Manufacturing Bits: Oct. 19

Solar mini-reactors; finding defects in solar cells; AFMs.


Solar mini-reactors
The University of Amsterdam has developed a standalone solar-powered mini-reactor. The technology could one day serve as an autonomous off-grid photochemistry system for remote locations.

The prototype solar reactor measures 0.25 square meters. The system is equipped with a solar cell, which provides the power for the pumps and control system. This solar cell is placed behind a flow reactor in a stacked configuration.

The mini-reactor is designed to produce chemicals in remote locations on Earth and even Mars. The new system is capable of synthesizing drugs and chemicals in volumes.

“(The system) shines in isolated environments and allows for the decentralization of the production of fine chemicals,” said Timothy Noël, a professor at the University of Amsterdam. “The mini-plant is based on the concept of photochemistry, using sunlight to directly ‘power’ the chemical synthesis. We employ a photocatalyst, a chemical species that drives the synthesis when illuminated. Normally powerful LEDs or other lighting equipment are used for the illumination, but we choose to use sunlight. For starters, this renders the synthesis fully sustainable. But it also enables stand-alone operation in remote locations. Our dream is to see our system used at a base on the Moon or on Mars, where self-sustaining systems are needed to provide energy, food and medicine. Our mini-plant could contribute to this in a fully autonomous, independent way.”

Finding defects in solar cells
The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and the Colorado School of Mines have utilized a metrology technique to identify defects in silicon solar cells that cause a drop in efficiency.

Researchers used a technique called electron paramagnetic resonance (EPR) to identify defects responsible for light-induced degradation (LID) in solar cells. EPR spectroscopy is a method for studying materials, which have unpaired electrons.

Solar cells made from silicon account for more than 96% of the solar market, according to NREL. The most common silicon for solar is made from boron-doped silicon, but this material is prone to LID.

LID is problematic. It reduces the efficiency of silicon solar cells by about 2%, according to researchers. LID could accumulate and contribute to a drop in power output over the lifespan of solar cells in the field.

LID has been studied for decades, but it’s unclear what’s behind it. Clearly, it’s important to gain an understanding of LID. Otherwise, solar may not live up to its promises.

That’s where EPR fits in. Researchers used it to identify defects responsible for the LID. Using EPR, researchers detected a defect signature as the sample solar cells became more degraded by light.

“Using electron paramagnetic resonance (EPR), we have identified the spin-active paramagnetic signatures of this phenomenon and gained insights into its microscopic mechanism. We found a distinct defect signature, which diminished when the degraded sample was annealed. The second signature, a broad magnetic field spectrum, due to the unionized B acceptors, was present in the annealed state but vanished upon light exposure,” said Abigail Meyer, a Ph.D. candidate at Colorado School of Mines and a researcher at NREL, in the journal of Energy & Environmental Science.

Solar AFMs
The National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences (CAS) has developed a way to investigate the surface energy distribution at the interface layer of organic solar cells.

Researchers used atomic force microscopy (AFM) to characterize the hole transporting layers in organic solar cells. These solar cells leverage the properties of conductive organic polymers or molecules. They are used for light absorption and charge transport to produce electricity from sunlight.

Surface energy plays a role in terms of developing bulk-heterojunction films in organic solar cells.

Measuring the surface energy is key here. This is obtained by measuring the contact angle using Owens-Wendt model, according to researchers.

But this method has some shortcomings. Researchers from CAS used the AFM-based peak-force quantitative nanomechanical mappings (PFQNM) technique to characterize the nanoscale surface energy distribution of hole transporting layers in organic solar cells, according to CAS.

AFM involves a standalone system that provides surface measurements on structures down to the angstrom level. (1 angstrom = 0.1nm.) Basically, an AFM system incorporates a cantilever with a tiny hard tip or needle. In operation, the tip scans the surface of a structure, providing three-dimensional measurements with resolutions from 100µm to 0.1nm.

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