January 3, 2012 — Ottawa, Ontario
NRC researchers have produced the world’s most efficient “inverted” organic photovoltaic (OPV) solar cells, outperforming a previous record established just a few months ago. Their feat could help to accelerate the adoption of OPV devices in novel solar power applications.
Using a new polymer developed by Dr. Jianping Lu in collaboration with Laval University, NRC has developed a series of increasingly efficient organic solar cells. In November 2011, the team’s inverted OPV cells were officially certified by an independent U.S. certification lab at a power conversion efficiency of 7.1 percent, which means they convert 7.1 percent of incoming solar light into electricity.
Dr. Ta-Ya Chu poses beside an organic solar cell on display at the Canada Science and Technology Museum in Ottawa. The solar cell was developed by an NRC team led by Dr. Ye Tao.
In September, the Belgian firm Imec had set the previous record of 6.9 percent power conversion efficiency on an inverted OPV cell with an aperture area measuring 0.08 cm2. Besides being more efficient, NRC’s 1.0 cm2 OPV cells are 12 times larger. “The larger their active area, the closer they are to manufacturing reality,” says NRC’s Dr. Ta-Ya Chu.
The world’s first solar cells were made from silicon, which is relatively efficient at converting sunlight to energy. But silicon is also expensive and brittle. A new generation of solar cells called organic photovoltaics (OPVs) are more flexible and less expensive to produce, but also less efficient at generating electricity.
“OPVs have the potential to provide power at less cost than silicon cells because you don’t need as much material, and devices can be produced using low-cost fabrication techniques, such as high-speed, roll-to-roll printing technology,” says Dr. Jianping Lu of NRC. “To cover one square metre with a thickness of 100 nanometres, you only need about 0.1 grams of active materials.”
Inverted OPV cells — so called because the positions of their anode and cathode are reversed —are usually less efficient at converting light into electricity than conventional OPV cells, but they have the optimum structure for roll-to-roll mass production. In addition, an inverted OPV cell is more stable and less prone to environmental degradation than a conventional OPV cell.
NRC began working on organic photovoltaic devices in 2005. Three years later, a research consortium — consisting of NRC, Laval University, St-Jean Photochemicals and U.S. firm Konarka Inc. — received funding from Sustainable Development Technology Canada, a not-for-profit foundation created by the Government of Canada. “When we started this project, the average efficiency of OPVs was around 3 percent, but we more than doubled their performance within a few years,” says Dr. Lu.
NRC’s solar cells are made of thin layers of an organic plastic. These lightweight, flexible layers can be “painted” or “printed” onto a thicker plastic backing layer — like an overhead projector transparency — that can be rolled or folded like tough, light, portable road maps. [See: Shaped to fit: flexible solar cells]
Due to their mechanical flexibility, OPV solar cells can make electricity when spread over irregular shapes such as briefcases, backpacks, military equipment, tents, store awnings — even entire buildings. They can also form a dual-purpose semi-transparent window coating that may be tinted any colour, producing power while creating an interior mood and/or exterior pattern. NRC’s industrial partner, Konarka Inc., already markets some flexible solar cell products.
Dr. Chu credits the high performance of NRC’s OPV devices to several scientific advances, including new materials, a new device fabrication process, as well as interdisciplinary teamwork between chemists and physicists with theoretical and experimental expertise.
