Upconversion emission using the interfacial charge transfer state

Abstract

Triplet–triplet annihilation (TTA) is the upconversion process in which one high-energy singlet exciton is created from two low-energy triplet excitons. TTA is widely utilized in the field of organic optoelectronics because it can emit light at almost twice the energy of the initial excited state. TTA is a two-photon process, thus the spatial density of triplet excitons must be large for efficient TTA. By sensitizing the triplet excitons of the emitter molecule using charge transfer (CT) states at the interface, we achieved highly efficient TTA emission in solid-state photon upconversion systems and organic LED devices with an ultralow driving voltage. This is attributed to the location and high concentration near the D/A interface of the triplet exciton sensitized by the CT state, responsible for the efficient concentration-dependent TTA process. This report explains the properties of CT states and introduces recent results related to TTA.