Nanosolar Solar Cells:
Cheaper than Milk?

Enter gallium, literally.  Some breakthroughs in science are well planned, others are accidents, and this was thought to be one of the accidents.  In one manner or another, it was found that the presence of the element gallium, replacing some of the indium in the CIS structure, stabilizes the desired phase without degrading the electronic or optical characteristics.  Now, the material becomes CIGS and no longer has the instability problem and in fact has some enhanced properties like a more optimum response to the sun’s spectrum here on earth.

Another issue related to the inability of getting a good p-side contact. Because of the low mobility of the p-type carriers and the inability to get a really high carrier density through native defect generation, making a good ohmic contact with the material, one which could carry a high enough current to make the device commercially feasible, was problematic.  A doping problem... a material scientist’s nightmare!

Enter sodium.  This time from the use of sodium bearing glass substrates.  It was thought (and I’m not sure if it has ever been actually substantiated) that the presence of sodium ions actually passivate (render harmless) stray bonds within the structure that caused some desirous impurities to become electrically active i.e. for p-type contact formation.  We will bump into the passivation issue again in discussing the silicon scramble.

So, by the early part of this decade, CIS became CIGS, gallium stabilized and passivated with sodium ions and all seemed to be working pretty well but still several years before large scale integration would be feasible.

That is, until the renegade Nanosolar and it’s bevy of talented whippersnappers came to town!
 

Nanosolar, the New Technology

As you may imagine, Nanosolar is quite vague about technologic details.  Vague is hardly the correct word here.  Nanosolar’s technology is more secret than the true ingredients in pepperoni. No one really knows what pepperoni is actually made of, and, outside of Nanosolar, few, if any, really have a clear grasp of their technology, of what actually goes on inside that great big Chamber of Surprise.  So secret is the technology that the company won’t even release product specifications without a non-disclosure agreement in place.  Hmmm! That’s harsh.  Initial deliveries are scheduled to European power producers and will be gated, fenced, guarded, and otherwise cut-off from prying eyes, so says Nanosolar’s CEO, in an effort to protect their intellectual property rights beyond mere patents.

Here’s what we do know: Gathered from press releases, it is clear that they employ a print-like deposition of a thin layer of a highly solar absorbent alloy of copper indium gallium diselenide (CIGS) onto a conductive but flexible aluminum foil like Reynolds Wrap.  The foil is coated with molybdenum or, by the time the film cools to room temperature, molybdenum diselenide (MoSe2).  Nanosolar refers to this layer as the bottom contact.  Molybdenum diselenide is used in other CIGS technologies for micro-structural stability and oriented grain growth.  It’s the interface between the flexible substrate and the thin-film.  In addition to micro-structural control this layer might be used to keep the thin-film from spalling off or delaminating under the varied environmental conditions a flexible solar cell may find itself.

The CIGS absorber material is produced from a carefully formulated (and highly, highly, secret) concoction of nanoparticles suspended in an organic solution that has ink-like characteristics.  It could very well be the key to the whole shebang.  Presumably, it sprays quickly, spreads out uniformly, and drops nanosized grains of CIGS at the proper concentration while the organic binder is most likely driven off prior to, or possibly in conjunction with, the next processing step somewhere in the Chamber of Surprise.  Once printed, the structure is sandwiched with a transparent (to the wavelengths of light absorbed by the CIGS layer) and mysterious PN junction layer.  Not much is said about this PN layer and it is my best guess that it is another CIGS layer with a different doping level that creates the junction.  But with whippersnappers in the research lab, you just never know.  Above that is the top contact layer, a transparent a zinc oxide film, standard fare in the CIGS world except new innovation extends the current carrying capacity dramatically making the overall structure more efficient.  The entire bundle is pressed and rolled onto rollers at several hundred feet per minute, a speed absolutely unheard of in semiconductor processing, which equates to many miles a day of solar cell strips. That’s a good bundle of solar cells.

Note that there is no mention anywhere of processing temperatures, pressures, and gaseous environments, the things that really count in process development circles... the parameters that take years of trial and error to optimize.

If the equipment is impressive, the innovation is sublime. But to put things in perspective, especially in terms of unexpected Murphy’s Law type things that can happen, we should look briefly at the new side of silicon.

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