Troy Van Voorhis, professor of chemistry (left), and Marc Baldo, professor of electrical engineering (right). Photo: MIT

The most basic principle of a solar cell is that it works by transferring the energy from an incoming photon (light) to a molecule, which causes one or more electrons to become displaced until an electrical current is formed. That’s the absolute gist of it, only besides electricity, some of the incoming photon energy gets lost as waste heat. Oddly enough, however, there are some organic materials that behave in the opposite way: when extra energy is given, more electrons form.

Weird physics

A team of researchers at MIT used both experiments and theoretical models to explain the mechanics of this phenomenon – called singlet exciton fission – and thus help solar cells become vastly more efficient.

The phenomenon was first observed in the 1960s, yet the exact mechanism involved has become the subject of intense controversy in the field. MIT’s Troy Van Voorhis, professor of chemistry, and Marc Baldo, professor of electrical engineering, led a team which investigated this odd behaviour. They synthesized and gathered materials made of four types of exciton fission molecules decorated with various sorts of “spinach” — bulky side groups of atoms that change the molecular spacing without altering the physics or chemistry. They then subjected these to various experiments to determine their fission rate.

The MIT team turned to experts including Moungi Bawendi, the Lester Wolfe Professor of Chemistry, and special equipment at Brookhaven National Laboratory and the Cavendish Laboratory at Cambridge University, under the direction of Richard Friend.

Experimental data and theoretical models confirm once and for all what was first proposed some 50 years ago: when excess energy is available in these materials, an electron in an excited molecule swaps places with an electron in an unexcited molecule nearby. The result: one photon in, two electrons out. “

“The simple theory proposed decades ago turns out to explain the behavior,” Van Voorhis says. “The controversial, or ‘exotic,’ mechanisms proposed more recently aren’t required to explain what’s being observed here.”

As such, the results provide a solid guideline for designing solar cells with these sort of exotic materials. They show that molecular packing is important in defining the rate of fission — but only to a point. When the molecules are very close together, the electrons move so quickly that the molecules giving and receiving them don’t have time to adjust. Indeed, a far more important factor is choosing a material that has the right inherent energy levels.

David Reichman, a professor of chemistry at Columbia University who was not involved in this research, considers the new findings “a very important contribution to the singlet fission literature. Via a synergistic combination of modeling, crystal engineering, and experiment, the authors have provided the first systematic study of parameters influencing fission rates,” he says. Their findings “should strongly influence design criteria of fission materials away from goals involving molecular packing and toward a focus on the electronic energy levels of selected materials.”

The results are reported in the journal Nature Chemistry.