In response to my first blog, Abel wrote: "I would like to know the present and future of A-Si efficiency, if this technology can get a better efficiency and how it could be possible."
Those are good questions. If you ask a number of people, you will get a number of different answers. There is a hesitancy to give hard numbers about efficiencies. This is understandable. Amorphous may be less efficient under ideal conditions, although in low light situations, cloudy, morning, evening etc, amorphous compares favorably with other technologies. Perhaps, if hard numbers are needed, one should stick to specific power and shy away from efficiency.
But Abel asked about efficiencies, so here are mine. Based on what I know, I would have to state that roll to roll mass produced, large (inside frame) aperture area, and weather stabilized, tandem a-Si modules on polyimide substrates would be hard pressed to be higher than about 7% efficiency in the near term. If the a-Si layers are tandemed with micro-X-Si, then 10% is possible, sometime in the future. (It's also difficult to compare micro-X-Si to X-Si material because of the reduced voltage output in the X-Si material. This is somewhat like comparing apples and oranges).
Note the qualifying statements: roll-to-roll mass produced; large aperture area; weather stabilized; tandem; polyimide substrate. These are all real world qualifiers.
Many if not most record setting efficiencies for a-Si, other thin film PV materials and
X-Si are done with laboratory, small area, as-deposited devices. The reader must be careful to find the weasel words, always, to make valid comparisons. Google solar cell efficiencies and check out articles like
http://en.wikipedia.org/wiki/Solar_cell
with graphs like Best Research-Cell Efficiencies:
http://en.wikipedia.org/wiki/File:PVeff%28rev100921%29.jpg
Here we see that the best a-Si multi-junction has stabilized out at about 12.5%, CdTe at about 16.7% and CIGS at about 20.3%. But a laboratory gadget does not a large area production module make.
It is interesting to highlight what it really means when news is released about cell efficiencies with CIGS or CdTe thin-films, or even X-Si. Specifically, analyze all the news about "record" efficiencies that are reached in lab and ponder how difficult and how long it would take to achieve in a manufacturing setting. For example, there are real world difficulties being seen at current manufacturers of CIGS to reach production scale. There are indications that some products on the market are under-performing the marketing claims.
I have a story to tell about my own personal involvement in CIGS research. Well, actually it was then CIS research (copper indium diselenide - CIGS minus the gallium). In the early 90's, before ITFT became PF, I became interested in CIS. We were largely a contract research company then. Thinking CIS fabrication had advanced to the cookbook stage, I applied for and won a Phase I NASA SBIR Award for CIS research on flexible substrates. Although we did make working solar cells, we quickly realized that there would be manufacturing difficulties with scale-up. When it came time to get the Phase II, we declined, much to the surprise of our NASA contract monitor, who said the award was pretty much ours for the taking. He had never had anyone turn down a Phase II award before! -- and probably not since either. It's not that we couldn't have used the funds. We just did not see the point in involving ourselves further in a technology where manufacture on polyimide would be difficult and where the raw materials (indium, and also gallium in the case of CIGS) would ultimately limit production volume, as compared to silicon.
Long term, ultra-high volume manufacturability is the Holy Grail. That is true for PV as well as other technologies. While there is no denying that a-Si production throughput is slower and its efficiency is lower than other technologies, still, it is a durable material with high production yield and the raw materials are very abundant. In short it is manufacturable on a very large scale. Manufacturing techniques are developing to improve the throughput as well. For example, this just crossed my radar:
New technique to help produce cheap solar cells 10x faster
http://www.solarnovus.com/index.php?option=com_content&view=article&id=2...
Whether this new modified process reaches its full potential again remains to be seen.
A-Si's lower efficiency is to be balanced against it's ultimate lower cost per watt based on raw material availability (vs. Te, In and Ga) in HUGE economies of scale.
Here I'm talking 100's of GWp per year. For example, the installed generating capacity of the US alone is around 980 GW (where W = Watts effective or Weff). For the PV equivalent with storage built in, and calculating on a 24 hour cycle, with an USA average of Weff = 6 x Wp (varying on climate, input/output power transformation, etc.), we would need 980 GW x 6 = 5880 GWp. If an a-Si PV module lifetime is 20 years, this means 294 GWp per year would need replacement. Worldwide installed electrical generating capacity in 2006 was over 4000GW, so using the same ballpark figures, this means 24,000 GWp of PV would be needed, or 1,200 GWp replacement per year.
If we are truly serious about a green/solar energy economy, there is no way that CdTe or CIGS will meet these numbers. Just google: tellurium supply ; then indium supply ; then gallium supply. Only solar modules based on the most abundant earth crust element, Si, will meet these goals. And since a-Si uses around 1/100 the amount of Si that X-Si does, the implications with scale are obvious: a-Si looks like it will be the "last one standing."
The nuclear fiasco in Japan, coupled with what appears to be the onset of peak oil, means hard choices are upon us all.
Hopefully, uranium/plutonium fission reactors will be phased out and replaced by thorium reactors -- non-weapons suitable/low radioactive waste/much safer-- (for the necessary 24/7 "storage"), in a final transition to a greener, PV-dominated energy supply.
Derrick P. Grimmer © 2011
