Converting Sunlight To Cheaper Energy

Assistant professor Qiquan Qiao in SDSU’s Department of Electrical Engineering and Computer Science said so-called organic photovoltaics, or OPVs, are less expensive to produce than traditional devices for harvesting solar energy.

Qiao and his SDSU colleagues also are working on organic light-emitting diodes, or OLEDs.

The new technology is sometimes referred to as “molecular electronics” or “organic electronics” — organic because it relies on carbon-based polymers and molecules as semiconductors rather than inorganic semiconductors such as silicon.

“Right now the challenge for photovoltaics is to make the technology less expensive,” Qiao said.

“Therefore, the objective is find new materials and novel device structures for cost-effective photovoltaic devices.

“The beauty of organic photovoltaics and organic LEDs is low cost and flexibility,” the researcher continued. “These devices can be fabricated by inexpensive, solution-based processing techniques similar to painting or printing."

“The ease of production brings costs down, while the mechanical flexibility of the materials opens up a wide range of applications,” Qiao concluded.

Organic photovoltaics and organic LEDs are made up of thin films of semiconducting organic compounds that can absorb photons of solar energy. Typically an organic polymer, or a long, flexible chain of carbon-based material, is used as a substrate on which semiconducting materials are applied as a solution using a technique similar to inkjet printing.

“The research at SDSU is focused on new materials with variable band gaps,” Qiao said.

“The band gap determines how much solar energy the photovoltaic device can absorb and convert into electricity.”

Qiao explained that visible sunlight contains only about 50 percent of the total solar energy. That means the sun is giving off just as much non-visible energy as visible energy.

“We’re working on synthesizing novel polymers with variable band gaps, including high, medium and low-band gap varieties, to absorb the full spectrum of sunlight. By this we can double the light harvesting or absorption,” Qiao said.

SDSU’s scientists plan to use the variable band gap polymers to build multi-junction polymer solar cells or photovoltaics.

These devices use multiple layers of polymer/fullerene films that are tuned to absorb different spectral regions of solar energy.

Ideally, photons that are not absorbed by the first film layer pass through to be absorbed by the following layers.

The devices can harvest photons from ultraviolet to visible to infrared in order to efficiently convert the full spectrum of solar energy to electricity.

SDSU scientists also work with organic light-emitting diodes focusing on developing novel materials and devices for full color displays.

“We are working to develop these new light-emitting and efficient, charge-transporting materials to improve the light-emitting efficiency of full color displays,” Qiao said.

Currently, LED technology is used mainly for signage displays. But in the future, as OLEDs become less expensive and more efficient, they may be used for residential lighting, for example.

The new technology will make it easy to insert lights into walls or ceilings. But instead of light bulbs, the lighting apparatus of the future may look more like a poster, Qiao said.

Qiao and his colleagues are funded in part by SDSU’s electrical engineering Ph.D. program and by National Science Foundation and South Dakota EPSCoR, the Experimental Program to Stimulate Competitive Research.

In addition Qiao is one of about 40 faculty members from SDSU, the South Dakota School of Mines and Technology and the University of South Dakota who have come together to form Photo Active Nanoscale Systems (PANS).

The primary purpose is developing photovoltaics, or devices that will directly convert light to electricity.