Abstract: Chemistry affects almost every aspect of our existence, so that it will be an essential component of solutions in global issues in health, materials, and energy. For this reason, the design and synthesis of novel molecular materials lies at the forefront of transformative research and has game-changing character. A leading example for such shifts in existing scientific paradigms is surpassing the Shockley-Queisser limit, which places an upper bound on solar conversion efficiency for a single p-n junction solar cell at slightly more than 30%, by means of singlet fission (SF) in molecular acenes, the molecular analog to multiple exciton generation (MEG). In an optimal SF process, the lowest singlet excited state of one molecule (S1) that is positioned next to a second molecule in its ground state (S0) is down-converted into two triplet excited states (T1) each residing on one of the two adjacent molecules. The two triplet states initially form a correlated pair state 1(T1T1), which then evolves into two separated triplet states (T1 + T1). As such, the energetic requirement for SF is E(S1) ≥ 2 ´E(T1). Shifting the focus to intramolecular SF in dilute solutions rather than intermolecular SF in crystalline thin films enabled the following important breakthroughs:
Firstly, we demonstrated that in a series of pentacene dimers, which were linked by a myriad of molecular spacers, SF takes place with quantum yields of up to 200%.
Secondly, we identified all key intermediates in the SF process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons.
Thirdly, we employed those parts of the solar spectrum, in which pentacene dimers lack absorptions, in non-resonant, indirect excitation of the SF materials via two-photon absorptions or intramolecular Förster resonance energy transfer.
Finally, we succeeded in showcasing the use of up to 200% triplet quantum yields by the realization of 130% carrier multiplication in solar cellsfor pentacene dimers immobilized onto semiconductors.