Pushing the Boundaries of Endothermic Singlet Fission in Organic Semiconductors through Structure and Energetic Control

Singlet exciton fission (SF) is an exciton multiplication process in organic semiconductors in which a photoexcited singlet exciton is converted to two triplet excitons. SF based solar cells help break the Shockley-Quisser limit as high energy photons are used to generate two electron-hole pairs per photon absorbed with power conversion efficiencies (PCE) ~40%. Energetics of the states involved in SF determine the nature of driving force for SF, making it either an entropy-driven endothermic system or enthalpy-driven exothermic system. Many efforts to understand and implement SF in photovoltaics (PV) have been undertaken in the recent years. However, a fundamental understanding of triplet formation and physical parameters that govern the process is incomplete, and of paramount importance. In this dissertation, we study how two key parameters influence endothermic SF: morphology and energetics. SF occurs via two steps in acenes: a photoexcited singlet, S1, decays to form TT, and TT breaks into two free triplets, T1. The role played by morphology or the local geometry of molecules in triplet generation via SF is vital, as the process depends on the placement of adjacent molecular pairs. Morphology, determined by molecular structure as well as sample preparation methods, is thus linked directly to the efficiency of SF. We first demonstrate this in a model system: endothermic SF nanoparticles. Further, we engineer these nanoparticles to suit triplet transfer into inorganics for PV applications. The effect of molecular and crystal structures on SF dynamics is further explored in a new class of photo-stable molecules, thienoanthracenes. We investigate the endothermicity of thienoanthracenes by assessing their singlet and triplet energies. Thereafter, we delve into thermodynamic and kinetic parameters that determine efficient SF in thienoanthracenes. We choose one of the thienoanthracenes as a model system to understand what drives endothermic SF efficiently in it. An entropy based model with a statistical mechanics perspective is built to predict key factors that help assess the thermodynamic feasibility of SF in endothermic systems.Dr. Manmohan Singh Scholarship (2015-2019) Winton Programme for the Physics of Sustainability ERC as part of the European Union’s Horizon 2020 research and innovation programme (grant agreement No 758826