The importance of accounting for non-radiative decay when screening materials for singlet fission: the case of Pigment Red 254

Abstract

Singlet exciton fission (SF) is a process in which two triplet (spin-1) excitons are generated in a material for every absorbed photon. If properly harnessed, SF could increase photovoltaic efficiency beyond the single-junction thermodynamic limit, from ≈30% to >40%. However, to date no solid-state SF sensitiser has proven ideal for this application. To solve this problem, recent work using quantum chemical calculations to screen materials deposited in the Cambridge crystallographic database [D. Padula, O. H. Omar, T. Nematiaram and A. Troisi, Energy Environ. Sci., 2019, 12, 2412–2416] has identified several candidates. Of them, Pigment Red 254, a common automotive dye pigment, appears an ideal candidate, with the correct energetics, crystal structure, stability, absorption properties and cost. Here we use steady-state and transient spectroscopy of films of Pigment Red 254 to test for the presence of SF. We find that rapid picosecond non-radiative decay out-competes SF, re-populating the ground-state within 20–40 ps. The rapid non-radiative decay is orders of magnitude faster than expected from Jortner's empirical gap laws. Further, we perform TRIR spectroscopy to investigate which molecular vibrations mediate this decay, and find high infra-red activity in the hydrogen bonds that form between molecules in the solid state. We propose a model for proton transfer mediated by a conical intersection with the ground state of an alternate molecular conformer. Our results highlight the importance of including non-radiative decay in calculations aimed at screening molecules for photonic or energy applications.