An experimental perspective on symmetry breaking and the singlet fission mechanism in solid-state materials.

Singlet fission (SF) materials have attracted attention in the field of solar cell research for decades. The deployment of novel and efficient SF molecules necessitates solution processibility and high SF yield in the solid state. SF materials can be integrated with silicon or organic photovoltaics, most commonly as a thin film layer on top of the semiconducting material. These films can be inhomogeneous, consisting of different structural domains with different crystalline structures or degrees of crystallinity, making the study of SF in the solid state highly challenging. Morphological inhomogeneity may also disturb the local molecular or crystal symmetry, leading to potential changes in the SF mechanism. In this perspective, we analyse and combine approaches in both selecting model systems and using advanced spectroscopic methods to devise an approach to rigorously analyse SF materials in the solid state, with special emphasis on discussing how symmetry breaking (SB) processes such as formal charge transfer and SB vibrations can impact the key mechanistic steps of SF. From a material perspective, we critically discuss the advantages and drawbacks of using dimers/oligomers, nanoparticles, and aggregates as model systems, as well as methods for manipulating and controlling molecular stacking environments. Then, we provide a brief review of recent developments in advanced spectroscopic methods and the key findings of research relevant to SF in the solid state.