Pursuit of singlet fission fulvene candidates using inverse design

Abstract

The pursuit of higher solar cell efficiency has sparked considerable interest in singlet fission materials, which offer the possibility to surpass the Shockley-Queisser limit. Singlet fission (SF) is a multiexciton process in which a singlet exciton is converted into two triplet excitons, doubling the number of low energy excitons per absorbed photon. A central bottleneck in the development of SF-enhanced solar cells is the limited number of chromophores fulfilling the strict energy-matching conditions required for SF. Tetracene and pentacene remain the most investigated SF materials despite their poor photostability. To address this challenge, we propose an inverse molecular design protocol to accelerate the discovery of suitable SF candidates based on fulvene derivatives. The excited-state energies of fulvenes can be modulated by the electronic nature of substituents and they behave as aromatic molecular chameleons with the ability to arrange their π-electrons to exhibit aromaticity in both the ground and the lowest triplet state. Using a best-first search algorithm, we explored the combinatorial chemical compound space to identify optimal substitution patterns satisfying the primary thermodynamic SF condition (i.e., E(S1) ≈ 2E(T1)). This approach represents a significant improvement over traditional design methods. Database analysis revealed 17 fulvene candidates fulfilling all the conditions for SF-enhanced silicon solar cells. Next to identifying potential SF chromophores, we derived general design rules highlighting the critical role of strategically positioning electron-donating and electron-withdrawing groups across the six substitution sites. The proposed protocol represents a significant step forward in the quest for efficient SF materials, offering a systematic approach to navigating chemical compound spaces and optimizing molecular structures with desired properties.