Singlet Fission

There is no doubt that solar energy will play an increasingly key role in the future of humankind. The ability to harvest, store, and utilize energy from the sun, however, poses a number of significant challenges that need to be met and overcome to efficiently exploit this resource. The past decades have seen significant efforts in the development of new materials, improved solar cell design, and increased device efficiencies. Organic photovoltaic solar cells (OPVs) have the potential to make a crucial contribution to our energy future through, for example, the possibility of low-cost device formation and the use of earth abundant elements. Realizing the potential of OPVs, however, requires that both the efficiencies and long-term stabilities be increased. This special collection in ChemPhotoChem describes a variety of approaches toward using singlet fission as a basis for an improved scheme of solar energy capture and conversion. These articles and a minireview provide exciting insight into new molecules, theoretical analyses, and mechanistic schemes at the forefront of research efforts to harness the potential of singlet fission. Why highlight singlet fission in a special collection of papers? The upper thermodynamic limit for single-junction solar cells is just above 30 %, due largely to the fact that excess energy from the absorbed photons is lost to heat rather that used for charge generation. To overcome this limit, creative and innovative strategies, such as singlet fission, are necessary. Singlet fission is a spin-allowed process in which, upon photoexcitation, the initial singlet state spontaneously splits into a pair of correlated triplet states. Current models suggest that solar cells based on singlet fission could achieve efficiencies of nearly 50 %. Singlet fission in systems based on organic molecules has been known for over 40 years, but only recently has it been embraced as a realistic scheme for OPVs. In the past decade or so, many significant advances have been made with respect to optimizing molecular materials, incorporation of materials into proof-of-concept devices, and understanding the mechanism of singlet fission. Nevertheless, challenges remain, including the ability to effectively “harvest” and convert triplet-excited states formed by singlet fission into useable forms of energy as well as the development of materials and devices that feature long-term stability in OPVs. The design of stable new molecules for singlet fission will undoubtedly play an important role toward solving these changes, including for example, electron-accepting and radical-based systems to complement the use of electronic donating acenes. We hope that this special collection will provide both a window into our current understanding of singlet fission for OPVs as well as provide inspiration toward providing answers for challenges that remain in the field. Rik R. Tykwinski is Professor and Chair of Chemistry at the University of Alberta (Canada). As a physical organic chemist, his research interests center on the synthesis of molecules that model the carbon allotrope carbyne and structure–function studies of carbon-rich conjugated systems. When not in the office, he is most likely entertaining his two sons or out on his mountain bike. Dirk M. Guldi is Professor of Chemistry at the University of Erlangen (Germany). His research interests are centered around light- and charge-management in solar energy conversion schemes using nano-carbons. While doing a full-time academic job, he fits in tri-training into a full and fulfilled life.

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