Technology Improves Solar PV Competitiveness

c si solar pv Technology Improves Solar PV Competitiveness

Recent climate targets supported by China and the U.S. state a commongoal of 17% emissions reduction, which is equal to slightly more than a40% 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 andcommercial rooftop markets because of higher efficiency and rapidlyfalling costs.

The largest contributor to the cost of crystallinesilicon solar installations is the highly purified polysilicon rawmaterial for the PV cell wafer substrate.

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

Thesereductions have helped drive panel costs down more than 50% since thefirst quarter of 2008: from more than $4 a watt to less than $1.75 awatt 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 duringprocessing 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 bringsolar energy closer to grid parity, the point at which on-site solarenergy 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% modules today to20% modules now entering production, installation costs can be reducedby more than 20%, 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 tomaintain the solar industry’s historical 7%, year-over-year,cost-reduction record.

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

Automation. Compared with today’s norm of 50-200 MW annual capacity, futurefactories are expected to reach between 500 MW and 1 GW, gainingeconomies of scale and supporting the increasing demand for solar cells. More complex and advanced processing needed to increase cellefficiencies will require 10-15 process steps, compared with seventoday. As a result, the total number of wafer movements will increase by a factor of 10. The resulting demand for factory automation to improvewafer handling will take place against a background of decreasing waferthickness from ~180?m today to 120-140?m.

High-productivity processing. Reducing the cost of each processing step requires high-productivity,low-maintenance systems optimized for the highest overall processingspeeds. The industry’s target for the next generations of tools is toincrease equipment throughput and yield.

Yield improvement and metrology. Cells from today’s factories are distributed across a range ofconversion efficiencies. Lower-efficiency cells and wafer breakage cause yield losses of up to 5%. Improved metrology, together with automaticprocess control (APC) and statistical process control (SPC)capabilities, can tighten the distribution of cells around the bestperformers 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 othersophisticated manufacturing industries, and those that do often dependon custom software to tie the process equipment to the MES system.Typical MES capabilities such as recipe controls and wafer/lot trackingare rare. Industrywide communication standards coupled with powerfulfactory control systems will allow the factory to manage more wafermoves while avoiding wafer misprocessing because of process recipesmismanagement.

Cell Technology Improvements

Recent efforts have brought cost per watt to about $1.2 for crystallinesilicon solar panel technology with some companies forecasting $1 perwatt 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% efficiency. These designsinclude improvements to:

Contact wiring. Areasshaded by the contact wires are not exposed to the sun and do notgenerate electricity. Tall, narrow contact lines reduce shading, andmoving contacts to the back surface eliminates it.

Coatings. Typical cells reflect as much as 30% of incident light. An anti-reflective coating can reduce this below 10%.

Cell designs. A solar cell is simply a photodiode. Optimizing the size and doping(with activating films) of the semiconductor structures can increaseconversion 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 thewafer that will conduct the light-generated electrons away from thecell.

This metallization process most commonly is done withscreen-printing technology in which a metal-containing conductive pasteis forced through the openings of a screen on to a wafer to form thecircuits 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 severallayers of silver paste on top of each other. Such wires preserve thecurrent-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 crystallinesilicon solar cells place all contacts on the wafer’s back surface.

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

Efficiency also can be improved by applying anti-reflective layers on the solarcell surface to maximize light absorption and improve electricalpassivation.

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% Barrier

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

Sanyo’s HIT cell recently demonstrated 23% 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% in mass production.

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

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

Suniva also announced its more than 20% cell efficiency road map withselective emitters, advanced metallization in the front, and pointcontacts 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, scalablemanufacturing, including fully automated manufacturing solutions withyield management and control systems.

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

This write up can is also available on Electric Light & Power’s web site.

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