The ultimate goal for a solar panel is to reach grid parity. In order to achieve this goal all the players in thesolar PV value chain have to do their part to lower the total system cost. The common view is that solar cell manufacturers play a critical role in achieving this goal. All the cost savings, design changes and performance improvements that a solar cell manufacturer can achieve has a multiplying effect across the value chain. This is a true statement, but the key enabling factors for all these improvements are advanced manufacturing and assembly materials.
One such example is a CIG Target for manufacturing of CIGS solar cells. Sputtering as we all know is a high throughput and high precision process. But this process requires the availability of high quality CIG target. A CIG target not only needs to be dimensionally accurate and highly dense but also needs to have homogeneous composition and grain sizes throughout. Moreover, adding to this complexity all the phases need to be tightly controlled inorder to obtain uniform sputtering. Innovations in hybrid consolidation processes have enabled high volume production of CIG sputtering targets bringing CIGS solarcells close to commercialization.
23rd European Photovoltaic Solar Energy Conference and Exhibition
The European Photovoltaic Solar Energy Conference and Exhibition is a premier venue for learning and networking in the Solar Industry. It has been one of the early shows dedicated to the solar industry.
This year the event will be held at Feria Valencia in Valencia, Spain. The event starts on Sept 1st and ends on Sept 5th.
Below is the link to find more information,
23rd European Photovoltaic Solar Energy Conference and Exhibition
Indium Corporation will be exhibiting at the Hall 3 / B-2. Indium Corporation will also host a press conference at 2:00 pm on Tuesday, Sept. 2nd. The press conference is focused at discussing our ability to support the industry with > 5 GW / year of solar PV production with a wide variety of materials including indium and gallium.
Some of the products we would be showcasing at the show are,
- OnSpec® LTTF-6363 Metallization Paste
- OnSpec® Tabbing Ribbon
- OnSpec® CIG Alloy Sputtering Target
- OnSpec® Solar Grade Fluxes
I will be glad to meet and host you at our exhibit booth, please plan on stopping by.
The choice of a transparent conductive oxide (TCO) as the top contact of a CIGS PV cell is critical to the long term performance of the cell. While most PV applications are forced to match the warranties of roofing contractors...25-30 years..., the life of the cells over that time period can be called into question by the choice of the TCO.
Aluminum Zinc Oxide (AZO) is a common choice of TCO because of cost and relatively good optical transmission performance in the solar spectrum. However, if even the slightest amount of moisture penetrates the cell, the AZO coating reacts with the water and ceases to operate as a TCO, rendering the cell useless.
I have not been privy to all environmental life testing of CIGS solar cells, but my belief is that it is virtually impossible to create a flexible seal on a thin film CIGS cell that will withstand 25-30 years of storms on a rooftop in my neighborhood. And if you can't withstand the storms and moisture for 25-30 years, you should not use AZO. Your cell will not survive.
The main concern about another TCO, indium tin oxide (ITO) is the cost. ITO can be priced at several times that of AZO. However, ITO does consistently defeat AZO in almost every performance category including chemical resistance to moisture. ITO is not affected by moisture and it can survive in a CIGS cell for 25-30 years on a rooftop.
While the sputtering target or evaporative material that is used to deposit the ITO is significantly more costly than AZO, consider that the amount of material placed on each cell is quite small. Therefore the cost penalty per cell is quite small too.
It is my belief that the cell lifetime benefits of ITO greatly outweigh the cost penalty of the material and I strongly recommend to all CIGS manufacturers that they use ITO as their TCO.
Plating Bath
Copper-indium-gallium-diselenide is one of the more promising thin film photovoltaic solar cell technologies. The "gold standard" for depositing the absorber layer in this photovoltaic is evaporation, and the current champion efficiency of 19.9% was achieved by vacuum evaporation of the absorber layer at NREL. Several companies are in pilot production of CIGS thin film photovoltaic solar cells using evaporation as well as sputtering, another physical vapor deposition (PVD) process.
Since both evaporation and sputtering require expensive and complex high vacuum equipment, other CIGS manufacturers are exploring non-PVD processes such as mixed oxide, mixed selenide or metal alloy nanoparticle printing. Another interesting, but less researched non-PVD process is electroplating. Controlled thicknesses of indium, copper, gallium and selenium can be sequentially be deposited onto a substrate using the respective individual plating bath, and the multilayer stack fused to form the CIGS alloy.
