James Montgomery, Associate Editor, RenewableEnergyWorld.com
December 03, 2013 | 0 Comments
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."