Cathodoluminescence spectroscopy of triplet excitons in organic emitters

Abstract

Singlet and triplet excited-state energies are fundamental to a variety of processes and applications of organic semiconductors, such as singlet fission, triplet–triplet annihilation, and thermally activated delayed fluorescence. Measuring the triplet energy via traditional optical methods is often challenging though, due to spin selection rules governing these excited states following optical excitation. Here we examine cathodoluminescence (CL) spectroscopy in an electron microscope as a candidate to circumvent these challenges. Both singlet and triplet luminescence bands are observed in a single continuous wave CL spectrum, as the electron beam directly excites singlets alongside plasmons that decay into electron–hole pairs forming triplets. This process removes the need for intersystem crossing to generate triplet excitons in the emitter molecule itself. It is estimated that on average 104 plasmon decay events are required to form a single exciton. The mechanism is in principle similar to electroluminescence as in OLED devices, with CL also allowing the target material to be investigated in its native state (or dilute in insulating media) without electrical biasing or adjacent transport layers. We propose that CL is potentially a powerful technique for organic semiconductor characterisation, although not yet capable of resolving triplet emission from more challenging emitters. Care must also be taken to minimise electron beam damage, since the luminescence from singlets is preferentially quenched, due to a longer diffusion length compared to triplets at room temperature.