However, it would be ideal if the CIGS alloy could be electroplated in a single step from one plating bath containing all metals. Such alloy electroplating is relatively straight forward, if the individual metals have similar electropotentials. For example, tin and lead have similar electropotentials, and the electroplating of 60% tin and 40% lead solder alloy from one solution is routine. However, copper, indium, gallium and selenium all have varying electropotentials. While the development of a CIGS plating bath is technically possible by the proper selection of chelating/complexing agents and other chemical additives, developing the formulation chemistry to produce a stable and robust production electroplating bath presents a challenging task. The company who meets this challenge will have a winning process.
LTTF-6363 Metallization Paste
Other Metallization Paste
One of the ways to increase the power output of a solar cell is to have a right balance between the number and size of conductive transmission lines on the top side (sun facing side) of the solar cell. Having the right number of transmission lines is quite easy but having consistency in the dimensions of the transmission lines is quite challenging. Even though the transmission lines serve the purpose of carrying the current, oversized and inconsistent transmission lines cause shadowing of the solar cell reducing the effective output.
Ability to screen print a metallization paste with <100 microns line width is half of the problem. The other half is using a low slump metallization paste. Here are two cross sectional images of the cured metallization past which forms a transmission line. Both paste are equivalent in terms of conductivity and contact resistance but one has higher slump factor than other. The shadowing caused by the high slump metallization paste can be easily observed (almost 30% more shadowing than the low slump paste).
In conclusion the cells made with low slump metallization paste will always have higher peak watt output and aggregate power output as compared to the equivalent ones made with high slump paste.
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?
Indium Corporation Fluxes
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.
Computer Brain Vs. Solar Photovoltaic
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.
Solar PV in Space
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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Tabbing Ribbon
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?
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.
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. Thats 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
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
Following the recent surge in the prices of crude oil futures and the rally of solar stocks, we are stuck with wide range views on the future of PV. How large will the market be in 2010? Will the shortage of materials ease or even turn into a glut? I believe the answer to most of these problems lies in the solar cell itself – conversion efficiencies.
But there are several challenges that one has to face to increase the conversion efficiency. Every slight improvement in the cell and its assembly makes a difference. The morphology of the absorber layer, the transparency or electrical conductivity of the top conductor layer, the interface resistance and line resolution of the metallization paste etc.; each of them counts. Depending on the starting point every 1% increase in solar conversion efficiency can result in 6%-20% increase in production capacity. In fact the incentives for increasing efficiencies are even higher for thin film solar manufacturers as compared to the established Si players. Indium Corporation tends to work extremely closely with customers to take care of every minute detail that helps in increasing conversion efficiency and longevity of solar cell. Looking at the opportunities in several steps of the manufacturing process and the acceleration of technological progress, I am sure the there is no stopping back on solar conversion efficiencies.
Image: NREL.gov
Last week I attended the Tech Valley Energy Forum at the College of Nanoscale Science and Engineering at Albany. The keynote speakers were Patrick J. Curran, Executive Director of the Energy Association of NYS and Gavin Donohue, President of the Independent Power Producers of New York. Although the forum was government policy related, it was encouraging to hear that the NY state government is actively trying to overcome the issues that are obstructing the adoption of solar photovoltaic in New York. In the short term solar industry definitely needs support from the governments in developed countries. Rebates, Tax credits, Feed-in-tariffs are some of the tools policymakers are using to support the solar photovoltaic industry. In the long term the solar industry needs to take advantage of this support, innovate, learn, grow and reduce costs. Personally I am glad that the government policies in various states in USA are slowly gathering momentum.
In the meanwhile Indium Corporation continues to help several solar players across the world with it's advanced materials and technical expertise. The goal being reduction of the total system cost through higher solar conversion efficiencies, lower system downtime and longer lasting solar panels.
Here are few links that are insightful about the rebates, incentives and savings estimate in USA.
Image: E2tac.org
Someone asked me to define the current state of the Solar Photovoltaic industry and I could think of two words evolving and explosive. These two words would describe the processes, needs, and markets of the solar industry. To add to the excitement there are unique challenges that every company in the industry has to overcome. This is probably the reason the industry is constantly attracting talent from various other mature industries.
Having been involved with the solar industry for last 20 years and having supported the infancy to maturity of various other industries; Indium Corporation is well equipped in this space. Whether it is about sputtering targets made from challenging materials or top conductor metallization pastes which need to perform over 20 years, Indium Corporation's scientists and engineers are leaping ahead every day. With the launch of several new advanced materials this year and the capacity rampup we have demonstrated our support. But looking at the explosive growth of the industry I know it's just a beginning.
Image: About.com
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