A Method to Realize Efficient Deep-Red Phosphorescent OLEDs with a Broad Spectral Profile and Low Operating Voltages

Organic light-emitting diodes (OLEDs) used as phototherapy light sources require sufficient spectral distribution in the effective wavelength ranges and low operating voltages. Herein, a double emitting layer structure consisting of a red-emitting Ir(piq)2acac and a deep-red Ir(fliq)2acac was designed to generate a broad electroluminescence spectrum. An efficient TCTA:CN-T2T exciplex system was used as the host of the emitting layer, facilitating effective energy transfer from the exciplex host to the red and deep-red phosphors. The materials used in the exciplex host were also used as the carrier transport layers to eliminate the energy barriers and thus increase the current density. The hole injection layer structures were varied to examine the hole injection capabilities and the carrier balance. The resulting optimized phosphorescent OLEDs with a broad spectral profile exhibit a 90% coverage ratio in the target ranges from 630 to 690 nm, together with a high peak efficiency of 19.1% (10.2 cd/A and 13.8 lm/W). The proposed device only needs 5.2 V to achieve a power density of 5 mW/cm2, implying that the device could be driven via two series-connected button cell batteries. These results illustrate the feasibility of our design concepts and demonstrate the realization of a portable and lightweight OLED phototherapy light source.

Red-Emitting Phosphor Ca2MgSi2O7: Sm3+, Eu3+ for UV White Light-Emitting Diodes

In this research, a series of Ca2MgSi2O7: 2% Sm3+, x% Eu3+ (x=4, 5, 6, 7, 8) red phosphors were synthesized using the high-temperature solid-phase method. The phase and luminescence properties of the samples were investigated using an X-ray diffractometer and a photoluminescence spectrometer. The results showed that the synthesized samples were of pure phase, and the introduction of small amounts of Sm3+ and Eu3+ had no significant impact on the crystal structure of the phosphors. It is observed from the phosphor spectra that the co-doped samples exhibited intense red-light emission corresponding to that of Eu3+ at 613 nm when excited at the strongest excitation peak of Sm3+ (401 nm); here, the emission peak intensity of Eu3+ increased by a factor of 5.3. It is found that resonant nonradiative energy transfer occurs from Sm3+ to Eu3+ in the sample, and the energy transfer efficiency reaches up to 44%. The calculated critical distance for energy transfer is 16.117 Å and the concentration interrupt mechanism is an electric dipole-dipole (d-d) interaction. The color coordinates of the samples are all located in the red region (0.6319, 0.3676) with a color purity of ~ 89.3%. The samples exhibited thermal stability of up to 68.6% of that at room temperature when heated to 150˚C . The LED samples packaged with the phosphors emitted warm white-light with a color temperature (CCT) of 5553 K and a color rendering index (Ra) of 84.8. The magnesium yellow feldspar silicate phosphor is suitable as the red component in white LED trichromatic phosphors and has potential applications in solid-state lighting.