A new equation will improve solar cell research

October 20, 2010

A new equation will improve solar cell researchA new equation that “describes the relationship of current to voltage at the junctions of organic semiconductors” has been developed at the University of Michigan.  That equation is sure to prove crucial to further research and development in the use of organic material, such as graphene, in such applications as solar cells.

Computers became possible when William Shockley developed his own equation that “describes the relationship between electric current and voltage in inorganic semiconductors such as silicon.”  That equation, known as the Shockley equation was developed in 1949, over a half a century ago.

The development of the new equation as yet unnamed, will lead to better organic semiconductors that could conceivably change the future as much as the computer has changed our lives in the last 61 years.  For example, “they could enable advanced solar cells, thin and intense OLED (organic light-emitting diode) displays, and high-efficiency lighting.”  That doesn’t even count the new applications that we can’t even imagine at this early stage of organic semiconductor development.

"The field of organic semiconductor research is still in its infancy. We’re not making complicated circuits with them yet, but in order to do that someday, we need to know the precise relationship of current and voltage. Our new equation gives us fundamental insights into how charge moves in this class of materials. From my perspective, it’s a very significant advance," said Steve Forrest, the William Gould Dow Collegiate Professor of Electrical Engineering and U-M vice president for research.

Before developing the current equation, researchers in Forrest’s lab described how to use the Shockley equation with organic semiconductors.  Even with their explanation, the application of the Shockley equation was imperfect and not as precise as needed to really advance the physics behind new organic semiconductors.

Using the new equation will allow scientists to improve their choice of organic materials for different devices as well as “describe and predict” how different organic semiconductors will work.  That ability to “describe and predict” will enable researchers to expand the use of semiconductors in a wide variety of current and new applications that could easily lead to improved clean energy sources and greener applications.

Forrest worked with two graduate students Noel Giebink (now at Argonne National Laboratories) and Brian Lassiter, in the U-M Department of Electrical Engineering and Computer Science to contribute to development of the equation.  Their work has led to publication of two papers in Physical Review B – "The Ideal Diode Equation for Organic Heterojunctions. I. Derivation and Application," and "The Ideal Diode Equation for Organic Heterojunctions. II. The Role of Polaron Pair Recombination."

Although the titles might not be the most exciting ever written, the research and results outlined certainly are electrifying. (Sorry, couldn’t resist the pun.)

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