Achieving Highly Efficient Fluorescent Blue Organic Light‐Emitting Diodes Through Optimizing Molecular Structures and Device Configuration

Abstract

Based on the results of first‐principles calculations of the electronic properties of blue light‐emitting materials, the molecular structures of oligofluorenes are optimized by incorporating electron‐withdrawing groups into the molecules to balance hole and electron injection and transport for organic light‐emitting diodes (OLEDs). The result is a remarkable improvement in the maximum external quantum efficiency (EQE) of the undoped device from 2.0% to 4.99%. Further optimization of the device configurations and processing procedures, e.g., by changing the thickness of the emitting layer and through thermal annealing treatments, leads to a very high maximum EQE of 7.40% for the undoped sky‐blue device. Finally, by doping the emitter in a suitable host material, 4,4’‐bis(carbazol‐9‐yl)biphenyl (CBP), at the optimal concentration of 6%, pure blue emission with extremely high maximum EQE of 9.40% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.147, 0.139) is achieved.

Highly efficient blue fluorescent organic light-emitting diodes with a high emitter/host ratio

“…[10][11][12] In particular, for high efficiency fluorescent OLEDs, the typical doping concentration is much lower than that for phosphorescent OLEDs, and the device performance is very sensitive to the doping concentration, e.g., a slight fluctuation of doping concentration can significantly change the device efficiency. [13][14][15] In this paper, however, we report that highly efficient blue fluorescent OLEDs with a maximum external quantum efficiency (EQE) of 6.1% can be made at extremely high doping concentrations of up to 50 wt. %, which is beyond the definition of doping.…”

“…It is a generally accepted theorem that in a guest-host system the energy transfer efficiency is proportional to the spectral overlap between the emission spectrum of a host and the absorption spectrum of a guest molecule. 12,15,17 The relatively small overlap (see Figure 3) indicates that the energy transfer, either via a F€ orster or a Dexter process, from Bepp 2 to BCzVBi should be less favorable than that from CBP to BCzVBi. Therefore, the high efficiency observed using Bepp 2 host suggests that additional physical process should be considered in this device structure.…”

“…The 6.1% EQE is among the highest efficiencies of blue OLEDs using BCzVBi as a dopant, [18][19][20][21] and is also among the highest EQEs ever reported for blue fluorescent OLEDs. 15,[22][23][24] Figure 5 shows the corresponding EL spectra of devices shown in Figure 4. It can be seen that there are two dominate EL peaks, one at $450 nm (peak-I) and another at $470 nm (peak-II).…”

Reverse intersystem crossing from upper triplet levels to excited singlet: a ‘hot excition’ path for organic light-emitting diodes

“…However, the high fabrication cost of POLEDs due to the utilization of rather expensive and unsustainable transition metals will critically restrict their extensive applications. Although the FOLEDs possess much lower fabrication cost, only singlet excitons (25% of the total excitons) can be used and triplet excitons decay to the ground state as non-radiative transition [37,38]. As a result, the maximum EQE of FOLEDs is limited to 5%.…”

Photoluminescence properties of new Zn(II) complexes with 8-hydroxyquinoline ligands: Dependence on volume and electronic effect of substituents

“…The photophysical properties of B1-B4 such as emission maxima wavelength (λ em ), fluorescence quantum yield (φ S ) and average fluorescence lifetime (τ) were all listed in Table 3. As shown in Table 3, all the four complexes emit yellow to reddish orange color in methanol solution with maximum emission peaks at 572 nm of B1, 583 nm of B2, 602 nm of B3, 606 nm of B4 respectively [20]. Due to the space effect and strong electron-donating effect of the methoxy group, Complex B1 gave the shortest emission wavelength (Fig.…”