EMISSION MECHANISM OF NEXT GENERATION ORGANIC LIGHT-EMITTING MATERIALS

Abstract

Unlike conventional fluorescence or phosphorescence materials, next generation organic lightemitting materials can harvest both the first excited singlet (S1) and first excited triplet (T1) states for light emission with a nearly 100% of emission efficiency. In particular, materials showing thermally activated delayed fluorescence (TADF), so-called TADF materials or TADF molecules, have attracted considerable attention because TADF materials can be consisted of only light elements, such as H, C, and N, having a potential to reduce a production cost of organic light-emitting diodes (OLEDs). Currently, a synthesis of highly efficient of TADF materials is a serious demand for the development of OLEDs. A lack of understand of the TADF emission mechanism is the bottleneck of the synthesis of TADF molecules. TADF, which is a radiative transition from S1 states produced via thermally up-conversion of T1 state, is generally known to rely on the energy difference between S1 and T1. Therefore, control of the energy difference enhances the TADF emission efficiency. However, the actual rate cannot be determined by the energy difference only. This fact suggests an importance of excited-state dynamics for the TADF emission mechanism. Ultrafast time-resolved spectroscopies are powerful tools to study excited-state dynamics of materials. Especially, transient absorption spectroscopies can directly detect time behaviour of both S and T state. In this talk, I give a general introduction of TADF materials and concept of thermal up-conversion of T state. Then, using our ultrafast spectroscopic techniques, I show our recent results of the detail emission mechanism of TADF. Our results aid in designing highly efficient TADF materials.