Enhancing Chemical Stability and Photovoltaic Properties of Highly Efficient Nonfullerene Acceptors by Chalcogen Substitution: Insights from Quantum Chemical Calculations

The chemical stability of nonfullerene acceptor (NFA) is the Achilles' heel of the research on state-of-the-art organic solar cells (OSC). The fragility of the NFA is essentially due to the weak bond that links the central donor core of the molecules with their acceptor moieties at the edges. Here we proposed the replacement of thiophene at the outer-core position of traditional NFAs for tellurophene, a hitherto unexplored modification. Since tellurium is a distinctive element among chalcogens, the basic features of Te compounds cannot be deduced straightforwardly from the properties of their lighter analogues, S and Se. The modeled Te-based NFAs presented interesting features like stronger intra- and intermolecular interactions induced by a distinctive secondary bond effect between the end acceptor moiety and the outer chalcogen atom. This design strategy resulted in stiffer molecules with red-shifted absorption spectra and less susceptible to degradation verified through stress tests and vibrational spectra analysis. Besides that, a weakened exciton binding energy has been found, opening the possibility of blends with a lower driving force. Our results shed light on several aspects of selenation and telluration of traditional NFAs, providing valuable insights into the possible consequences for OSCs applications.