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Power/Performance Bits: Sept. 18

Etching photovoltaics; solar panel degradation; photoelectrode for water splitting.


Etching photovoltaics
Researchers at Michigan Technological University and Aalto University found a way to reduce production costs of black silicon solar cells by more than 10%. The first prototype modules have been manufactured on an industrial production line.

Typically, the silicon used for solar cells is etched to reduce reflected light, although some light is still lost. Nano-texturing silicon creates ‘black silicon,’ which is much more efficient at capturing light. However, the current black silicon used in industry consists of shallow nanostructures that lead to sub-optimal optical properties and requires a separate antireflection coating.

The new approach to black silicon uses dry etching to create deep needle-like nanostructures that make an optically perfect surface, eliminating the need for the antireflection coatings. To reduce the effects of surface defects on electrical performance, the silicon is also treated with an appropriate atomic layer deposition (ALD) coating.

Black PERC solar cell without AR-coating produced at industrial line. Obs. the substrate is multicyrstalline silicon despite the rounded corners. (Source: Hele Savin’s research group / Aalto University)

Previously, the combination of dry etching and ALD has been thought too expensive for practical use. However, the team found that while production of individual black-Si passive emitter rear cells (PERC) were between 15.8% and 25.1% more expensive than making conventional cells, the efficiency gains and the ability to use less-expensive multicrystalline silicon outweighed those extra costs: overall the cost per unit power dropped by 10.8%.

The best module produced energy with more than 20% efficiency. The cells also showed a high tolerance towards impurities and much better long-term stability as compared to the industry standard reference cells.

Solar panel degradation
Researchers at Purdue University developed a methodology to detect solar panels degradation in the field. The team used public solar panel data provided by the National Renewable Energy Laboratory to pull together parameters of how well the panels are generating electricity, such as resistance and voltage. When fed into the algorithm, a curve generates to show the power output of a solar cell.

Real-time diagnostics using data generated by active solar farms would ultimately inform better panel designs, the team argues. “If you look at solar modules on the market, their designs hardly differ no matter where they are in the world, just like how iPhones sold in the U.S. and China are almost identical,” said Xingshu Sun, a recent doctoral graduate of Purdue. “But solar modules should be designed differently, since they degrade differently in different environments.”

Degradation in humid environments, for example, comes in the form of corrosion, but high altitudes with no humidity cause degradation through the increased concentration of UV light. Without knowing where the panels will be placed, companies tend to compensate for different weather conditions by under- or over-designing solar panels, driving up manufacturing costs.

The Solar PV Diagnosis dataset stores all the files (input, output and simulation code) and provides an easy access to users for their perusal. A pre-processing tool has been added to System 50 files to show how the raw field data and weather data are converted into appropriately formatted input files. (Source: Xingshu Sun, Raghu Vamsi Krishna Chavali, Muhammad Ashraful Alam / Purdue)

In the long term, the researchers hope the algorithm could show how much energy a solar farm produces in 30 years by looking at the relationship between weather forecast data and projection of electric circuit parameters. Integrating the algorithm with other physics-based models could eventually predict the lifetime of a solar farm.

The algorithm is in an experimental stage, but is downloadable for other researchers to use through the National Science Foundation-funded Digital Environment for Enabling Data-driven Science (DEEDS) platform.

Photoelectrode for water splitting
Scientists at Hokkaido University developed a photoelectrode for artificial photosynthesis systems that can harvest 85% of visible light, converting light energy more efficiently than previous methods.

The team sandwiched a 30nm titanium dioxide thin-film semiconductor between a 100nm gold film and gold nanoparticles to enhance light absorption. When the system is irradiated by light from the gold nanoparticle side, the gold film worked as a mirror, trapping the light in a cavity between the two gold layers and helping the nanoparticles absorb more light.

Left: The newly developed photoelectrode, a sandwich of semiconductor layer (TiO2) between gold film (Au film) and gold nanoparticles (Au NPs). The gold nanoparticles were partially inlaid onto the surface of the titanium dioxide thin-film to enhance light absorption. Right: The photoelectrode (Au-NP/TiO2/Au-film) with 7nm of inlaid depth traps light making it nontransparent (top). An Au-NP/TiO2 structure without the Au film are shown for comparison (bottom). (Source: Misawa H. et al., Nature Nanotechnology, July 30, 2018)

Gold nanoparticles are known to exhibit a phenomenon called localized plasmon resonance which absorbs a certain wavelength of light. “Our photoelectrode successfully created a new condition in which plasmon and visible light trapped in the titanium oxide layer strongly interact, allowing light with a broad range of wavelengths to be absorbed by gold nanoparticles,” said Hiroaki Misawa, a professor in the Research Institute for Electronic Science at Hokkaido University.

When gold nanoparticles absorb light, the additional energy triggers electron excitation in the gold, which transfers electrons to the semiconductor. “The light energy conversion efficiency is 11 times higher than those without light-trapping functions,” Misawa explained. The boosted efficiency also led to an enhanced water splitting for artificial photosynthesis: the electrons reduced hydrogen ions to hydrogen, while the remaining electron holes oxidized water to produce oxygen.