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MIT model lets solar cells gather more electricity

08 May 2014  | Nancy W. Stauffer

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Much of the incoming energy from sunlight ends up as waste heat rather than electrical current on today's solar cells. However, in some materials, extra energy produces extra electrons. This behaviour can significantly increase solar-cell efficiency.

An MIT team has now identified the mechanism by which that phenomenon happens, yielding new design guidelines for using those special materials to make high-efficiency solar cells. The results are reported in the journal Nature Chemistry by MIT alumni Shane R. Yost and Jiye Lee, and a dozen other co-authors, all led by MIT's Troy Van Voorhis, professor of chemistry, and Marc Baldo, professor of electrical engineering.

In most photovoltaic (PV) materials, a photon (a packet of sunlight) delivers energy that excites a molecule, causing it to release one electron. But when high-energy photons provide more than enough energy, the molecule still releases just one electron—plus waste heat.

A few organic molecules don't follow that rule. Instead, they generate more than one electron per high-energy photon. That phenomenon, known as singlet exciton fission, was first identified in the 1960s. However, achieving it in a functioning solar cell has proved difficult, and the exact mechanism involved has become the subject of intense controversy in the field.

For the past four years, Van Voorhis and Baldo have been pooling their theoretical and experimental expertise to investigate this problem. In 2013, they reported making the first solar cell that gives off extra electrons from high-energy visible light, which makes up almost half the sun's electromagnetic radiation at the Earth's surface. According to their estimates, applying their technology as an inexpensive coating on silicon solar cells could increase efficiency by as much as 25 per cent.

Van Voorhis and Baldo

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

While that's encouraging, understanding the mechanism at work would enable them and others to do even better. Exciton fission has now been observed in a variety of materials, all discovered—like the original ones—by chance. "We can't rationally design materials and devices that take advantage of exciton fission until we understand the fundamental mechanism at work, until we know what the electrons are actually doing," Van Voorhis says.

To support his theoretical study of electron behaviour within PVs, Van Voorhis used experimental data gathered in samples specially synthesised by Baldo and Timothy Swager, MIT's John D. MacArthur Professor of Chemistry. The samples were 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.

To detect fission rates, which are measured in femtoseconds (10-15 seconds), 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.

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