Technology Improves Solar PV Competitiveness

by Kevin Chen, Applied Materials

Recent climate targets supported by China and the U.S. state a common goal of 17 percent emissions reduction, which is equal to slightly more than a 40 percent reduction in emissions intensity by 2020.

Energy from renewable sources, including solar photovoltaics (PV), is expected to play a key role in reaching this target.

The current market for solar PV is dominated by crystalline silicon solar panels.

This technology likely will continue to dominate the residential and commercial rooftop markets because of higher efficiency and rapidly falling costs.

The largest contributor to the cost of crystalline silicon solar installations is the highly purified polysilicon raw material for the PV cell wafer substrate.

It accounts for as much as half of the solar module cost and more than 10 percent of the total installation cost.

Supply shortages in 2008 drove the price of this material to more than $300 per kilogram.

Since then, silicon refining capacity has caught up with the solar industry’s growth, reducing prices to less than $50 a kilogram.

These reductions have helped drive panel costs down more than 50 percent since the first quarter of 2008: from more than $4 a watt to less than $1.75 a watt today.

The industry also is benefiting from a drive to thinner wafers and advances in wafer slicing.

Cutting thinner wafers at higher yields while reducing the silicon lost during processing increases the number of solar cells per kilogram of silicon.

Thinner cutting wire helps reduce the polysilicon loss.

Overall, the cost reductions in crystalline silicon panels have helped bring solar energy closer to grid parity, the point at which on-site solar energy generation costs the same as electricity from the grid.

Solar energy already has reached grid parity during peak demand.

The European Photovoltaic Technology Platform group expects it to reach overall grid parity in most of Europe within 10 years.

As crystalline solar panel efficiencies improve from 15 percent modules today to 20 percent modules now entering production, installation costs can be reduced by more than 20 percent, making solar PV even more competitive.

Factory of the Future—Improving Productivity

Solar installations do not consume fuel and incur minimal maintenance costs. Thus, manufacturing efficiency is one of the few levers available to maintain the solar industry’s historical 7 percent, year-over-year, cost-reduction record.

The solar fabs of the future increasingly will depend on automation to deliver high throughput and high yield and on improved cell designs to deliver superior performance.

  • Automation. Compared with today’s norm of 50-200 MW annual capacity, future factories are expected to reach between 500 MW and 1 GW, gaining economies of scale and supporting the increasing demand for solar cells. More complex and advanced processing needed to increase cell efficiencies will require 10-15 process steps, compared with seven today. As a result, the total number of wafer movements will increase by a factor of 10. The resulting demand for factory automation to improve wafer handling will take place against a background of decreasing wafer thickness from ~180μm today to 120-140μm.
  • High-productivity processing. Reducing the cost of each processing step requires high-productivity, low-maintenance systems optimized for the highest overall processing speeds. The industry’s target for the next generations of tools is to increase equipment throughput and yield.
  • Yield improvement and metrology. Cells from today’s factories are distributed across a range of conversion efficiencies. Lower-efficiency cells and wafer breakage cause yield losses of up to 5 percent. Improved metrology, together with automatic process control (APC) and statistical process control (SPC) capabilities, can tighten the distribution of cells around the best performers and increase the average cell efficiency. In addition, incoming sorting can eliminate wafers with microcracks and other defects likely to cause breakage.
  • Factory control systems. Today’s solar cell factories follow few industry standards. Few use the manufacturing execution systems (MES) typical of most other sophisticated manufacturing industries, and those that do often depend on custom software to tie the process equipment to the MES system. Typical MES capabilities such as recipe controls and wafer/lot tracking are rare. Industrywide communication standards coupled with powerful factory control systems will allow the factory to manage more wafer moves while avoiding wafer misprocessing because of process recipes mismanagement.

 
 C-Si solar PV is reaching grid parity in certain applications. Courtesy Deutsche Bank, May 2008, LCOE projection

Cell Technology Improvements

Recent efforts have brought cost per watt to about $1.2 for crystalline silicon solar panel technology with some companies forecasting $1 per watt around 2011-12.

As noted, thinner wafers reduce cost directly by reducing raw material consumption.

Raising cell efficiency also can lower the cost per watt, and several companies are shipping cells with better than 20 percent efficiency. These designs include improvements to:

  • Contact wiring. Areas shaded by the contact wires are not exposed to the sun and do not generate electricity. Tall, narrow contact lines reduce shading, and moving contacts to the back surface eliminates it.
  • Coatings. Typical cells reflect as much as 30 percent of incident light. An anti-reflective coating can reduce this below 10 percent.
  • Cell designs. A solar cell is simply a photodiode. Optimizing the size and doping (with activating films) of the semiconductor structures can increase conversion efficiency and reduce resistive losses.

One of the most crucial steps for producing crystalline silicon solar cells is creating the grid of very fine circuit lines on the front and back sides of the wafer that will conduct the light-generated electrons away from the cell.

This metallization process most commonly is done with screen-printing technology in which a metal-containing conductive paste is forced through the openings of a screen on to a wafer to form the circuits or contacts.

Screen-printed wires achieve very low electrical resistance. The silver wires, however, are not transparent to light.

The area they cover is shadowed from the sun, reducing the amount of electricity produced.

One solution is to create very thin but very tall wires by placing several layers of silver paste on top of each other. Such wires preserve the current-carrying capacity while reducing shadowing.

This is difficult to do with standard screen-printing equipment.

Depositing such lines requires equipment and pastes optimized for double printing.

As mentioned, the most efficient, commercially available crystalline silicon solar cells place all contacts on the wafer’s back surface.

Back-contact designs reduce shadowing to virtually zero. Improved screen-printing technology used in place of expensive photolithography can achieve the complete back-side patterning process at much reduced cost.

Efficiency also can be improved by applying anti-reflective layers on the solar cell surface to maximize light absorption and improve electrical passivation.

Advanced multilayer films can co-optimize anti-reflection and passivation properties.

This type of film stack has demonstrated high carrier effective lifetime and high cell efficiency.

Multilayer passivation films will become a standard for advanced technology cell designs.

Breaking the 20 Percent Barrier

Several companies already have broken the 20 percent cell efficiency barrier, and likely more will follow.

Sanyo’s HIT cell recently demonstrated 23 percent efficiency at the research level and is moving to mass production.

SunPower’s all-back-contact cell design and novel manufacturing processes has reached cell efficiency of 21 percent in mass production.

Suntech’s Pluto line boasts 17.2 percent efficiency in a multicrystalline cell and close to 20 percent in mono-crystalline cell performance.

In 2009 at the PV industry’s leading conference, PVSEC, the Fraunhofer Institute published results for 23 percent cell efficiency.

Suniva also announced its more than 20 percent cell efficiency road map with selective emitters, advanced metallization in the front, and point contacts on the back, all using screen-print techniques.

Conclusion

The drive to reduce carbon emissions is creating high demand for green technologies.

Opening the electricity market for solar PV requires low-cost, scalable manufacturing, including fully automated manufacturing solutions with yield management and control systems.

Advanced technologies such as thinner wafers, double-printing techniques, advanced passivation and advanced cell architectures will reduce the cost per watt further.

Author

Kevin Chen is the chief marketing officer for the energy and environmental solutions division at Applied Materials. Reach him at 408-727-5555. For more information, visit the website, http://appliedmaterials.com. 

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