Highly efficient single-stack hybrid cool white OLED utilizing blue thermally activated delayed fluorescent and yellow phosphorescent emitters

Abstract

Highly efficient single-stack hybrid cool white organic light-emitting diodes (OLEDs) having blue-yellow-blue multiple emitting layers (EMLs) are designed and constructed by utilizing blue thermally activated delayed fluorescent (TADF) and yellow phosphorescent emitters. The out-coupling efficiencies of yellow and blue emissions are maximized by tuning the ITO and total device thickness that satisfies both of antinode positions for yellow and blue emissions in a limited multiple EML thickness. To obtain a cool white emission, the exciton formation ratio in the blue-yellow-blue multiple EML system is controlled by manipulating the recombination zone through charge conductivity variation of host medium in the blue TADF EML. The resulting device exhibits cool white emission with very high maximum external quantum efficiency of 23.1% and CIE color coordinates of (0.324, 0.337). We anticipate that the studied approach will raise the viability of single-stack hybrid cool white OLEDs for high performance display applications.

Molecular Design Strategies for Color Tuning of Blue TADF Emitters

“…The optimized high dopant concentration (35%) of I–III in DPEPO was chosen after screening concentrations of 15–50% and is consistent with the optimized 30% of DDMA-TXO2, compound I , in a mixed host reported by Lee and Lee . It has been shown that the hole transport from the TADF material is a reason for needing such high concentrations of TADF material, as DPEPO does not transport holes, and so the TADF emitter must also perform this role. , …”

Bond Rotations and Heteroatom Effects in Donor–Acceptor–Donor Molecules: Implications for Thermally Activated Delayed Fluorescence and Room Temperature Phosphorescence

“…It has been shown that the hole transport from the TADF material is a reason for needing such high concentrations of TADF material, as DPEPO does not transport holes, and so the TADF emitter must also perform this role. 36,37 The same device architecture was employed for I−III to enable a meaningful comparison within the whole series, rather than comparing with previously reported devices of I, which used different architectures. 20,21 For all devices, the maximum EQEs achieved were in agreement with the measured PLQYs (approximated using EQE = PLQY × charge balance × outcoupling = 0.25 × PLQY), assuming there is no horizontal molecular alignment.…”

Color-Tunable Thermally Activated Delayed Fluorescence in Oxadiazole-Based Acrylic Copolymers: Photophysical Properties and Applications in Ratiometric Oxygen Sensing

“…The development of efficient emitters exhibiting thermally activated delayed fluorescence (TADF) has generated significant research interest due to their utility in the fabrication of high-performance organic light-emitting diodes (OLEDs). − TADF allows for efficient reverse intersystem crossing (RISC) of triplet excitons to singlets via ambient thermal energy, allowing a purely organic material to harvest 100% of the excitons in an electroluminescent device. While many examples of high-quantum-yield TADF emitters spanning the visible spectrum have now been reported, − the majority of these require costly high-vacuum sublimation to deposit thin films for devices. , OLED technology offers the potential for durable, high-efficiency, and high-contrast displays; yet the cost of OLED fabrication has, to date, largely limited the application of this technology to comparatively expensive consumer electronics such as televisions and mobile phones. , As such, there is a significant need for inexpensive methods for generating high-quality thin films exhibiting TADF, which would increase the uptake of OLEDs in low-cost applications such as large-area lighting. , Furthermore, many TADF emitters exhibit broad emission spectra as a result of their donor–acceptor architecture. , Although a challenge to the application of TADF in display technologies where high color purity is essential, broad emission can be an asset in lighting applications where white light is desired. − …”