Novel Techniques Examine Solar Cells with Nanoscale Precision

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Using dual novel techniques, researchers during a National Institute of Standards and Technology (NIST) have for a initial time examined, with nanometer-scale precision, a variations in chemical combination and defects of widely used solar cells. The new techniques, that investigated a common form of solar dungeon done of a semiconductor element cadmium telluride, guarantee to assist scientists in improved bargain a tiny structure of solar cells and might eventually advise ways to boost a potency during that they modify object to electricity.

Schematic of cadmium telluride examined by a photothermal prompted inflection technique. Image credit: Yoon et al./NIST

Even yet customary methods to impersonate solar cells have prolonged proven useful in running their phony and design, these evidence collection “give us usually a singular bargain of because a inclination work during sub-optimal efficiency,” pronounced NIST physicist Nikolai Zhitenev. For instance, nonetheless a process famous as electron-beam prompted current, that analyzes samples regulating a lamp of an nucleus microscope, provides information on nanoscale variations in solar dungeon efficiency, it gives tiny information on a underlying clear defects and impurities that reduce efficiency. Two other methods, photoluminescence and cathodoluminescence, that satisfy light glimmer from a samples, yield usually deficient or surreptitious information on a mechanisms of potency losses.

To tighten that believe gap, “we’ve now grown new techniques to inspect a microstructure of solar cells and demonstrated that we can daydream defects by their visual signature,” pronounced lead author Yohan Yoon of NIST and a University of Maryland in College Park. He and his colleagues during NIST, a University of Maryland and a University of Utah described their work in a new Nanoscale paper.

Topographic picture (a) of a skinny cut of a cadmium-telluride representation and images (b-f) of same cut taken with a direct-transmission near-field scanning visual microscopy technique. Image credit: Yoon et al./NIST

In their study, a scientists used dual interrelated methods that rest on an atomic force microscope (AFM). Photothermal prompted inflection (PTIR) provides information on a solar cell’s combination and defects during a nanometer-size scale by measuring how most light a representation absorbs over a extended operation of wavelengths, from manifest light to a mid-infrared. The other method, famous as direct-transmission near-field scanning visual microscopy (dt-NSOM), creates minute nanoscale images that constraint variations in a combination of a solar cells and defects in their structure by recording how most light is transmitted during specific sites within a cell. The process produces crook images than PTIR.

The setup for PTIR, fabricated by NIST researcher Andrea Centrone, resembles a finely tuned chronicle of a Rube Goldberg contraption. First, light pulses from a laser irradiate a representation of cadmium telluride. When a representation absorbs a laser light, it heats adult and expands. The enlargement nudges a pointy tip of an AFM that is in hit with a sample. The tip translates a heat-induced enlargement into automatic motion, causing a cantilever on that it is mounted to vibrate. Finally, a quivering is rescued by bouncing light from another laser off a cantilever into a AFM detector.

Because a border of a cantilever’s vibrations is proportional to a appetite engrossed by a cadmium telluride sample, PTIR measurements yield pivotal information about a material. For instance, when a tip is hold during one plcae on a representation though a wavelength of pulsed laser light is varied, PTIR generates information on a spectra of deviation that is engrossed during opposite points along a sample, with nanoscale resolution. When a AFM tip moves over a representation though a laser’s wavelength stays fixed, PTIR yields an fullness map of a element that reveals variations in chemical combination from one partial of a representation to a other. Notably, a tiny distance of a examine tip provides fullness information with a spatial fortitude smaller than a laser wavelength used in a experiments.

In a dt-NSOM technique, light from a pointy tip of an AFM examine illuminates a tiny partial of a sample. A photodetector in hit with a representation measures a volume of light transmitted by a element as a examine scans over a sample.

Critical to a success of a dual techniques, records Zhitenev, was not usually entrance to an AFM, though to other modernized apparatus accessible during NIST’s Center for Nanoscale Science and Technology (CNST), where a experiments were performed. This includes a focused ion lamp that could cut slices of a cadmium telluride element a small 350 nanometers (350 billionths of a meter) in thickness.

“Without a ability to obtain such ultrathin slices, it would not have been probable to take full advantage of a high-resolution techniques and exhibit excellent sum of a solar dungeon material,” he said. The wavelengths of light used to examine a semiconductor would routinely dig to a abyss of several millionths of a meter. By slicing slices thinner than that depth, a density of a slices determines a effective spatial fortitude of a analysis, explained Zhitenev.

The experiments showed that defects in a clear arrangement of a element are associated to impurities in a chemical composition, propagated along and from a bounds between adjoining clear grains. The group also demonstrated that techniques can magnitude a spatial movement of supposed low defects in cadmium telluride samples. These defects, that means electrons and holes (positively charged particles) in cadmium telluride and other semiconductors to recombine instead of generating electricity, are one of a pivotal reasons that solar cells do not perform as good as fanciful models.

Although a new measurements are presented as a explanation of judgment in study cadmium telluride, a well-characterized material, a commentary “are of extended qualification and will assist solar dungeon research, heading to a improved bargain of a accumulation of photovoltaic materials, and consequently, operative them for larger efficiency,” a researchers concluded.

Source: NIST

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