New equation could advance research in solar cell materials

A new equation developed, in part, by researchers at the University of Michigan could help to enable a wider adoption of organic semiconductors.

The equation describes the relationship of current to voltage at the junctions of the organic semiconductors - carbon-rich compounds that don’t necessarily come from a biological source, but resemble them.

The semiconductors present special challenges for researchers because they are more disordered than their inorganic counterparts. But they could enable advanced solar cells, thin and intense OLED displays, and high-efficiency lighting.

Steve Forrest, the William Gould Dow Collegiate Professor of Electrical Engineering and U-M vice president for research, and his doctoral students Noel Giebink (now at Argonne National Laboratories) and Brian Lassiter, in the U-M Department of Electrical Engineering and Computer Science, contributed to this research. Two papers on the work are published in the current edition of Physical Review B.

About six years ago, researchers in Forrest’s lab realised that they could use Shockley’s equation to describe the current/voltage relationship in their organic solar cells to a degree.

Their findings were published and, from that time on, many physicists and engineers used the Shockley equation for organic semiconductors even though it didn’t describe the physics perfectly. The new equation does.

Forrest says it will allow researchers to better describe and predict the properties of the different organic semiconductors they’re working with. And in that way, they'll be able to more efficiently choose which material best suits the needs of the device they’re working on.

The papers are titled, ‘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’.