Singlet Fission's Two-for-One Potential

Singlet Fission's Two-for-One Potential

NREL scientists confirm an exciting first for singlet fission.

Consumers like to get two-for-one deals. So do scientists at NREL, who are working in collaboration with scientists at the University of Colorado, Boulder and Northwestern University, to search for innovative scientific concepts that may lead to solar cells having very high conversion efficiencies.

Singlet fission is just such a concept, and has the potential to significantly boost our nation's ability to create more abundant and lower-cost solar-generated electricity and hydrogen.

Series of four small rectangular panels (a through d). Energy Level increases upward for all. Each chart has two halves, with Molecule 1 to left and Molecule 2 to right. For Panel a, each half has two small arrows (one points up, the other down) on lower line labeled S0. For Panel b, left half has wavy red line angling in from left side; one short arrow points down on lower line, one short arrow points up on upper line labeled S1. Long arrow points from lower and upper line. Right half is same as left half of Panel a.  For Panel c, left half, one small arrow points down on lower line and another points down on middle line labeled T1. Longer arrow points from upper to middle line. Right half, one small arrow points up on lower line and another points up on middle line labeled T1. Longer arrow points from lower to middle line. For Panel d: left half, red h+ label on lower line, red e- label on middle line; same for right half. Enlarge image

The basics of singlet fission: (a) Two neighboring organic molecules in their unexcited ground states S0 (thick arrows indicate electron spin direction); (b) A high-energy photon excites the first molecule, creating a singlet state S1; (c) The first molecule relaxes to a lower-energy triplet state T1 and transfers energy to the second coupled molecule, which is excited to a triplet state; (d) The two T1 states each generate a free electron (e-) and hole (h+). Hence, one photon produces two electron-hole pairs—a two-for-one transaction.
Source: NREL

In the fission process, one singlet state excited by a photon of the high-energy portion of sunlight creates two triplet states. In turn, these triplets potentially produce two electron-hole pairs per absorbed photon—the "two-for-one" possibility—in a solar cell, which boosts the energy conversion efficiency.

The collaborating scientists used spectroscopy—on a thin-film of the compound 1,3-diphenylisobenzofuran (DPIBF)—to measure a yield of triplets of 200% (±30%) at a temperature of 77 kelvin. This yield represents nearly perfect efficiency, with two triplets created per one absorbed photon. Also, this result confirms theoretical work that has been conducted on molecules designed with specific properties that favor singlet fission.

Thermodynamic modeling indicates that a simple multilayered solar cell based on singlet fission could increase the photovoltaic power conversion efficiency by more than 43% above the maximum theoretical limit—the so-called Shockley-Queisser limit. The resulting conversion efficiency would be about 46%.

Evolution of a Disruptive Concept

The singlet-fission phenomenon has been known for nearly three decades, but had not been formally considered as a light-harvesting scheme until it was highlighted in a 2006 NREL publication. This work led to an extensive search, guided by principles of quantum mechanics, for appropriate candidate materials. One of these materials, DPIBF, essentially has a perfect ratio of singlet-to-triplet excitation energies of 2 to 1. This means that photons with energy at least twice that of DPIBF's "bandgap" energy can produce two positive and two negative charges, and therefore can potentially double the electrical current of a solar cell.

In addition, through their search, NREL researchers and collaborators determined two design principles with promising results. The first identifies a new class of high-potential materials (biradicaloids), and the second establishes a geometry between the two coupled molecules needed for singlet fission that fosters the fission phenomenon. NREL's theoretical, experimental, and characterization work is expanding the potential application of singlet fission to greatly enhance the conversion efficiency of organic-based solar cells.

This concept is an extension of NREL work from the mid 1970s on what are now generally referred to as "third-generation" solar photon conversion approaches. The early work focused on hot-carrier conversion, which is a process in solar cells that increases solar conversion efficiency by using more of the sun's energy rather than losing it as heat. Then, in 2000, an NREL research team identified multiple-exciton generation (MEG) in quantum dots, which are semiconductor nanocrystals that can produce two or more electron-hole pairs (excitons) from one high-energy photon. Singlet fission is the molecular analog of MEG in inorganic quantum dots.

Taking the Concept to the Next Level

The results in singlet-fission research represent a basic science advance by NREL, but it will take much applied research to find ways to use this effect in a commercial solar cell. Singlet fission has been demonstrated spectroscopically to be efficient under the right circumstances. But challenges ahead include understanding how the process functions in a wider range of compounds and how to incorporate these organic molecules into devices.

Consequently, there are probably another ten years of research ahead to prepare these sorts of singlet-fission cells for prime time. Specifically, researchers need to identify optimal materials to use, and then develop these within actual solar devices, with the goal of collecting the electrons produced by the triplet states. The effort will be intense, but the result is that more of the sun's energy will be available to generate solar electricity and produce solar fuels. This is a key step toward making solar electricity and fuels more efficient and cost competitive as a conventional power source.

The research at NREL was sponsored by the Hydrogen Fuel Initiative within the U.S. Department of Energy (DOE) Office of Basic Energy Sciences and that at the University of Colorado and Northwestern University by the Solar Energy Technologies Program within DOE's Office of Energy Efficiency and Renewable Energy.

—Don Gwinner

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