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.

Organic light-emitting diodes (OLEDs) have attracted considerable attention for display and lighting applications. However, improving device stability and operational lifetime remains a major issue. In this study, exciplex upconversion-type OLEDs (ExUC-OLEDs) utilizing a donor/acceptor interface were investigated to achieve low-voltage operation and enhanced device stability. Compared with conventional OLEDs, the ExUC-OLED exhibited superior voltage stability and a 35 times longer operational lifetime. Displacement current measurements confirmed that charge accumulation was effectively suppressed even after long-term operation and ideal carrier recombination at the interface was achieved. Analysis focusing on carrier dynamics provides new insights into the degradation processes of OLEDs.

The development of optical devices using organic semiconductor materials, such as organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic photodiodes (OPDs), is actively being pursued. The operational mechanisms of OLEDs, OPVs, and OPDs are essentially inverse processes, and by combining appropriate materials, a single device can exhibit both electrical-To-optical and optical-To-electrical conversion functions. We have been developing organic multifunction devices (MFDs) that possess both functions as a new advancement in organic optical devices. This paper introduces the donor/acceptor (D/ A) stacked MFDs we have been working on recently. © 2025 International Society of Functional Thin Film Materials & Devices (FTFMD).