Highly efficient blue electroluminescence based on thermally activated delayed fluorescence

Abstract

Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.

Thermally Activated Delayed Fluorescence Emitters for Deep Blue Organic Light Emitting Diodes: A Review of Recent Advances

“…Notably, they demonstrated the decay rate of the delayed fluorescence to be greatly improved by extending the π-conjugation of both the electron-donating and electron-accepting parts. By elongating the π-conjugation, the oscillator strength could be increased and ∆E ST simultaneously lowered even for molecules where a small overlap is observed between the HOMO and LUMO energy level [80]. These design rules constitute a second key-element for the design of TADF emitters.…”

Red Organic Light‐Emitting Diode with External Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed Fluorescence Emitter

“…Since highly efficient thermally activated delayed fluorescence (TADF) emitters were introduced into organic light‐emitting diodes (OLEDs), TADF‐based OLEDs have attracted great attention by their promising full exciton utilization and are considered as the third generation of OLEDs 1, 2, 3, 4. Generally, efficient reverse intersystem crossing (RISC) process and high fluorescence quantum yield ( Φ F ) are two essential requirements for efficient TADF emitters 1, 5.…”

“…Generally, efficient reverse intersystem crossing (RISC) process and high fluorescence quantum yield ( Φ F ) are two essential requirements for efficient TADF emitters 1, 5. For the former one, an extremely small singlet‐triplet energy split (Δ E ST ) between lowest singlet excited state (S 1 ) and lowest triplet excited state (T 1 ) is highly desired to up‐convert triplet excitons to singlet excitons through thermal excitation 4, 6, 7, 8, 9, 10.…”

“…Thus, we assumed all exciton loss on DPXZ‐BPPZ is via the nonradiative transitions of triplet excitons. And the main kinetic parameters of DPXZ‐BPPZ can be estimated according to the following equations1, 16, 17, 18 $k_{\mathrm{normalp}}=\hspace{0.17em}\frac{1}{{\tau }_{\mathrm{normalp}}}$ $k_{\mathrm{normalF}}\hspace{0.17em}=\hspace{0.17em}{k}_{\mathrm{normalp}}{\mathrm{normal\Phi }}_{\mathrm{normalF}}$ $k_{\mathrm{normalp}}\hspace{0.17em}=k_{\mathrm{normalF}}\hspace{0.17em}+k_{\mathrm{ISC}}$ $k_{\mathrm{TADF}}=\hspace{0.17em}\frac{{\mathrm{normal\Phi }}_{\mathrm{TADF}}}{\hspace{0.17em}{\mathrm{normal\Phi }}_{\mathrm{ISC}}{\tau }_{\mathrm{TADF}}}$ $k_{\mathrm{RISC}}=\frac{k_{\mathrm{normalp}}{k}_{\mathrm{TADF}}}{k_{\mathrm{ISC}}}\hspace{0.17em}\frac{{\mathrm{normal\Phi }}_{\mathrm{TADF}}}{{\mathrm{normal\Phi }}_{\mathrm{normalF}}}$ $k_{\mathrm{nr}}^{\mathrm{normalT}}=k_{\mathrm{TADF}}\hspace{0.17em}-\left(1-\frac{k_{\mathrm{normalISC}}}{k_{\mathrm{F}}+k_{\mathrm{normalISC}}}\right)k_{\mathrm{RISC}}$where Φ TADF is the delayed fluorescence quantum efficiency; Φ ISC is the ISC efficiency; k p , k RISC , and k TADF are the rate constants of prompt fluorescence, the RISC, and delayed fluorescence decay, respectively; $k_{\mathrm{nr}}^{\mathrm{normalT}}$ is the nonradiative decay rate constant of triplet excitons. Φ F and Φ TADF are estimated from the Φ PL with a relative ratio of 2:3 which was calculated from the transient PL spectra according to the literature 34.…”

The (Non-)Local Density of States of Electronic Excitations in Organic Semiconductors

“…Deep-blue emitters for OLEDs, for example, are still vulnerable to degradation processes related to the long lifetime of the highly energetic excited triplet state [36]. In this context, thermally activated delayed fluorescence is a mechanism that repopulates singlet via triplet states [37], thus not only maintaining high external quantum efficiencies, but also rendering heavy metal complexation for spin-orbit coupling redundant. The triplet-to-singlet conversion rate may, however, still be too slow to effectively prevent degradation and maintain high efficiencies at high luminances.…”