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TCO choices for CIGS manufacturing
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.
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CIGS Absorber Layer Electroplating
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. -
No Slump Metallization Paste
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.
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Meet the Bloggers
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
- Wafer Flux Spin-Coating Topography
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CIGS - Can sputtering make a breakthrough?
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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Fluxes for Soldering Tabbing Ribbon
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.
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Computer Brain vs. Solar Photovoltaic
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.
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Beam it down from space
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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Selection of the Optimum Lead-Free Solder for Solar Tabbing Ribbon
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?
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Record Makes Thin-Film Solar Cell Competitive with Silicon Efficiency
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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