Power/Performance Bits: Aug. 1

Concentrating photovoltaics
Engineers at Penn State University and the University of Illinois at Urbana-Champaign tested a new concentrating photovoltaic solar system, which they say can produce over 50% more energy per day than standard silicon solar cells.

In contrast to silicon solar panels, which currently dominate the market at 15 to 20 percent efficiency, concentrating photovoltaics (CVPs) focus sunlight onto smaller, but much more efficient solar cells like those used on satellites, to enable overall efficiencies of 35% to 40%. Current CPV systems are large — the size of billboards — and have to rotate to track the sun during the day. These systems work well in open fields with abundant space and lots of direct sun.

“What we’re trying to do is create a high-efficiency CPV system in the form factor of a traditional silicon solar panel,” said Chris Giebink, Assistant Professor of Electrical Engineering, Penn State.

To do this, the researchers embed tiny multi-junction solar cells, roughly half a millimeter square, into a sheet of glass that slides between a pair of plastic lenslet arrays. The whole arrangement is about two centimeters thick and tracking is done by sliding the sheet of solar cells laterally between the lenslet array while the panel remains fixed on the roof. An entire day’s worth of tracking requires about one centimeter of movement.

The concentrating photovoltaic system showing the top lenslet; the little black square visible near the middle is the solar cell and the lines running away from it are the contact wires. Source: Giebink Lab, Penn State)

The CVP prototype contained a single microcell and a pair of lenses that concentrated sunlight more than 600 times and was tested over an entire day alongside a commercial silicon solar cell.

The CPV system reached 30% efficiency, in contrast to the 17% efficiency of the silicon cell. All together over the entire day, the CPV system produced 54% more energy than the silicon and could have reached 73% if microcell heating from the intense sunlight were avoided, according to the team.

Giebink notes that major challenges still lie ahead in scaling the system to larger areas and proving that it can operate reliably over the long term, but he remains optimistic.

“With the right engineering, we’re looking at a step-change in efficiency that could be useful in applications ranging from rooftops to electric vehicles — really anywhere it’s important to generate a lot of solar power from a limited area.”

Antireflective solar cells
Researchers at Osaka University found a way to increase light harvesting and power conversion efficiency in solar cells by modifying the cells’ surface texture to reflect less light, without significantly adding to the cost.

“Unmodified silicon solar cells throw away light energy in the form of reflection, so most solar cells have some kind of antireflective coating,” said first author Daichi Irishika. However, traditional antireflective coatings are expensive to produce, especially for covering large areas.

“To avoid using these extra coatings we fabricated a submicron structure using a simple wet treatment directly into the silicon surfaces to give the cell its own antireflective coating,” said Irishika.

Ultralow reflectance polycrystalline silicon wafers before and after treatment. (Source: Osaka University)

Previously, the team made low reflection silicon cells using a much cheaper process based on the surface structure chemical transfer (SSCT) method to fabricate so-called black silicon. Chemically treating the front side of silicon cells produces tiny submicron silicon structures, which prevent light reflection and give a black appearance. The team has also developed a method to passivate the submicron silicon structures with huge surface area to prevent the recombination loss by deposition of phosphosilicate glass followed by heat treatment. This method can simultaneously form pn-junction to separate photo-generated electrons and holes, and therefore isn’t an additional process.

Building on their method, the team created rougher light-trapping microstructures on the back side of the silicon cells to capture even more infrared light.

According to Hikaru Kobayashi, a professor at Osaka, “Making very high efficiency solar cells is important but we should also consider the economics and practicality of any processes used to increase efficiency. The wet processes we have developed are simple yet effective, and our work with black silicon has real-world applications in making cost-effective silicon solar panels.”

New perovskite record
Researchers at UNIST, Korea Research Institute of Chemical Technology (KRICT), and Hanyang University set a new record for perovskite solar cell efficiency, reaching performance of 22.1% in small cells and 19.7% in 1 square centimeter cells.

The previous record was 21.1% efficiency, set by a team at EPFL.

The key behind the latest achievement was a method of reducing defects in inorganic-organic hybrid perovskite solar cells (PSCs).

The formation of a dense and uniform thin layer on the substrates is crucial for the fabrication of high-performance PSCs. The concentration of defect states, which reduce a cell’s performance by decreasing the open-circuit voltage and short-circuit current density, needs to be as low as possible.

The team carefully controlled the growth conditions of the perovskite layers, introducing additional iodide ions into the organic cation solution, which are used to form the perovskite layers through an intramolecular exchanging process, decreasing the concentration of deep-level defects.

“The key to manufacturing high-performance solar cells to reduce defects in materials that generate energy loss when converting sunlight to electricity,” said Sang-Il Seok, Professor of Energy and Chemical Engineering at UNIST. “Our study presents a new method that suppresses the formation of deep-level defects, thereby setting a new record efficiency for PSCs.”