Harnessing exciton fission gives solar cell efficiency boost
When sunlight shines on solar cells, much of the incoming energy is given off as waste heat rather than electrical current. In a few materials, however, extra energy produces extra electrons – behavior that could boost solar-cell efficiency.
The research 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 delivers energy that excites a molecule, causing it to release one electron. However, when high-energy photons provide more than enough energy, the molecule still releases just one electron – plus waste heat.
A few organic molecules do not 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 the 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 percent.
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,” explained Van Voorhis.
To support the theoretical study of electron behavior within PVs, Van Voorhis used experimental data gathered in samples specially synthesized 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.
Van Voorhis’ new first-principles formula predicts the fission rate in materials with vastly different structures. In addition, it confirms once and for all that the mechanism is the ‘classic’ one proposed in 1960s: 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 excited electron brings some energy along and leaves some behind, so that both molecules give off electrons. 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.”
The results also provide practical guidelines for designing solar cells with these materials. They show that molecular packing is important in defining the rate of fission – but only to a point. When the molecules are close together, the electrons move so quickly that the molecules giving and receiving them do not have time to adjust. A far more important factor is choosing a material that has the right inherent energy levels.
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