High Efficiency Non-Doped Organic Light Emitting Diodes Based on Pure Organic Room Temperature Phosphorescence by High-Lying Singlet Exciton Fission

Heavy-metal-free pure organic room temperature phosphorescence (ORTP) holds great potential in the field of organic optoelectronic devices owing to 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 internal physical mechanism based on these ORTP molecules. Here, a high-performance OLED induced by an unexpected interfacial spin-mixing process between the ORTP molecule and interlayers is demonstrated, and the high efficiency electroluminescence (EL) mechanism is studied 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 itself. Further, the HLSF process and the corresponding energy level position are confirmed by the incident wavelength- and temperature-dependent PL spectra and the magnetic-field-dependent transient PL. Finally, by optimizing the interfacial material adjacent to the emissive layer to utilize this interfacial spin mixing effect, a high-efficiency non-doped ORTP-OLED with external quantum efficiency of 16% and CIE coordinates of (0.27, 0.49) is developed. The proposed mechanism during the EL process will give insight to produce more efficient OLEDs based on ORTP materials in the future.