Indium Corporation’s industry leading semiconductor bloggers are hosting a Meet the Bloggers session on Tuesday, July 15, 2008 at Indium Corporation’s Semicon West exhibit, booth #7834, from 2-3pm PST.

The technology experts will lead discussions on topics including:  

·         Flux deposit measurement using non-contact metrology

·         Two upcoming white papers (currently under development):

• Wafer Flux Spin-Coating Topography
• Wafer-Level Flux Printing

·         Recent hot semiconductor blog topics, including:

• Semiconductor assembly materials
• Future trends in first- and second-level assembly
• Halogen-free semiconductor assembly materials
• Engineered solders in MEMS assembly
• Thermal interface issues
• Solar device assembly  

Indium personnel who will be discussing these topics include Jim Hisert, Paul Socha, Fez Sayed, Dr. Andy Mackie, and Rick Short.

All attendees are welcome to participate in, or observe, the session. Snacks will be served, too! 

Indium’s blogs can be seen at www.indium.com/blogs

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The techniques for depositing the thin film absorber layer of a CIGS device are well known, with evaporation and sputtering leading the way and printing pushing hard to catch up.

However the data from recent production runs indicates that evaporation has a commanding lead in cell efficiency.  While companies using evaporation are all reporting efficiencies of 14-17% (including NREL's record of over 19%), all those companies that are using sputtering are reporting less than 10% efficiency in production.

The reasons for this difference are unclear and have spawned a variety of theories, including:

1. Sputtering and Evaporation form thin films of different structures and stress levels.  What part does this fact play in the efficieny struggle?

2. Sputtering could be causing substrate damage or thin film dislocation due to higher kinetic energy.  Is the damage real?  How bad is the damage?  Could this damage be hurting efficiencies?

3. Sputtering does not lend itself to a process that creates complete and uniform Selinization as does evaporation.  It is well known that the complete and uniform incorporation of Se into the thin film matrix is critical to the formation of a good CIGS absorber.  Could this be a significant factor in efficiency differences?

There are several other anecdotal ideas that have yet to be studied or quantified that could hold all or part of the truth as well.

I would invite any and all theories to be discussed here.  The more information that can be disseminated on this topic, the faster we will be able to discover the reason for this discrepancy and perhaps cause the breakthrough that is so urgently needed.

Therefore, I pose a question to all of you in the CIGS industry; Can sputtering have a breakthough that will propel this technology into the efficiency levels of evaporation or beyond, or are we stuck in the doldrums of a segmented market with sputtering bringing up the rear?

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Solder fluxes facilitate solder wetting by dissolving the oxides present on the surface of the tabbing ribbon as well as the silver metallization bonding stripes on the top and bottom of the solar cell.  Typically liquid fluxes consist of a chemical activator package, rosin or a synthetic resin and a solvent system.

 

The solar industry has historically used fluxes formulated with alcohol solvents, but newer formulations are available formulated with low VOC solvents.  These newer low VOC fluxes are safer to use and have less environmental impact.

 

In both electronics assembly and the manufacture of solar cells, long term reliability is of paramount importance, and care must be taken to insure that the flux selected for soldering will be non-corrosive.   It is important the activator/resin system be designed to volatize or decompose during the peak temperature of soldering.  This insures that no corrosive by-products remain, and therefore the flux residue can safely remain on the substrate.  Such fluxes are known as “no-clean” and the formulation technology and reliability testing were developed for electronics assembly and microelectronics applications by flux manufacturers serving these industries.  In these industries, circuitry line width and spacing are significantly less than used in solar cells and even minute amounts of corrosive residues negatively impact on SIR (surface insulation resistance) performance.   Therefore it is prudent for the module assembler to select a tabbing ribbon flux supplier that also supplies to the electronics assembly and microelectronics industry.  

 

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Computers need to catch up, they need to catch up to the speed and processing capabilities of human brain. All the progress in computers and smart devices can be fairly described using Moore's law. It states "the number of transistors that can be inexpensively placed on an integrated circuit is increasing exponentially, doubling approximately every two years." But even at this pace computers have a long way to go.

On the other hand, solar pv panels are quite close to their ultimate goal. The ultimate goal isn't being 100% efficient and converting all incident sunlight into electricity. But it is to be at grid parity. I was attempting to define the Sayed's Law for Solar PV industry when Jim Slattery brought to my attention the Moore's law equivalent for Solar PV industry.

Even though it can be debated whether the Moore's law equivalent is on track or not, but the reality is that the Solar PV industry is getting one step closer every day to achieving grid parity. Currently there are quite a few companies who are quite confident of reaching grid parity before 2012.

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Have you recently thought about any creative ways to harness solar power?

