and thicker oxide films, annealing interface layers to reduce defects, and improving transparent oxide film uniformity. RASIRC Steamers provide a superior solution for addition of water vapor to manufacturing processes. RASIRC Steamers deliver safety, low cost of purchase and ownership, ease of use, and high flow capability. The steam delivered is the only purified steam available today. Using these products, process engineers have achieved increased growth rate, process uniformity, and contamination/particle-free films. References 1) Basic Research Needs for Solar Energy Utilization - Report on the Basic Energy Sciences Workshop on Solar Energy Utilization. Sciences Workshop on Solar Energy Utilization April 18-21, 2005 DOE 2) Chemical Vapor Deposition of Tin Oxide: Fundamentals and Applications by Mol, et.al, Thin Solid Films 502 (2006) ) Effect of high temperature steam annealing for SiO 2 Passivation by Abe, Y. et.al, Solar Energy Materials & Solar Cells 65 (2001) ) High-efficiency P-I-N a-si: H solar cells with low boron cross-contamination prepared in a large-area single-chamber PECVD reactor U. Kroll et al. Thin Solid Films (2204) ) Boron Doping Effects on the Electro-optical Properties of Zinc Oxide Thin Films Deposited by Low-Pressure Chemical Vapor Deposition Process J. Steinhauser, et.al., Mater. Res. Soc. Symp. Proc. Vol MRS 6) Silicon Oxide/ Silicon Nitride Stack System for 20% Efficient Silicon Solar Cells by Schultz et.al. 31 st IEEE PVSC Orlando, Fl, ) Fraunhofer ISE Annual Report 2005 RASIRC, Inc. Page 6 of 6