High efficiency non-doped organic light emitting diodes based on pure organic room temperature phosphorescence by high-lying singlet exciton fission

Abstract Heavy-metal-free pure organic room temperature phosphorescence (ORTP) holds great potential in the field of organic optoelectronic devices owing to their low economic cost, simple preparation techniques and high exciton utilization. However, it is still filled with challenges in realizing high efficiency organic light-emitting diodes (OLEDs) and exploring the behind internal physical mechanism based on these ORTP molecules. Here, we demonstrate a high performance OLED induced by an efficient interfacial spin-mixing process based on an ORTP molecule, and study the electroluminescence (EL) mechanism through magneto-electroluminescence (MEL) and magneto-photoluminescence (MPL) measurements. The steady-state and transient PL properties imply that the interfacial effect is related to a high-lying singlet fission (HLSF) process in the ORTP molecule, which will not only increase the number of triplet excitons, but also largely enhance the fluorescence lifetime, thus improving the intersystem crossing (ISC) efficiency. Furthermore, the HLSF process and the corresponding energy level position are confirmed by investigating the incident wavelength- and temperature-dependent PL spectra. Based on this interfacial spin mixing effect, the fabricated non-doped OLEDs based on the ORTP molecule as emissive layer exhibit high efficiency, and the external quantum efficiency is as high as 16% with CIE coordinates of (0.27, 0.49) by optimizing the interfacial material adjacent to the emissive layer. The proposed mechanism during EL process will give insight to produce more efficient OLEDs based on ORTP materials in the future.