Power/Performance Bits: Feb. 10

Recent solar cell developments bring nanocrystals out of the dark and concentrating photovoltaic systems to your roof.

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Solar power technology progresses at a snappy pace and the diversity of approaches keeps expanding. In this edition, investigations in two aspects of solar energy design: understanding a potential solar cell material and a design to make those we use now more effective.

Unravelling the peculiarities of nanocrystals

Researchers at ETH Zurich conducted an extensive study of nanocrystal solar cells, considered a promising possibility for third generation solar cells. Nanocrystal solar cells have a higher absorption coefficient than the current bulk silicon solar cells and can be designed to absorb a larger fraction of the solar light spectrum. But developing them is no straightforward task. Even though scientists can easily control nanocrystal size in the fabrication process, the physics of electron transport in the complex material system of nanocrystal cells has not been understood – making it impossible to systematically engineer them better.

“These solar cells contain layers of many individual nano-sized crystals, bound together by a molecular glue. Within this nanocrystal composite, the electrons do not flow as well as needed for commercial applications,” explains ETH Zurich professor Vanessa Wood.

Nanocrystals are small enough that effects of quantum physics come into play. Changing their size changes their physical properties, including a bandgap that varies based on crystal size. Yet in spite of being able to maximize the spectrum that nanocrystal solar cells can absorb, efficiency has topped at just 9 percent, far below the 15 percent efficiency needed to be considered for commercial application.

A solar cell chip based on nanocrystals fabricated by the ETH researchers. (Source: Deniz Bozyigit / ETH Zurich)

A solar cell chip based on nanocrystals fabricated by the ETH researchers. (Source: Deniz Bozyigit / ETH Zurich)

Using temperature-dependent current–voltage characterization and thermal admittance spectroscopy on a series of diodes fabricated from variously sized lead sulfide nanocrystals the researchers were able to separate the influence of the bandgap, trap states and temperature to identify governing physical processes and describe the electron transport in these types of cells, producing a generally applicable physical model for the first time. “Our model is able to explain the impact of changing nanocrystal size, nanocrystal material, or binder molecules on electron transport,” said Wood. The team used the findings to propose a set of guidelines to achieve higher power conversion efficiencies, removing some of the darkness around nanocrystal solar cell engineering.

Rooftop solar panels get a makeover

Those big black solar panel slabs may not end up as the defacto rooftop covering after all. Researchers at Penn State and the University of Illinois have developed a new microscale solar concentration technology to take solar farm techniques to the neighborhood.

“Concentrating photovoltaic (CPV) systems leverage the cost of high efficiency multi-junction solar cells by using inexpensive optics to concentrate sunlight onto them,” said Noel C. Giebink, assistant professor of electrical engineering at Penn State. “Current CPV systems are the size of billboards and have to be pointed very accurately to track the sun throughout the day. But, you can’t put a system like this on your roof, which is where the majority of solar panels throughout the world are installed.”

In making CPV feasible for rooftops, the researchers combined miniaturized gallium arsenide photovoltaic cells, 3D-printed plastic lens arrays and a moveable focusing mechanism to reduce the size, weight and cost of the CPV system and to create something similar to a traditional solar panel.

Photograph of the prototype panel being tested outdoors. The small black squares seen under each lenslet in the close-up are the solar cells. (Source: Nature Communications)

The solar cells themselves are less than 1 square millimeter, arrayed on a glass or plastic sheet and then sandwiched between a pair of plastic lenslet arrays that act as a magnifying glass and concave mirror. As the sun moves the sheet of solar cells does too, keeping them at the sun’s focal point while moving.

The researchers say a prototype panel tested over the course of a day showed a solar concentration of over 100x. Solar cells make up only one percent of the panel and with the bulk being acrylic plastic this CPV system just might be cheap, too.