More Efficient Solar Cells – In Theory

Wednesday, January 29, 2014 @ 02:01 PM gHale

A new theoretical model holds the key to methods for developing better materials for solar cells.

This new model could lead to new solar cell materials made from improved blends of semiconducting polymers and fullerenes, said Eric Bittner, a John and Rebecca Moores Professor of Chemistry and Physics in the University of Houston’s College of Natural Sciences and Mathematics, and Carlos Silva, an associate professor at the Université de Montréal and Canada Research Chair in Organic Semiconductor Materials.

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The researchers describe their findings in a paper titled “Noise-Induced Quantum Coherence Drives Photo-Carrier Generation Dynamics at Polymeric Semiconductor Heterojunctions.”

“Scientists don’t fully understand what is going on inside the materials that make up solar cells. We were trying to get at the fundamental photochemistry or photophysics that describes how these cells work,” Bittner said.

Solar cells consist of organic semiconductors – typically blends of materials. However, solar cells made of these materials have about 3 percent efficiency. Bittner added the newer materials, the fullerene/polymer blends, only reach about 10 percent efficiency.

“There is a theoretical limit for the efficiency of the ideal solar cell – the Shockley-Queisser limit. The theory we published describes how we might be able to get above this theoretical limit by taking advantage of quantum mechanical effects,” Bittner said. “By understanding these effects and making use of them in the design of a solar cell, we believe you can improve efficiency.”

“In polymeric semiconductors, where plastics form the active layer of solar cells, the electronic structure of the material is intimately correlated with the vibrational motion within the polymer chain,” Silva said. “Quantum-mechanical effects due to such vibrational-electron coupling give rise to a plethora of interesting physical processes that can be controlled to optimize solar cell efficiencies by designing materials that best exploit them.”

Bittner said the benefit of their model is it provides insight into what is happening in solar cell systems.

“Our theoretical model accomplishes things that you can’t get from a molecular model,” he said. “It is mostly a mathematical model that allows us to look at a much larger system with thousands of molecules. You can’t do ordinary quantum chemistry calculations on a system of that size.”

The calculations prompted a series of new experiments by Silva’s group to probe the outcomes predicted by their model.

Bittner and Silva’s next steps involve collaborations with researchers who are experts in making polymers and fabricating solar cells.

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