Time-resolved x-Ray spectroscopy to study luminophores with relevance for OLEDs

Abstract

Organic light-emitting diodes (OLEDs) are employed in innovative display technologies and have the potential to be used widely in large area lighting. Luminophores containing 4d or 5d metals are efficient in electro-luminescent devices due to their phosphorescent properties but are scarce and expensive. Alternatives take advantage of temperature-activated delayed fluorescence (TADF), e. g. in Cu(I) complexes. We show modern X-ray spectroscopy methods exemplified on Ru, Ir and Cu-based luminophores offering insight into the capabilities of such techniques to researchers from various fields. Knowledge of structural rearrangements in the excited state is crucial to understand non-radiative energy losses and to develop efficient luminophores. Pump-probe X-ray absorption spectroscopy (XAS) gives information about the molecular structure in the excited state and the electronic structure. We show how this information can be complemented with X-ray emission spectroscopy at X-ray free-electron lasers (XFEL) and discuss the potential of time-resolved X-ray excited optical luminescence (TR-XEOL) to provide additional selectivity of XAS to luminescent sites.