Organic Luminescent Molecule with Energetically Equivalent Singlet and Triplet Excited States for Organic Light-Emitting Diodes

We demonstrate an organic molecule with an energy gap between its singlet and triplet excited states of almost zero ($\Delta E_{\mathrm{ST}}\sim 0\mathrm{eV}$). Such separation was realized through proper combination of an electron-donating indolocarbazole group and a diphenyltriazine electron-accepting moiety. Calculated and measured $\Delta E_{\mathrm{ST}}$ were 0.003 and 0.02 eV, respectively. A total photoluminescence efficiency of $59\%\pm 2\%$ with $45\%\pm 2\%$ from a delayed component and $14\%\pm 2\%$ from a prompt component was obtained for a doped film. Organic light emitting diodes containing this molecule as an emitting dopant exhibited an unexpectedly high external electroluminescence efficiency of ${\eta }_{\mathrm{EQE}}=14\%\pm 1\%$.

Figure 1

Conceptual energy diagram of an organic luminescent molecule with a zero gap between the singlet ($S_1$) and triplet ($T_1$) excited states under (a) optical and (b) electrical excitations, including intersystem crossing forward and backward between the $S_1$ and $T_1$ excited states. In the photoluminescence process, singlet exciton formation occurs after photoexcitation. Some of the excitons decay radiatively and some convert to the $T_1$ state through intersystem crossing. Some of the triplet excitons then decay radiatively or nonradiatively to the ground state ($S_0$) and some upconvert to the $S_1$ state again through thermal upconversion. This process continues back and forth until the excitons reach a final thermal equilibrium. The upconverted singlet excitons decay through TADF. Under electrical excitation, excitons are directly formed as 75% triplet and 25% singlet, and the same exciton decay processes are followed. Thus, the large branching ratio of triplet excitons under electrical excitation is well harvested as light emission through TADF. ($k_r$ is the fluorescence decay rate, $k_{\mathrm{r}}$ the nonradiative decay rate from the $S_1$ excited state, $k_p$ the phosphorescence decay rate, $k_{\mathrm{np}}$ the nonradiative decay rate from the $T_1$ excited state, $k_{\mathrm{ISC}}$ the intersystem crossing rate, $k_{\mathrm{RISC}}$ the reverse intersystem crossing rate, $M_1{}^{*}$ the singlet exciton, $M_3{}^{*}$ the triplet exciton).

Figure 2

Molecular structures of PIC-TRZ and PIC-TRZ2 with their HOMOs and LUMOs calculated at the PBE0/6-31G(d) level of theory. The calculated energy difference between the $S_1$ and $T_1$ excited states is $\Delta E_{\mathrm{calc}}=0.080\mathrm{eV}$ for PIC-TRZ and 0.003 eV for PIC-TRZ2.

Figure 3

(a) Photoluminescence decay characteristics of 6 wt % PIC-TRZ2: mCP, and (b) emission spectra of prompt (black line) and delayed (red line) components at $T=5\mathrm{K}$. Spectra corresponded well with each other, indicating that the delayed component is caused by TADF. (c) Photoluminescence decay characteristics of 6 wt % PIC-TRZ2: mCP at room temperature ($T=300\mathrm{K}$), indicating the presence of clear two exponential decays with transient lifetimes of 83 ns and $2.7\mu \mathrm{s}$. (d) Temperature dependence of the radiative decay rate of the TADF component, indicating $\Delta E_{\mathrm{ST}}=0.02\mathrm{eV}$.

Figure 4

(a) Photoluminescence quantum efficiency (PLQE) of PIC-TRZ2 at $T=300\mathrm{K}$ in various host layers with different triplet energies. Total PLQE (black) is the sum of QE based on prompt fluorescence efficiency (red) and QE based on TADF (green). (b) Corresponding transient decay curves in mCP (red), TPT1 (green), and TAPC (blue) host layers and a neat film (black).

Figure 5

OLED characteristics of five devices using PIC-TRZ2 as an emitter under steady-state conditions. ${\eta }_{\mathrm{ext}}\mathrm{\text{−}}J$ characteristics for devices (a) I–IV, and (b) V–VII. The highest external EL efficiency of $14.0\%\pm 1\%$ was achieved for device V.