Putting renewable back into renewable energy

Salem Zahmi grew up under a very hot sun. So it’s no wonder that this grad­uate stu­dent from the United Arab Emi­rates is studying solar energy. He hopes to con­tribute to a growing research cam­paign in the UAE that is looking to take advan­tage of the country’s renew­able resources as it rec­og­nizes the unsus­tain­able nature of an oil-​​based economy.

Zahmi, who works in the research lab of chem­ical engi­neering pro­fessor Eliz­a­beth Pod­laha, is looking for ways to make solar panels less expen­sive, and, believe it or not, more sus­tain­able. This weekend, he pre­sented new find­ings in this endeavor at the 221st meeting of the Elec­tro­chem­ical Society in Seattle.

The most effi­cient solar cell on the market today uses four ele­ments in its so-​​called “p-​​type” (that’s the pos­i­tive layer of a solar cell, which has the job of trans­porting free elec­trons to the elec­trode). These ele­ments are copper, indium, gal­lium and sele­nium, giving this par­tic­ular type of solar cells the name CIGS.

Gal­lium and indium are both expen­sive, said Zahmi, and rather hard to come by. Just like oil, there isn’t a huge supply of indium in the world.

Zinc and tin on the other hand, are cheaper and more abun­dant. So, researchers have attempted to replace the gal­lium and indium in CIGS solar cells with zinc and tin, giving this new type of cell the name CZTS. But the problem now is that the effi­ciency of these cells isn’t all that great.

Zahmi sus­pects that by adding a little bit of indium back into the CZTS system, he might be able to increase effi­cien­cies by a sig­nif­i­cant amount, a sort of com­pro­mise between cost/​sustainability and effi­ciency. After all, what’s the point of making these things if they don’t work that well? If it takes a bazil­lion solar cells to do the job of one gas tank, it might not be as good for the planet and the pock­et­book as it sounds.

His efforts in this area have so far been suc­cessful. At the ECS meeting this weekend, he showed that using a com­bi­na­tion of indium, copper, and tin gives a higher “reduc­tion rate” than each of them alone or any other duet, so to speak.

The reduc­tion rate is impor­tant because it tells you how fast the p-​​type will be able to grab elec­trons and move them over to the elec­trode. Higher reduc­tion rates should trans­late into higher solar cell efficiencies.

Zahmi also explored another area of solar cell pro­duc­tion, which has to do with the way in which the mate­rials are deposited. They can be deposited all together in one fell swoop, or as indi­vidual films stacked one on top of the other. They can also be deposited with or without a “com­plexing agent,” which can, as the name implies, cause the var­ious ele­ments to aggre­gate together.

Zahmi found that co-​​depositing tin and indium together enhances the overall reduc­tion rate of the indium, meaning that Zahmi may be on to a method that is both more effi­cient (as implied by the higher reduc­tion rate) and less expen­sive (since less indium is required), than the CIGS method.

Finally, Zahmi looked at the impact of the com­plexing agent on the dif­ferent ele­ments across a gra­dient of pH ranges. When ele­ments are bound up in a com­plex, they aren’t of much use in the whole system. From his cal­cu­lated pH data, Zahmi devel­oped a model that pre­dicts the amount of each ele­ment at a given pH. Since the com­plexing agent has a direct impact on the pH, Zahmi could poten­tially use this model to iden­tify the ideal pH con­di­tions (and thus amount of com­plexing agent required) to tailor the system to have exactly the right con­cen­tra­tion of each ele­ment, thereby opti­mizing the effi­ciency through the simple addi­tion of citric acid, for example (one such com­plexing agent).

At this point it’s still too soon to say whether adding indium back into the CZTS system has any ben­efit.
But the team hopes their depo­si­tion efforts will pro­vide a cheap alter­na­tive to pro­duce either CIGS or CZTS mate­rials, said Pod­laha. The ulti­mate goal is to pro­vide a common solu­tion, which engi­neers could draw from, mixing and matching dif­ferent com­bi­na­tions of these ele­ments in the search for a better solar panel.

If this doesn’t make sense, please fire off some ques­tions. Zahmi will also be pre­senting even more results at the October ECS meeting, so stay tuned!

Photo: Living Off Grid, “Solar Panels” April 6, 2008 via Flickr. Cre­ative Com­mons Attribution.