Tandem Cells, Portable Power, Concentrators, Solar Trackers, Internal Refelecters, Back Contact etc. are all concepts of past. While the solar panels continue to spread and cover the rooftops, radio towers and land there are others working on raising the bar even further. "A single kilometer-wide band of geosynchronous Earth orbit experiences enough solar flux in one year to nearly equal the amount of energy contained within all known recoverable conventional oil reserves on Earth today." Sounds like a quote from a scifi novel but may soon be a reality in near future.

Below is an article from CNN that talks about an interesting concept which may very well come true one day.

Cnn.com - How to harvest solar power?


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Solar tabbing ribbon typical consists of 10-15 micrometers of solder alloy coated on OFHC or ETP copper strip. SN60, (60% tin and 40% lead) or SN62, (62% tin, 36% lead and 2% silver) typically are the two standard lead-containing solder alloy choices. Both solder alloys have excellent wettabilty and have a history of use dating back to the early days of radio assembly, and therefore the reliability is well established for both alloys.

Inherent solder alloy wettability is particularly important because the silver thick film metallization on photovoltaics is more difficult to wet, compared to the metallizations on printed circuit boards used in electronics assembly.

Since the implementation of RoHS, a European directive that severely restricted the use of lead containing solders in Europe in 2006, the use of lead-containing solder in electronics has rapidly declined. In general, lead-free solders have poorer wettablity compared to lead-containing solders and the wettablity varies among different lead-free solder alloys. Of the several lead-free solder alloys available, 96.5% tin, 3.0% silver and 0.5% copper has been become the defacto standard in electronics assembly. Commonly known as SAC305, this alloy has better wettability and a lower melting point compared to SN96, (96.5% tin and 3.5% silver), the lead-free solder currently most commonly used today in tabbing ribbon. So the question is why isn’t SAC305 used more often in solar tabbing ribbon?

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It was great to learn that the team at NREL have broken their own record and achieved 19.9% efficient CIGS solar cell.

http://www.nrel.gov/news/press/2008/574.html

Kudos to the team at NREL for this remarkable achievement.

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Apart from independence from silicon, thin-film solar cells have several advantages. On perfect day and perfect conditions crystalline silicon solar cells would out-perform thin-film solar cells in terms of efficiency. Some of these perfect conditions include direct sunlight, angle of solar rays and operating temperatures. Weather conditions and time of the day / year have considerable impact on efficiencies of solar cells. Thin-film solar cells are usually more adaptable and resilient than their counterparts. Even though there have been issues regarding longevity of thin-film solar cells, these issues have been more related to their packaging and encapsulation. The intrinsic semiconductor layers are quite stable in varied conditions and thermal cycles. Several advanced materials and technological developments have increased the life of thin-film solar cells. That’s why I believe thin-film solar cells are here to stay and not just a stop-gap alternative to the shortage of silicon.

Image: Innolas.com]]>

It was interesting to learn that CIGS semiconductor is catalyzing breakthroughs in advanced image sensors for digital cameras.

According to The Nikkei Business Daily (Tuesday, Feb 5 ’08 edition) National Institute of Advanced Industrial Science and Technology and Rohm Co. have jointly developed a new image sensor by fabricating a thin film of copper indium gallium di-selenide (CIGS) above the silicon substrate. The inventors were able to overcome the current leakage problem associated with CIGS. This sensor is more than six times as sensitive as a conventional silicon-based device and is also capable of detecting light across a broader spectrum, from visible light to near infrared light (up to a wavelength of 1,300 nanometers). 90% of the surface of this sensor is capable of detecting light which is triple the normal amount.

According to Nikkei “this combination of features can boost the shutter speed of digital cameras and provide the kind of nighttime vision useable for monitoring cameras and car-mounted safety systems”.

Image: www.tribcsp.com

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Many people do not consider either semiconductors or solar cells as having any moving parts. But this is not completely true. Indeed, every solar cell, and semiconductor has parts which are moving all the time. Not only the atoms, electrons and holes are moving but also the different layers that form the solar cell. There is continuous movement and diffusion between layers in response to temperature and pressure. These diffusions and movements are quite substantial during the actual formation of the solar cell.

Selection of appropriate assembly materials that can be processed within the thermal budget of various layers that form a solar cell is extremely important. Proper selection of high quality assembly materials (such as sputtering targets, metallization pastes, tabbing ribbons, fluxes and solders) has a direct correlation to increased efficiency and usable life of a solar cell. We at Indium Corporation are continuously applying thought to uncover different ways to help make our customer’s solar cell more efficient and reliable.

If you are looking at pushing efficiencies beyond 20% and reliability over 30 years, call us to find out – how we can synchronize your solar cell.

Image: Coeshow.com

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