Newswise — Converting sunlight to electricity through photovoltaic devices will provide an abundant source of clean energy for future generations, according to associate professor Qiquan Qiao of the South Dakota State University Electrical Engineering and Computer Science Department. Though the cost of producing solar energy has dropped from $4 per watt to below $1 per watt, Qiao admitted, “we are still in research status.”

When he came to the SDSU Center for Advanced Photovoltaics in 2007, Qiao began establishing a lab devoted to studying organic electronic materials and devices. His research focuses on improving the efficiency of organic solar cells. In 2010, Qiao received an NSF Faculty Early Career Development award that has provided more than $436,000 during the last five years to advance his work. In addition, he was the SDSU lead on a recently completed three-year, $750,000 NASA-EPSCoR grant collaborating with the South Dakota School of Mines and Technology, government laboratories and commercial entities to develop a photovoltaic materials research cluster in South Dakota. Use of multiple layersOrganic solar cells are less expensive to produce than silicon-based technologies, Qiao explained. The flexible, plastic materials use “solution-based processing without high temperature or high vacuum requirements, which decreases the cost of production,” he said. “That’s the main advantage of organic solar cells.”

Solar cells are made of two semiconductor materials layered on a glass substrate. The donor material absorbs sunlight that excites electrons to produce electricity, while the acceptor material captures this energy via a loose bonding mechanism, Qiao explained. These single-junction solar cells are able to convert up to 10 percent of the available light into electricity, he noted. However, significant energy losses occur in the single junction between the two materials. In addition, each single junction donor-acceptor unit can absorb light only within a specific wavelength or bandwidth, limiting the amount of solar energy that can be harvested. Using multiple units tuned to different bandwidths via multi-junction solar cells will help capture a greater percentage of the available light, Qiao explained, but the additional interfacial layers also increase energy losses.

Reducing energy lossesTo do this, the researchers provide a way to interconnect these multiple donor-acceptor layers to facilitate charge transport. Tunnel junctions provide effective pathways through which charge can travel, thus reducing the energy losses, according to Qiao. The SDSU team, including one doctoral student and three master’s students, is developing an interfacial layer that will more efficiently transfer the energy.

He and his team will create a tunneling layer in the organic solar cells. “Efficient tunneling at the interconnecting layer can improve overall photovoltage and photocurrent output by reducing any charge accumulation and better matching current from the subcells,” he pointed out.

Through this method of charge transport, the researchers hope to achieve up to 15 percent efficiency. About South Dakota State UniversityFounded in 1881, South Dakota State University is the state’s Morrill Act land-grant institution as well as its largest, most comprehensive school of higher education. SDSU confers degrees from eight colleges representing more than 175 majors, minors and specializations. The institution also offers 32 master’s degree programs, 15 Ph.D. and two professional programs. The work of the university is carried out on a residential campus in Brookings, at sites in Sioux Falls, Pierre and Rapid City, and through Cooperative Extension offices and Agricultural Experiment Station research sites across the state.