Concentration dependence of triplet-triplet upconversion under electrical excitation

Abstract Triplet–triplet upconversion (TTU) is a promising strategy to enhance the efficiency of organic light-emitting diodes (OLEDs) by converting non-radiative triplet excitons into radiative singlet excitons. While much research has investigated TTU under photoexcitation, its behavior under electrical excitation remains insufficiently understood. In this study, the concentration dependence of TTU materials near the donor/acceptor interface was systematically examined using exciplex upconversion-type OLEDs (ExUC-OLEDs). By adjusting the donor composition or utilizing polymer blends, the triplet diffusion was effectively controlled. It was found that reducing the TTU material concentration below a certain threshold suppressed diffusion of triplet excitons, resulting in the confinement of triplet excitons. As a result, the current density at which TTU becomes dominant over monomolecular decay decreased. These findings provide valuable insights into the design of TTU-based OLEDs and contribute to a deeper understanding of TTU processes under electrical excitation.