news digest ♦ Solar
continuously over a wavelength range of about 2000 nm. That broad range is the “super” in super- continuum.
Collimated output of the solar simulator illuminates a small solar cell. Electrical probes are used to measure cell efficiency
“The conventional light source for testing PV materials is the xenon arc lamp,” says Tasshi Dennis of the Quantum Electronics and Photonics Division at NIST’s Boulder, CO campus. “It has plenty of energy, a decent spectral match to sunlight after some shaping, and good uniformity. But its light is spatially incoherent - it is emitted in every direction - and thus quite difficult to focus or propagate. Moreover, it’s not ideal for testing recently developed multi-junction materials in which individual sections are tuned to respond only to a particular spectral band.”
Dennis’ co-worker, John Schlager, came up with the idea of exploiting a technology that had just become commercially available; a “super-continuum” white-light laser system. Dennis and Schlager produced a design that makes controllable spectral modifications to the super-continuum light and uses the output to illuminate different PV materials.
“From the start,” says Dennis, “there was one big question to answer: Does our light really look like the sun?” The answer to that question would depend critically on two factors.
The first factor is the pulsed nature of the light from a newly available laser system that produces a super-continuum beam in a two-stage process. First, light is generated in an optical, fibre-based, mode-locked, multi-watt laser that emits pulses of several hundred femtoseconds duration at a rate that is controllable between 1 MHz and 80 MHz. That output is then amplified and sent into a photonic-crystal fibre. In the crystal medium, non- linear effects cause the spectrum to broaden out
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www.compoundsemiconductor.net October 2012
Diagram of lab apparatus. The laser output is sent to a prism which splits the light into its constituent wavelengths. Masks are inserted into the prism beam to shape the spectrum by modulating certain wavelengths. The resulting spectrum is recombined and routed to a collimator and focused onto PV samples
“The shaped spectrum was a very good match to sunlight over our wavelength range,” Dennis says, “but we worried that the pulsed beam might not have a quasi-continuous effect. But we found that it produces photovoltaic responses that are very close to continuous xenon light. We also wanted to see if the samples were sensitive to the pulse repetition rate, so we tested them at 20 MHz, 40 MHz and 80 MHz. As it turns out, the variation in response was only about 1 percent in PV cell efficiency. So it appears that the pulsed nature of the light doesn’t matter for PV testing purposes.”
The second factor, still an ongoing concern, is the absence of ultraviolet (UV) light in the super-
“A key advantage of this method,” says Dennis, “is that the light from this fibre is single-mode” - that is, all the component frequencies have the same spatial distribution and form one single ray. That fibre output is then directed into a prism which splits the light into its spectral components and directs them at a mirror. Because the different wavelengths are spread out in space, inserting masks at selected points in the light before it hits the mirror will shape the spectrum to resemble sunlight by subtracting out specific wavelengths. The reflected light is then recombined into a single beam and focused onto PV samples.
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