Mixed Solar Cell Brings Efficiency

Wednesday, May 27, 2015 @ 04:05 PM gHale

Because of lower production costs, polymer solar cells could become a viable alternative to conventional solar cells with silicon substrates when they achieve a power conversion efficiency — a measure that indicates how much electricity they can generate from a given amount of sunlight — of between 10 and 15 percent.

Now, using carefully designed materials and an “inverted” architecture, a team of scientists has achieved efficiency of 10 percent, bringing these cells close to the threshold of commercial viability.

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Polymer-based solar cells offer a number of potential advantages. They consist of polymers that are inexpensive and flexible, and can end up deposited on glass or plastic substrates, allowing the construction of large-scale structures. They are cheaper to manufacture, and more environmentally-friendly, than their silicon counterparts. The catch is, though, they have lower power efficiency due to their structure and also tend to degrade more quickly.

That may soon change as a team including Itaru Osaka and Kazuo Takimiya of the RIKEN Center for Emergent Matter Science in Japan created a type of polymer solar cell called a bulk-heterojunction solar cell, where the electron donor and acceptor layers mix together. The power conversion efficiency of 10 percent is close to what will allow these materials to be commercially viable.

“While private firms have been able to develop cells with similar efficiency, they have done so using proprietary technology, so that it was not possible to know why things were working the way they were,” Osaka said. “We began experimenting with a substance called PNTz4T, which we had previously developed, and were able initially to achieve a power conversion efficiency of about 8 percent, with a fairly thick active layer of about 300 nanometers. Surprisingly, though, we found that when we used an inverted architecture, where the light enters through a transparent negative electrode, in our case made of zinc oxide, we found that the cell with the inverted architecture had better efficiency, which is abnormal for cells of the type we built. We believe that it is due to the alignment of molecules inside the mixed layers.”

The researchers analyzed the composition of the materials using the SPring-8 synchrotron facility in Harima, and found in the inverted model, the orientation of the molecules within the active layer was very commonly “face-on,” an orientation well suited to the transport of electron holes through the material.

“We surmised that this was the secret to the success in the experiment,” Takamiya said. “It turns out that by trying something that might seem unusual, we got a surprising result, and through this were able to understand something about what makes cells more or less efficient.”

“This is an exciting result because we now have an understanding of how we can move forward to create polymer solar cells with greater efficiency,” said Professor Hideyuki Murata of the Japan Advanced Institute of Science and Technology, who participated in the research. “We hope that researchers around the world will be able to make use of these results to create commercially viable cells.”

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