Author
Seiichiro Izawa, Materials and Structures Laboratory, Institute of Integrated Research, Institute of Science Tokyo
Keywords
Organic light-emitting diode, Organic semiconductor interface, Upconversion, Charge transfer state, Triplet excited state, Energy transfer
Abstract
Organic light-emitting diodes (OLEDs) are already commercially available and used in smartphones and large-screen televisions because they can display vivid images. However, blue OLEDs typically require high driving voltages due to the high energy of blue light, which often limits device stability and power efficiency. To address this issue, blue upconversion (UC)-OLEDs that utilize triplet-triplet annihilation (TTA) to achieve emission at an ultra-low voltage of approximately 1.5 V have been developed. This study investigates the fundamental single-electron transfer process from the interfacial charge-transfer (CT) state to the triplet excited state (T1) of the blue TTA emitter molecule, which is relevant to the emission process of blue UC-OLED. Firstly, the CT state and T1 energies of 45 different combinations of anthracene-based donors and naphthalene diimide-based acceptors were estimated by the optical methods. As a result, a correlation between the energy difference (between the CT state and T1) and TTA-UC emission efficiency that is mainly governed by the single electron transfer process between these states is discussed based on Marcus theory. The results demonstrate that the transfer process is most efficient when the energy difference is near zero. This finding indicates that facilitating efficient transitions and minimizing voltage loss could be realized at the same time. These insights offer a strategic framework for optimizing low-voltage blue OLEDs for future energy-saving display and lighting technologies