Solar cell efficiency round-up: silicon, CIGS, and sonics

New Hampshire, USA -- During the past few weeks we've been tracking some announcements about solar cell efficiency improvements, both for silicon and thin-film PV. While research continues to push both traditional and newer materials' performance, let's pause to scan the latest announcements.

For reference here's NREL's multistrand solar-cell efficiency chart, one of our favorite solar energy graphics, showing the progression of every solar energy technology over the past four decades.

Choose Your Silicon: P-Type or N-Type

Fraunhofer ISE says it has created silicon solar cells using n-type material that achieves 24 percent conversion efficiency. That's slightly less than a percent gain over the past four years.

Most of today's solar cells use boron-doped p-type silicon, mainly piggy-backing off of what the semiconductor industry has been using for years, but that specific type of silicon material isn't ideal for solar cells; it's susceptible to defectivities and impurities that lead to more recombination and gradually degraded conversion efficiency. N-type silicon, doped with phosphorous, is less susceptible to light-induced degradation and recombination from metal impurities, making it a better-quality material that can achieve higher conversion efficiencies. Passivation with n-type silicon doesn't work with conventional materials (e.g. silicon dioxide), but that's being addressed by switching to different materials such as aluminum oxide.

ISE researchers now say also they have devised a better way to construct the rear contact of this high-efficiency solar cell. Metal contacts patterned on the backside of solar cells limit solar cell efficiency (they take up space that otherwise harvests sunlight). One way to do this is passivated emitters and rear-cell (PERC) structure that minimizes the area of the metal contact. ISE researchers say they've come up with a new selective passivated contact, called tunnel oxide passivated contact ("TOPCon") that combines an ultrathin tunnel-oxide and thin silicon layer to contact the entire rear area of the solar cell, allowing more charge carriers to pass through and fewer carriers to recombine. They presented their work this fall at EU PVSEC in Paris.

Simulated current density distribution and current flux for a solar cell with local rear contacts (left)
and for TOPCon with a passivated rear contact covering the entire surface. Credit: Fraunhofer ISE

Meanwhile, fellow European research center imec is pushing ahead with p-type solar using the aforementioned PERC emitting process and a new laser doping step to make a thin aluminum oxide (Al₂O₃) layer for the local back surface field (BSF), a lower-temperature process vs. the typical firing step that avoids degradation of the rear layer material. (The thinly deposited Al₂O₃ can act as the passivation layer and doping source, meaning the laser both does contact patterning and forms the local BSF. another step-saver.) A nickel/copper plating process was used to form the front contact. The cell's average conversion efficiency topped 20.2 percent, on a standard-sized 156 × 156 mm cell, with fill factor of up to 80 percent indicating "excellent contact quality," they claim.

Jozef Szlufcik, silicon PV program director at imec, called the work "a substantial simplification of the i-PERC manufacturing process" and "an important step towards reducing the cost-of-ownership of i-PERC technology."

imec and partners described these technologies and steps also at this year's EU PVSEC.

CIGS Improvements Served Up Two Ways

The new record holder for thin-film CIGS (copper indium gallium di-selenide) is in Germany: the Zentrum für Sonnenenergie und Wasserstoff-Forschung Baden-Württemberg (ZSW, Center for Solar Energy and Hydrogen Research) have made a 20.8 percent efficient cell, a few ticks above their previous mark of 20.3 percent set in August 2010. The new mark also beats the best performance of 20.4 percent for multicrystalline solar cells, points out Michael Powalla, head of ZSW's photovoltaics unit. ZSW and Manz proclaim commercial CIGS modules with "16 to 18 percent" efficiency could be possible in roughly four years, vs. today's 14-15 percent efficient CIGS modules.

Key to this new mark was a simultaneous evaporation process codeveloped with German equipment supplier Manz, which holds exclusive right to implement the process in its CIGS line in Schwäbisch-Hall and scale it up. The two state somewhat less solidly, though, that the process "in principle can be transferred into industrial production processes."

ZSW isn't focusing just on CIGS, though — they're also developing new materials to push the efficiency of similar thin-film solar cells that use more earth-abundant Kesterite materials, in this case tin and zinc, achieving a 10.3 efficient cell, close to the 11.1 percent world record cell but with a simpler process.

Swiss institute EMPA, meanwhile, is coming at thin-film CIGS from a different direction, saying it's made 20.4 percent efficient cells on plastic foils that would allow for more streamlined roll-to-roll manufacturing. In their work published in the journal Nature Materials, the researchers describe how they added sodium and potassium fluoride to the CIGS layer to alter the material's chemical composition, reducing the thickness of the CdS buffer layer without losing its electronic properties. That hiked the cell's efficiency by nearly two full points from the team's previous mark of 18.7 percent set in 2011 — and at 20 percent and higher the cells "compete with the best polycrystalline silicon cells," according to the group. They also note that their process uses lower temperature processes and a non-rigid substrate than the ZSW CIGS process described above.

A New "Record" for Solar PV

While others tinker with solar cell designs, materials, and manufacturing processes to tweak their conversion efficiencies — what if the answer lies at the other end of the proverbial spectrum, with the input to the device itself? And we're not talking about sunlight, we're talking about party rock.

Researchers at Queen's Mary University and Imperial College London built a solar cell in their labs using zinc oxide nanorods grown in a solution, and covered with a solution-deposited active polymer. Then, turning to the concept of piezoelectrics — applying pressure or strain, or more commonly some kind of vibration, to manipulate voltage output — they thought to hit the material with soundwaves to see what would happen to the power output. They admittedly weren't expecting much, but to their surprise it actually worked — dull flat sounds didn't do much, but playing music with different sound pitches did.

Ultimately they determined that sound levels as low as 75 decibels, equivalent to roadside ambient noise or an office printer, could "significantly" improve the solar cell's performance — up to a 40 percent increase in efficiency. And pop music made the biggest difference of all, which they attribute to the material's sensitivity to higher-pitched sounds that the pop music had in spades. (No word about what they counted as "pop" music, if Robin Thicke spiked the cells’ activity, or if Justin Bieber made them shrivel up inside.)

Ultimately they're not going after the music business, though — they see a market for small devices that could be powered by soaking in the acoustic vibration energy of their surrounding environments, such as air conditioning units, vehicles and even video advertising displays in trains. Their work was published online in Advanced Materials.

http://www.renewableenergyworld.com/rea/news/article/2013/12/solar-cell-efficiency-round-up-silicon-cigs-and-sonics?page=4