"Nanograss" boosts the efficiency of organic solar cells

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An array of nanoscale pillars ("nanograss") structure could make organic solar cells more efficient by capturing light (Image: UMass Amherst)

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Researchers at the University of Massachusetts Amherst, Stanford University and the Dresden University of Technology have developed a long sought-after nanostructure that can significantly increase the efficiency of organic solar cells. Their "nanograss," a dense array of vertical nanopillars, can capture photons at a very high efficiency and could also lead to cheaper and more advanced 3D transistors, photodetectors and charge storage devices.

Solar cells are built using two different types of semiconductors ("p-type" and "n-type"), each with a slightly different composition; when the two come in close contact, they form a so-called "PN junction." This junction is a critical component of any solar cell because it generates an electric field that causes charge inside the cell to flow in a set direction, creating a voltage. Voltage times current equals (solar) power.

After decades of trial and error, scientists now believe that the ideal geometry for a PN junction would consist of a series of vertical nanoscale pillars made from one type of semiconductor (either p- or n-type) and surrounded by a semiconductor of the opposite type. This shape is extremely effective at trapping light without reflecting it, resulting in a greater amount of charge being collected, while also allowing the use of cheaper, lower-grade materials in smaller volumes, which decreases the overall cost of the cell.

This "Holy Grail" structure has already been achieved in inorganic solar cells, but has been elusive for their organic counterpart due to some of the unique challenges they present. Now, however, a team led by Prof. Alejandro Briseno at UMass Amherst has developed a new simple and highly adaptable technique that can produce "nanograss" for use in organic solar cells, which could lead to a significant boost in their efficiency.

To grow the nanopillars, the researchers placed the source semiconductor material and a graphene substrate in a near-vacuum inside a temperature gradient furnace. Lower temperatures initially formed an ultrathin film; then, as the temperature was gradually increased, the semiconductor compound started to stack like a pile of coins, growing into an array of 3D, single-crystalline nanopillars.

According to the scientists, the process is highly adaptable. It can be made to work with a number of organic semiconductors (both p-type and n-type), as well as a wide range of common substrate materials such as graphene, zinc oxide and copper iodide.

"For decades scientists and engineers have placed great effort in trying to control the morphology of p-n junction interfaces in organic solar cells," says Briseno. "This work is a major advancement in the field of organic solar cells because we have developed what the field considers the 'Holy Grail' of architecture for harvesting light and converting it to electricity."

The researchers tested their "nanograss" structure on an organic solar panel and observed a 32 percent increase in power conversion efficiency over thin films of the same materials – from 2.2 to 2.9 percent.

Briseno says this simple and inexpensive technique could apply to low-end energy applications such as gadgets, toys, and disposable devices with a short lifetime. The advance could also help build better 3D transistors, photodetectors and energy storage devices.

A paper describing the research was published in a recent issue of the journal Nano Letters.

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